Monday, March 14, 2016

[Paleontology • 2016] Extinction of Fish-shaped Marine Reptiles associated with reduced Evolutionary Rates and Global Environmental Volatility

Two ichthyosaurs Pervushovisaurus bannovkensi wander through a middle Cenomanian low latitude ecosystem that will prevail for most of the Late Cretaceous: high sea level and sea temperatures, rudist reefs (Ichthyosarcolites, Hippurites), newly radiating neoselacian sharks and acanthomorph fishes Aipichthyoides.
Artwork by Andrey Atuchin 

Despite their profound adaptations to the aquatic realm and their apparent success throughout the Triassic and the Jurassic, ichthyosaurs became extinct roughly 30 million years before the end-Cretaceous mass extinction. Current hypotheses for this early demise involve relatively minor biotic events, but are at odds with recent understanding of the ichthyosaur fossil record. Here, we show that ichthyosaurs maintained high but diminishing richness and disparity throughout the Early Cretaceous. The last ichthyosaurs are characterized by reduced rates of origination and phenotypic evolution and their elevated extinction rates correlate with increased environmental volatility. In addition, we find that ichthyosaurs suffered from a profound Early Cenomanian extinction that reduced their ecological diversity, likely contributing to their final extinction at the end of the Cenomanian. Our results support a growing body of evidence revealing that global environmental change resulted in a major, temporally staggered turnover event that profoundly reorganized marine ecosystems during the Cenomanian.

Figure 1: Phylogeny and ecological diversity of parvipelvian ichthyosaurs.
(a) Time scaled strict consensus tree arising from equal weight maximum parsimony analysis. Numbers denote >1 Bremer decay indices. Grey bars denote range extensions by specimens identified at the generic level. Colour coding of taxa refers to the results of b. (b) Cluster dendrogram based on the ecological data set, with gut-content data and the general features of each guild. (c) Teeth representative of each guild across the Late Albian–Cenomanian interval, illustrating the ecological extinction at the beginning of the Cenomanian. ‘Platypterygius campylodon’ and ‘Platypterygius’ sp. from the US are early Cenomanian in age, Pervushovisaurus bannovkensis is Middle Cenomanian in age and ‘Platypterygius’ sp. from Germany is Late Cenomanian in age. *denotes taxa from the Stoilensky/Kursk fauna. Scale bar, 50 mm.

Valentin Fischer, Nathalie Bardet, Roger B. J. Benson, Maxim S. Arkhangelsky and Matt Friedman. 2016. Extinction of Fish-shaped Marine Reptiles associated with reduced Evolutionary Rates and Global Environmental Volatility. Nature Communications. 7, Article number: 10825 DOI: 10.1038/ncomms10825

Slower evolution and climate change drove ichthyosaurs to extinction via @physorg_com