A morphometric analysis of Cambrian trilobites shows that extinction events sharply reduced the morphological diversity of ptychopariid trilobites living on the shelf at the lower boundaries of the Marjumiid, Pterocephaltid, and Ptychaspid biomeres (Middle to Upper Cambrian). These extinction events not only separate the trilobite assemblages that characterize each biomere, but they also separate similar sequences of morphological diversification. During the initial deposition of each biomere, the shelf was repopulated by a limited number of ptychopariid species that had similar morphologies of very limited range and type. These initial faunas then underwent a morphological diversification evolving similar morphologies during the deposition of the remainder of each biomere. This pattern of repeating episodes that begin with similar morphologies of trilobites which then undergo similar morphological diversifications suggests that the extinction events at the base of each biomere limited the range of morphologies from which new taxa could evolve. Each new biomere assemblage was unable to build upon the adaptations of the specialized taxa of the previous biomere. Only the limited range of similar morphologies of the initial repopulating trilobites was the source from which new taxa could evolve. As a result, new taxa “reevolved the wheel” each time they became adapted to the vacated habitats that were previously occupied in the underlying biomere.
In contrast, near the Cambrian-Ordovician boundary, the extinction event at the base of the Lower Ordovician Symphysurinid biomere did not reduce the range of morphologies as drastically as the previous biomere extinctions in the Cambrian. A more diverse initial fauna occurs in the Symphysurinid biomere. This fauna did not undergo a morphological diversification during deposition of the biomere and some regions of morphospace previously occupied in the Cambrian were not reoccupied in this biomere. Taxa of the lower Ordovician were relatively canalized in their morphology.
In the Marjumiid biomere (Middle to lower Upper Cambrian), the morphological diversification of ptychoparid trilobites was not significantly influenced by either paleogeography, depositional settings, or migration of taxa from outside North America. The morphological diversification of the ptychopariid assemblage is also seen in a subset of ptychopariids that consists of the Superfamilies Raymondinacea, Asaphiscacea, and Marjumiacea. These superfamilies are the numerically abundant and most diverse taxonomic groups in the Marjumiuid biomere. The patterns of morphological diversification in both the Order Ptychopariida and the combined assemblages of the Superfamilies Raymondinacea, Asaphiscacea, and Marjumiacea were relatively consistent among different paleogeographic regions and were not influenced by general lithofacies. The consistency of the regional patterns and the lack of influence of general lithofacies on these patterns indicate that the overall pattern of morphological diversification is the result of in situ evolution of taxa within the North American Craton and not the result of shifting biofacies or the migration of taxa from outside North America.
The morphological diversification of ptychoparids in the Marjumiid Biomere is composed of two distinct phases. The morphological expansion of the Superfamily Ptychopariacea (the ehmaniellids) in the Ehmaniella Biozone is an early phase of diversification. The morphological expansion of the Superfamilies Norwoodiacea, Solenopleuracea (exclusive of the Family Solenopleuracea), Asaphiscacea, Raymondinacea, and Marjumiacea in the Bolaspidella to Crepicephalus biozones represent the later phase of diversification. This second phase is also composed of two morphological expansions, an early expansion in the Superfamilies Norwoodiacea and Solenopleuracea in the Bolaspidella and Cedaria biozones, and a later expansion in the Superfamilies Asaphiscacea, Raymondinacea, and Marjumiacea in the Cedaria and Crepicephalus biozones. The ptychopariid assemblages of the younger Ptychaspid biomere (Upper Cambrian) also display a two-phase morphological diversification. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/38742 |
| Date | 10 July 2007 |
| Creators | Sundberg, Frederick Allen |
| Contributors | Geology, Bambach, Richard K., Gilinsky, Norman L., McLean, Dewey M., Read, J. Fred, Sepkoski Jr., J. John |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | 2 volumes, BTD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 22356095, LD5655.V856_1990.S934.pdf |
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