ICHTHYOSTEGA

genus of extinct amphibians found as fossils in Late Devonian rocks in eastern Greenland (the Devonian Period began about 408,000,000 years ago and lasted about 48,000,000 years). Ichthyostega is among the oldest known vertebrates that can be called amphibians; the genus retains many fishlike characteristics. Ichthyostega was about one metre (three feet) long and had a small dorsal fin along the margin of its tail; the tail itself possessed a series of bony supports, typical of the tail supports that are found in fish. Other fishlike traits include the relatively short snout region, the presence of a preopercular bone in the cheek region (which serves as part of the gill cover in fish), and many small scales on the body. Advanced traits shared with more advanced amphibians include typical early amphibian limbs and limb supports, the lack of gills, and strong ribs. Ichthyostega and its relatives represent forms that appear to be near the evolutionary line leading to the more advanced amphibians. It is possible that the ichthyostegids persisted into the following Mississippian Period.