Receptaculites has been described by an author of a college geology textbook as "a double-spiral radiating pattern of rhombus-shaped plates supported by spindle-like meroms that grew on the seafloor. Fossils can usually be identified by the intersecting patterns of clockwise and counterclockwise rows of plates or stalk spaces."
(Translation of the above definition for the layman: In geometry, a rhombus is a quadrilateral shape with four sides of equal length. In zoology, meroms are tiny structures, made of calcium carbonate, secreted by tiny lifeforms that provided a stable structure for the colony. Curiously, meroms are not found in any other group of organisms, living or extinct.)
"Receptaculitids are the least known fossils," according to a report by Dr. Matthew H. Nitecki, former curator of the University of Chicago's Field Museum in Chicago. "Their demise was gradual in the fossil record, but they were a major component of massive organic buildups and were an important rock-building element. Beyond these facts, it is an unexplainable fossil group."
However, in the opinion of Dr. Char Mehrtens, professor and chairwoman of the University of Vermont's Department of Geology, the Receptaculites mystery has been solved. She has been studying fossils of the Champlain Valley for 28 years, most of her academic career.
The veteran, award-winning Vermont geologist says the unique local fossil-found in rocks here and in Russia, China, Japan and Australia-is neither sponge nor coral.
"Receptaculites is found in Panton Stone, a Middle Ordovician limestone," she said. "Paleontologists can tell the difference between the wall structures of sponges and calcareous algae to determine the origin of this fossil. Calcareous algae make little 'plates' of calcite, fused together. Sponges have very loosely constructed walls of little spikes, called spicules." And that's why Mehrtens believes Receptaculites was made by sea algae.
Mehrtens said her current research at UVM is focused on the evolution of the northern Appalachians, notably the Green Mountain range.