COMPARISONS, HOMOLOGY AND PHYLOGENY OF VERTEBRATES
Biology W3002y -- Structure and function of the vertebrates
This material is to provide a guide to Chapters 1-3 in Walker and Liem Functional anatomy of the vertebrates, and to provide supplementary material. The discussion of the classification, relationships and evolution of the Chordata given in this handout supersedes that given in the textbook. For background purposes, I have included an extensive discussion of some basic principles of evolution, classification and phylogeny.
A) Evolution: Evolution simply change with respect to time. Organic evolution may be defined as: Any modification in the characteristics of organisms with respect to time. Four major points must be noted, namely:
1) Evolutionary change in organisms is the modification in features from that present in one generation of organisms to that present in future generations. Organic evolution is not change observed in the same object (= the same organism). Thi s distinguishes organic evolution from transformational changes seen in the same object, such as the evolution of the solar system or ontogenetic change during the life of an organism.
2) The modification observed in organisms does not have to be hereditary, that is, does not have to be change based upon genetic modification and variation. Any observed change, or difference between chronologically successive populations whi ch is a result of modification from an ancestral stock must be regarded as an evolutionary change no matter whether it is one based upon differences in hereditary or not. Environmentally determined modifications - physiological adaptation, phenocopies, so matic changes and etc. - are all valid evolutionary changes. Such modifications are generally adaptive changes. Moreover, it is not possible to separate observed modifications in the fossil record, between living taxa or between populations of the same sp ecies into those that are genetically determined and those that have been environmentally induced. This distinction becomes especially difficult when dealing with single features of organisms which interact with other features and may modify because of ch anges in these other features.
3) The minimum time period for an evolutionary change is one generation. Thus the difference between organisms and their offspring may be regarded as an evolutionary change. This excludes differences that appear during the life span of a sing le individual organism from evolutionary changes. Any minimum time period longer than one generation would necessitate choice of arbitrary delimitation of evolutionary change.
4) Evolutionary change, especially in sexually reproducing organisms, is a change observed in populations, not change observed from single individual organisms to single descendent organisms.
Evolutionary change refers to the modifications that organisms undergo with the passage of time. Evolution is not the same as phylogeny which is the history of life. Organisms evolve through time and leave trails [= time lines] of phylogeny b ehind them.
Evolution is not necessarily speciation which is the formation (multiplication) of new species. A clear distinction must be maintained between the two processes of organic evolution which are (a) phyletic evolution and (b) speciatio n.
a) Phyletic evolution is evolutionary change occurring in a single phyletic lineage of organisms with respect to time. A single phyletic lineage is the time line of a species [= chronological series of interbreeding populations]. Speci es evolve, phyletic lineages are the result of the evolution of species. Phyletic lineages represent history; they are not subject to causes, do not evolve or change in any way. A species is the cross-section of a single phyletic lineage at one point in t ime.
The mechanisms of phyletic evolution are:
i) Production of genetic variation in the population by (1) recombination of all sorts by crossing over, segregation of chromosomes, translocation, etc, which forms new combinations of existing genetic material; (2) influx of genetic material from other populations by gene flow; and (3) by mutations which adds, slowly and continuously new genetic material to the population.
ii) Production of genetically-based phenotypic variation in the population which depends on the genotype of each individual organism and on the environmental influences during ontogenetic development.
iii) Over production of offspring during reproduction which is a property of all organisms. The result of over production of offspring is the eventual, and generally rapid, increase in the size of the population beyond the supporting capacity of their environment. One of the major roles of over production of offspring is to permit the population to rebuild its numbers quickly after a disaster, be it a regular winter kill or an accidental environmental catastrophe.
iv) Maintenance of a stable level of a population close to the carrying capacity of its environment, and hence elimination of most of the over production of individuals during each breeding period. This elimination of excessive individuals ha s two components, one of which is random, accidental death in which the causation of the death of the individual is not related to its phenotypical properties. The other component results in a nonrandom survival and reproduction of individuals whose pheno typical characteristics permit to cope best with conditions of their environment. The individuals possessing the best phenotypical properties for life in their environment are those that survive and hence breed and contribute their genetic material to the successive generation. Environmental factors, the agents of selective demands, interact only with phenotypical features of individual organisms. a distinction must be made between selective demands and natural selection. Natural selection is now defined as nonrandom differential reproduction of genes -- a result or outcome definition. This definition does not state what are the causes which result in natural selection. Natural selection is, therefore, change in gene pool of the entire interbreeding popul ation. Selective demands are causes arising from the external environment and interact with individual organisms, differentially favoring and not favoring individuals. Selective demands interact with the phenotype of whole individual organisms, not with g enes or genotypes. Selective demands can result in natural selection.
The unit on which selective demands act is the individual organism. But, the interbreeding population, not the individual, is the unit of evolutionary change. All causes of evolutionary change act up individual organisms.
v) The genotype is the genetic material possessed by a single individual.
The gene pool is the total genetic material possessed by a single interbreeding population. Frequency of genes in the population is an essential aspect of the gene pool.
The phenotype is the manifested aspects, morphological, physiological, etc. of the organism. The phenotype is the result of the interaction between the genotype and the environment in which the individual develops. Any particular genotype doe s not give rise to a single phenotype, but to a spectrum of phenotypical expressions depending upon the range of environments in which the individual develops. Modification of the phenotype after completion of development because of change in the environm ent, e.g,, physiological adaptation, is an aspect of the general notion that the phenotype is a consequence of the genotype interacting with the environment during development.
vi) Selective demands are an important but not the only mechanism of evolution. Selective demands act only in the present, never in the past or the future. Selective demands cannot act with forethought. It can only choose between those phenot ypical variants that exist in the population at the present time and must choose the better of the available variants. Selective demands can reject or it can accept all variants, but it cannot defer its action because the best possible choice has inherent defects. Selective demands cannot predetermine the phenotypic variants offered to it; its mechanism is only to choose.
Selective demands acts according to definite relationships between the phenotypical properties of organisms and the requirements of the environment. These demands may be regarded as the design aspect of evolution.
A clear distinction must be made between selective demands and natural selection. Selective demands are a cause of phyletic evolutionary change. In modern evolutionary theory, natural selection is defined as "nonrandom differential reproducti on of genes" which is an outcome, not a cause. Natural selection is the result of the action of causes which are unstated in this definition. These causes include the production of genetically-based phenotypic variation, selective demands and possibly oth er causes such as differential reproduction, genetic drift and other causes resulting from chance events in small populations.
vii) The production of genetic variation that underlies phenotypical variation is an accidental or chance-based mechanism with respect to the selection forces acting on the organism at the present time or in the future. This accidental aspect of evolution determines which phenotypical variants are produced and offered to selection for choice.
Other accidental factors of evolution exist such as the timing of a change, the geographical and ecological location of the species in which a change occurs, and the other features of the species in which a change occurs.
viii) All evolutionary changes depend upon the simultaneous actions of the dual mechanisms of the accidental origin of genetic variation and the design action of natural selective demands. Both mechanisms determine the course of evolutionary change. Therefore, the origin and modification of any feature has an accidental component and a design one. Part of the answer to why a particular feature evolved is that is the feature that appeared because of the accident mechanism of the production of genetic variation. The other part of the answer is that particular featured was favored by the selection forces acting on the organism.
2) Speciation is the formation of new species, and more precisely the multiplication of species. The consequence of speciation is the splitting of a single phyletic line into two or more separate phyletic lines, each of which is underg oing its own phyletic evolution by the mechanisms discussed above.
a) The generally accepted definition of a species in sexually reproducing organisms is: A species is composed of groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.< /B> I will advocate a slightly different definition, namely: A species is composed of groups of actually or potentially interbreeding natural populations which are genetically isolated from other such groups. A species represents the cross-s ection of a single phyletic line at one point in time. The species concept applies only to sexually reproducing organisms and is most readily applied to organisms coexisting geographically and temporally. If organisms are studied over a larger and larger geographical area, the species concept "breaks down" in some cases; that is, it is not possible to determine whether geographically displacing populations are members of the same species taxon or members of different species taxa. In the study of organism s over longer time periods, the concept of species loses all meaning. Instead, the concept of phyletic lineages and their splitting becomes important.
