“Protospinax carried features that are found in both sharks and rays today,” explains study author Patrick L. Jambura. Protospinax lived some 150 million years ago and was a 1.5-m-long, dorso-ventrally flattened cartilaginous fish with expanded pectoral fins and a prominent fin spine in front of each dorsal fin. Although known from well preserved fossils, the phylogenetic position of Protospinax has puzzled researchers ever since it was first described in 1918. “Of particular interest,” Jambura continued, “is whether Protospinax represents a transition between sharks and rays as a ‘missing link’ -- a hypothesis that has gained considerable appeal among experts over the past 25 years.” Alternatively, Protospinax could have been a very primitive shark, an ancestor of rays and sharks, or an ancestor of a certain group of sharks, the Galeomorphii, which includes the great white shark today -- all of which are exciting ideas whose plausibility has now been clarified by scientists.