Does shape matter? Morphological diversity and differential survivorship among Triassic ammonoid genera

Abstract

Variation in longevity of taxa in the fossil record has been recognized, but few studies have tested for correlation between position in morphospace and differential survivorship. A sample of 322 Triassic ammonoid species, each one representing a genus, was studied to test whether longer-lived genera were significantly further from the centre of morphospace than shorter-lived genera. Two empirical morphospaces were constructed from morphological data, and the deviation of each genus from the “average form” (centroid) was calculated. Spearman Rank Correlation and Kruskal-Wallis tests were used to test for any significant relationships between distance from the centre of morphospace and longevity. Some longer-lived taxa tended to plot further from the centre of morphospace, but the amounts of variance in longevity accounted for were small and largely statistically non-significant. Ammonoid clade-level morphological stasis appears to be the product of repeated reoccupation of the centre of morphospaces after taxonomic turnover events.

Keywords:

• Triassic
• ammonoid
• survivorship
• morphospace
• disparity

Acknowledgements

The work presented in this paper was part of my Ph. D. thesis at the University of Chicago and I thank the Department of Geophysical Sciences for support and facilities during the course of my research. I thank my committee (D. Jablonski, M. Foote, S. M. Kidwell, A. M. Ziegler, and W. B. Saunders) for their advice and suggestions that improved the manuscript. L. H. Liow discussed the ideas in this paper and her own ideas about the relationship between morphology and longevity with me. I thank her for her thoughtful and constructive review. K. Jones advised me on the neotological literature on extinction risk and disparity. Museum trips to collect data for this work were supported by the Geological Society of America, the Sylvester-Bradley Fund of the Palaeontological Association, the Schuchert Fund of the Yale Peabody Museum, and the Paleontological Society. This work was completed with support from Leverhulme Trust grant A20040143 to A. B. Smith.