Ignacio Alejandro Cerda

The paleobiology of sauropodomorph dinosaurs has historically garnered considerable interest, primarily because this clade includes the largest terrestrial vertebrates. Bone and tooth microstructures are crucial sources of paleobiological data, and many paleohistological studies have focused on this group. Here we review and synthesize the main advances in an understanding of sauropodomorph paleobiology derived from osteological and dental histology. We explore a wide range of topics, including age assessment, ontogenetic stages, growth rates, growth dynamics and life histories, skeletal element formation, soft tissue reconstructions, paleopathologies, paleocytology, and tooth implantation and replacement rates. We also provide a comprehensive bibliographic review of the published literature detailing sauropodomorph bone and/or tooth histology.

The sauropod dinosaurs were the largest terrestrial animals to have inhabited the Earth (Carballido et al., 2017; Upchurch et al., 2004; Wilson, 2002), with an adult body mass exceeding 50 metric t...

The Pseudosuchia (Diapsida, Archosauria) arose during the Early Triassic and were the dominant large to medium‐sized tetrapods in continental ecosystems until the Triassic–Jurassic extinction. Despite this important palaeoecological role, the group remains poorly examined. Here, we report several palaeobiological, palaeoecological and phylogenetic implications based on the bone histology of appendicular elements of seven species of non‐crocodylomorph Pseudosuchia from South America, and provide a review of the current state of the osteohistological research on this group. Saurosuchus galilei (Paracrocodylomorpha, Loricata) and Sillosuchus longicervix (Paracrocodylomorpha, Poposauroidea) exhibit fibrolamellar complexes, showing extremely rapid growth. Similarly, Aetobarbakinoides brasiliensis (Aetosauria) and Gracilisuchus stipanicicorum (Gracilisuchidae) display woven‐fibred bone, indicative of fast growth. However, Aetosaourides scagliai (Aetosauria) and Tarjadia ruthae (Erpetosuchidae) developed both woven‐fibred and parallel fibred bone, which implies an intermediate or moderate growth rate. Finally, Riojasuchus tenuisceps (Ornithosuchidae) records parallel‐fibred bone mostly, marking a slower growth rate. Nevertheless, when compared with other pseudosuchians worldwide, this diversity in growth rate does not indicate a reduction in growth rate within the lineage from stem towards crown Pseudosuchia (i.e. a phylogenetic signal). Neither does it correspond with estimated global palaeoclimatic patterns but instead seems to respond to more local environmental influences (e.g. seasonality) and to intrinsic physiological issues. In general, the diversity seen in the growth dynamics of the Triassic non‐crocodylomorph Pseudosuchia from South America is similar to that reported for North America and Europe.

Alvarezsauroids are an enigmatic clade of predominantly small-bodied theropod dinosaurs that are known mainly from the Jurassic to Cretaceous periods of Asia and South America^1,2,3. Late Cretaceous alvarezsauroids possess specialized forelimbs adapted for digging^4,5, minute supernumerary teeth and heightened sensory capacities⁶, and are interpreted as myrmecophagous. They are hypothesized to exhibit evolutionary miniaturization coupled to their dietary specialization². Fragmentary South American taxa are traditionally arrayed as a paraphyletic grade with respect to the Late Cretaceous Asian subclade Parvicursorinae^2,3, invoking dispersal to explain their disjunct distributions. Here we describe a skeleton of the alvarezsauroid Alnashetri cerropoliciensis⁷ representing to our knowledge the most complete and smallest South American taxon to date. We also recognize two alvarezsauroids among historic taxa from the Northern Hemisphere. Phylogenetic analysis recovers Alnashetri among basal non-alvarezsaurids, rendering South American taxa polyphyletic. Combined with the new taxa recognized here, our biogeographical analyses infer a Pangaean ancestral distribution for Alvarezsauroidea, with vicariance dominating the early history of the clade. The early branching position of Alnashetri among larger-bodied relatives revises best-fit models of body size evolution in alvarezsauroids—we find no support for evolutionary miniaturization but, rather, find support for repeated evolution within a narrow body size range.

Manidens condorensis , a small‐bodied heterodontosaurid from the Lower Jurassic Cañadón Asfalto Formation of Patagonia, represents the most complete ornithischian dinosaur currently known from South America for this interval. This study presents the first detailed description of its postcranial skeleton, based on high‐resolution micro‐computed tomography and 3D reconstructions of the holotype specimen (MPEF‐PV 3211). We describe the first recorded proatlas in a heterodontosaurid, together with the description of vertebrae from all regions, pectoral and pelvic girdles, and preserved ribs and tendons, which offer new comparative insights into early ornithischians. Histological sections from thoracic ribs indicate a subadult ontogenetic stage and provide information on growth dynamics and tissue structure. Taphonomic analysis of the holotype, informed by disarticulation patterns and bone orientations, indicates low‐energy depositional conditions and a left‐side‐down post‐mortem resting position. Updated anatomical data were incorporated into both comprehensive and targeted phylogenetic analyses, with an emphasis on heterodontosaurid relationships. Results support the placement of Manidens as a transitional form between more basal Laurasian and more derived Gondwanan heterodontosaurids, combining plesiomorphic and specialized features, identify novel postcranial autapomorphies for Manidens , and highlight increased similarities with more complete taxa. These findings strengthen its role as a key taxon for understanding early ornithischian diversification and the evolution of herbivorous adaptations. The integration of osteology, taphonomy, histology and phylogenetics in this study significantly expands our knowledge of Manidens and its evolutionary context within Heterodontosauridae.

