Polychaete worms of the order Eunicida possess microscopic jaws, called scolecodonts, that are common in the fossil record (unlike the soft-bodied worms themselves).
In Baltoscandian Ordovician and Silurian carbonate rocks the scolecodonts are well preserved and better studied than any other place in the world. They can sometimes be found on bedding planes but far larger collections can be obtained by acid extraction technique. Thus scolecodonts belong to acid-resistant microfossils, together with chitinozoans, conodonts and acritarchs.