Stethacanthus altonensis
- Rodrigo
- Mar 19
- 3 min read
Age
Devonian（Frasnian）-Carboniferous（Gzhelian）
382.7-298.9 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: Symmoriiformes
Family: Stethacanthidae
Genus: Stethacanthus
Species: Stethacanthus altonensis
Morphological description
Stethacanthus altonensis possessed a slender body with a moderate size. The most distinctive feature in males was the so-called "ironing board-shaped" dorsal structure located anterior to the dorsal fin, consisting of a basal plate and a brush-like complex. Microscopic studies by Rainer Zangerl revealed that the dorsal spine was composed of trabecular dentine, containing internal hollow chambers and fibrous connective tissue, and was closely integrated with the trunk musculature.
The brush-like surface was formed by regularly arranged hollow tubes and connective tissue, possibly capable of expansion. Its surface was covered by thick dermal denticles, with tooth cusps inclined anteriorly. A corresponding denticle-covered plate was also present on the top of the head, oriented opposite to the brush surface. Viewed from the front, these opposing surfaces resembled an open mouth lined with sharp teeth. This structure was highly developed in males but greatly reduced in females, representing clear sexual dimorphism.
Dentitionally, Stethacanthus altonensis exhibited the typical tooth morphology of Symmoriiform sharks. The teeth were tricuspid, with a tall central cusp flanked by two smaller lateral cusps. The crown surface bore fine longitudinal and transverse ridges. The tooth base was narrow, rectangular or trapezoidal, and relatively tall, differing from the flattened bases seen in most modern sharks. This configuration was suited for piercing and gripping prey rather than crushing hard shells. The teeth were arranged in discrete rows and continuously replaced, maintaining functional sharpness.
Etymology
The genus name Stethacanthus derives from the Greek stēthos ('chest') and akanthos ('spine'), referring to the unusual spine-like structure on the dorsal region.
The species name altonensis refers to Alton, Illinois, the locality where the fossils were first discovered.
Biological description
Ecologically, the "ironing board-shaped" dorsal structure likely had profound implications for the behavior of Stethacanthus altonensis. Fossils from the Bear Gulch Lagerstätte indicate that it inhabited a diverse shallow marine lagoon environment. However, such a large and rough dorsal structure would have significantly increased hydrodynamic drag, suggesting that it was not a fast swimmer. Zangerl proposed that it was likely a slow-moving, possibly benthic shark, relying on near-bottom positioning or slow mid-water cruising.
Functionally, this unusual structure was likely not limited to courtship display or aggressive mating behavior. It may have played a defensive role. When the male contracted its dorsal musculature, the head could be raised and the dorsal spine tilted forward, while the brush-like structure expanded laterally. The denticle-covered surfaces on the head and dorsal structure would then face each other. From the front, this configuration resembled a wide-open mouth filled with sharp teeth, creating an intimidating display that mimicked a much larger predator.
This complex and energetically costly structure implies an evolutionary trade-off: reduced swimming efficiency in exchange for enhanced display and deterrence capabilities.
（Author: Rodrigo）
References
Ginter, M. (2018). Symmoriiform sharks from the Pennsylvanian of Nebraska. Acta Geologica Polonica, 68(2), 179–197.
Ginter, M., Hampe, O., & Duffin, C. J. (2010). Handbook of Paleoichthyology, Vol. 3D: Paleozoic Elasmobranchii—Teeth. Munich: Verlag Dr. Friedrich Pfeil.
Lund, R., Grogan, E. D., & Greenfest-Allen, E. (2014). Ecomorphology of the Mississippian fishes of the Bear Gulch Limestone. Munich: Verlag Dr. Friedrich Pfeil.
Zangerl, R. (1984). On the microscopic anatomy and possible function of the spine-“brush” complex of Stethacanthus (Elasmobranchii: Symmoriida). Journal of Vertebrate Paleontology, 4(3), 372–378. https://doi.org/10.1080/02724634.1984.10012016
38.89, -90.18
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