Fossil of the month: Isotelus
Trilobites are a favorite of fossil collectors. One of the most sought-after trilobites in Kentucky is this month’s fossil of the month, the giant trilobite, Isotelus. While most trilobites are only a few centimeters long, Isotelus may be tens of centimeters in length. Trilobites were marine arthropods, distantly related to modern crabs and crustaceans. Trilobite bodies can be divided into three lobes from side to side (two pleural lobes and an axial lobe) and three parts from front to back (cephalon, thorax, and pygidium). Whole Isotelus fossils are relatively rare, but dark brown molt fragments are common in several Upper Ordovician rock units in central Kentucky. Isotelus is the official state invertebrate fossil of neighboring Ohio.
Description. Isotelus is a large fossil trilobite. It is generally oval in shape and is wider than most trilobites found in Kentucky. It’s cephalon (head) and pygidium (tail) are both large, and similar in size to the thorax (middle body). Isotelus’ cephalon and pygidium are subtriangular to subrounded in outline, and usually are slightly wider than long. The central region of its cephalon (called a glabella) gently slopes toward the front and widens in front of the eyes. Its eyes are large, compound types (called holochroal), which are elevated slightly from the cephalon. Some species have genal spines projecting behind the outer rear margins of the cephalon, while others have rounded to pointed tips. The thorax is divided into eight segments and rings. The axial lobe in the thorax is usually wider than the lateral pleural lobes. Each pleura or segment has an elongate depression, called a pleural furrow. The underside of this trilobite (where preserved) has a plate (called a hypostome) in front of its mouth, which is distinctly forked into two backward-projecting prongs, like large fangs (Rudkin and Tripp, 1951; Harrington and others, 1959; Babcock, 1996).
Species. Two species of Isotelus are reported from central and northern Kentucky, southwestern Ohio, and southeastern Indiana: I. gigas, and I. maximus. In Latin, “gigas” means “gigantic,” and “maximus,” means “maximum;” so both species are named for their large size. Another species, I. brachycephalus, was historically reported in Kentucky, but subsequently determined to be a variation (junior synonym) of I. maximus (Hall, 1993; Babcock, 1996).
Isotelus gigas is the type species of the genus. It was one of the first trilobites described in North America. The type specimen is from New York, but this trilobite has been found in Upper Ordovician strata in many eastern U.S. states (Rudkin and Tripp, 1951). It has a more triangular cephalon and pygidium than I. maximus. Adult specimens (larger) lack genal spines, although juvenile (small) specimens may have short spines. It is the most common species found in Kentucky. It may be more than 24 centimeters (9 inches) long (Rudkin and Tripp, 1951; Shrake, 1990; Babcock, 1996). Several specimens from Kentucky are in museums around the country.
Isotelus maximus is wider and often larger than I. gigas. The largest specimens are more than 40 centimeters (16 inches) long, although most specimens are 30 centimeters (12 inches) or less in length. It has a more rounded cephalon and pygidium than I. gigas. It also has genal spines at the rear of its cephalon, which I. gigas usually lacks (Shrake, 1990; Babcock, 1996; Rudkin and others, 2003). Isotelus maximus is perhaps more common in southwestern Ohio than in Kentucky.
Other species of Isotelus occur in other parts of the U.S. The largest is Isotelus rex from the western United States. It has a maximum length of 70 centimeters (27 inches) (Rudkin and others, 2003), which is three times larger than I. gigas!
Molt fragments. Isotelus fossils are fossils of the exoskeleton (outer shell). Whole fossils are rare. Fragments of various sizes and completeness are rare to common. Like many other arthropods, Isotelus trilobites had to shed their exoskeletons as they grew in a process called molting. Modern crabs for example, molt monthly to annually.
The three-fold division of trilobite exoskeletons (in both directions) and various sutures in the exoskeleton, created natural lines of breakage during molting. Most specimens appear to have exited their old shell from the front (Wilmot, 1988). Discarded exoskeleton could be further fragmented by scavenging, bottom currents, and storms. Hence, fragments of Isotelus fossils are more common than whole specimens.
