Ateleaspis tessellata Osteostracan Fossil Fish part and counter part
Ateleaspis tessallata is the oldest known Osteostracan.
This part and counterpart fossil is a well preserved specimen. It shows the head of the fish with all the fine details. These fossils are one of the rarest fishes found in the Silurian of Scotland. Like the other fishes from the Scottish Silurian these cannot be collected anymore. The skull is 3.5cm long by 3.5cm wide (inch).
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Ateleaspis tessellata is an osteostracan. This is an armoured fish characterised by a flattened bony head shield with two orbits, a pair of pectoral fins, a pair of dorsal fins, and a heterocercal tail. Its body was covered by thick scales which, together with the armoured head, provided protection against predators like eurypterids.
Age and Distribution
Ateleaspis tessellata occurs in the Lower Silurian Fish Bed Formation of Scotland, which is approximately 430 million years old. Other fishes occurring at this horizon are the anaspids Lasanius problematicus and Birkenia elegans, the thelodonts Lanarkia horrida, Lanarkia spinulosa, Lanarkia lanceolata, and Shielia taiti, and the euphaneropid Ciderius cooperi. The eurypterid Lanarkopterus dolichoschelus also occurs at this fauna, as well as many enigmatic fossils like Taitia catena and Dictyocaris sp.
About the osteostracans
Evolutionary significance & taxonomy
Ateleaspis is significant in that it is the oldest known member of the osteostracans. What makes A.tessellata remarkable is that, despite it being the most basal osteostracan, it has more advanced traits than later osteostracans. This is because later osteostracans would go on to lose their pectoral fins and dorsal fins. In essense, Ateleaspis therefore represents the pinnacle of osteostracan evolution.