Skeletal development ofMacrochelys temminckii (Reptilia: Testudines: Chelydridae)

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.

Ontogenetic variation in the cranium of Mixosaurus cornalianus, with implications for the evolution of ichthyosaurian cranial development

Relatively complete ontogenetic series are comparatively rare in the vertebrate fossil record. This can create biases in our understanding of morphology and evolution, since immaturity can represent a source of unrecognized intraspecific variation in both skeletal anatomy and ecology. In the extinct marine reptile clade Ichthyopterygia, ontogenetic series were widely studied only in some Jurassic genera, while the ontogeny of the oldest and most basal members of the clade is very poorly understood. Here, we investigate cranial ontogeny in Mixosaurus cornalianus, from the Middle Triassic Besano Formation of the Swiss and Italian Alps. This small-bodied taxon is represented by a wealth of material from multiple size classes, including fetal material. This allows us to assess ontogenetic changes in cranial morphology, and identify stages in the ontogenetic trajectory where divergence with more derived ichthyosaurs has occurred. Early ontogenetic stages of Mixosaurus show developmental patterns that are reminiscent of the presumed ancestral (early diverging sauropsid) condition. This is prominently visible in the late fetal stage in both the basioccipital, which shows morphology akin to basal tubera, and in the postorbital, which has a triradiate head. The ontogenetic trajectory of at least some of the cranial elements of Mixosaurus is therefore likely still very akin to the ancestral condition, even though the adult cranium diverges from the standard diapsid morphology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13358-023-00289-z.

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.

Snake-like limb loss in a Carboniferous amniote

Among living tetrapods, many lineages have converged on a snake-like body plan, where extreme axial elongation is accompanied by reduction or loss of paired limbs. However, when and how this adaptive body plan first evolved in amniotes remains poorly understood. Here, we provide insights into this question by reporting on a new taxon of molgophid recumbirostran, Nagini mazonense gen. et sp. nov., from the Francis Creek Shale (309-307 million years ago) of Illinois, United States, that exhibits extreme axial elongation and corresponding limb reduction. The molgophid lacks entirely the forelimb and pectoral girdle, thus representing the earliest occurrence of complete loss of a limb in a taxon recovered phylogenetically within amniotes. This forelimb-first limb reduction is consistent with the pattern of limb reduction that is seen in modern snakes and contrasts with the hindlimb-first reduction process found in many other tetrapod groups. Our findings suggest that a snake-like limb-reduction mechanism may be operating more broadly across the amniote tree.

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.

Disentangling the correlated evolution of body size, life history, and ontogeny in miniaturized chelydroid turtles

Organismal miniaturization is defined by a reduction in body size relative to a large ancestor. In vertebrate animals, miniaturization is achieved by suppressing the energetics of growth. However, this might interfere with reproductive strategies in egg-laying species with limited energy budgets for embryo growth and differentiation. In general, the extent to which miniaturization coincides with alterations in animal development remains obscure. To address the interplay among body size, life history, and ontogeny, miniaturization in chelydroid turtles was examined. The analyses corroborated that miniaturization in the Chelydroidea clade is underlain by a dampening of the ancestral growth trajectory. There were no associated shifts in the early sequence of developmental transformations, though the relative duration of organogenesis was shortened in miniaturized embryos. The size of eggs, hatchlings, and adults was positively correlated within Chelydroidea. A phylogenetically broader exploration revealed an alternative miniaturization mode wherein exceptionally large hatchlings grow minimally and thus attain diminutive adult sizes. Lastly, it is shown that miniaturized Chelydroidea turtles undergo accelerated ossification coupled with a ~10% reduction in shell bones. As in other vertebrates, the effects of miniaturization were not systemic, possibly owing to opposing functional demands and tissue geometric constraints. This underscores the integrated and hierarchical nature of developmental systems.

Origin and Evolution of the Turtle Body Plan

The origin of turtles and their uniquely shelled body plan is one of the longest standing problems in vertebrate biology. The unfulfilled need for a hypothesis that both explains the derived nature of turtle anatomy and resolves their unclear phylogenetic position among reptiles largely reflects the absence of a transitional fossil record. Recent discoveries have dramatically improved this situation, providing an integrated, time-calibrated model of the morphological, developmental, and ecological transformations responsible for the modern turtle body plan. This evolutionary trajectory was initiated in the Permian (>260 million years ago) when a turtle ancestor with a diapsid skull evolved a novel mechanism for lung ventilation. This key innovation permitted the torso to become apomorphically stiff, most likely as an adaption for digging and a fossorial ecology. The construction of the modern turtle body plan then proceeded over the next 100 million years following a largely stepwise model of osteological innovation.

