Credit: A. JERVE

Adorned with spiny fins and diamond-shaped scales, the fossilized 'acanthodian' fishes resemble two of today's fish groups: bony fishes, which include eels and salmon, and cartilaginous fishes such as sharks. Palaeontologists have long puzzled over which family these ancient fishes — which hail from the Devonian period, between around 415 million and 360 million years ago — belong to, because few clues have been gleaned from the limited fossils available. It was thought that the discovery of new fossils might help to set the record straight. Martin Brazeau, a palaeontology graduate student at Uppsala University in Sweden, found that that we have had the answer all long — we just didn't know it.

“Many of these fishes had a cartilagenous skeleton like a shark's and so deteriorated easily,” says Brazeau. “I sometimes describe the fossils that we have as 'fish-shaped patches of scales', because we have the scales but no internal skeletal bones. It's hard to compare them to anything else.”

In addition to these fossils was a palaeontological prize: the braincase of a fish of the genus Acanthodes. This group lived late in acanthodian history, during the Permian period 290 million to 248 million years ago. Many palaeontologists took this fossil to be the representative example of the whole group, and because it has more bony-fish features, ascribed acanthodians to the bony fishes. However, discoveries during the past 15 years of acanthodian fossils with shark-like scales and teeth have called the grouping into question once again.

“These recent fossils started to make us question: are these a natural group or are we looking at a bunch of organisms closely related to the common ancestor of all jawed vertebrates?” says Brazeau. “It's tempting to put them all into one group; however, they might come from different groups but all look very similar.” This, Brazeau adds, is a common problem.

While searching the literature at London's Natural History Museum, Brazeau stumbled on a 1973 monograph describing a well-preserved acanthodian called Ptomacanthus anglicus from the Welsh border region. The specimen stood out to him. It presented a ventral view of a flattened head that provided a glimpse into the roof of the mouth — a rare opportunity, because most known fossils capture fish side-on. The fossil hailed from the acanthodian heyday of the early Devonian period, and is more than 100 million years older than that of Acanthodes. Brazeau checked to see whether the fossil had been further described in the literature. The only references he could find were to its whorled teeth, which, at a superficial level, resemble those of sharks.

Brazeau obtained a rubber cast of the fossil from the museum's collection and found that the brain case was most similar to sharks and placoderms, or bony-headed fish. He then performed a phylogenetic analysis, comparing the characteristics of fossils from 47 groups including bony fish, sharks, acanthodians, placoderms, and jawless fish. According to this, acanthodians as a sole group fell apart. Some members appeared in the bony fish lineage, and others in the shark lineage (see page 305).

Brazeau remarks, “For decades, paleontologists have been holding out for a braincase in addition to Acanthodes and there it was sitting in the literature for 30 years.”