It should be emphasized that a problem exists because the single word "species" is used for the species concept, for the species category and for the species taxon. The species concept is part of basic biological theory. Only sexually reprodu cing organisms are organized into species. The species category specified the rank of the species in the taxonomic hierarchy and must be defined to cover all organism, both sexually and asexually reproducing. The species taxon is the actual taxonomic grou p of individual organisms distinguished and described in taxonomic work.
Species are not the same as the phyletic lineage which is the time course of a species as it reproduces itself generation after generation through time. The species is a section of the phyletic lineage at any point in time. As such spe cies at different time sections of the phyletic lineage are not separated from one another by a definite boundary. Species do not have a beginning and do not have an age -- all presently existing species can be said to be equally old. Species have boundar ies with respect to one another with respect to horizontal comparisons -- at the same period of time, but do not have vertical boundaries with respect to one another -- along the same phyletic lineage.
Each species taxon (considering sexually reproducing organisms only) represents a complex of mutually adapted complex of features adapted to a particular set of environmental factors. Under the biological species concept, species taxa possess three major attributes or can be regarded as representing three different types of units; these attributes or units are:
1) Genetic isolation (a genetic unit) in that members of a species taxon form a genetic community which is genetically isolated from other species taxa. Gene flow exists among members of a species, but not from one species to another u nder natural conditions. The species is the largest unit in which gene flow or genetic recombination can take place. Genetic isolation between members of different species taxa is maintained by genetic isolating mechanisms which result from phenotypic pro perties of the individuals of the species taxa. The set of genetic isolating mechanisms is not identical with the set of reproductive isolating mechanisms although there is a broad overlap between the two sets. Unfortunately biologists have not distinguis hed between the set of genetic isolating and reproductively isolating mechanisms, but have combined them together under the heading of intrinsic isolating mechanisms.
2) Reproductive isolation (a reproductive unit) in that members of a species taxon form an interbreeding community which is reproductively isolated from other species taxa. Members of one species taxon do not interbreed or attempt to i nterbreed with members of another species taxa under natural conditions regardless of the barriers to gene flow between these species taxa. Reproductive isolation between species taxa is not the same as genetic isolation. Two species taxa could be genetic ally isolated without being reproductively isolated such as horses and donkeys which can reproduce, but produce sterile mules. Most evolutionists have not made any distinction between genetic isolation and reproductive isolation. Interbreeding between mem bers of different species taxon is prevented by certain, but not all intrinsic isolating mechanisms which can be called reproductive isolating mechanisms.
3) Ecological isolation or separation (an ecological unit) in that members of a species taxon form an ecological community in which the organisms possess similar ecological requirements that differ from those of members of other specie s taxa. Because species taxa are different ecological units means that competition between sympatric members of fully evolved different species taxa will be greatly reduced or will not exist. The fact that members of a species taxon constitute an ecologic al unit depends on the possession of similar "ecological" features in these organisms which differ from the "ecological" features present in members of other species taxa. The phenotypic features resulting in ecological isolation insure that individual or ganisms of a species taxa are functional, integrated whole organisms that are viable in the normal environment occupied the species taxon. These features can be grouped together under the general heading of the adaptive features of the species taxon.
Species taxa in asexually reproducing organisms are characterized only by the last property, and the limits of these species taxa are largely arbitrary, corresponding roughly to the ecological limits of species taxa in sexually reproducing ta xa.
b) Species taxa are maintained by the existence of isolating mechanisms which are defined as biological properties of individuals that prevent the interbreeding of populations that are actually or potentially sympatric.
1) Geographic isolation or extrinsic isolating barriers are ones that prevent populations from interbreeding during a period when they would do so in the absence of such a barrier. Geographical (extrinsic) barriers are essential for th e process of speciation. Such barriers are not a (intrinsic) isolating mechanism and must be carefully distinguished from the set of intrinsic isolating mechanisms.
2) Intrinsic isolating mechanisms for genetic isolation may be classified as:
a) Mechanisms that prevent interspecific crosses (pre-mating).
*1) Potential mates do not meet (seasonal and habitat isolation).
*2) Potential mates meet but do not mate (ethological isolation).
*3) Copulation attempted, but no transfer of sperm takes place (mechanical isolation).
b) Mechanisms that reduce full success of interspecific crosses (post-mating mechanisms).
*1) Sperm transfer takes place, but egg is not fertilized (gametic mortality).
*2) Egg is fertilized but zygote dies (zygote mortality).
3) Zygote produces an Fl hybrid of reduced viability (hybrid inviability).
4) F1 hybrid zygote is fully viable, but partially or completely sterile, or produces deficient F2 (hybrid sterility).
The subset of these isolating mechanisms marked with an asterisk (*) are those which also result in reproductive isolation.
The result of the genetic intrinsic isolating mechanisms is prevention of gene flow between two species. Individuals of different species may mate and even produce hybrid offspring, but without resulting gene flow (exchange of genetic materia l) between the two species. Reproductive isolating mechanisms prevent reproduction between members of different species.
c) Speciation among sexually reproducing occurs by external barriers that isolate two subgroups of a species for a sufficiently long period to permit the evolution of intrinsic isolation mechanisms in each subgroup. The basic external barrier in sexually reproducing animals and many plants is by the existence of a geographical barrier which separates the original species into two allopatric segments.
Allopatric: of populations or species, occupying mutually exclusive geographic areas.
Sympatric: of two or more populations occupying the same geographic area; more precisely, the existence of one population in breeding condition within the geographical area (cruising range of individuals) of another population.
Evolution of intrinsic genetic isolating mechanisms occurs as a byproduct of other evolutionary changes. Intrinsic genetic isolating mechanisms are never favored by selective demands acting to improve them as intrinsic isolating mechanisms. R eproductive isolating mechanisms can evolve as the result of selective demands if the two species are separated by genetic isolating mechanisms.
The intrinsic genetic isolating mechanisms must be 100% effective when the geographical barrier disappears and the two allopatric populations are able to invade each other's range and become sympatric. Selection of intrinsic isolating mechani sms after the two newly evolved species become sympatric is not to improve them as isolating mechanisms, but to reduce their reproductive cost -- that is selective demands act on these intrinsic isolating mechanisms only as reproductive isolating mechanis ms. Post-mating intrinsic isolating mechanisms are as effective as pre-mating intrinsic isolating mechanisms, but involve a much higher reproductive cost. Selective demands can act to reduce this cost and hence modify the isolating mechanisms after the in itial sympatry, but these selective demands are not for isolating mechanisms as such.
Speciation is not completed with the breakdown of the geographical barrier, but has a sympatric phase during which the two newly appeared species exert mutual selective demands on each other because of ecological interactions and breeding int eractions, with resulting divergence of the two species. The mutual selective demands each species exert on the other during their initial period of sympatry may be strong and the rate of divergence is high. As the species become more different, they no l onger interact during the breeding season (no longer attempt to form interspecific pair bonds) and no longer compete; hence the mutual selective demands decline to zero.
B) Phylogeny is the history of life. It is the lineages of organisms arranged chronologically, showing the branching of phyletic lines and the sequences of events and changes in the characteristics of the organisms. Phylogeny is not th e same as evolution, but is the result of the evolution of organisms. Obviously only correct phylogeny exists for any group of organisms, but no one has ever observed a phylogeny directly. Paleontologists and neontologists must use the same types of obser vations and make the same conclusions in studies of phylogeny. Any statement about the presumed phylogeny of a group of organisms is a scientific hypothesis that is offered for disproof; statements about phylogeny can never be proven. If repeated attempts to disprove a particular phylogeny have failed, then one's confidence in that particular phylogeny is strengthened. The confidence in a particular phylogeny depends upon the types of observations used in the attempts to disprove it as well as the continu ed failure to disprove the phylogeny.
