Early Tetrapods Research Articles

Amphibia: Disparity and Diversification of Early Tetrapods

Fossils conventionally classified as amphibians comprise a diverse, paraphyletic assemblage of roughly 15 distinctive lineages (=clades; Figure 1) of non–amniote tetrapods whose phylogenetic interrelationships continue to defy resolution (Carroll, 1992; Figure 2). The historical accumulation of morphological data, new paleontological discoveries, better understanding of characters and transformation series, and refinement of rigorous cladistic analytical methodology have stimulated a resurgence of interest in the phylogenetics of early tetrapods (Bolt, 1991; Bolt and Lombard, 1991; Clack, 1988, 1991; Duellman, 1988; Gauthier et al., 1988b; Lombard and Bolt, 1988; Milner, 1988, 1990, 1993; Panchen, 1991; Panchen and Smithson, 1987, 1988; Smithson, 1985; Trueb and Cloutier, 1991a, 1991b). However, most investigations have done little more than substantiate the distinctiveness of the major non–amniote tetrapod clades and clarify within–clade relationships. Differences in parsimony between alternative phylogenies proposed for the early lineages are generally too minor to consider any as strongly corroborated (Carroll, 1990, 1992; e.g. Panchen and Smithson, 1988).

A proteolipid protein gene family: Expression in sharks and rays and possible evolution from an ancestral gene encoding a pore-forming polypeptide

The myelin proteolipid proteins (PLP and DM20) are believed to act as “adhesive struts” in the extracellular apposition of the CNS myelin sheath. These proteins have been considered late evolutionary developments, which arose de novo in the antecedents of early tetrapods. However, PCR primed with degenerate oligonu-cleotides corresponding to common segments of rat PLP/DM20 revealed three novel mRNAs in the brains of two elasmobranchs. These mRNAs are closely related to each other and to mammalian DM20, but lack the sequence that distinguishes PLP from DM20. We term the novel proteolipid proteins DM α, DM β, and DM γ. At least DM α and DM γ are highly expressed in white matter in myelinating shark brain. The DMs not only are highly homologous to each other, but also contain regions bearing similarities with segments of channel-forming regions of the nicotinic acetylcholine receptor and the glutamate receptor macromolecular complexes. Significantly, we find that across these segments, DM α and DM γ are more similar to the channel proteins than the two channel proteins are to each other.

Recent Progress in Understanding Early Tetrapods

This paper reviews significant discoveries and interpretations made for Paleozoic tetrapods over the past twenty-five years. In that span twelve significant, new localities have been found, including the oldest ever at about 370 million years in age. About 60 new genera have been described; five providing important insight into the early evolution of land vertebrates. The number of exceptionally well known taxa with multiple, excellently preserved specimens has doubled to eight from four. The very earliest tetrapods have been discovered to have been polydactylous, the ear region to have had a complicated early evolution and the specialized tooth type found in Recent amphibians has been found in a group of Lower Permian temnospondyl amphibians, indicating an evolutionary relationship

Origins of the higher groups of tetrapods: controversy and consensus

This edited volume explores the various views on the origins of tetrapods-amphibians, reptiles, birds, and mammals-views that agree or differ depending in part on how certain fossil animals are classified and which methodology is used for classification. Eighteen chapters by an international group of paleontologists and neontologists here present current hypotheses, emphasizing the kinds of data needed to answer controversial questions, as well as the variety of solutions that emerge from diferent analyses of the same data set. The book is arranged in five sections, each of which contains an overview essay that either describes the development of various schools of thought regarding the origin of the tetrapod group in question or critically summarizes the arguments presented in the section. The first section addresses the origins of tetrapods as a group, focusing on lobe-finned fishes and early tetrapods. Next is a section dealing with amphbians, followed by one on reptiles. The fourth section concerns avian origins, and the final section treats the origins and early diversification of mammals. With an overall goal of stimulating critical evaluation by the reader rather than providing unequivocal answers, this volume will be of particaular interest to vertebrate paleontologists, evolutionary morphologists, and ichthyological, herpatological, avian, and mammalian systematists.

