Increasing morphological complexity in multiple parallel lineages of the Crustacea

*Division of Biology, Imperial College London, Silwood Park Campus, Ascot SL5 7PY, United Kingdom; and
^‡Department of Biology and Biochemistry, University of Bath, The Avenue, Claverton Down, Bath BA2 7AY, United Kingdom

Edited by James W. Valentine, University of California, Berkeley, CA, and approved January 31, 2008 (received for review October 2, 2007)

Abstract

The prospect of finding macroevolutionary trends and rules in the history of life is tremendously appealing, but very few pervasive trends have been found. Here, we demonstrate a parallel increase in the morphological complexity of most of the deep lineages within a major clade. We focus on the Crustacea, measuring the morphological differentiation of limbs. First, we show a clear trend of increasing complexity among 66 free-living, ordinal-level taxa from the Phanerozoic fossil record. We next demonstrate that this trend is pervasive, occurring in 10 or 11 of 12 matched-pair comparisons (across five morphological diversity indices) between extinct Paleozoic and related Recent taxa. This clearly differentiates the pattern from the effects of lineage sorting. Furthermore, newly appearing taxa tend to have had more types of limbs and a higher degree of limb differentiation than the contemporaneous average, whereas those going extinct showed higher-than-average limb redundancy. Patterns of contemporary species diversity partially reflect the paleontological trend. These results provide a rare demonstration of a large-scale and probably driven trend occurring across multiple independent lineages and influencing both the form and number of species through deep time and in the present day.

Footnotes

^†To whom correspondence may be sent at the present address:
Department of Biology, University of Waterloo, 200 University Avenue West, Waterloo, ON, Canada N2L 3G1.
E-mail: sadamowi{at}gmail.com
^§To whom correspondence may be addressed. E-mail: m.a.wills{at}bath.ac.uk
Author contributions: S.J.A., A.P., and M.A.W. designed research; S.J.A. and M.A.W. performed research; S.J.A., A.P., and M.A.W. analyzed data; and S.J.A., A.P., and M.A.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0709378105/DC1.