Palaeobiological inferences of "rauisuchians" Fasolasuchus tenax (Los Colorados Fm., Argentina) and Prestosuchus chiniquensis (Santa Maria Super sequence, Brazil) from the Middle-Upper Triassic of South America based on microstructural analyses

The first ontogenetic model for non-crocodylomorph loricatans, based on osteohistology of the ontogenetic series of Prestosuchus chiniquensis from the Middle Triassic of Brazil

Prestosuchus chiniquensis is an iconic non-crocodylomorph loricatan from the Brazilian Triassic beds and the best-known taxon, represented by several specimens. The completeness and preservation of its skeleton make it a valuable taxon for paleobiological studies. We explore the microstructure of bone tissues of appendicular elements and ribs of three specimens of Prestosuchus to access a variety of aspects of its paleobiology, such as histovariability, ontogeny, and growth patterns. Integrating our data and other osteohistologically studied P. chiniquensis specimens, we proposed for the first time an ontogenetic model for non-crocodylomorph loricatans. The model encompasses six distinct age classes (I-VI) that allow us to infer the growth patterns of P. chiniquensis and possibly expand to other phylogenetically close taxa. During early ontogeny (age classes I-II), sustained fast growth was maintained by a fibrolamellar complex. In mid ontogeny (age classes III-IV), highly vascularized parallel-fibered bone predominates, suggesting intermediary growth rates. A change for a poorly vascularized parallel-fibered/lamellar bone would mark the attainment of sexual (age classes IV-V) and skeletal maturity, comprising the age class VI. An external fundamental system (EFS) present in the outermost cortex is the main histological feature that characterize the age class VI. Major histovariability features are present between appendicular bones and ribs of skeletally immature individuals. The most prominent of them is the presence of fibrolamellar complex and highly vascularized parallel-fibered bone in appendicular bones and poorly vascularized parallel-fibered bone in ribs. In advanced ontogenetic stages, the histovariability between appendicular bones and ribs tends to be minor. Our data also support previous hypothesis of the presence of one new taxon among the specimens assigned to P. chiniquensis, increasing the diversity of non-crocodylomorph loricatans. The new taxon, represented by the specimen UFRGS-PV-0152-T, awaits a formal anatomical description. Our study advances the preliminary understand of the ontogeny and growth patterns of non-crocodylomorphs loricatans and Pseudosuchia as a whole.

A fast start: Evidence of rapid growth in Trialestes romeri, an early Crocodylomorpha from the Upper Triassic continental beds of Argentina based on osteohistological analyses

The palaeobiology aspects of early Crocodylomorpha during their origin in the Triassic are poorly known, despite occupying an important palaeoecological role in continental environments. In this study, we report the microstructural features of appendicular bones of two specimens of Trialestes romeri, a non-Crocodyliformes Crocodylomorpha from the Upper Triassic from the NW of Argentina. Our goals are to infer aspects of life history (i.e. age estimation and maturity events), inter-elemental variation and the growth dynamics within a phylogenetic framework. The samples include the humerus and the ulna of the holotype (PVL 2561) and the humerus and the femur of a referred specimen (PVL 3889). All elements are mostly composed by the fibrolamellar complex with a variable distribution of parallel-fibred bone in their cortex. Furthermore, they possess a uniform and homogeneous vascularisation, formed mainly by a laminar pattern. The humerus of the PVL 2561 records two lines of arrested growth (LAGs) and no cyclical growth marks were registered in the ulna. Meanwhile, the humerus and the femur of PVL 3889 exhibit a single LAG. Hence, a slight disparate inter-elemental variation is reported in both individuals. The absence of an external fundamental system (EFS) and a homogenous distribution of the bone matrix and vascularisation suggest that both specimens did not attain sexual and somatic maturity. Overall, the histological features of T. romeri indicate a rapid growth rate just like other early crocodylomorphs (Terrestrisuchus, Saltoposuchus), whereas others possess a rather moderate (Crocodylomorpha indet. BP/1/8484 specimen) or slower (Hesperosuchus) growth rate. These findings reveal that rapid growth rates were widely present among early crocodylomorphs and their early occurrence in the evolutionary history of Crocodylomorpha might suggest that it is the plesiomorphic condition.

