Evolution Of The Amniote

One view for the evolution of amniotes from anamniotes (Carroll, 1969, 1970, 1982, 1988) concludes that the transition was made by species of relatively small adult size that presumably laid eggs similar in many respects to the direct developing eggs of a variety of extant amphibians. The oocyte produced by the predecessor to amniotes, like the oocyte produced by contemporary amphibians Figure 1. Highly schematic representation of evolution of the early amniote egg from an anamniotic...

Skin As Protective Covering Gaining Speed By Losing Peripheral Bone

A substantial full-body covering of dermal bone in the form of scales, osteoderms, or heavy plates is the ancestral condition for vertebrates as evidenced by its presence in Paleozoic fish (Carroll, 1988 Romer, 1966) and its continued presence in all modern fish groups. Many early tetrapods have been found with bony scales (Clack, 1989 Godfrey, 1989 Godfrey et al., 1981 Holmes, 1984 Hook, 1983 Panchen, 1972) and their absence in some cases is simply an artifact of preservation. Among those...

Info

Synapsida Diadectomorpha ( ) Sauropsida amphibian (anamniote)-grade taxon has ever managed to invade the niche of terrestrial herbivore, and a causal reason has been suggested for this. The endosymbionts required for digestion of high fiber vegetation must be obtained immediately after hatching fromterrestrial microbial decomposers (Modesto, 1992 Zug, 1993 Hotton and Beerbower, 1994 Hotton et al., this volume). This constraint therefore prevents anamniote (aquatically reproducing) taxa from...

Acknowledgments and Dedication

The genesis of this book required far more than the ideas of the editors alone. The original impetus was the invitation to organize a symposium, Paleontological and Neontological Approaches to the Origin of Amniotes, for the Fourth International Congress of Vertebrate Morphology. We thank speakers and the audience for their enthusiastic response. We are most grateful to Susan Herring and James Hanken for their support, interest, and suggestions, and to Susan Abrams and R. Eric Lombard for their...

Literature cited

E. Lombard. 1992. Reinterpretation of the temporal and occipital regions in Diadectes and the relationships of the diadectomorphs. Journal of Paleontology, 66 481-499. Bremer, K. 1988. The limits of amino acid sequence data in angiosperm phylogenetic reconstruction. Evolution, 42 795-803. Bryant, H. N. 1994. Comments on the phylogenetic definition of taxon names and conventions regarding the naming of crown clades. Systematic Biology, 43 124-130. Clack, J. A....

Skin As Mechanical Support From Crossed Fibers To A Dermal

The functional and evolutionary morphology of the dermal layer of the skin has been particularly neglected by comparative vertebrate biologists. It is widely recognized that the dermis, whether ossified or not, provides structural integrity to the skin (Harkness, 1968). However, biomechanical study of vertebrate support and movement has focused largely on the bony internal skeleton, whereas the mechanics of the external dermis have been largely ignored. In the few cases in which dermal...

Conclusion

Gans and Pough (1982, p. 8) wrote, We suggest that the absence of a unifying morphological scheme, rather than being an incidental by-product, is an important aspect of the reptilian grade. The lives of Recent reptiles are shaped by a set of shared characteristics that need not produce obvious structural features. The features they suggest as definitive are physiological features The amniotic egg, ectothermy, low metabolic rate (compared with endotherms), reliance on anaerobiosis for activity,...

Amniotes Introduction

The past three decades have seen a revolution in the way terrestrial vertebrate fossils are studied. This is due, primarily, to two significant influences (1) the advent of cladistic methods of phylogenetic analysis, and (2) the acceptance of plate tectonics and plate movement (Lombard and Sumida, 1992). Marking the beginning of this period of innovative study were Romer's (1966) classic textbooks in vertebrate paleontology and Carroll's (1969a,b,c, 1970) series of landmark studies of the...

Origin of the amniote egg

The absence of recognizable antecedents to most structural components of the egg of amniotes, particularly the extraembryonic membranes, has forced speculation on the origin of the amniote egg to focus on functional analogs among modern anamniotes and on the ecological and physiological advantages of specific structures to modern amniotes. The resulting scenarios have emphasized three questions (1) Which came first, the amniote or the amniote egg (Romer, 1957 Carroll, 1970) (2) what was the...

