Jun Wang, Rui Pei, Jianye Chen, Zhenzhu Zhou, Chongqin Fenga & Su-Chin Chang (2019)
New age constraints for the Middle Triassic archosaur Lotosaurus: Implications for basal archosaurian appearance and radiation in South China
Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication)
Highlights
238.0âÂâ1.4âMa as the maximum age for Lotosaurus in NW Hunan, China
Precise age constraints firstly provided for Lotosaurus in the Badong Formation
Lotosaurus older than shuvosaurids, younger than pre-Ladinian poposauroids
Providing age constraints on the radiation of archosaurians in the Early Triassic
Abstract
Archosaurians were the dominant terrestrial tetrapods during the Mesozoic and became one of the most successful tetrapod clades on Earth following the end-Permian mass extinction. The lower Badong Formation in northwestern Hunan Province, South China hosts fossils representing the basal archosaurian Lotosaurus. The Badong Formation has been previously interpreted as Anisian-Ladinian (247â237âMa) in age based on biostratigraphic correlations. This study presents new detrital zircon UPb age constraints for sandstones hosting Lotosaurus fossils in Sangzhi County of northwestern Hunan. Our data give a weighted mean age of 238.0âÂâ1.4âMa (Ladinian Stage) as the maximum depositional age for the fossil-bearing layer in the lower Badong Formation. This age is consistent with younger estimates from previous biostratigraphic studies. It is also more precise and stratigraphically constrained than previously published radiometric dating results for the units. This age estimate provides critical geochronologic constraint on the radiation of early archosaurians from China into other areas of Pangea beginning in the Early Triassic.
Morgan L. Guignard, Agustin G. Martinelli & Marina B. Soares (2019)
Postcranial anatomy of Riograndia guaibensis (Cynodontia: Ictidosauria)
Geobios (advance online publication)
Postcranial remains of non-mammaliaform cynodonts provide crucial information documenting the anatomical steps and locomotor advances that led to the mammalian pattern. Here, we present a detailed description of the first postcranial specimens referable to Riograndia guaibensis (Probainognathia, Ictidosauria), an abundant non-mammaliaform cynodont from the Late Triassic of Brazil (Riograndia Assemblage Zone, CandelÃria Sequence) that is closely related to Mammaliaformes. Functional and ecological implications of these fossils are also investigated, based on scapular and humeral morphology and muscular reconstructions. The pectoral girdle and forelimb show fossorial adaptations, with numerous specializations increasing the mechanical advantages of retractor muscles. Riograndia guaibensis retained a semi-sprawling forelimb, with well-developed adductor muscles to hold the body off the ground. These results confirm that the forelimb posture of non-mammaliaform probainognathians was still similar to that of basal synapsids and differed from the more erect posture and parasagittal function of therians.