Morphological differentiation of avicularia and the proliferation of species in mid-cretaceous Wilbertopora Cheetham, 1954 (Bryozoa : Cheilostomata)

摘要

Discovery of avicularium-like polymorphs in Wilbertopora mutabilis Cheetham, 1954 has provided not only a new opportunity for revising the genus Wilbertopora Cheetham, 1954, but also a more detailed basis for documenting the series of morphological changes by which avicularia differentiated from ordinary feeding zooids in what appears to be the first occurrence of these characteristic cheilostome bryozoan structures in the fossil record. Eighteen of a total 60 quantitative characters measured on avicularia and ordinary and ovicell-bearing autozooids were sufficient to distinguish eight species of Wilbertopora by discriminant function analysis of zooid data from 93 colonies from the mid-Cretaceous (Albian-Cenomanian) Washita Group in northeastern Texas and southeastern Oklahoma. Eighteen of a total of 20 of the quantitative characters that could be statistically coded for cladistic analysis proved to be informative with respect to parsimony, providing two maximally parsimonious trees for the eight species. Two-thirds of the diagnostic characters involve avicularia. An additional 55 colonies too poorly preserved for morphometric analysis could then be assigned to species qualitatively, with 170 more colonies lacking species-diagnostic characters. The cladistic trees strongly suggest that most or all of the species diverged before the end of the Albian, but stratigraphic resolution is insufficient to test this hypothesis. Nevertheless, the series of morphological changes differentiating avicularia from ordinary autozooids in these species, based on the cladistic relationships, is highly significant statistically, and may be a pattern later repeated in other cheilostomes.