Osteology and skeletal development of Apalone spinifera (Reptilia: Testudines: Trionychidae)

Skull osteology, neuroanatomy, and jaw-related myology of the pig-nosed turtle Carettochelys insculpta (Cryptodira, Trionychia)

The osteology, neuroanatomy, and musculature are known for most primary clades of turtles (i.e., "families"), but knowledge is still lacking for one particular clade, the Carettochelyidae. Carettochelyids are represented by only one living taxon, the pig-nosed turtle Carettochelys insculpta. Here, we use micro-computed tomography of osteological and contrast-enhanced stained specimens to describe the cranial osteology, neuroanatomy, circulatory system, and jaw musculature of Carettochelys insculpta. The jaw-related myology is described in detail for the first time for this taxon, including m. zygomaticomandibularis, a muscular unit only found in trionychians. We also document a unique arterial pattern for the internal carotid artery and its subordinate branches and provide an extensive list of osteological ontogenetic differences. The present work provides new insights into the craniomandibular anatomy of turtles and will allow a better understanding of the evolutionary history of the circulatory system of trionychians and intraspecific variation among turtles.

The topological organization of the turtle cranium is constrained and conserved over long evolutionary timescales

The cranium of turtles (Testudines) is characterized by the secondary reduction of temporal fenestrae and loss of cranial joints (i.e., characteristics of anapsid, akinetic skulls). Evolution and ontogeny of the turtle cranium are associated with shape changes. Cranial shape variation among Testudines can partially be explained by dietary and functional adaptations (neck retraction), but it is unclear if cranial topology shows similar ecomorphological signal, or if it is decoupled from shape evolution. We assess the topological arrangement of cranial bones (i.e., number, relative positioning, connections), using anatomical network analysis. Non-shelled stem turtles have similar cranial arrangements to archosauromorph outgroups. Shelled turtles (Testudinata) evolve a unique cranial organization that is associated with bone losses (e.g., supratemporal, lacrimal, ectopterygoid) and an increase in complexity (i.e., densely and highly interconnected skulls with low path lengths between bones), resulting from the closure of skull openings and establishment of unusual connections such as a parietal-pterygoid contact in the secondary braincase. Topological changes evolutionarily predate many shape changes. Topological variation and taxonomic morphospace discrimination among crown turtles are low, indicating that cranial topology may be constrained. Observed variation results from repeated losses of nonintegral bones (i.e., premaxilla, nasal, epipterygoid, quadratojugal), and changes in temporal emarginations and palate construction. We observe only minor ontogenetic changes. Topology is not influenced by diet and habitat, contrasting cranial shape. Our results indicate that turtles have a unique cranial topology among reptiles that is conserved after its initial establishment, and shows that cranial topology and shape have different evolutionary histories.

Embryonic development and cranial ossification of the Japanese Aodaishō, Elaphe climacophora (Serpentes: Colubridae): with special reference to the prootic bone and auditory evolution in snakes

Snakes show remarkably deviated "body plan" from other squamate reptiles. In addition to limb loss, they have accomplished enormous anatomical specialization of the skull associated with the pit organs and the reduction of the tympanic membranes and auditory canals in the outer ears. Despite being the most diverse group of snakes, our knowledge of the embryonic staging for organogenesis and cranial ossification has been minimal for Colubridae. Therefore, in the present observation, we provide the first embryonic description of the Japanese rat snake Elaphe climacophora. We based our study on the Standard Event System (SES) for external anatomical characters and on a description of the cranial ossification during post-ovipositional development. We further estimated the relative ossification timing of each cranial bony element and compared it with that of selected other snakes, lizards, turtles, and crocodilians. The present study shows that the relative ossification timing of the palatine and pterygoid bones is relatively early in squamates when compared to other reptiles, implying the developmental integration as the palate-pterygoid complex in this clade and functional demands for the unique feeding adaptation to swallow large prey with the help of their large palatine and pterygoid teeth. Furthermore, unlike in species with pit organs, the prootic bone of Ela. climacophora is expanded to provide articulation with the supratemporal, thereby contributing to the hearing system by detecting substrate vibration. We also demonstrate that the relative timing of the prootic ossification is significantly accelerated in colubrids compared to snakes with pit organs. Our finding suggests that the temporal changes of the prootic ossification underpin the evolution of the perception of the ground-bourne sound signals among snakes.

Chondrocranial anatomy of Testudo hermanni (Testudinidae, Testudines) with a comparison to other turtles

Using histological cross-sections, the chondrocranium anatomy was reconstructed for two developmental stages of Hermann's tortoise (Testudo hermanni). The morphology differs from the chondrocrania of most other turtles by a process above the ectochoanal cartilage with Pelodiscus sinensis being the only other known species with such a structure. The anterior and posterior processes of the tectum synoticum are better developed than in most other turtles and an ascending process of the palatoquadrate is missing, which is otherwise only the case in pleurodiran turtles. The nasal region gets proportionally larger during development. We interpret the enlargement of the nasal capsules as an adaption to increase the surface area of the olfactory epithelium for better perception of volant odors. Elongation of the nasal capsules in trionychids, in contrast, is unlikely to be related to olfaction, while it is ambiguous in the case of Sternotherus odoratus. However, we have to conclude that research on chondrocranium anatomy is still at its beginning and more comprehensive detailed descriptions in relation to other parts of the anatomy are needed before providing broad-scale ecological and phylogenetic interpretations.

Regionalization, constraints, and the ancestral ossification patterns in the vertebral column of amniotes

The development of the vertebral column has been studied extensively in modern amniotes, yet many aspects of its evolutionary history remain enigmatic. Here we expand the existing data on four major vertebral developmental patterns in amniotes based on exceptionally well-preserved specimens of the early Permian mesosaurid reptile Mesosaurus tenuidens: (i) centrum ossification, (ii) neural arch ossification, (iii) neural arch fusion, and (iv) neurocentral fusion. We retrace the evolutionary history of each pattern and reconstruct the ancestral condition in amniotes. Despite 300 million years of evolutionary history, vertebral development patterns show a surprisingly stability in amniotes since their common ancestor. We propose that this stability may be linked to conservatism in the constraints posed by underlying developmental processes across amniotes. We also point out that birds, mammals, and squamates each show specific trends deviating from the ancestral condition in amniotes, and that they remain rather unchanged within these lineages. The stability of their unique patterns demonstrates a certain homogeneity of vertebral developmental constraints within these lineages, which we suggest might be linked to their specific modes of regionalization. Our research provides a framework for the evolution of axial development in amniotes and a foundation for future studies.

A systematic compendium of turtle mandibular anatomy using digital dissections of soft tissue and osteology

Turtles are a charismatic reptile group with a peculiar body plan, which most notably includes the shell. Anatomists have often focused descriptive efforts on the shell and other strongly derived body parts, such as the akinetic skull, or the cervical vertebrae. Other parts of turtle osteology, like the girdles, limbs, and mandibles, are documented with less rigor and detail. The mandible is the primary skeletal element involved in food acquisition and initial food processing of turtles, and its features are thus likely linked to feeding ecology. In addition, the mandible of turtles is composed of up to seven bones (sometimes fused to as little as three) and has thus anatomical complexity that may be insightful for systematic purposes and phylogenetic research. Despite apparent complexity and diversity to the mandible of turtles, this anatomical system has not been systematically studied, not even in search of characters that might improve phylogenetic resolution. Here, we describe the mandibular osteology for all major subclades of extant turtles with the help of digitally dissected 3D models derived from high-resolution computed tomography (μCT) scans of 70 extant species. We provide 31 fully segmented mandibles, as well as 3D models of the mandibular musculature, innervation, and arterial circulation of the cryptodire Dermatemys mawii. We synthesize observed variation into 51 morphological characters, which we optimize onto a molecular phylogeny. This analysis shows some mandibular characters to have high systematic value, whereas others are highly homoplastic and may underlie ecological influences or other factors invoking variation.

