Metrarabdotos Canu, 1914 and the related genera Escharoides Milne Edwards, 1836b, Adeonellopsis MacGillivray, 1886, and Reptadeonella Busk, 1884 were key taxa in the decline of Bryozoa with erect, arborescent colonies and concomitant increase in numbers of species with encrusting colonies in the late Paleogene and Neogene of tropical America. In particular, the abundance, continuity of occurrence, and diversity of Metrarabdotos before its decline have made this genus, over the past 20 years, a model taxon for detailed morphometric studies of fine-scale evolutionary tempo and mode. During the same period, significant new occurrences of both Metrarabdotos and its near relatives have been documented with detailed collections from tropical American areas not represented in the original studies.
In this paper we present a complete reanalysis of the original morphometric data (Cheetham, 1986a), incorporating the new tropical American material and comparative material of selected Metrarabdotos species from Europe and Africa in order to formalize the taxonomy of the genus, many species of which have been in open nomenclature since 1986, and to explore their possible phylogenetic relationships to each other and to Escharoides, Adeonellopsis, and Reptadeonella. The new analysis, with procedures slightly modified from those used in the original analysis, is based on more than twice the number of specimens and 20% more morphological characters. Although the results include changes in species assignments for 13% of the specimens in the original analysis, the pattern of intraspecific morphological stasis previously identified in the tropical American Neogene species, and thus the concomitant interpretation of evolutionary tempo and mode, are unaltered.
Cladistic analysis resulted in a single most parsimonious tree for the 22 tropical American Metrarabdotos species, arranged in two monophyletic crown groups and a paraphyletic stem group. The stem group, ranging from latest Eocene to late Early or early Middle Miocene in age, includes four species, two of which are new: M. aguilerai from Venezuela and M. hispaniolae from the Dominican Republic and Haiti. Crown group A comprises seven species occurring in deposits of Late Miocene to Recent age, four of which are new: M. arawakorum from Venezuela; M. boldi and M. saundersi from the Dominican Republic; and M. coatesi, which occurs in the Dominican Republic, Costa Rica, and Panama. Crown group B, comprising 11 species, ranges from Early Miocene to Recent and includes five new species: M. cubaguaense from Venezuela; M. vokesorum from the Dominican Republic, Costa Rica, and Panama; and M. tainorum, M. jungi, and M. lopezense from the Dominican Republic. Incorporation of six European and African (eastern Atlantic) species, ranging in age from Early Miocene to Recent, into cladograms with the tropical American species produced more variable results in terms of both numbers of alternative trees and the positions of the tropical American species in them. One of these species, M. thomseni from the Pleistocene of Greece, and possibly a second, M. cf. M. maleckii Cheetham, 1968 from the Miocene of the Czech Republic, are new. With one exception, the eastern Atlantic species were placed in more or less basal positions in one or the other of the crown groups, suggesting that, as hypothesized by Cheetham (1968), the “Old World” and “New World” species of Metrarabdotos represent largely independent radiations. The exception is the living species M. cookae Cheetham, 1968, from West Africa and the Cape Verde Islands, which is placed unequivocally in a terminal position in crown group B. Although the three groups of tropical American species correspond in part to subgenera of Metrarabdotos erected by Cheetham (1968), the paraphyly of the stem group and the unresolved identity of the type species of the subgenus apparently corresponding to crown group A suggest that formalizing the taxonomy of these groups is unwarranted at present.
A further cladistic analysis, incorporating the three groups of Metrarabdotos species and Escharoides, Adeonellopsis, and Reptadeonella, yielded two trees, both of which support Cheetham's (1968) hypothesis that Metrarabdotos is more closely related to Escharoides than to Adeonellopsis and Reptadeonella. Escharoides is represented in tropical American Neogene deposits by two species, one of which, E. guraboensis from the Dominican Republic, is new. Of the six tropical American Neogene species of Adeonellopsis, four are new: A. cribrospiramen and A. guraboensis from the Dominican Republic, A. limonensis from Costa Rica, and A. antilleana from Cuba and Jamaica. Three of the six tropical American Neogene species of Reptadeonella, R. buddae from the Dominican Republic, R. collinsae from the Dominican Republic, Panama, Costa Rica, and the southeastern United States, and R. buricaensis from Panama, are also new.