Stethacanthus

Stethacanthus was an extinct genus of shark-like cartilaginous fish. These animals lived during the Late Devonian to the Late Carboniferous epoch. They eventually went extinct around 298.9 million years ago. Fossils of these creatures have been discovered across the globe. They are found in North America, Europe, and Asia. They have also been found in Australia. The name Stethacanthus comes from Ancient Greek words. It means "chest spine" or "chest thorn." This name refers to a very distinct feature found on mature males.

The most famous part of this fish is the spine-brush complex. This structure is located where the first dorsal fin usually sits. It consists of a basal plate and a dorsal spine. The spine is made of a material called trabecular dentine. This spine can move in an anterio-posterior direction. This means it can move forward and backward. The "brush" part is made of many parallel, membranous tubules. These tubules are composed of globular calcified cartilage. Scientists believe these tubules might be inflatable, similar to erectile tissues. The entire complex is covered in rows of large denticles, which are small, tooth-like scales.

Scientists have several theories about why this complex existed. One idea is that it helped with mating rituals. Another theory suggests it helped the fish clamp onto the bellies of larger marine animals. The denticles on the spine and head might have served a defensive purpose. Because the denticles point forward, they could simulate the mouth of a much larger fish. This would frighten away potential predators. The spine-brush complex is only found in mature males. It is not yet known if females possessed this structure.

Stethacanthus also possessed unique anatomy in its fins and teeth. The pectoral fins included a long structure called a fin whip. This whip contained at least 22 axial cartilages. These structures extended past the pelvic fins. The teeth of Stethacanthus were of the cladodont variety. Each tooth had one large central cusp and four smaller accessory cusps. This gave each tooth a total of five cusps. These teeth were used by the fish to eat a carnivorous diet. They likely fed on small fish, brachiopods, and crinoid ossicles.

Learning about this genus has been a slow process for paleontologists. In the late 1800s, scientists only found isolated spines. John Strong Newberry studied these and thought they were part of the pectoral fins. He did not realize they were part of a unique dorsal complex. It was not until 1974 that Richard Lund defined the family Stethacanthidae. He recognized that these fish were very different from other elasmobranchs. Since then, new discoveries have expanded our knowledge of their range. We have found remains in Scotland, China, and the United States.

Because of the large spine-brush complex, Stethacanthus was likely a slow swimmer. The structure would have created a drag force during fast movement. Their small fins and teeth also suggest they were bottom-dwellers. Many specimens come from the Bear Gulch Limestone in Montana. This suggests the area might have been a breeding ground. This could mean that Stethacanthus were migratory animals. The different species varied in size. For example, S. altonensis was smaller than S. productus.

Stethacanthus belongs to the group Chondrichthyes. This group includes modern sharks and rays. However, its exact classification is a subject of scientific debate. Some researchers place them in the order Symmoriida. Others suggest they are more closely related to chimaeras. This is based on recent reclassifications of the Symmoriiformes group. The presence of globular calcified cartilage is also very significant. It is the first large record of this tissue in chondrichthyans. This helps scientists understand how the skeletons of sharks evolved over millions of years.