A large and unusually thick-shelled turtle egg with embryonic remains from the Upper Cretaceous of China

Turtle eggs containing embryos are exceedingly rare in the fossil record. Here, we provide the first description and taxonomic identification, to our knowledge, of a fossilized embryonic turtle preserved in an egg, a fossil recovered from the Upper Cretaceous Xiaguan Formation of Henan Province, China. The specimen is attributed to the Nanhsiungchelyidae (Pan-Trionychia), an extinct group of large terrestrial turtles (possibly the species Yuchelys nanyangensis). The egg is rigid, spherical, and is one of the largest and thickest shelled Mesozoic turtle eggs known. Importantly, this specimen allowed identification of other nanhsiungchelyid egg clutches and comparison to those of Adocidae, as Nanhsiungchelyidae and Adocidae form the basal extinct clade Adocusia of the Pan-Trionychia (includes living soft-shelled turtles). Despite the differences in habitat adaptations, nanhsiungchelyids (terrestrial) and adocids (aquatic) shared several reproductive traits, including relatively thick eggshells, medium size clutches and relatively large eggs, which may be primitive for trionychoids (including Adocusia and Carrettochelyidae). The unusually thick calcareous eggshell of nanhsiungchelyids compared to those of all other turtles (including adocids) may be related to a nesting style adaptation to an extremely harsh environment.

On the “cartilaginous rider” in the endocasts of turtle brain cavities

Abstract

In recent years, paleoneurology became a very popular research field and hundreds of brain-endocasts were described. The interpretation of a dorsal protuberance of the brain-endocast puzzled researchers for a long time, the so-called (cartilaginous) rider. This is mainly because of technical limitations in the past and due to non-accessibility of comparative material. Using turtles as a case-study, we conducted a literature review and studied embryological data in addition to fossil and extant species’ endocasts. We assessed three hypotheses on the origin of the rider as relating to 1) the pineal gland, to 2) the blood vessel system, and to 3) skull roof elements. Based on our integrated anatomical observations, we refute the pineal gland hypothesis (1) and an exclusive blood vessel explanation (2). However, we show that, in most cases, the cartilaginous origin applies (3). The related cartilages, mainly the anterior process of the chondrocranial tectum synoticum, can persist until adulthood. Its diversity is interpreted in regard to the mechanical support for the temporal skull region, the shape of which has been shown to be in turn related to neck retraction and jaw mechanics. Finally, we highlight the value of embryological data to provide profound hypotheses for evolutionary research despite its low quantitative evaluability. We argue that it should be studied in conjunction with modern computer-aided data acquisition whenever possible.

Regulation of the limb shape during the development of the Chinese softshell turtles

Interdigital cell death is an important mechanism employed by amniotes to shape their limbs; inhibiting this process leads to the formation of webbed fingers, as seen in bats and ducks. The Chinese softshell turtle Pelodiscus sinensis (Reptilia: Testudines: Trionychidae) has a distinctive limb morphology: the anterior side of the limbs has partially webbed fingers with claw‐like protrusions, while the posterior fingers are completely enclosed in webbings. Here, P. sinensis embryos were investigated to gain insights on the evolution of limb‐shaping mechanisms in amniotes. We found cell death and cell senescence in their interdigital webbings. Spatial or temporal modulation of these processes were correlated with the appearance of indentations in the webbings, but not a complete regression of this tissue. No differences in interdigital cell proliferation were found. In subsequent stages, differential growth of the finger cartilages led to a major difference in limb shape. While no asymmetry in bone morphogenetic protein signaling was evident during interdigital cell death stages, some components of this pathway were expressed exclusively in the clawed digit tips, which also had earlier ossification. In addition, a delay and/or truncation in the chondrogenesis of the posterior digits was found in comparison with the anterior digits of P. sinensis, and also when compared with the previously published pattern of digit skeletogenesis of turtles without posterior webbings. In conclusion, modulation of cell death, as well as a heterochrony in digit chondrogenesis, may contribute to the formation of the unique limbs of the Chinese softshell turtles.

Cell death and senescence shape the interdigital webbings of Pelodiscus sinensis.

Delayed chondrogenesis/ossification and truncated tips are found in posterior digits, as well as differential expression of bone morphogenetic proteins and Msh homeobox 1 transcription factors.

Comparative morphology and allometry of select extant cryptodiran turtle kidneys

Reptile, avian, and mammalian species all possess a metanephric kidney to maintain fluid homeostasis. The physiology of the kidney is intimately related to tissue organization and gross morphology, which is dependent upon organ size, animal habitat, and body plan. Reptiles have significant variations in body plan and as a result have differences in visceral organ placement and morphology. One organ that appears to show great morphological variation is the reptilian kidney found in Crocodylia, Testudines, and Squamata (Sauria and Ophidia). However, limited research has been conducted to evaluate and compare kidney morphology in reptiles and more specifically, in turtles. Here we have examined multiple cryptodiran turtle species from the families Chelydridae, Emydidae, Kinosternidae, and Trionychidae. Detailed descriptions of kidney morphology along with comparative allometry are provided. Significant differences in external renal morphology were found between and within turtle families as well as differences in scaling of kidney mass with body mass. Our study provides a foundation for understanding differences in organ development and tissue architecture as well as potential differences in physiology.

