#cladida

Carabocrinus is a kind of sea lily, a marine animal that used its arms to filter food. Between 359 and 488 million years ago, the number of species of sea lilies boomed and so did the number of shapes they took. For example, their arms can be branched or unbranched. Those that branch can do so in dozens of ways.

One scientist recorded every difference in shape between all the species of sea lilies known during that time. He found 73 features that varied between skeletons of different species. He used those features to compare shape diversity to species diversity.

Unexpectedly, shape diversity didn’t increase in step with species diversity. At first, both types escalated as new species appeared with shapes never seen before.

Then, around the time the specimen in the photograph was alive, the number of shapes levelled off even though species diversity kept increasing. New shapes were only minor variation on shapes that already existed. There appeared to be some limitation to shape diversity and a finite range of shapes available to sea lilies.

Specimen Number: UT1121TX108

References: Foote, Mike. “Morphology of Ordovician - Devonian Crinoids.” Contributions from the Museum of Paleontology, University of Michigan 29, no. 1 (1994): 1-39.

Not all spots on the ocean floor are equal. Some are calm. Some are tumultuous. High-energy water is closer to shore, where the tides and wind-driven waves move water faster than in deeper, calmer parts of the ocean.

Scientists suspected that differences in ocean location might matter for crinoids because there's a mathematical link between how much energy water has and how fast it can move through filters and nets with certain-sized holes. The arms of individuals of Graffhamicrinus and other crinoid animals acted like filters, or fishing nets for microscopic animals, during life.

If scientists' model were correct, then crinoids with particular arm shapes, and therefore particular sized holes in their fishing nets, should have lived only where food-filled water moved at the right speed.

The dense fan of specimens of Graffhamicrinus should be found close to prehistoric seashores, where high-energy water used to be. Certain rock structures only form on the ocean floor close to shore. Researchers went out to find more specimens of Graffhamicrinus, and when they found the fossils, they also found those near-shore rock structures. The evidence matched the predictions of the mathematical model.

Specimen Number: NPL 43968

References:

Holterhoff, Peter F. "Filtration models, guilds, and biofacies: Crinoid paleoecology of the Stanton Formation (Upper Pennsylvanian), midcontinent, North America." Palaeogeography, Palaeoclimatology, Palaeoecology 130(1997):177-208.

Based on the rocks the skeleton came from, researchers know that it belonged to an animal that lived in the ocean. On the left side of the photo is a column of thin, circular plates. Each plate has holes through the middle where cords of tissue ran through the center of the stack. This stack of plates made the animal’s stalk flexible, like the stack of bones that make up our spine.

Fewer pieces of the skeleton made up the animal’s calyx, or the lumpy bowl in the center of the photograph. Those plates interlocked, and provided protection for the creature’s gut and other organs.

The arms, partially visible on the right, contained the majority of pieces of the skeleton. This animal had ten arms. Each arm had central stack of paired, wedge-shaped plates. Like the joints in our body, each meeting of plates was an opportunity for bending. With so many plates, each arm could roll and twist in the water like a centipede. That flexibility allowed the animal to snatch bits of food out of the water.

Specimen Number: NPL 21582

Sources:

Mirantsev, G. V., and S. V. Rozhnov. “First records of the genus Ulocrinus Miller et Gurley (Crinoidea, Cromyocrinidae) from the Upper Carboniferous Beds of the Moscow Syneclise and Oka-Tsna Swell.” Paleontological Journal 45(2011):52-59.

Ubaghs, Georges, R. C. Moore, H. Wienberg Rasmussen, N. Gary Lane, Albert Breimer, H. L. Strimple, J. C. Brower, Russell M. Jeffords, James Sprinkle, R. E. Peck, D. B. Macurda, Jr., D. L. Meyer, Michel Roux, Hertha Sieverts-Doreck, R. O. Fay, and R. A. Robison. Treastise on Invertebrate Paleontology: Part T: Echinodermata 2, Volume 1. Paleontological Institute, Lawrence, Kansas (1978), 402 pp.