Lotosaurus is an extinct genus of sail-backed poposauroid known from Hunan Province of central China.[1]
Lotosaurus is known from the holotype IVPP V 4881 (or possibly V 4880), an articulated and well-preserved skeleton. Other referred specimens include IVPP V 48013 (a skull) as well as many articulated and disarticulated skeletal remains concentrated in a bonebed which is almost completely composed of Lotosaurus bones. All known specimens of this genus were collected from this bonebed, known as the Lotosaurus site, which belongs to the Batung Formation (or alternatively Xinlingzhen Formation of the Badong Group[2]).
Further excavations in 2018 revealed many more specimens, as well as geological and environmental details of the Lotosaurus site. At least 38 individuals of various ages died within this one location.[3]
Lotosaurus was first named by Fa-kui Zhang in 1975 and the type species is Lotosaurus adentus. Lotosaurus was originally placed in its own family, Lotosauridae, which named by Zhang in 1975. The specific name is derived from the Greek a and denta, meaning "toothless", in reference to its toothless beak.[4]
Lotosaurus was 1.5 to 2.5 m (4.9 to 8.2 ft) long and a heavily built quadruped. It was a herbivore, shearing off leaves with its toothless, beaked jaws. Lotosaurus, like some other members of the Poposauroidea, had a sail on its back, granting it an appearance superficially similar to that of Permian pelycosaurs like Dimetrodon and Edaphosaurus, although not as high.[5]
Lotosaurus was originally thought to be a thecodont, probably related to Ctenosauriscus and other archosaur taxa with elongated neural spines (=Ctenosauriscidae).[4] Although modern cladistic analyses have abandoned the order Thecodontia, they have also supported the theory that Lotosaurus was a distant relative of ctenosauriscids. Nesbitt (2007) was the first to suggest that Lotosaurus is more closely related to Shuvosaurus (a shuvosaurid) than to Arizonasaurus (a ctenosauriscid).[6] In his massive revision of archosaurs which included a large cladistic analysis, Sterling J. Nesbitt (2011) found Lotosaurus to be a member of the group Poposauroidea. Poposauroids were part of Pseudosuchia, the lineage of reptiles which also includes aetosaurs, raisuchids, and modern crocodilians.[1]