BABIES WERE BIGGER IN THE JURASSIC: SIZE EVOLUTION IN EARLY DEVELOPMENTAL STAGES OF CYCLOSTOME BRYOZOANS

Cyclostomes are the only order of stenolaemate bryozoans that have survived to the present-day. Therefore, they are important for obtaining a better understanding of the paleobiology of the extinct stenolaemate orders that dominated Paleozoic biotas. The non-feeding larvae of modern cyclostomes metamorphose on settlement and produce a dome-shaped protoecium, the first-formed part of the skeleton of the founder zooid of the new colony. Available evidence, albeit limited, suggests that the diameter of cyclostome protoecia correlates with larval size, a relationship mirroring that known between molluscan protoconch diameter and larval size. Here we use this relationship to estimate larval size in a suite of tubuliporine cyclostome species and to test whether larval size has changed from the Jurassic to the Recent. Our results suggest that Jurassic cyclostomes had typically larger larvae than Recent cyclostomes, mean protoecial diameter being 20% greater at 204 µm in our Jurassic sample than the value of 169 µm in our Recent sample. This pattern may reflect a more general decrease in body size through time, with modern cyclostomes generally having smaller zooids and colonies. A plausible explanation for the trend of decreasing size is that the appearance in the Late Jurassic, and subsequent radiation in the mid-Cretaceous, of the dominant cheilostome bryozoans drove cyclostomes into a small-size refuge. However, more data is required to test for the influence of biogeographical, ecological and taxonomic factors on size changes through geological time in early developmental stages of cyclostome bryozoans.