A new aetosaur from the Upper Triassic of the Santa Maria Formation, southern Brazil

Abstract

Aetosaurs are armored pseudosuchian archosaurs widespread in Upper Triassic units. In South America, four taxa were previously recorded: Aetosauroides scagliai, Neoaetosauroides engaeus, Aetobarbakinoides brasiliensis, and Chilenosuchus forttae. Herein we describe a new Late Triassic juvenile aetosaur from the Santa Maria Formation of southern Brazil, Polesinesuchus aurelioi gen. et sp. nov., increasing the paleobiodiversity of this interesting group to five taxa in Western Gondwana. The holotype is composed of cranial (parietal and braincase) and postcranial elements (cervical, dorsal, sacral, caudal vertebrae, both scapulae, a humerus, ilium, pubis, ischium, tibia, a partial right pes, and anterior and mid-dorsal paramedian osteoderms). It belongs to a juvenile individual, as its neurocentral sutures are open in all vertebrae, and also due to its small size. However, future paleohistological investigation is necessary to fully corroborate this assumption. This new taxon is distinguished from all other aetosaurs by the presence of an unique combination of character states (not controlled by ontogeny) such as: cervical vertebrae with prezygapophyses widely extending laterally through most of the anterior edge of the diapophyses; absence of hyposphene articulations in both cervical and mid-dorsal vertebrae; presence of a ventral keel in cervical vertebrae; anterior and mid-dorsal vertebrae without a lateral fossa in their centra; expanded proximal end of scapula; anteroposteriorly expanded medial portion of scapular blade; a short humerus with a robust shaft; and a dorsoventral and very low iliac blade with a long anterior process which slightly exceeds the pubic peduncle. Regarding its phylogenetic relationships, the present analysis placed Polesinesuchus as the sister taxon of Aetobarbakinoides and both as sister taxa of the unnamed monophyletic clade Desmatosuchinae plus Typothoracisinae.

Early evolution of sauropodomorphs: anatomy and phylogenetic relationships of a remarkably well-preserved dinosaur from the Upper Triassic of southern Brazil

“…The attribution of these three additional specimens to B. schultzi is uncertain, but plausible for topotypic reasons. In paleoecological terms, if all specimens correspond to B. schultzi, this taxon was relatively abundant in comparison to other taxa from the Buriol outcrop and surrounding localities, such as Polesinesuchus aurelioi and Prozostrodon brasiliensis, which are recorded in the area based on a single specimen each (Roberto-da-Silva et al, 2014;Pacheco et al, 2017). On the contrary, Hyperodapedon sp.…”

“…2). The presence of specimens ascribed to the rhynchosaur Hyperodapedon (Dias-da-Silva, Cabreira & Roberto-da-Silva, 2011;Roberto-da-Silva et al, 2014;Müller et al, 2017) allows the correlation with the Ischigualasto Formation, in northwest Argentina, radioisotopically dated as 231.4 ± 0.3 Mya (Martínez et al, 2011). In addition to B. schultzi and Hyperodapedon sp., the Buriol outcrop has also yielded the lagepetid Ixalerpeton polesinensis (Cabreira et al, 2016), whereas fishes (dipnoi plate, hybodontiform shark spine and actinopterygian remains), aetosaurs, temnospondyls and cynodonts were found in close surrounding outcrops (Perez & Malabarba, 2002;Toledo & Bertini, 2005;Richter & Toledo, 2008;Dias-da-Silva et al, 2011, 2012Roberto-da-Silva et al, 2014;Pacheco et al, 2017).…”

Novel data on aetosaur (Archosauria, Pseudosuchia) osteoderm microanatomy and histology: palaeobiological implications

“…Roberto‐da‐Silva et al . ). We consider that the ventral osteoderms of PVL 2052 actually correspond to another juvenile individual, which died at an earlier ontogenetic stage.…”

“… Each row of osteoderms in the dorsal armour is formed by two paramedian and two lateral osteoderms which are articulated at their medial (both paramedian and lateral osteoderms) and lateral (only the paramedian ones) margins, forming a symmetrical structure. The intimate contact between successive osteoderms in the dermal armour is maintained since early ontogeny, as evidenced by recent discoveries (Roberto‐da‐Silva et al . ; Paes Neto et al . ). Considering a possible arrangement of the paramedian and lateral osteoderms in life position, the width of the dermal armour in a particular row can be estimated summing the width of two of these bones (e.g.…”

Revised phylogenetic analysis of the Aetosauria (Archosauria: Pseudosuchia); assessing the effects of incongruent morphological character sets

“…For this study all previously recommended characters used for determination of aetosaurian systematics were reviewed ( Parrish, 1994 ; Heckert, Hunt & Lucas, 1996 ; Heckert & Lucas, 1999 ; Desojo, 2005 ; Parker, 2007 ; Desojo, Ezcurra & Kischlat, 2012 ; Roberto-Da-Silva et al, 2014 ; Heckert et al, 2015 ). Characters were discarded if found to be generally uninformative or ambiguously scored.…”