Osteohistology and growth dynamics of the Brazilian noasaurid Vespersaurus paranaensis Langer et al., 2019 (Theropoda: Abelisauroidea)

Osteohistology of the unusually fast-growing theropod dinosaur Ceratosaurus

Ceratosaurus is a large-bodied non-avian theropod dinosaur known from the Upper Jurassic Morrison Formation of North America and is remarkable both for its exceptionally fast annual growth rate and its status as the only theropod currently known with postcranial osteoderms. We describe the osteohistology of three hind limb bones, two dorsal ribs, and one osteoderm representing four individuals of Ceratosaurus. In addition to describing the tissues of these bones, we compared the annual growth rates from three individuals in our sample to those of five other ceratosaurians. We fit seven growth models to two of the specimens in our sample and compared the results of the best-fit model(s) to those of two other ceratosaurians (Masiakasaurus knopfleri and Majungasaurus crenatissimus) for which sufficient growth data were available. The bone tissue of hind limbs in Ceratosaurus is highly vascularized, with dense plexiform or reticular vascular complexes and alternating strips of parallel or woven-fibered matrix. Few lines of arrested growth were recorded in hind limbs prior to specimens achieving asymptotic body size. Both sampled dorsal ribs are highly remodeled, with only small portions of primary bone visible in each section, revealing parallel-fibered bone with sparse primary osteons. Both dorsal ribs contain numerous lines of arrested growth throughout the cortex that allowed for more accurate estimates of individual age when paired with the data from hind limbs. The osteoderm is composed of a core of large Haversian canals and a perimeter of lamellar bone with dense Sharpey's fibers along the internal surface of the bone. Multiple LAGs are also present within the lamellar bone along the exterior margins. Maximum annual growth rates in Ceratosaurus were on average nine-fold faster than those of other ceratosaurians. Our sample lacks data from juveniles so confidence in inferred growth models is limited. Thus, to begin to constrain Ceratosaurus growth patterns, we averaged the results of all models that possessed an Akaike Information Criterion score corrected for small sample size (AICc) within 10 of the lowest scoring model. We found that the monomolecular model exhibited the lowest AICc value, with the von Bertalanffy and Gompertz models possessing AICc values within 10 units of it. In contrast, the logistic and Gompertz models were confidently selected for Masiakasaurus and Majungasaurus, respectively. Irrespective of growth model, maximum relative annual growth rates for Ceratosaurus were several-fold greater than those of Masiakasaurus and Majungasaurus. Both histological and growth model estimates of life history support an evolutionary trend towards more prolonged development in Ceratosauria through evolutionary time.

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.

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.

Developmental strategies underlying gigantism and miniaturization in non-avialan theropod dinosaurs

In amniotes, the predominant developmental strategy underlying body size evolution is thought to be adjustments to the rate of growth rather than its duration. However, most theoretical and experimental studies supporting this axiom focus on pairwise comparisons and/or lack an explicit phylogenetic framework. We present the first large-scale phylogenetic comparative analysis examining developmental strategies underlying the evolution of body size, focusing on non-avialan theropod dinosaurs. We reconstruct ancestral states of growth rate and body mass in a taxonomically rich dataset, finding that contrary to expectations, changes in the rate and duration of growth played nearly equal roles in the evolution of the vast body size disparity present in non-avialan theropods-and perhaps that of amniotes in general.

The first edentulous ceratosaur from South America

The recognition of ontogenetic edentulism in the Jurassic noasaurid Limusaurus inextricabilis shed new light on the dietary diversity within Ceratosauria, a stem lineage of non-avian theropod dinosaurs known for peculiar craniomandibular adaptations. Until now, edentulism in Ceratosauria was exclusive to adult individuals of Limusaurus. Here, an exceptionally complete skeleton of a new toothless ceratosaur, Berthasaura leopoldinae gen. et sp. nov., is described from the Cretaceous aeolian sandstones of the Bauru Basin, Southern Brazil. The specimen resembles adult individuals of Limusaurus by the absence of teeth but based on the unfused condition of several elements (e.g., skull, vertebral column) it clearly represents an ontogenetically immature individual, indicating that it might never have had teeth. The phylogenetic analysis performed here has nested Berthasaura leopoldinae as an early-divergent Noasauridae, not closely related to Limusaurus. It represents the most complete non-avian theropod from the Brazilian Cretaceous and preserves the most complete noasaurid axial series known so far. Moreover, the new taxon exhibits many novel osteological features, uncommon in non-avian theropods, and unprecedented even among South American ceratosaurs. These include not only toothless jaws but also a premaxilla with cutting occlusal edge, and a slightly downturned rostral tip. This indicate that B. leopoldinae unlikely had the same diet as other ceratosaurs, most being regarded as carnivorous. As the ontogenetically more mature specimens of Limusaurus, Berthasaura might have been herbivorous or at least omnivorous, corroborating with an early evolutionary divergence of noasaurids from the ceratosaurian bauplan by disparate feeding modes.