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Early Palaeozoic Biogeography |
University of Texas at Arlington, Arlington, Texas 76019-0049, USA
The biogeography of Cambro-Devonian nautiloid cephalopods is documented from their origin during Late Cambrian through to the end of the Devonian. The biogeography developed as the consequence of two related sets of events with different magnitudes. Events controlling the biogeography of first order magnitude were geotectonic, either Gondwana glaciation due to plate movement or contraction in size of oceanic ridges related to decreased rates of sea floor spreading, manifested as first and second order eustatic fall in sea level. These first order events separate nautiloid biogeography into four episodes: (1) Late Cambrian; (2) Ordovician; (3) Silurian through Early Devonian; and (4) Middle through Late Devonian. Each began with an expansion in terms of generic diversity and ended with a crisis of some proportion. Only the end of the Late Cambrian episode cannot be explained by geotectonic events.
Events of the second order are both biologic and geotectonic and involve the interaction between the dispersal characteristics of nautiloid cephalopods and plate movements. Due to lack of a planktonic larva and depth restrictions as a function of shell design, the ability of nautiloids to disperse and colonize shallow shelf seas separated by expanses of ocean or depths exceeding shell design limits was minimal. When the width of oceans or depths decreased through some form of geotectonics, nautiloids rapidly radiated and colonized regions as they became available. Thus second order biogeographic episodes tend to evolve from a collection of disjunct patterns with high faunal similarity among faunas of particular landmasses, but with little similarity among separate landmasses, to rapid development of high similarities both among and within faunas of converging landmasses once the physical barrier of distance and water depth is eliminated.
