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

Developmental evolution of the distal ankle in the dinosaur-bird transition

The adult ankle of early reptiles had five distal tarsal (dt) bones, but in Dinosauria, these were reduced to only two: dt3 and dt4, articulated to metatarsals (mt) mt3 and mt4. Birds have a single distal tarsal ossification center that fuses to the proximal metatarsals to form a new adult skeletal structure: the composite tarsometatarsus. This ossification center develops within a single large embryonic cartilage, but it is unclear if this cartilage results from fusion of earlier cartilages. We studied embryos in species from four different bird orders, an alligatorid, and an iguanid. In all embryos, cartilages dt2, dt3, and dt4 are formed. In the alligatorid and the iguanid, dt2 failed to ossify: only dt3 and dt4 develop into adult bones. In birds, dt2, dt3, and dt4 fuse to form the large distal tarsal cartilage; the ossification center then develops above mt3, in cartilage presumably derived from dt3. During the entire dinosaur-bird transition, a dt2 embryonic cartilage was always formed, as inferred from the embryology of extant birds and crocodilians. We propose that in the evolution of the avian ankle, fusion of cartilages dt3 and dt2 allowed ossification from dt3 to progress into dt2, which began to contribute bone medially, while fusion of dt3 to dt4 enabled the evolutionary loss of the dt4 ossification center. As a result, a single ossification center expands into a plate-like unit covering the proximal ends of the metatarsals, that is key to the development of an integrated tarsometatarsus.

The evolution of the manus of early theropod dinosaurs is characterized by high inter- and intraspecific variation

The origin of the avian hand, with its reduced and fused carpals and digits, from the five-fingered hands and complex wrists of early dinosaurs represents one of the major transformations of manus morphology among tetrapods. Much attention has been directed to the later part of this transition, from four- to three-fingered taxa. However, earlier anatomical changes may have influenced these later modifications, possibly paving the way for a later frameshift in digit identities. We investigate the five- to four-fingered transition among early dinosaurs, along with changes in carpus morphology. New three-dimensional reconstructions from computed tomography data of the manus of the Triassic and Early Jurassic theropod dinosaurs Coelophysis bauri and Megapnosaurus rhodesiensis are described and compared intra- and interspecifically. Several novel findings emerge from these reconstructions and comparisons, including the first evidence of an ossified centrale and a free intermedium in some C. bauri specimens, as well as confirmation of the presence of a vestigial fifth metacarpal in this taxon. Additionally, a specimen of C. bauri and an unnamed coelophysoid from the Upper Triassic Hayden Quarry, New Mexico, are to our knowledge the only theropods (other than alvarezsaurs and birds) in which all of the distal carpals are completely fused together into a single unit. Several differences between the manus of C. bauri and M. rhodesiensis are also identified. We review the evolution of the archosauromorph manus more broadly in light of these new data, and caution against incorporating carpal characters in phylogenetic analyses of fine-scale relationships of Archosauromorpha, in light of the high degree of observed polymorphism in taxa for which large sample sizes are available, such as the theropod Coelophysis and the sauropodomorph Plateosaurus. We also find that the reduction of the carpus and ultimate loss of the fourth and fifth digits among early dinosaurs did not proceed in a neat, stepwise fashion, but was characterized by multiple losses and possible gains of carpals, metacarpals and phalanges. Taken together, the high degree of intra- and interspecific variability in the number and identities of carpals, and the state of reduction of the fourth and fifth digits suggest the presence of a 'zone of developmental variability' in early dinosaur manus evolution, from which novel avian-like morphologies eventually emerged and became channelized among later theropod clades.

Ontogeny of the Postcranial Axial Skeleton of Melanosuchus niger (Crocodylia, Alligatoridae)

This study proposes the description of the development of the postcranial axial skeleton, including vertebrae, gastralium, ribs, sternum, and interclavicle, in Melanosuchus niger. Six nests were marked and two eggs removed from each nest at 24-hr intervals until hatching. For posthatching evaluation, 30 hatchlings were kept in captivity and one exemplar was euthanized at three-day intervals. Samples were diaphanized using potassium hydroxide (KOH), alizarin red S, and Alcian blue. A routinely generally used method was applied for histological evaluation. It was difficult to define in which vertebrae the development of cartilaginous centers began, but it was possible to observe that this condensation advanced in the craniocaudal direction. The condensation started in the vertebral arches and was visibly stronger in the cervical and dorsal regions, advancing to the lumbar, sacral and, last, to the caudal region. The atlas showed a highly different morphology compared with the other cervical vertebrae, with a short intercenter, two neural arches, and a proatlas. The ossification process began in the body of cervical vertebrae III to VIII and alizarin retention decreased in the last vertebrae, indicating a craniocaudal direction in bone development, similar to cartilage formation. In the histological sections of gastralium and interclavicles of M. niger at several development stages, it was possible to observe that these elements showed intramembranous development. Anat Rec, 301:607-623, 2018. © 2017 Wiley Periodicals, Inc.