This odd-looking structure is called a holdfast and is part of a fossil crinoid. Crinoids are a group of marine animals in the Class Crinoidea, Phyllum Echinodermata. Echinoderms are sea animals with radial symmetry, such as starfish, sea urchins, sand dollars, and sea cucumbers. There are only about 600 crinoid species living today but they were much more common and diverse in the past. In fact, crinoid fossils can be very common in Paleozoic limestone beds, and are found in the Devonian limestone of central Ohio. Many of the fossil species, and some of the living species, have long stalks that attach to the sea floor. Crinoids also have multiple arms that surround their mouth and wave in the water and pull in organic material. They can somewhat resemble plants, and some are even called Sea Lilies. The crinoids stalk attaches to the sea bottom or other substrate with a disc-like sucker. Some species will also have root-like structures that allow it a tighter grip on the substrate.
A stalked crinoid from the Gulf of Mexico.
So what is the function of this grappling hook-like structure of this crinoid!? Well, rather than have suckers or roots to attach to the sea floor, paleontologists think this structure probably swiveled on the sea floor and hooked on to coral, seaweed, or other objects and thus held the animal in place. The specimen in the photo, called Ancyrocrinus, was collected in Clark Co., Indiana and is from the mid-Devonian (385 – 406 million years ago).