Studies on skeleton formation in reptiles: Patterns of ossification in the skeleton of Chelydra serpentina (Reptilia, Testudines)

Patterns and sequence of ossification are described throughout the skeleton of Chelydra serpentina Linnaeus. Evidence is adduced documenting the decoupling of ossification processes from sequence and patterns of chondrification. Convergence of ontogenetic repatterning in the ossification of the axial skeleton in Chelydra and Squamata is discussed, as are problems of adaptive modification of ossification patterns. The development of a carapace may be correlated with changes of ossification patterns in the postcranial axial skeleton of turtles, but the most striking evidence for the adaptive modification of ossification sequence obtains from a comparison of the limb skeleton and its ossification in Chelydra and in sea turtles     

Studies on skeleton formation in reptiles, v. Patterns of ossification in the skeleton of Alligator mississippiensis DAUDIN (Reptilia, Grocodylia)

Patterns of ossification are described in the endo-and exoskeleton of Alligator mississippiensis. The occurrence of a dermo-supraoccipital is discussed in light of the independence of dermal and endochondral bone. The development of the bony secondary palate is discussed in light of Haeckelian recapitulation. The sequence of ossification in the limb skeleton is shown to differ from the sequence of chondrification of the cartilaginous precursors. Patterns of ossification in Alligator are compared to lepidosaurs in terms of sequence and timing. Important differences relate to ossification patterns in the limb skeleton: lepidosaurs show a dominance of digit III > IV > II > I > V, whereas Alligator shows a dominance of digits III > II> IV > I > V in the ossification process. Ontogenetic repatterning in the ossification of the axial skeleton is discussed as it bears on the serial homology of dorsal ribs, sacral ribs and caudal ribs (transverse processes).     

Skeletal development in the Chinese soft‐shelled turtle Pelodiscus sinensis (Testudines: Trionychidae)

We investigated the development of the whole skeleton of the soft‐shelled turtle Pelodiscus sinensis, with particular emphasis on the pattern and sequence of ossification. Ossification starts at late Tokita‐Kuratani stage (TK) 18 with the maxilla, followed by the dentary and prefrontal. The quadrate is the first endoskeletal ossification and appears at TK stage 22. All adult skull elements have started ossification by TK stage 25. Plastral bones are the first postcranial bones to ossify, whereas the nuchal is the first carapacial bone to ossify, appearing as two unstained anlagen. Extensive examination of ossification sequences among autopodial elements reveals much intraspecific variation. Patterns of ossification of cranial dermal elements are more variable than those of endochondral elements, and dermal elements ossify before endochondral ones. Differences in ossification sequences with Apalone spinifera include: in Pelodiscus sinensis the jugal develops relatively early and before the frontal, whereas it appears later in A. spinifera; the frontal appears shortly before the parietal in A. spinifera whereas in P. sinensis the parietal appears several stages before the frontal. Chelydrids exhibit an early development of the postorbital bone and the palatal elements as compared to trionychids. Integration of the onset of ossification data into an analysis of the sequence of skeletal ossification in cryptodirans using the event‐pairing and Parsimov methods reveals heterochronies, some of which reflect the hypothesized phylogeny considered taxa. A functional interpretation of heterochronies is speculative. In the chondrocranium there is no contact between the nasal capsules and planum supraseptale via the sphenethmoid commissurae. The pattern of chondrification of forelimb and hind limb elements is consistent with a primary axis and digital arch. There is no evidence of anterior condensations distal to the radius and tibia. A pattern of quasi‐ simultaneity is seen in the chondrogenesis of the forelimb and the hind limb. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Survival and physiological responses of hatchling blanding's turtles (Emydoidea blandingii) to submergence in normoxic and hypoxic water under simulated winter conditions

Overwintering habits of hatchling Blanding's turtles (Emydoidea blandingii) are unknown. To determine whether these turtles are able to survive winter in aquatic habitats, we submerged hatchlings in normoxic (155 mmHg Po2) and hypoxic (6 mmHg Po2) water at 4 degrees C, recording survival times and measuring changes in key physiological variables. For comparison, we simultaneously studied hatchling softshell (Apalone spinifera) and snapping (Chelydra serpentina) turtles, which are known to overwinter in aquatic habitats. In normoxic water, C. serpentina and A. spinifera survived to the termination of the experiment (76 and 77 d, respectively). Approximately one-third of the E. blandingii died during 75 d of normoxic submergence, but the cause of mortality was unclear. In hypoxic water, average survival times were 6 d for A. spinifera, 13 d for E. blandingii, and 19 d for C. serpentina. Mortality during hypoxic submergence was probably caused by metabolic acidosis, which resulted from accumulated lactate. Unlike the case with adult turtles, our hatchlings did not increase plasma calcium and magnesium, nor did they sequester lactate within the shell. Our results suggest that hatchling E. blandingii are not particularly well suited to hibernation in hypoxic aquatic habitats.     