Morphogenetic and constructional differences of the carapace of aquatic and terrestrial turtles and their evolutionary significance

The postembryonic development of the turtle carapace was studied in the aquatic Еmys orbicularis and the terrestrial Тestudo graeca. Differences in the structure of the bony shell in aquatic and terrestrial turtles were shown to be associated with varying degrees of development of epidermal derivatives, namely, the thickness of the scutes and the depth of horny furrows. Sinking of the horny furrows into the dermis causes local changes in the structure of the collagen matrix, which might precondition the acceleration of the ossification. Aquatic turtles possess a relatively thin horny cover, whose derivatives are either weakly developed or altogether absent and thus make no noticeable impact on the growth dynamics of bony plates. Carapace plates of these turtles outgrow more or less evenly around the periphery, which results in uniform costals, relatively narrow and partly reduced neurals, and broad peripherals extending beyond the marginal scutes. In terrestrial turtles (Testudinidae), horny structures are much more developed and exert a considerable impact on the growth of bony elements. As a result, bony plates outgrow unevenly in the dermis, expanding fast in the zones under the horny furrows and slowly outside these zones. This determines the basic features of the testudinid carapace: alternately cuneate shape of costals, an alternation of broad octagonal and narrow tetragonal neurals, and the limitation of the growth of peripherals by pleuro-marginal furrows. The evolutionary significance of morphogenetic and constructional differences in the turtle carapace, and the association of these differences with the turtle habitats are discussed.

Ontogeny of the skull of the Black Caiman (Melanosuchus niger) (Crocodylia: Alligatoridae)

We describe the formation of the chondrocranium and the ossification pattern of the skull of the Black Caiman (Melanosuchus niger (Spix, 1825)). The embryos were cleared and double-stained with Alizarin Red S and Alcian Blue 8GX. Additionally, they were visualized by histological hematoxylin and eosin staining and computed tomography imaging. The chondrocranium of M. niger comprised the nasal capsule, orbitotemporal, and optic–occipital regions. Its development began at stage 9, with the chondrification of the acrochordal cartilage, trabeculae, and mandibular cartilage. The optic capsule was formed in the caudolateral portion of the chondrocranium at stage 13. The basal plate appeared at stage 14, with foramina for the hypoglossal. The chondrocranium was completely formed at stage 16. The first osteogenic events were noted at stage 13, in the bones, maxilla, jugal, postorbital, and pterygoid. The quadratojugal, prefrontal, frontal, and squamosal began their ossification at stage 14. The parietal bone began to ossify only at stage 20. The basisphenoid began at stage 15 and the parasphenoid began at stage 16. The jaw bones ossified between stages 13 and 16. The dermal elements started their ossification prior to the endochondral bones.

Chondrification and Character Identification in the Skull Exemplified for the Basicranial Anatomy of Early Squamate Embryos

The neurocranium of vertebrates is mainly derived from early cartilaginous anlagen, the so-called chondrocranium, the base of the future skull. Two initial bar-shaped and paired chondrifications flank the notochord, the rostral trabecles and the caudal parachordals. In most reptiles, there is an additional component, the transverse acrochordal, which is placed between trabecles and parachordals. All these elements compose the base of the future chondrocranium. There are several drastically different hypotheses concerning the development and interrelationship of these elements. We reexamined the basicranial development in four squamates and found that all species show very similar conditions of early chondrocranial development. The anterior part of the notochord is not embedded into the basal plate as it was previously reported. It remains free. The medial edges of the parachordals form the lateral walls of the basicranial fenestra. Only the posterior portions of the parachordals fuse and form the basal plate. The space in-between the parachordals is filled with a thin layer of cells, which never chondrify. The anterior tips of the parachordals later fuse with the posterior edge of the acrochordal, which ultimately delimitates, as crista sellaris, the basicranial fenestra anteriorly. We consider the observed processes a common development at least in lizards and review a variety of methodological approaches and differences in data interpretation as reasons for the anatomical differences reported in the literature. Moreover, based on our data we argue that the acrochordal is of mesodermal origin, which coincides with results of fate map experimental studies.

