Wooster’s Fossils of the Week: A calcareous sponge with a crinoid holdfast (Matmor Formation, Middle Jurassic, Israel)
Mark Wilson April 8th, 2012
The Class Calcarea of the Phylum Porifera is a group of sponges characterized by spicular skeletons made of calcium carbonate (calcite in this case). The spicules (small elements of the skeleton) are often fused together, causing the sponges to look a bit like corals or bryozoans. They are among the most common fossils in the Matmor Formation (Middle Jurassic, Callovian) of southern Israel. Melissa Torma and I collected this particular fossil on our expedition last month. It is another indication that the Matmor Formation was deposited in very shallow waters.
This is the underside of the Matmor calcareous sponge. (I wish we had a name for it, but the taxonomy is in considerable flux right now.) You can see the way it grew radially around an encrusting center. In the lower right a circular oyster attachment is visible.
A close view of the top surface of the calcareous sponge showing radiating canals called astrorhizae. They were used to channel water currents for the sponge’s filter-feeding system.
This crinoid holdfast (the base of an attaching stem) locked onto the calcareous sponge after its death. We can tell this because it is bound to the spicular skeleton itself, which was only exposed after the sponge’s soft tissues rotted away. It is not possible to identify the crinoid, but it is likely in the genus Apiocrinites.
The Class Calcarea was named by James Scott Bowerbank in 1864. Bowerbank (1797-1877) was an English naturalist born in London. He helped run a distillery with his brother, making enough money to support his diverse interests in natural history. He collected many fossils in his life, specializing in the London Clay (Eocene). His various publications gained him membership in the Royal Society in 1842. His greatest work was probably a four-volume set titled “A Monograph of the British Spongidae”. (You can read at least part of this work online.) He was well known as a strong supporter of young scientists, opening his home and collections (and use of his valuable microscopes) to all those seriously interested in natural history. I like to think he would have been happy as a liberal arts geology professor!