The evolution of the Eucarida, (Crustacea, Eumalacostraca), in relation to the fossil record

Abstract

Eumalacostracan fossils from the Mississippian indicate the beginnings of Recent superorders other than the Eucarida; which latter probably also differentiated around this time, from a primitive shrimp with carapace sculpture like that of Palaeopalaemon, in a marine form not yet discovered There is no valid evidence to suggest Eucarid polyphyly. The only Paleozoic record of the superorder seems to be the peculiar form Palaeopemphix from the Permian, which is certainly not a Glypheid but may represent an early, calcified offshoot of the stem-form of the order Decapoda (the family P ALEOPEMPHICIDAE, n.). In the early Triassic, not only are the known Eucarids definitive Decapods, but this order was already differentiated into the suborders Dendrobranchiata and PLEOCYEMATA ( n.), which were themselves already subdivided (e.g., the Peneidae were presumably already separated from the AEGERIDAE, n. fam.). The available Mesozoic representation undoubtedly includes a disproportionate frequency of forms specialized by calcification; and this record seems too late, fragmentary and non-consecutive to supply crucial evidence either for or against the present view of Decapod phylogeny founded on evidence from Recent forms. The habits and characteristics to be expected of Paleozoic fossils representing the hypothetical stems of Eucarid groups are outlined.