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.

Polarized Sonic Hedgehog Protein Localization and a Shift in the Expression of Region-Specific Molecules Is Associated With the Secondary Palate Development in the Veiled Chameleon

Secondary palate development is characterized by the formation of two palatal shelves on the maxillary prominences, which fuse in the midline in mammalian embryos. However, in reptilian species, such as turtles, crocodilians, and lizards, the palatal shelves of the secondary palate develop to a variable extent and morphology. While in most Squamates, the palate is widely open, crocodilians develop a fully closed secondary palate. Here, we analyzed developmental processes that underlie secondary palate formation in chameleons, where large palatal shelves extend horizontally toward the midline. The growth of the palatal shelves continued during post-hatching stages and closure of the secondary palate can be observed in several adult animals. The massive proliferation of a multilayered oral epithelium and mesenchymal cells in the dorsal part of the palatal shelves underlined the initiation of their horizontal outgrowth, and was decreased later in development. The polarized cellular localization of primary cilia and Sonic hedgehog protein was associated with horizontal growth of the palatal shelves. Moreover, the development of large palatal shelves, supported by the pterygoid and palatine bones, was coupled with the shift in Meox2, Msx1, and Pax9 gene expression along the rostro-caudal axis. In conclusion, our results revealed distinctive developmental processes that contribute to the expansion and closure of the secondary palate in chameleons and highlighted divergences in palate formation across amniote species.

Comparative cranial morphology of the Late Cretaceous protostegid sea turtle Desmatochelys lowii

BACKGROUND

The phylogenetic placement of Cretaceous marine turtles, especially Protostegidae, is still under debate among paleontologists. Whereas protostegids were traditionally thought to be situated within the clade of recent marine turtles (Chelonioidea), some recent morphological and molecular studies suggest placement along the stem of Cryptodira. The main reason why the evolution of marine turtles is still poorly understood, is in part due to a lack of insights into the cranial anatomy of protostegids. However, a general availability of high-quality fossil material, combined with modern analysis techniques, such as X-ray microtomography, provide ample opportunity to improve this situation. The scope of this study is to help resolve its phylogenetic relationships by providing a detailed description of the external and internal cranial morphology of the extinct protostegid sea turtle Desmatochelys lowii Williston, 1894.

MATERIAL AND METHODS

This study is based on the well-preserved holotype of Desmatochelys lowii from the Late Cretaceous (middle Cenomanian to early Turonian) Greenhorn Limestone of Jefferson County, Nebraska. The skulls of two recent marine turtles, Eretmochelys imbricata (Linnaeus, 1766) (Cheloniidae) and Dermochelys coriacea Lydekker, 1889 (Dermochelyidae), as well as the snapping turtle Chelydra serpentina (Linnaeus, 1758) (Chelydridae) provide a comparative basis. All skulls were scanned using regular or micro CT scanners and the scans were then processed with the software program Amira to create 3D isosurface models. In total, 81 bones are virtually isolated, figured, and described, including the nature of their contacts. The novel bone contact data is compiled and utilized in a preliminary phenetic study. In addition, an update phylogenetic analysis is conduced that utilizes newly obtained anatomical insights.

RESULTS

The detailed examination of the morphology of the herein used specimens allowed to explore some features of the skull, to refine the scoring of Desmatochelys lowii in the recent global matrix of turtles, and develop five new characters. The alleged pineal foramen in the type skull of Desmatochelys lowii is shown to be the result of damage. Instead, it appears that the pineal gland only approached the skull surface, as it is in Dermochelys coriacea. Whereas the parasphenoid in confirmed to be absent in hard-shelled sea turtles, ist possible presence in Desmatochelys lowii is unclear. The results of the phenetic study show that Desmatochelys lowii is least similar to the other examined taxa in regards to the nature of its bone contacts, and therefore suggests a placement outside Americhelydia for this protostegid sea turtle. The phylogenetic study results in a placement of Protostegidae along the stem of Chelonioidea, which is a novel position for the group.

