Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution

The origin of turtles: a paleontological perspective

The origin of turtles and their unusual body plan has fascinated scientists for the last two centuries. Over the course of the last decades, a broad sample of molecular analyses have favored a sister group relationship of turtles with archosaurs, but recent studies reveal that this signal may be the result of systematic biases affecting molecular approaches, in particular sampling, non-randomly distributed rate heterogeneity among taxa, and the use of concatenated data sets. Morphological studies, by contrast, disfavor archosaurian relationships for turtles, but the proposed alternative topologies are poorly supported as well. The recently revived paleontological hypothesis that the Middle Permian Eunotosaurus africanus is an intermediate stem turtle is now robustly supported by numerous characters that were previously thought to be unique to turtles and that are now shown to have originated over the course of tens of millions of years unrelated to the origin of the turtle shell. Although E. africanus does not solve the placement of turtles within Amniota, it successfully extends the stem lineage of turtles to the Permian and helps resolve some questions associated with the origin of turtles, in particular the non-composite origin of the shell, the slow origin of the shell, and the terrestrial setting for the origin of turtles.

Emerging from the rib: resolving the turtle controversies

Two of the major controversies in the present study of turtle shell development involve the mechanism by which the carapacial ridge initiates shell formation and the mechanism by which each rib forms the costal bones adjacent to it. This paper claims that both sides of each debate might be correct-but within the species examined. Mechanism is more properly "mechanisms," and there is more than one single way to initiate carapace formation and to form the costal bones. In the initiation of the shell, the rib precursors may be kept dorsal by either "axial displacement" (in the hard-shell turtles) or "axial arrest" (in the soft-shell turtle Pelodiscus), or by a combination of these. The former process would deflect the rib into the dorsal dermis and allow it to continue its growth there, while the latter process would truncate rib growth. In both instances, though, the result is to keep the ribs from extending into the ventral body wall. Our recent work has shown that the properties of the carapacial ridge, a key evolutionary innovation of turtles, differ greatly between these two groups. Similarly, the mechanism of costal bone formation may differ between soft-shell and hard-shell turtles, in that the hard-shell species may have both periosteal flattening as well as dermal bone induction, while the soft-shelled turtles may have only the first of these processes.

Biological armors under impact--effect of keratin coating, and synthetic bio-inspired analogues

A number of biological armors, such as turtle shells, consist of a strong exoskeleton covered with a thin keratin coating. The mechanical role upon impact of this keratin coating has surprisingly not been investigated thus far. Low-velocity impact tests on the turtle shell reveal a unique toughening phenomenon attributed to the thin covering keratin layer, the presence of which noticeably improves the fracture energy and shell integrity. Synthetic substrate/coating analogues were subsequently prepared and exhibit an impact behavior similar to the biological ones. The results of the present study may improve our understanding, and even future designs, of impact-tolerant structures.

Comparative study of the shell development of hard- and soft-shelled turtles

The turtle shell provides a fascinating model for the investigation of the evolutionary modifications of developmental mechanisms. Different conclusions have been put forth for its development, and it is suggested that one of the causes of the disagreement could be the differences in the species of the turtles used - the differences between hard-shelled turtles and soft-shelled turtles. To elucidate the cause of the difference, we compared the turtle shell development in the two groups of turtle. In the dorsal shell development, these two turtle groups shared the gene expression profile that is required for formation, and shared similar spatial organization of the anatomical elements during development. Thus, both turtles formed the dorsal shell through a folding of the lateral body wall, and the Wnt signaling pathway appears to have been involved in the development. The ventral portion of the shell, on the other hand, contains massive dermal bones. Although expression of HNK-1 epitope has suggested that the trunk neural crest contributed to the dermal bones in the hard-shelled turtles, it was not expressed in the initial anlage of the skeletons in either of the types of turtle. Hence, no evidence was found that would support a neural crest origin.

