The recovery and radiation of Early Jurassic ammonoids: morphologic versuspalaeobiogeographical patterns

The recovery and radiation of the Early Jurassic ammonoid morphospace, as r epresented by nine morphologic groups identified in an earlier study, are t raced through the first 36 subzones of the Hettangian to Domerian time inte rval. A quantitative survey of the dispersion of 436 species over 15 palaeo geographical areas reveals seven palaeobiogeographical patterns, each corre sponding to an exclusive set of species exhibiting similar characteristics in terms of distribution and abundance. This study combines morphologic, ch ronostratigraphical, and palaeobiogeographical data in an attempt to invest igate possible connections between morphologic recovery and radiation patte rns and the history of ammonoid distribution on a global scale. Two out of nine morphologic groups are found to be prevalent in a single palaeobiogeog raphical pattern through the major part of the studied time interval, and a single morphologic group prevails in all the marine areas considered durin g Domerian times. All the other relationships identified are less tightly c onstrained in space and time and indicate loose ties over more limited peri ods and narrower distributions. The present results suggest that such compl ex relationships can be deciphered more successfully by using analytical pa laeobiogeographical patterns as proposed here rather than classical palaeob iogeographical units (biochores). (C) 2001 Elsevier Science B.V. All rights reserved.