Integration of morphological data sets for phylogenetic analysis of Amniota: the importance of integumentary characters and increased taxonomic sampling

Several mutually exclusive hypotheses have been advanced to explain the phylogenetic position of turtles among amniotes. Traditional morphology-based analyses place turtles among extinct anapsids (reptiles with a solid skull roof), whereas more recent studies of both morphological and molecular data support an origin of turtles from within Diapsida (reptiles with a doubly fenestrated skull roof). Evaluation of these conflicting hypotheses has been hampered by nonoverlapping taxonomic samples and the exclusion of significant taxa from published analyses. Furthermore, although data from soft tissues and anatomical systems such as the integument may be particularly relevant to this problem, they are often excluded from large-scale analyses of morphological systematics. Here, conflicting hypotheses of turtle relationships are tested by (1) combining published data into a supermatrix of morphological characters to address issues of character conflict and missing data; (2) increasing taxonomic sampling by more than doubling the number of operational taxonomic units to test internal relationships within suprageneric ingroup taxa; and (3) increasing character sampling by approximately 25% by adding new data on the osteology and histology of the integument, an anatomical system that has been historically underrepresented in morphological systematics. The morphological data set assembled here represents the largest yet compiled for Amniota. Reevaluation of character data from prior studies of amniote phylogeny favors the hypothesis that turtles indeed have diapsid affinities. Addition of new ingroup taxa alone leads to a decrease in overall phylogenetic resolution, indicating that existing characters used for amniote phylogeny are insufficient to explain the evolution of more highly nested taxa. Incorporation of new data from the soft and osseous components of the integument, however, helps resolve relationships among both basal and highly nested amniote taxa. Analysis of a data set compiled from published sources and data original to this study supports monophyly of Amniota, Synapsida, Reptilia, Parareptilia, Eureptilia, Eosuchia, Diapsida, Neodiapsida, Sauria, Lepidosauria, and Archosauriformes, as well as several more highly nested divisions within the latter two clades. Turtles are here resolved as the sister taxon to a monophyletic Lepidosauria (squamates + Sphenodon), a novel phylogenetic position that nevertheless is consistent with recent molecular and morphological studies that have hypothesized diapsid affinities for this clade.     

Evolution of amniote dentine apposition rates

In amniotes, daily rates of dentine formation in non-ever-growing teeth range from less than 1 to over 25 μm per day. The latter value has been suggested to represent the upper limit of odontoblast activity in non-ever-growing teeth, a hypothesis supported by the lack of scaling between dentine apposition rates and body mass in Dinosauria. To determine the correlates and potential controls of dentine apposition rate, we assembled a dataset of apposition rates, metabolic rates and body masses for ca 80 amniote taxa of diverse ecologies and diets. We used phylogenetic regression to test for scaling relationships and reconstruct ancestral states of daily dentine apposition across Amniota. We find no relationship between body mass and daily dentine apposition rate (DDAR) for non-ever-growing teeth in Amniota as a whole or within major clades. Metabolic rate, the number of tooth generations, diet and habitat also do not predict or correspond with DDARs. Similar DDARs are found in large terrestrial mammals, dinosaurs and marine reptiles, whereas primates, cetaceans and some smaller marine reptiles independently evolved exceptionally slow rates. Life-history factors may explain the evolution of dentine apposition rates, which evolved rapidly at the origin of major clades.     

Germline development in amniotes: A paradigm shift in primordial germ cell specification

In the field of germline development in amniote vertebrates, primordial germ cell (PGC) specification in birds and reptiles remains controversial. Avians are believed to adopt a predetermination or maternal specification mode of PGC formation, contrary to an inductive mode employed by mammals and, supposedly, reptiles. Here, we revisit and review some key aspects of PGC development that channelled the current subdivision, and challenge the position of birds and reptiles as well as the ‘binary’ evolutionary model of PGC development in vertebrates. We propose an alternative view on PGC specification where germ plasm plays a role in laying the foundation for the formation of PGC precursors (pPGC), but not necessarily of PGCs. Moreover, inductive mechanisms may be necessary for the transition from pPGCs to PGCs. Within this framework, the implementation of data from birds and reptiles could provide new insights on the evolution of PGC specification in amniotes.     

