Hindfins ofIchthyosaurus: effects of large sample size on ‘distinct’ morphological characters

Abstract

The abundance of specimens of Ichthyosaurus provides an opportunity to assess morphological variation without the limits of a small sample size. This research evaluates the variation and taxonomic utility of hindfin morphology. Two seemingly distinct morphotypes of the mesopodium occur in the genus. Morphotype 1 has three elements in the third row: metatarsal two, distal tarsal three and distal tarsal four. This is the common morphology in Ichthyosaurus breviceps, I. conybeari and I. somersetensis. Morphotype 2 has four elements in the third row, owing to a bifurcation. This morphotype occurs in at least some specimens of each species, but it has several variations distinguished by the extent of contact of elements in the third row with the astragalus. Two specimens display a different morphotype in each fin, suggesting that the difference reflects individual variation. In Ichthyosaurus, the hindfin is taxonomically useful at the genus level, but species cannot be identified unequivocally from a well-preserved hindfin, although certain morphologies are more common in certain species than others. The large sample size filled in morphological gaps between what initially appeared to be taxonomically distinct characters. The full picture of variation would have been obscured with a small sample size. Furthermore, we have found several unusual morphologies which, in isolation, could have been mistaken for new taxa. Thus, one must be cautious when describing new species or genera on the basis of limited material, such as isolated fins and fragmentary specimens.

A prevalence ofArthropterygius(Ichthyosauria: Ophthalmosauridae) in the Late Jurassic—earliest Cretaceous of the Boreal Realm

“…Recently Massare & Lomax (2018) demonstrated the effect of large sample sizes on the identification of taxonomically distinct morphological characters in Ichthyosaurus . This is what is actually needed for ophthalmosaurids.…”

A taxonomic approach on diagnostic characters used to define new pterosaur taxa and an estimation of pterosaur diversity

“…Another well-known problem concerning the erection of new species regards the limited sample size of fossils, which was recently addressed by Massare & Lomax (2018) for Ichthyosaurus. Aquatic reptiles, as pterosaurs, bear unique morphological characters and did not have living representatives or suitable analogs.…”

“…In the case of Ichthyosaurus, if just fewer specimens would have been available, variation in hindfin morphology could have appeared less variable and more distinct. This shows that the more reduced sample size, the less variation will be recognized, which hampers morphotypes identification (Massare & Lomax 2018). As morphotypes are defined as different types of individuals that belong to the same species (morph), recognition of morphotypes relies upon many quite complete specimens of a species, otherwise intermediate morphologies could not be identified.…”

A new specimen ofPalvennia hoybergeti: implications for cranial and pectoral girdle anatomy in ophthalmosaurid ichthyosaurs

“…This resembles the situation interpreted for? I. somersetensis (NHMUK OR14563 in McGowan & Motani, 2003 ; Massare & Lomax, 2018 ) and Platypterygius hercynicus ( Kolb & Sander, 2009 ). The elongated and ventral facets are placed at a more equal distance between the anterior and posterior margin, whereas the circular facets touches the anterior margin.…”