The early diversification of dinosaurs produced a major ecological change in the terrestrial ecosystems, culminating with tetrapod assemblages dominated in abundance by dinosaurs by the Triassic/Jurassic boundary (~201 million years ago (Ma)). Therefore, studying the initial diversification of dinosaurs is crucial to understand the establishment of Mesozoic assemblages. However, the lack of stratigraphically continuous fossil data in the few geological units that preserve the oldest known dinosaurs (~233–227 Ma, Carnian age) obscures our understanding of this initial diversification. The Ischigualasto Formation in northwestern Argentina (231.4–225.9 Ma) yields a rich vertebrate assemblage and new studies resulted in an abundant and stratigraphically near-continuous fossil record, which offers new insights into the early diversification of dinosaurs. Among the discoveries, we report Anteavis crurilongus gen. et sp. nov., an early-diverging theropod, which supports the notable diversity of small- to medium-sized dinosaurs during the late Carnian. Anteavis is recovered outside Neotheropoda, but it has features previously thought to be exclusive to that group. We show that dinosaur diversity and abundance in the Ischigualasto Formation were higher than previously recognized, particularly among small herbivores (<30 kg) and medium-sized (30–200 kg) predators. This diversification occurred in Ischigualasto during a climatic shift to semi-arid conditions, but the return of more humid conditions resulted in a gap in the dinosaur record that started at 228.91 ± 0.14 Ma. Only 15 million years (Myr) later, in the middle Norian age, the dinosaur record recovered its abundance and diversity in the basin, but now it was characterized by larger-bodied species. Our findings demonstrate an early dinosaur diversification probably punctuated by a climate-driven faunal turnover in, at least, southwestern Pangaea.

Recent fossil discoveries have cast considerable light on the palaeobiology of Megaraptora, a group of large-clawed carnivorous theropod dinosaurs known from Cretaceous deposits in Asia, Australia, and especially South America. Nevertheless, many important aspects of megaraptoran morphology and evolution remain poorly understood, due in large part to the fragmentary nature of most fossils of these theropods and the scarcity of anatomically overlapping skeletal elements among the known taxa. Here we report a previously unknown megaraptoran genus and species represented by a partially articulated partial skeleton recovered from an uppermost Cretaceous stratum of the Lago Colhué Huapi Formation of south-central Chubut Province in central Patagonia, Argentina. Pertaining to the derived megaraptoran subclade Megaraptoridae, the taxon is among the most completely represented and latest-surviving megaraptorans. Its stratigraphic occurrence indicates that these dinosaurs likely persisted to the Cretaceous/Palaeogene boundary; moreover, the preservation of a crocodyliform humerus between the dentaries of the new theropod may provide information on megaraptoran dietary preferences and feeding strategies. Megaraptorids appear to have been the apex predators in central and southern Patagonian palaeoecosystems approaching the end of the Cretaceous, in contrast to more northerly areas of South America where these niches were occupied by other non-avian theropod groups.

In contrast to other cartilaginous fishes (Chondrichthyes), the dentition of the Holocephali consists of six dental plates, four in the upper jaw (two vomerines and two palatines) and two in the lower jaw (mandibulars). While the microstructure of holocephalan dentition has been extensively studied in extant species, similar analyses in fossil taxa are less common. Here, we provide a detailed description and interpretation of the microanatomy and histology of dental plates of the extinct taxon †Ischyodus dolloi Leriche (Chimaeriformes: ‘Edaphodontidae’). The study is based on two palatine and two mandibular plates from the Eocene of the Antarctic Peninsula. Our main objectives were to (1) identify the presence and distribution of different tissues; (2) analyze morphological diversity in hypermineralized structures (i.e., tritors) within the dental plates; (3) compare the microanatomy and histology of †I. dolloi with other living and fossil holocephalans; and (4) infer the sequence and mode of tissue formation within the plates. The dental plates are composed of an outer shell enclosing a framework of trabecular dentine, in which hypermineralized dentine (i.e., whitlockin) is deposited to form the tritors. Trabecular dentine becomes compacted by centripetal deposition of circumvascular dentine around vascular canals. The tritors exhibit two morphological types: tritoral rods (first report for ‘Edaphodontidae’) and tritoral pads. Additionally, a distinct tissue, which we term ‘parallelitubular dentine’, is formed in the aboral and symphyseal regions. Regarding the growth of dental plates, we identified a developmental pattern in which the full formation of tritors and complete compaction of trabecular dentine follows a mesial‐to‐lingual, oral‐to‐aboral, and labial‐to‐symphyseal sequence. This growth pattern aligns with the spatial order in which the dentinal tissues are progressively removed due to feeding‐related wear.