Rolled specimens. Some specimens of Isotelus are found enrolled with the bottoms of the cephalon and pygidium touching each other. Like many segmented trilobites (and other arthropods), Isotelus could roll up into a ball for protection. Rolling up its outer shell was likely a defensive response, similar to the response of tiny pill bugs when you pick up the rock they were hiding under. When rolled up, the outer hard body armor of the Isotelus exoskeleton completely surrounded and protected the soft tissues and limbs beneath. Many of these rolled specimens are found in shales that represent sediment flows or storm deposits on the ancient sea floor. The trilobites probably rolled up for protection against (1) large cephalopods (shelled squids), which were the top of the Ordovician sea food chain, and (2) sudden sedimentation events.
Isotelus eyes. Trilobites are one of the few ancient organisms in which major parts of the eyes are preserved as fossils. That’s because the cornea of the eyes were actually composed of minerals. Both the exoskeleton and eyes were composed of the mineral calcite. The cornea was composed of closely packed, tiny calcite facets (like crystal faces) precisely oriented so that they reflected an image of the surrounding environment to the trilobite’s optical nerves (Towe, 1973; Schmidt ad Wagermaier, 2017). Different types of trilobites had different kinds of eyes, and some were even blind. Isotelus had compound holochroal eyes. These consisted of hundreds to thousands of closely packed calcite facets. Scientists have compared Isotelus eyes to the eyes of modern arthropods and found that they are similar to arthropods that live in dim light, such as deep-sea mantis shrimp (Fordyce and Cronin, 1993). The large size of Isotelus eyes might also suggest eyes adapted to reduced light conditions.
Other senses. The rear of the cephalon and middle to upper segments (pleura) of Isotelus trilobites are commonly covered with very tiny pits. Modern crabs and crustaceans (and many other arthropods) have similar pitting on their exoskeletons. In modern marine arthropods, surface pits like these contain sensory receptors, from which tiny hairs (called setae) protrude. These hairs are used for sensing various aspects of the environment such as temperature and currents (Wilmot, 1988; Babcock, 1996). The hairs are soft parts, so are difficult to fossilize, but the pits in the exoskeleton (hard part) provide evidence for their position on the trilobite.
Range. Isotelus lived from the Middle to Late Ordovician (Harrington and others, 1959). In Kentucky, Isotelus gigas fossils have been reported in the Late Ordovician Lexington Limestone, and the Clays Ferry through Bull Fork Formations. I. maximus is reported from the Kope Formation to the top of Ordovician strata. These units are exposed at the surface in central and north-central Kentucky. Most whole or near-complete Isotelus fossils are found in shales rather than limestones, but molt fragments are relatively common in limestones. Molt fragments tend to have a dark brown or orange-brown color, which stands out from the gray limestones. The Late Ordovician units in which Isotelus are found in Kentucky are 445 to 450 million years old.
Paleoecology. All trilobites were marine organisms. Some trilobites were swimmers (benthic), others crawled on the sea floor (epifaunal) and some burrowed beneath the sea floor (infaunal). The large, broad flat shape of Isotelus suggests it lived on the sea floor (epibenthic) or occasionally burrowed just beneath the sediment (shallow infaunal) with its eyes exposed just above the sediment.
Isotelus is mostly found in shales which were deposited as muds in relatively deeper waters of the Ordovician seas that once covered Kentucky. The deeper waters may have been more dimly lit than the shallows, and periodically clouded by suspended sediment. Such conditions would favor a trilobite with large eyes able to see in low light, like Isotelus.
