"Rock of the Month # 218, posted for August 2019" ---

From modern beaches to fossiliferous strata hundreds of millions of years old, echinoids or "sea urchins" are familiar seaside creatures. Together with crinoids ("sea lilies") and other organisms, echinoids are members of a larger invertebrate group, the echinoderms. The oldest echinoids are found in late Ordovician strata, some 450 million years old.

This example, Eupatagus antillarium is an echinoid of late Eocene age, circa 38-40 million years old. The two tests (shells) were cleaned by expert fossil preparator and dealer Kevin Brett of Ontario. This species is found in the Ocala Formation of Florida. Eupatagus is an easy-to-recognize fossil, one that fits nicely in the hand, 2-3 inches (50-75 mm) in diameter. It has apparently been proposed as the "state fossil". Florida has a state stone, which is Eocene silicified coral, but no formal state fossil (cf. Ohio and Pennsylvania, with their trilobites, New York with a sea scorpion, and Michigan with a mastodon).

Geological Setting

The oldest bedrocks at surface in Florida, a geologically youthful state, are three formations of Eocene age. The Ocala Formation, the middle of the three, includes an upper member that is highly fossiliferous. It is composed of marine carbonate sediments, predominantly limestone, variably dolomitized, with local development of chert. The upper member has abundant echinoids, foraminifera (large and small), bryozoans and molluscs. Another distinctive fossil is the large foraminiferan genus, Lepidocyclina, a "monster microfossil". The limestone has been variably dissolved over the centuries, yielding a karst topography, in which surface and groundwater play hide-and-seek with the air, along disappearing streams and emergent springs. The Ocala is bedrock to much of Dixie, Levy and Marion counties, west and south from Gainesville, on the western (Gulf of Mexico) side of the northern half of the Florida peninsula, north of Sarasota.

The occurrence of echinoids (often with foraminifera and other fossil forms) is not limited to Florida, but is worldwide. Some other Eocene examples can be found in the Caribbean region (Schuchert, 1935), in the Horn of Africa (Somalia, Somaliland, Ethiopia: Dainelli, 1943a,b) and in Saudi Arabia (Beydoun, 1966)

More on Echinoderms

The echinoderms are a long-lived phylum, dating back to the lower Cambrian. They are all marine, most are bottom-dwellers, and they have an internal skeleton of small plates, each plate a single crystal of calcite. Most have pentameral (five-sided) symmetry, with either bilateral (mirror) or radial arrangement of columns of plates. Famous echinoids include Micraster (in the Cretaceous Chalk) and Clypeaster (found to this day in tropical waters). Early forms were the blastoids and cystoids. Echinoids and crinoids are the best-known, most widely-distributed fossils of the phylum. There are also asteroids (the familiar starfish ["sea stars"], not the extraterrestrial "rocks" like Vesta and Ceres!), the morphologically- similar ophiuroids ("basket stars", "serpent stars", and "brittle stars"), and holothurians ("sea cucumbers"). Today, the Echinodermata are represented by some 5,300 species worldwide. See Barnes (1968) and Black (1972) for more detailed descriptions.

Echinoids are covered by spines when alive: these are generally lost when the dead animal is tossed about in the tidal zone, so common fossils such as these examples lack the spines. Some forms are flattened, hence the popular terms "sea biscuit" and "sand dollar". Nebelsick and Kroh (2002) describe instances where large accumulations of fossil echinoids are preserved. Echinoderms are an important component of carbonate sediments, as illustrated in the petrographic descriptions by Scholle and Ulmer-Scholle (2003).

Figs. 2-3: Close-up views of echinoid tests, dorsal and ventral (top and bottom) sides.

References

Barnes,RD (1968) Invertebrate Zoology. W.B. Saunders Company, Philadelphia, 2nd edition, 743pp., pp.605-669.

Beydoun,ZR (1966) Geology of the Arabian peninsula: Eastern Aden protectorate and part of Dhufar. USGS Prof.Pap. 560-H, 49p. plus 5 maps and sections.

Black,RM (1972) The Elements of Palaeontology. Cambridge University Press, 339pp., pp.104-142.

Dainelli,G (1943a) Geologia Dell'Africa Orientale, Vol. I. Il Progresso delle Conoscenze. Reale Accademia d'Italia, Rome, 464pp. (in Italian).

Dainelli,G (1943b) Geologia Dell'Africa Orientale, Vol. III. La Successione Terziaria e i Fenomeni del Quaternario. Reale Accademia d'Italia, Rome, 748pp. (in Ital.).

Nebelsick,JH and Kroh,A (2002) The stormy path from life to death assemblages: the formation and preservation of mass accumulations of fossil sand dollars. Palaios 17, 378-393.

Scholle,PA and Ulmer-Scholle,DS (2003) A Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis. AAPG Memoir 77, xii+459pp.

Schuchert,C (1935) Historical Geology of the Antillean-Caribbean Region, or The Lands Bordering the Gulf of Mexico and the Caribbean Sea. John Wiley and Sons, Inc., New York, 811pp.