Page 368 - Introduction to Paleobiology and The Fossil Record
P. 368

SPIRALIANS 2: MOLLUSKS  355


                              post-spawning mortality model: Antarctic example
                          shelf                                                         basin
                                    spawning ground
                                                             migration to spawning grounds


                              post-spawning mortality
                                                                            limited mortality

                                                      turbidity-current triggered
                            accumulation and concentration
                            by shallow-marine processes
                                                                          belemnite-rich turbidite


                                                                               sparse belemnites
                            (a)


                             predation concentration model


                                                          predation






                                                                    regurgitates

                               foundered vertebrate with intact gastric mass




                          (b)
             Figure 13.23 Belemnite battlefields and their possible origin: (a) post-spawning mortality model and

             (b) predation concentration model. (From Doyle & MacDonald 1993.)

             Watsonella dominated rostroconch faunas of      probably derived independently from a sepa-
             the Tommotian. The rostroconchs diversifi ed     rate monoplacophoran ancestor. In some
             during the Ordovician to reach an acme in the   respects, the rostroconchs may represent a
             Katian when all seven families were repre-      missing link between the univalved and
             sented. They probably occupied similar eco-     bivalved molluskan lineages, while their
             logical niches to those of the bivalves.        unlikely morphology may have contributed
             However, there followed a decline in abun-      towards their late discovery.
             dance and diversity until fi nal  extinction  at
             the end of the Permian when only conocar-
             diodes such as Arceodomus were still extant.    EVOLUTIONARY TRENDS WITHIN
               The rostroconchs occupy a pivotal position    THE MOLLUSCA
             in molluskan evolution (Runnegar & Pojeta       A spectacular variety of mollusk morphotypes
             1974). The group developed from within the      and life modes evolved during the Phanero-
             monoplacophoran plexus with a loss of seg-      zoic, from the simple body plan of the arche-
             mentation; the rostroconchs themselves gen-     mollusk. Despite the diversity of early mollusks
             erated both the bivalves and the scaphopods     in the Cambrian, the phylum was not notably
             whereas the gastropods and cephalopods were     conspicuous in the tiered suspension-feeding
   363   364   365   366   367   368   369   370   371   372   373