Thelodonti

Thelodonti are an extinct class of jawless fishes that lived during the Palaeozoic era. Their name comes from the Greek words for "nipple teeth," which describes their unique covering. Unlike many other jawless fishes of their time, they did not have large, heavy armor plates. Instead, they were covered in small, spiny scales. These animals first appeared during the Late Ordovician period. They lived for a long time before perishing after the Frasnian–Famennian extinction event in the Late Devonian.

The most famous feature of these fish is their thelodont scales. Each scale is shaped like a tiny teardrop sitting on a bulky base. These scales are very small, usually measuring between 0.5 and 1.5 mm. They were arranged to point backward along the body. This helped the fish move in a streamlined direction through the water. The scales were made of several layers. They had a non-growing crown of dentine and an enameloid upper surface. Below that was a base made of aspidine, which is a type of acellular bony tissue. The growing part of the scale was made of cell-free bone.

Because these scales were so tough, they are the most common vertebrate fossils from that era. Scientists study the histology, or the microscopic structure, of these scales to understand the animals. The scales were formed and shed throughout the fish's life. After a thelodont died, the scales detached easily and spread across the seafloor. This makes them excellent biostratigraphic markers. This means scientists can use them to help date and correlate different layers of rock and sediment around the world.

Finding a complete thelodont fossil is very difficult for researchers. Most fossils are just scattered scales because the fish lacked an internal bony skeleton. This lack of bone meant the bodies did not preserve well. However, some rare specimens were discovered in 1993 that were preserved in three dimensions. These fossils changed what we knew about thelodonts. They showed that the fish were not actually flat. These fossils also revealed that thelodonts had a visible stomach. This was a major discovery because scientists previously thought stomachs evolved only after jaws appeared.

Recent studies have changed how we think about their lifestyle. Scientists once thought thelodonts were mostly bottom dwellers that fed on debris. Now, we know they occupied many different roles in the water column. Some species were active swimmers, much like modern sharks or bony fishes. Many species preferred living in reef ecosystems. Their small, flexible scales likely helped them navigate these environments. The scales protected them from abrasion while allowing for more movement than heavy armor would permit.

There is still much debate about how thelodonts are related to other animals. Some scientists believe they are a monophyletic group, meaning they all share a single common ancestor. Others think they are polyphyletic. This would mean the group is made of different lineages that just happened to evolve similar scales. Some researchers see a close link between thelodonts and the families of fish with jaws, known as gnathostomes. This idea comes from finding internal scales that look like tooth-whorls. They also had fork-shaped tails, which are common in jawed fishes.

Because complete fossils are rare, scientists use scale shapes to organize the group. This is called taxonomy. They have proposed different ways to group them. One framework uses three main groups based on scale structure. Other systems use the names of specific orders, such as Archipelepidiformes or Furcacaudiformes. Some families, such as the Lanarkiidae, are known only from their tiny scale fossils. Even though scale shape can vary on a single fish, these tiny structures remain the best tool for measuring the diversity of this ancient class.