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Hindfins of Ichthyosaurus; effects of large sample size on "distinct" morphological characters

Judy A. Massare and Dean R. Lomax
Hindfins of Ichthyosaurus; effects of large sample size on "distinct" morphological characters
Geological Magazine (April 2019) 156 (4): 725-744


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.

ISSN: 0016-7568
EISSN: 1469-5081
Coden: GEMGA4
Serial Title: Geological Magazine
Serial Volume: 156
Serial Issue: 4
Title: Hindfins of Ichthyosaurus; effects of large sample size on "distinct" morphological characters
Affiliation: State University of New York, College at Brockport, Department of the Earth Sciences, Brockport, NY, United States
Pages: 725-744
Published: 201904
Text Language: English
Publisher: Cambridge University Press, London, United Kingdom
References: 44
Accession Number: 2019-036034
Categories: Vertebrate paleontology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
N37°57'00" - N40°30'00", W07°30'00" - W04°40'00"
Secondary Affiliation: University of Manchester, GBR, United Kingdom
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Cambridge University Press. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201919
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