YAMAGUCHI, S; OAKLEY, TH; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara: Ostracod crustaceans as a key to testing an early Paleozoic diversification

Did an explosive period of animal divergences, the so-called “Cambrian explosion” , occur in the early Paleozoic? Although molecular phylogenic data can address this question, the taxa used often have poor fossil records, resulting in difficulties for accurately estimating divergence times. For two reasons, the Ostracoda (Crustacea) are key to understanding early Paleozoic divergences. First, several divergent ostracod fossils appeared just after the Cambrian (in the early Ordovician). Second, ostracods have a rich fossil record throughout the Phanerozoic. Taken together, these facts strongly suggest that ostracods diversified rapidly in the early Paleozoic. We are using the estimated phylogenetic relationships and divergence times of ostracods to distinguish between two interesting hypotheses. First, if molecular estimates of divergences are concordant with first fossil appearance, ostracods may have undergone an explosive divergence pattern. Alternatively, the origin of ostracods might be long before their first appearance in the fossil record. We performed molecular phylogenetic analyses using ostracods from four extant lineages (orders) – Podocopida, Platycopida, Myodocopida, and Manawa (palaeocopidan or another unknown lineage) – plus closely related crustaceans from Branchiura and Pentastomida. We used standard methods to infer the phylogeny based on the 18S and 28S rDNA sequences and examined divergence times with relaxed molecular clock method. Interestingly, relaxed clock methods suggest that the ostracod lineages diverged at least in or before the early Cambrian. This result has broad implications because an accurate divergence time of ostracod origins provides a minimum date for divergences of major animal lineages.