Life history and growth dynamics of a peirosaurid crocodylomorph (Mesoeucrocodylia; Notosuchia) from the Late Cretaceous of Argentina inferred from its bone histology

Notosuchia were a successful lineage of Crocodyliformes that achieved a remarkable diversity during the Cretaceous of Gondwana, particularly in South America. Although paleohistology has expanded our knowledge of the paleobiology of notosuchians, several clades of this lineage remain poorly understood in this aspect. Here we help to address this gap by conducting the first histological analysis of appendicular bones of a peirosaurid. To increase our knowledge about growth dynamics and examine intraeskeletal and interspecific histological variation, we analyze the microstructure of a tibia, fibula, phalanx, fragment of ornamented element (possible osteoderm or skull bone) and a possible long bone of an individual assigned to Peirosauridae indet. (MAU-Pv-437). The peirosaurid studied here appears to have reached sexual but not somatic maturity and the minimum age inferred from appendicular bones results in a lower estimated than the age inferred from osteoderms in a previous study on the same individual. The cortical bone in MAU-Pv 437 is formed by vascularized parallel fibered bone/lamellar bone which indicates that this individual experienced a moderate growth rate. This indicates different growth dynamics from what has been observed for other notosuchians specimens, suggesting a lack of a uniform growth pattern for this clade.

Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries

Dinosauria debuted on Earth's stage in the aftermath of the Permo-Triassic Mass Extinction Event, and survived two other Triassic extinction intervals to eventually dominate terrestrial ecosystems. More than 231 million years ago, in the Upper Triassic Ischigualasto Formation of west-central Argentina, dinosaurs were just getting warmed up. At this time, dinosaurs represented a minor fraction of ecosystem diversity. Members of other tetrapod clades, including synapsids and pseudosuchians, shared convergently evolved features related to locomotion, feeding, respiration, and metabolism and could have risen to later dominance. However, it was Dinosauria that radiated in the later Mesozoic most significantly in terms of body size, diversity, and global distribution. Elevated growth rates are one of the adaptations that set later Mesozoic dinosaurs apart, particularly from their contemporary crocodilian and mammalian compatriots. When did the elevated growth rates of dinosaurs first evolve? How did the growth strategies of the earliest known dinosaurs compare with those of other tetrapods in their ecosystems? We studied femoral bone histology of an array of early dinosaurs alongside that of non-dinosaurian contemporaries from the Ischigualasto Formation in order to test whether the oldest known dinosaurs exhibited novel growth strategies. Our results indicate that the Ischigualasto vertebrate fauna collectively exhibits relatively high growth rates. Dinosaurs are among the fastest growing taxa in the sample, but they occupied this niche alongside crocodylomorphs, archosauriformes, and large-bodied pseudosuchians. Interestingly, these dinosaurs grew at least as quickly, but more continuously than sauropodomorph and theropod dinosaurs of the later Mesozoic. These data suggest that, while elevated growth rates were ancestral for Dinosauria and likely played a significant role in dinosaurs' ascent within Mesozoic ecosystems, they did not set them apart from their contemporaries.

Bone histology supports gregarious behavior and an early ontogenetic stage to Decuriasuchus quartacolonia (Pseudosuchia: Loricata) from the Middle-Late Triassic of Brazil

