Crustacean disparity through the Phanerozoic: comparing morphological and stratigraphic data

Crustaceans have been an important component of marine diversity and biomas s since the earliest Phanerozoic. With a relatively well-documented fossil record, they provide an excellent subject for a continuous study of dispari ty (approximate to bodyplan variety) from the Cambrian to the Recent. A dat a base of 135 morphological characters Forms the basis for cladistic and mo rphospace studies at the ordinal and sub-ordinal level. Gross cladistic top ology is: (Eumalacostraca + Hoplocarida vs Maxillopoda) vs Phyllopoda (para phyletic). Each of these groups is of approximately equal disparity, and oc cupies a distinct region of the morphospace plot. A few problematical fossi ls (e.g. Waptia and Odaraia) fall close to the base of the tree. Comparison of the cladogram with stratigraphic range data indicate the location of pr obable ghost lineages, and randomization procedures provide a statistical t est of the goodness of fit of a given set of stratigraphic ranges to a give n tree topology. Disparity indices are calculated at series and stage inter vals. Observed range data indicate that Cambrian disparity was approximatel y one third its present level. The Earliest Ordovician saw a marked decreas e, with an increase and subsequent plateau through rest of the period. Incr eases through the Silurian and Devonian corresponded to the radiation of br anchiopods, cephalocarids, and latterly the Eumalacostraca and Hoplocarida. By the end of the Carboniferous, observed disparity had reached ol er four fifths of Recent levels, and die remaining history of the group saw a grad ual but slightly irregular increase up until the end of die Tertiary. Indic es of disparity incorporating ghost lineages exhibit less marked peaks and troughs, with fewer perturbations overall. Cladistically-implied disparity in the Lower Cambrian is estimated at three quarters of that ill the Recent . Rarefaction is used to compare actual levels of disparity at each time in terval with the mean for a similar number of taxa selected randomly from th e: list of all realized bodyplans. Mosi intervals preserved a range of form s more disparate than the mean of random samples drawn from the pool of all the taxa considered. From the Triassic to the Recent this difference was i ntermittently significant. Once occupied, extremes of morphospace tend nor to fall vacant again. (C) 1998 The Linnean Society of London.