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From ur- (original) +‎ bilaterian.


urbilaterian (plural urbilaterians)

  1. (biology) The hypothetical last common ancestor to all bilaterians.
    • 2007, Aaron G. Filler, The Upright Ape: A New Origin of the Species, page 153,
      There is no fossil record to show that the urbilaterian existed. However, the echoes of the existence of this creature are recorded in stunning clarity in the genes of every living descendant, from fruit fly to spider to human.
    • 2008, Philosophical Transactions: Biological Sciences, Volume 363, Issues 1493-1496, page 1523,
      This way, different aspects of the molecular architecture of the urbilaterian nervous system are reconstructed and yield insight into the degree of centralization that was in place in the bilaterian ancestors.
    • 2011, Wallace Arthur, Evolution: A Developmental Approach, unnumbered page,
      The competing hypotheses of simple and complex urbilaterians are still being debated at the time of writing.