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).     

Ossification heterochrony in the therian postcranial skeleton and the marsupial-placental dichotomy.

Postcranial ossification sequences in 24 therian mammals and three outgroup taxa were obtained using clear staining and computed tomography to test the hypothesis that the marsupial forelimb is developmentally accelerated, and to assess patterns of therian postcranial ossification. Sequence rank variation of individual bones, phylogenetic analysis, and algorithm-based heterochrony optimization using event pairs were employed. Phylogenetic analysis only recovers Marsupialia, Australidelphia, and Eulipotyphla. Little heterochrony is found within marsupials and placentals. However, heterochrony was observed between marsupials and placentals, relating to late ossification in hind limb long bones and early ossification of the anterior axial skeleton. Also, ossification rank position of marsupial forelimb and shoulder girdle elements is more conservative than that of placentals; in placentals the hind limb area is more conservative. The differing ossification patterns in marsupials can be explained with a combination of muscular strain and energy allocation constraints, both resulting from the requirement of active movement of the altricial marsupial neonates toward the teat. Peramelemorphs, which are comparatively passive at birth and include species with relatively derived forelimbs, differ little from other marsupials in ossification sequence. This suggests that ossification heterochrony in marsupials is not directly related to diversity constraints on the marsupial forelimb and shoulder girdle.     

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.     

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.     

Sequence of ossification in the skeleton of growing and metamorphosing tadpoles of Rana pipiens

The order of ossification of bones in the skeleton of Rana pipiens during larval growth and metamorphosis has been determined from observations on specimens fixed in 70% alcohol and stained with alizarin red S. The axial skeleton ossifies in a generally cephalo-caudal sequence, beginning with the parasphenoid bone at Taylor-Kollros stages IV-IX, followed by vertebrae (V-IX) and then the urostyle (IX-XIV). Exoccipitals (VII-IX), frontoparietals (XI-XII) and prootics (XIII-XVII) are additional cranial bones which successively ossify before metamorphosis. With the onset of metamorphosis at stage XVIII jawbones and rostral bones of the skull ossify in the following succession: premaxilla, maxilla, septomaxilla, nasal, dentary, angular, squamosal, pterygoid, prevomer, mentomeckelian, quadratojugal, palatine, columella, posteromedial process of “hyoid.” The sphenethmoid does not ossify until after metamorphosis. Ossification of limbbones begins with the femur or humerus at stages X-XII and progresses proximo-distally to the phalanges by stages XIII-XV. Carpals, however, do not ossify until stage XXV or after metamorphosis. The ilium of the pelvic girdle begins to ossify at stages X-XII, but the ischium is delayed until stages XX-XXIII. Scapula and coracoid of the pectoral girdle undergo initial ossification at stages XII-XIV, suprascapula and clavicle at stages XIII-XV. The sternum does not begin to ossify until stage XXIV. The possible role of thyroid hormones in stimulating osteogenesis is discussed.     

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.     

Ontogeny and phytogeny of the mesopodial skeleton in mosasauroid reptiles

Current phylogenics of mosasauroid reptiles are reviewed and a new phylogeny examining aigialosaur interrelationships presented. Patterns of mesopodial ossification and overall limb morphology are described for adult mosasauroids. Ossification sequences are mapped onto a phylogeny in order to assess the distribution of ontogenetic characters. Consistent and ordered distributions are found. Based on the phylogenetic distribution of ossification patterns, an overall mesopodial ossification sequence for mosasaurs is proposed. Carpal sequence: ulnare—distal carpal four (dc4)—intermedium—dc3—radiale or dc2—de1 or pisiform and dc5. Tarsal sequence: astragalus—distal tarsal four or calcaneum. Skeletal paedomorphosis is recognized as a dominant pattern in the evolution of mosasauroid limbs. Apomorphic characters of skeletal paedomorphosis, apparent in most taxa, reach extremes in tylosaurs. Arguments for the presence of a single proximal cartilage in the tarsus of mosasaurs are made. This cartilage is presumed to include ossification centres from which both the astragalus and calcaneum will ossify.     

OSSIFICATION HETEROCHRONY IN THE THERIAN POSTCRANIAL SKELETON AND THE MARSUPIAL–PLACENTAL DICHOTOMY

Postcranial ossification sequences in 24 therian mammals and three outgroup taxa were obtained using clear staining and computed tomography to test the hypothesis that the marsupial forelimb is developmentally accelerated, and to assess patterns of therian postcranial ossification. Sequence rank variation of individual bones, phylogenetic analysis, and algorithm-based heterochrony optimization using event pairs were employed. Phylogenetic analysis only recovers Marsupialia, Australidelphia, and Eulipotyphla. Little heterochrony is found within marsupials and placentals. However, heterochrony was observed between marsupials and placentals, relating to late ossification in hind limb long bones and early ossification of the anterior axial skeleton. Also, ossification rank position of marsupial forelimb and shoulder girdle elements is more conservative than that of placentals; in placentals the hind limb area is more conservative. The differing ossification patterns in marsupials can be explained with a combination of muscular strain and energy allocation constraints, both resulting from the requirement of active movement of the altricial marsupial neonates toward the teat. Peramelemorphs, which are comparatively passive at birth and include species with relatively derived forelimbs, differ little from other marsupials in ossification sequence. This suggests that ossification heterochrony in marsupials is not directly related to diversity constraints on the marsupial forelimb and shoulder girdle.     

