Meeting Abstract

S8-1.1  Saturday, Jan. 5  The origin of Decapoda SCHRAM, F.R.; Burke Museum, University of Washington fschram@u.washington.edu

Any effort to elucidate a decapod ancestor runs into several problems. First, while there is general agreement about relationships within Decapoda, there is considerable difference of opinion about the sister group of Decapoda. Some advocate Euphausiacea; others promote a clade of all other Eumalacostraca; still others see a link to Hoplocarida; a few wonder about a range of �eocarids� in the middle to late Paleozoic. Considerations of various possible out-groups could affect notions of a decapod ground pattern. Second, the earliest fossil decapod in the Late Devonian, Palaeopalaemon newberryi, is actually an advanced reptant; there are no known natant decapods earlier than that. Hence, we see no convergence in the fossil record back in time towards an ancestral type. Third, known fossils of Late Paleozoic Eumalacostraca are found in strata left by the epicontinental epeiric seas of that time. If decapods took origin elsewhere, we are at a loss. Fossils from alternative ancestral habitats are few and far between, such as the deep sea of the open oceans or in groundwater habitats. Fourth, our search for an ancestral type might actually be misdirected by our search image, i.e., assumptions that decapod origins must lie among natant �shrimp.� Evidence from developmental genes indicates that a creature with a pleon of at least 8 segments be sought. We might actually postulate an alternative ancestor: some minute, early Paleozoic, inhabitant of either ground water or the surface flocculent layers, lacking a carapace, with incipient specializations of anterior thoracopods towards maxillipeds, an 8 segment pleon, and a broad terminal telson or pleotelson. We might seek such a creature in early and middle Paleozoic localities that preserve micro-crustaceans.