Phylogenies are ascertained by determination of homologies, arrangements of morphological characteristics into sequences, attempts to ascertain primitive and advanced features, recognition of unique paradaptations, correlations of features an d sequences and so forth. All methods depend upon the consequences of evolutionary mechanisms and phenomena.
C) Homology is a concept applied to features of organisms and connotes a certain type of relationship, namely features that stem from the same feature in the common ancestor and possess certain shared characteristics because of this de scent from the same feature in the common ancestor. Hence, homology means common phylogenetic origin and may be defined as: A feature (or condition of a feature) in one organism is homologous to a feature (or condition of a feature) in another organism if the two features (or conditions) stem (= can be traced back) phylogenetically from the same feature or condition in the immediate common ancestor of both organisms. This is a phylogenetic definition of homology and is strictly noncircular in that homology is defined in terms of phylogeny and phylogeny is defined in terms of evolution.
Please note that in this definition of homology that:
(a ) No mention of morphological similarity appears in the definition of homology.
(b) No mention of similarity in ontogenetic development appears in the definition of homology.
The definition of homology has nothing to do with the similarity of features. Degree of resemblance and common origin are quite distinct problems of phylogenetic study and must be kept separate. Homology is not an intrinsic property of a feat ure, such as its color or mass, but a relationship depending upon the existence of corresponding features in other organisms. Homology can be applied to any attribute of organisms - morphological features, developmental sequences, behavioral displays and so forth - so long as the definition applies to them.
Nonhomology, the opposite of homology, applies to features or conditions in two or more organisms that do not stem phylogenetically from the same feature or condition in the immediate common ancestor of these organisms. Frequently the term an alogy is used in place of the term nonhomology, but such a usage should be discouraged because of past and continued ambiguity and multiple use of analogy. The most consistent definition for analogy is for two features that are similar morphologically bec ause of similar functions (= actually similar biological roles). Thus homologous features can be analogous and nonhomologous features can be analogous. This term has the identical or a similar meaning as convergence which is unambiguous and should be used for features that have become more similar to one another during the course of their evolution. And, if one examines the use of the term analogy, it is always used in the meaning of convergence.
Statements about homology should never be put in the form "The skull of the human is homologous" which does not express a relationship, or in the form "The arm of the gorilla is homologous to the arm of the chimpanzee" or "The quadrate of bir ds is homologous to the incus of mammals" which are meaningless because they do not state what the homologous relationship is. A proper statement about homology must always include a "conditional phrase" which states the conditions of the homology. Thus t he correct form of a statement on homology is: "The wing of birds is homologous to the wing of bats as the forelimb of vertebrates", or "The pectoral flipper of whales and the pectoral fin of sharks are homologous as vertebrate fore limbs".
The conditional phrase in a statement about homologous features describes the presumed characteristics of the feature in the presumed common ancestor. The homologous features in different organisms would agree in sharing these properties. The oretically, one would have to describe in detail the characteristics in the conditional phrase, which is actually done when one knows little about the phylogeny and classification of the group of organisms under study. In better known groups, a short cut is used if the conditional phrase is given "as the tetrapod forelimb", and this abbreviated style should be recognized as such.
After a proper comparative study, decisions must be reached whether features in different organisms are homologous. Repeated determinations of homologues must be undertaken using more and more restricted conditional phrases until one has a hi erarchial series of homologies. Any decision about homologous or nonhomologous features is a scientific statement that may be disproven, but can never be proven. Decisions must be made about the homology of features with the realization that many of them may be in error. The lack of a sharp demarkation between homologous and nonhomologous features must also be recognized. A broad gray zone exists between the two limits, but one does not have the luxury of deciding that features are "probably homologous", or "probably nonhomologous".
The methods by which homologous features are recognized must be in agreement with the general principles of evolution and phylogeny. But these methods cannot be based upon earlier conclusions on the presumed phylogeny or the relationships of organisms under study, Any such methods for recognizing homologues would result in circular reasoning. The basic methods for recognizing homologous features are founded on similarities of various sorts between the features. Thus morphological similarity, functional similarity, the same ontogenetic development, a similar pattern of relationships to other features (e.g., nerves innervating muscles, pattern of articulation of bones, etc.) and so forth. The rationale for using methods based upon similarities for determining homologues is based upon the argument that if two features stemmed phylogenetically from the same feature in their common ancestor, then these features were identical at that time. Each feature might change somewhat during the evolution of each lineage from the common ancestor. But the features will still be the same for those aspects that did not change during the evolution of either line. And these shared similar aspects would provide evidence for the homology of the features. Moreover, the longer the period of time between the species under study and their common ancestor, the more chance for change. Therefore, the conditional phrase would be less detailed. Thus, if one concludes that the features are still homologous with increasingly restrictive conditional phrases, then one can conclude that the common ancestor had been more recent in time. And in general, species belonging to the same taxon of a lower categorical rank (e.g., a genus or family as compared to an order or class) would have had a more recent common ancestor and possess homologues of more restrictive conditional phrases.
The pattern of study in ascertaining presumed phylogenies is:
a) Comparative study of features among the species under question.
b) Determination and testing of individual homologues, establishing hierarchial series with ever more restrictive conditional phrases. A large number of homologies must be ascertained because of the low resolving power of the methods, and hen ce the good chance of erring in both directions. Each homologous feature must be tested independently of the testing of other homologues.
c) Testing phylogenetic hypotheses about groups of organisms based on these homologies. Presumably the pattern of individual homologues should agree with the phylogeny of the species. Conflicts will be found in almost all studies which can be resolved in several ways. A larger number of homologous features can be used in the phylogenetic study with the notion that decision on the phylogeny can be reached by a correlation between the numbers of homologues that show the same pattern of distribu tion. Or each feature can be studied in more detail to discover which homologues appear to be valid (i.e., one has more confidence in them).
Statements about phylogenies are scientific statements that may be disproven, but never proven. Confidence in a phylogeny is gained after repeated failed attempts to disprove the phylogeny using features and comparisons best designed to dispr ove the phylogeny.
The study of actual phylogenies is one of the most demanding of biological disciplines. In spite of the two hundred years of work on the phylogeny and classification of organisms, great gaps in our knowledge still exist. One should, for examp le, examine most statements about the phylogeny of vertebrates with great care. Once one attempts statements any more precise than reptiles arose from amphibians and gave rise to birds and reptiles, difficulties start to appear. Perhaps the best informati on is known about the phylogeny of mammals during the Cenozoic for which a superb fossil record is available.
D) Comparisons: In biology, comparisons of species should be separated into two types.
1) Vertical comparisons are between members ("species" at different time levels) of the same phyletic lineage.
2) Horizontal comparisons are between members (different species) of different phyletic lineages whether or not these species exist at the same time.
The observations, conclusions and interpretations which may result from each of these comparisons are different, and those of horizontal comparisons are not necessarily applicable to vertical comparisons and vice versa.
Differences observed in horizontal comparisons of features evolved under the control of the same selective demand (different adaptive answers) are "paradaptive" (= besides adaptation) with respect to that selective demand. Different paradapta tions arise because of different accidental evolutionary mechanisms associated with the production of genetic variation. Differences observed in vertical comparisons are adaptive if the evolution of the feature is under the control of selective demands. P>
At the onset of a comparative study, one does not know which comparisons are horizontal ones and which are vertical ones. These conclusions must be reached after a proper study of the features and whether their properties are ones that may be regarded to represent different paradaptations.
E) Classification is the arrangement of species into monophyletic taxonomic groups or taxa (taxon, singular). These taxa are arranged into hierarchial, non-overlapping sets with each taxon containing all members of the contained taxa o f the next lower level. The level of taxa correspond to one categorical level of the taxonomic hierarchy. A category is a level in this hierarchy such as genus, family or order; it is not a group of organisms. Categories are defined e.g., the genus repres ents the level between species and family, and contains one or more species. Taxa are natural groups of organisms and hence are diagnosed, not defined.
