ENDEMISM AND DIVERSIFICATION OF MIDDLE AND LATE DEVONIAN RHOMBIFERANS

Glyptocystitoid rhombiferans are a small but successful clade of advanced blastozoan echinoderms with a known stratigraphic range from the Late Cambrian through the Late Devonian. An initial rapid diversification resulted in a global distribution by the Middle Ordovician, which was retained until the clade experienced a dramatic decrease in diversity by the end of the Early Devonian. By the Middle to Late Devonian (Givetian to Frasnian) only Callocystitids remained, and even these appear to be geographically restricted to shallow-water, mixed carbonate-clastic facies of the North American mid-continent in present day Iowa, Michigan and Missouri. Of the seven species known, six are placed into three genera (Strobilocystites, Lipsanocystis, and Adocetocystis) which comprise a distinct Middle to Late Devonian clade characterized by paired hydropores. The seventh species, "Lipsanocystis" oblatus, represented by a single specimen, shows affinities to the primitive (Silurian to Early Devonian) callocystitid Lepocrinites.

Despite extreme reduction in species diversity, Middle to Late Devonian, double-hydropore rhombiferans show a tremendous amount of morphological disparity. Ancestrally, this clade is derived from species that possessed approximately 100 short, thin brachioles borne on wide, unbranched ambulacra. In the Middle to Late Devonian, Strobilocystites evolved branching ambulacra with upwards of 250 short, thin brachioles, although brachiole size retained the primitive condition. This adaptation may have favored increased food collection and potentially increased brachiolar respiration. Lipsanocystis and Adocetocystis, on the other hand, reduced the number of brachioles to between 15 and 50, while greatly increasing brachiole width and length – an adaptation thought to favor collection and ingestion of significantly larger food particles. Together, these observations suggest that even as glyptocystitoids were suffering worldwide extinction, the evolution of fundamentally different feeding strategies amongst paired-hydropore Callocystitids may have allowed the extended survival of these rhombiferans to their last known occurrence in the Lower Frasnian of Iowa.