Comparative patterns of postcranial ontogeny in therian mammals: an analysis of relative timing of ossification events

Data on the relative sequence of ossification of postcranial elements for eight therian mammals (Myotis lucifugus, Homo sapiens, Rattus norvegicus, Mus musculus, Mesocricetus auratus, Cavia porcellus, Didelphis albiventris, and Sminthopsis macroura) and three outgroups (Chelydra serpentina, Alligator mississippiensis, and Lacerta vivipara) were taken from the literature. For each species, a matrix was constructed in which the relative timing of the onset of ossification in 24 elements was summarized. This resulted in 276 event pairs (characters) for each species. Thirty-three (33.3)% of the characters examined are uniform across all taxa, 16.3% are variable but uninformative in the phylogeny, and 50.4% potentially deliver diagnostic features for clades of two or more taxa. In all species examined, the clavicle is the first bone to appear. Placentalia is not unequivocally diagnosed by the state of any event pair, while Marsupialia has the largest amount of autapomorphies with 18. The acceleration in the timing of ossification of the scapula in relation to the hindlimb in marsupials is most probably causally correlated to movements after birth and during early phases of pre-weaning life. Marsupials are almost unique among amniotes in that the earliest onset of ossification of at least one element among carpals and among tarsals is simultaneous. Three parsimony analyses, each one with a different reptilian taxon as outgroup, were performed using event pairs as characters. In all cases, results were incongruent with the phylogeny of the studied taxa.     

Ossification patterns in the tetrapod limb – conservation and divergence from morphogenetic events

Two different patterns of the condensation and chondrification of the limbs of tetrapods are known from extensive studies on their early skeletal development. These are on the one hand postaxial dominance in the sequential formation of skeletal elements in amniotes and anurans, and on the other, preaxial dominance in urodeles. The present study investigates the relative sequence of ossification in the fore‐ and hindlimbs of selected tetrapod taxa based on a literature survey in comparison to the patterns of early skeletal development, i.e. mesenchymal condensation and chondrification, representing essential steps in the late stages of tetrapod limb development. This reveals the degree of conservation and divergence of the ossification sequence from early morphogenetic events in the tetrapod limb skeleton. A step‐by‐step recapitulation of condensation and chondrification during the ossification of limbs can clearly be refuted. However, some of the deeper aspects of early skeletal patterning in the limbs, i.e. the general direction of development and sequence of digit formation are conserved, particularly in anamniotes. Amniotes show a weaker coupling of the ossification sequence in the limb skeleton with earlier condensation and chondrification events. The stronger correlation between the sequence of condensation/chondrification and ossification in the limbs of anamniotes may represent a plesiomorphic trait of tetrapods. The pattern of limb ossification across tetrapods also shows that some trends in the sequence of ossification of their limb skeleton are shared by major clades possibly representing phylogenetic signals. This review furthermore concerns the ossification sequence of the limbs of the Palaeozoic temnospondyl amphibian Apateon sp. For the first time this is described in detail and its patterns are compared with those observed in extant taxa. Apateon sp. shares preaxial dominance in limb development with extant salamanders and the specific order of ossification events in the fore‐ and hindlimb of this fossil dissorophoid is almost identical to that of some modern urodeles.     

Early skeletal development in Talpa europaea, the common European mole

An ontogenetic series of 22 cleared and double-stained prenatal specimens was used to study the sequence of ossification of selected postcranial skeletal elements of Talpa europaea. Results were compared with nine other therian mammals, with Alligator, Chelydra, and Lacerta as outgroups. Using the event-pairing method, shifts in the onset of ossification in T. europaea, Sus, and Homo were identified. In T. europaea, the ossification of the cervical vertebrae starts before the metatarsals. In Homo and Sus, the tarsals ossify before the pubic bone. These shifts in the sequence of ossification are unique among the mammals examined, whereas many other changes, characterising monophyletic groups and/or evolving convergently, were also identified. Particular attention was given to some peculiar calcified elements of the hand in T. europaea, which were identified as accessory ;sesamoid bones', and do not display a chondrified precursor. They start to calcify before all others of the hand and later fuse. They appear in all fingers and function as reinforcement for the distal phalanges, most likely as an adaptation for burrowing. The development of the sesamoid bones was examined using histological sections and macerated adults.     

