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

Abstract

The dicynodonts are an emblematic group of herbivorous therapsids, which survived the Permo-Triassic (P-Tr) crisis. Laotian dicynodonts from stratigraphically constrained beds, recently dated using the U-Pb zircon method, yield new insights into terrestrial faunas of Southeast Asia during latest Permian and the earliest Triassic. Summarily described, they were attributed to the genus Dicynodon. We provide a new phylogenetic analysis for Laotian dicynodonts, based on three well-preserved skulls indicating that they belong to two new species: Counillonia superoculis gen. et sp. nov. and Repelinosaurus robustus gen. et sp. nov. Our phylogenetic analysis within Dicynodontia indicate that 1) Counillonia is closely related to some "Dicynodon"-grade taxa; and 2) Repelinosaurus is a kannemeyeriiform. The phylogenetic affinities of these new Laotian dicynodonts allow discussing the survivorship of multiple lineages (Kannemeyeriiformes and "Dicynodon"-grade dicynodontoids) across the P-Tr crisis. The Laotian dicynodonts also shed new light on the paleobiogeography of the Southeast Asia from the late Paleozoic to the early Mesozoic, particularly about the timing of collisions between the Indochina, the South China and the North China blocks. The presence of dicynodonts in Laos most likely in the Early Triassic thus implies that the connection between the Indochina Block and South China Block occurred no later than the latest Permian or earliest Triassic (i.e., when the dicynodonts provide direct evidence for a connection).

Revision of the Tanzanian dicynodontDicynodon huenei(Therapsida: Anomodontia) from the Permian Usili Formation

“…In their phylogenetic analyses, Kammerer, Angielczyk & Fröbisch (2011) never recovered Daptocephalus leoniceps and Dicynodon lacerticeps as sister-taxa, supporting recognition of a separate genus for the former. More recent studies have maintained the distinction between Dicynodon lacerticeps and Daptocephalus leoniceps (Botha-Brink, Huttenlocker & Modesto, 2014; Jasinoski et al, 2014; Angielczyk & Kammerer, 2017; Olivier et al, 2019), and detailed analysis of the stratigraphic distributions of these two taxa indicates that they had somewhat different ranges (Viglietti et al, 2016; Viglietti, Smith & Rubidge, 2018).…”

“…These OTUs were added (see Supplemental Information) to the most recent iteration of the anomodont character matrix originally published by Kammerer, Angielczyk & Fröbisch (2011), namely that of Kammerer et al (2019). The emydopoid Thliptosaurus imperforatus (Kammerer, 2019) was also added to that analysis, but the recently-described Laotian dicynodontoids Counillonia superoculis and Repelinosaurus robustus (Olivier et al, 2019) were not included, as their holotypes were not available for study during the production of this paper. The data were analyzed using parsimony in TNT v1.5 (Goloboff, Farris & Nixon, 2008) using New Technology search parameters (tree drifting, parsimony ratchet, and tree fusing), starting at level 65 and forced to find the shortest tree at least 20 times.…”

Peer Review #3 of "Revision of the Tanzanian dicynodont Dicynodon huenei (Therapsida: Anomodontia) from the Permian Usili Formation (v0.1)"

“…In their phylogenetic analyses, Kammerer et al (2011) never recovered D. leoniceps and D. lacerticeps as sister-taxa, supporting recognition of a separate genus for the former. More recent studies have maintained the distinction between Dicynodon lacerticeps and Daptocephalus leoniceps (e.g., Botha-Brink et al, 2014;Jasinoski et al, 2014;Angielczyk & Kammerer, 2017;Olivier et al, 2019), and detailed analysis of the stratigraphic distributions of these two taxa indicates that they had somewhat different ranges (Viglietti et al, 2016(Viglietti et al, , 2018. Kammerer et al (2011) considered Dicynodon lacerticeps and Daptocephalus leoniceps to be restricted to the Karoo Basin of South Africa, and considered extra-basinal Permian dicynodontoid records to mostly represent distinct, locally-endemic taxa (e.g., Jimusaria and Turfanodon in China; Peramodon and Vivaxosaurus in Russia; Gordonia in Scotland).…”

“…These OTUs were added (see Supplemental Information) to the most recent iteration of the anomodont character matrix originally published by Kammerer et al (2011), namely that of . The emydopoid Thliptosaurus imperforatus was also added to that analysis, but the recently-described Laotian dicynodontoids Counillonia superoculis and Repelinosaurus robustus (Olivier et al, 2019) were not included, as their holotypes were not available for study during the production of this paper. The data were analyzed using parsimony in TNT v1.5 (Goloboff et al, 2008) using New Technology search parameters (tree drifting, parsimony ratchet, and tree fusing), starting at level 65 and forced to find the shortest tree at least 20 times.…”

“…The Dicynodon clade is supported by two characters (continuous character 14, shape of mandibular fenestra, and discrete state character 51, postorbital contribution to intertemporal bar relatively flat) and the Daptocephalus clade is supported by four characters (continuous characters 8, median pterygoid plate width, and 11, relative area of internal nares, and discrete state characters 92, ventral surface of median pterygoid plate smooth and flat, and 111, absence of central circular depression on occipital condyle) (refer to Supplementary Information for lists of all character states). Manuscript to be reviewed The discovery that multiple taxa of large dicynodontoids are present in the Usili Formation should not be surprising-this clade is common in the late Permian, and sympatric dicynodontoid taxa are known within more poorly-sampled, probably coeval Laurasian basins (Li et al, 2008;Kurkin, 2012;Olivier et al, 2019) in addition to the heavily sampled Karoo Basin of South Africa (Kammerer et al, 2011). What is notable about the Usili species, however, is their close similarity to (and recovery as sister-taxa of) particular Karoo species (Dicynodon lacerticeps and Daptocephalus leoniceps), to the extent that they are here considered congeneric.…”