Replacement tooth in mesosaurs and new data on dental microanatomy and microstructure.

The Permian mesosaurs are well known for being the earliest amniotes to exhibit adaptations for living in a marine environment (Irati-Whitehill Sea). In addition to their set of skeletal features associated with aquatic dwelling life, their dentition includes important characteristics related to feeding in this habitat, which is described in this work, based on the analysis of mesosaur specimens from the Lower Permian Irati Formation of Brazil. Mesosaurs have several slender, conical teeth bordered by enamel apicobasal ridges, a feature predominantly found in aquatic amniotes. Internally, the dentine walls are formed by the arrangement of layers of orthodentine and globular dentine. To prevent tooth loss, the basal area is equipped with plicidentine, a particular type of orthodentine, allied with cementum, alveolar bone trabeculae, and periodontal space that reinforces anchorage and provides some flexibility. The teeth are replaced in a labio-vertical path, and the dentition replaces alternately. This feature is regarded as plesiomorphic, and it ensures the oral cavity is supplied with enough teeth. However, these features do not the assessment of whether mesosaurs teeth were capable of piercing prey with resistant tegument. Instead, we interpret this adaptation as a mechanism for catching prey, at least in adults, and we endorse a possible ontogenetic dietary shift from small to large forms.

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.

Weighing in on miniaturization: New body mass estimates for Triassic eucynodonts and analyses of body size evolution during the cynodont-mammal transition.

Body size influences most aspects of an animal's biology, consequently, evolutionary diversification is often accompanied by differentiation of body sizes within a lineage. It is accepted that miniaturization, or the evolution of extremely small body sizes, played a key role in the origin and early evolution of different mammalian characters in non-mammaliaform cynodonts. However, while there are multiple studies on the biomechanical, behavioral, and physiological consequences of smaller sizes, few explore the evolutionary processes that lead to them. Here, we use body mass as a universal size measurement in phylogenetic comparative analyses to explore aspects of body size evolution in Cynodontia, focusing on the cynodont-mammal transition, and test the miniaturization hypothesis for the origin of Mammaliaformes. We estimated the body masses of 29 species, ranging from Theriocephalia to Mammaliaformes, providing the largest collection of Triassic cynodont body mass estimates that we know of, and used these estimates in analyses of disparity through time and RRphylo . Unexpectedly, our results did not support the miniaturization hypothesis. Even though cynodont body size disparity fell during the Late Triassic, and remained lower than expected under a purely Brownian motion model of evolution up until the Early Jurassic, we found that rates of body size evolution were significantly lower in prozostrodontians leading to the first Mammaliaformes than in other lineages. Evolution rates were higher in medium and large-sized taxa, indicating that size was changing more rapidly in those lineages and that small sizes were probably a persistent plesiomorphic character-state in Cynodontia.

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.

Old fossil findings in the Upper Triassic rocks of southern Brazil improve diversity of traversodontid cynodonts (Therapsida, Cynodontia).

In this contribution we describe a new genus and species of gomphodontosuchine cynodont from the Late Triassic Hyperodapedon Assemblage Zone (AZ) of the Santa Maria Supersequence of southern Brazil, based on material collected 20 years ago. The new taxon, Santagnathus mariensis gen. et sp. nov., is based on numerous cranial and postcranial remains, which altogether provide data on several parts of the skeleton. Santagnathus mariensis is closely related to Siriusgnathus niemeyerorum and Exaeretodon spp. and expands the knowledge about gomphodontosuchine cynodonts. Morphologically, the new species has a skull bauplan particularly similar to S. niemeyerorum and E. riograndensis, differing from them by a unique combination of traits such as three upper incisors, the absence of a descending process in the jugal, a more posterior position of the postorbital bar and a preorbital region larger than the temporal one. The new traversodontid was found associated with the rhynchosaur Hyperodapedon sp., corroborating the assignment of the cynodont fossils to the Hyperodapedon AZ. We also provide comments on the status of the Argentine traversodontid cynodont Proexaeretodon vincei, typically considered a junior synonym of Exaeretodon argentinus, and here accepted as a valid taxon.

Reassessment of Faxinalipterus minimus, a purported Triassic pterosaur from southern Brazil with the description of a new taxon.

