A fresh look at the genus Ichthyosaurus: species characteristics, morphometrics, and phylogeny

Abstract

Ichthyosaurs are among the best known of Mesozoic marine reptiles, ranging from the Early Triassic (252-247 Ma) to the end of the Cenomanian (93.9 Ma; Fischer et al., 2016). The genus Ichthyosaurus was one of the first genera to be named and is arguably one of the most iconic of ichthyosaurs. Due to this abundance of specimens, and to initial inadequacies in the definition of the genus and its constituent species, Ichthyosaurus has become a waste-basket taxon; a poorly classified taxon with many morphologically variable specimens (Smith and Radley, 2007).
Ichthyosaurus is known from an enormous number of specimens, many of which possess confusing combinations of anatomical features. Both the quantity of data and distribution of anatomical characters make it difficult to recognise distinct species within our understanding of this taxon.
To study this genus in depth, a modern problem of inaccessible specimens displayed in museums is an obstacle that needed to be overcome. A new method of parallel laser photogrammetry to bridge to gap of inaccessibility and scientific study was explored. This method has been used in other fields, but has not transitioned into palaeontology yet. The marine reptile gallery of the Natural History Museum, in London was utilised to test this method. The findings from that method were utilized in the other analyses performed. This new method allows researchers to get some scientific data from specimens that would otherwise not be utilized, but it is not a complete substitute for an up close in person examination of specimens.
To better understand the current breadth of the species Ichthyosaurus, multiple morphometric and phylogenetic studies were performed with a starting dataset of over 320 specimens and 66 different measurements. Each of these studies was done at the specimen level, which is a rarity. These analyses also help to determine if there is more diversity present in Ichthyosaurus than is currently recognized. During the course of this PhD 3 new species have been

named to this genus confirming that there is a larger amount of diversity than originally thought at the beginning of this project (Lomax and Massare, 2015 and 2016). The morphometric study focused on the measurements used in ratio characters, to determine if they were useful in separation of species. The analyses show that just measurements used in the previously defined ratios are not sufficient to separate species out by themselves. The phylogenetic study was done at the specimen level to truly see how much variation there is in the genus and species. The characters used in that study were also defined in detail as to produce replicable results. The phylogenetic results show that the species of this genus need to be more strictly defined, or that there are more species present in the genus still to be identified.

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ORCID for Ian Harding: orcid.org/0000-0003-4281-0581

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