416-359 million years ago


Rhipidistian fish adapted to freshwater environments and evolved a number of features which would be retained in their tetrapod descendants.

The fossil sarcopterygian Styloichthys seems to be intermediate between the most primitive sarcopterygians (Psarolepis and Achoania) and the more advanced rhipidistians (Zhu, 2002). Rhipidistians were the dominant late Paleozoic freshwater sarcopterygians that became extinct in the Permian Period. There are a number of characteristics they share with the first amphibians, which seem to have descended from them. They have a large number of bones (up to 30) around the eye in a sclerotic ring while ray finned fish have only 4 such bones. The nostrils open into the oral cavity via an internal naris while in ray finned fish the nasal cavity formed a blind sac. There is a tube that is comparable to the lacrimal duct (tear duct) of land vertebrates. They possess lateral line organs (involving grooves along the skull) for detecting aquatic prey that were still present in the first amphibians. The organization of the skull bones was extremely similar to that of the first amphibians. The pineal foramen is very prominent between the two parietal bones, as it was in the early amphibians. Although the notochord is still large and functions in support, thin walled vertebral centra surround it. Since living sarcopterygians have lungs, it is presumed that their ancient relatives which would evolve into amphibians had them also. The frontal-ethmoid ethmoid region of the skull and nasal cavity were longer, as in early amphibians (Carroll, 1988; Yu, 1998)