Info Ybv

The following is a list of autapomorphies of the clades and terminal taxa. In this list, ambiguous characters are indicated by an asterisk and the state of the character is indicated in parentheses when it is not 1. Reversals (any transition to a state of lower numerical value than the state present at the next more inclusive node) are indicated by a minus and by a number in parentheses when the reversal is not to the state 0. Node A -6*, 31, 36, 43*, 55, 64*, 69*, 80, 115, 119*, 120, 129, 130,...

Primitive Amniote Feeding Mechanisms

Although amniote taxa display considerable diversity in their feeding mechanisms, a number of authors have abstracted from the large base of comparative data several general features of amniote jaw function that are believed to be primitive for the clade as a whole (Bels et al., 1994a Bramble and Wake, 1985 Delheusy and Bels, 1992 Hiiemae and Crompton, 1985 Reilly and Lauder, 1990a Schwenk and Throckmorton, 1989). Many of these characteristics of amniote feeding are well illustrated by the...

Outgroup Patterns Amphibians

Amphibian taxa represent an important clade for understanding amniote feeding mechanisms. Within the Amphibia are species that exhibit aquatic feeding, terrestrial feeding, and (in some taxa) ontogenetic and or ecological transitions between these two feeding modes. At first glance, it seems, one could hardly ask for a better out-group clade on which to conduct experimental analyses of feeding mechanisms. By studying aquatic feeding in amphibians, one can hold the environment constant and...

Plasma Osmolarity Of Vertebrates

For the illustrative purposes of this chapter, we have chosen to analyze plasma osmolarity as a representative physiological trait. No fossil indicator of this trait has ever been proposed, so inferences about its value in extinct organisms must be based on consideration of living forms. Plasma is the fluid component of blood it can vary widely in solute concentration. The ability of solutes to cause osmotic pressure and osmosis is measured in terms of osmoles, the osmole being a measure of the...

Discussion and speculation

As suggested at the beginning of this survey, this study is not intended to define synapomorphies of successively nested taxa. Rather, it has demonstrated that the traditionally accepted notion of a transition to a relatively more terrestrially adapted amniote is not appropriate. The anamniote sister groups to the Amniota are also clearly terrestrial taxa. Anthracosauroids and seymouriamorphs share many locomotor features with amniotes. The primitive tetrapod Westlothiana combines a mosaic of...

Discussion

The Late Paleozoic radiation of tetrapods reflected their successful exploitation of the newly accessible and expanding terrestrial biosphere. The diversity of terrestrial niches accessible to these tetrapods was coupled to a strong selection pressure for an increased performance capacity not only for locomotion, but also for a variety of processes such as feeding, growth, reproduction, and sensory physiology. We have suggested that atmospheric hyperoxia may have played a role in tetrapod...

Plasma Osmolarity milliosmols

Frequency histogram of plasma osmolarities of 172 vertebrate taxa (data from Fig. 1 and Appendix 1). with habitat occupancy, which is to some extent a behavioral trait. Also other physiological and biochemical properties can be expected to have coadapted (sensu Huey and Bennett, 1987) with plasma osmolarity. That is, natural selection should have favored the correlated evolution of appropriate combinations of (1) osmoregulatory strategies (e.g., regulating versus conforming), (2) the...

Highfiber Herbivory

High-fiber herbivory has probably been entirely limited to the cotylosaur clade and is widely distributed among the various subclades, carrying to the extreme the adoption of omnivory and low-fiber herbivory outlined earlier. A diadectid, Desmatodon, provides the oldest evidence for primary high-fiber diets with its presence in mid-Upper Carboniferous assemblages (probably Stephanian B) from Pennsylvania and southwestern North America (Berman and Sumida, 1995) another, more derived...

Appendix

List of Characters Used in this Study 1 Lateral-line canals location in adults surrounded by bone (0) in grooves at the surface (1) in soft tissues or absent (2). 2 Lateral-line canal grooves in ontogeny always present (0) in larvae only, lost in adults (1) never present (2). 3 Dermal sculpturing high ridges and pits, temnospondyl pattern (0) cosmine (1) shallow, widely spaced pits, anthracosaur pattern (2) low rugosities, pustules (3) none, smooth bone (4). 4 Rostrum absent (0) short, high (1)...