Ontogeny of the Appendicular Skeleton in Melanosuchus niger (Crocodylia: Alligatoridae)

The objective of the present study was to analyze chondrogenesis and the ossification pattern of the limbs of Melanosuchus niger in order to contribute with possible discussions on homology and the fusion pattern of autopodial elements and phylogeny. In the Reserva Extrativista do Lago Cuniã, Rondônia, Brazil, six nests were marked and two eggs removed from each nest at 24-hour intervals until hatching. Embryos were cleared using KOH; bone tissue was stained with alizarin red S and cartilage with Alcian blue. Routine staining with HE was also performed. In the pectoral girdle, the scapula showed ossification centers before the coracoid process. In the pelvic girdle, the ilium and the ischium were condensed as a single cartilage, although ossification took place through two separate centers, forming distinct elements in the adult. The pubis developed from an independent cartilaginous center with free end, which reflects its function in breathing. In the initial stages, the stylopodium and the zeugopodium developed from the condensation of a Y-shaped cartilage in the limbs, and differentiation of the primary axis and digital arch were observed. The greatest changes were observed in the mesopodia. In their evolution, Crocodylia underwent a vast reduction in the number of autopodial elements as a consequence of fusions and ossification of some elements. This study shows that the chondrogenesis and ossification sequences are dissociated. Moreover, the differences between M. niger and other species show clear variation in the patterns for these events in Alligatoridae.

Palaeoamyda messeliana nov. comb. (Testudines, Pan-Trionychidae) from the Eocene Messel Pit and Geiseltal localities, Germany, taxonomic and phylogenetic insights

Background

Abundant pan-trionychid (soft-shell) turtles specimens have been found in Eocene sequences of central Europe, particularly from two localities in Germany, the Messel Pit (a UNESCO World Natural Heritage Site) and Geiseltal, traditionally attributed to Trionyx messelianus or Rafetoides austriacus. Over the last two decades new specimens of this taxon from these two localities have been discovered and fully prepared. However, they have remained unstudied, as well as their phylogenetic position inside Pan-Trionychidae is unknown.

Results

Five new specimens of Palaeoamyda messeliana nov. comb. from Messel Pit and Geiseltal localities are fully described here. A revised diagnosis for the species is also presented here, together with its inclusion in a phylogenetic analysis of Pan-Trionychidae that shows that this species is sister to the extant Amyda cartilaginea, one of the most abundant pan-trionychid (soft-shell) turtles from Asia, both members of the clade Chitrini. The specimens described in here are among the best and most complete fossil pan-trionychid skeletons so far known.

Osteologia de Melanosuchus niger (Crocodylia: Alligatoridae) e a evidência evolutiva

RESUMO: O objetivo foi realizar a descrição anatômica do esqueleto de Melanosuchus niger, com o intuito de contribuir com informações evolutivas sobre a espécie. Utilizaram-se três espécimes adultos de M. niger, com comprimento médio de 2,40m, provenientes da coleção biológica do Lapas-UFU. Na cintura peitoral, a escápula é maior do que o coracóide. Já nos elementos da cintura pelvina, o púbis não participa da formação do acetábulo, o contato com o ilío, ocorre por ligamentos, e sua articulação com o ísquio, permite movimentos dorso-ventrais. Nos membros torácicos, o úmero figura como elemento do estilopódio, a ulna e rádio como elementos do zeugopódio. No carpo há o ulnar do carpo, fusão do radial+intermédio, fusão dos distais do carpo 3+4+5 e o pisiforme; possui cinco metacarpos, numerados lateromedialmente e a fórmula falângica 2:3:4:3:2. Nos membros pelvinos, o estilopódio é formado pelo fêmur e o zeugopódio pela tíbia e fíbula. No tarso há a fusão do intermédio+central, fibular do tarso, distal do tarso 3, distal do tarso 4; possui quatro metatarsos longos I, II, III e IV, sendo os metatarsos II e III maiores que os demais. O metatarso V é um osso bastante reduzido e o pé possui a fórmula falângica 2:3:4:4. No crânio, a abertura nasal é única, o palatino, vômer, pterigóide, pré-maxila e maxila formam a estrutura óssea do palato secundário; o osso parietal é o único elemento no teto craniano. No esqueleto pós- axial em pares de costelas distintas que se articulam com as vértebras cervicais, dorsais, lombares, sacrais e caudais. A gastrália é formada por sete fileiras de ossos finos localizados entre o púbis e a região caudal do esterno.

