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A Bridge Too Many? Phylogeny and Biogeography of the Upper Cambrian Saukiid Trilobites

  • Author(s): Collette, Joseph Henry
  • Advisor(s): Hughes, Nigel C.
  • et al.
Abstract

The family Saukiidae Ulrich and Resser, 1933 was one of the largest groups of late Cambrian trilobites, with representatives occurring on all major paleogeographic plates. However, inconsistent systematic practices that have been historically applied to this group have resulted in a muddled concept of the Saukiidae and an exaggerated species count. This has led to erouneous placement of many taxa due to a lack of distinct characters with which to diagnose some saukiid genera and species. To mitigate past systematic digressions and to re-elevate Saukiidae to a useful concept, a series of cladistic analyses were performed that incorporated all global occurrences of the Saukiidae for which there is a good fossil record. As a further test of the familial relationships of the Saukiidae, members of the Ptychaspididae and Pterocephaliidae were included to test what effect, if any, different outgroups would have on character polarity and the branching point with the Saukiidae. Results vary with outgroup choice, but suggest that some of the characters uniting Ptychaspididae and Saukiidae may be convergent. Because the 'upside-down' stratigraphic placement of taxa recovered with this outgroup is inconsistent with stratigraphic evidence, the Pterocephaliidae outgroup is preferred. The preferred phylogeny indicates that the Saukiidae is monophyletic, and is composed of two distinct groups of subfamilial rank: the Saukiinae, n. subfam., defined by the absence of a preglabellar field (Calvinella, Danzhaisaukia, Eosaukia, Linguisaukia, Lophosaukia, Mictosaukia, Saukia, Sinosaukia, and Tellerina); and the Prosaukiinae n. subfam., defined by possession of a preglabellar field (Anderssonella, Hoytaspis, Lichengia, Pacootasaukia, Prosaukia, and Saukiella). The ancestral geographic area(s) for each node on the preferred cladogram was then computed using Fitch optomization. The Fitch analysis revealed that toward the base of the tree, descendants that had originated from ancestors living in Laurentia likely moved westward towards Australia and North China, most likely as larvae. Further up the tree, a single dispersal event appears to have moved eastward toward South China; all taxa in a more derived position to this dispersal occur in Gondwana indicating a radiation across Gondwana from this event, and a likely Laurentian origin of the family Saukiidae.

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