Ontogenetic Series Research Articles

Morphological variations within the ontogeny of Deinonychus antirrhopus (Theropoda, Dromaeosauridae).

This research resulted from the determination that MCZ 8791 is a specimen of Deinonychus antirrhopus between one and two years of age and that the morphological variations within particular growth stages of this taxon have yet to be described. The primary goal of the research is to identify ontogenetic variations in this taxon. Histological analyses determined that the Deinonychus specimens AMNH 3015 and MOR 1178 were adults. Comparisons are made between MCZ 8791 and these adult specimens. The holotype, YPM 5205, and the other associated specimens of this taxon within the YPM collection are similar in size and morphology to AMNH 3015. Further comparisons were made with the three partial specimens OMNH 50268, MCZ 4371, and MOR 1182. Although these specimens represent only a partial ontogenetic series, a number of morphological variations can be described. One secondary goal of this research is to compare the known pattern of variable, informative, ontogenetic characters in MCZ 8791 to a similar pattern of morphological characters in the sub-adult dromaeosaurid specimen Bambiraptor feinbergorum, AMNH FR: 30556. If the characters that have been determined to represent variable juvenile morphology in the ontogeny of Deinonychus are exhibited in Bambiraptor, this study will begin the process of determining whether a similar, conservative, ontogenetic pattern exists throughout the rest of Dromaeosauridae. If defensible, it may reduce the number of sympatric taxa within this clade. The other secondary goal relates to the forelimb function. The approximate body size, forelimb length, wrist development, and the presence of a more prominent olecranon on the ulna of MCZ 8791 support the hypothesis that juveniles of this taxon possessed some form of flight capability.

Postnatal development of franciscana's (Pontoporia blainvillei) biosonar relevant structures with potential implications for function, life history, and bycatch

AbstractFranciscana dolphins (Pontoporia blainvillei) are the most endangered species of the western South Atlantic Ocean. The major cause of their vulnerability is incidental bycatch in fishery gill nets. Ontogenetic changes of biosonar relevant structures in Pontoporia were analyzed in five specimens (one female neonate, two male neonates and two male adults) using digital imaging technology (MRI, CT) and macroscopic dissections to compare structures involved in sound production and reception. These data were compared to an ontogenetic series of 69 macerated skulls of Pontoporia in order to elucidate the correlation between soft tissue structures and bones of the epicranial complex and to describe the development‐related changes in the mandible. Postnatal developmental shape changes of the posterior part of the right vestibular air sac followed bone formation and the melon with its right branch elongated, paralleling the flatter facial depression of adults. Minor postnatal developmental modifications were verified in the tympano‐periotic complex but a shape change of the mandible was visible by a ventral deviation of the posterior part of the mandible in adults. These results reveal postnatal changes in allometry and shape of biosonar relevant structures that may be one of the causes that increase bycatch of neonate and young Pontoporia individuals.

Seahorses: Osteogenesis and Caudal Fin Loss

Seahorses (Family: Syngnathidae) are well known for their highly derived head shape, prehensile tail and armoured body, but their osteogenesis is poorly described. Here we compare the osteogenesis of Hippocampus reidi over an ontogenetic series spanning the first 93 days after release from the brood pouch to that of a smaller series of Hippocampus; namely H. subelongatus. We also compared our results to those published for a dwarf species, H. zosterae. We show that ossification onset in H. subelongatus is earlier than in H. reidi, despite similar sizes at parturition. Interestingly, osteogenesis in H. zosterae is similar to that of the larger species, H. subelongatus. Also, the growth rate of all three species is similar initially but at one month of age, the size of the dwarf species H. zosterae remains relatively constant whilst the other two species continue growing with an accelerated growth phase. Putting these observations within a phylogenetic context suggests that there has been a heterochronic shift in the timing of osteogenesis in H. reidi and H. zosterae. We conclude that H. zosterae is a proportioned dwarf rather than a developmentally truncated dwarf species. Furthermore, we show that caudal fin loss is incomplete in Hippocampus seahorses.

A neonatal perspective on Homo erectus brain growth

The Mojokerto calvaria has been central to assessment of brain growth in Homo erectus, but different analytical approaches and uncertainty in the specimen's age at death have hindered consensus on the nature of H. erectus brain growth. We simulate average annual rates (AR) of absolute endocranial volume (ECV) growth and proportional size change (PSC) in H. erectus, utilizing estimates of H. erectus neonatal ECV and a range of ages for Mojokerto. These values are compared with resampled ARs and PSCs from ontogenetic series of humans, chimpanzees, and gorillas from birth to six years. Results are consistent with other studies of ECV growth in extant taxa. There is extensive overlap in PSC between all living species through the first postnatal year, with continued but lesser overlap between humans and chimpanzees to age six. Human ARs are elevated above those of apes, although there is modest overlap up to 0.50 years. Ape ARs overlap throughout the sequence, with gorillas slightly elevated over chimpanzees up to 0.50 years. Simulated H. erectus PSCs can be found in all living species by 0.50 years, and the median falls below the human and chimpanzee ranges after 2.5 years. H. erectus ARs are elevated above those of all extant taxa prior to 0.50 years, and after two years they fall out of the human range but are still above ape ranges. A review of evidence for the age at death of Mojokerto supports an estimate of around one year, indicating absolute brain growth rates in the lower half of the human range. These results point to secondary altriciality in H. erectus, implying that key human adaptations for increasing the energy budget of females may have been established by at least 1 Ma.

Morphology and development of the eodiscoid trilobite Tsunyidiscus yanjiazhaiensis from the Cambrian (Stage 3, Series 2) of South China

The post-protaspid ontogenetic series of the eodiscinid trilobite Tsunyidiscus yanjiazhaiensis S. Zhang et al. in Yin & Li, 1978 is described, on the basis of crack-out specimens including numerous articulated specimens and a large number of disarticulated sclerites from the lower Cambrian (Stage 3, Series 2) Shuijingtuo Formation, Zhenba County, Shaanxi Province, South China. In addition, hundreds of phosphatized sclerites assigned to this species were obtained by dilute acetic acid dissolution of thin-bedded limestones from the same horizon. These sclerites exhibit considerable morphological detail and even some delicate structures, such as the axial pores and marginal spines on the pygidium and the bacculae that can be retained from the protaspid to the holaspid period. Ontogenetic study of T. yanjiazhaiensis provides new evidence to support and reconfirm the unique developmental pattern shared by early Cambrian eodiscoid trilobites, i.e. the processes of liberation of the thoracic segment and segment insertion into the pygidium are separate from one another. This represents a highly unusual growth pattern among the Trilobita and offers fresh insights into segmentation during postembryonic development of eodiscinid trilobites.

Ontogenetic changes in the olfactory antennules of the shore crab, Hemigrapsus oregonensis, maintain sniffing function during growth.

Malacostracan crustaceans capture odours using arrays of chemosensory hairs (aesthetascs) on antennules. Lobsters and stomatopods have sparse aesthetascs on long antennules that flick with a rapid downstroke when water flows between the aesthetascs and a slow return stroke when water is trapped within the array (sniffing). Changes in velocity only cause big differences in flow through an array in a critical range of hair size, spacing and speed. Crabs have short antennules bearing dense arrays of flexible aesthetascs that splay apart during downstroke and clump together during return. Can crabs sniff, and when during ontogeny are they big enough to sniff? Antennules of Hemigrapsus oregonensis representing an ontogenetic series from small juveniles to adults were used to design dynamically scaled physical models. Particle image velocimetry quantified fluid flow through each array and showed that even very small crabs capture a new water sample in their arrays during the downstroke and retain that sample during return stroke. Comparison with isometrically scaled antennules suggests that reduction in aesthetasc flexural stiffness during ontogeny, in addition to increase in aesthetasc number and decrease in relative size, maintain sniffing as crabs grow. Sniffing performance of intermediate-sized juveniles was worse than for smaller and larger crabs.

Functional geometric morphometric analysis of masticatory system ontogeny in papionin primates.

The three-dimensional configuration of the primate masticatory system is constrained by the need to maximize bite forces while avoiding distraction of the temporomandibular joint (TMJ). Within these bounds, shape variation has predictable effects on functional capacities such as mechanical advantage and gape. In this study, geometric morphometric analysis is used to investigate the ontogeny of masticatory function in papionin monkeys and test the hypothesis that biomechanical constraints determine the location of molar eruption. This "constrained eruption hypothesis" predicts that the distalmost molar (DMX) will occupy a consistent location anterior to the TMJ and that jaw adductor muscles will maintain consistent positions relative to both DMX and TMJ. Craniometric landmarks were digitized on cross-sectional ontogenetic series of nine papionin species. Form-space PCA of Procrustes residuals, visualization of Bookstein shape coordinates, and nonparametric ANOVA were used to identify ontogenetic shape trends and test for significant ontogenetic changes in relative landmark positions. In most taxa, DMX maintains a consistent position relative to the TMJ while the anterior dentition migrates anteriorly. Where significant intraspecific ontogenetic differences occur, they involve anterior migration of DMX in later dental stages, likely due to late adolescent growth of the posterior palate. Attachments of the anterior temporalis and deep masseter also maintain consistent positions relative to the TMJ; however, the superficial masseter migrates anteriorly throughout ontogeny. All muscle attachments migrate laterally relative to the TMJ, reflecting positive scaling of adductor PCSA. Overall, results support the constrained eruption hypothesis and suggest mechanisms by which functional capacity is maintained during ontogeny.

Morphological scaling of body form in four shark species differing in ecology and life history

Body form can change across ontogeny, and can influence how animals of different sizes move and feed. Scaling data on live apex predatory sharks are rare and, therefore, we examined patterns of scaling in ontogenetic series of four sympatric shark species exhibiting a range of sizes, ecologies and life histories (tiger, bull, blacktip, and nurse shark). We evaluated 13 linear morphological variables and two areas (caudal and dorsal) that could influence both animal condition and locomotor performance. These measurements included dimensions of the dorsal, pectoral, and caudal fins, as well as several dimensions of body circumference, and of the head. For all four species, the body axis (eye-to-eye, lateral span, frontal span, proximal span) scaled close to isometry (expected slope of 1.0). The two largest sharks (tiger and bull sharks) also showed significant negative allometry for elements of the caudal fin. We found significant negative allometry in the lengths of the upper lobe of the caudal fin (caudal fin 1) and the overall height of the caudal fin (caudal fin 2) in tiger and bull sharks, with slopes ranging from about 0.60 to 0.73. Further, tiger sharks showed negative allometry in caudal fin area. These results suggest that in terms of overall body dimensions, small sharks are roughly geometrically similar to large sharks, at least within the species we examined. However, juvenile tiger (and to a lesser extent bull sharks) are notable in having proportionately larger caudal fins compared to adult sharks. As the caudal fin contributes to generating thrust during forward locomotion, this scaling implies differences among adult and juvenile sharks in locomotor ability. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114, 126–135.

Description of the Tadpole ofPtychadena uzungwensis(Amphibia: Anura: Ptychadenidae) from the Southern Udzungwa Mountains, Tanzania

Abstract The tadpole of Ptychadena uzungwensis is described from material collected at Mufindi, southern Udzungwa Mountains, Tanzania. We provide a detailed description of the tadpole based on an ontogenetic series, including color photographs of living specimens and buccopharyngeal morphology. Identity was confirmed by rearing tadpoles through metamorphosis and through DNA bar coding. The tadpole has a keratodont formula of 1/2 and generally resembles other species of Ptychadena. Diagnostic features of known tadpoles of East African species of Ptychadena are provided and discussed.

The crinoid Eugeniacrinites cariophilites from the Late Jurassic of southern Germany: babies, cripples and enigmatic wing plates

The material for the present paper is based on 50 kg of processed sediment from the Early Kimmeridgian of Geisingen. The most frequent crinoids comprise cups and brachials of the roveacrinid Saccocoma quenstedti while Crassicoma feifeli is represented by a few cups only; these remains are not described. Cups and primibrachials of Eugeniacrinites cariophilites are the commonest cyrtocrinids, while cups of Pilocrinus moussoni are rather rare and not further treated, as are columnals of the two species. The ontogenetic series of cups and axillary primibrachials of E. cariophilites starts from ossicles of about 0.5 mm in height; and growth is allometric in both types. Postlarval cups resemble phyllocrinids; the distinctive shape with lanceolate interradial extensions and wide articular facets appears at a later stage. The spearhead-shaped axillary primibrachials are synapomorphic. The youngest of these ossicles with a muscular facet proximally were able to form a flat roof over the radial cavity. First primibrachials with synostosial distal facet appeared at a slightly later stage with a width of 1 mm; growth is isometric. A significant number of cups are mutilated by bite marks. The predatory attacks were sublethal in most cases as shown by secondary stereom growth and regenerated facets. First primibrachials are not deformed, but deformations occur in a small number of second primibrachials. The results suggest that the predators, presumably fishes, targeted the soft parts in the cup. Enigmatic brachials with a large wing to one side and a facet to an arm are thought to belong to E. cariophilites and presumably attached to deformed cups; the wings may have been an antipredatory adaptation providing some shelter to the cup. The various antipredator morphologies in cyrtocrinid history are discussed. The presently available growth series of Eugeniacrinites invites discussion of early cyrtocrinid phylogeny. Eugeniacrinites seems to have been derived from Phyllocrinidae; and Sclerocrinidae with Cyrtocrinus as the first representative in the Middle Jurassic is thought to have originated from Early Jurassic Tetracrinidae such as Bilecicrinus.

Ontogenetic development and sexual dimorphism of franciscana dolphin skull: A 3D geometric morphometric approach.

The aim of this work was to study the postnatal ontogenetic development of Pontoporia blainvillei skull, identifying major changes on shape, and relating them to relevant factors in the life history of the species. We analyzed a complete ontogenetic series (73♂, 83♀) with three-dimensional geometric morphometric techniques. Immature dolphins showed a very well-developed braincase and a poorly developed rostrum, and the principal postnatal changes affected the rostrum and the temporal fossa, both structures implied functionally to the feeding apparatus, thus suggesting a specialized mode for catch fast prey in P. blainvillei. Osseous elements associated with sound production were already well developed on immature dolphins, suggesting the importance of this apparatus since the beginning of postnatal life. Sexual dimorphism was detected on both shape and size variables. Females were bigger than males, in accordance with previous studies. Shape differences between sexes were found on the posterior part of premaxillaries and external bony nares (P < 0.01), suggesting that this sexual dimorphism is related to differences on vocalization capabilities.

Long bone cortices in a growth series ofApatosaurussp. (Dinosauria: Diplodocidae): geometry, body mass, and crystallite orientation of giant animals

Comparative studies of body mass, cortical bone geometry, and preferred crystallographic orientation (texture) of bone mineral in an ontogenetic series of the sauropod, Apatosaurus sp., and recent animals are reported. The cortical bone geometry shows that sauropods have an expansion of the cortical thickness as a result of their heavier mass. It is shown that a universal power-law relationship connects body mass to bone length, suggesting the validity of Wolff's law for sauropods. Moreover, sauropod and mammalian bones seem to be exposed to the same compressive bone stress. X-ray texture measurements of long bones were used to study the orientation of carbonated hydroxyapatite (HA) crystals. All bones investigated showed a 001-fibre texture, in which the c-axis of the HA crystals is oriented along the bone axis. Texture strength (defined by the alignment of HA crystals) appears to be of only minor importance regarding the mechanical strength and seems to be influenced by the loading mode of the bone. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112, 782–798.

The three-front model: a developmental explanation of long bone diaphyseal histology of Sauropoda

The bone histology of non-avian dinosaurs is enlightening for understanding many aspects of the growth and development of these long-extinct animals. The rate of bone apposition and remodelling in the shaft of long bones appears to be accelerated in some groups and decelerated in others. We propose a developmental model to illustrate these fundamental aspects of long bone diaphyseal histology at different growth stages. We developed the model based on an ontogenetic series of long bones of the sauropod dinosaur Apatosaurus. The model describes the histology and microanatomy based on three fronts that move radially: the apposition front, the Haversian substitution front, and the resorption front. When applied to additional sauropod dinosaurs, differences and similarities observed in the microstructure of the different taxa could be explained with the model. The benefit of this model is that it is not limited to Sauropoda but appears to be applicable to a broad range of terrestrial amniote long bones and thus could provide unique insights into life history and evolutionary patterns of bone development. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112, 765–781.

Heterochronic growth of ostracods (Crustacea) from microbial deposits in the aftermath of the end-Permian extinction

The Kokarkuyu Formation (Early Triassic) from the Çürük Dağ section, Western Taurus, Antalya Nappes, Turkey was carefully sampled for a taxonomic study of the ostracods. A total of 57 species belonging to 15 genera are recognized. Three species are newly described: Monoceratina hussonae sp. nov., Reviya sylvieae sp. nov. and Eukloedenella adcapitisdolorella sp. nov. The present data are the first Neo-Tethyan illustration of ostracod survival in the aftermath of the end-Permian extinction in a refuge of microbial origin. Ostracods are abundant within the microbialites at the base of the formation and allow the reconstruction of ontogenetic series for nine species. Shape variations through ontogeny are described for seven species: Bairdia? kemerensis, Praezabythocypris? ottomanensis, Liuzhinia antalyaensis, Paracypris gaetanii, Reviya curukensis, R. sylvieae and Eukloedenella adcapitisdolorella. Paedomorphosis through deceleration and peramorphosis through acceleration are identified as secondary survival strategies following the end-Permian extinction.http://zoobank.org/urn:lsid:zoobank.org:pub:6849AB0D-B085-47BE-950A-BC98CC50C4E3

Mandibular development in Australopithecus robustus

Australopithecus robustus has a distinct mandibular anatomy, with a broad and deep corpus and a tall, relatively upright ramus. How this anatomy arose through development is unknown, as gross mandibular size and shape change have not been thoroughly examined quantitatively in this species. Herein, I investigate A. robustus mandibular growth by comparing its ontogenetic series with a sample of recent humans, examining age-related size variation in 28 linear measurements. Resampling is used to compare the amount of proportional size change occurring between tooth eruption stages in the small and fragmentary A. robustus sample, with that of a more complete human skeletal population. Ontogenetic allometry of corpus robusticity is also assessed with least squares regression. Results show that nearly all measurements experience greater average increase in A. robustus than in humans. Most notably, A. robustus corpus breadth undergoes a spurt of growth before eruption of M1 , likely due in part to delayed resorption of the ramus root on the lateral corpus. Between the occlusion of M1 and M2 , nearly all dimensions experience greater proportional size change in A. robustus. Nested resampling analysis affirms that this pattern of growth differences between species is biologically significant, and not a mere byproduct of the fossil sample size. Some species differences are likely a function of postcanine megadontia in A. robustus, although the causes of other differences are less clear. This study demonstrates an important role of the postnatal period for mandibular shape development in this species.

