A putative ancestor for the swordfish-like ichthyosaur Eurhinosaurus

Abstract

Ichthyosaurs, extinct marine reptiles superficially similar to sharks and dolphins, flourished throughout most of the Mesozoic, but are best represented in the early Jurassic^1–5. Hundreds of skeletons are known, principally from the Lower Liassic (Hettangian, Sinemurian and Lower Pleinsbachian) of south-west England and the Upper Liassic (Toarcian) of southern Germany^6–9. Among the most specialized is Eurhinosaurus, unique for its long rostrum and shortened mandible, which confers a striking resemblance to the modern swordfish, Xiphias^1,5,10. Known only from the Upper Liassic of Germany, this monotypic genus, like most fossil species, appears suddenly, without antecedents^11–14. The recent discovery of a geologically older (Lower Liassic) ichthyosaur with a partially extended rostrum is therefore of considerable interest. As described here, this specimen, which is too large to be part of a growth series of Eurhinosaurus, appears to be ancestral, and could have given rise to Eurhinosaurus through changes in growth rates of the rostrum and mandible.