Fossil of the month: Hebertella
This month’s fossil of the month is the Upper Ordovician brachiopod Hebertella. Brachiopods are the official state fossil of Kentucky. Hebertella is an easy-to-find, common example of a brachiopod from central Kentucky. Brachiopods are a phylum of invertebrate (lacking a backbone) marine organisms that have a shell. They look somewhat similar to clams (a type of mollusk), but clams have two valves (shells) that are mirror images of each other, whereas brachiopods have two valves that are different from each other. In life, brachiopods attach to the seafloor with a tissue called a pedicle, which extends out through a hole in one of the valves of the shell, called the pedicle valve.
Genus description. Hebertella is a medium-size, brachiopod found in upper Ordovician strata. It belongs to the brachiopod order Orthida. Hebertella has a subquadrate to subelliptical outline, and a biconvex (usually unequally biconvex), to convexo-concave, lateral profile. The dorsal (brachial) valve is generally somewhat hemispherical, thicker, and more convex than the ventral (pedicle) valve. Sulcus and fold are moderately (but variably) developed. The sulcus is a depression along the midline of the pedicle (ventral) valve, and the fold is a corresponding raised area along the midline of the brachial (dorsal) valve. Valves are ornamented by fine costae or costellae (thin to very thin ribbing). The genus is distinguished by the shape of muscle scars on the inside of its valves (Walker, 1982; Wright and Stigall, 2013).
Species. Five species of Hebertella are identified in Ordovician rocks of central Kentucky (see table). Not all specimens can be identified to species level. Hebertella occidentalis is the most common. Differences between several species are subtle. In general, species can be differentiated based on (1) the relative size and overall shape of shells, (2) the relative width, spacing, and branching of costae and costellae (thin and very thin ribbing), and (3) the relative development of the fold and sulcus.Distinctions between species are also made based on the muscle fields on the interior of the valves, but these are hidden in whole specimens.
H. frankfortensis is most different from the other species because of its more rounded outline. Some reports have considered H. subjugata a junior synonym of H. occidentalis (it was originally described as a more finely ribbed H. occidentalis), but statistical analysis of distinguishing features by Wright and Stigall (2013) shows they are separate species. On the other hand, Hebertella sinuata, described in many historical reports (e.g., McFarlan, 1931), has been considered a subspecies of H. occidentalis (H. occidentalis sinuata) and is now considered a junior synonym of H. occidentalis (Walker, 1982; Wright and Stigall, 2013). Detailed descriptions of the different species can be found in publications by Walker (1982) and Wright and Stigall (2013).
Range and geographic occurrence. Hebertella is found in the Lexington Limestone and younger Late Ordovician strata (445 to 450 million years old). Different species occur in different units, with some overlap. You can use the Kentucky Geological Survey’s online geologic map to determine rock units in which you find specimens.
The Lexington Limestone is exposed in the Inner Bluegrass Region of central Kentucky, including the area around Lexington and Frankfort, and the Clays Ferry through Drakes Formations are exposed in the Outer Bluegrass Region. The uppermost members of the Drakes Formation are exposed along the outer rim of the Bluegrass Region. In all, rocks containing Hebertella occur in 41 counties in central Kentucky.
Life and paleoecology. Upper Ordovician strata of the Bluegrass Region were deposited in shallow tropical seas. Hebertella lived on the seafloor. It is usually found in diverse and abundant fossil assemblages with other brachiopods, bryozoa, cephalopods, gastropods, and crinoids. Hebertella appears to have lived in water depths between normal and storm wave base (Holland and others, 1996). In life, Hebertella was oriented with its commissure (opening between the valves) upright and vertical, and its proximal (hingeline) side resting on the seafloor. The wide interarea along its hingeline may have aided stability on the seafloor (Alexander, 1984). Its relatively robust shell would likely have been an advantage in storms. Like other brachiopods, it fed by filtering nutrients from seawater.
Text and illustrations by Stephen Greb