PANTON-If you're familiar with Vermont's famous Champlain Black or Panton Stone-a much sought after landscaping stone found in deposits along Lake Champlain from Vermont and New York to the Canadian border-you may have admired the ubiquitous marine fossils embedded in its dense gray matrix. These ancient reef creatures include a variety of seashells, crablike animals called trilobites, and other invertebrate denizens of the prehistoric deep.
Among the Panton Stone's ancient reef fauna are distinctive, disk-shaped objects commonly called "sunflower coral". These sunflower-like fossil disks were a big part of the local reef community and are frequently found in western Addison County-some grew up to three feet or more in diameter.
Regarding the ancient makers of the unusual disks, scientists have been debating the origin question since this fossil was first discovered in the early 19th century. While some have identified the fossil as coral, others have identified it as a kind of hard porifera or sponge built up by tiny, protozoa-like critters.
Today, most fossil experts believe "sunflower coral" was the product of green sea algae-unicellular and colonial plants.
If their theory is correct, then the layered, accretionary disks found in Vermont were built up by prehistoric algae absorbing minerals and nutrients via sea water and then expelling the waste to build up porous mounds.
A few fossil-collecting mavericks consider "sunflower coral" as a kind of mysterious quasi-sponge, but they are unable to pin down exactly what tiny vanished critters created the calcite structures. But even with most researchers now favoring green algae as the source of "sunflower coral" found in the Panton Stone-with a few porifera holdouts-it remains to be officially classified to any biological phyla.
In 1830, French zoologist Henri Marie Ducrotay de Blainville gave the fossil its Latin, scientific name, Receptaculites (pronounced: receptacle-eye-tees), named for the hundreds of tiny receptacle-like chambers found in the disks. De Blainville also helped date the Panton Stone fossil-and its fellow turned-to-stone reef lifeforms-to approximately 480-450 million years ago.
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.
"I am particularly interested in how the rocks of our region record the history of ocean-basin opening, closing, and the formation of mountains," she said. "Right now, I have a graduate student starting her master's research on the Middlebury, Orwell, and Panton limestones-she will study the age, environment of deposition, and burial history of these rocks."
Check it Out: For safe, accessible examples of Vermont's Panton Stone, visit the exhibits of the UVM Perkins Museum of Geology. For a fine example of Panton Stone used in attractive landscape design, look for the low rock wall near the entrance to Middlebury College's new main library. Middlebury's downtown bridge, built in the 1800s is made up, in part, of Panton Stone. A few fossils, including fragments of Receptaculites, may be seen in these rocks.
Warning: Fossil collecting without the landowner's permission is illegal in Vermont. State property also has severe restrictions regarding rock, mineral and fossil collecting. Before collecting any natural object, ask permission.
Special thanks to Dr. Char Mehrtens of UVM and the research staffs of the Middlebury College Armstrong Science Library and Ilsley Public Library for assistance in preparing this article.