Development and evolution of the tetrapod skull–neck boundary

The origin and evolution of the vertebrate skull have been topics of intense study for more than two centuries. Whereas early theories of skull origin, such as the influential vertebral theory, have been largely refuted with respect to the anterior (pre‐otic) region of the skull, the posterior (post‐otic) region is known to be derived from the anteriormost paraxial segments, i.e. the somites. Here we review the morphology and development of the occiput in both living and extinct tetrapods, taking into account revised knowledge of skull development by augmenting historical accounts with recent data. When occipital composition is evaluated relative to its position along the neural axis, and specifically to the hypoglossal nerve complex, much of the apparent interspecific variation in the location of the skull–neck boundary stabilizes in a phylogenetically informative way. Based on this criterion, three distinct conditions are identified in (i) frogs, (ii) salamanders and caecilians, and (iii) amniotes. The position of the posteriormost occipital segment relative to the hypoglossal nerve is key to understanding the evolution of the posterior limit of the skull. By using cranial foramina as osteological proxies of the hypoglossal nerve, a survey of fossil taxa reveals the amniote condition to be present at the base of Tetrapoda. This result challenges traditional theories of cranial evolution, which posit translocation of the occiput to a more posterior location in amniotes relative to lissamphibians (frogs, salamanders, caecilians), and instead supports the largely overlooked hypothesis that the reduced occiput in lissamphibians is secondarily derived. Recent advances in our understanding of the genetic basis of axial patterning and its regulation in amniotes support the hypothesis that the lissamphibian occipital form may have arisen as the product of a homeotic shift in segment fate from an amniote‐like condition.     

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 and homology of the astragalus in early amniotes: new fossils, new perspectives

The reorganization of the ankle in basal amniotes has long been considered a key innovation allowing the evolution of more terrestrial and cursorial behavior. Understanding how this key innovation arose is a complex problem that largely concerns the homologizing of the amniote astragalus with the various ossifications in the anamniote tarsus. Over the last century, several hypotheses have been advanced homologizing the amniote astragalus with the many ossifications in the ankle of amphibian-grade tetrapods. There is an emerging consensus that the amniote astragalus is a complex structure emerging via the co-ossification of several originally separate elements, but the identities of these elements remain unclear. Here we present new fossil evidence bearing on this contentious question. A poorly ossified, juvenile astragalus of the large captorhinid Moradisaurus grandis shows clear evidence of four ossification centers, rather than of three centers or one center as posited in previous models of astragalus homology. Comparative material of the captorhinid Captorhinikos chozaensis is also interpretable as demonstrating four ossification centers. A new, four-center model for the homology of the amniote astragalus is advanced, and is discussed in the context of the phylogeny of the Captorhinidae in an attempt to identify the developmental transitions responsible for the observed pattern of ossification within this clade. Lastly, the broader implications for amniote phylogeny are discussed, concluding that the neomorphic pattern of astragalus ossification seen in all extant reptiles (including turtles) arose within the clade Diapsida.     

Evolution of posture in amniotes–Diving into the trabecular architecture of the femoral head

