Fossil of the month: Ctenacanthus
What is this strange curved fossil? It might surprise you to discover it's not some type of plant stem or horn coral, but rather part of an ancient shark. Also, although its curving shape might make you think it's some type of internal bone like a rib, it's not. Sharks' skeletons are composed of cartilage rather than bone, and cartilage does not fossilize well, so shark ribs and most other skeletal parts are usually not fossilized. Bony hard parts like teeth, and in some cases, fin spines, are all that remain of ancient sharks, except in very rare cases. Few modern sharks have fin spines, but during the Paleozoic, several types of sharks and shark-like fish had fin spines like this.
Description. Ctenacanthus is a fossil fin spine. It is a form-genus, which means it is a name given to a variety of similar-appearing body parts, rather than to a specific organism. Whole spines are 4 to 7 inches (10 to 18 cm) in length, although smaller broken fragments of less than 2 inches are more common. Spines are slightly curving, and gradually taper to a point. Spines are ornamented with narrow, closely spaced, longitudinal (oriented along the long axis) ridges or ribs. Ribs are commonly ornamented with comb-like projections of tiny bumps (tubercules) across part of their length, although they may be lacking. In cross section, the front (anterior) side of the spine is slightly rounded, and the back (posterior) side is indented with a long furrow. The furrow is where the spine attached to the fin when the shark was living. The lateral margins of the posterior side of the spines are ornamented by a row of low, pointed to sawtooth-like protrusions (denticles).
Not all fossil fin spines are shark spines. Maisey (1981) stressed the similarity of many supposed Ctenacanthus spines to other Paleozoic shark and fish spines, as well as the historical tendency to label any fin spine as the genus Ctenacanthus, even if it differed in ornamentation. Some expertise is needed in differentiating fossil spine genera. Other fin spine genera include Amelacanthus, Bythiacanthus, Eunemacanthys, Goodrichthys, and Sphenacanthus. All of these fin-spine genera are classified based on subtle differences in ornamentation and overall shape. These differences are generally more apparent on whole spine fossils, than more commonly found spine fragments. Amelacanthus and Sphenacanthus are most similar to Ctenacanthus; they have an ornamentation dominated by longitudinal ridges, but the ridges and grooves between ridges differ slightly from Ctenacanthus, among other details. Maisey's (1981, 1982a, 1982b) research articles on Ctenacanthus contain descriptions and pictures of a variety of different types of fin spines for comparisons.
Species. If expertise may be needed to differentiate genera, it certainly is needed for species-level identification. Many species of Ctenacanthus have historically been reported across the globe, and many are invalid because they are not different enough from previously named species, original specimens were lost or not described well enough, or because new genera were named subsequent to the spines’ original designation as a type of Ctenacanthus.
Maisey (1981, 1982a, 1982b, 1984) reviewed historical reports and compared specimens of Ctenacanthus in many of the world's museums. His 1984 publication lists valid species names for the genus. Learn more about Ctenacanthus species from Kentucky.
Learn more about Ctenacanthus species from Kentucky.
Range and distribution. Ctenacanthus spines are found nearly worldwide in Devonian through Permian strata. The long range of the spines means they may have belonged to different genera and species of sharks through time. In Kentucky, they have been reported in Devonian through Pennsylvanian strata, including in the Late Devonian Ohio Shale and Berea Sandstone (Newberry, 1889), possibly the Early Mississippian Sunbury Shale (Miller, 1916), the Late Mississippian Slade Formation, and the late Middle Pennsylvanian Brush Creek and Ames Limestone Members of the Conemaugh Formation. Many of the historically reported spines, however, have subsequently been referred to other fin-spine genera, Ctenacanthus spines might not be found in all of these units in Kentucky. In surrounding states, Ctenacanthus spines have been found in correlative units, as well as in the mid-Mississippian Salem and St. Louis Limestones, unspecified Late Mississippian Chesterian limestones, and unspecified Pennsylvanian coal measures (Maisey, 1984).
What did the Ctenacanthus shark look like? A rare, partial shark body fossil with Ctenacanthus fin spines is known from the Late Devonian Cleveland Shale Member of the Ohio Shale (Hussakof, 1908; Dean, 1909; Maisey, 1981, 1984). The fossil represents the anterior (front) upper part of a shark. It is a partial body, with leathery skin (shagreen), jaws, teeth, and pectoral fins and a partial impression of a fin spine (Maisey, 1984). The shark fossil was originally called Ctenacanthus clarkii (Hussakof, 1908), and later renamed Ctenacanthus compressus (Dean, 1909; Maisey, 1981). Hence, the genus name Ctenacanthus is used for both the fin spines and the shark itself.