Faxinalipterus minimus was originally described as a purported pterosaur from the Late Triassic (early Norian) Caturrita Formation of southern Brazil. Its holotype comprises fragmentary postcranial elements, whereas a partial maxilla was referred to the species. The assignment of Faxinalipterus minimus to Pterosauria has been questioned by some studies, but the specimen has never been accessed in detail after its original description. Here we provide a reassessment of Faxinalipterus minimus after additional mechanical preparation of the holotype. Our interpretations on the identity of several bones differ from those of the original description, and we found no support favoring pterosaur affinities for the taxon. The maxilla previously referred to Faxinalipterus minimus is disassociated from this taxon and referred to a new putative pterosauromorph described here from a partial skull and fragmentary postcranial elements. Maehary bonapartei gen. et sp. nov. comes from the same fossiliferous site that yielded Faxinalipterus minimus, but the lack of overlapping bones hampers comparisons between the two taxa. Our phylogenetic analysis places Faxinalipterus minimus within Lagerpetidae and Maehary bonapartei gen. et sp. nov. as the earliest-diverging member of Pterosauromorpha. Furthermore, the peculiar morphology of the new taxon reveals a new dental morphotype for archosaurs, characterized by conical, unserrated crowns, with a pair of apicobasally oriented grooves. These two enigmatic archosaurs expand our knowledge on the Caturrita Formation fauna and reinforce the importance of its beds on the understanding of Late Triassic ecosystems.

Intraspecific variation in the axial skeleton of Aetosauroides scagliai (Archosauria: Aetosauria) and its implications for the aetosaur diversity of the Late Triassic of Brazil.

Aetosauria represents a remarkable clade of armored pseudosuchians in which some of its oldest members are recovered from late Carnian units of Brazil. Three species are known: the mid-sized aetosaur Aetosauroides scagliai, which also occurs in Argentina, and two small-sized species, Aetobarbakinoides brasiliensis and Polesinesuchus aurelioi. We provide a detailed description and comparative analysis of the axial skeleton of Aetosauroides, identifying some diagnostic features as variable. These include the deep pocket pit lateral to the base of the neural spine, the presence of the infradiapophyseal laminae and the lateral fossa ventral to the neurocentral suture. These features are not found in smaller and immature Aetosauroides specimens, resembling the condition found in Polesinesuchus, which is based solely on a juvenile individual, as revealed by osteoderm microstructure analysis. As Polesinesuchus cannot be anatomically differentiated from other small individuals of Aetosauroides, we propose it as a junior synonym of Aetosauroides scagliai. Our results shrink the number of putative 'dwarf' aetosaurs, indicating that morphological variation related to ontogeny affects aetosaur taxonomy and phylogeny.

Osteology of the first skull of Aetosauroides scagliai Casamiquela 1960 (Archosauria: Aetosauria) from the Upper Triassic of southern Brazil (Hyperodapedon Assemblage Zone) and its phylogenetic importance.

Aetosauria, which includes 30 species, is a diverse group of armored pseudosuchian archosaurs restricted to Upper Triassic beds. Three species occur in Brazil, and one of these, Aetosauroides scagliai Casamiquela, 1960, also occurs in Argentina. The specimen UFSM 11505, found at Faixa Nova-Cerrito I Outcrop, Santa Maria Formation (Hyperodapedon Assemblage Zone), Santa Maria, Rio Grande do Sul State, Brazil, is here referred to as Aetosauroides scagliai. This specimen preserves most of the skull with both hemimandibles in association with most of the postcranium, thus representing one of the most complete aetosaur skeletons found in Brazil. The premaxilla, one of the key elements of the cranial morphology of aetosaurs, along with the posterior portion of the mandible, was not described until now for A. scagliai. In contrast to the typothoracinae aetosaurs, the premaxilla of UFSM 11505 presents a shovel-shaped tip, but it is not as prominent as the lateral expansion of desmatosuchian aetosaurs, including both species of Stagonolepis, S. robertsoni Agassiz, 1844 and S. olenkae Sulej, 2010. The retroarticular process of the mandible is elongate and not tall, as in Stenomity huangae Small & Martz, 2013 and other typothoracinae aetosaurs. Unlike previous descriptions of A. scagliai, the maxillary teeth are recurved ziphodont-like with serrations on the entire length of both margins. Premaxillary teeth are also present, being less recurved than the maxillary teeth and cylindrical. We recovered Aetosauroides scagliai as the most basal taxon within Aetosauria, like previous phylogenetic analyses. Furthermore, our analyses reinforce that recurved and unconstricted maxillary teeth, the shovel-shaped premaxilla and the presence of a tuber on the surangular are plesiomorphic features of Aetosauria.