The recovery and radiation of Early Jurassic ammonoids: morphologic versus palaeobiogeographical patterns

The recovery and radiation of the Early Jurassic ammonoid morphospace, as represented by nine morphologic groups identified in an earlier study, are traced through the first 36 subzones of the Hettangian to Domerian time interval. A quantitative survey of the dispersion of 436 species over 15 palaeogeographical areas reveals seven palaeobiogeographical patterns, each corresponding to an exclusive set of species exhibiting similar characteristics in terms of distribution and abundance. This study combines morphologic, chronostratigraphical, and palaeobiogeographical data in an attempt to investigate possible connections between morphologic recovery and radiation patterns and the history of ammonoid distribution on a global scale. Two out of nine morphologic groups are found to be prevalent in a single palaeobiogeographical pattern through the major part of the studied time interval, and a single morphologic group prevails in all the marine areas considered during Domerian times. All the other relationships identified are less tightly constrained in space and time and indicate loose ties over more limited periods and narrower distributions. The present results suggest that such complex relationships can be deciphered more successfully by using analytical palaeobiogeographical patterns as proposed here rather than classical palaeobiogeographical units (biochores).