My academic work and employment have covered a number of different branches of Geology. This particular vignette falls within the science of Paleontology - the study of fossils. Studying fossils such as the Class Echinoidea (at left) can also extend into the study of layered rocks and the study of ancient ecosystems.
Echinoids compose a particular class within the Phylum Echinodermata. Echinoderms, in broad sense include Asteroids (starfish), Crinoids (sea lillies, sea feathers), Echinoids (sand dollars, sea urchins), etc., and some extinct, weird critters such as Blastoids and Cystoids.
They are characterized by a pentameral symmetry, shown in the five "petals" of the fossil sand dollar at left. The echinoids are divided into Regular Echinoids (sea urchins, pencil urchins) and Irregular Echinoids (sand dollars, sea biscuits, sea cookies, and heart urchins). And they are one of my geo-hobbies". I have been lucky enough to have had a couple of good collecting sites in my field mapping areas in SW Georgia.
The Regular Echinoids appeared in the fossil record during the Ordovician Period. Generally the Paleozoic urchins are discovered as crushed specimens with some of their spines and plates present. Carefully cleaned and restored examples appear on eBay from time to time. Many of these types come from the Pennsylvanian shales in the Brownwood, TX area. The Irregular Echinoids (heart urchins) began appearing in the fossil record during the Mesozoic Era, in the Jurassic Period. The sand dollars began appearing in the fossil record during the latest part of the Cretaceous Period or early Tertiary Period.
The specimen above Periarchus pileussinensis is one that I collected from the Late Eocene Tivola Limestone (approx. 40 million years old) in the old Medusa Cement Company quarry near Clinchfield, Houston County, Georgia, on the inner Coastal Plain. It was during one of my undergraduate field trips, probably in 1973 or 1974. The Eocene Epoch (56 to 34 million years ago) of the Tertiary Period was a time of global warmth (palm trees in Alaska, crocodiles in the Dakotas), high sea levels, and great echinoid biodiversity.
Late Eocene sedimentary rock units on the Georgia Coastal Plain include the Clinchfield Sand, the Tivola Limestone, the Twiggs Clay, the Ocmulgee Formation, the Sandersville Limestone, and others, extending from south of Augusta southwestward to the Bainbridge, GA area. Much of the Florida Peninsula is underlain by Eocene limestones, such as the Ocala Limestone and in many surface exposures of the Ocala Limestone, echinoid collectors find their idea of heaven.
The Periarchus genus is found in North Carolina, Georgia, Alabama, Florida, and Mississippi and was present from the earliest Late Eocene until the end of the Eocene, when the genus, along with others, became extinct, perhaps due to the impact of the "Chesapeake Bolide".
In Georgia, the preceding species to P. pileussinensis was the Periarchus lyelli in the Clinchfield Sand (and elsewhere). Also in Georgia, the next succeeding species in the lineage was Periarchus quinquefarius in the Sandersville Limestone. Most of the Periarchus specimens found in Georgia measure about 3.5 to 4 inches in diameter, the size of the modern day Atlantic Coast sand dollars. The smaller Protoscutella in the older Middle Eocene of the Carolinas, Alabama, and Mississippi may represent an ancestor to Periarchus, though most Protoscutella specimens are generally 1/2 to 1/3 the diameter of Periarchus. To my knowledge, Protoscutella has not yet been found in Georgia (though I have tried). And to my knowledge Protoscutella and Periarchus have not been found in the same layers. When there are multiple fossil-bearing layers present in an area, Periarchus is always found in units younger than Protoscutella.
[Another time, I will post more photos of some of my Favorite Things.]
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