Rhx

Edaphosauridae, Haptodontidae (a polyphyletic and paraphyletic cluster of taxa), and Sphenacodontidae (Hopson, 1994). The early reptiles encompass two subclades the Captorhinidae and probably the Bolosauridae in one and the Protorothyrididae and Araeoscelidae in the other (Gauthier, 1994, Laurin and Reisz, 1995). Most authors view the diadectomorphs (including the Limnoscelidae and Diadectidae) as the immediate sister group of amniotes, forming with them the Cotylosauria (Gauthier, 1994...

Introduction

One of the areas of vertebrate structure and function that has received the most attention during the past 20 years is the study of the feeding system. Due to the relatively good fossil record of bones, the many characters within the jaws used for systematic diagnoses, and interest in the mechanisms used by vertebrates to obtain resources from the environment, functional morphologists and paleontologists have devoted considerable effort to analyzing the vertebrate skull (Bels et al, 1994b...

Structural summary

The preceding survey demonstrates that there is no clear and distinct transitional point in locomotor features between anamniote and amniote sides of the transition. Rather, there is a continuum of changes. Following is a summary of that continuum of changes mapped onto the hybrid hypothesis of relationships presented in Figure 1. In all of the taxa considered the pleurocentrum is the dominant element of the vertebral centrum, possibly facilitating stiffening the vertebral column to aid in...

Outgroup Patterns Fishes

The monophyletic clades of extant fishes that form out-group taxa to tetrapods and amniotes are the sharks and relatives (Elasmobranchiomorpha), ray-finned fishes (Actinopterygii), coelacanths (Actinistia), and lungfishes (Dipnoi). The feeding mechanisms of members of all of these taxa have been studied in some form or other during recent years, and a comparative analysis of feeding morphology and function in these clades provides the basis for our subsequent consideration of tetrapod feeding...

The First Amniote Animals

Feder (1992) suggests that in amphibians, fluxes seemingly permit only limited independence from the immediate environment. Accordingly, the internal milieu of amphibians may be far less fixed than that of many other vertebrates. The first amniote, we suggest, accomplished the transition from amphibian anamniote to reptilitan amniote mainly by altering its style of water balance. Restriction of skin permeability permitted greater control over the state of body hydration. Early amphibians...

Skin As Permeability Barrier Debunking The Myth Of A Need For Terrestrial Waterproofing

The permeability of membranes-be they cell membranes, respiratory surfaces, or an integument-is an extremely labile property. Many organisms routinely encounter drastic changes in the concentration gradients across their membranes, be it for oxygen, carbon dioxide, water vapor or dissolved ions. Most organisms accommodate to these changes physiologically at least in part by altering the permeability of their membranes. Evolution rily, membrane permeability is also extremely labile. Most...

Appendix Parsimony In Systematic And Comparative Biology

The English word parsimony is derived from the Latin stem pars-, which denotes to spare, save (Oxford English Dictionary, 1971). As a logical principle, parsimony dictates, in essence, that simple explanations are generally to be preferred. Parsimony is often used as a procedure to infer the phylogenetic relationships of organisms and, as we discuss in this chapter, the evolution of particular characters in the context of an (independent) hypothesis of phylogenetic relationships. Use of...

Axial skeleton

The vertebral column of most late Paleozoic tetrapods is composed of multipartite elements with varying degrees of fusion (Fig. 3). In amniotes and their closest sister taxa the vertebral body is composed of two elements a pleurocentrum that dominates in size and an anteriorly placed intercentrum which is usually reduced to a smaller, crescent-shaped wedge. Laterally directed transverse processes which provide articular surfaces for the ribs vary in size depending on their position more...

Evolutionary patterns of egg morphology

The eggs of Reptilia and Monotremata undergo meroblastic partial cleavage and are surrounded by tertiary shell membranes that are proteinaceous secretions of the oviduct Hughes, 1977 . The large yolk mass is enclosed by a choriovitelline membrane as a stage in yolk sac formation, amniogenesis proceeds by a process of folding, and a large allantoic vesicle develops. These characteristics are likely ancestral for amniotes and represent a combination of primitive and derived traits. The tertiary...

Results

Only three most parsimonious tree were found. They require 654 steps and have a consistency index of 0.52. The character optimizations and discussions below are all based on the strict consensus tree Fig. 1 . The consensus tree differs substantially from the commonly accepted phylogeny of tetrapods and has important implications for hypotheses of amniote origins. For instance, Lepospondyli includes Lissamphibia and is closely related to diadectomorphs and amniotes. Previously, seymouriamorphs...