Kinematic and Electromyographic Analysis of the Functional Role of the Body Axis During Terrestrial and Aquatic Locomotion in the Salamander Ambystoma Tigrinum

ABSTRACT Aquatic neotenic and terrestrial metamorphosed salamanders (Ambystoma tigrinurn) were videotaped simultaneously with electromyographic (EMG) recording from five epaxial myotomes along the animal’s trunk during swimming in a flow tank and trotting on a treadmill to investigate axial function during aquatic and terrestrial locomotion. Neotenic and metamorphosed individuals swim using very similar axial wave patterns, despite significant differences in axial morphology. During swimming, both forms exhibit traveling waves of axial flexion and muscle activity, with an increasing EMG-mechanical delay as these waves travel down the trunk. In contrast to swimming, during trotting metamorphosed individuals exhibit a standing wave of axial flexion produced by synchronous activation of ipsilateral epaxial myotomes along the trunk. Thus, metamorphosed individuals employ two distinct axial motor programs -one used during swimming and one used during trotting. The transition from a traveling axial wave during swimming to a standing axial wave during trotting in A. tigrinurn may be an appropriate analogy for similar transitions in axial locomotor function during the original evolution of terrestriality in early tetrapods.

Experiment and Comprehending the Evolution of Function

Experimentation involves the manipulation of variables in vertebrate form-functionsystems. Two general experimental approaches are used most often in attempts at defining the relationship of form and function in animals: holding the form constant while varying the experimental regime, and the reverse. Either real or abstract forms and experimental regimes canbe productive, though all variations have limitations. The place of such experiments in the understanding of the evolution of form-function is explored in a narrative tracing the change in our conception of function and evolution for the tetrapod ear over the past twenty years. Combining experimental functional anatomy and studies of structural evolution and phylogeny creates an environment of reciprocal illumination. The experimental determination of function illuminates details of structure otherwise not noticed—creating new characters or suggesting reinterpretations for those already known. Attempts to understand the evolution of structure and function in a clade can suggest experiments or key taxa for experimental study. This combinedapproach is relatively new and is being led by vertebrate morphologists.

The Order Adelospondyli: Carboniferous lepospondyl amphibians

AbstractThe Order Adelospondyli, an isolated group of amphibians from the Carboniferous of the Edinburgh area, is redescribed and distinguished from other early tetrapods. Although systematics of genera and species is unsatisfactory, the group seems monophyletic, since as far as they are known, all conform to a single overall structural plan. There is an entire skull roof with a reduced complement of bones in an unique pattern, a highly characteristic marginal dentition, a massive hyoid apparatus, at least 70 holospondylous trunk vertebrae, and (like some other Lower Carboniferous tetrapods) a shoulder region with a heavy dermal girdle but no trace of ossified limbs or endochondral girdle. Adelogyrinids show no special affinity with other Palaeozoic amphibians: the vertebral characters in which they resemble microsaurs, lysorophids, aïstopods, and nectrideans may have evolved more than once among early tetrapods, and the shoulder girdle and neural arch characters shared with labyrinthodonts are probably primitive. The adelogyrinids thus illustrate the more general problems of establishing interrelationships among early tetrapods. Their way of life is discussed.

Biogeography of early Mesozoic continental tetrapods: patterns and implications

The stratigraphic framework for Triassic and Early Jurassic continental strata has greatly changed in recent years. These revised correlations necessitate a review of traditional views of early Mesozoic continental faunal succession and biogeography. We have examined the relationship between tetrapod distribution and paleogeographic context during the Triassic and Early Jurassic on the basis of a data base comprising updated faunal lists for major early Mesozoic assemblages of continental tetrapods. Analysis of these data supports the hypothesis that there were few barriers to biotic interchange among continental tetrapods throughout the Triassic and Early Jurassic. Early Mesozoic tetrapod assemblages are dominated by widely distributed, often cosmopolitan families. Late Triassic patterns of latitudinal variation among tetrapod assemblages appear to be correlated to those seen among terrestrial plants and contrast with the extremely uniform distribution of Early Jurassic continental biotas.