Extant amniotes show remarkable postural diversity. Broadly speaking, limbs with erect (strongly adducted, more vertically oriented) posture are found in mammals that are particularly heavy (graviportal) or show good running skills (cursorial), while crouched (highly flexed) limbs are found in taxa with more generalized locomotion. In Reptilia, crocodylians have a “semi-erect” (somewhat adducted) posture, birds have more crouched limbs and lepidosaurs have sprawling (well-abducted) limbs. Both synapsids and reptiles underwent a postural transition from sprawling to more erect limbs during the Mesozoic Era. In Reptilia, this postural change is prominent among archosauriforms in the Triassic Period. However, limb posture in many key Triassic taxa remains poorly known. In Synapsida, the chronology of this transition is less clear, and competing hypotheses exist. On land, the limb bones are subject to various stresses related to body support that partly shape their external and internal morphology. Indeed, bone trabeculae (lattice-like bony struts that form the spongy bone tissue) tend to orient themselves along lines of force. Here, we study the link between femoral posture and the femoral trabecular architecture using phylogenetic generalized least squares. We show that microanatomical parameters measured on bone cubes extracted from the femoral head of a sample of amniote femora depend strongly on body mass, but not on femoral posture or lifestyle. We reconstruct ancestral states of femoral posture and various microanatomical parameters to study the “sprawling-to-erect” transition in reptiles and synapsids, and obtain conflicting results. We tentatively infer femoral posture in several hypothetical ancestors using phylogenetic flexible discriminant analysis from maximum likelihood estimates of the microanatomical parameters. In general, the trabecular network of the femoral head is not a good indicator of femoral posture. However, ancestral state reconstruction methods hold great promise for advancing our understanding of the evolution of posture in amniotes.     

Inference of the Protokaryotypes of Amniotes and Tetrapods and the Evolutionary Processes of Microchromosomes from Comparative Gene Mapping

Comparative genome analysis of non-avian reptiles and amphibians provides important clues about the process of genome evolution in tetrapods. However, there is still only limited information available on the genome structures of these organisms. Consequently, the protokaryotypes of amniotes and tetrapods and the evolutionary processes of microchromosomes in tetrapods remain poorly understood. We constructed chromosome maps of functional genes for the Chinese soft-shelled turtle (Pelodiscus sinensis), the Siamese crocodile (Crocodylus siamensis), and the Western clawed frog (Xenopus tropicalis) and compared them with genome and/or chromosome maps of other tetrapod species (salamander, lizard, snake, chicken, and human). This is the first report on the protokaryotypes of amniotes and tetrapods and the evolutionary processes of microchromosomes inferred from comparative genomic analysis of vertebrates, which cover all major non-avian reptilian taxa (Squamata, Crocodilia, Testudines). The eight largest macrochromosomes of the turtle and chicken were equivalent, and 11 linkage groups had also remained intact in the crocodile. Linkage groups of the chicken macrochromosomes were also highly conserved in X. tropicalis, two squamates, and the salamander, but not in human. Chicken microchromosomal linkages were conserved in the squamates, which have fewer microchromosomes than chicken, and also in Xenopus and the salamander, which both lack microchromosomes; in the latter, the chicken microchromosomal segments have been integrated into macrochromosomes. Our present findings open up the possibility that the ancestral amniotes and tetrapods had at least 10 large genetic linkage groups and many microchromosomes, which corresponded to the chicken macro- and microchromosomes, respectively. The turtle and chicken might retain the microchromosomes of the amniote protokaryotype almost intact. The decrease in number and/or disappearance of microchromosomes by repeated chromosomal fusions probably occurred independently in the amphibian, squamate, crocodilian, and mammalian lineages.     

Recent Progress in Understanding Early Tetrapods

This paper reviews significant discoveries and interpretationsmade 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 importantinsight into the early evolution of land vertebrates. The numberof exceptionally well known taxa with multiple, excellentlypreserved specimens has doubled to eight from four. The veryearliest tetrapods have been discovered to have been polydactylous,the ear region to have had a complicated early evolution andthe specialized tooth type found in Recent amphibians has beenfound in a group of Lower Permian temnospondyl amphibians, indicatingan evolutionary relationship. Perhaps the most significant advancein understanding the evolution of early tetrapods is that thebasal amniote groups have become better characterized and astart has been made in providing a defensible hypothesis fortheir relationships. The ascendancy of cladistics, functionalmorphology and plate tectonics has changed the way paleontologistsview fossils resulting in more defensible phylogenies and behavioraland biogeographical scenarios. These approaches to understandinghave, perhaps, had a more profound impact than any of the newfossil discoveries.     

