Choristoderes are freshwater aquatic diapsid reptiles of uncertain phylogenetic position. Although the lineage probably diverged from other diapsids in the Permo-Triassic, choristoderes apparently never reached Gondwana. Within Laurasia, however, they have been recovered from Japan, China, Mongolia, Trans-Baikal Russia, Central Asia, Western Europe, and North America, reaching extreme northern latitudes. Setting aside controversial Triassic records, their known temporal range currently extends from the Middle Jurassic (Britain, Kyrgyzstan) to the Miocene (Czech Republic). However, although small choristoderes are known to span the entirety of this period, the larger, more derived neochoristoderes are recorded only from the Early Cretaceous through to the earliest Eocene. The gavial-like neochoristodere Champsosaurus is the most familiar taxon, characterised by large size, a long rostrum and flared temporal fenestrae, but research over the last three decades has revealed many new genera and exposed an unexpected diversity in terms of body size (small to relatively large), neck length (long v. short) and skull morphology (longirostrine v. brevirostrine, open v. closed lower temporal fenestrae). Typically choristoderes occur as part of a mesic assemblage that includes fish, lizards, mammals, turtles, frogs, salamanders, small dinosaurs and, usually, crocodiles. At maturity, Jurassic choristoderes were generally smaller than co-occuring crocodiles and were relatively unspecialized postcranially. The Early Cretaceous of Asia saw a dramatic diversification of choristoderes, including the appearance of much larger neochoristoderes. Perhaps significantly, the relevant Asian horizons (e.g. Yixian Formation, China; Okurodani and Kuwajima formations, Japan) yield no crocodiles. In Late Cretaceous and Paleogene horizons however, across Euramerica, large gavial-like neochoristoderes came to share freshwater ecosystems with a diversity of crocodiles, the neochoristoderes apparently occupying a specialist (gavial-like) piscivorous niche as long as it was available and resources were adequate.

Choristodera are freshwater aquatic reptiles known from the Middle Jurassic to the Miocene. Their record shows a peak in diversity in the Early Cretaceous of eastern Asia, most notably in the Jehol Biota of China but also in Japan and Mongolia. However, until now, the only Jurassic records from Asia have been rare disarticulated elements from Middle Jurassic microvertebrate sites in Siberia and Transbaikalian, with a possible jaw fragment from the Late Jurassic of Xinjiang, China. Here we describe a new, fully articulated, choristodere skeleton from the Tiaojishan Formation of China, considered to be of Late Jurassic (Oxfordian) age. As such, the new specimen represents the first complete Jurassic choristodere recovered worldwide, as well as providing important information on the pre-Cretaceous history of the group in eastern Asia. In its proportions and postcranial characters, the new taxon resembles previously described Asian taxa such as Philydrosaurus, but it is distinct from them in possessing several apparently plesiomorphic characters including a short antorbital region, paired external nares, and an open lower temporal fenestra. Phylogenetic analysis places the new choristodere as the sister taxon of a non-neochoristoderan clade comprising Asian and European taxa, whereas the European Jurassic Cteniogenys becomes the sister taxon to all other Choristodera.

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The presence of a palatal dentition is generally considered to be the primitive condition in amniotes, with each major lineage showing a tendency toward reduction. This study highlights the variation in palatal tooth arrangements and reveals clear trends within the evolutionary history of tetrapods. Major changes occurred in the transition between early tetrapods and amphibians on the one hand, and stem amniotes on the other. These changes reflect the function of the palatal dentition, which can play an important role in holding and manipulating food during feeding. Differences in the arrangement of palatal teeth, and in their pattern of loss, likely reflect differences in feeding strategy but also changes in the arrangement of cranial soft tissues, as the palatal dentition works best with a well-developed mobile tongue. It is difficult to explain the loss of palatal teeth in terms of any single factor, but palatal tooth patterns have the potential to provide new information on diet and feeding strategy in extinct taxa.