Fossil of the Week

4/7/10 – Lobolith

Think you know crinoids? Well, here’s one that you might not know about. This is a lobolith from the Silurian-Devonian boundary (ca. 444 million years ago) of Oklahoma. A lobolith is a float at the surface of the ocean – kind of like a bobber that a fisherman would use – from which a stalked crinoid hangs, upside down, spending its life as a pelagic organism. This makes perfect sense – what better place to gather plankton, than hanging freely in the water? It is unusual but not rare for normally-benthic organisms to have a close relative in the plankton. Gastropod (snails) have one – the Blue Ocean Slug, Glaucus atlanticus, is a pelagic nudibranch or sea slug.

Most crinoids (Phylum Echinodermata, Class Crinoidea, AKA “feather stars”) – both living and extinct, and either stalked or sessile – are flower-like benthic animals that filter-feed from the bottom of the ocean, usually attached by a holdfast to a rocky perch with their feathery-armed “crowns” extended into the water column, capturing plankton as it drifts by. [FYI - Some crinoids are not as sedentary as you might think, and have been observed to “walk” from rock to rock.] Planktonic crinoids (family Scyphocrinitidae) occur almost worldwide at the Silurian-Devonian boundary formations of Europe, Asia, North Africa, and North America. These are the only known pelagic members of the Crinoidea, past or present. This specimen, which is approximately 3 inches (75 mm) in width, has been halved – the surface facing you is part of the interior, showing several of the spaces inside the lobolith that were filled with gas or air to provide buoyancy.

You might ask, how did this adaptation evolve? The answer is: we do not know for sure. One hypothesis is an intermediate “pseudoplanktonic” stage or a crinoid that lived attached to floating driftwood; we have such fossils and they were also really, really huge – stalks 20 meters (60 feet) long with crowns one meter (3 feet) in diameter. Fossil evidence also suggests that the life history of lobolith-bearers includes a juvenile that remains attached to its parent until its lobolith is of sufficient size to float independently. Another obvious question is: why wasn’t this life strategy successful enough to persist until today? Again we do not know for certain, but it could have something to do with the evolution of fishes, which experienced their greatest radiation in the Devonian – perhaps floating crinoids were just too delicious to endure!

Text by Paula Mikkelsen