Reflections on the origin of the amniote egg in the light of reproductive strategies and shell structure

On the basis of evolutionary pattern of reproductive strategies (r‐ and K‐selected animals) and the ultrastructure of modern amniote eggshells, a new model of the origin of the amniote egg is presented. In contrast to the well‐known idea of Romer (1957) that the egg came first while adult reptiles in the Carboniferous remained in water, it is suggested that the early evolution of the amniote egg (including the gradual formation of membranes) happened within the aquatic realm. Increasing enlargement of eggs and yolk is interpreted as an adaptation of reproduction strategies to lakes with poor nutrient contents. The first accumulation of Ca‐ions in the outer membrane, paralleled by many modern noncalcified lepidosaurian eggs, was a process of detoxification, according to new ideas in biomineralization. The function of the shell, to protect the embryo against microbial activity and to prevent water loss, which is necessary for the preamniote egg to become a fully terrestrial egg, was the terminal step in this story. Yolk‐rich eggs enclosed by a more or less calcified shell can be interpreted as an essential preadaptation for tetrapods to have become fully terrestrial during the Late Paleozoic.     

The origin and early evolutionary history of amniotes

Recent phylogenetic analyses of Paleozoic tetrapods have yielded startling new insights into the origin and early evolutionary history of amniotes. The origin of this successful group involves evolutionary innovations that are associated with the development of the cleidoic egg and related reproductive strategies, and are therefore not represented directly in the fossil record. Despite this obvious difficulty, recent studies have been able to distinguish Paleozoic amniotes from their anamniotic tetrapod relatives to determine major patterns of interrelationships.     

Prey processing in amniotes: biomechanical and behavioral patterns of food reduction

In this paper we examine the biomechanics of prey processing behavior in the amniotes. Whether amniotes swallow prey items whole or swallow highly processed slurries or boluses of food, they share a common biomechanical system where hard surfaces (teeth or beaks) are brought together on articulated jaws by the actions of adductor muscles to grasp and process food. How have amniotes modified this basic system to increase the chewing efficiency of the system? To address this question we first examine the primitive condition for prey processing representative of many of the past and present predatory amniotes. Because herbivory is expected to be related to improved prey processing in the jaws we review patterns of food processing mechanics in past and present herbivores. Herbivory has appeared numerous times in amniotes and several solutions to the task of chewing plant matter have appeared. Birds have abandoned jaw chewing in favor of a new way to chew--with the gut--so we will detour from the jaws to examine the appearance of gut chewing in the archosaurs. We will then fill in the gaps among amniote taxa with a look at some new data on patterns of prey processing behavior and jaw mechanics in lizards. Finally, we examine evolutionary patterns of amniote feeding mechanism and how correlates of chewing relate to the need to increase the efficiency of prey processing in order to facilitate increased metabolic rate and activity.     

Paleogenomic data suggest mammal-like genome size in the ancestral amniote and derived large genome size in amphibians

An unsolved question in evolutionary genomics is whether amniote genomes have been expanding or contracting since the common ancestor of this diverse group. Here, we report on the polarity of amniote genome size evolution using genome size estimates for 14 extinct tetrapod genera from the Paleozoic and early Mesozoic Eras using osteocyte lacunae size as a correlate. We find substantial support for a phylogenetically controlled regression model relating genome size to osteocyte lacunae size (P of slopes <0.01, r2=0.65, phylogenetic signal λ=0.83). Genome size appears to have been homogeneous across Paleozoic crown-tetrapod lineages (average haploid genome size 2.9-3.7 pg) with values similar to those of extant mammals. The differentiation in genome size and underlying architecture among extant tetrapod lineages likely evolved in the Mesozoic and Cenozoic Eras, with expansion in amphibians, contractions along the diapsid lineage, and no directional change within the synapsid lineage leading to mammals.     

