Fossil of the month from the KGS collections: Blastoids
Blastoids are a type of marine echinoderm. They have a theca or calyx (covering or sheath) that is usually shaped somewhat like a nut. In life, the theca was sometimes attached to a stem (similar to most ancient crinoids), which attached to the seafloor. In other cases, blastoids were stemless (like modern crinoids) and attached directly to the seafloor. Blastoids filtered food from seawater through a series of arms or bristles that extended up from a series of structures on the top of the theca, which look like five petals of a flower.
The first fossil blastoid ever described was from Kentucky. In an 1808 book, British doctor and scientist James Parkinson described a fossil from western Kentucky as an “Asterite,” which means star-like. In the figure caption for the sketch of the fossil he provided, Parkinson described the fossil as the “asterial fossil from America.” (James Parkinson also wrote many medical papers, including a description of a shaking palsy, which now bears his name, “Parkinson’s disease.”) The name “Asterite” and description “asterial” came from the five-point star or flower-like structure on the theca. Five-point or pentagonal symmetry is characteristic of echinoderms. Modern echinoderms include crinoids, starfish, and sea urchins.
The original Asterite fossil and similar fossils found later were determined to be a new type of fossil echinoderm, termed a blastoid, and were assigned to the blastoid genera and species Pentremites godoni (Say, 1820; Galloway and Kaska, 1957; Fay, 1961). The specimens described by Parkinson were considered the type for this genera (Say, 1820). A type specimen is a fossil from which a new genera or species is based. The original fossils that Parkinson sketched were subsequently lost, so Fay (1961) designated replacement types (called neotypes) in the U.S. National Museum collection (no. 139103), which also were found in Kentucky, near Bowling Green. Many genera and species of blastoids have been reported in Kentucky, especially from Upper Mississippian strata in western Kentucky.
- Defrance, J.M.L., 1819, Dictionnaire des Sciences Naturelles, v. 14, EA–EQE, p. 467.
- Fay, R.O., 1961, Blastoid studies: University of Kansas Paleontological Contributions, Echinodermata, Article no. 3, 147 p.
- Galloway, J.J., and Kaska, H.V., 1957, Genus Pentremites and its species: Geological Society of America Memoir 69, 130 p.
- Parkinson, J., 1808, Organic remains of a former world: London, Noraville & Fell, v. 2, p. 235–236, plate 13, Figs. 36–37.
- Say, Thomas, 1820, Observations on some species of zoophytes, shells, etc., principally fossil: American Journal of Science, v. 2, no. 2, p. 34–45.