Extreme Modification of the Tetrapod Forelimb in a Triassic Diapsid Reptile

The tetrapod forelimb is one of the most versatile structures in vertebrate evolution, having been co-opted for an enormous array of functions. However, the structural relationships between the bones of the forelimb have remained largely unchanged throughout the 375 million year history of Tetrapoda, with a radius and ulna made up of elongate, paralleling shafts contacting a series of shorter carpal bones. These features are consistent across nearly all known tetrapods, suggesting that the morphospace encompassed by these taxa is limited by some sort of constraint(s). Here, we report on a series of three-dimensionally preserved fossils of the small-bodied (<1 m) Late Triassic diapsid reptile Drepanosaurus, from the Chinle Formation of New Mexico, USA, which dramatically diverge from this pattern. Along with the crushed type specimen from Italy, these specimens have a flattened, crescent-shaped ulna with a long axis perpendicular to that of the radius and hyperelongate, shaft-like carpal bones contacting the ulna that are proximodistally longer than the radius. The second digit supports a massive, hooked claw. This condition has similarities to living "hook-and-pull" digging mammals and demonstrates that specialized, modern ecological roles had developed during the Triassic Period, over 200 million years ago. The forelimb bones in Drepanosaurus represent previously unknown morphologies for a tetrapod and, thus, a dramatic expansion of known tetrapod forelimb morphospace.

Morphology, development and heterochrony of the carapace of Giant Amazon River Turtle Podocnemis expansa (Testudines, Podocnemidae)

Abstract: With aim to report the ontogeny of the osseous elements of the carapace in Peurodiras, 62 embryos and 43 nestlings of Podocnemis expansa were collected and submitted to the clearing and staining technique of bones and cartilages and study of serial histological slices. The carapace has mixed osseous structure of endo and exoskeleton, formed by 8 pairs of costal bones associated with ribs, 7 neural bones associated with neural arches, 11 pairs of peripheral bones, 1 nuchal, 1 pygal and 1 suprapygal. This structure begins its formation in the beginning of stage 16 with the ossification of the periosteal collar of the ribs. With exception of the peripheral bones, the other ones begin their ossification during the embrionary period. In histologic investigation it was found that the costal bones and neural bones have a close relation to the endoskeleton components, originating themselves as intramembranous expansions of the periosteal collar of the ribs and neural arches, respectively. The condensation of the mesenchyme adjacent to the periosteal collar induces the formation of spikes that grow in trabeculae permeated by fibroblasts below the dermis. The nuchal bone also ossifies in an intramembranous way, but does not show direct relation to the endoskeleton. Such information confirms those related to the other Pleurodira, mainly with Podocnemis unifilis, sometimes with conspicuous variations in the chronology of the ossification events. The formation of dermal plates in the carapace of Pleurodira and Criptodira follow the same pattern.

Development of the turtle plastron, the order-defining skeletal structure

The dorsal and ventral aspects of the turtle shell, the carapace and the plastron, are developmentally different entities. The carapace contains axial endochondral skeletal elements and exoskeletal dermal bones. The exoskeletal plastron is found in all extant and extinct species of crown turtles found to date and is synaptomorphic of the order Testudines. However, paleontological reconstructed transition forms lack a fully developed carapace and show a progression of bony elements ancestral to the plastron. To understand the evolutionary development of the plastron, it is essential to know how it has formed. Here we studied the molecular development and patterning of plastron bones in a cryptodire turtle Trachemys scripta We show that plastron development begins at developmental stage 15 when osteochondrogenic mesenchyme forms condensates for each plastron bone at the lateral edges of the ventral mesenchyme. These condensations commit to an osteogenic identity and suppress chondrogenesis. Their development overlaps with that of sternal cartilage development in chicks and mice. Thus, we suggest that in turtles, the sternal morphogenesis is prevented in the ventral mesenchyme by the concomitant induction of osteogenesis and the suppression of chondrogenesis. The osteogenic subroutines later direct the growth and patterning of plastron bones in an autonomous manner. The initiation of plastron bone development coincides with that of carapacial ridge formation, suggesting that the development of dorsal and ventral shells are coordinated from the start and that adopting an osteogenesis-inducing and chondrogenesis-suppressing cell fate in the ventral mesenchyme has permitted turtles to develop their order-specific ventral morphology.