Floral Evolution in the Detarieae (Leguminosae): Phylogenetic Evidence for Labile Floral Development in an Early-Diverging Legume Lineage

Premise of research. We examine floral evolution and phylogenetic relationships in the monophyletic Detarieae and related lineages of Caesalpinioideae. Tribe Detarieae (82 genera) includes nearly half of the genera in subfamily Caesalpinioideae and represents some of the most diverse legumes with respect to floral morphology.Methodology. A total of 67 floral ontogenetic and morphological characters were combined with DNA sequences from the plastid trnL-F and matK regions of 34 Detarieae species and representatives of Cassieae, Cercideae, Caesalpinieae, Papilionoideae, and Mimosoideae, for which we have near-complete ontogenetic series. The morphological and ontogenetic characters were optimized onto the resulting most parsimonious phylogenetic trees and Bayesian topology to study character evolution.Pivotal results. Our study supports previously proposed relationships within the tribe Detarieae and among caesalpinioid lineages and indicates that certain features (bracteole and hypanthium characters, sepal initiation, anther position in bud, overlap in timing of initiation of organ whorls) are phylogenetically informative for particular clades whereas others (reductions in petal and stamen number, sepal and petal initiation patterns) have evolved multiple times in parallel in the Detarieae and other Caesalpinioideae. These analyses suggest that modifications that occur early in ontogeny can be good phylogenetic characters for distinguishing both major taxonomic groups and more closely related taxa and that morphological differences that differentiate species within genera can be caused by changes that occur at all stages of ontogeny. Phylogenetic distribution of character states and ontogenetic evidence suggest that in the Caesalpinioideae, loss or suppression of organs within a whorl, both of which are very common, usually does not affect development in subsequent whorls.Conclusions. Our analyses reveal several switches from zygomorphy to actinomorphy (and vice versa), but in Caesalpinioideae (contrary to Papilionoideae), zygomorphy is likely not clearly associated with higher diversification rates. This study suggests that floral initiation patterns are much more variable in Caesalpinioideae than in the other two subfamilies. Although particular patterns may be canalized in certain lineages of the subfamily (Cercideae, Caesalpinieae), in other clades (Detarieae, Dialiinae) floral development is more labile, explaining the high diversity in floral morphology encountered overall in the paraphyletic Caesalpinioideae.

Interspecific comparison of ontegenetic change in temporal bone pneumatization (918.22)

Comparative studies of temporal bone pneumatization in adult chimpanzees and modern humans have shown that human temporal bones have reduced pneumatization compared to chimpanzee temporal bones, following the general trend towards reduction in pneumatization over the course of hominid evolution. Although the adult pattern of pneumatization in chimpanzees is well studied, extensive quantitative comparisons of the ontogenetic change in pneumatization of the temporal bone in chimpanzees and modern humans have not been completed. The purpose of this study is to compare patterns of ontogenetic change in temporal bone pneumatization in humans and chimpanzees.High resolution computed tomography scans were acquired from cross‐sectional ontogenetic series of human (n=29) and chimpanzee (n =30) temporal bones. The sample included individuals ranging in age from neonate to adult for both species. Surface areas and volumes of the temporal bone air cell tracts were calculated using Amira for both species.Results indicate that, from an early age, chimpanzees show larger surface areas and volumes of the temporal bone air cell tracts compared to modern humans. Although there are some differences in timing of pneumatization in the two species, these differences in size may result from the progression of the air cells from both the mastoid and squamous antrums in chimpanzees. This study highlights the early establishment of adult size differences in pneumatization.

