Lower Cretaceous fossils from China shed light on the ancestral body plan of crown softshell turtles (Trionychidae, Cryptodira)

Abstract

Pan-trionychids or softshell turtles are a highly specialized and widespread extant group of aquatic taxa with an evolutionary history that goes back to the Early Cretaceous. The earliest pan-trionychids had already fully developed the “classic” softshell turtle morphology and it has been impossible to resolve whether they are stem members of the family or are within the crown. This has hindered our understanding of the evolution of the two basic body plans of crown-trionychids. Thus it remains unclear whether the more heavily ossified shell of the cyclanorbines or the highly reduced trionychine morphotype is the ancestral condition for softshell turtles. A new pan-trionychid from the Early Cretaceous of Zhejiang, China, Perochelys hengshanensis sp. nov., allows a revision of softshell-turtle phylogeny. Equal character weighting resulted in a topology that is fundamentally inconsistent with molecular divergence date estimates of deeply nested extant species. In contrast, implied weighting retrieved Lower Cretaceous Perochelys spp. and Petrochelys kyrgyzensis as stem trionychids, which is fully consistent with their basal stratigraphic occurrence and an Aptian-Santonian molecular age estimate for crown-trionychids. These results indicate that the primitive morphology for soft-shell turtles is a poorly ossified shell like that of crown-trionychines and that shell re-ossification in cyclanorbines (including re-acquisition of peripheral elements) is secondary.

The shell morphology of the latest Cretaceous (Maastrichtian) trionychid turtleHelopanoplia distincta

“…Pan-Trionychidae, soft-shelled turtles, is a diverse clade of turtles with an extensive fossil record that extends from the Early Cretaceous (Aptian) to the Recent. Although trionychids are currently restricted to Asia, North America, and Africa, fossils document their former presence in Australia, Europe, and South America as well ( Vitek & Joyce, 2015 ; Georgalis & Joyce, 2017 ; Brinkman, Rabi & Zhao, 2017 ). Extant representatives of the clade are easily recognized externally by their flat shell covered by a leathery skin, well-developed, soft paddles, and an elongate proboscis ( Ernst & Barbour, 1989 ).…”

Tracing the patterns of non‐marine turtle richness from the Triassic to the Palaeogene: from origin to global spread

“…Trionychids first appeared in the North American record during the Santonian (Brinkman et al . ), as pleurodires did in Europe, perhaps again due to the warmer conditions facilitating immigration from Asia via high latitude dispersal routes. The terrestrial Nanhsiungchelyidae also appeared here in the Late Cretaceous ( Basilemys ; Holroyd & Hutchison ).…”

“…Perochelys ) and a diverse array of xinjiangchelyids (Brinkman et al . ). The separately plotted Jehol Group (between K2 and K3) had comparable, but lower, richness to that of the cumulative Asian record at this time.…”

“…In the Asian Late Cretaceous, soft‐shelled trionychids remained diverse and important components of the fauna (Brinkman et al . ), alongside terrestrial nanhsiungchelyids (Danilov & Syromyatnikova ) and stem testudinoids (e.g. Lindholmemys ; Hirayama et al .…”

Characteristics of Shell Bone as an Identification Tool for Turtle Species (Reptiles: Testudines) in Java, Borneo, and Sumatra

“…The plastron consists of epiplastron, entoplastron, hyoplastron, hypoplastron, xiphiplastron, and bridge. Each fragment is connected by a suture (Brinkman et al, 2017;Pritchard et al, 2009;Sobolik & Steele, 1996).…”