Fossil of the month: Pentremites
This month’s fossil is the nut-shaped fossil, Pentremites. It’s a somewhat unusual fossil that looks like it has a small starfish embedded on top. The first place this fossil was ever discovered in the world was Kentucky, and it was discovered by the doctor who also discovered Parkinson’s disease.
Pentremites is a type of blastoid. Blastoids are an extinct type of marine echinoderm, similar to, but different than crinoids. Starfish and sea urchins are examples of living echinoderms. Blastoids had a theca (also called a calyx) that is usually shaped somewhat like a nut, with a five-petaled flower or small starfish shape embedded in the top. Pentremites thecas are usually small (1 to 2 cm wide or long), but some are larger. The starfish- or flower-like structures are firmly attached to the theca and are called ambulacra. Each ambulacra has a groove down the axis, called the main food groove. If you look closely at the ambulacra (or with the aid of a magnifying glass), you can see many, thin, closely-spaced grooves and ridges transverse to the axis. You can also see a tiny star-shaped hole at the center of the starfish shape, which was the mouth of the blastoid. The mouth is surrounded by 4 smaller holes and 1 larger hole. The smaller holes are called spiracles. The larger hole is the blastoid’s anus. In life, bristle-like structures, called brachioles, extended from the margins of the ambulacra. The bristles are rarely preserved as fossils. The blastoid’s theca was attached to a column (stem), similar to crinoids, but only rarely are Pentremites thecas found attached to their stems (Galloway and Kaska, 1957; Beaver and others, 1967).
More than 40 species of Pentremites have been reported from Mississippian rocks in Kentucky (and surrounding states). Many of these species, however, have proven to be similar enough that they are no longer considered valid. Less than ten are common or useful for correlation, and only five or six are regularly cited in research (Horowitz and others, 1981; Chesnut and Ettensohn, 1988; Ettensohn and others, 2003). Species of Pentremites are mostly differentiated based on slight differences in their shapes including (1) length-to-width ratios, (2) vault-to-pelvis ratios, (3) concavity vs. convexity of ambulacra, and (4) the angle of the sides of the pelvis (Galloway and Kaska, 1957; Waters and others, 1985).
Pentremites species can be grouped into four basic types similar to (1) P. conoideus, (2) P. pyriformis, (3) P. godoni, and (4) P. sulcatus. Waters and others (1985) quantitatively analyzed a large number of Pentremites species and suggested two basic shapes were dominant: (1) somewhat rounded or globular, called godoniform (like P. godoni), and (2) more extended or somewhat pear-shaped, called pyriform (like P. pyriformis). Some rock units, such as the Upper Mississippian Paint Creek and Glen Dean Limestones, contain several species. Species-level identification generally requires good specimens, some familiarity with known species, and familiarity with echinoderm paleontology to make accurate identifications. Changes in proportions of thecas as individual species grew in their individual lives (think juvenile, teenager, adult), called ontogeny, can complicate some species identifications.
Originally found in Kentucky!
The first fossil blastoid ever described in the world was from Kentucky, and it was later named a species of Pentremites. In 1804, 1808, and 1811, British doctor and scientist James Parkinson published three volumes of Organic Remains of a Former World, in which he described and illustrated fossils from Britain and around the world. In the second volume, he described and illustrated a fossil from western Kentucky as an Asterite, which means star-like (Say, 1820; Galloway and Kaska, 1957; Fay, 1961a, 1961b). Parkinson’s Asterite fossil was later used to found a new class of fossil echinoderm, termed a blastoid, and assigned to the blastoid genera Pentremites, and the species, P. godoni (Say, 1820; Galloway and Kaska, 1957; Fay, 1961b). Parkinson made many other contributions to science, but he’s most famous for first describing the disease which now bears his name, Parkinson’s disease.
The blastoid 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 (and in this case, a class of echinoderm as well). The original fossils that Parkinson sketched were subsequently lost, so Fay (1961b) designated replacement types (called neotypes) of Pentremites godoni from Kentucky. Those specimens are in the U.S. National Museum collection (no. 139103). They were found near Bowling Green, Kentucky. Many genera and species of blastoids have been reported in Kentucky, especially from Upper Mississippian strata in western Kentucky.
Range and geographic occurrence
Blastoids ranged from the Silurian through the Permian periods. The blastoid genera, Pentremites, lived from the Middle Mississippian to Early Pennsylvanian in North America (Waters and others, 1985), but is only found from the Middle to Late Mississippian in Kentucky. In western Kentucky, different species of Pentremites are reported from the Salem-Warsaw Formation (and its equivalents) to the top of Mississippian strata, and are perhaps most common in the Paint Creek through Glen Dean Limestones. In eastern Kentucky, they can be found in the Slade and Paragon Formations, and equivalent Newman Limestone and Pennington Formation. These rock units are approximately 325 to 340 million years old and occur throughout the Mississippian Plateaus region in more than 30 Kentucky counties.
Pentremites is the most common Middle to Late Mississippian blastoid found in Kentucky and surrounding states, but several other genera of blastoids occur in Lower Mississippian and older strata. In the Middle Mississippian Warsaw and Salem Formations (often mapped together in Kentucky as Salem-Warsaw Formation), Pentremites (P. conoidus), occur with two other blastoids, Diploblastus sp. (possibly D. glaber), and Metablastus (several species). Another blastoid, Tricoelocrinus was historically reported from the Somerset Shale Member of the Warsaw Formation in Kentucky (Feldman, 1989; Chesnut and Ettensohn, 1988), but that genera was later interpreted as a growth variant of Metablastus so now is considered a species of Metablastus, rather than a separate genera (Ciampaglio and others, 2017). Diploblastus is smaller and more globe-shaped than Pentremites conoidus. The two blastoids are diffrentiated based on details of the holes (spiracles) on top of their thecas. Metablastus is easily distinguished from Pentremites by its length, longer (taller) pelvis, and different shaped ambulacra. In Mississippian strata above the Ste. Genevieve Limestone, Pentremites is the only known blastoid genera in Kentucky. This is one of the reasons it is important to record where fossils are found. If you know the location, you can determine the bedrock unit, which often can narrow down the possibilities of fossils which are known from that unit!
