CINCINNATI, Ohio -- When some scientists first floated the idea that an ancient crinoid called Uintacrinus, could float on the top of the sea, other scientists were skeptical. Modern day relatives such as sea stars all live on the ocean bottom in tightly packed colonies.
David Meyer, a professor of geology at the University of Cincinnati, will present evidence at the Geological Society of America annual meeting Monday, October 20 supporting the idea that Uintacrinus could float. However, Meyer's model goes further by proposing a brand new model to explain the 85 million-year-old Cretaceous crinoid's unexpected buoyancy. It's a model that began with a few random thoughts.
"In thinking about this, I got to thinking about insects like water striders," said Meyer. "There are aquatic insects that live on the surface of the water and use surface tension. They walk on water."
As Meyer explored this phenomenon, he discovered that insects that can walk on water have very long legs compared with their body size. Uintacrinus has similar properties. It looks a bit like a tiny octopus with a golf-ball sized body and ten long arms stretching out from the center.
"I tried to calculate how long the arms of the Uintacrinus would be. We found the length of their arms could be one meter long, so they'd have a diameter of two meters and that's just the arms alone. The little extensions of the arms called the pinnules give you even more length. We found these crinoids have much greater arm length than any known crinoids."
Using a well established formula involving surface tension, Meyer discovered that all those arms could easily keep Uintacrinus afloat if the arms were spread out over the surface of the water. In fact, that arrangement would support an animal nearly six times heavier than Uintacrinus. It would also make it easy for the crinoid to pull food particles and microscopic plankton out of the water, since that food tends to accumulate right at the sea's surface. Finally, the model explains why Uintacrinus fossils are always found in huge slabs with hundreds of individuals per colony.
"We know they lived together by the hundreds in these groups," said Meyer. "The idea would be that they link together, and it may even be a collective property that adds to their surface tension."
Viewed from above, Meyer's model would reveal a huge spider web type complex floating on the top of the sea. That's much different than the old textbook model which shows Uintacrinus bobbing with its arms hanging down into the sea. Meyer rejected that idea two years ago, noting there were not special structures to help the organism float and that it would be easier to collect food if Uintacrinus sat on the sea floor and waved its arms upward into the currents.
After two years of comparing Uintacrinus slabs from field sites and museums around the world. Meyer flipped his thinking around once more. The determining factor was an odd pattern revealed in the fossilized slabs. Fossils swept by river currents or tides generally get preserved pointing in a single direction. The Uintacrinus fossils, on the other hand, were arranged in a radial pattern like spokes on a bicycle wheel.
Meyer thinks he can explain that pattern. Numerous major volcanic eruptions are well documented throughout the Cretaceous, and modern-day crinoids have a well documented response to stress. They fold up their arms like an umbrella. If Uintacrinus reacted the same way, the organisms would suddenly lose their buoyancy and go sinking to the sea floor.
"It's almost like teardrops sinking," said Meyer. "Gradually the center of the mass sags, and they fall into this pattern of spokes. We haven't tested that yet, but we've got a model we plan to test."
The followup work will be done by graduate student Andrew Webber. Meyer's research is supported by the National Geographic Society.