Abstract

This study considers the biogeographic origins, evolutionary affinities, and patterns of diversification and extinction in a portion of the Lower and Middle Devonian proetid trilobite fauna of Eastern North America. Four generic clades comprising about 45 species are known from the strata of the upper Emsian (Sawkillian) Bois Blanc Limestone and Schoharie Grit, the Eifelian (Southwoodian) Onondaga Limestone, and the Givetian (Cazenovian, Tioughniogan, Taghanic) Hamilton Group. These taxa have traditionally been assigned to the subfamilies Proetinae Salter, 1864, and/or Dechenellinae Pribyl, 1946. These are Crassiproetus Stumm, 1953 a, Basidechenella Richter, 1912, Dechenella Kayser, 1880, and Monodechenella Stumm, 1953a, which were originally considered to be closely related. A higherlevel phylogenetic analysis of the Proetinae is conducted to see if these taxa were indeed closely related and thus represent a single endemic radiation of species in Eastern North America or rather a series of independent lineages in that region. In the course of discerning characters that defined the Proetinae, it was discovered that Monodechenella lacks several of the characters that define the Proetinae, and the members of this genus therefore must be excluded from this subfamily. They instead appear to belong to a larger group informally referred to as the "Thebanaspis clade," which appears to be closely related to the Proetinae. A phylogenetic analysis is performed on proetine ingroup taxa using 21 taxa and 53 characters, and several of the major generic clades in the Proetinae are considered. The phylogenetic analysis of the Proetinae is used to ascertain the ancestral biogeographic states for the three genera in the Proetinae that form an important component of the Lower and Middle
Devonian trilobite fauna of Eastern North America. This information is used to determine if these taxa are ancestrally present in Eastern North America or rather represent a series of invasions from other biogeographic regions. Other taxa occurring in Eastern North America at this time appear to represent elements that invaded from Armorica. This invasion of taxa has been related to the collision between plates that produced the
Acadian Orogeny during the Middle Devonian. Patterns in these proetid taxa are compared with those known for other groups to ascertain what control the Acadian Orogeny as a biogeographic event may have had on the appearance of these taxa in Eastern North America.Phylogenetic analysis is then performed on all available species in each of these generic clades that occur in Eastern North America. Species that belong to these clades but which hail from otherbiogeographic regions are also considered. These phylogenies were used to assess macroevolutionary patterns such as diversification and extinction within each ofthese clades in Eastern North America. In addition to being an important paleogeographic event, the Acadian Orogeny also caused major paleoenvironmental changes. The impact of these changes on the proetid fauna of Eastern North America is assessed. It appears that a phenomenon analogous to Vrba's (1985, 1992) Turnover Pulse Hypothesis may have mediated elevated speciation rates in the proetid taxa over the period considered. However, eventually the profound changes in environment appear to have led to the extinction of much of the proetid trilobite fauna of Eastern North America. Information on patterns of occurrence in different geographic regions is combined with information from the phylogenetic analyses of the individual generic clades to consider large-scale biogeographic patterns in the late Lower and Middle Devonian. A method for considering biogeographic patterns using cladistic information is developed. This method is based on Brooks Parsimony Analysis, but it allows multiple events of range expansion and subsequent vicariant splitting to occur within each generic clade. This biogeographic method was used to evaluate the relationships between the Arctic, Armorican, and Eastern North American paleobiogeographic regions. The relationships between the different major sedimentary basins in Eastern North America, the Appalachian, Illinois, and Michigan basins, are also considered. Finally, the origin of major evolutionary faunas in the fossil record is discussed. Included is a brief discussion of a depauperate proetid fauna of the Emsian and Eifelian whose members do not belong to the four generic clades considered in detail. In this work four new genera are recognized: Plesiowensus, Arcticormistonia, Aayemenaytcheia, and Milesdavis. In addition, 12 new species are described: Arcticormistonia edgecombei, Crassiproetus halliturgidus, C. neoturgidus, C. stummi, C. schohariensis, Basidechenella cartwrightae, B. timwhitei, Dechenella perscheii, D. carvalhoae, Pedinodechenella modelli, Milesdavis eldredgei, and Monodechenella legrandsmithi. Diagnoses and discussions for all of the taxa considered are presented.