ITERATIVE PELMATOZOAN COMMUNITY REORGANIZATION: THE KEY TO BLASTOID SUCCESS?

Crinoid genera of the subclass Camerata generally dominated Middle and Late Paleozoic pelmatozoan (stemmed) echinoderm assemblages. Camerate crinoids typically have large, boxy thecae and occupied the upper tier of the pelmatozoan communities. At various times during the Paleozoic, major changes in dominance and diversity of crinoids occurred, often associated with fundamental shifts in sedimentation patterns. During these times the dominance of typical camerate morphologies gave way to camerates that were cladid homeomorphs (e.g., Platycrinites), and pelmatozoan communities often experienced a dramatic rise in abundance and diversity of blastoids and flexible crinoids. This pattern can be documented for the Late Famennian echinoderm faunas from China, Germany, and England and from Early Carboniferous echinoderm communities from eastern North America and Europe. Difficulties in stratigraphic correlation cloud our ability to delineate a third occurrence of this pattern in the Permian of Timor. Blastoid and camerate-crinoid diversity curves through the Paleozoic generally parallel each other as blastoids were able to occupy middle- and lower-level tiering niches in the camerate-dominated communities found on carbonate platforms. However, dramatic increases of blastoid diversity or increases in abundance of particular taxa in the Paleozoic are usually associated with pelmatozoan community reorganization. In the specific cases considered in this research, the blastoids were members of different clades, suggesting that they had the ability to exploit decreased competition during the crinoid reorganizations. Flexible crinoids show a similar pattern. The community reorganizations are associated with fundamental changes in sedimentation regimes from broad, stable carbonate platforms to clastic-carbonate cyclicity, suggesting that the key to the success of blastoids (and perhaps flexible crinoids) lay more the external influences of sedimentation than in internal patterns of evolutionary innovation.