FREEMAN, G.: A Developmental Basis for the Cambrian Explosion: Evidence from Extant Brachiopods

Paleontological evidence indicates that a large number of new animal phyla, in addition to classes and orders within phyla, were first observed during the Cambrian. One possible basis for the genesis of new morphologies during this period may have been the ease with which developmental programs responsible for regional specification during early development could be changed. Subsequently these developmental programs are postulated to have become more tightly integrated, making changes in early development more difficult to effect and thereby reducing the variation that natural selection can operate on. Within the Linguliformea and Craniiformea, extant lingulids, discinids and craniids belong to orders that originated during the Cambrian. Within the Rynchonelliformea, extant rhynchonellids belong to the pentameride lineage that also originated during the Cambrian. The other order, Terebratulida, with extant families, originated via a series of intermediate forms, from a pentameride ancestor at the base of the Devonian. By comparing cleavage patterns, fate maps, patterns of morphogenetic movements during gastrulation, the timing of regional specification during early development, and the mechansisms responsible for regional specification in extant species one can assess whether there is more variation in the ways of going about early development in descendants of orders that originated during the Cambrian versus descendants of the Terebratulida which originated later. There are major differences in embryogenesis in groups that originated during the Cambrian. There are essentially no differences in the early embryologies of the rhynchonellid Hemithyris, and two representatives of the Terebratulida, although these forms are morphologically disparate.