A test of Simpson's "rule of the survival of the relatively unspecialized" using fossil crinoids

Am Nat. 2004 Oct;164(4):431-43. doi: 10.1086/423673. Epub 2004 Sep 7.


Temporally long-ranging (=long-lived) taxa have been postulated to have unusual properties that aid their prolonged geologic survival. Past studies have examined dispersal capabilities, geographic ranges, and single-character morphological adaptations as factors that may contribute to geologic longevity. Here, I test whether long-lived fossil crinoid taxa are morphologically unusual using a whole suite of morphological characters. I define long-lived taxa in several explicit, comparative ways. I find that long-lived crinoid genera and families are often less distant from mean morphologies of their crinoid orders than their shorter-lived relatives; that is, they are relatively less specialized. I also compare the morphology of crinoid genera relative to basal members of their respective orders; mean morphological distances of long-lived genera from basal morphologies are seldom distinct from those of their shorter-lived relatives. I observe that long-lived crinoid genera are less distant from mean morphologies of their temporal cohorts compared with shorter-lived genera but not in a statistically significant manner. I conclude that long-lived crinoids are relatively unspecialized, in the sense that they are relatively closer to mean morphologies of their taxonomic groups.

MeSH terms

  • Animals
  • Echinodermata / anatomy & histology
  • Echinodermata / classification
  • Echinodermata / physiology*
  • Extinction, Biological
  • Fossils*
  • Models, Biological*
  • Phylogeny