Footprints of marine reptiles from the Middle Triassic (Anisian-Ladinian) Guanling Formation of Guizhou Province, southwestern China: The earliest evidence of synchronous style of swimming

Abstract

Subaqueously-registered trackways of marine tetrapods are only rarely preserved; some of the best examples so far come from the Shizishan Member (Member II) of the Guanling Formation (Middle Triassic, Anisian-Ladinian) of Yunnan Province in southwestern China. The unit is well-known for the Luoping Biota including abundant skeletons of marine reptiles. The described footprints consist of impressions of fin-like autopodia comprising numerous broad trackways that have been named Dikoposichnus luopingensis and interpreted as having been made by foraging nothosaurs. Morphologically identical tracks and trackways have now been discovered near the city of Anshun in Guizhou Province, in slightly older strata and on a large surface of argillaceous dolostone belonging to the Songzikan Member (Member I) of the Guanling Formation. Additionally, a second morphotype is abundant on the same surface, with long trackways that show semi-plantigrade to plantigrade foot imprints with four distinct digit impressions. All tracks are similar in shape and positioned symmetrically, in broad trackways on either side of the trackway midline. Based on several unique features this morphotype is assigned here to the new ichnotaxon Anshunpes aquacursor ichnogen. nov. ichnosp. nov. It is interpreted as the trackway of a buoyant and punting tetrapod, using its forefeet only. The trackmaker is inferred to be similar to some diapsid placodonts and saurosphargids whose skeletons have been found in the Guanling Formation (Middle Triassic, Anisian-Ladinian) and in the overlying Falang Formation (Middle–Upper Triassic, Ladinian–Carnian). Comparative measurements of trackways and skeletons support this. Additional associated ichnofossils are enigmatic traces such as isolated large oval impressions and cord-like knobby structures. The palaeoenvironment was a shallow near-coast marine habitat, possibly a lagoon. The ichnofauna from Anshun is important, because it is the first record of subaqueous progression activity by marine diapsid placodonts and/or saurosphargids, and also indicates the early adoption of a synchronous style of swimming by marine reptiles.