Studies on skeleton formation in reptiles. I. The postembryonic development of the skeleton in Cyrtodactylus pubisulcus (Reptilia: Gekkonidae)

The study of ossification during postembryonic development of the lizard Cyrtodactylus pubisulcus reveals consistent patterns in the skeleton of the body axis and of the limbs. The vertebral column shows a distinct antero-posterior gradient in ossification; the serial homology of sacral ribs and caudal transverse processes with dorsal ribs requires further scrutiny. The sequence of ossification of carpal and tarsal elements is constant, yet different from the pattern of chondrification as described in the literature. The homology of a separate 'intermedium' in the ossified lizard carpus requires further discussion. The development of the lizard astragalus is discussed in detail, as is the ossification of epiphyses in the limbs.     

Intramembranous ossification of scleral ossicles in Chelydra serpentina

Scleral ossicles are present in many reptiles, including turtles and birds. In both groups the sclerotic ring situated in the eye is composed of a number of imbricating scleral ossicles or plates. Despite this gross morphological similarity, Andrews (1996. An endochondral rather than a dermal origin for scleral ossicles in Cryptodiran turtles. J. Herpetol. 30, 257-260) reported that the scleral ossicles of turtles develop endochondrally unlike those in birds, which develop intramembranously after a complex epithelial-mesenchymal inductive event. This study re-explores one of the species examined by Andrews in order to determine the mode of ossification of scleral ossicles in turtles. A growth series of Chelydra serpentina embryos, including the stages examined by Andrews, were examined by staining separately for cartilage and bone. Results clearly contradict Andrews (1996) and show that the scleral ossicles of Chelydra serpentina develop similarly to those in birds. That is, they develop intramembranously without a cartilage precursor and are likely induced by transient scleral papillae. The sequence of scleral papillae development is broadly similar, but the papillae themselves are not as distinct as those seen in chicken embryos. This study has important consequences for understanding the homology of scleral ossicles among tetrapods.     

Patterns of chondrification and ossification in the skull of Graptemys pseudogeographica,the false map turtle (Emydidae)

Patterns of ossification and chondrification are well‐described for several species of turtles, but details of the chondrocranial anatomy are known for only a handful of species. Cleared and double‐stained embryos of Graptemys pseudogeographica were used to examine the fully formed chondrocranium and the formation, chondrification, and ossification of the cranium. The chondrocranium of G. pseudogeographica possesses an unusually large, irregularly shaped foramen epiphaniale that is joined with the fenestra olfactoria. As in other emydids, and many turtles generally, the taenia marginalis is present only as a small projection and the taenia medialis is lacking in mature stages of embryonic development. Ossification data for G. pseudogeographica are consistent with those of other Testudines in that the dentary and maxilla (dermal elements of the upper and lower jaws) ossify early, whereas the articular (an endochondral bone of the lower jaw) ossifies relatively late. Additionally, comparative ossification shows that the vomer is quite variable in its relative timing of ossification across Testudines.     

Description of the adult skeleton and developmental osteology of the hyperossified horned frog,Ceratophrys cornuta (Anura: Leptodactylidae)