The evolutionary origin of the turtle shell and its dependence on the axial arrest of the embryonic rib cage

Turtles are characterized by their possession of a shell with dorsal and ventral moieties: the carapace and the plastron, respectively. In this review, we try to provide answers to the question of the evolutionary origin of the carapace, by revising morphological, developmental, and paleontological comparative analyses. The turtle carapace is formed through modification of the thoracic ribs and vertebrae, which undergo extensive ossification to form a solid bony structure. Except for peripheral dermal elements, there are no signs of exoskeletal components ontogenetically added to the costal and neural bones, and thus the carapace is predominantly of endoskeletal nature. Due to the axial arrest of turtle rib growth, the axial part of the embryo expands laterally and the shoulder girdle becomes encapsulated in the rib cage, together with the inward folding of the lateral body wall in the late phase of embryogenesis. Along the line of this folding develops a ridge called the carapacial ridge (CR), a turtle-specific embryonic structure. The CR functions in the marginal growth of the carapacial primordium, in which Wnt signaling pathway might play a crucial role. Both paleontological and genomic evidence suggest that the axial arrest is the first step toward acquisition of the turtle body plan, which is estimated to have taken place after the divergence of a clade including turtles from archosaurs. The developmental relationship between the CR and the axial arrest remains a central issue to be solved in future.

The endoskeletal origin of the turtle carapace

The turtle body plan, with its solid shell, deviates radically from those of other tetrapods. The dorsal part of the turtle shell, or the carapace, consists mainly of costal and neural bony plates, which are continuous with the underlying thoracic ribs and vertebrae, respectively. Because of their superficial position, the evolutionary origins of these costo-neural elements have long remained elusive. Here we show, through comparative morphological and embryological analyses, that the major part of the carapace is derived purely from endoskeletal ribs. We examine turtle embryos and find that the costal and neural plates develop not within the dermis, but within deeper connective tissue where the rib and intercostal muscle anlagen develop. We also examine the fossils of an outgroup of turtles to confirm that the structure equivalent to the turtle carapace developed independently of the true osteoderm. Our results highlight the hitherto unravelled evolutionary course of the turtle shell.

The evolutionary origins of the costal and neural bony plates of the turtle shell have long remained elusive. Here the authors show, through comparative morphological and embryological analyses, that the most of the carapace is derived from endoskeletal ribs.

Divergent palate morphology in turtles and birds correlates with differences in proliferation and BMP2 expression during embryonic development

During embryonic development, amniotes typically form outgrowths from the medial sides of the maxillary prominences called palatal shelves or palatine processes. In mammals the shelves fuse in the midline and form a bony hard palate that completely separates the nasal and oral cavities. In birds and lizards, palatine processes develop but remain unfused, leaving a natural cleft. Adult turtles do not possess palatine processes and unlike other amniotes, the internal nares open into the oral cavity. Here we investigate craniofacial ontogeny in the turtle, Emydura subglobosa to determine whether vestigial palatine processes develop and subsequently regress, or whether development fails entirely. We found that the primary palate in turtles develops similarly to other amniotes, but secondary palate ontogeny diverges. Using histology, cellular dynamics and in situ hybridization we found no evidence of palatine process development at any time during ontogeny of the face in the turtle. Furthermore, detailed comparisons with chicken embryos (the model organism most closely related to turtles from a molecular phylogeny perspective), we identified differences in proliferation and gene expression patterns that correlate with the differences in palate morphology. We propose that, in turtles, palatine process outgrowth is never initiated due to a lack of mesenchymal bone morphogenetic protein 2 (BMP2) expression in the maxillary mesenchyme, which in turn fails to induce the relatively higher cellular proliferation required for medial tissue outgrowth. It is likely that these differences between turtles and birds arose after the divergence of the lineage leading to modern turtles.

Rapid evolution of Beta-keratin genes contribute to phenotypic differences that distinguish turtles and birds from other reptiles

