Osteolepis, 30 cm. 

Supervised by Philippe Janvier. Thanks to Jeff Liston

Osteolepis is an extinct genus of lobe-finned fishes from the Devonian period. It lived in the Lake Orcadie of northern Scotland, and grew 20 cm long. A key character of this prehistoric fish is the large square scales and rhombic plates on its head. These scales and plates were covered in a thin layer of spongy, bony material called cosmine. The sponginess of cosmine may have helped with buoyancy of the heavy plates on the otherwise small lobe-finned fish.

The cosmine layer contained canals, which were connected from pores on the surface of the face to sensory cells deeper in the skin, meaning that the sensory organs underneath still had contact with the water. This made Osteolepsis able to sense vibrations in the water, allowing it to detect even minute changes in water pressure caused by passing aquatic animals.

Osteolepsis are grouped with other lobe-finned fish, that are intermediate steps of tetrapod development: Its upper tail lobe was more muscular than that of the lower tail lobe, resembling an intermediate step towards a tetrapod tail. It also had a choanae; a posterior nasal passage between the nasal cavity and the throat, allowing for breathing when the mouth is closed. This is a feature of tetrapods (as well as humans), but not commonly found in fish.