Thelodonts are an enigmatic group of fishes known Ordovician to Devonian deposits. They are characterised by the possession of a body covering of minute tooth-like scales, also called denticles. Most species are known from isolated denticles, but some, and most notably the Scottish species, are known from articulated material. Due to their body covering of scales little is known about the internal anatomy of these fishes. Distinguishing different thelodont species from each other is usually done by comparing the shape of their denticles.
Lanarkia horrida Traquair, 1898
Lanarkia horrida can be discerned from other Lanarkia species their striated denticles which end with a single spine. In cross section these denticles are circular. Strangely, there are several different types of juvenile Lanarkia horrida, as seen below. Some possess only small denticles, others possess only large denticles, while yet others display a mixture of large and small denticles. Its size ranges from 1cm to approximately 25cm.
Lanarkia lanceolata Märss, 1998
Lanarkia lanceolata can be distinguished from other Lanarkia species by the lance shaped scales which look like a cross in cross section.
Lanarkia spinulosa Traquair 1898
Lanarkia spinulosa differs from other Lanarkia species by the presence of trident-shaped denticles which end in three points.
Loganellia scotica Traquair, 1898
Loganellia scotica occurs at Patrick Burn Formation and equivalent localities. It can be found partly- or entirely enclosed within a concretion. Concretions from the fossil locality Birk Knowes SSSI in particular have a unique preservation capacity. By carefully etching split nodules with diluted acetic acid we encountered three types of internally situated denticle groups. The ones near the gill apparatus were fused into oblong tooth plates. Our discovery of tooth plates within a jawless fish has given rise to debate in the field of early vertebrates. Thelodonts are now regarded by some as forerunners of fishes with jaws.
Shielia taiti Stetson, 1931
Shielia taiti is readily distinguished from other thelodonts because of the very small and short denticles that end in five points.