Hedgehog regulation of superficial slow muscle fibres in Xenopus and the evolution of tetrapod trunk myogenesis

In tetrapod phylogeny, the dramatic modifications of the trunk have received less attention than the more obvious evolution of limbs. In somites, several waves of muscle precursors are induced by signals from nearby tissues. In both amniotes and fish, the earliest myogenesis requires secreted signals from the ventral midline carried by Hedgehog (Hh) proteins. To determine if this similarity represents evolutionary homology, we have examined myogenesis in Xenopus laevis, the major species from which insight into vertebrate mesoderm patterning has been derived. Xenopus embryos form two distinct kinds of muscle cells analogous to the superficial slow and medial fast muscle fibres of zebrafish. As in zebrafish, Hh signalling is required for XMyf5 expression and generation of a first wave of early superficial slow muscle fibres in tail somites. Thus, Hh-dependent adaxial myogenesis is the likely ancestral condition of teleosts, amphibia and amniotes. Our evidence suggests that midline-derived cells migrate to the lateral somite surface and generate superficial slow muscle. This cell re-orientation contributes to the apparent rotation of Xenopus somites. Xenopus myogenesis in the trunk differs from that in the tail. In the trunk, the first wave of superficial slow fibres is missing, suggesting that significant adaptation of the ancestral myogenic programme occurred during tetrapod trunk evolution. Although notochord is required for early medial XMyf5 expression, Hh signalling fails to drive these cells to slow myogenesis. Later, both trunk and tail somites develop a second wave of Hh-independent slow fibres. These fibres probably derive from an outer cell layer expressing the myogenic determination genes XMyf5, XMyoD and Pax3 in a pattern reminiscent of amniote dermomyotome. Thus, Xenopus somites have characteristics in common with both fish and amniotes that shed light on the evolution of somite differentiation. We propose a model for the evolutionary adaptation of myogenesis in the transition from fish to tetrapod trunk.     

The relationship between body mass and relative investment in testes mass in amniotes and other vertebrates

In terrestrial placental mammals, there is a well‐known negative allometric relationship between body mass and relative investment in testes mass. Such a negative relationship means that males of relatively monogamous small species invest proportionately more in their reproductive tissues than males of more polyandrous larger species. The selective pressure responsible for this relationship remains unclear and is it not known if this is a general allometric relationship that is similar across all vertebrate lineages. To investigate this, we conducted the first comparison of relationships between body mass and testes mass (using percentage testes mass as the dependent variable) across a variety of vertebrate groups. In all amniote lineages examined, the allometric relationship between body mass and testes mass was relatively strong and negative. We show, for the first time, that reptiles, birds and terrestrial placental mammals followed the same allometric relationship and, contrary to previous expectations, this relationship is sigmoidal rather than linear. Within this data set, there was no significant difference between this general amniote relationship and any of the 13 orders of reptiles, birds and terrestrial placental mammals examined. As a result, we propose that a sigmoidal relationship should be considered the default assumption for the form of the body mass – testes mass relationship within the amniote lineage. However, we also identify significant differences within some additional mammal groups (marsupials, bats and cetaceans). In each of these cases, only some sub‐groupings differed significantly from the general amniote relationship. In contrast to the amniotes, the relationship is relatively weak and positive in teleost fish and frogs suggesting that a negative allometric relationship is not universal in vertebrates. We explore whether variation in the body mass – testes mass relationships can be linked to sperm competition or a variety of ecological characteristics, either for amniotes in particular or vertebrates in general.     

Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high-resolution x-ray microcomputed tomography,and the origin of the amniote crown group

The origin of amniotes was a key event in vertebrate evolution, enabling tetrapods to break their ties with water and invade terrestrial environments. Two pivotal clades of early tetrapods, the diadectomorphs and the seymouriamorphs, have played an unsurpassed role in debates about the ancestry of amniotes for over a century, but their skeletal morphology has provided conflicting evidence for their affinities. Using high-resolution X-ray microcomputed tomography, we reveal the three-dimensional architecture of the well preserved endosseous labyrinth of the inner ear in representative species belonging to both groups. Data from the inner ear are coded in a new cladistic matrix of stem and primitive crown amniotes. Both maximum parsimony and Bayesian inference analyses retrieve seymouriamorphs as derived non-crown amniotes and diadectomorphs as sister group to synapsids. If confirmed, this sister group relationship invites re-examination of character polarity near the roots of the crown amniote radiation. Major changes in the endosseous labyrinth and adjacent braincase regions are mapped across the transition from non-amniote to amniote tetrapods and include: a ventral shift of the cochlear recess relative to the vestibule and the semicircular canals; cochlear recess (primitively housed exclusively within the opisthotic) accommodated within both the prootic and the opisthotic; development of a distinct fossa subarcuata. The inner ear of seymouriamorphs foreshadows conditions of more derived groups, whereas that of diadectomorphs shows a mosaic of plesiomorphic and apomorphic traits, some of which are unambiguously amniote-like, including a distinct and pyramid-like cochlear recess.     

