Inferring larval type in fossil bryozoans

Pachut, J.F. & Fisherkeller, P. 2010: Inferring larval type in fossil bryozoans. Lethaia, Vol. 43, pp. 396–410. Larval type in extinct organisms might be recognizable because larvae of living marine invertebrates are approximately of the same size as the initial post‐larval organism. Two larval types typically occur. Planktotrophic larvae feed on other members of the plankton, potentially prolonging their larval existence and producing broad geographic distributions. Conversely, lecithotrophic larvae feed on yolk supplied by the fertilized egg, often settle quickly after release, and display more restricted distributions. However, some lecithotrophic bryozoans undergo embryonic fission forming multiple, small, polyembryonic larvae. The relationship between post‐larval size and larval type was evaluated in bryozoans by comparing the size of the ancestrula, the founding individual of a colony, to the sizes of extant planktotrophic, lecithotrophic and polyembryonic lecithotrophic larvae and ancestrulae. The sizes of larvae and ancestrulae in extant lecithotrophic and planktotrophic cheilostome (gymnolaemate) species are statistically the same. They are, however, statistically larger than the polyembryonic larvae of extant cyclostomes (stenolaemates). In turn, the sizes of cyclostome larvae are indistinguishable from the ancestrulae of extant and fossil cyclostomes, the ancestrulae of other fossil stenolaemate species measured from the literature, and the ancestrulae of three of four genera from North American Cincinnatian strata. Ancestrulae of a fourth genus, Dekayia, are the same size as cyclostome ancestrulae but are statistically smaller than the ancestrulae of other stenolaemates. With few exceptions, stenolaemates have statistically smaller larvae and ancestrulae than both lecithotrophic and planktotrophic cheilostomes. We infer that the sizes of fossil ancestrulae permit the discrimination of taxa that had polyembryonic lecithotrophic larvae from those possessing other larval types. This inference is strengthened, in several cases, by the co‐occurrence of brood chambers (gynozooecia) and restricted palaeobiogeographic distributions. The presence of cyclostomes in Early Ordovician strata suggests that polyembryony may have been acquired during the initial radiation of Class Stenolaemata. Polyembryony appears to be a monophyletic trait, but confirmation requires the demonstration that species of several stenolaemate suborders lacking skeletally expressed brood chambers possessed polyembryonic larvae. □Ancestrulae, evolution, fossil bryozoans, gynozooecia, larvae.