Evidence of determinate growth in an American alligator (Alligator mississippiensis) based on long-term recapture and osteohistological confirmation

Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: Typothorax coccinarum)

Maximum individual body size in pseudosuchian archosaurs is not well constrained in the fossil record, but it may be influenced by a variety of factors including basal metabolic rate, evolutionary relationships, and environmental conditions. Body size varies among the Aetosauria in which estimated total length ranges between 1 m (e.g., Coahomasuchus kahleorum) and 5 m (e.g., Desmatosuchus spurensis). A new, very large specimen of the aetosaurian Typothorax coccinarum from Petrified Forest National Park in northeastern Arizona is nearly twice the size of all other known specimens of Typothorax and is the largest aetosaur specimen currently known worldwide. The specimen lacks co-ossified neurocentral sutures in the trunk vertebrae which may suggest that the individual had not yet reached skeletal maturity, yet smaller specimens of T. coccinarum exhibit partially or fully co-ossified neurocentral sutures in the same region. If body size correlates with skeletal maturity in aetosaurs, this discrepancy warns that timing of neurocentral suture co-ossification in aetosaurs may not be a reliable indicator of ontogenetic stage. Osteohistological observations of a trunk rib demonstrate that although PEFO 42506 shows a large body size, the specimen did not deposit an external fundamental system despite depositing as many as 19 growth lines, further indicating that it had not yet reached skeletal maturity. Thus, at least within Aetosauria, neurocentral suture co-ossification and skeletal maturity may correlate, whereas body size can be incongruent in comparison. Furthermore, this specimen indicates that non-desmatosuchin aetosaurs could exhibit large body sizes and suggests that some aetosaurs may have experienced indeterminate growth.

Ontogenetic and interelemental study of appendicular bones of Caiman latirostris Daudin, 1802 sheds light on osteohistological variability in crocodylians

The osteohistology of vertebrates provides a reliable source to deduce biological information, particularly regarding growth and development. Although osteohistological studies in Neosuchia (Crocodyliformes, Mesoeucrocodylia) are relatively numerous, the number of species studied within the group is still small. Extant crocodilians are known to exhibit intraspecific variability linked to environmental conditions, habitat, feeding, and other intrapopulation factors. Here, we analyzed the osteohistology of the living South American Caiman latirostris throughout posthatching ontogeny. The histology of several appendicular bones of 13 different-sized captive and wild individuals were examined. Although some thin sections revealed the classic lamellar, parallel-fibered, or woven bone matrices, others showed a variation and a mix between the organization of the bone tissue. These histological differences are likely related to variability in the growth dynamics of caimans. In some bones of the juveniles studied, remnants of embryonic bone were observed. Osteohistological variation related to prevailing environmental conditions is documented. Furthermore, our results show ontogenetic variation in the type of bone tissues deposited throughout the development of C. latirostris. This study offers a broad framework for life history interpretations for C. latirostris and provides insight into the evolutionary history and ontogenetic growth of extinct crocodylian lineages.

Dispatches from the age of crocodiles: New discoveries from ancient lineages

Crocodilians inspire researchers and the public alike with their explosive hunting methodologies, distinct craniofacial and dental morphology, and resplendent fossil record. This special issue highlights recent advances in the biology and paleontology of this fascinating lineage of vertebrates. The authors in this volume bring crocodylians and their extinct ancestors to life using a variety of approaches including fieldwork, imaging, 3D modeling, developmental biology, physiological monitoring, dissection, and a host of other comparative methods. Our journey begins with early crocodylomorphs from the Triassic, carries us through the radiation of crocodyliforms during the rest of the Mesozoic Era, and finally celebrates the diversification development and biology of extant crocodylians. Crocodyliform science has grown appreciably the past few decades. New fossil species and genetic evidence continue to keep phylogenies and our understanding of relationships wavering in key places of the tree such as the relationships of the extinct marine thalattosuchians as well as still living species like gharials. The application of imaging approaches and 3D modeling to both preserved tissues as well as living specimens is now revealing patterns in brain and lung evolution and function, growth strategies, and feeding and locomotor behaviors across the lineage. Comparative anatomical studies are offering new data on genitals, cephalic venous drainage and thoracoabdominal pressures. The new discoveries found here only reveal there is far more work to be done to understand the biology and behavior responsible for the great radiation extinct suchians and their crocodylian descendants experienced during their conquest of Mesozoic and Tertiary ecosystems.