Morphology of the temporal skull region in tetrapods: research history,functional explanations,and a new comprehensive classification scheme

The morphology of the temporal region in the tetrapod skull traditionally has been a widely discussed feature of vertebrate anatomy. The evolution of different temporal openings in Amniota (mammals, birds, and reptiles), Lissamphibia (frogs, salamanders, and caecilians), and several extinct tetrapod groups has sparked debates on the phylogenetic, developmental, and functional background of this region in the tetrapod skull. This led most famously to the erection of different amniote taxa based on the number and position of temporal fenestrae in their skulls. However, most of these taxa are no longer recognised to represent natural groupings and the morphology of the temporal region is not necessarily an adequate trait for use in the reconstruction of amniote phylogenies. Yet, new fossil finds, most notably of parareptiles and stem-turtles, as well as modern embryological and biomechanical studies continue to provide new insights into the morphological diversity of the temporal region. Here, we introduce a novel comprehensive classification scheme for the various temporal morphotypes in all Tetrapoda that is independent of phylogeny and previous terminology and may facilitate morphological comparisons in future studies. We then review the history of research on the temporal region in the tetrapod skull. We document how, from the early 19th century with the first recognition of differences in the temporal region to the first proposals of phylogenetic relationships and their assessment over the centuries, the phylogenetic perspective on the temporal region has developed, and we highlight the controversies that still remain. We also compare the different functional and developmental drivers proposed for the observed morphological diversity and how the effects of internal and external factors on the structure of the tetrapod skull have been interpreted.     

Molecular characterization of the gastrula in the turtle Emys orbicularis: an evolutionary perspective on gastrulation

Due to the presence of a blastopore as in amphibians, the turtle has been suggested to exemplify a transition form from an amphibian- to an avian-type gastrulation pattern. In order to test this hypothesis and gain insight into the emergence of the unique characteristics of amniotes during gastrulation, we have performed the first molecular characterization of the gastrula in a reptile, the turtle Emys orbicularis. The study of Brachyury, Lim1, Otx2 and Otx5 expression patterns points to a highly conserved dynamic of expression with amniote model organisms and makes it possible to identify the site of mesoderm internalization, which is a long-standing issue in reptiles. Analysis of Brachyury expression also highlights the presence of two distinct phases, less easily recognizable in model organisms and respectively characterized by an early ring-shaped and a later bilateral symmetrical territory. Systematic comparisons with tetrapod model organisms lead to new insights into the relationships of the blastopore/blastoporal plate system shared by all reptiles, with the blastopore of amphibians and the primitive streak of birds and mammals. The biphasic Brachyury expression pattern is also consistent with recent models of emergence of bilateral symmetry, which raises the question of its evolutionary significance.     

The evolution of the mechanical properties of amniote bone

162 specimens from 19 species of amniote were tested for various mechanical and physical properties to ascertain whether there were characteristic differences between different groups. All mechanical properties showed very great variation. In general the reptiles were not inferior to the mammals and birds. The histology of living forms was compared to that of fossil forms, to see whether 'weak' histology was more characteristic of primitive amniotes. The earliest reptiles probably had rather complaint bone, but it was probably tough. Modern types of bone appeared over two hundred million years ago. Very specialised bone, like that of the bullae of whales and antlers, may have evolved only in the mammals, but the fossil record is not complete enough to assert this confidently.