Isotelus shared the sea floor with other marine creatures, such as brachiopods marine bivalves, bryozoans, and crinoids. It was likely a scavenger or predator on the sea floor (Rudkin and Tripp, 1951). Fossilized tracks of probable Isotelus above a worm burrow in Ordovician limestone, appears to preserve a trilobite digging for a soft-bodied worm in its burrow (Brandt and others, 1995; English and Babcock, 2007). Also, the forked mouth plate (hypostome) of Isotelus is shaped somewhat like a “claw hammer” so may have allowed this trilobite to pry prey out of burrows or dig into sediment to get to prey (Devera, 2013). Hegna (2010) suggested it may also have had a grinding function, and would have been most useful against smaller, soft-shelled organisms in the sediment.
Other websites for Isotelus information:
- Babcock, L.E., 1996, Chapter 8-Phylum Arthropoda, Class Trilobita, in Feldman, R.M., and Hackathorn, M., eds., Fossils of Ohio: Ohio Geological Survey, Bulletin 70, p. 90-113.
- Brandt, D.S., Meyer, D.L. and Lask, P.B., 1995, Isotelus (trilobita) “hunting burrow” from Upper Ordovician strata, Ohio: Journal of Paleontology, v. 69, no. 6, p.1079-1083.
- Devera, J.A., 2013, Death by common household tools; mechanical analogy and the functional morphology of the hypostome in genus Isotelus (Dekay) evidence from Isotelus iowensis (Owen): Geological Society of America, Abstracts with Programs, p. 59.
- English, A.M., and Babcock, L.E., 2007, Feeding behavior of two Ordovician trilobites inferred from trace fossils and non-biomineralized anatomy, Ohio and Kentucky, USA: Memoirs of the Association of Australasian Palaeontologists, v. 34, p. 537-544.
- Fordyce, D., and Cronin, T.W., 1993, Trilobite vision: a comparison of schizochroal and holochroal eyes with the compound eyes of modern arthropods: Paleobiology, v. 19, no. 3, p. 288-303.
- Hall, C.P., 1993, Biometric and taxonomic analysis of the genus Isotelus (Trilobita) from Cincinnatian (Upper Ordovician) rocksof Ohio, Kentucky, and Indiana: Ohio State University, Bachelor’s thesis, 35 p.
- Harrington, H.J., Henningsmoen, G., Howell, B.F., Jaanuson, V., Lochman-Balk, C., Moore, R.C., Paulsen, C., Rasetti, F., Richter, E., Richter, R., Schmidt, H., Sdzuy, K., Struve, W., Størmer, L., Stubblefield, C.J., Tripp, R., Weller, J.M., and Whittington, H.B., 1959, Part O, Arthropods 1, Trilobita, in Moore, R.C., ed., Treatise on Invertebrate Paleontology: Geological Society of America and University of Kansas, p. 38-560.
- Hegna, T.A., 2010, The function of forks: Isotelus-type hypostomes and trilobite feeding: Lethaia, v. 43, no. 3, p. 411-419.
- Rudkin, D.M., and Tripp, R.P., 1951, The type species of the Ordovician trilobite genus Isotelus, I. gigas Dekay, 1824: Toronto, Royal Ontario Museum, Life Sciences Contributions, v. 152, 12 p.
- Rudkin, D.M., Young, G.A., Elias, R.J., and Dobrzanski, E.P., 2003, The world's biggest trilobite-Isotelus rex new species from the Upper Ordovician of northern Manitoba, Canada: Journal of Paleontology, v. 77, no. 1, p. 99-112.
- Schmidt, I. and Wagermaier, W., 2017, Tailoring calcium carbonate to serve as optical functional material: Examples from biology and materials science: Advanced Materials Interfaces, v. 4, no. 1, p.1600250.
- Shrake, D.L., 1990, Common trilobites of Ohio: Ohio Department of Natural Resources, Division of the Geological Survey, Ohio Geology Newsletter, 8 p.
- Towe, K.M., 1973, Trilobite eyes: calcified lenses in vivo: Science, v. 179, no. 4077, p.1007-1009.
- Wilmot, N.V., 1988, Design and function of trilobite exoskeletons: Birmingham, England, Ashton University, Ph.D. dissertation, 207 p.
Text and illustrations by Stephen Greb (KGS).