Thelodonti

Thelodonti is an extinct subclass of jawless fish that lived during the Silurian and Devonian periods. They are characterized by their small, flattened bodies and the presence of placoid scales, which are tooth-like denticles that covered their skin. Unlike modern fish, thelodonts lacked paired fins and possessed a cartilaginous skeleton. Their mouths were typically ventral and lacked true teeth, though they had keratinous denticles within their pharynx.

Fossil evidence suggests that thelodonts were bottom-dwellers, feeding on detritus and small invertebrates found in marine and freshwater environments. Their scales are abundant in the fossil record and are often used by paleontologists to identify different species and geological strata. The diversity of thelodont scales, ranging in shape and size, indicates a variety of ecological niches and adaptations.

The exact phylogenetic position of Thelodonti within the early vertebrate lineage has been a subject of ongoing debate. However, they are generally considered to be a group of primitive jawless fish, distinct from other known groups like ostracoderms. Thelodonts became extinct by the end of the Devonian period, likely due to competition from more advanced jawed fishes that were rapidly diversifying at the time. Their extinction marks the end of a significant branch in early vertebrate evolution.