In placoderms, some researchers say, 430 to 360 mya:
Placoderms were the world’s first megapredators, with ancient behemoths such as Dunkleosteus reaching the same size as today’s Great White Shark.
They were also the first vertebrate animals on Earth to have a complex form of sexual reproduction – copulation – where males fertilised the females internally rather than just spawning in water.
To achieve this, the males bore bony paired structures – the claspers – which were previously thought to be similar to the cartilaginous claspers of modern sharks.
However,
Unlike the claspers of modern sharks and rays that are a part of the paired pelvic fins, the claspers and female basal plates in placoderms were not at all connected to that fin.
Instead they developed as an extra pair of limbs further down the body.
This discovery implies that the first vertebrates did not conform to the typical tetrapod-like pattern of having just two pairs of paired limbs (arms, or pectoral fins, and legs, or pelvic fins) that typifies the basic jawed vertebrate body pattern from fishes through to mammals. More.
In that case, it was convergent evolution that caused the placoderm claspers to seem like those of sharks.
That said, the claim that some vertebrates were six-legged will (and should) give many pause for thought. Was it ever seen elsewhere?
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