The cranial endocast of Dipnorhynchus sussmilchi (Sarcopterygii: Dipnoi) and the interrelationships of stem-group lungfishes

The first virtual cranial endocast of a lungfish from the Early Devonian, Dipnorhynchus sussmilchi, is described. Dipnorhynchus, only the fourth Devonian lungfish for which a near complete cranial endocast is known, is a key taxon for clarifying primitive character states within the group. A ventrally-expanded telencephalic cavity is present in the endocast of Dipnorhynchus demonstrating that this is the primitive state for “true” Dipnoi. Dipnorhynchus also possesses a utricular recess differentiated from the sacculolagenar pouch like that seen in stratigraphically younger lungfish (Dipterus, Chirodipterus, Rhinodipterus), but absent from the dipnomorph Youngolepis. We do not find separate pineal and para-pineal canals in contrast to a reconstruction from previous authors. We conduct the first phylogenetic analysis of Dipnoi based purely on endocast characters, which supports a basal placement of Dipnorhynchus within the dipnoan stem group, in agreement with recent analyses. Our analysis demonstrates the value of endocast characters for inferring phylogenetic relationships.