Monophyletic is applied to a group of organisms that descended from a single common ancestor. A monophyletic taxa contains species that descended from an ancestral taxa of the same or lower categorical level. Thus a monophyletic taxon contains species that descended from an ancestral order, family, genus, or species. If possible, the breadth of the ancestral taxon is restricted as much as possible. Thus it is preferable to recognize an order that descended from an ancestral genus than an ancestral order. It is not necessary to include all descendants of an ancestral taxon in a monophyletic descendant taxon If one lineage becomes markedly different, then it may be set off as another taxon of equal rank.
A polyphyletic taxon is an unnatural or artificial group and contains species which have descended from two or more ancestral taxa of the same or higher categorical rank. Hence if members of an order descended from two ancestral orders , than that taxon would be polyphyletic.
It should be clear that monophyletic and polyphyletic taxa are not separated by a sharp boundary, but grade into one another with a large gray intermediate zone. This gradual gradation between monophyly and polyphyly corresponds exactly to th e gradual gradation between homology and nonhomology.
F) Phylogeny and classification of the Chordata: -- This section is to supplement the material in Walker and Liem, Chapters, 2 and 3. You are responsible for knowing the taxa that are in bold. You should know their major characteristic s (including the diagnostic attributes), when these features appeared and underwent major modifications in each group during the history of the chordates, the phylogenetic relationships of these taxa and when they appeared in the fossil record. You should have some notion of the major factors in the evolution of vertebrate features.
1) Chordate relatives: The phylum closest to the Chordata is the Echinodermata. The Hemichordata are often considered to be a separate phylum, but will be treated as a subphylum of the Chordata in this course. The Hemichordata contain two main groups, the Enteropneusta (or acorn worms) and the Pterobranchia ("moss animals"). Another group that may belong here are the Pogonophora, a group that has become much better known in recent years. A pecu liar group of fossil animals -- the Calcichordata -- are sometimes placed in the Echinodermata and sometimes in the Chordata, but are perhaps better placed in the Echinodermata. Some workers have argued that these fossils may represent the intermed iate stock between echinoderms and chordates. Some of the basic similarities of Chordates and Echinodermata are ontogenetic development, larvae morphology and energy-rich phosphorus compounds.
2) Phylum Chordata: The Chordata are characterized by the possession of gill slits, notochord, dorsal hollow nerve cord and a post-anal tail at some stage of their life history. Structures as gill slits and dorsal, hollow nerve cords are peculiar features among animals, and unique to the Chordata. Not all members of the Chordata possess all of these features, but may have lost one or another during their evolution. Chordates probably evolved from a worm-like animal possessing a segmented body (musculature and coelom), circular, transverse and longitudinal muscles, and a non-rigid skeleton comprised of muscles acting on other muscles or perhaps on tubes of a gel-like material. The Chordata includes the follow ing subphyla:
a) The Cephalochordata (amphioxus or lancelets): The Cephalochordata are perhaps the most generalized of the lower chordate groups and may be taken as representative of the ancestral chordate group. They are filter feeders, similar to the larva of lampreys. In addition to the typical chordate features, lancelets possess a muscular tail, with the muscles arranged in myotomes, a large branchial chamber with endostyle, and cartilage bars in the gill arches, blood vessels and diffuse pumpi ng mechanisms. They possess a peculiar type of excretory mechanism - nephridia or flame cells - found in invertebrates and quite distinct from the kidney present in all chordates. Amphioxus differs from vertebrates in its lack of a heart, blood cells and blood pigments, neural crest (and all features derived from the neural crest (hence the cartilage in gill arches has a different origin in amphioxus), bone and a complex skin. The notochord in lancelets extends to the anterior tip of the nose, further ant erior than in the vertebrates. This difference may result from anterior growth of sense organs and the head in vertebrates, and not from an anterior growth of the notochord in lancelets.
Lancelets burrow in sandy bottoms and must have a strong muscular system in order to move through the sand. They are poor swimmers and lack structures to control movement through the water. They lie buried in the sand with just their mouth pr ojecting into the water and filter feed. The waste water exiting from their gills moves away from the animal through the spaced between the sand granules.
Some workers believe that amphioxus is a neotenic descendent of agnathan fishes because the ammocoete larva of lampreys is very similar to the adult lancelet. Two points that argue against this idea are the presence of nephridia (flame cells) in lancelets and hence absence of kidneys, and the lack of the neural crest in lancelets. Neither feature should have been lost in a neotenic descendent of agnathan fishes.
The cephalochordates are important in analyzing the early phylogeny of the vertebrates because they possess many features similar to those in the vertebrates while still retaining features of other early chordates and chordate relatives. They may represent an ancestral marine chordate group which invaded fresh water and gave rise to the vertebrates. Such a sequence would be in agreement with the difference in excretory organs in amphioxus and the vertebrates. Reasons for the origin of the neu ral crest are still obscure, but it is of interest to compare amphioxus and the vertebrates to ascertain the presence and structure of neural crest derivatives in amphioxus.
b) The Urochordata (tunicates or sea squirts): The Urochordata are typically sessile (in the adult stage), filter feeders, covered with a tunic, without definite symmetry, and a nerve net. Most of the body is composed of a large branch ial chamber [= pharynx] with numerous gill slits and an endostyle. The notochord, dorsal, hollow nerve cord and post-anal tail have been lost in the adult. The larva is free swimming, with a muscular tail, notochord, dorsal hollow nerve cord and gill slit s. Some tunicates never attach to the bottom and metamorphosis into the adult form, but remain as free-swimming sexual-reproducing adults. Paedogenesis or neoteny is the loss of the adult stage (or breeding as a larva) during evolution, and may permit a n ew radical course of evolution in the environment and life habit of the larva instead of the adult. In these tunicates, the sessile, asymmetrical adult stage is lost, and the larval, free-swimming stage, with bilateral symmetry and a definite anterior hea d, forms the basis of further evolutionary changes. This particular neotenic modification has been considered by some workers as the basic event that change the course of chordate phylogeny from a sessile animal to a bilateral free-swimming organism that led to vertebrates. However, other evidence argues against this conclusion, and that the neotenic, free-swimming pelagic turnicates are dead-end evolutionary lineages.
Adult tunicates are sessile, fixed to a hard substrate and obtain food by filter feeding. Hence they need a tough protective sheath around themselves. Larval tunicates are free swimming for a period of time, during which time they disperse. S ome species do not feed as larvae, others with a longer larval period will feed. At the end of the larval period, the animal swims toward the bottom of the water, locates a site for attachment and attaches to the bottom. Then the larva metamorphoses into the adult form, losing the tail, the notochord, the dorsal hollow nerve cord and becomes a sessile animal with roughly radial symmetry. Free-living tunicates retain the larval stage throughout their life.
c) The Hemichordata (acorn worms and pterobranchs): Hemichordates are classified by some workers as a subphylum of the Chordata and by others as a separate phylum; I will accept the former arrangement. In either case, these animals are closely related to other chordates. The two major groups of the Hemichordata -- the Enteropneusta (acorn worms) and the Pterobranchia (moss animals) -- which are very different from one another and it is sometimes difficult to comprehend wh y these two groups are placed together in the same taxon.
The acorn worms have a long worm-like body, are sedentary mud burrowers and filter feeders. They possess numerous gill slits, a small dorsal nerve cord with hollow spaces and a short hollow pouch in the base of the proboscis which has been ho mologized with the notochord. This hollow tube is continuous with the cavity of the gut which agrees with the close relationship of the notochord to the endoderm and gut cavity in the ontogeny of amphioxus and the vertebrates. Features such as the probosc is and collar are specializations of the acorn worms, associated with their burrowing habits. Acorn worms possess only very long fibered longitudinal muscles which contract rapidly and pull the animal quickly into the burrow in case of danger. The muscula r proboscis serves as a locomotory device to pull the animal slowly forward to the anterior end of the burrow so that the head and mouth of the animal can project out into the water and obtain water for filter feeding.
