How did surviving phylloceratid ammonoids contribute to Jurassic cephalopod diversity?

The phylloceratid ammonoids were critical to the recovery of cephalopod diversity following the end-Triassic extinction. While many other ammonoid groups, such as the ceratitids, suffered total extinction during this global event, the phylloceratids—a group of smooth-shelled ammonites that first appeared in the Early Triassic—were the sole survivors.12

As the earliest true ammonites, these surviving phylloceratids served as the essential rootstock for the subsequent adaptive radiation of ammonites during the Jurassic Period. Their ability to persist through the mass extinction allowed them to diversify rapidly into numerous new forms, filling the ecological niches left vacant by the extinction of other marine groups. Because of their complex suture lines and rapid evolution, these descendants became highly effective index fossils used by scientists to sequence and correlate Jurassic rock strata.2134