Discovery of the earliest-known tetrapod stapes.

The evolution of the middle ear is central to the discussion of how the first tetrapods adapted to life on land as well as their phylogeny. Here I report the discovery of the stapes of Acanthostega gunnari, from the Upper Devonian of east Greenland. This is the earliest tetrapod stapes so far described, and it throws new light on both these aspects of early tetrapod biology. It has been assumed that the common inheritance of all early tetrapods was a light, rod-like stapes associated with a temporal notch in the otic region that was thought to have supported a tympanum, or eardrum. The stapes would have conducted vibrations from the tympanum to the otic capsule. By contrast, the stapes of Acanthostega was stout with a broad distal ramus associated with the temporal notch. I suggest that the temporal notch of Acanthostega and other early tetrapods supported a spiracular opening rather than a tympanum, and that the stapes controlled palatal and spiracular movements in ventilation.

Functional interpretations of the vertebral structure in paleozoic labyrinthodont amphibians

Previous attempts to analyze structure‐function relationships of vertebral centrum patterns in Paleozoic amphibians have been too simplistic and led to vague conclusions. Vertebral movements, as in the human spine, were coupled. Movements and flexibility of the column were correlated with zygapophysis orientation. The essentially notochordal centrum of early tetrapods permitted several widely divergent patterns to arise without compromising load‐bearing capacity. As the osseous centrum became more robust to assume a greater supportive role in later tetrapods, there was less opportunity to remodel its structure. The seymouriamorph pattern permitted limited axial rotation in association with lateral flexure, while the rhachitomous pattern permitted extended axial rotation in association with lateral flexure by distributing movements within its multipartate centrum. The persistence of widely divergent vertebral configurations, regardless of habitat, in later Paleozoic amphibians can be explained at least par...

Evolution of Cranial Kinesis in Lower Tetrapods

The earliest tetrapods had pleurokinetic skulls (with movable palatoquadrates and associated dermal bones of palate and cheeks). Pleurokinesis developed from the splanchnokinesis of the crossopterygian skull; the crossopterygian neurokinesis was lost in the primitive tetrapods. Recent amphibians also have pleurokinetic skulls. The stegocrotaphic Gymnophiona retain the primitive pattern of pleurokinesis; in other extant amphibians this is modified and combined with the rhynchokinesis (Anura) or prokinesis (Urodela). Primitive reptiles attained a pleurometakinetic condition of the skull. Cranial kinesis was most advanced in the diapsid reptiles. The improvement of kinesis in the amphikinetic skull of lizards included either development of hyperstreptostyly from simple streptostyly, or a flexible palate from a firm palate. Snakes show both of these trends. Cranial kinesis involves multiple adaptive functions of the entire skull, with the improvement and changes of its adaptive roles in tetrapod phylogeny discussed here. The most important evolutionary mechanisms for increased cranial kinesis include morpho-functional and ontogenetic protoadaptations. Evolution of these occurred by means of ontogenetic heterochrony (especially fetalization), and were controlled by stabilizing selection, which is genetic reflection of modifications, arising due to the influence of altered mechanical tasks.

In search of earliest tetrapods

In search of earliest tetrapods

A new Lower Carboniferous tetrapod locality in Iowa

The earliest tetrapods known are from two or three Upper Devonian1–3 and some 20 Lower Carboniferous localities in Scotland4 and North America5–8. Most sites yield few and fragmentary specimens; well-preserved and even partially articulated material is exceedingly rare. This report discusses a middle Lower Carboniferous site rich in amphibian and fish remains discovered near Delta, south-east Iowa, and represents the first Lower Carboniferous tetrapod locality found in mid-continental North America. The bones occur within collapse-structures or depressions, and appear to represent a fresh- or brackish-water pond fauna. The Delta site contains the oldest well-preserved tetrapod fauna in North America, and one of the oldest in the world. Several hundred tetrapod fossils have been collected to date, with excavation somewhat more than half completed. Specimens range from isolated bones to articulated, nearly complete skeletons of at least two apparently new amphibian species. This material will make an important contribution to knowledge of the mor-phology and interrelationships of early tetrapods.

