Description

About Osteolepis macrolepidotus Devonian Osteolepids

Physical Description

These are small and slender fishes, generally not exceeding 135 mm in length. They are the smallest known osteolepids occurring within the Orcadian Basin. The dorsal fin is located on the middle of the back instead of closer to the tail as with the osteolepid genera Thursius and Gyroptychius. Most O. panderi fossils are disarticulated due to anaerobic rotting. Therfore, intact specimens are a rarity.

Age and Distribution

Osteolepis  is from the Middle Devonian of Scotland. This fish lived approximately 385 million years ago. At the time this area of Scotland was part of the Orcadian Basin, which was a huge complex of lakes that stretched for hundreds of miles. During this period plants started to rapidly colonize the land and fishes dominated nearly every niche of seas and lakes, hence why this period is also known as the ‘Age of Fishes’. O. macrolepidotus co-inhabited the lake with other fishes like the acanthodians Mesacanthus and Diplacanthus, the osteolepids Gyroptychius and Thursius, the porolepid Glyptolepis and the placoderms Dickosteus and Homostius.

About the Osteolepidae Devonian Osteolepids

Evolutionary Significance

It is believed that the Osteolepiformes gradually evolved into the first amphibians. Osteolepids have certain features that are also found in the oldest known amphibians. One of the most important is that the bones of their pectoral and pelvic fins were the precursors of terrestrial limbs. A few evolutionary stages further and these fins would evolve into legs used to travel on land. These then became amphibians (tetrapods), which in turn became reptiles, mammals, and eventually us. Fishes like Osteolepis can therefore be regarded as our ancestors.

Another important indication that osteolepids are ancestral to tetrapods is that they most probably possesed an air bladder which also could be used as a respiratory organ. Osteolepids are regarded as a sister-group of the Dipnoi (lungfishes).

Taxonomy

Osteolepis macrolepidotus belongs to the Osteolepidae which is a subclass of the Crossopterygii (lobe-finned fishes). Other Osteolepidae known from the Middle Devonian of Scotland are: Osteolepis panderi, Thursius macrolepidotus, Thursius pholidotus, Gyroptychius agassizi and Gyroptychius milleri.