Acorn worms live in a burrow in the mud; this burrow has two ends. The head of the animal projects from one end of the burrow and the animal filter feeds. The collar between the mouth and the gill slits blocks the tube, thereby preventing was te water from leaving the anterior end of the burrow and mixing with the new food-laden water. Hence the waste water must exit from the posterior end of the borrow.
Pterobranchs are small sessile animals, living in a hard tube attached to the substrate. At least some forms still possess gill slits, but then only one or two pairs. They have a collar and a proboscis "similar" to those of acorn worms. Tenta cle-like structures - lophophores - project from the collar region; these are branched and possess cilia. The lophophores served to capture food and transport it to the mouth. The digestive tube is U-shaped somewhat like that in tunicates; this similarity may, however, be the result of sessile life.
d) The Pogonophora: These worm-like animals may be related to the Hemichordata, but their affinities are still unclear. Also the Calcichordata (a fossil group) may be related to the Hemichordata or possibly to the most ancestral members of the Echinodermata. An important change from the Chordata to the Echinodermata would be the shift from gill filter feeding to tentacle (arm) filter feeding with the appearance and evolution of tentacles from the collar of the Hemichordata.
It is a matter of decision where the limits of the phylum Chordata are drawn - below or above the Hemichordata. The best place is probably at the separation between those groups still possessing gill slits and those groups which lost gill sli ts completely and use only tentacles for feeding. That is between the Hemichordata and the Echinodermata.
What is important is that the Echinodermata appear to have evolved from an advanced member of the early Chordata -- namely from the Hemichordata, rather than the Chordata evolving from an echinodermata-like ancestor. Please note the ph ylogeny (Figure 2-10) in Walker and Liem shows the reverse of this evolutionary change and the Hemichordata as a separate phylum.
e) The Vertebrata: The vertebrates are regarded as the "higher" chordates and are the dominant group of this phylum in every respect. In addition to possessing all of the typical chordate features, the vertebrates are distinguished fro m the other chordates by the following characteristics. The vertebrates are that part of early chordate radiation which left the oceans and invaded fresh waters. Invading fresh-waters required several major sets of evolutionary modifications, which are: P>
a) Better swimming ability because these animals had to swim upstream against the current of the rivers.
b) Development of a mechanism (kidney) to eliminate excess water from the body.
c) Development of a exoskeletal dermal bony armor as protection against large invertebrate predators existing in these fresh-waters at this time.
The features which evolved with the origin of the Vertebrata are:
[Features associated with "a"]
1) A vertebral column which may be most rudimentary in some primitive groups of fishes, but replaces the notochord and becomes the major support for the body.
2) A skeletal cranium (skull) of cartilage or bone which is a box about the brain and the major sensory organs with gill arches. The jaws and hyomandibular are later modifications of the anterior gill arches.
3) A neural crest and all features developing from neural crest cells. These features include the neuroglia cells of the brain, sheathing cells of the peripheral nervous system, all pigment cells, the second neuron of the autonomic nervous system , the cartilages of the gills and parts of the skull, cortex of the adrenal gland, etc.
4) A true skin formed of epidermis and a dermis, each several layers thick. The dermis is especially thick and complex.
5) A heart, ventral in position and containing several chambers.
6) Blood cells, both white and red; only a few fishes have lost red blood cells.
7) A definite liver (which is a peculiar organ in that the cells are uniform and relatively simple in appearance, but perform a large number of biochemical tasks) and a pancreas.
8) Specialized sense organs, such as eyes (of a definite type), olfactory organs, a lateral line system in fishes and amphibians and derived features such as the ear, one to three semicircular canals.
9) Several unique endocrine organs such as the pituitary thyroid (homologous with the endostyle of lower chordates, parathyroids, etc.
10) Well developed brain, cranial nerves and autonomic nervous system.
[Features associated with "b"]
11) Kidneys, with the nephron as the basic unit.
[Features associated with "c"]
12) Presence of bone as a skeletal element. Bone is not found in the very earliest vertebrates, but is found in the first vertebrates present in the fossil record. Bone first appears as exoskeletal bone, as a dermal armor in the skin.
Little is known about the earliest vertebrates because of a lack of fossils. They apparently evolved in fresh-water (at least all known groups descended from a fresh-water group), probably in the Cambrian period. Presumably vertebrates arose from a lancelet-like ancestor.
H) Vertebrate phylogeny: The following is a summary and comment of the discussion presented by walker and Liem. I shall try to emphasize problems, unknown areas and interesting topics. In case of any differences, follow the material in this handout and in the lectures.
A) Class Agnatha: These are the jawless fish which are the most primitive vertebrates and the earliest vertebrates found in the fossil record. All other vertebrates are grouped together as the Gnathostomata or "jawed-mouthed". The agna than fishes are found first in the Ordovician, some 500 million years ago. The major radiation of the Agnatha lasted into the Devonian with a few lines surviving until the present day. These extant forms are the cyclostomes - lampreys and hagfish - which have a very poor fossil record. One good fossil lamprey is known from the Carboniferous; this form does not differ significantly in its morphology from living lampreys.
The characteristics of the Agnatha are:
1) Lack of jaws.
2) Lack of paired fins - medial fins, including a tail fin, are present.
3) Rudimentary vertebral column (cartilage only)
4) Lack of lungs
Features that appeared in this group and retained in later vertebrate evolution are:
5) Internal skeleton, which is comprised of cartilage in the Agnatha.
6) Dermal bony armor
7) Tail fin with heterocercal condition (vertebral column lies in dorsal lobe).
8) Some have a "pectoral" fin that lacked an internal skeleton. The fin may be only a projection of the bony dermal armor as it is unclear whether this structure contains a soft component, including muscles. Moreover it appears that this structur e is not homologous with the pectoral fin of the later vertebrates.
9) Kidney with nephron of fresh-water type (large Bowman's capsule).
10) Extraembryonic membranes in the form of a trilaminate yolk sac may have evolved either in the agnathan fish or in the placoderms.
Most agnathan fishes are bottom dwelling, with a few being free-swimming pelagic fish. They are probably poor swimmers, but were probably the best adapted free-swimming fishes because they were the only ones. Basically the agnathan fish remained fi lter feeders, with some becoming scavengers, or parasitic. Some may have been active predators.
The nature of the deposits in which early agnathan fishes are found and the structure of the nephron in living forms suggests strongly that these animals arose in fresh waters. Moreover, the core phyletic line in fish evolution up to the orig in of tetrapods and the origin of the teleosts lived in fresh waters. Once the vertebrates left the oceans, the main evolutionary lines were fresh-water and terrestrial.
Vertebrates were not found as fossils until they possessed bone in their skeleton. Primitive bone was dermal, membrane bone in the form of a dermal head shield and large plates over the body. It had been argued that bone preceded cartilage in vertebrate phylogeny as a reaction to the earlier idea that the cartilaginous sharks represent the primitive vertebrates. Clearly the earliest fossil vertebrates are fully evolved vertebrates and clearly no fossils would be found so long as the skeleton was (uncalcified) cartilage. In the head shield of early agnathans, epichondral bone (laid down on the surface of a cartilaginous block) is found outlining every canal and foramen, suggesting strongly that cartilage was present before bone. Hence, the mos t reasonable conclusion is that cartilage preceded bone in vertebrate phylogeny. Bone presumably evolved because of two reasons. One is as a reservoir of mineral salts. The second and presumably the most important is as mechanical protection against large r invertebrate predators.
When the Agnatha appear in the fossil record, they are not only fully evolved members of this class, but have undergone considerable radiation as the earliest fossil agnathans are quite diversified. This suggests that the evolutionary origin of the Vertebrata occurred considerably earlier, perhaps even in the Cambrian.
The classical subdivision of the Agnatha is into two subclasses; namely:
1) The Ostracodermi (bony, fossil forms):
2) The Cyclostomata (lamprey and hagfish):
This is admittedly an artificial classification, but is sufficient for our purposes. A recent classification divides the class into two subclasses and six orders. Hagfish and lampreys are placed in different orders and superorders, suggesting that these eel-like agnathan fish evolved separately and have converged toward each other.
