alphynix:

For many years, paleontologists were finding strange tooth-like fossils called “conodonts” in marine rocks all over the world, dating from the Late Cambrian all the way to the end of the Triassic (~500-200 mya). They were tiny, some as small as 200µm long (0.008″) and even the very largest only 2cm (0.8″), and came in a variety of different shapes, from simple cones to long bars with multiple serrations.

But despite them being common enough to be used for biostratigraphic dating, and identifying potential oil- and gas-bearing rock layers for the petroleum industry, nobody actually knew what sort of animal they belonged to.

It wasn’t until the 1980s that soft-tissue fossils were found – completely by accident. Scientists looking for shrimp fossils in an uncatalogued Scottish museum collection discovered several small eel-shaped creatures containing clusters of the tooth-like elements in their heads, finally solving the mystery.

The conodont animals turned out to be an early branch of “jawless” vertebrates, related to modern hagfish and lampreys. They had long thin bodies with a ray-finned tail, proportionally large eyes, and an arrangement of multiple “teeth” in their mouths and throats forming a complex feeding apparatus. Most had a total body length of only a few centimeters, but some ranged up to around 40cm (16″).

Since they existed for so long, and were found in almost every type of marine habitat all over the world, different conodonts probably inhabited a variety of ecological niches. Some species’ teeth have been interpreted as being used for filter
feeding, while others seem to have been more adapted for grasping and
crushing. But, despite their obvious success, they eventually started declining and were finally finished off entirely during the Triassic-Jurassic mass extinction event.

(This image doesn’t depict any specific type of conodont, but is mainly based on one of the best-known groups, the ozarkodinids.)