Sequencing of vertebrate genomes permits changes in distinct protein families, including gene gains and losses, to be ascribed to lineage-specific phenotypes. A prominent example of this is the large-scale duplication of beta-keratin genes in the ancestors of birds, which was crucial to the subsequent evolution of their beaks, claws, and feathers. Evidence suggests that the shell of Pseudomys nelsoni contains at least 16 beta-keratins proteins, but it is unknown whether this is a complete set and whether their corresponding genes are orthologous to avian beak, claw, or feather beta-keratin genes. To address these issues and to better understand the evolution of the turtle shell at a molecular level, we surveyed the diversity of beta-keratin genes from the genome assemblies of three turtles, Chrysemys picta, Pelodiscus sinensis, and Chelonia mydas, which together represent over 160 Myr of chelonian evolution. For these three turtles, we found 200 beta-keratins, which indicate that, as for birds, a large expansion of beta-keratin genes in turtles occurred concomitantly with the evolution of a unique phenotype, namely, their plastron and carapace. Phylogenetic reconstruction of beta-keratin gene evolution suggests that separate waves of gene duplication within a single genomic location gave rise to scales, claws, and feathers in birds, and independently the scutes of the shell in turtles.

Late-emigrating trunk neural crest cells in turtle embryos generate an osteogenic ectomesenchyme in the plastron

BACKGROUND

The turtle plastron is composed of a keratinized epidermis overlying nine dermal bones. Its developmental origin has been controversial; recent evidence suggests that the plastral bones derive from trunk neural crest cells (NCCs).

RESULTS

This study extends the observations that there is a turtle-specific, second wave of trunk NCC delamination and migration, after the original NCCs have reached their destination and differentiated. This second wave was confirmed by immunohistochemistry in whole-mounts and serial sections, by injecting DiI (1,1', di-octadecyl-3,3,3',3',-tetramethylindo-carbocyanine perchlorate) into the lumen of the neural tube and tracing labeled cells into the plastron, and by isolating neural tubes from older turtle embryos and observing delaminating NCCs. This later migration gives rise to a plastral ectomesenchyme that expresses NCC markers and can be induced to initiate bone formation.

CONCLUSIONS

The NCCs of this second migration have properties similar to those of the earlier NCCs, but also express markers characteristic of cranial NCCs. The majority of the cells of the plastron mesenchyme express neural crest markers, and have osteogenic differentiation capabilities that are similar or identical to craniofacial ectomesenchyme. Our evidence supports the contention that turtle plastron bones are derived from a late emigrating population of cells derived from the trunk neural crest.

Micro-structure and mechanical properties of the turtle carapace as a biological composite shield

Turtle shell is a multi-scale bio-composite in which the components are arranged in various spatial patterns, leading to an unusually strong and durable structure. The keratin-coated dorsal shell, termed the carapace, exhibits a flat bone, sandwich-like structure made up of two exterior cortices enclosing a cancellous interior. This unique structure was developed by nature to protect the reptile from predator attacks by sustaining impact loads and dissipating energy. In the present study we attempt to correlate the micro-scale architecture with the mechanical properties of the carapace sub-regions of the red-eared slider turtle. The microscopic structural features were examined by scanning electron microscopy and micro-computed tomography. Nanoindentation tests were performed under dry and wet conditions on orthogonal anatomical planes to evaluate the elastic modulus and hardness of the various carapace sub-regions. The mineral content was also measured in the different regions of the carapace. Consequently, we discuss the influence of hydration on the carapace sub-regions and the contribution of each sub-region to the overall mechanical resistance of the assemblage.

Natural flexible dermal armor

Fish, reptiles, and mammals can possess flexible dermal armor for protection. Here we seek to find the means by which Nature derives its protection by examining the scales from several fish (Atractosteus spatula, Arapaima gigas, Polypterus senegalus, Morone saxatilis, Cyprinius carpio), and osteoderms from armadillos, alligators, and leatherback turtles. Dermal armor has clearly been developed by convergent evolution in these different species. In general, it has a hierarchical structure with collagen fibers joining more rigid units (scales or osteoderms), thereby increasing flexibility without significantly sacrificing strength, in contrast to rigid monolithic mineral composites. These dermal structures are also multifunctional, with hydrodynamic drag (in fish), coloration for camouflage or intraspecies recognition, temperature and fluid regulation being other important functions. The understanding of such flexible dermal armor is important as it may provide a basis for new synthetic, yet bioinspired, armor materials.