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.

The hooked element in the pes of turtles (Testudines): a global approach to exploring primary and secondary homology

The hooked element in the pes of turtles was historically identified by most palaeontologists and embryologists as a modified fifth metatarsal, and often used as evidence to unite turtles with other reptiles with a hooked element. Some recent embryological studies, however, revealed that this element might represent an enlarged fifth distal tarsal. We herein provide extensive new myological and developmental observations on the hooked element of turtles, and re-evaluate its primary and secondary homology using all available lines of evidence. Digital count and timing of development are uninformative. However, extensive myological, embryological and topological data are consistent with the hypothesis that the hooked element of turtles represents a fusion of the fifth distal tarsal with the fifth metatarsal, but that the fifth distal tarsal dominates the hooked element in pleurodiran turtles, whereas the fifth metatarsal dominates the hooked element of cryptodiran turtles. The term 'ansulate bone' is proposed to refer to hooked elements that result from the fusion of these two bones. The available phylogenetic and fossil data are currently insufficient to clarify the secondary homology of hooked elements within Reptilia.

Skeletal gene expression in the temporal region of the reptilian embryos: implications for the evolution of reptilian skull morphology

Reptiles have achieved highly diverse morphological and physiological traits that allow them to exploit various ecological niches and resources. Morphology of the temporal region of the reptilian skull is highly diverse and historically it has been treated as an important character for classifying reptiles and has helped us understand the ecology and physiology of each species. However, the developmental mechanism that generates diversity of reptilian skull morphology is poorly understood. We reveal a potential developmental basis that generates morphological diversity in the temporal region of the reptilian skull by performing a comparative analysis of gene expression in the embryos of reptile species with different skull morphology. By investigating genes known to regulate early osteoblast development, we find dorsoventrally broadened unique expression of the early osteoblast marker, Runx2, in the temporal region of the head of turtle embryos that do not form temporal fenestrae. We also observe that Msx2 is also uniquely expressed in the mesenchymal cells distributed at the temporal region of the head of turtle embryos. Furthermore, through comparison of gene expression pattern in the embryos of turtle, crocodile, and snake species, we find a possible correlation between the spatial patterns of Runx2 and Msx2 expression in cranial mesenchymal cells and skull morphology of each reptilian lineage. Regulatory modifications of Runx2 and Msx2 expression in osteogenic mesenchymal precursor cells are likely involved in generating morphological diversity in the temporal region of the reptilian skull.

Forelimb kinematics during swimming in the pig-nosed turtle, Carettochelys insculpta, compared with other turtle taxa: rowing versus flapping, convergence versus intermediacy

Animals that swim using appendages do so by way of rowing and/or flapping motions. Often considered discrete categories, rowing and flapping are more appropriately regarded as points along a continuum. The pig-nosed turtle, Carettochelys insculpta, is unusual in that it is the only freshwater turtle to have limbs modified into flippers and swim via synchronous forelimb motions that resemble dorsoventral flapping, traits that evolved independently from their presence in sea turtles. We used high-speed videography to quantify forelimb kinematics in C. insculpta and a closely related, highly aquatic rower (Apalone ferox). Comparisons of our new data with those previously collected for a generalized freshwater rower (Trachemys scripta) and a flapping sea turtle (Caretta caretta) allow us to: (1) more precisely quantify and characterize the range of limb motions used by flappers versus rowers, and (2) assess whether the synchronous forelimb motions of C. insculpta can be classified as flapping (i.e. whether they exhibit forelimb kinematics and angles of attack more similar to closely related rowing species or more distantly related flapping sea turtles). We found that the forelimb kinematics of previously recognized rowers (T. scripta and A. ferox) were most similar to each other, but that those of C. insculpta were more similar to rowers than to flapping C. caretta. Nevertheless, of the three freshwater species, C. insculpta was most similar to flapping C. caretta. 'Flapping' in C. insculpta is achieved through humeral kinematics very different from those in C. caretta, with C. insculpta exhibiting significantly more anteroposterior humeral motion and protraction, and significantly less dorsoventral humeral motion and depression. Based on several intermediate kinematic parameters and angle of attack data, C. insculpta may in fact represent a synchronous rower or hybrid rower-flapper, suggesting that traditional views of C. insculpta as a flapper should be revised.