Life and paleoecology
Blastoids like Pentremites were marine invertebrates. They lived in shallow marine seas that once covered Kentucky and surrounding areas. Pentremites shared the sea floor with other marine organisms like brachiopods, corals, fenestrate bryozoa, and crinoids. Pentremites filtered food from the seawater through the bristles (brachioles) on top of its theca. The food was carried down the ambulacra to the mouth in the center of the theca (Beaver and others, 1967). Thecas were attached to narrow stems which attached to the sea floor, giving Pentremites blastoids a flower-like appearance, similar to their cousins, crinoids. Unfortunately, only a few Pentremites thecas have ever been found attached to their columns, so we really don’t know much about how these blastoids attached to the sea floor. Two columns reported by Strimple (1977) were 10 and 20 cm long, and had cirri attached. Cirri are root-like extensions of the columns. They may have helped anchor the column to the sea floor. Whether the column was mostly upright or mostly dragged on the sea floor with multiple cirri anchors is uncertain. The fact that so few columns have been found attached to their thecas indicates that Pentremites fell apart relatively easily after death.
Pentremites are commonly found in groups or clusters with crinoids. In the Middle to Late Mississippian, gardens of crinoids and blastoids would have covered the sea floor with delicate fenestrate bryozoan fans and many types of brachiopods.
- Atwood, J.W., and Sumrall, C.D., 2012, Morphometric investigation of the Pentremites fauna from the Glen Dean Formation, Kentucky: Journal of Paleontology, v. 86, no. 5, p. 813–828.
- Bassler, R.S. and Moodey, M.W., 1943, Bibliographic and faunal index of Paleozoic pelmatozoan echinoderms: Geological Society of America Special Paper 45, 734 p.
- Beaver, H.H., Fay, R.O., Macurda, D.B., Moore, R.C., and Wanner, J., 1967, Part S–Blastoids, in Moore, R.C., ed., Echinoderms: Geological Society of America and University of Kansas Press, Treatise of Invertebrate Paleontology, part S, v. 2, p. S298–S445.
- Beaver, H.H., and Fabian, A.J., 1998, Color patterns in Mississippian (Chesterian) blastoids: Journal of Paleontology, v. 72, no. 2, p. 332–338.
- Chesnut, D.R., Jr., and Ettensohn, F.R., 1988, Hombergian (Chesterian) echinoderm paleontology and paleoecology, south–central Kentucky: Bulletins of American Paleontology, v. 95, no. 330, 102 p.
- Ciampaglio, C.N., Jacquemin, S.J., and Fabian, A.J., 2017, Morphological analysis of the blastoid taxa Metablastus wortheni and Tricoelocrinus woodmani with emphasis on taxonomic implications: Southeastern Geology, v. 52, no. 4, p. 205–222.
- Defrance, J.M.L., 1819, Dictionnaire des Sciences Naturelles, v. 14, EA–EQE, p. 467.
- Ettensohn, F.R., Ausich, W.I., Kammer, T.W., Johnson, W.K., and Chesnut, Dr., Jr., 2009, Carboniferous echinoderm succession in the Appalachian basin in Greb, S.F., and Chesnut, D.R., Jr., eds., Carboniferous [Geology and Biostratigraphy] of the Appalachian and Black Warrior Basins: Kentucky Geological Survey, Special Publication 10, p. 85–93.
- Fay, R.O., 1961a, Blastoid studies: University of Kansas Paleontological Contributions, Echinodermata, Article no. 3, 147 p.
- Fay, R.O., 1961b, The type of Pentremites Say: Journal of Paleontology, v. 35, p. 868–873.
- Feldman, H.R., 1989, Echinoderms of the Somerset Shale Member, Salem Limestone (Mississippian), in Indiana and Kentucky: Journal of Paleontology, v. 63, no. 6, p. 900–912.
- Galloway, J.J., and Kaska, H.V., 1957, Genus Pentremites and its species: Geological Society of America Memoir 69, 130 p.
- Horowitz, A.S., Macurda, D.B., and Waters, J.A., 1981, Taxonomic revision of Pentremites Say (Blastoidea): Geological Society of America, Abstracts with Programs, v. 13, p. 281.
- Jillson, W.R., 1965, The geology of Casey County, Kentucky: Frankfort, Kentucky, Roberts Printing Co., 108 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.
- Strimple, H.L., 1977, Unusual morphological features in the blastoid genus Pentremites: Geological Magazine, v. 114, no. 1, p. 9–13.
- Taylor, R.S., 1968, Paleontology and lithology of the Salem Limestone (Mississippian) in south-central Kentucky: The Compass of Sigma Gamma Epsilon, v. 46, p. 46–54.
- Waters, J. A., Horowitz, A.S., and Macurda, D.B., 1985, Ontogeny and phylogeny of the Carboniferous blastoid Pentremites: Journal of Paleontology, v. 59, no. 3, p. 701–712.
- Weller, 1931, Mississippian fauna, in Jillson, W.R., ed., Paleontology of Kentucky: Kentucky Geological Survey, v. 36, p. 250–291.
Text, photographs, and illustrations by Stephen Greb (unless otherwise cited)