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.     

Food competition in captive juvenile snapping turtles, Chelydra serpentina

Food-competitive hierarchies and their stability in snapping turtles, Chelydra serpentina, were studied. Nine hatchling turtles were paired against each other in independent tests of food-obtaining dominance. Turtles differed in ability to obtain food. These differences appeared stable over time, and were linear. Neck-stretching, snapping, and clawing were the major behavioural patterns involved in food-appetitive and aggressive encounters.     

Pecularities of bony skeleton development in Asian clawed salamanders (Onychodactylus, Hynobiidae) related to embryonization

We studied skull, vertebral column, and limb skeleton development in Japanese clawed salamander Onychodactylus japonicus (Hynobiidae). The study is based on the ontogenetic series of embryos and larvae obtained from wild-captured adults by artificial induction of breeding using hormonal stimulation. The first stages of the skeleton formation in O. japonicus are shifted to the late embryonic period and hatching larvae already possess a well-ossified vertebral column, large number of skull ossifications and show signs of ossification in the forelimb skeleton. Compared to the primitive pattern of the skeleton development typical for other hynobiid salamanders, O. japonicus shows a number of heterochronies related to embryonization. In particular, this species is characterized by an earlier ossification of the vertebral column compared to that of the skull and by the delayed development and early reduction of the coronoid. Our results, along with the previously reported data on the skeleton development in the Fischer’s clawed salamander O. fischeri (Smirnov and Vassilieva, 2002), indicate that the genus Onychodactylus is characterized by the loss or reduction of several skeletal features typically found at early larval stages in other Hynobiidae species. In particular, provisional bones (especially the coronoid) and their dentition are underdeveloped. In addition, it is corroborated that the first tooth generation is absent in Onychodactylus, whereas such monocuspid nonpedicellate tooth generation normally develops at the early larval stages of other caudate amphibians. Since similar patterns of skeleton ontogeny are observed in other caudate groups with different extent of embryonization, it is proposed that, in different lineages of Urodela, the evolution of ontogeny followed similar pathways and was accompanied by the same changes in skeletogenesis.     

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.     

Experimental regeneration of the caudal skeleton of the glass knifefish,Eigenmannia virescens (Rhamphichthydae,Gymnotoidei)

This paper describes the regeneration of the caudal axial skeleton after amputation of the tail, including about 20 vertebrae, in the gymnotoid fish Eigenmannia virescens. Seven days after amputation, a regeneration blastema developed and soft tissues degenerated. A cylinder of cartilage developed at the end of the notochord. When this cartilage was about 10 mm long (21 days), perichondral ossification began. The cartilage continued to elongate and ossification increased while osteoclasts began to destroy the cartilage ventrally. Finally, a bony rod formed and at its tip the cartilage persisted as a rod, 2 to 3 mm long. The anal fin also regenerated: Endoskeletal cartilage developed first, following by differentiation of the lepidotrichia, and finally ossification of the endoskeleton.     

Temperature during embryonic and juvenile development influences growth in hatchling snapping turtles,Chelydra serpentina

Embryonic temperature influenced subsequent growth in juvenile snapping turtles, Chelydra serpentina: incubation temperatures of 24 and 26.5°C enhanced growth relative to a temperature of 29°C. Although embryonic temperature normally determines gonadal sex in this species, experimental manipulations revealed that temperature effects on growth were independent of sex. Ambient temperature also affected growth: juvenile turtles grew slowly in a cool (19°C) versus a warm (28°C) environment. In a parallel experiment, turtles from different embryonic temperatures displayed different patterns of temperature choice in response to nutritional status or time of day. We tentatively conclude that embryonic temperature has both direct and indirect (i.e., through temperature choice) effects on growth in snapping turtles.     

Postnatal skeletal ontogeny in Callisaurus draconoides and Uta stansburiana (Iguania: Phrynosomatidae).

The mineralization of the skeleton from hatching to near maximum size in two phrynosomatid lizards, Callisaurus draconoides and Uta stansburiana, is described in detail. Observed patterns in the appearance of epiphyseal secondary centers, ossification centers, apophyseal ossifications, and calcifications, the distribution of sesamoids, and the timing of fusions, are compared and contrasted with observations of other squamates available in the literature. Overall, Callisaurus and Uta exhibit an advanced state of ossification in the hatchling relative to other squamate neonates and share a similar sequence of braincase fusions and appearance of secondary centers. Preliminary observations suggest that patterns of postnatal skeletal development are highly conserved and independent of patterns of prenatal morphogenesis, and thus a potentially rich source of character data for systematic investigations.     

Prenatal ossification in rabbits as indicative of fetal maturity.

During the prenatal period of development of the rabbit skeleton (days 21-30) the successive phases of fetal maturity can be distinguished by reference to the progress of ossification, particularly in the distal limbs and sternum. In the present study absent or incomplete ossification of sternebra 5 occurred in 8% of term fetuses. The incidence of anomalies of prenatal ossification of the axial skeleton was 1.9%. The relevance of delayed and disturbed ossification with regard to teratological tests is discussed.