A basal ichthyosauriform with a short snout from the Lower Triassic of China

Abstract

The incompleteness of the fossil record obscures the origin of many of the more derived clades of vertebrates. One such group is the Ichthyopterygia, a clade of obligatory marine reptiles that appeared in the Early Triassic epoch, without any known intermediates. Here we describe a basal ichthyosauriform from the upper Lower Triassic (about 248 million years ago) of China, whose primitive skeleton indicates possible amphibious habits. It is smaller than ichthyopterygians and had unusually large flippers that probably allowed limited terrestrial locomotion. It also retained characteristics of terrestrial diapsid reptiles, including a short snout and body trunk. Unlike more-derived ichthyosauriforms, it was probably a suction feeder. The new species supports the sister-group relationships between ichthyosauriforms and Hupehsuchia, the two forming the Ichthyosauromorpha. Basal ichthyosauromorphs are known exclusively from south China, suggesting that the clade originated in the region, which formed a warm and humid tropical archipelago in the Early Triassic. The oldest unequivocal record of a sauropterygian is also from the same stratigraphic unit of the region.

The evolution and extinction of the ichthyosaurs from the perspective of quantitative ecospace modelling

“…Cartorhyncus is plotting so far from the rest of the genera due to its unique ecospace; Cartorhyncus was a very small (and proposed demersal) suction feeder [2]. This ecotype was attained only once, early in the history of the Ichthyosauriformes, and was lost by the Middle Triassic [2]. Ecotype B, representing the majority of EarlyMiddle Triassic genera, represents the ecospace occupied by the basal Ichthyopterygia.…”

“…Ecotype A represents a recently described and unusual basal ichthyosauriform genus, Cartorhyncus [2]. Cartorhyncus is plotting so far from the rest of the genera due to its unique ecospace; Cartorhyncus was a very small (and proposed demersal) suction feeder [2]. This ecotype was attained only once, early in the history of the Ichthyosauriformes, and was lost by the Middle Triassic [2].…”

“…Figure 1 shows the results of the non-metric MDS analysis, showing ecotype distribution through time. Ecotype A represents a recently described and unusual basal ichthyosauriform genus, Cartorhyncus [2]. Cartorhyncus is plotting so far from the rest of the genera due to its unique ecospace; Cartorhyncus was a very small (and proposed demersal) suction feeder [2].…”

“…Ecospace modelling can be used to recognize convergent behavioural and ecological phenomena that may be missed by standard phylogenetic or morphospace modelling [1]. Here, we demonstrate how applying a novel ecospace framework to the major genera of the Ichthyosauriformes [2] through the Mesozoic allows the degree of ecological diversity, niche narrowing and convergence seen to be placed in an evolutionary framework, and provides insights into the evolutionary history of this clade. We show how continued loss of unique ecotypes, resulting in gradual contraction onto a highly derived and potentially unsustainable ecospace, produced an ecological bottleneck potentially contributing to their eventual extinction in the Late Cretaceous.…”

How giant marine reptiles terrorized the ancient seas

“…It dates to 248 million years ago, only 4 million years after the end-Permian extinction. A nearby quarry yielded a snub-nosed fish-lizard of roughly the same age, Cartorhynchus lenticarpus 6 . About as long as a rainbow trout (Oncorhynchus mykiss), this primitive ichthyosaur may have heaved itself around on land atop its big flippers in much the same way that sea turtles do.…”