Hanosaurus is an extinct genus of marine reptiles that existed during the Triassic period in what is now China. The type species is Hanosaurus hupehensis. It was a small animal, with specimens measuring 79.4 cm (31.3 in) long in total body length,[2] which likely fed on soft-bodied prey.[3]
Hanosaurus (lizard of Han River) was discovered from the Sonshugo locality of Hubei, China. It was found at the second member of the Jialingjiang Formation.[2] The type specimen consisted of a skull, complete hindlimbs and pelvis, incomplete shoulder girdle represented by coracoid and clavicle, and an articulated but incomplete vertebral column.[4] It was initially described as a thalattosaur but subsequent research showed it was more likely to be a sauropterygia.[4]
A more complete referred specimen was described in 2022 from the Yingzishan locality of Hubei, China. The specimen was also found at the second member of the Jialingjiang formation. It consisted of almost the entire skeleton preserved with its ventral side up.[2] However, a later study has cast doubt on the referral of this specimen to Hanosaurus.[5]
Rieppel's 1998 reassessment recovered Hanosaurus within a monophyletic pachypleurosaurid in a phylogenetic analysis featuring diapsids.[4] Its placement within Sauropterygia, however, was rather inconsistent between different subsequent studies. Some studies support Rieppel 1998's pachypleurosaur placement;[6] some found it to be more closely related to Nothosauridae than other pachypleurosaurs;[7] some found it to be the sister taxon to a clade consisted of Nothosauridae and Pachypleurosauridae;[3] some found it to be the sister taxon to eosauropterygia;[5] while some recovered it to be a basal sauropterygia[8] or outside of Sauropterygia altogether.[2][9]
A 2024 study by Hu, Li & Liu recovered Hanosaurus as the sister taxon of the clade formed by the Nothosauroidea and the Pachypleurosauria. The results of their phylogenetic analyses are shown in the cladogram below:[10]
Following the description of the referred specimen and the placement of the taxon being the most basal sauropterygiform (a clade that includes sauropterygia, Helveticosaurus, and Saurosphargidae), Wang et al. (2022) diagnosed Hanosaurus as a medium sized sauropterygiform with the following characteristics:[2]