NAUTILID MORPHOLOGY AND EVIDENCE FOR ECOLOGICAL RELEASE FOLLOWING THE END-CRETACEOUS EXTINCTION OF AMMONOIDS

Extinctions can be agents of long term ecological change by eliminating previously dominant groups and opening ecological niche space through competitive release. Competitive release has been recognized in Cenozoic mammals and ray-finned fishes, but the effects of ecological release on shape, size, and abundance are not well understood. Both ammonoids and nautilids were cephalopods that shared characteristics such as external coiled shells, suggesting they could have ecologically overlapped and competed prior to the end-Cretaceous extinction of ammonoids. Here, we test whether the extinction of ammonoids released competitive pressures and allowed shifts in morphology or increases in abundance among nautilids. We used the paleoTS R package to test for shifts in nautilid evolutionary mode caused by the end-Cretaceous extinction. Pre-extinction size was similar to ammonoids and the best supported model indicated stasis in size evolution. Despite the extinction of ammonoids, there was no shift in mean size at the K/Pg boundary and stasis continued. In contrast, nautilid sutures underwent a punctuated increase in complexity at the extinction event. In the Cretaceous, nautilids and ammonoids occupied distinct parts of morphospace and nautilid shape evolved in a regime of stasis, consistent with competitive interactions acting as a constraint. Beginning in the Eocene, nautilid shape evolution shifted from stasis to a directional trend toward more compressed shell shapes. The evidence for competitive release is mixed, but shape parameters may be more affected by ecological constraints.