Severe soft tissue ossification in a southern right whale Eubalaena australis

The carcass of a stranded southern right whale Eubalaena australis, discovered on the coast of Golfo Nuevo in Península Valdés, Argentina, exhibited extensive orthotopic and heterotopic ossification, osteochondroma-like lesions, and early degenerative joint disease. Extensive soft tissue ossification led to ankylosis of the axial skeleton in a pattern that, in many respects, appeared more similar to a disabling human genetic disorder, fibrodysplasia ossificans progressiva (FOP), than to more common skeletal system diseases in cetaceans and other species. This is the first reported case of a FOP-like condition in a marine mammal and raises important questions about conserved mechanisms of orthotopic and heterotopic ossification in this clade.

Evolution of the turtle bauplan: the topological relationship of the scapula relative to the ribcage

The turtle shell and the relationship of the shoulder girdle inside or 'deep' to the ribcage have puzzled neontologists and developmental biologists for more than a century. Recent developmental and fossil data indicate that the shoulder girdle indeed lies inside the shell, but anterior to the ribcage. Developmental biologists compare this orientation to that found in the model organisms mice and chickens, whose scapula lies laterally on top of the ribcage. We analyse the topological relationship of the shoulder girdle relative to the ribcage within a broader phylogenetic context and determine that the condition found in turtles is also found in amphibians, monotreme mammals and lepidosaurs. A vertical scapula anterior to the thoracic ribcage is therefore inferred to be the basal amniote condition and indicates that the condition found in therian mammals and archosaurs (which includes both developmental model organisms: chickens and mice) is derived and not appropriate for studying the developmental origin of the turtle shell. Instead, among amniotes, either monotreme mammals or lepidosaurs should be used.

Fixed cervical count and the origin of the mammalian diaphragm

Why is mammalian cervical count fixed across the historically long and ecologically broad mammalian radiation? Multiple lines of evidence, considered together, suggest a link between fixed cervical count and the muscularization of the diaphragm, a key innovation in mammalian history. We test this hypothesis by documenting the anteroposterior (AP) movement of the diaphragm, a lateral plate derivative, relative to that of the somitic thoracolumbar transition in unusually patterned mammals, by comparing the temporal occurrence of an osteological proxy for the diaphragm and fixed cervical counts in the fossil record, and by quantifying morphological differentiation within the mammalian cervical series. We then integrate these anatomical observations with details of diaphragm function and development to propose a sequence of innovations in mammalian evolution that could have led to fixed cervical count. We argue that the novel commitment of migratory muscle precursor cells (MMPs) of mid-cervical somites to a fate in the abaxial diaphragm defined these somites as a new and uniquely mammalian modular subunit. We further argue that the coordination of primaxial abaxial patterning constrained subsequent AP migration of the forelimb, thereby secondarily fixing cervical count.

Pleated turtle escapes the box--shape changes in Dermochelys coriacea

Typical chelonians have a rigid carapace and plastron that form a box-like structure that constrains several aspects of their physiology and ecology. The leatherback sea turtle, Dermochelys coriacea, has a flexible bony carapace strengthened by seven longitudinal ridges, whereas the plastron is reduced to an elliptical outer bony structure, so that the ventrum has no bony support. Measurements of the shell were made on adult female leatherbacks studied on the feeding grounds of waters off Nova Scotia (NS) and on breeding beaches of French Guiana (FG) to examine whether foraging and/or breeding turtles alter carapace size and/or shape. NS turtles exhibited greater mass and girth for a given curved carapace length (CCL) than FG turtles. Girth:CCL ratios rose during the feeding season, indicating increased girth. Measurements were made of the direct (straight) and surface (curved) distances between the medial longitudinal ridge and first right-hand longitudinal ridge (at 50% CCL). In NS turtles, the ratio of straight to curved inter-ridge distances was significantly higher than in FG turtles, indicating distension of the upper surfaces of the NS turtles between the ridges. FG females laid 11 clutches in the breeding season; although CCL and curved carapace width remained stable, girth declined between each nesting episode, indicating loss of mass. Straight to curved inter-ridge distance ratios did not change significantly during the breeding season, indicating loss of dorsal blubber before the onset of breeding. The results demonstrate substantial alterations in size and shape of female D. coriacea over periods of weeks to months in response to alterations in nutritional and reproductive status.

