When: Permian (300-250 million years ago)
Where: North America
What: Diplocaulus is an amphibian. It reached lengths of roughly 3 feet (~1 meter long), making it the largest of its clade. We have an exceptional fossil record of these animals, including wonderful ontogenetic series of juveniles, which show that the two-phase metamorphosis of modern amphibians was not present in these taxa. Instead juvenile Diplocaulus looked mostly like small adults. The biggest difference is that the head shield became more and more elaborate as the animal grew, juveniles barely had any ‘boomerang’ shape to their heads. Most Nectridea had head shields, but they were among their most developed in Diplocaulus. These animals were carnivorous and lived in the many rivers, lakes, and swamps covering North America during the Permian.
How Diplocaulus is related to extant amphibians is a topic of much debate. It, along with a number of other extinct amphibian clades, have in the past been grouped as the Lepospondyli. However, modern interpretations are divided as to if this is a real group, with all of its members descended from a common ancestor, or if it is an artificial collection of taxa. It is not likely that they are found within the Lissamphibia (the modern amphibians), but they have been proposed to be stem taxa to this clade. Some members have also been proposed to be more closely related to amniotes (reptiles - including birds, and mammals) than to these living amphibians. Detailed cladistic studies incorporating all relevant taxa are needed for a firm understanding of this part of the evolutionary tree.