FOULING FILTRATION FANS: HOW CHANGING SEDIMENTOLOGICAL REGIMES CHANGED FAN ORIENTATION IN BOTTOM DWELLING BLASTOIDS

Blastoids were members, sometimes minor and sometimes major, of Paleozoic echinoderm communities. Patterns of blastoid abundance, diversification, and major trends in morphologic adaptation reflect changing climatic, biogeographic, and sedimentological patterns in the Paleozoic. In this study, I consider these factors on the distribution and autecology of bottom-dwelling blastoids. Silurian through middle Mississippian blastoids were members of the Middle Paleozoic crinoid fauna. Bottom dwelling blastoids from these faunas were eleutherozoic with tetralobate, bilaterally symmetrical, stemless thecae and a plane of bilateral symmetry passing through the AB-D axis. All sat on the sea floor on the D ray. The filtration fan was oriented perpendicular to the bottom. Typically these blastoids lived in carbonate environments and fouling of the filtration fan at the sediment – water interface was minimal.

Blastoid abundance and diversity was greatly reduced in the Late Paleozoic crinoid faunas, particularly in North American and Europe. However, they were abundant in Paleo-Tethyan Permo-Carboniferous crinoid communities. Bottom dwelling blastoids from these faunas lived in a greater range of sedimentological settings and were far more abundant and diverse than earlier bottom-dwelling genera. Apparently they were able to tolerate greater clastic sediment input by orienting the oral-aboral axis perpendicular to the bottom. The filtration fans were oriented parallel to the sediment water interface. Morphologic strategies to accomplish the strategy varied. Some taxa sat directly on the seafloor with ambulacra confined to the adoralmost surface. Others expanded the lower part of the theca into radiate, star-shaped extensions in a lunar module strategy. Still others used a rocket strategy with the theca resting on the aboral tips of the radials. Many of these taxa have long deltoids and ambulacra. Brachiole attachment scars are typically missing aborally on the ambulacra indicating that the filtration fan was elevated. Hydrospires often do not extend below the radio-deltoid suture in these taxa. All of these taxa have diminutive stems that may reflect an ontogenetic shift to bottom dwelling in adulthood.