B) Class Placodermi: The placoderms represent the only vertebrate class that is extinct and hence the placoderms do not have any common names associated with them. They appeared in the Silurian and disappeared in the Permian. As the os tracoderms, the placoderms were mainly fresh-water, bottom dwelling forms. Some, the spiny sharks (the Acanthodii), were free swimming, pelagic forms. Most placoderms were small but the arthrodires were giant forms with a huge head shield and massi ve bony jaws.
The placoderms are important because many of the basic vertebrate features appeared in this group. Yet they should be regarded as an archaic group of fishes that was replaced by the radiations of "modern" fishes. It should be emphasized that no goo d evidence exists as to which group(s) of agnathan fish gave rise to the placoderms, as little is known of the detailed radiation of the placoderms and relationships of the major subtaxa, and which groups of placoderms gave rise to the later bony and cart ilaginous fishes.
The major attributes of the placoderms represent advances over the agnathans can be grouped into four major sets namely:
a) Better feeding apparatus associated with evolution of feeding on larger food items rather than filter-feeding.
b) Better swimming ability.
c) Respiratory system with the evolution of a lung to obtain atmospheric oxygen.
d) Urea retention system (if this did not evolve in the agnathan fish).
New vertebrate features evolving with the origin and specialization of the placoderms are:
[Features associated with "a"]
1) Development of jaws by the modification of the first gill (mandibular) arch into upper and lower jaws. The upper jaw is not fused with the brain case, but is mobile and forms the basis of the property of cranial kinesis.
2) In later placoderms (or in one advanced group that gave rise to modern fishes), the second gill (hyomandibular) arch became modified into the hyomandibular which provided support (the suspensorium) for the mandible. The ventral components of t he hyomandibular arch evolved into hyoid bones.
3) With the evolution of the hyomandibular, the gill slit between the mandibular arch and the second arch became restricted from ventrally until only the most dorsal part remained. This remnant became the spiracle present in many groups of fishes and evolved into the middle ear cavity and the eustachian tube in tetrapods.
4) Teeth are modified bony scales and presumably evolved in placoderms, although some forms lack ("lost" or "never had",?) teeth.
[Features associated with "b"]
5) The large bony plates of ostracoderms were "broken up" into smaller placoderm scales which are thick and contain several layers of bone, dentine and enamel.
6) Paired fins and girdles, both pectoral and pelvic, evolve with an internal skeleton (archipterygium) in the fin. This skeletal arrangement is termed the "archipterygium" and basic to future evolution of the skeleton in fins of fish and the limbs of tetrapods. The pectoral girdle skeleton is attached solidly to the posterior end of the skull, while the pelvic skeleton lies in the trunk musculature. Some variation exists in the number of paired fins in some placoderms (acanthodians), but two sets, the pectoral and pelvic, become fixed in the most other placoderms and in the lineage leading to advanced fishes.
[Features associated with "c"]
7) Lungs apparently appeared in this group as a pouch from the ventro-posterior end of the pharynx to serve as an accessory oxygen obtaining mechanism. As soon as the lung appeared, it also served as a buoyancy structure. The lung was retained in some groups of fish, evolved into the swim bladder in others and lost in yet others.
8) Subdivision of the atrium into a right and a left atria to separate the different pressures of the returning pulmonary and systemic bloods
[Features associated with "d"]
9) Conversion of ammonia to urea and retention of urea as part of the water-balance system.
[Other features; reproduction]
10) If it had not appeared in the Agnatha, a trilaminate yolk sac.
Most placoderms are bottom dwelling, poorly swimming fishes, but possessing decidedly more control than in the agnathans. One group, the Acanthodii or spiny sharks, are free swimming and quite different from other placoderms. Some workers feel that the Acanthodii should constitute a separate class of vertebrates because they do not share a definite set of characteristics with other placoderms. Other workers believe that the spiny sharks are the ancestral stock of the bony fishes and include them as a subclass of the Osteichthyes (as does Walker and Liem, p. 64). Evidence supporting this arrangement is weak. Features of the head that are shared by spiny sharks and later bony fishes may be convergent because of similar pelagic habits. Moreover, the A canthodii lack any internal skeleton in the fins. (The skeleton shown in Walker and Liem, p. 278, Fig. 8-9 is questionable). The fins in these fish are supported only by the large anterior spine. The fin structure in the Acanthodii is hard to align with t he basic fin skeleton of bony fishes, cartilaginous fishes and most placoderms. If the Acanthodii are a primitive group of the Osteichthyes, then one must conclude that the archipterygium fin skeleton evolved several times in the early vertebrates, or thi s skeleton was lost in the spiny sharks and re-evolved in the advanced bony fishes.
The large gaps in our knowledge of early evolution of vertebrates cannot be glossed over. We know little about the relationships of groups within and between the Agnatha, the Placodermi and the early groups of modern fishes. Little is gained by ere cting many new classes and subclasses because we do not know the detailed relationships of distinct groups, or to shift taxa from one class to another. The origin of the modern fishes is just as poorly known. The lack of fossils results from a scarcity of fresh-water deposits during the Silurian. At the onset of the Devonian, fresh-water deposits are once again widespread, and many fossils are found of placoderms, sharks and chimaeras, ray-finned fishes, lung fishes and crossopterygian fishes. For each gr oup, the earliest Devonian fossils represent fully evolved members of each higher taxon. No intermediate forms between these major groups are known, and few hints of trends in the evolution of individual features are known in these fossils. In addition, t he first tetrapods appeared in the middle Devonian. Clearly much evolution of higher fishes took place in the Silurian. But this evolution presumably took place in fresh water, and fresh water deposits are rare in the Silurian. With the evolution of the p lacoderms, most vertebrate features have evolved. Many major modifications in these features occurred with the evolution of the higher fish and the tetrapods, but very few new vertebrate features appeared after the evolution of the Placodermi. Hence in th is aspect, vertebrate evolution was largely over with the origin and evolutionary radiation of the Placodermi.
New vertebrate features evolving after the Placodermi:
1) Internal nares (Sarcopterygii).
2) A neck appeared (Amphibia).
3) The vertebral column started to differentiate into definite segments (Amphibia).
4) External gills evolved in the larval stage (Amphibia).
5) Keratinization of the skin (epidermis) and the evolution of horny, epidermal scales (Reptilia) which evolved into feathers in the Aves and into hair in the Mammalia.
6) Shelled egg with allantois and egg-white (Reptilia).
7) The method for eliminating nitrogenous wastes is using uric acid (Reptilia).
8) Homoiothermy, with a four chambered heart (independently in the Aves and Mammalia, with a different subdivision of the ventricle in these groups and different loss of half of the double dorsal aorta).
9) After birth, the neonate is nourished with a secretion (milk) from the mammary glands (Mammalia).
10) A dentary-squamosal jaw articulation (Mammalia).
C) Class Chondrichthyes: The sharks, rays and chimaeras, or cartilaginous fish, appeared in the earliest Devonian and remained as a major group of fish until the present day. Most likely the Chondrichthyes originated and underwent their earl y radiation in the Silurian. These fish are characterized by:
1) A complete lack of endoskeletal bone; their skeleton is composed of cartilage which is calcified in larger forms. Bone remains only in base of the dermal scales and teeth.
2) A kidney with nephrons with a large Bowman's capsule (fresh-water type), and use urea retention for water balance.
3) Loss of the air (or swim) bladder.
4) The spiracle is large in most species, but smaller to lost in pelagic sharks.
The Chondrichthyes are primarily marine forms with a few fresh water species (both fossil and recent). Most species rest on the bottom when not swimming actively; bottom-resting individuals must pump water actively through their gills. They have a typical heterocercal tail, flattened ventral surface and ventral subterminal mouth. The paired fins are large and have an internal skeleton. All forms have internal fertilization with the pelvic fins in most forms (males only) modified into claspers for t ransfer of sperm. Some species bear live young; most lay hard-shelled eggs with much yolk.
