Choristoderes and the freshwater assemblages of Laurasia

Abstract

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.

Microvertebrates from the classic Rhaetian bone beds of Manor Farm Quarry, near Aust (Bristol, UK)

“…Pachystropheus was the only known choristodere not found in freshwater deposits. However, this identification has been queried: Matsumoto and Evans (2010) noted that the diagnostic characters of choristoderes are in the skull, and yet only one cranial bone of P. rhaeticus has been found. Renesto (2005) suggested that Pachystropheus might be related to Endennasaurus from the Late Triassic of Italy, a possible thalattosaur, based on the common occurrence of 22 characters regarded by Storrs and Gower (1993) as unique to choristoderes; thalattosaurs are marine.…”

A marine vertebrate fauna from the Late Triassic of Somerset, and a review of British placodonts

“…Storrs and Gower (1993) assigned it to Choristodera, a group of crocodile-like semi-aquatic diapsids, found in freshwater habitats. However, this is problematic because the skull contains the diagnostic features of choristoderes (Matsumoto and Evans, 2010), but only one cranial bone has been found of P. raeticus. Renesto (2005) suggested that Pachystropheus might be related to Endennasaurus from the Late Triassic of Italy, a possible thalattosaur, based on the common occurrence of 22 characters regarded by Storrs and Gower (1993) as unique to choristoderes.…”

Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover

“…The timing of the radiation of the non-neochoristodere group, Monjurosuchidae, is a point of ongoing study, although their origin might be in the Early Cretaceous of Asia (Gao & Fox, 2005;Averianov et al, 2006;Richter et al, 2010;Gao et al, 2013). The earliest records of the major lineage Neochoristodera occur in Barremian (Early Cretaceous) deposits of Asia (Matsumoto & Evans, 2010) and North America (Britt et al, 2006), and this lineage persisted well past the K/Pg boundary (Evans & Klembara, 2005). There appears to have been an ecological transition around the J/K boundary, from smaller basal forms (Late Jurassic), to larger taxa, primarily representing neochoristoderes (Early Cretaceous onwards), with non-neochoristoderes seemingly becoming extinct in Euamerica (Matsumoto & Evans, 2010).…”