Lanarkia horrida is a small fish called a ‘thelodont’ whose body is covered with skin-teeth called denticles. These denticles have different sizes and shapes depending on their locality on and in the body. Lanarkia horrida fossils have relatively large denticles, which meant that the body was well-protected against predators. The body shape is flat and broadly resembles a modern Angel Shark. L. horrida had a small dorsal fin and like all fishes from this Silurian locality their tail is upside down compared to current fishes, so the shorter lobe at the top and the longer lobe at the bottom. At the side of its head it had an area which accommondated its gills. These archaic fishes are normally between 3-12 cm but based on fragmentary fossils it is evident that some specimens could grow larger than that.
Age and Distribution
Lanarkia horrida occurs in the Lower Silurian Fish Bed Formation of Scotland, approximately 430 million years old. Other fishes occurring at this horizon are the anaspid Lasanius problematicus, Birkenia elegans, the thelodonts Lanarkia spinulosa, Lanarkia horrida, and Lanarkia lanceolata, the osteostracan Ateleaspis tessellata, and the recently-discovered euphaneropid Ciderius cooperi. A eurypterid called Lanarkopterus dolichoschelus also occurs at this fauna, as well as several enigmatic fossils such as Taitia catena and Dictyocaris sp.
About the thelodonts
Evolutionary significance & taxonomy
Lanarkia horrida is a member of the Thelodonti. At the moment, little is known about thelodonts other than their scale covering. They are known from Silurian and Devonian deposits throughout the world. Most species are known from isolated scales, with only a handful represented by articulated material. Almost nothing is known about their internal anatomy. Their relationship to other groups of fishes is quite unclear, although their body shape and skin-teeth are somewhat reminiscent of sharks. This is why they are sometimes referred to as ‘proto-sharks’.