Diademodus is an extinct genus of phoebodontid elasmobranch that lived during the Late Devonian Period. The genus was first described by paleontologist John E. Harris based on a body fossil from the Cleveland Shale formation of Ohio. Teeth belonging to this genus have also been uncovered in Utah, Nevada, Western Australia, and Russia.
Description
According to Harris, Diademodus was a long, slender cartilaginous fish with a body length of up to 40 cm (1.3 ft). He also stated that Diademodus' body may have been slightly wider than it was tall, making it comparable to the modern catshark genus Scyliorhinus. Like most cartilaginous fish, its body was covered in tooth-like dermal denticles. These denticles were shorter near the animal's head and larger in rows along its lateral line. The tail lacked an exaggerated keel like that seen in the contemporaneous genus Cladoselache.
Fins
Diademodus possessed unusual fin morphology compared to other known chondrichthyans. Its pectoral fins were positioned unusually far back along the animal's body, approximately 14 cm (5.5 in) from the tip of its head. They were also extremely small, extending only 5.2 cm (2.0 in) along the body and projecting 2.5 cm (0.98 in) outward. The dorsal fin was located far forward, 2 cm (0.79 in) ahead of the pectoral fin's front edge. Compared to the rest of its fins, Diademodus' pelvic fins were proportioned like typical sharks, with a length of 2.9 cm (1.1 in). The caudal fin was heterocercal, with the upper lobe being longer than the bottom one. There is no evidence of fin spines, an anal fin, or a second dorsal fin in the only described body fossil of the genus, although it is assumed a second dorsal fin was present. Males had well-developed pelvic claspers.
Teeth
The teeth of Diademodus were very small, only about 1 mm (0.039 in) wide at the base. These teeth contained nine to seventeen cusps, depending on the species, with the innermost and outermost cusps being significantly longer than the rest. Between the large cusps were at least three smaller cusps. Some teeth also had additional, minuscule cusps between the other cusps in the dentition.
Classification
Diademodus was originally classified as a member of the family Coronodontidae, a group that only included the genera Coronodus and Diademodus. However, this classification was disputed in 2010, and Diademodus was tentatively reassigned to the family Phoebodontidae within the larger order Phoebodontiformes, potentially placing it alongside Phoebodus and Thrinacodus. The phoebodonts themselves are believed to represent early members of the subclass Elasmobranchii, which also includes modern sharks and rays.
Paleobiology and paleoecology
Diademodus' small fins, heterocercal tail, and elongated body imply that it was most likely a bottom-dwelling fish. When the animal was first described, paleontologist John E. Harris proposed that its small teeth indicated a scavenging lifestyle. He also suggested that D. hydei was piscivorous, as bony fish scales belonging to palaeoniscoids were preserved in its stomach. In the 2008 description of D. utahensis, it was found that the teeth of this species would have been weakly connected to the jaws, which would have limited the animal's ability to catch large prey. As a result, paleontologist Michał Ginter proposed that Diademodus may have been a filter-feeder, using its teeth as a sieve to trap small organisms while seawater was pushed through its mouth.
References
External links
- 3D model of scanned Diademodus hydei type specimen at Sketchfab
