Fossil of the Month: Grewingkia
Is it a dinosaur tooth? A claw? This month’s fossil of the month is Grewingkia canadensis. Grewingkia may look like a tooth or claw to the public, but it lacks details that are seen in fossil teeth and claws. Instead, details of Grewingkia’s structure show it is a rugose coral, more commonly termed a “horn” coral because of its shape.
Description. Grewingkia is one of three Ordovician solitary rugose corals found in Kentucky, and is the only large Ordovician horn coral. In small specimens its corallum (skeleton) is conical with a slightly curving base. In larger specimens the upper half of the corallum may have relatively straight sides. Many specimens exhibit faint, closely spaced lines longitudinally (parallel to long axis) along the outside of the corallum (called costae), as well as wrinkling across the corallum. Lengths are commonly between 4 and 8 centimeters, but can be as much as 13 centimeters. Widths are commonly 2 to 3 centimeters, but can be more than 4 centimeters.
At the top of the widest part of the corallum is a cup, called a calyx. The calyx is where the coral animal (polyp) lived. In Grewingkia, the calyx has more than 80 closely spaced septae (skeletal plates), arranged in some specimens as alternating major (40 to 50 longer) and minor (40 to 50 shorter) septae. Septae may curve inward toward the axis, but in adult specimens, septa may be fused toward the axis into a featureless mass called an axial boss (Elias, 1983). The boss may look like a structureless axial region surrounded by a medial or outer ring with septa. In many specimens of Grewingkia, the cup is infilled and septae are not visible.
Species. Only one species of Grewingkia is known from Kentucky: G. canadensis.
Range and geographic occurrence.In Kentucky and surrounding areas, Grewingkia canadensis fossils are found in the upper part of the Upper Ordovician (Richmondian Substage of the Cincinnatian Stage), in strata approximately 450 million years old. The coral has been reported from the Rowland, Bardstown, Preachersville, and Saluda Dolomite Members of the Drakes Formation and equivalent parts of the upper Bull Fork Formation. The largest Grewingkia specimens are reported from the Bardstown and Saluda Dolomite Members (Elias, 1983). These units occur in a belt around the outer edge of the Bluegrass Region in Kentucky.
Life and paleoecology. Grewingkia canadensis corals lived in relatively stable waters in the shallow tropical sea that covered central Kentucky and neighboring Indiana and Ohio during the Late Ordovician. Most Grewingkia fossils are found on their sides, but in life they lived with the calyx pointed upward. The living coral animal used its small tentacles to feed in the water column.
Many specimens have small holes in them (called Trypanites), which are thought to represent borings of bristle worms (polychaetes). Because some specimens have borings on all sides of the coral, the borings are interpreted to have been made while the corals were upright, before toppling and being buried. Where borings are only on one side of the coral, they were made after the coral fell over. Microscopic algal borings are also common (Elias, 1983).
The dead corals provided a hard surface for marine organisms to grow on. Some Grewingkia have other fossils such as bryozoans and brachiopods attached to one side of them. If you find a Grewingkia fossil, look closely on its sides for other fossils that may be attached.
- Elias, R.J., 1983, Middle and Late Ordovician solitary rugose corals of the Cincinnati Arch region: U.S. Geological Survey Professional Paper 1066-N, 13 p., plus plates.