A newly discovered skull of the temnospondyl amphibian Dendrerpeton acadianum Owen

A virtually complete three-dimensional skull of a subadult of the temnospondyl amphibian Dendrerpeton acadianum provides new information on the structure of the palate and lower jaw. The left stapes appears to lie in a natural position on the quadrate ramus of the pterygoid. The proportionately large otic ossicle probably precluded it from having acted as a transmitter of airborne vibrations from a tympanum to the inner ear. The use of the term "otic notch" should be restricted to amphibians or reptiles in which the embayment of the posterior margin of the cheek is accompanied by the presence of a slender rod-like stapes that could have functioned as part of an impedance matching system. In those species in which a notch is present but the stapes is a massive "supporting" element, the term "squamosal embayment" should be used, rather than "otic notch." The squamosal embayment in early tetrapods may have been inherited directly from their fish ancestors and had no functional relationship to hearing.

The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint

Endothermic tetrapods differ dramatically from ectothermic tetrapods in having a great capacity to sustain vigorous locomotion. I suggest that this difference reflects alternative adaptive responses to a mechanical constraint that was an inherent consequence of the vertebrate transition from aquatic to terrestrial modes of locomotion and respiration. The earliest tetrapods may not have been able to walk and breathe at the same time. Their sprawling gait and lateral vertebral bending would have required unilateral contractions of the thoracic musculature that may have interfered with the bilateral movements necessary for breathing. Modern lizards, whose locomotor and respiratory anatomy resembles that of the early tetrapods, provide support for this hypothesis because their breathing is greatly reduced during locomotor activity. Tetrapod lineages that gave rise to modern ectotherms apparently retained the constraint, becoming either highly specialized for burst activity based on anaerobic metabolism or specialized in passive mechanisms of defense against predators. The lineages from which birds and mammals are derived have undergone morphological changes that enable simultaneous running and breathing. In modern tetrapods upright posture is correlated with endothermic metabolism. This correlation may have arisen to circumvent ancestral constraints on locomotor stamina.

Vertebrate paleontology: new approaches and new insights

In the past fifteen years, the biological interpretation of vertebrate fossils has been markedly advanced by the application of experimental techniques in functional morphology, biomechanical and allometric modelling, more rigorously-derived empirical correlations between form and function in living species, and studies of modern community structure and of the taphonomic processes by which fossil assemblages are formed. Traditional paleontological methods of phylogenetic analysis are under attack by the advocates of cladistics; the resulting debate is leading to increased rigor in paleontological systematics. Paleobiogeographical studies of vertebrates has become an extremely active area of research due to the need to reanalyze past distribution patterns in the light of plate tectonics.New information has extended the fossil record of vertebrates back to the Late Cambrian, but little more is known about the nature of the earliest vertebrates or of the pre-vertebrate ancestors. Much has been learned of the fishes antecedent to tetrapods but little of the earliest tetrapods. The early reptiles are now well known, but the nature of their amphibian ancestors is uncertain. Restudy ofArchaeopteryxhas suggested a dinosaurian ancestry for birds. The origin and early evolution of mammals is the subject of extensive research as a result of greatly augmented recent collecting. The most active and innovative research is on the biology of dinosaurs, especially on their physiology. Among Cenozoic mammals, the fossil record of primates has been greatly expanded and knowledge of primate history, from the basal prosimians of the Paleocene to early man, is increasing rapidly.

Ecological succession among late palaeozoic and mesozoic tetrapods

Natural selection and the development of new taxa are associated with ecological replacement and the increase in number of niches with time. Continental faunal interchange was possible globally because of the existence of the super-continent Pangaea during much of the Upper Palaeozoic and Mesozoic. Figures of tetrapod niches vs. time and discussion of this concept for that period are presented for the first time. Four habitat divisions are used, namely marine, fresh-water, lowland and upland. The marine habitat was colonised rather late by tetrapods and these may have been the first predators on the early bony fishes which had diversified in the Permian. The radiation of bony fishes in the Jurassic was followed by a further increase in variety of their reptilian predators. Predators seem to develop some time after the radiation of a new potential prey group. Most early amphibians occupied fresh-water habitats in “crocodile” or “frog” niches, but from the Triassic tetrapods moved from fresh-waters and lowlands into the uplands also. In terrestrial habitats, the replacement of mammal-like reptiles by dinosaurs is tentatively explained in terms of palaeoclimatology and thermoregulatory physiology. Ornithischians capable of dealing with tough vegetation evolved to occupy the new niches produced by the radiation of conifers in the Jurassic. The extinction of dinosaurs appears to have been connected with temperature and habitat changes. Conclusions are supported by a summary of published opinions on the palaeoecological roles of early tetrapods.

