HR: 1340h
AN: B33B-0255    [Abstracts]
TI: The Urbilateria: Insights From Comparative Developmental Biology Into The Cambrian Radiation
AU: * Erwin, D H
EM: erwind@si.edu
AF: Smithsonian Institution, Dept. of Paleobiology, MRC-121, Washington, DC 20560 United States
AB: A critical issue in understanding the patterns of morphological and developmental evolution during the Cambrian radiation of animal body plans is the morphological complexity of the last common ancestor of the two great bilaterian clades, the protostomes and deuterostomes. Often known as the Urbilateria or the protostome-deuterostome ancestor (PDA) this node in metazoan phylogeny distinguishes the sponges and cnidarians from the bilaterian clades. Highly conserved developmental genes and complexes suggests that the PDA was a relatively complex organism, with dorsal-ventral and anterior-posterior patterning, eyes, appendages, heart, central nervous system, segmentation and other relatively complex morphological features. If this view of the PDA is correct, it has important implications for understanding the Cambrian radiation, for such a complex animal is likely to leave traces in the sedimentary record even if it lacked a durable skeleton. However, this interpretation of conserved developmental sequences may overestimate the extent to which homologies of developmental sequences implies morphological conservation. In other words, does the presence of the Nkx 2.5 gene in the PDA necessitate the presence of a heart, or simple the appearance of contractile muscles? If the latter, then the PDA may have been a much less complex organism. This latter view of developmental evolution suggests the PDA possessed a developmental toolkit, but that the specific implementation of these tools in specific body plans occurred after the PDA, within specific clades. Moreover, this view also strongly suggests that changes in the physical environment, or in ecological relationships, was primarily responsible for the evolutionary innovations seen in the Cambrian, rather than developmental innovations.
DE: 9614 Paleozoic
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting