Experimental taphonomy of embryo preservation in a Cenozoic brooding bivalve

To distinguish between alternative explanations for the presence of synchronous broods in the Miocene-Pliocene bivalve, Transenriella species. we performed in situ burial experiments of the Recent species T. corfusa . All Recent Transennella species are asynchronous brooders; a single brood contains all or most developmental stages. Specimens from Miocene—Pliocene deposits of California suggest that some members of this taxon were synchronous brooders, i.e., all the embryos of a brood develop simultaneously with only one developmental stage represented at any time. The presence of synchronous Transennella broods in the Miocene—Pliocene could indicate that an evolutionary change in mode of reproduction has occurred in this genus. Alternatively, asynchronous brooding in this taxon may he conservative and preferential preservation of later stages of development, or seasonal variation in reproduction, could result in a taphonomic overprint. Our burial experiments indicate that the earliest stages of development are almost entirely lost; however. there is enough preservation of the later stages of development to distinguish the two modes of reproduction. Additionally. we discovered a single fossil specimen with an asynchronous brood. Based primarily on this specimen, and observations from the burial experiments, we conclude that the fossil synchronous broods are an artifact of preservation and asynchronous brooding in Transenella is conservative.