Evolution of the tetrapod ear: an analysis and reinterpretation

The dominant view of tetrapod otic evolution–the “standard view”–holds that the tympanum developed very early in tetrapod history and is homologous in all tetrapods and that the opercular process of the rhipidistian hyomandibula is homologous to the tympanic process of the stapes in lower tetrapods. Under that view, the labyrinthodont amphibians of the Paleozoic are usually considered ancestral to reptiles, and thus the “otic notch” of labyrinthodonts and the tympanum it presumably contained form the starting-point for middle ear evolution in reptiles. Four problems have classically been identified with the standard view: the differing relationships of the internal mandibular branch of N. VII (chorda tympani) to the processes of the stapes in amniotes and anurans; the differing orientations of the stapes in key fossil and living groups; the location of the tympanum in early fossil reptiles; and the transferral of the tympanum, during the origin of mammals, from the stapes to the articular bone of the lower jaw. An examination of these problems and of the solutions proposed under the standard view reveals the ad hoc, and therefore unsatisfactory, nature of the proposed solutions. To organize and review alternative hypotheses of otic evolution an analytical table is constructed, using three characters (tympanic process, Nerve VII, tympanum), each with two possible states. A total of eight hypotheses about middle ear evolution are possible under this system, one of which is the standard view. The seven “non-standard” hypotheses, only five of which have been argued in the literature, are briefly examined. Six of the “non-standard” hypotheses appear unattractive for various reasons, including reliance on ad hoc arguments. The seventh was first proposed by Gaupp in 1898. It is today almost universally ignored but apparently largely for historical rather than scientific reasons. This hypothesis, her called the “alternative view”, appears to rest on assumptions equally as plausible as those of the standard view. Moreover, it offers a solution of the problems associated with the standard view without, apparently, raising any similarly serious problems. This paper compares the standard and alternative views of middle ear evolution in detail. Comparison proceeds on two levels. On one level, they are compared in terms of the hypotheses of phyletic tetrapod relationships each promotes and how strongly each supports its hypothesis. Both views promote the same hypothesis of tetrapod relationships. The alternative view is the more parsimonious, but the difference is not considered sufficient to provide a choice. On another level, the two views are compared in terms of their implications for: (1) the evolution of relative and absolute auditory perceptive ability; (2) the origin of reptiles; (3) the evolution of the suspensorium and cranial kinesis; and (4) the origin and evolution of recent amphibians. The nature of the data required for a test of the implications of the two views is specified in each case. Where data are available. the alternative view is consistent and the standard view is inconsistent with these data. We conclude that the alternative view is the preferable hypothesis of middle-ear evolution. This conclusion implies the following: the tympanic membranes and the tympanic processes of the stapes in recent mammals, reptiles + birds. and frogs. are not homologous; the evolution of “special periotic systems” in the ancestors of amphibians and amniotes were independent events and preceded the evolution of tympanic membranes; the amphibian tympanic membrane. probably including that of labyrinthodonts. is not ancestral to that of amniotes. and that labyiinthodonts with an otic notch are not suitable as amniote ancestors; the stapes of early reptiles functioned primarily as part of the jaw suspension rather than in hearing; the mechanisms and abilities of sound perception in recent tetrapods are likely to be diverse rather than forming parts of a cline; and the lack of a tympanum in Gymnophiona and Caudata may be a retention of a primitive condition.