Hepatocyte growth factor is crucial for development of the carapace in turtles

Turtles are characterized by their shell, composed of a dorsal carapace and a ventral plastron. The carapace first appears as the turtle-specific carapacial ridge (CR) on the lateral aspect of the embryonic flank. Accompanying the acquisition of the shell, unlike in other amniotes, hypaxial muscles in turtle embryos appear as thin threads of fibrous tissue. To understand carapacial evolution from the perspective of muscle development, we compared the development of the muscle plate, the anlage of hypaxial muscles, between the Chinese soft-shelled turtle, Pelodiscus sinensis, and chicken embryos. We found that the ventrolateral lip (VLL) of the thoracic dermomyotome of P. sinensis delaminates early and produces sparse muscle plate in the lateral body wall. Expression patterns of the regulatory genes for myotome differentiation, such as Myf5, myogenin, Pax3, and Pax7 have been conserved among amniotes, including turtles. However, in P. sinensis embryos, the gene hepatocyte growth factor (HGF), encoding a regulatory factor for delamination of the dermomyotomal VLL, was uniquely expressed in sclerotome and the lateral body wall at the interlimb level. Implantation of COS-7 cells expressing a HGF antagonist into the turtle embryo inhibited CR formation. We conclude that the de novo expression of HGF in the turtle mesoderm would have played an innovative role resulting in the acquisition of the turtle-specific body plan.

Levels of biological organization and the origin of novelty

The concept of novelty in evolutionary biology pertains to multiple tiers of biological organization from behavioral and morphological changes to changes at the molecular level. Identifying novel features requires assessments of similarity (homology and homoplasy) of relationships (phylogenetic history) and of shared developmental and genetic pathways or networks. After a brief discussion of how novelty is used in recent literature, we discuss whether the evolutionary approach to homology and homoplasy initially formulated by Lankester in the 19th century informs our understanding of novelty today. We then discuss six examples of morphological features described in the recent literature as novelties, and assess the basis upon which they are regarded as novel. The six are: origin of the turtle shell, transition from fish fins to tetrapod limbs, origination of the neural crest and neural crest cells, cement glands in frogs and casquettes in fish, whale bone-eating tubeworms, and the digestion of plant proteins by nematodes. The article concludes with a discussion of means of acquiring novel genetic information that can account for novelty recognized at higher levels. These are co-options of existing genetic circuitry, gene duplication followed by neofunctionalization, gene rearrangements through mobile genetic elements, and lateral gene transfer. We conclude that on the molecular level only the latter category provides novel genetic information, in that there is no homologous precursor. However, novel phenotypes can be generated through both neofunctionalization and gene rearrangements. Therefore, assigning phenotypic or genotypic "novelty" is contingent on the level of biological organization addressed.

Evolution and palaeoecology of early turtles: a review based on recent discoveries in the Middle Jurassic

In recent years, no less than five new species of early turtles have been described worldwide. Among them are three new turtles from Middle Jurassic deposits that partially fill a previous temporal and morphological gap in our knowledge of the early evolution of these shelled amniotes: Heckerochelys romani, Condorchelys antiqua and Eileanchelys waldmani. For the first time, the phylogenetic position of these three new species is tested in the context of the two presently competing cladistic models of turtle evolution. The addition of these taxa to each matrix does not favour or alter any of the two opposed hypotheses. However, it is demonstrated here that, by documenting yet unknown stages in the evolution of several morphological structures, these three species give stronger support to the model of an extended phylogenetic stem for turtles. These new lines of evidence include the structure of the vomer, the position of the aditus canalis stapedio-temporalis and of the posterior opening of the canalis cavernosus, and the morphology of the processus interfenestralis of the opisthotic.

Recent discoveries also reinvigorate the debate about the palaeoecology of early turtles. Whereas simple morphological characters (e.g., shell fontanelle, ligamentous bridge, flattened carapace) can be misleading, forelimb proportions and shell bone histology have led to the conclusion that most stem turtles (i.e., Proganochelys quenstedti, Palaeochersis talampayensis, Proterochersis robusta, Kayentachelys aprix and meiolaniids) were terrestrial forms. At least two out of the five recently described early turtles have been convincingly interpreted as having aquatic habits: Odontochelys semitestacea and Eileanchelys waldmani. More investigation is needed, but this will undoubtedly trigger further debate on the primitive ecology of turtles and on the origin of aquatic habits in testudines (i.e., the crown-group), respectively.