H

0.1 1 mg 10 100 1 g 10 Ovum or Egg Mass Figure 2. Relationship between the rate of oxygen consumption in hatching stage embryos of amphibians Seymour and Bradford, 1995 and reptiles Vleck and Hoyt, 1991 . The equation for amphibians is V02 1.06 M0 52 and that for reptiles is V02 0.68 M 82 in which rate of oxygen consumption V02 has units of il hr and egg mass M has units of mg. Some of the data for amphibian eggs were corrected to 30 C assuming a Q10 of 1.5. The data for reptilian eggs were...

Characteristics of specific taxa

Platypus Extraembryonic Coelom

Griffiths 1978 provides a detailed summary of monotreme reproduction and development. Luckett 1977 and Hughes 1993 are excellent descriptions of the development of the extraembryonic membranes of echidna, Tachyglossus aculeatus, and platypus, Ornithorhynchus anatinus, that incorporate original observations into classical literature accounts. There is great similarity in the development of the extraembryonic membranes of these two species. An intrauterine phase of development is followed by...

Distribution Of Diadectomorpha

The suborder Diadectomorpha consists of three families. The Late Pennsylvanian-Early Permian Limnoscelidae and Early Permian Tseajaiidae have a strictly North American record, whereas members of the Late Pennsylvanian-Early Permian Diadectidae are found in both North America and central Europe. It is important to note here two taxonomic reassessments of reported limnoscelids. Initially, Martens 1989 reported the occurrence of a limnoscelid-like form from the Bromacker locality in the Lower...

Appendicular skeleton

Chasmatosaurus Shoulder Girdle Skeleton

With the conspicuous exception of Holmes 1977, 1980 , few since the early work of Romer 1922 have concentrated on both the osteological and muscular components of the basal tetrapod limb. Figure 5. Left lateral views of the atlas-axis complex of taxa near the origin of amniotes. Except where noted, all illustrations are after Sumida et al. 1992 . A The embolomerous anthracosaurian Proterogyrinus B the seymouriamorph Seymouria C the diadectomorph Limnoscelis D the diadectomorph Diadectes E the...

Defining Amniota Keycharacter And Crown Concepts

Given the above described phylogeny, applying the crown-clade definition of Amniota is very straightforward. Amniota would consist of the most recent common ancestor of clade E and all its descendants. Thus, the boundaries of this clade are very clear, and this definition is therefore very precise Fig. 2 . Drawing the boundary of Amniota under the traditional definition is more problematical, because it requires knowledge of when the amniote egg arose. With fossil taxa, this requires making...

Origin of Amniotes

The significance of this new phylogeny to the origin of amniotes lies in the recognition of an entirely new set of outgroups beyond diadectomorphs. The new phylogeny indicates that future phylogenetic studies of amniote phylogeny will have to include lepospondyls as outgroups. This has not been a common practice, but it will be necessary if the phylogeny presented here is accepted. Using lepospondyls could certainly help to polarize several characters used in phylogenetic studies of amniotes...

Distribution Of Captorhinomorpha

The suborder Captorhinomorpha contains two, for the most part easily distinguishable, families the Protorothyrididae and Captorhinidae. Whereas the Protorothyrididae is mainly a Late Carboniferous family, with a few Early Permian members, the Captorhinidae is restricted to the Early and Late Permian, though only the Early Permian distributions are plotted here. The Early Permian captorhinids are known to occur only in the southwestern and Figure 7. Late Carboniferous distributions of the...

Phylogenetic Framework

Our revised phylogeny of Anthracosauria sensu Gauthier et al., 1988b , the clade consisting of amniotes and their nearest amphibian-grade relatives, is depicted in Figure 1. This is based on a cladistic analysis of anthracosaurians summarised in the appendix. This study considered 45 characters for seven terminal taxa Solenodonsauridae, Diadectomorpha, Synapsida, and Sauropsida. It was extracted from a much larger cladistic analysis that considered both the external relationships of amniotes...

Candidates For Omnivory

Among the early amniotes, sundry captorhinids, protorothyridids, araeoscelids, and primitive ophiacodonts and the edaphosaurians are likeliest candidates for omnivory, consuming nutrient-rich plant materials along with invertebrates Fig. 1 . Some of their shared characteristics, such as relatively short coupled bodies, robust limbs, and big feet, indicate a primitive emphasis on wide foraging in terrestrial habitats. Their jaw mechanics embody a derived form of the static-pressure bite marked...