A new sphenodontian (Reptilia, Lepidosauria) from the Lower Cretaceous of Southern Italy and the phylogenetic affinities of the Pietraroia Plattenkalk rhynchocephalians

The Pietraroia Plattenkalk (Albian, Lower Cretaceous) of Southern Italy is known for well-preserved fossil vertebrates, including the rhynchocephalian lepidosaur Derasmosaurus pietraroiae. A previously described Pietraroian rhynchocephalian differs from Derasmosaurus in vertebral, pelvic and foot morphology. A third Pietraroian rhynchocephalian is described for the first time. The new specimen is smaller and more gracile than the other Pietraroian rhynchocephalians, shows a broad unpaired parietal with a small foramen, and robust dorsal neural spines. Comparison with the ontogenetic series of Sphenodon suggests that small body size and relatively broad parietal in the new rhynchocephalian indicate immaturity, although the complete obliteration of the interparietal suture and the relatively small size of the parietal foramen may support an ontogenetically mature condition for that specimen. The morphology of the dorsal vertebrae excludes referral to Derasmosaurus. Phylogenetic analyses placed the second specimen among the basal branch of Opisthodontia, and the third specimen and Derasmosaurus among Sphenodontinae. The phylogenetic reconstruction supports the interpretation of the Pietraroian rhynchocephalians as late-surviving members of Jurassic lineages.

Postnatal ontogeny of limb proportions and functional indices in the subterranean rodentCtenomys talarum(Rodentia: Ctenomyidae)

Burrow construction in the subterranean Ctenomys talarum (Rodentia: Ctenomyidae) primarily occurs by scratch-digging. In this study, we compared the limbs of an ontogenetic series of C. talarum to identify variation in bony elements related to fossorial habits using a morphometrical and biomechanical approach. Diameters and functional lengths of long bones were measured and 10 functional indices were constructed. We found that limb proportions of C. talarum undergo significant changes throughout postnatal ontogeny, and no significant differences between sexes were observed. Five of six forelimb indices and two of four hindlimb indices showed differences between ages. According to discriminant analysis, the indices that contributed most to discrimination among age groups were robustness of the humerus and ulna, relative epicondylar width, crural and brachial indices, and index of fossorial ability (IFA). Particularly, pups could be differentiated from juveniles and adults by more robust humeri and ulnae, wider epicondyles, longer middle limb elements, and a proportionally shorter olecranon. Greater robustness indicated a possible compensation for lower bone stiffness while wider epicondyles may be associated to improved effective forces in those muscles that originate onto them, compensating the lower muscular development. The gradual increase in the IFA suggested a gradual enhancement in the scratch-digging performance due to an improvement in the mechanical advantage of forearm extensors. Middle limb indices were higher in pups than in juveniles-adults, reflecting relatively more gracile limbs in their middle segments, which is in accordance with their incipient fossorial ability. In sum, our results show that in C. talarum some scratch-digging adaptations are already present during early postnatal ontogeny, which suggests that they are prenatally shaped, and other traits develop progressively. The role of early digging behavior as a factor influencing on morphology development is discussed.

Postnatal Development of the Skull of Dinilysia patagonica (Squamata‐Stem Serpentes)

The snake skull represents a profound transformation of the ancestral squamate cranium in which dermal skull roof bones were integrated with the braincase, in a manner convergent with that which occurred during the origin of mammals. However, the ontogeny of snake characters at the origin of the clade has until now been inaccessible. Here we describe a postnatal ontogenetic series of the Late Cretaceous stem snake Dinilysia patagonica and compare it to that of extant lizards and snakes. Comparative analysis indicates notable ontogenetic changes, including advanced state of ossification, isometric growth of the otic capsule, fusion of the stylohyal to the quadrate, and great posterior elongation of the supratemporal. Of these transformations, the unfused condition of braincase bones and the retention of a large otic capsule in adults are examples of paedomorphic and peramorphic processes, respectively. Some ontogenetic transformations detected, in particular those present in middle ear, skull roof and suspensorium, are strikingly similar to those present in extant snakes. Nevertheless, Dinilysia retains a lizard-like paroccipital process without an epiphyseal extremity, and a calcified epiphysis that caps the sphenoccipital tubercle. Finally, the integration of the dermal skull roof with the braincase is similar to that seen in mammals with regard to the overall closure of the braincase, but the two evolutionary and developmental modules appear less integrated in snakes in that the parietal bone of the dermal skull roof progressively overlaps the supraoccipital of the chondrocranial braincase.