Mechanical Function of a Complex Three-dimensional Suture Joining the Bony Elements in the Shell of the Red-eared Slider Turtle

Certain design strategies appear repeatedly in a variety of biological structures. One such motif consists of a soft and pliable interface joining much larger and stiffer elements. Examples include the craniofacial sutures between the bones of the skull, the sutures between the bony plates in shell of turtles and the periodontal ligament between teeth and their sockets. Yet the detailed mechanics of these systems are not fully understood.

Turtles are believed to have existed already in the early Triassic, about 200 million years ago. They are thus one of the oldest non-extinct vertebrates. Their shell is therefore a particularly attractive subject for investigation since it has developed and conserved through such an extremely long evolutionary process and has achieved a highly optimized structure.

The turtle shell has a ‘sandwich’ structure typical of flat bones like the skull of vertebrates. It consists of two external, relatively thin sheets of dense bone (internal endocortical and external exocortical bone plates) which contain very few voids, and between them a thick and very porous spongy bone layer. At the mid-distance between adjacent ribs the dermal bones are separated by soft sutures which have a unique and complex 3-D shape.

The primary function of the shell is to protect the turtle from external trauma, and therefore it has to be stiff. However excessive stiffness may result in microdamage accumulation as a result of everyday activities like minor impact, and decrease the efficiency of respiration and locomotion. We speculate that the structure and architecture of the sutures allow easy deformation of the shell at small loads but cause it to become considerably more rigid at larger loads, reminiscent of composite materials with interlocking elements. We hypothesize that this mechanical property is related to the putative function of the suture in the turtle shell.

In order to examine this hypothesis we studied samples obtained from shells of the red eared slider turtle (Chrysemys scripta elegans). We used several imaging techniques (micro-computed tomography, scanning electron microscopy and light microscopy), histology and mechanical testing. Based on these observations we present a concept of the structure-mechanics relationship of the shell, and present a simple mathematical model of the deformation pattern of the suture-containing samples in 3-point bending tests and compare its predictions to our experimental results.

Comparative development of the crocodylian interclavicle and avian furcula, with comments on the homology of dermal elements in the pectoral apparatus

The pectoral apparatus (shoulder girdle plus sternum) of amniotes plesiomorphically includes an unpaired element of dermal origin. In crocodylians, lepidosaurs, and nontherian synapsids (monotremes and their ancestors) this element is identified as the interclavicle, in Testudines (turtles and tortoises) as the entoplastron, and in Aves as the furcula. We investigated embryonic development of the interclavicle in Alligator mississippiensis (American alligator) and of the furcula in Gallus gallus (domestic chicken). The interclavicle and furcula are among the first skeletal elements to ossify, beginning at Ferguson stage 19 (Alligator) and Hamburger and Hamilton stage 33 (Gallus). Both elements: occupy a similar mid-ventral position within the pectoral apparatus; develop from paired (bilateral) cell condensations; never coexist at anytime during ontogeny or in the adult; and undergo intramembranous (i.e., direct) ossification. For both the interclavicle and the furcula, the initial onset of ossification is concomitant with mineralization of elements of the dermatocranium, and occurs in advance of mineralization of the replacement bones (e.g., scapula, metacoracoid) of the pectoral apparatus. Shortly after the initiation of ossification the paired condensations of both elements fuse. For each of Alligator and Gallus, only one pair of skeletogenic condensations is present during embryonic development. Based on these data and a review of the evolution and development of dermal elements in the pectoral apparatus, we conclude that the interclavicle is equally parsimonious as a homolog of the furcula.

A locomotor innovation enables water-land transition in a marine fish

BACKGROUND

Morphological innovations that significantly enhance performance capacity may enable exploitation of new resources and invasion of new ecological niches. The invasion of land from the aquatic realm requires dramatic structural and physiological modifications to permit survival in a gravity-dominated, aerial environment. Most fishes are obligatorily aquatic, with amphibious fishes typically making slow-moving and short forays on to land.