Anatomy of the fully formed chondrocranium of Emydura subglobosa (Chelidae): a pleurodiran turtle

The chondrocranium is a cartilaginous structure that forms around and protects the brain and sensory organs of the head. Through ontogeny, this skeletal structure may become more elaborate, remodeled and reabsorbed, and/or ossified. Though considerable attention has been given to the formation of the chondrocranium and a great amount of data has been gathered on the development of this structure among many craniates, the anatomy of this structure in turtles often is neglected. We describe the mature chondrocranium of the pleurodiran turtle, Emydura subglobosa (Chelidae) based on hatchling specimens. Though formation and ossification of bony elements has been studied previously in this species, a detailed description of the chondrocranium of this pleurodiran turtle has not been presented. Anatomy of the chondrocranium was described for E. subglobosa by examination of cleared and double-stained specimens. The orbitotemporal region of E. subglobosa is dramatically different from that of other described turtles (e.g., Apalone spinifera, Pelodiscus sinensis, Chelydra serpentina, Macrochelys temminckii, Trachemys scripta, Chrysemys picta, and Eretmochelys imbricata) in that a prominent taenia marginalis spans the space between the planum supraseptale and otic capsules, and the pila antotica (which becomes modified and ossified through ontogeny to form the processus clinoideus) is greatly reduced and essentially absent in hatchling specimens. The morphology seen in E. subglobosa is similar to that of Caretta caretta, particularly as it relates to the taenia marginalis. Variation in the orbitotemporal region is briefly discussed in the context of the taenia marginalis, taenia medialis, pila metoptica, and pila antotica.

Unraveling the influences of soft-tissue flipper development on skeletal variation using an extinct taxon

Adaptation to an aquatic habitat results in dramatic changes in tetrapod limb morphology as limbs take on the roles of propulsion and steering and lose their weight-bearing function. Changes include enclosure of the limb in a soft-tissue flipper and proportional lengthening of the distal limb, often accomplished through the addition of skeletal elements (hyperphalangy). The flipper structure itself and changes to the developmental architecture permitting hyperphalangy are hypothesized to increase observed limb variation, based on a cetacean model. These hypotheses are examined in the ichthyosaurs Stenopterygius and Mixosaurus. Hyperphalangy combined with high levels of variation in phalangeal counts were observed in both genera. The amount of variation was not proportional to the number of phalanges in a digit, but was related to functional digit length. In addition, qualitative variants were catalogued in both genera. Polyphalangy, phalangeal fusion, and additional ossifications in the zeugopodial row were not observed in Mixosaurus, but were common in Stenopterygius, even though both genera exhibited a similar degree of hyperphalangy. These results suggest that while the flipper structure and processes resulting in hyperphalangy may increase observed variation in phalangeal counts, these factors are unlikely to be causing high levels of qualitative variation in ichthyosaurs. Patterns of variation in ichthyosaur limbs, and thus variability, are unique to species but can change over evolutionary time.

Timing of ossification in duck, quail, and zebra finch: intraspecific variation, heterochronies, and life history evolution

Skeletogenic heterochronies have gained much attention in comparative developmental biology. The temporal appearance of mineralized individual bones in a species - the species ossification sequence - is an excellent marker in this kind of study. Several publications describe interspecific variation, but only very few detail intraspecific variation. In this study, we describe and analyze the temporal order of ossification of skeletal elements in the zebra finch, Taeniopygia guttata, the Japanese quail, Coturnix coturnix japonica, and the White Pekin duck, a domestic race of the mallard Anas platyrhynchos, and explore patterns of intraspecific variation in these events. The overall sequences were found to be conserved. In the duck, variability is present in the relative timing of ossification in the occipital, the basisphenoid and the otic regions of the skull and the phalanges in the postcranium. This variation appears generally in close temporal proximity. Comparison with previously published data shows differences in ossification sequence in the skull, the feet, and the pelvis in the duck, and especially the pelvis in the quail. This clearly documents variability among different breeds.