Climatius reticulatus

Climatius reticulatus from Wikipedia

Name: Climatius reticulatus [Gradational network]

When: Early Devonian Period, about 415 million years ago

Where: Numerous localities in Europe

Claim to fame: Let's nerd out!

Climatius and its relatives have long been known to paleontologists as members of a group called acanthodians, meaning "spiny ones." They get the name for their conspicuous feature - fin spines on absolutely all fins, often accompanied by extra spines separate from any fins. Climatius is special only for being an especially well known member. Acanthodians are among the earliest vertebrates with proper jaws, but what exactly they were has been a long-standing mystery. In the last century they were jokingly called "spiny sharks" because their body-shape was sort of shark-like, but not out of any belief that they were really closely related to them. As of 2000, most paleontologists actually viewed them as relatives of proper bony fish, but not members of the group, themselves.

In the 21st century, however, acanthodians like Climatius have come into better focus! Some worker suggested that they were simply the basic morphotype for both bony fish and cartilaginous fish like actual sharks. Maybe some were closer relatives of bony fish, some closer to cartilaginous fish, and others distantly related to both. But the real breakthrough came in 2013, when Min Zhu and colleagues convincingly showed that all acanthodians are close relatives of cartilaginous fish. What a surprise! It turns out that calling them "spiny sharks" really wasn't such a joke!