METHODOLOGY/PRINCIPAL FINDINGS

Here I describe the behaviors and movements of a little known marine fish that moves extraordinarily rapidly on land. I found that the Pacific leaping blenny, Alticus arnoldorum, employs a tail-twisting movement on land, previously unreported in fishes. Focal point behavioral observations of Alticus show that they have largely abandoned the marine realm, feed and reproduce on land, and even defend terrestrial territories. Comparisons of these blennies' terrestrial kinematic and kinetic (i.e., force) measurements with those of less terrestrial sister genera show A. arnoldorum move with greater stability and locomotor control, and can move away more rapidly from impending threats.

CONCLUSIONS/SIGNIFICANCE

My results demonstrate that axial tail twisting serves as a key innovation enabling invasion of a novel marine niche. This paper highlights the potential of using this system to address general evolutionary questions about water-land transitions and niche invasions.

The integumentary skeleton of tetrapods: origin, evolution, and development

Although often overlooked, the integument of many tetrapods is reinforced by a morphologically and structurally diverse assemblage of skeletal elements. These elements are widely understood to be derivatives of the once all-encompassing dermal skeleton of stem-gnathostomes but most details of their evolution and development remain confused and uncertain. Herein we re-evaluate the tetrapod integumentary skeleton by integrating comparative developmental and tissue structure data. Three types of tetrapod integumentary elements are recognized: (1) osteoderms, common to representatives of most major taxonomic lineages; (2) dermal scales, unique to gymnophionans; and (3) the lamina calcarea, an enigmatic tissue found only in some anurans. As presently understood, all are derivatives of the ancestral cosmoid scale and all originate from scleroblastic neural crest cells. Osteoderms are plesiomorphic for tetrapods but demonstrate considerable lineage-specific variability in size, shape, and tissue structure and composition. While metaplastic ossification often plays a role in osteoderm development, it is not the exclusive mode of skeletogenesis. All osteoderms share a common origin within the dermis (at or adjacent to the stratum superficiale) and are composed primarily (but not exclusively) of osseous tissue. These data support the notion that all osteoderms are derivatives of a neural crest-derived osteogenic cell population (with possible matrix contributions from the overlying epidermis) and share a deep homology associated with the skeletogenic competence of the dermis. Gymnophionan dermal scales are structurally similar to the elasmoid scales of most teleosts and are not comparable with osteoderms. Whereas details of development are lacking, it is hypothesized that dermal scales are derivatives of an odontogenic neural crest cell population and that skeletogenesis is comparable with the formation of elasmoid scales. Little is known about the lamina calcarea. It is proposed that this tissue layer is also odontogenic in origin, but clearly further study is necessary. Although not homologous as organs, all elements of the integumentary skeleton share a basic and essential relationship with the integument, connecting them with the ancestral rhombic scale.

The lateral somitic frontier in ontogeny and phylogeny

The vertebrate musculoskeletal system comprises the axial and appendicular systems. The postcranial axial system consists of the vertebrae, ribs and associated muscles, and the appendicular system comprises the muscles and skeleton of the paired appendages and their respective girdles. The morphology, proportions, and arrangements of these parts have undergone tremendous variation during vertebrate history. Despite this vertebrate diversity, the cells that form all of the key parts of the musculoskeletal system during development arise from two populations of embryonic mesoderm, the somites and somatic lateral plate. Nowicki et al. (2003. Mech Dev 120:227-240) identified two dynamic domains in the developing chick embryo. The primaxial domain is populated exclusively by cells from the somites. The abaxial domain includes muscle and bone that develop within lateral plate-derived connective tissue. The boundary between the two domains is the lateral somitic frontier. We hypothesize that the primaxial and abaxial domains are patterned independently and that morphological evolution of the musculoskeletal system is facilitated by partially independent developmental changes in the abaxial and primaxial domain. Here we present our hypothesis in detail and review recent experimental and comparative studies that use the concept of the lateral somitic frontier in the analysis of the evolution of the highly derived chelonian and limbless squamate body plans.