EVOLUTION OF BRAIN AND BEHAVIOR

page (p. 52) he tells us that most Agamidae replace some of teeth on premaxilla (posteriorly) and anterior ends of maxilla and dentary. function of this statement is lost when, over a series of pages, he points out that species with which he is concerned do not replace teeth. Finally (p. 62) he reveals that some carnivorous species replace teeth and others, or herbivores, do not. upshot of a long and difficult account is that acrodonty correlates with precise occlusion and, whereas this was not case in living Sphenodon, it was in fossil sphenodontids. More or less isolated in this collection of morphologically oriented papers are those of Swain and Olson. Swain suggested that extinction of ruling archosaurs could be correlated with rise of angiosperms in late Cretaceous and unpalatability or even poisonous nature of these plants. He discusses tannins and toxic alkaloids which may have had such effects as shell-thinning. argument is attractive as Cretaceous reptile economy was largely based on large, hungry herbivores. Olson's contribution is titled The exploitation of land by early tetrapods, but little is said about origin of amniote egg or anatomical modifications involved. style is indicated by such a statement as (pp. 2-3) Rapid emergence [onto land] . . . is strongly supported by current data on morphology of earliest reptiles and their immediate predecessors, and is necessary to explain ecological complexes of vertebrates that are encountered during late Cretaceous and Permian. This rich rhetoric assumes much more than I believe is known in terms of predecessors, faunas and ecological conditions. A recent find of Namurian material in Great Britain suggests possibility that reptiles, terrestrial organisms, were already present. earliest known reptiles, Westphalian, are already of diverse types. As is often case in papers by Olson, some of confused thinking of past is destroyed. In this instance, he condemns order (p. 9, suborder, p. 14) Cotylosauria. This is personally pleasing, as I never could comprehend this group as a taxonomic entity. This paper, again like others of his, uses largely undefinable taxonomic terminology which one hopes will eventually become more real. illustrations in this instance seem incidental to text. Of a physiological sort are accounts by Avery and Spellerberg concerning thermoregulation in reptiles and adaptations to cold climates. Similarly isolated is paper by Westphal on the dermal armour of some triassic placodont reptiles. I do agree with writer's conclusion that there is a basic hexagonal dermal plate element in vertebrates which is seen in one form or another from fish to fowl. For readers of Evolution, I would give this volume one star or at most two.

DEVONIAN AMPHIBIANS: DID THEY EXCRETE CARBON DIOXIDE VIA SKIN, GILLS, OR LUNGS?

An unresolved problem of vertebrate evolution concerns the means by which Devonian amphibians ancestral to the Reptilia excreted the carbon dioxide produced in intermediary metabolism. One school of thought is that a moist integument in these ancient tetrapods was the primary avenue for escape of carbon dioxide (Howell, 1970; Hughes, 1966, 1967; Rahn, 1967; Thomson, 1969); a second school holds that internal gills, which presumably persisted in Devonian amphibians, were the site of carbon dioxide exchange (Romer, 1972a); a third school maintains that the lungs accommodated both the uptake of oxygen and the elimination of carbon dioxide, and that a high rate of pulmonary ventilation prevented the accumulation of carbon dioxide in the body fluids (Gans, 1970a, b); and yet a fourth school contends that the lungs were the primary site for carbon dioxide exchange, but that early amphibians had already acquired a tolerance for high carbon dioxide tensions in arterial blood, thus enabling them to accomplish carbon dioxide exchange without high rates of pulmonary ventilation (see Garey and Rahn, 1970; Johansen, 1971: 595; McMahon, 1970). Each of the first three theories (but not the fourth) has been clearly formulated, and each has won support within the scientific community. Nonetheless, these first three theories can all be criticized on theoretical grounds, and none of the three receives strong support from the large body of physiologic information available for several contemporary fishes which seem to be in transition from water breathing to air breathing. My objectives are to summarize the major objections to each of the first three theories concerning the mode of excretion of carbon dioxide used by Devonian amphibians, and then to offer some very rough calculations in support of the fourth theory-calculations which indicate that a relatively low rate of pulmonary ventilation in these early tetrapods would have been sufficient to eliminate carbon dioxide via the lungs without concomitant acidosis.