Decuriasuchus quartacolonia is a middle-sized basal "rauisuchian" (Pseudosuchia, Loricata) from the Triassic beds of Brazil, whose original description was based on 10 specimens of equivalent size found in aggregation. In this contribution, we explore the osteohistology of its appendicular bones and a rib, aiming to infer growth patterns and ontogeny. In all analyzed bones (except in the rib) from three individuals, we found fibrolamellar bone (FLB) throughout the cortices, a pattern shared with other histologically sampled basal loricatans. We found evidence that suggests an early ontogenetic stage for all studied specimens: absence of secondary osteons, lack of transition from an inner highly vascularized FLB matrix to a poor vascularized parallel-fibred bone, one line of arrested growth, open vascular canals in the external surface of the cortex and absence of an external fundamental system. In addition, we observed that the neurocentral sutures in the caudal to cervical vertebrae of the holotype are unfused, strongly suggesting that these individuals were not skeletally mature, as had been previously assumed. In addition, our data support the prior hypothesis that Decuriasuchus is the oldest-known archosaur to show evidence of gregarious behavior and adds a probable cause of this as a strategy to obtain food and avoid predation until becoming independent as adults. Furthermore, our results open the possibility that Decuriasuchus may represent an earlier growth stage of the larger Prestosuchus chiniquensis, with two specimens recovered from the same stratigraphic level and paleontological site. Since the adult form of Decuriasuchus remains unknown, a deeper anatomical study is needed to discuss the validity of the species. This study case exemplifies the importance of paleohistology as an essential complementary tool to describe a new morphospecies in vertebrate paleontology.

Paleoneurology and neuroanatomical notes on the South American "rauisuchian" Saurosuchus galilei from the Ischigualasto formation, San Juan, Argentina

Non-crocodylomorph loricatans, traditionally known as "rauisuchians," are considered as the top predators of the Triassic continental faunas that reigned before the emergence of the well-known theropod dinosaurs. In particular, Saurosuchus galilei is a large quadrupedal prestosuchid loricatan found in the Ischigualasto Formation from northwestern Argentina. Here, we reevaluated the braincase of S. galilei and present the first paleoneurological study based on the partial natural casts of the holotype and the digital cranial endocast of the referred specimen PVSJ 32. The braincase of S. galilei was here reinterpreted, identifying the sutures of the supraoccipital, otoccipital, basioccipital, parabasisphenoid, prootic, and laterosphenoid. A unique feature identified in the braincase of S. galilei is the presence of deep paracondylar recesses associated with pharyngotympanic pneumaticity, which has not been identified in any other "rauisuchians" so far. Most of the structures of the encephalon were recognized in the cranial endocast and natural casts including cranial nerves V-XII, olfactory bulbs, main blood vessels and sinuses, and inner ear. These structures allowed us to quantify some of its sensorial capacities and recognize that S. galilei had an enhanced olfactory acuity, with coefficients higher than those expected for its body size, a condition previously observed in living crocodilians and tyrannosaurid dinosaurs. An improved sense of smell might have allowed Saurosuchus to track its prey from long distances and compensate for the poorer development of its other senses like vision and balance.

Postcranial anatomy of Prestosuchus chiniquensis (Archosauria: Loricata) from the Triassic of Brazil

Prestosuchus chiniquensis is the best represented pseudosuchian archosaur from the Pinheiros-Chiniquá Sequence, Middle-Late Triassic (Ladinian/Carnian) of the Santa Maria Supersequence, Southern Brazil. Several incomplete specimens attributed to this species have been described, but the morphology of the postcranial skeleton of P. chiniquensis is poorly known. In this contribution we present the postcranial material of the UFRGS-PV-0629-T specimen, concluding its description, as its skull and endocast have already been described. Additionally, histological data provided new information on the poorly known ontogenetic series of P. chiniquensis, and on its growth patterns suggesting a longer period of slow growth when compared to other basal Loricata species. A phylogenetic analysis placed UFRGS-PV-0629-T in a group composed by the lectotype, paralectotype, and other described P. chiniquensis specimens, further corroborating our taxonomic hypothesis, that specimens of basal Loricata collected in Brazil are closely related to each other. Due to the association of characters found in the phylogenetic analysis, the specimen UFRGS-PV-0629-T is attributed as the most complete material ever found for P. chiniquensis. As such, it is clear that the material presented here provides important new information on P. chiniquensis. Based on the results presented here, we revised the diagnosis for P. chiniquensis. However, it also evidences the need for new discoveries and studies of other specimens seeking to understand this and other closely related species, which were important components of worldwide trophic webs of the Triassic biotas.