Laosuchus nagagen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology

Abstract

Chroniosuchians were a clade of non-amniotic tetrapods known from the Guadalupian (middle Permian) to Late Triassic, mainly from Russia and China. The rarity of complete or articulated remains means that relatively little is known about this group in terms of its anatomy, palaeobiology, or evolutionary history. Based on a nearly complete skull with a left hemimandible, we describe the first chroniosuchian from Laos from rocks preserving the Permian-Triassic boundary in the Luang Prabang Basin, which is located on the tectonic Indochina Block. This specimen is referred to a new genus and species, Laosuchus naga, based on numerous diagnostic features, including an extremely reduced pineal foramen; absence of palatal dentition; well-developed transverse flange of the pterygoid that contacts the maxilla; internal crest on and above the dorsal side the palate; otic notch closed by the tabular horn and the posterior part of the squamosal, forming a continuous wall; thin and high ventromedial ridge on parasphenoid. A phylogenetic analysis of 51 characters and 25 taxa reveals a basal position for Laosuchus naga among Chroniosuchia. In addition, CT scan data reveal internal structures and provide new insights about the anatomy and palaeobiology of chroniosuchians. Laosuchus naga was likely amphibious and spent most of its time in water, rather than in terrestrial environments. As chroniosuchians are nonmarine tetrapods previously known on the North China Block, South China Block, and Laurussia, the occurrence of Laosuchus naga on the Indochina Block supports the hypothesis of physical connections between all these tectonic plates by the time of the Permo-Triassic boundary.

New dicynodonts (Therapsida, Anomodontia) from near the Permo-Triassic boundary of Laos: implications for dicynodont survivorship across the Permo-Triassic mass extinction and the paleobiogeography of Southeast Asian blocks

“…Such a hypothesis reconciles an Early to Middle Triassic collision between the SCB and the Indochina Block with the paleobiogeographic distribution of dicynodonts. This is further corroborated with the discovery of a new chroniosuchian in non-marine rocks in the Purple Claystone Fm (Arbez et al, 2018), which is inferred to have had an amphibious lifestyle (e.g., Buchwitz et al, 2012;Golubev, 2015;Arbez et al, 2018) and which supports a connection between Eurasia and the Indochina Block at that time.…”

“…The Limestone and Sandstone Fm is overlain by the Purple Claystone Fm, from which various fossil remains have been excavated (dicynodonts and a chroniosuchian; Steyer, 2009;Arbez et al, 2018). The Purple Claystone Fm is mainly composed of homogeneous silty-claystones, silts, and more rarely clays (Bercovici et al, 2012).…”

“…Blanchard et al, 2013). Sample LP03 was collected at the dicynodont site, sample LP04 in an unfossiliferous site, and sample LP05 at the chroniosuchian site (Arbez et al, 2018). Blanchard et al, 2013).…”

The youngest occurrence of embolomeres (Tetrapoda: Anthracosauria) from the Sunjiagou Formation (Lopingian, Permian) of North China

“…The clade is generally considered to be a stem member of the Reptiliomorpha, taxa that are more closely related to amniotes than to lissamphibians (Ruta et al, 2003;Vallin and Laurin, 2004;Ruta and Coates, 2007;Clack and Klembara, 2009;Klembara et al, 2014;Marjanovic and Laurin, 2019). Its relationship with the Permian-Triassic Chroniosuchia is still uncertain, but it is generally accepted that Chroniosuchia are not a subclade of the Embolomeri (Carroll, 2009;Clack and Klembara, 2009;Klembara et al, 2010;Schoch et al, 2010;Clack, 2012;Arbez et al, 2018;Marjanovic and Laurin, 2019). The distribution and diversity of Embolomere are closely related to the paleogeography and climate changes of the late Paleozoic.…”

“…One is Acheria, represented by abundant material from Texas and Oklahoma of North America (Holmes, 1989), and the other is the much less-known Aversor, presumably an eogyrinid, from the Kungurian Inta Formation (∼ 273 Ma) of Russia (Gubin, 1985;Steyer, 2000). The decline of the embolomeres was probably related to the gradual aridification of Euramerica during the Permian (Roscher and Schneider, 2006;Roscher et al, 2011;Bernardi et al, 2017), which resulted in the decrease of suitable habitats.…”