Worthenopora: an unusual cryptostome (Bryozoa) that looks like a cheilostome

Abstract

Worthenopora is a Mississippian cryptostome bryozoan genus with certain characteristics that so closely resemble those of some cheilostomes (Jurassic–Recent) that previous bryozoan experts speculated whether Worthenopora should be assigned to the order Cheilostomata. This raises the question of possible affinities between cryptostomes (class Stenolaemata) and cheilostomes (class Gymnolaemata). Based on wall structure, growth sequence, zooecial shape, and secondary structures such as stylets, Worthenopora is here shown to be a stenolaemate, assignable to the suborder Ptilodictyina with other bifoliate cryptostomes. Although the exterior appearance of Worthenopora superficially resembles some cheilostomes, it represents rather limited divergence from more typical bifoliate growth patterns. Additionally, presence of characteristics such as elongate zooecia with thin exozones, cuticular medial plane, and distinctive exterior ridges justifies retention of the monogeneric family Worthenoporidae. Three species (W. spatulata, W. spinosa, and W. valmeyerensis), known only from North America and primarily restricted to the Illinois Basin, are recognized. Analysis of apertural spacing in these species suggests a trade-off between feeding efficiency and occupation of water-column space. Worthenopora valmeyerensis is considered a hypermorphic derivative of W. spinosa.