The Chondrichthyes are divided into two distinct subclasses; no intermediate forms exist. Some workers believe that these two subclasses arose independently from lower fishes (placoderms), but these two taxa share a number of features suggesting th at the Chondrichthyes are a good monophyletic taxon.
The subclass Elasmobranchii contains the typical sharks and rays. Their upper jaw is free and mobile on the brain case, small bony scales are present in the skin and the gill slits open externally, not covered by an operculum. The skates and rays are flattened with an expansion of the pectoral region of the body and gradual loss of the tail and posterior part of the body.
The second subclass Holocephali contains the chimaeras or rat-tailed fishes. Their upper jaw is fused solidly to the brain case and their gill slits are covered by a flap of skin. The males possess a peculiar frontal spine on the middle of t he forehead, just anterior to the eyes.
D) Class Osteichthyes: This class contains all the higher bony fishes and is a large a extremely diverse group. A good diagnosis of this taxon is difficult to provide. All members possess a partly bony to completely bony endoskeleton, and a series of other features that are shared by members of the Placodermi. At one time, it was believed that the placoderms lacked a specialized hyomandibular and spiracle, that is, the second arch was still an unmodified gill arch and had a complete gill sli t anterior to it. But, strong evidence suggest that some of the placoderms had acquired these features.
Romer and other workers (Walker and Liem, p. 64) place the spiny sharks -- the Acanthodii -- as a primitive subclass of the bony fishes. I prefer to retain them in the Placodermi.
The subdivision of the Osteichthyes is still debated. A classical system which I shall follow is to divide the class into two subclasses, namely:
a) The Actinopterygii or ray-finned fishes:
b) The Sarcopterygii or lobed-finned fishes:
The Sarcopterygii are divided into the Crossopterygii and the Dipnoi (lungfish). Fossil representatives of the Actinopterygii, Crossopterygii and Dipnoi are found in the early Devonian without any intermediate forms between these grou ps. Some workers regard the Dipnoi as a taxon of equal rank as the Crossopterygii and Actinopterygii. The origin of the Osteichthyes and of the major subdivisions, including the crossopterygians, the lungfish and the actinopterygians took place sometime i n the Silurian as fossils of all of these groups are found in the very beginning of the Devonian.
Subclass Actinopterygii: The ray-finned fishes are characterized:
1) Their fins being supported by spines and rays. Primitive forms have an internal skeleton in their fins.
2) They lack internal nares.
3) Possess a long or air bladder; the latter evolves into an air-bladder and subsequently into a swim-bladder in the more advanced groups.
4) Loss of the urea-retaining mechanism of water balance; hence marine groups had to evolve a specialized mechanism for water-balance.
Primitive members of the Actinopterygii are fresh-water and many present-day groups still are; many advanced forms represent phyletic lineages that never left fresh water. Evolution in the actinopterygian fishes resulted in one radiation being repl aced by a latter one. The major groups are difficult to distinguish from one another because of the continuous modification of characteristics. Earlier groups grade gradually into later ones. Evolution in the actinopterygians is largely associated with mo difications in swimming and in feeding.
Three major subdivisions of the Actinopterygii will be recognized.
1) The Chondrostei are the most primitive ray-finned fishes. They are fresh water, their jaws are movable on the brain case, tail is heterocercal, possess scales covered with ganoid, some fins (e.g. pectoral fin in Polypterus) possess es a lobed base with internal skeleton, a typical lung. Living chondrostean fishes are the sturgeon (Acipenser) of the northern hemisphere, the paddle-fish (Polyodon) of southern North America, and Polypterus of Africa. The latter is most interesting as it appears to be the closest representative of the palaeoniscoids, which are the large, earliest radiation of chondrosteans, and hence actinopterygian, fishes. Sturgeons and paddle-fish are specialized forms with a largely cartilaginou s skeleton, a sensitive rostrum and weak jaws. Their tail is heterocercal, like early palaeoniscoids, unlike the advanced almost symmetrical tail of Polypterus.
2) The Holostei represent a later radiation of ray-finned fishes that replaced the Chondrostei. The heterocercal tail shortened, becoming more symmetrical, the ganoid covering of the scales lost, and the jaws still movable, but shorter. The holosteans invaded salt water in the Mesozoic. Living forms are the fresh water gar pikes (Lepisosteus) and bowfin (Amia) of North America.
3) The Teleostei are the last and major radiation of fish and represent most bony fishes as we know them. They appeared in the Mesozoic (almost certainly in fresh water, not the oceans) and remained the dominant group of fish ever since. The tail is symmetrical in outer shape, although the skeleton still shows the remains of the primitive heterocercal condition. The bony scales are thin and flexible, having lost all of the outer ganoid layer. Fins are small, with the pelvic fins displaced an teriorly to the level of the pectorals in many groups.
The air bladder has modified to a swim bladder, losing all direct connection to the pharynx in advanced forms. In marine groups, the nephron has modified to one with a small Bowman's capsule, or even lost completely. The jaws are highly modified, b ut with the upper jaw still movable on the brain case.
Subclass Sarcopterygii: The lobed-finned fishes are characterized:
1) The fins possessing a heavy fleshy base with an internal skeleton.
2) Lungs or an air bladder are present, as are internal nares.
3) All forms have nephrons with large Bowman's capsules and use the urea retention method of water balance (also in the living marine Latimeria).
The Sarcopterygii are primitively fresh-water fish, with some lines always remaining in fresh water. The two main groups of sarcopterygian fish (which may be quite separate subclasses) are:
1) The Crossopterygii possess a movable upper jaw (cranial kinesis), including a most peculiar hinge that separates the brain case into two segments, strong fins. The crossopterygians include two groups, the rhipidistians and the coelacanths . The former gave rise to the tetrapods, and the latter are a specialized side group. The rhipidistians disappeared before the end of the Paleozoic, while the coelacanths lasted into the Mesozoic before apparently disappearing in the Cretaceous. However, at least one lineage of coelacanths remained until recent times. A living coelacanth (Latimeria) was discovered in 1939 off the east coast of Africa. This species is important as it provides evidence on the soft anatomy and physiology of crossopter ygians which gave rise to the tetrapods. But, one must remember that Latimeria is a coelacanth, not a rhipidistian which gave rise to tetrapods, and that these two crossopterygian groups had separated in the earliest Devonian.
2) The Dipnoi, or the lungfish, also appeared fully evolved in the fossil record in the early Devonian. Their upper jaw is firmly fused to the brain case, and the fins are weaker. All lungfish can breath air, the South American and African s pecies can survive drought conditions by burrowing in the mud. They use urea retention for water balance when encased in this "mud-ball".
E) Class Amphibia: The amphibians are the first tetrapods and appeared in the middle Devonian and exist to the present. Modern amphibians are markedly different from the earliest forms and those of the early major radiations. Although major changes occurred with the origin of tetrapods, most of these modifications were loss of fish features as the basic tetrapod features had already evolved in fishes - some quite early in fish phylogeny. Very few new features appeared with the origin of the Amphibia in spite of the major change of environment from water to land.
The changes include:
1) Paired fins disappeared as the vertebrate appendages were modified into the pectoral and pelvic limbs. But the basic skeleton and musculature of the limbs were already present.
2) The distal end of the limb modified as the "rays" were lost and digits evolved. Digits can be considered as a new vertebrate feature.
3) The pectoral girdle lost its attachment with the skull and hence the axial skeleton. The pectoral girdle became smaller. Connection between the pectoral girdle and the axial skeleton and transfer of weight is by muscular slings.
4) The pelvic girdle increased in size and gains a solid attachment (bony fusion) with the vertebral column.
5) Fish gills disappeared and the gill slits closed over. Respiration was via lungs or lungs plus the skin. In many amphibians much gas exchange takes place via the skin (CO2 diffuses out even if O2 is obtained in the lungs). Some specialized fully terrestrial salamanders have lost the lung completely and breath via the skin.
6) A neck appeared, and the head became movable with respect to the trunk.
