Identifying Larval Type in the Fossil Bryozoans

 Joseph F. Pachut and Margaret M. Fisherkeller

Department of Geology, Indiana University-Purdue University at Indianapolis, 723 West Michigan Street, Indianapolis, IN 46202, and
Indiana State Museum, 202 North Alabama Street, Indianapolis, IN 46204 <PFisherkeller@dnr.state.in.us>

ABSTRACT--The larvae of living marine invertebrates appear to be the same size as the initial post-larval organism permitting the recognition of larval type for extinct, fossilized, organisms. Planktotrophic larvae of living marine invertebrate species are small ecological generalists that generally have a long larval existence and a broad geographic range. Conversely, living lecithotrophic larvae are larger, often settle quickly after release, and typically have more restricted species distributions. The larvae of some lecithotrophic bryozoans complicate the relationship between larval size and type by undergoing fission forming polyembryonic larvae. These larvae are smaller than non-polyembryonic lecithotrophic larvae and, based on size alone, could be confused with planktotrophs.
   The relationship between size and larval type was evaluated in bryozoans by measuring the sizes of ancestrulae, the founding individual of a colony. Fifty-seven fossil and Recent specimens selected from the literature and forty-five fossil specimens collected from a portion of the Upper Ordovician, Cincinnatian, strata of southeastern Indiana were measured. Cincinnatian trepostome (stenolaemate or tubular) genera include representatives of Dekayia, Parvohallopora, Heterotrypa, and Homotrypella. Cyclostomes are the only living stenolaemate suborder and possess polyembryonic larvae. No statistically-significant difference in ancestrular sizes could be detected between living cyclostome larvae, the ancestrulae of fossil cyclostomes, the ancestrulae of the majority of fossil stenolaemate species measured from the literature, and the ancestrulae of each of the four genera of Cincinnatian trepostomes. All of these groups are smaller, statistically, than the larvae or ancestrulae of cheilostomes. It appears that the sizes of fossil ancestrulae may be useful in distinguishing between taxa that had non-polyembryonic lecithotrophic larvae and those that had small planktotrophic or polyembryonic lecithotrophic larvae. However, size alone does not permit clear discrimination between taxa that had a planktotrophic larva and those that had polyembryonic lecithotrophic larvae. Small sizes coupled with restricted paleogeographic distributions suggest that the larvae of most stenolaemates were polyembryonic lecithotrophs. Exceptions include specimens of Paleschara, Prasopora, and Peronopora that most likely possessed large lecithotrophic larvae.
   Brood chambers have been used as indicators of polyembryony in the fossil record and they have been reported in thirty-one Paleozoic species. Paleogeographic distributional data exist for at least four of these species and indicates restricted distributions, strengthening the inference of polyembryony.
   The presence of Early Ordovician cyclostomes suggests that polyembryony may have been acquired during the initial radiation of the phylum. However, the taxonomic distribution of brood chambers currently indicates that polyembryony represents a polyphyletic attribute unless species of several other stenolaemate suborders possessed polyembryonic lecithotrophic larvae in the absence of skeletally expressed brooding structures. If true, then the acquisition of polyembryony in the Stenolaemata would represent a monophyletic trait with exceptions to this pattern reflecting the later reacquisition of full-sized, non-polyembryonic, lecithotrophic larvae.