Fossil of the month: Cryptolithus
This month’s fossil is Cryptolithus tessellatus, the “lace collar” trilobite. Fragments of Cryptolithus tessellatus are common in the Upper Ordovician Clays Ferry and Kope Formations in central Kentucky, but whole fossils of this trilobite are rare. Cryptolithus fragments are usually found concentrated in beds along with fragments of bryozoans, crinoids, and the brachiopod Sowerbyella. These fossils were deposited in shallow tropical seas that once covered what is now central Kentucky. The Late Ordovician Clays Ferry and Kope Formations are 450 million years old.
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 (cepahlon, thorax, and pygidium). Trilobites and other arthropods have an exoskeleton, or an outer hard covering. As trilobites grew, they shed their outer shell in a process called molting. Many fragments of Cryptolithus are likely molts that were shed.
Cryptolithus was a tiny trilobite, only a few centimeters long at most. It had a large cephalon (head shield) relative to its body (thorax) and tail shield (pygidium). Its cephalon had a large glabella (central region) and inflated cheeks. Two long spines extended backward from the cephalon. Although many trilobites had large eyes, Cryptolithus lacked eyes and is interpreted to have been a blind trilobite. Its cephalon was characterized by a broad, flat frill that was perforated with rows of tiny holes. These holes are the reason the trilobite is known by the nickname “lace-collar” trilobite. Fragments of lace collars are the most common fossils of this trilobite.
Scientists are uncertain what function the holes on the frill provided for the trilobite. The holes in the frill may have (1) acted as some sort of sensory device for this blind arthropod (Campbell, 1975; Shaw, 1991), (2) been a filter that allowed water to pass through the perforations in the frill from above to the mouth beneath (Cisnes, 1970), or (3) been a filter in which food was trapped beneath the frill as water was pushed from beneath up through the holes in the frills (Fortney and Owens, 1999). Perhaps the frill allowed the trilobite to more quickly cover itself with a thin layer of sediment when burrowing for protection from predators.
Several species of Cryptolithus were historically identified in Upper Ordovician strata of eastern North America (see, for example, Whittington, 1968). At least two species, C. tessellatus and C. bellulus, were recognized in Kentucky and surrounding states. Differences in the various species were based on the number of arcs of perforations or holes in the frill or slight shape differences. Subsequent statistical analysis of specimens found that slight variations in shape and the number of arcs or pits varied in all species and that the specimens were better considered variations (morphs) of one species rather than multiple species. Presently, all of the Upper Ordovician Cryptolithus from the eastern United States are considered Cryptolithus tessellatus (Shaw and Lespérance, 1994).
To learn more about trilobites in Kentucky, read “Tracking Trilobites: Adventures in Paleontology,” by Judy Lundquist (2005), a book about Kentucky trilobites published by the Kentucky Geological Survey and available through the KGS Publications and Maps Catalog.
- Campbell, K.S.W., 1975, The functional morphology of Cryptolithus: Fossils and Strata, v. 4, p. 65–86.
- Cisne, J. L. 1973, Beecher's trilobite bed revisited: ecology of an Ordovician deepwater fauna: Postilla, Peabody Museum of Natural History, Yale University, v. 160, p. 1–25.
- Fortney, R., and Owens, R., 1999, Feeding habits in trilobites: Palaeontology, v. 42, no. 3, p. 429–465.
- Lundquist, J., 2005, Tracking trilobites: Adventures in paleontology: Kentucky Geological Survey, ser. 12, Special Publication 4, 70 p.
- Shaw, F.C., 1991, Viola Group (Ordovician, Oklahoma) cryptolithinid trilobites: Biogeography and taxonomy: Journal of Palontology, v. 65, p. 919–935.
- Shaw, F.C., and Lespérance, P.J., 1994, North American biogeography and taxonomy of Cryptolithus (Trilobita, Ordovician): Journal of Paleontology, v. 68, no. 4, p. 808–823.
- Whittington, H.B., 1968, Cryptolithus (Trilobita): Specific characters and occurrence in Ordovician of eastern North America: Journal of Paleontology, v. 42, no. 3, p. 702–714.