Based on the partial body fossil and comparison to other rare body fossils in Devonian shales, Ctenacanthus had a shark-like body with two dorsal (upper) fins, two pectoral (lower, anterior) fins, two pelvic (lower, posterior) fins, and a caudal (rear, tail) fin (Maisey, 1975; Zangerl, 1981; Colleary, 2020). Spines on the two dorsal fins may have had slightly different sizes and ornamentation (Hlavin, 1976).
Paleoecology. The partial skeleton and soft parts of the ctenacanthid-spine-bearing shark from Ohio and all known Ctenacanthus spines are from marine strata, so these sharks were marine sharks. Based on the relatively small size of spines and rare body fossil fragments, these sharks were likely less than six feet long. The Ohio body fossil had cladodont teeth. Cladodont teeth are a type of fossil shark tooth consisting of multiple prongs, with a long middle, sharp, conical prong. Cladodont teeth suggest Ctenacanthus sharks were predatory meat eaters. Sharp, prong-like or spike-like teeth are good for holding and grasping food rather than slicing chunks of food from larger prey, or crushing food. Hence, this ancient shark’s diet may have consisted of smaller organisms it could eat whole, such as small fish, possibly cephalopods (squid-like organisms), and arthropods (shrimp and crustaceans).
Modern spiny dogfish sharks and bullhead sharks have spines in front of both of their dorsal fins. The spines on the spiny dogfish are for protection and are slightly poisonous (Jerve, 2016). Ctenacanthus also appears to have had spines on both dorsal fins, which could have offered protection from larger sharks and other predatory fish, although there is no way to know if they were poisonous. In the Late Devonian seas, larger predators included other sharks and large arthrodires (armored fish) such as Dunkleosteus.
- Colleary, C., 2020, The Cleveland sharks: Cleveland Museum of Natural History website, https://www.cmnh.org/science-news/blog/august-2020/the-cleveland-sharks, accessed 7/6/2021.
- Dean, B., 1909, Studies on fossil fishes (sharks, chimaeroids, and arthrodires): Memoirs American Museum of Natural History, v. 9, p. 209-287.
- Eastman, C.R., 1902, Some Carboniferous cestraciont and acanthodian sharks: Bulletin Museum of Comparative Zoology, v. 39, p. 55-99.
- Hlavin, W.J., 1976, Biostratigraphy of the Late Devonian black shales on the cratonal margin of the Appalachian geosyncline: Boston, Boston University, Ph.D. dissertation, 194 p.
- Hussakof, L., 1908, Catalogue of the type and figured specimens of fossil vertebrates in the American Museum of Natural History. Part I. Fishes: Bulletin of the American Museum of Natural History, v. 25, p. 1-103.
- Jerve, A., 2016, Development and three-dimensional histology of vertebrate dermal fin spines: Sweden, Uppsala Universitet, Ph.D. dissertation, 53 p.
- Maisey, J.G., 1975, The interrelationships of phalacanthous selachians: Neues Jahrbuch Geologie Paliontologie, v. 9, p. 553-567.
- Maisey, J.G., 1981, Studies on the Paleozoic selachian genus Ctenacanthus Agassiz. No. 1, Historical review and revised diagnosis of Ctenacanthus, with a list of referred taxa: American Museum Novitates, No. 2718, 22 p.
- Maisey, J.G., 1982a, Studies on the Paleozoic selachian genus Ctenacanthus Agassiz. No. 2. Bythiacanthus St. John and Worthen Amelacanthus, New Genus, Eunemacanthus St. John and Worthen, Sphenacanthus Agassiz, and Wodnika Miunster: American Museum Novitates, No. 2722, 24 p.
- Maisey, J.G., 1982b, Studies on the Paleozoic selachian genus Ctenacanthus Agassiz. No. 3. Nominal species referred to Ctenacanthus: American Museum Novitates, No. 2774, 20 p.
- Maisey, J.G., 1984, Studies on the Paleozoic selachian genus Ctenacanthus Agassiz. No. 3, Nominal species referred to Ctenacanthus: American Museum Novitates; no. 2774, 20 p.
- Miller, A.M., 1916, Some historic fish remains from Vanceburg, Kentucky [abs.]: Science, new ser., v. 44, p. 71-72.
- Newberry, J.S., 1875, Description of fossil fishes: Ohio Geological Survey, v. 2, part 2, p. 1-64.
- Newberry, J.S., 1889, Paleozoic fishes of North America: U.S. Geological Survey, Monograph 16, 340 p.
- Newberry, J.S., and Worthen, A.H.,1866, Description of vertebrates: Illinois Geological Survey, v. 2, Palaeontology, p. 9-141.
- Zangerl, R. 1981, Chondrichthyes I, Paleozoic Elasmobranchii, in H. P. Schultze, H.P., ed., Handbook of Paleoichthyology, v.3A: Gustav Fischer, Stuttgart, p. 1-115.
Text and images by Stephen Greb