Evolutionary History of the Ostracoda and the Origin of Nonmarine Faunas

Abstract

The Ostracoda are one of the most diverse arthropod groups alive today; they also have a tremendous fossil record. Because of their widespread environmental distributions, small size and carbonate shell, they have become extremely useful biostratigraphic and paleoenvironmental proxy indicators, particularly in nonmarine environments. Despite this utility, little is known about the phylogenetic history of this important group. We reconstructed a phylogenetic history of the major orders and suborders of Ostracoda in order to test the legitimacy of current classification schemes, determine if it is possible for ostracodes to have a Precambrian origin, and test the fidelity of some of the major morphological characters that have documented trends of either increased complexity, such as the hinge and marginal pore canals, or reduction in segments, such as the adductor muscle scar.In our phylogenetic analysis to test taxonomic fidelity, we coded seven morphological hard part characters for nine taxa from the orders Archaeocopida, Leperditicopida, Palaeocopida, Podocopida, and Myodocopida. A parsimony analysis was performed using PAUP (v. 4.0) yielding 4 trees of 17 steps with low levels of homoplasy and a strong phylogenetic signal. A majority rule consensus tree indicates there is not complete agreement between the standard classification scheme and the phylogeny produced by the characters used to establish the classification. In our complete analysis of Ostracoda, we coded 28 morphological characters that included 14 hard part and 14 soft part characters for twelve taxa that include the Archaeocopida, Leperditicopida, Podocopida, and Myodocopida. A parsimony analysis was completed using PAUP (v. 4.0) yielding 1 tree of 125 steps with low levels of homoplasy and a strong phylogenetic signal. An unrooted analysis of this character set has the Cambrian Archaeopodocopida and the Ordovician-Devonian Leperditicopida in an unresolved polytomy with much younger groups such as the Myodocopina, suggesting a much deeper split in the lineage and a possible Precambrian origin for the Ostracoda. Testing the various character state acquisitions over the tree indicates that the hinge does not show an increase in complexity within a phylogenetic context, while the adductor muscle scars do show a significant trend of decrease in complexity across the tree topology. The marginal pore canals, which are functionally tied to osmoregulation as well as carapace secretion, are extremely homoplastic, indicating that this character, which is related to nonmarine invasions and tolerances, was acquired many times throughout the evolutionary history of Ostracoda.By creating an evolutionary framework for the Ostracoda such as is presented here, we can further assess character state acquisition, and how it functionally and evolutionarily relates to ostracode paleoenvironmental tolerances. The framework will not only allow us to understand the overall evolution ofthis group but will also allow us to compare the history of the ostracode clade with other groups that also have a history ofmarine and nonmarine transitions.