Roughly 360 million years ago, one group of lobe-finned fish began evolving into four-legged, land-living animals that resulted in reptiles, amphibians and mammals like us.
Does it fit our evolutionary picture? (Brian Choo)
However, a fossil discovered in a quarry in Ningxia, north China, now threatens that stability. It was discovered in 2002 byMin Zhuat the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing andPer Ahlbergat Uppsala University in Sweden.
The fossil belongs to a new species of lobe-finned fish, namedHongyuchowi. It was about 1.5 metres long, and lived 370 to 360 million years ago.
This fish doesn’t fit
But when the researchers tried to fitH. chowiinto the existing evolutionary tree, it didn’t fit easily.
That’s because in some respects,H. chowilooks like an ancient predatory fish called rhizodonts. These are thought to have branched off from lobe-finned fish long before the group gave rise to four-legged land animals.
But Ahlberg saysH. chowihas aspects that look surprisingly like those seen in early four-legged animals and their nearest fishy relatives –an extinct group called the elpistostegids. These include the shoulder girdle and the support region for its gill covers.
The fossil of Hongyu chowi could change out picture of the move from sea to land
Nature
This implies one of two things, the researchers say. The first possibility is thatH. chowiis some sort of rhizodont that independently evolved the shoulders and gill cover supports of a four-legged animal.
Alternatively, the rhizodonts may be more closely related to the four-legged animals and the elpistostegids than we thought. But this would also imply a certain amount of independent evolution of similar features, because the rhizodonts would then sit between two groups that have many features in common – features the two groups would have had to evolve independently.
Ambush hunter
H. chowidoes look a lot like an early four-legged animal or an elpistostegid, saysMartin Brazeauat Imperial College London. That could imply that different groups of lobe-finned fish independently evolved similar features, he says.
The find confirms an earlier suspicion thatthere was independent or “parallel” evolutionbetween the rhizodonts, the elpistostegids and the first four-legged animals, saysNeil Shubinat the University of Chicago. “It has been a recurrent theme in the field.”
H. chowiis also interesting for its unusual lifestyle, says Ahlberg. Its anatomy suggests it was an ambush predator that lurked just above the bottom, snapping at any smaller fish that swam too close – like some anglerfish do today.
“Hongyuis one of the earliest examples of this lifestyle,” he says.