Aspects of gorgonopsian paleobiology and evolution: insights from the basicranium, occiput, osseous labyrinth, vasculature, and neuroanatomy

Synapsida, the clade including therapsids and thus also mammals, is one of the two major branches of amniotes. Organismal design, with modularity as a concept, offers insights into the evolution of therapsids, a group that experienced profound anatomical transformations throughout the past 270 Ma, eventually leading to the evolution of the mammalian bauplan. However, the anatomy of some therapsid groups remains obscure. Gorgonopsian braincase anatomy is poorly known and many anatomical aspects of the brain, cranial nerves, vasculature, and osseous labyrinth, remain unclear. We analyzed two gorgonopsian specimens, GPIT/RE/7124 and GPIT/RE/7119, using propagation phase contrast synchrotron micro-computed tomography. The lack of fusion between many basicranial and occipital bones in GPIT/RE/7124, which is an immature specimen, allowed us to reconstruct its anatomy and ontogenetic sequence, in comparison with the mature GPIT/RE/7119, in great detail. We explored the braincase and rendered various skull cavities. Notably, we found that there is a separate ossification between what was previously referred to as the “parasphenoid” and the basioccipital. We reinterpreted this element as a posterior ossification of the basisphenoid: the basipostsphenoid. Moreover, we show that the previously called “parasphenoid” is in fact the co-ossification of the dermal parasphenoid and the endochondral basipresphenoid. In line with previous descriptions, the anatomy of the osseous labyrinth is rendered in detail, revealing a unique discoid morphology of the horizontal semicircular canal, rather than toroidal, probably due to architectural constraints of the ossification of the opisthotic and supraoccipital. In addition, the orientation of the horizontal semicircular canal suggests that gorgonopsians had an anteriorly tilted alert head posture. The morphology of the brain endocast is in accordance with the more reptilian endocast shape of other non-mammaliaform neotherapsids.

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.

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.

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.

Sequência de ossificação do sincrânio e hioide em embriões de Caiman yacare (Crocodylia, Alligatoridae)

O crânio representa uma estrutura única e complexa dos vertebrados, sendo foco relevante objeto de estudos morfológicos e sistemáticos. Embora os crocodilianos constituam um importante grupo representante dos Archosauria, nossos conhecimentos acerca de seu desenvolvimento e homologias ainda são escassos. Aqui descrevemos uma sequência detalhada de ossificação dos ossos do crânio de Caiman yacare (Daudin, 1802), objetivando contribuir com informações de foco anatômico. Coletaram-se ao acaso embriões em intervalos regulares durante todo o período de incubação, sendo estes posteriormente submetidos a protocolo de diafanização e coloração de ossos. O padrão de ossificação em C. yacare segue parâmetros gerais em répteis e outros tetrápodes. Os primeiros centros de ossificação correspondem aos ossos dérmicos, envolvidos com funções primárias como a alimentação e respiração (e.g. maxila, dentário, esplenial, angular, pterigoide, ectopterigoide e jugal, incluindo ainda os dentes). Os ossos da porção dorsal do neurocrânio se ossificam posteriormente, evidenciando uma fontanela cranial que permanece até o momento da eclosão. Os ossos parietal, frontal e opstótico possuem mais de um centro de ossificação que se fundem durante a ontogenia. O centro de ossificação do parisfenoide está ausente, e apenas um centro de ossificação está presente para o basisfenoide. A porção posterior do crânio é formada por centros de substituição do condrocrânio que se ossificam em estágios posteriores.

Assessing the phylogenetic utility of sequence heterochrony: evolution of avian ossification sequences as a case study

The evolution of developmental sequences, or sequence heterochrony, is an emerging field of study that addresses the temporal interplay between evolution and development. Some phylogenetic signal has been found in developmental sequence data, but sampling has generally been limited to small numbers of taxa and few developmental events. Here we present the largest ossification sequence dataset to date. The sequences are composed of ossification events throughout the avian skeleton, and are used to address the evolutionary signal of ossification sequence data within this clade. The results indicate that ossification sequences are conserved in birds, and show a stronger phylogenetic signal than previous studies, perhaps due to the volume of data. Phylogenetic signal is not strong enough, however, to consider ossification sequence data to be any better at resolving phylogenetic hypotheses than other morphological data and just as prone to evolutionary convergence. There is no one-to-one correlation between ossification sequence and developmental stage. We discuss some methodological implications of our findings, as well as commonalities in avian ossification sequences such as early ossification of the long bones relative to the dermatocranium, and of the hindlimb over the forelimb.