New information on Riograndia guaibensis Bonaparte, Ferigolo & Ribeiro, 2001 (Eucynodontia, Tritheledontidae) from the Late Triassic of southern Brazil: anatomical and biostratigraphic implications

Abstract

The tritheledontid Riograndia guaibensis was the first cynodont described for the "Caturrita Formation" fauna from the Late Triassic of southern Brazil (Santa Maria 2 Sequence). The type materials did not preserve anatomical information regarding braincase, occiput, basicranium, zygomatic arch, postdentary bones and craniomandibular joint. Here new materials are described and supply the missing information. Riograndia shows a suite of important anatomical features quite derived among the non-mammaliaform eucynodonts, such as the partial closure of the medial orbital wall and braincase, extensive secondary osseous palate, wide primary palate, basicranium with jugular foramen separated from the periphery of fenestra rotunda, narrow zygomatic arch and much reduced postdentary bones. Many of these features constitute synapomorphies shared only with the other members of mammaliamorpha. Thus, the almost complete cranial, mandibular and dental information from the new fossils of Riograndia can bring a significant improve in the understanding of the anatomy and phylogenetic relationships of the tritheledontids and help to elucidate the transformational steps involved in the cynodont-mammal transition. Additionally, Riograndia is a key taxon in refining the "Caturrita Formation" biostratigraphy, enabling the connection of several fossiliferous outcrops that have a rich tetrapod fauna that can be correlated with other Triassic faunas from Gondwana and Laurasia.

New information about Irajatherium hernandezi Martinelli, Bonaparte, Schultz & Rubert 2005 (Eucynodontia, Tritheledontidae) from the upper triassic (Caturrita Formation, Paraná Basin) of Brazil

“…In Riograndia guaibensis, the most basal ictidosaur (the sistertaxon of Tritheledontidae), the jugular foramen and the fenestra rotunda are completely separated (Soares 2004;Soares et al 2005) as occurs in the Jurassic Pachygenelus monus (Wible and Hopson 1993). Because the basicranial region of Irajatherium is poorly preserved the separation of these openings cannot be discarded with confidence.…”

A new non-mammaliaform eucynodont from the Carnian-Norian Ischigualasto Formation, Northwestern Argentina

“…Until now, the record of Carnian cynodonts showed a rich taxonomic representation in the Santa Maria Formation from Brazil (Abdala & Ribeiro, 2010;Soares et al, 2011), contrasting with the lower diversity in the Ischigualasto Formation from Argentina (Martinez et al, 2011b). This is true considering the record of the whole formations, but the scenario changes when contrasting two supposedly contemporary faunal zones with equivalent fossil richness.…”

“…Chiniquodon Huene, 1936, Probainognathus, Therioherpeton Bonaparte & Barberena, 1975Huene, 1936;Abdala & Giannini, 2002;Romer, 1970;Bonaparte & Barberena, 1975;Oliveira, 2006). The paracanine fossae are located anteromedial to the upper canine ( Figure 2B) as in the majority of Triassic cynodonts; in contrast, Massetognathus (Romer, 1967;Liu et al, 2008) and Exaeretodon (Bonaparte, 1962;Abdala et al, , 2006 have the fossae located medially and posteromedially respectively, and in Riograndia Bonaparte, Ferigolo & Ribeiro, 2001Soares et al, 2011) and Pachygenelus Watson, 1913 are absent (Watson, 1913). The paracanine fossae of Diegocanis gen. nov. are dorsally closed, different from Ecteninion ( Figures 4C-F) and Trucidocynodon in which the fossae are dorsally opened allowing the lower canines to protrude through the snout roof.…”

“…The lacrimal shows a ventral process directed posteriorly, forming a pronounced ridge along the orbital margin. This process is in contact with the jugal ventrally and extends posteriorly as far as the posterior end of the maxilla ( Figure 2C), as in Ecteninion and Riograndia Soares et al, 2011), differing from Trucidocynodon that shows a reduced condition and others eucynodonts in which the process is absent (e.g. Probainognathus, Chiniquodon).…”

A juvenile sauropodomorph tibia from the 'Botucaraí Hill', Late Triassic of Southern Brazil

“…The fossil record of the locality includes the dicynodont Jachaleria candelariensis Araújo & Gonzaga, 1980, the prozostrodontian cynodont Botucaraitherium belarminoi Soares, Martinelli & Oliveira, 2014, as well as an indeterminate phytosaur, a stereospondyl amphibian, and several isolated archosaur teeth (Araújo & Gonzaga, 1980;Dornelles, 1990;Kischlat & Lucas, 2003;Dias-da-Silva et al, 2009;Soares et al, 2014). An isolated tooth referred to Riograndia guaibensis was also reported by Soares et al (2011) from the upper portions of the outcrop. The fossil occurrences, as well as the lateral correlation to nearby outcrops, suggest that the site is representative of the Riograndia Assemblage Zone .…”