7) With the appearance of a neck and pelvic girdle attachment, the vertebral column started to differentiate into definite segments.
8) External gills evolved in the larval stage; these external gills are different from the internal gills of fish. These secondary gills can be considered as a new vertebrate feature.
9) Bony scales are lost.
10) The heart remained three chambered with flow of blood to the lungs (condition in salamanders that lost their lungs?).
11) Presumably the hemoglobin (at least in early forms) is a fish-type in its oxygen dissociation curve. Presumably a terrestrial hemoglobin evolved in later amphibians, whether once or more times.
12) The kidney now serves to eliminate nitrogenous wastes in the terrestrial forms. The urea retaining mechanism of water balance remains and is used to eliminate nitrogenous wastes.
13) The hyomandibular of fish evolved into the stapes which transmits sound to the inner ear.
Reproduction and development are still like those in fishes with external fertilization in most forms. Reproductive systems differ quite considerably in diverse groups of living amphibians. Only a single extra embryonic membrane is present -- the t rilaminate yolk sac. A larval stage is present with metamorphosis (some forms do not metamorphosis and breed as larva). Almost all forms had and still live in fresh water. Only a few species of living amphibians can live in brackish waters.
A very early split occurred in the evolutionary history of the amphibians with one line leading to the labyrinthodonts and major radiations of amphibians, including the groups of living amphibians, and the other to the embolomeres and the reptiles. Tracing tetrapod features in the evolution of the amphibians must be done with care because important modifications occurred early in the labyrinthodont line, and were already different in early excellent and well studies fossils from that present in the first amphibians. Many groups of amphibians (some labyrinthodonts) never really left the water and some became specialized once again for a strictly aquatic life. Relationships between major groups of fossil amphibians is still obscure, and the origins o f the living groups (frogs, salamanders and apodans) from fossil taxa are not known.
F) The Reptilia: Reptiles appeared in the earliest Carboniferous, under went many major radiations, gave rise to birds and mammals and still exist but is a much smaller diversity than had existed during the Mesozoic. The reptiles represent t he final stage in the conquest of land by vertebrates.
Major evolutionary changes in the origin of the Reptilia include:
1) Keratinization of the skin (epidermis) and the evolution of horny, epidermal scales.
2) Limbs, girdles, vertebral column (full differentiation into regions), etc. had all modified for more efficient terrestrial locomotion.
3) Shelled egg with three new extra embryonic membranes, chorion, amnion and allantois, with the yolk sac modified into a bilaminate form. Fertilization is internal. A distinct larval stage is lost.
4) If not already modified in the amphibians, the hemoglobin had modified to one that can pick up and transport oxygen in the presence of high levels of CO2.
5) In some groups (including all living reptiles), the method for eliminating nitrogenous wastes is using uric acid. Hence the kidney is not specialized. Marine groups evolved salt-secreting glands.
A major split exists in the early evolution of the reptiles in the early Permian. One line led to the Synapsids, mammal-like reptiles and mammals. The other led to the major reptilian radiations, the dinosaurs, crocodiles, lizards, snakes, and bird s. Turtles are an old, old side group with no apparent close connections to any living reptiles.
Note that the phylogenetic split between birds and mammals is in the early Permian, about 250 million years ago. This is about half of the time period since the earliest vertebrates appeared in the fossil record.
Much of the evolution of reptiles and the relationships between major groups is far less known and based upon weaker evidence than one is told in beginning texts.
G) Class Aves: The birds, are warm blooded, feathered, flying tetrapods. They appeared in the late Triassic or mid-Jurassic (depending on the correctness of the identification of the earliest fossil) having evolved from some group within the major archosaurian radiation of the Reptilia. Birds underwent major radiations during the Cretaceous, and the modern birds at the end of the Mesozoic.
The major evolutionary changes in the origin of birds are:
1) Homoiothermy, with a four chambered heart (subdivided ventricle in addition to the divided atrium) with single right aorta.
2) Reptilian scales modified into feathers as an insulating layer and further specialization into flight (wing) and tail feathers.
3) The fore limb is modified into a wing. Large pectoral girdle with a bony sternum and large medial keel (reduced or lost in flightless forms).
4) Specialized bipedialism with a large solidly fused pelvic girdle and large synsacrum.
5) Long, flexible neck with specialized heterocoelous (saddle-shaped) articular surfaces, and a rather rigid trunk usually with uncinate processes on the ribs.
6) The tail is short with the caudal vertebrae reduced in number and fused into a pygostyle onto which the tail feathers are attached.
7) The jaws are modified into a beak covered with a horny rhamphotheca and the teeth are lost (in later birds -- present in early groups to the end of the Mesozoic). The upper jaw is kinetic.
6) Small lung with air tubes, and nonrespiratory air sacs extending to many parts of the body including the bones in some species.
7) Use the uric acid method to eliminate nitrogenous wastes; hence the kidney is not specialized. Evolve a salt-secreting gland from one of the nasal glands.
8) All species lay hard shelled eggs, no species is live bearing.
9) The brain is highly developed, but differently from mammals (much innate behavior but also learned behavior). Highly specialized parental care. Highly specialized directional abilities and migratory abilities with detection of the earth's magnetic f ield, of polarized light, and ability to use the sun's position.
H) Class Mammalia: The mammals are warm blooded, haired (or furry), live bearing (most) tetrapods that nurse their young. Females possess mammary glands (which are nothing more than specialized sweat glands). Mammals appeared in the late Tri assic having evolved from the mammal-like reptiles of the subclass Synapsida of the Reptilia. Mammals underwent major radiations during the Cretaceous, separating into the Prototheria (leading to the Monotremata) and the Theria (leading to the Marsupialia and the Eutheria) and with the modern mammalian orders evolving at end of the Mesozoic to early part of the Cenozoic. Most modern orders appeared in the Eocene, about 60 million years ago long after the origin of mammals about 150 million years ago.
The major evolutionary changes in the origin of mammals are:
1) Homoiothermy, with a four chambered heart (subdivided ventricle in addition to the divided atrium) with single left aorta.
2) Reptilian scales modified into hair as an insulating layer.
3) Red blood cells lack a nucleus when fully mature.
4) The teeth have differentiated into several specialized types with a deciduous and a permanent set.
5) The female reproductive system is specialized for nourishing the fetus in the uterus; a placenta evolved from the extra embryonic membranes. Monotremes lay shelled eggs which are incubated outside of the body of the female. But the neonate is nursed with milk from the mammary gland.
6) After birth, the neonate is nourished with a secretion (milk) from the mammary glands.
7) Short, less-flexible neck with usually seven cervical vertebrae, and a rather flexible trunk .
8) Limbs and girdles have modified compared to reptiles, with the limbs brought under the body.
9) A secondary bony palate developed, and the jaw articulation changed from the quadrate-articular one found in reptiles to a dentary-squamosal one. The dentary is the sole bone of the mandible.
10) Bones of the old jaw articulation modified into extra ear ossicles, the articular into the malleus and the quadrate into the incus. Basically, less change is seen in the skull and limbs of mammals from reptiles than the evolution of birds, because of the extreme specialization of birds for flight.
11) Large, collapsible lung with a diaphragm.
12) Use the urea method to eliminate nitrogenous wastes; hence the kidney is highly specialized with a long Loop of Henle in the nephron.
13) The brain is highly developed, but differently from birds with specialized with specialization of the cerebrum from centers associated with the olfactory sense (much learned behavior but also innate behavior). Highly specialized parental care.
Many or all of the features which appear to be similar in mammals and in birds evolved independently in these two groups and are similAr because of the evolution of homoiothermy and all of the associated features in the two groups. Remember that al though these two classes of vertebrates evolved from the reptiles, they evolved from very different groups within the Reptilia. These two reptilian groups separated early in the evolutionary history of the Reptilia, at least in the early Permian and perha ps back into the Carboniferous period. Hence in comparative studies, there may be little reason to study birds as a model for mammals except for the affect of homoiothermy on the comparison.