The adult skeleton and tadpole chondrocranium of the leptodcatylid frog, Ceratophrys cornuta (Ceratophryinae), are described in detail, including the ontogenetic development of the chondrocanium and the ossification sequence of the skeleton. The chondrocranium of the carnivorous larvae is unique in lacking a frontoparietal fontanelle and possessing a complete dorsal roof of cartilage. Furthermore, the chondrocranium is extremely robust, particularly those elements involved in the feeding mechanism; these include large palatoquadrate cartilages, stout Meckel's, supra- and infrarostral cartilages, and short, wide, cornua trabeculae. The chondrocranium of C. cornuta resembles that described for Ceratophrys cranwelli, but differs from the chondrocrania reported for the species of Lepidobatrachus. The large adult skull is hyperossified; most elements are fused into a single unit, and nearly all dermal elements are ornamented, casqued, and co-ossified. Calcification is present in nearly every cartilaginous element of the skeleton in larger (older) adults. Several osteological characters previously used in ceratophryine systematics, such as the otic ramus of the squamosal and the columella, are reassessed. Contrary to previous reports, the ossified, dorsal dermal shield above the vertebral column in many ceratophryine anurans is absent in C. cornuta. With few exceptions, the ossification sequence relative to metamorphosis is consistent with those that are known for other anurans. The squamosal arises from three distinct centers of ossification, including an otic element. The frontoparietal arises from two centers of ossification that fuse early in development. A robust postorbital arch is formed primarily by the otic flange of the frontoparietal, which articulates laterally with the medial border of the otic ramus of the squamosal. Changes in the timing of development, or heterochrony, are involved with the evolution of the unusual skull and skeleton of ceratophryine frogs. J Morphol 232:169–206, 1997. © 1997 Wiley-Liss, Inc.     

Development of the dermal skeleton in Alligator mississippiensis (Archosauria, Crocodylia) with comments on the homology of osteoderms

The dermal skeleton (=exoskeleton) has long been recognized as a major determinant of vertebrate morphology. Until recently however, details of tissue development and diversity, particularly among amniotes, have been lacking. This investigation explores the development of the dermatocranium, gastralia, and osteoderms in the American alligator, Alligator mississippiensis. With the exception of osteoderms, elements of the dermal skeleton develop early during skeletogenesis, with most initiating ossification prior to mineralization of the endoskeleton. Characteristically, circumoral elements of the dermatocranium, including the pterygoid and dentigerous elements, are among the first to form. Unlike other axially arranged bones, gastralia develop in a caudolateral to craniomedial sequence. Osteoderms demonstrate a delayed onset of development compared with the rest of the skeleton, not appearing until well after hatching. Osteoderm development is asynchronous across the body, first forming dorsally adjacent to the cervical vertebrae; the majority of successive elements appear in caudal and lateral positions. Exclusive of osteoderms, the dermal skeleton initiates osteogenesis via intramembranous ossification. Following the establishment of skeletal condensations, some preossified spicules become engorged with many closely packed clusters of chondrocyte-like cells in a bone-like matrix. This combination of features is characteristic of chondroid bone, a tissue otherwise unreported among nonavian reptiles. No secondary cartilage was identified in any of the specimens examined. With continued growth, dermal bone (including chondroid bone) and osteoid are resorbed by multinucleated osteoclasts. However, there is no evidence that these cells contribute to the rugose pattern of bony ornamentation characteristic of the crocodylian dermatocranium. Instead, ornamentation develops as a result of localized concentrations of bone deposited by osteoblasts. Osteoderms develop in the absence of osteoblastic cells, osteoid, and periosteum; bone develops via the direct transformation of the preexisting dense irregular connective tissue. This mode of bone formation is identified as metaplasia. Importantly, it is also demonstrated that osteoderms are not histologically uniform but involve a range of tissues including calcified and uncalcified dense irregular connective tissue. Between taxa, not all osteoderms develop by homologous processes. However, it is concluded that all osteoderms may share a deep homology, connected by the structural and skeletogenic properties of the dermis.     

Development of the mandibular, hyoid arch and gill arch skeleton in the Chinese barb Puntius semifasciolatus: comparisons of ossification sequences among Cypriniformes

The morphogenesis and sequence of ossification and chondrification of skeletal elements of the jaws, and hyoid arch and gill arches of Puntius semifasciolatus are described. These data provide a baseline for further studies and enable comparisons with other described cypriniforms. Some general patterns of ossification in the hyoid arch and branchial arches in cypriniforms were notable. First, the overall development is from anterior to posterior, with the exception of the fifth ceratobranchial bone, which ossifies first. Second, where ossification of iterated elements is sequential, it tends to proceed from posterior to anterior, even when more posterior chondrifications are the smallest in the series. Ossification of the ceratobranchial, epibranchial and pharyngobranchial bones tends to proceed from ventral to dorsal. The comparisons revealed small sets of skeletal elements whose ossification sequence appears to be relatively conserved across cyprinid cypriniforms. Several potentially key timing changes in the ossification sequence of the jaws, hyoid arch and gill arches were identified, such as the accelerated timing of ossification of the fifth ceratobranchial bone, which may be unique to cypriniforms.