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

Abstract

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.

Skull osteology of Aetosauroides scagliai Casamiquela, 1960 (Archosauria: Aetosauria) from the Late Triassic of Brazil: New insights into the paleobiology of aetosaurs

“…However, the latter species was recently reinterpreted as a junior synonym of A. scagliai, with apparent differences being attributable to the immature state of the holotype (Paes Neto et al, 2021b). In phylogenetic analyses, A. scagliai has been recovered as either the sister-taxon of Stagonolepis robertsoni Agassiz, 1844(e.g., Parrish, 1994Heckert et al, 1996;Heckert and Lucas, 1999;Harris et al, 2003;Parker, 2007) or as the earliest-diverging aetosaur (i.e., the only non-stagonolepidid aetosaur; e.g., Desojo et al, 2012;Heckert et al, 2015;Schoch and Desojo, 2016;Parker, 2016a;Brust et al, 2018;Hoffman et al, 2018;Reyes et al, 2021). Recent research on A. scagliai has provided a better understanding of its intraspecific variation Taborda et al, 2013;Taborda et al, 2015;Paes Neto et al, 2021b) and cranial osteology (Brust et al, 2018;Paes Neto et al, 2021a).…”

“…In phylogenetic analyses, A. scagliai has been recovered as either the sister-taxon of Stagonolepis robertsoni Agassiz, 1844(e.g., Parrish, 1994Heckert et al, 1996;Heckert and Lucas, 1999;Harris et al, 2003;Parker, 2007) or as the earliest-diverging aetosaur (i.e., the only non-stagonolepidid aetosaur; e.g., Desojo et al, 2012;Heckert et al, 2015;Schoch and Desojo, 2016;Parker, 2016a;Brust et al, 2018;Hoffman et al, 2018;Reyes et al, 2021). Recent research on A. scagliai has provided a better understanding of its intraspecific variation Taborda et al, 2013;Taborda et al, 2015;Paes Neto et al, 2021b) and cranial osteology (Brust et al, 2018;Paes Neto et al, 2021a). These and other studies (e.g., Desojo, 2005) have firmly established the Gondwanan A. scagliai as a valid taxon, distinct from the Laurasian S. robertsoni and Calyptosuchus wellesi Ballew, 1985 (contra Heckert andLucas, 2000;Lucas and Heckert, 2001;Heckert and Lucas, 2002), a status recognized in most recent work on the group (e.g., Desojo et al, 2012Desojo et al, , 2013Robertoda-Silva et al, 2014;Heckert et al, 2015;Parker, 2016a,b;Schoch and Desojo, 2016;Ezcurra et al, 2017;Brust et al, 2018;Hoffman et al, 2018;Parker, 2018a,b;Desojo et al, 2020a,b;Marsh et al, 2020;…”

“…Recent research on A. scagliai has provided a better understanding of its intraspecific variation Taborda et al, 2013;Taborda et al, 2015;Paes Neto et al, 2021b) and cranial osteology (Brust et al, 2018;Paes Neto et al, 2021a). These and other studies (e.g., Desojo, 2005) have firmly established the Gondwanan A. scagliai as a valid taxon, distinct from the Laurasian S. robertsoni and Calyptosuchus wellesi Ballew, 1985 (contra Heckert andLucas, 2000;Lucas and Heckert, 2001;Heckert and Lucas, 2002), a status recognized in most recent work on the group (e.g., Desojo et al, 2012Desojo et al, , 2013Robertoda-Silva et al, 2014;Heckert et al, 2015;Parker, 2016a,b;Schoch and Desojo, 2016;Ezcurra et al, 2017;Brust et al, 2018;Hoffman et al, 2018;Parker, 2018a,b;Desojo et al, 2020a,b;Marsh et al, 2020;Czepiński et al, 2021;Reyes et al, 2021).…”

“…2021a Aetosauroides scagliai: Heckert and Lucas, 2002;Taborda et al, 2015) and by the low number (5) of lines of arrested growth in osteoderm sections when compared with other individuals (e.g., Cerda and Desojo, 2010;Taborda et al, 2013Taborda et al, , 2015Cerda et al, 2018). Diagnosis (adapted from Brust et al, 2018). Medium-sized aetosaur (up to 2.45 m in length) distinguished from other aetosaurs by the following combination of characters (autapomorphies with asterisk): maxilla excluded from the margin of the external nares; ventral margin of the dentary convex and without a sharp inflexion (shared with Typothorax); dorsal margin of the surangular with the presence of a rounded tuber (shared with Stenomyti, Stagonolepis, and Longosuchus); recurved tooth crowns (shared with Coahomasuchus) with denticles on both mesial and distal margins, without wear facets or marked constriction between root and crown; cervical and trunk centra with oval fossae ventral to the neurocentral suture on the lateral sides of the centra; mid-and posterior trunk vertebrae with well-developed posterior infradiapophyseal lamina, directly below the diapophysis (shared with Stagonolepis, Lucasuchus, Longosuchus, Desmatosuchus, and Typothorax); mid-and posterior dorsal vertebrae with posterolaterally divergent postzygapophyses, ratio between the entire length of the postzygapophyses and the width between the distal-most tips of the postzygapophyses less than or equal to 0.75*; anterior tip of premaxilla slightly expanded laterally (incipiently forming a shovel-shaped tip).…”

“…Referred materials. Five referred specimens have been reported from the lower levels of the Ischigualasto Formation, San Juan, Argentina: (1) PVL 2052, a large specimen with much of the posterior portion of the postcranial skeleton well preserved, but also some skull elements preserved as natural casts (Casamiquela, 1967;Heckert and Lucas, 2002;Desojo, 2005;Parker, 2014;Brust et al, 2018); (2) PVL 2059, a small specimen with a partially preserved skull, with the anterior portion of the carapace preserved in articulation with the corresponding portion of the axial skeleton (Casamiquela, 1960(Casamiquela, , 1961Heckert and Lucas, 2002;Desojo, 2005;Taborda et al, 2013Taborda et al, , 2015Parker, 2014;Brust et al, 2018); (3) PVSJ 326, a large specimen with a partially preserved skull, isolated trunk and caudal vertebrae, several appendicular elements, and dorsal paramedian and lateral osteoderms (Desojo, 2005;Parker, 2014Parker, , 2016aEzcurra, 2016;Parker, 2016a); (4) PVL 2091, a large specimen with poorly preserved postcranial elements including cervical vertebrae, humerus, and several osteoderms (holotype of 'Argentinosuchus bonapartei'; see Ezcurra, 2016); (5) CRILAR-Pv 580, a small specimen preserving fragmentary teeth, vertebrae, limb elements, and dorsal osteoderms (Desojo et al, 2020b).…”