A crocodile-sized sea creature called an anomalocaridid, that swam the seas 480 million years ago was, in its day, the largest animal on Earth and its discovered sheds new light on the evolution early evolution of arthropods.
The Aegirocassis benmoulae, evolved from aggressive carnivorous monsters but was itself a gentler behemoth – one of the first filter feeders that were to lead to the plankton-eating giants such as the blue whale.
“There seems to be an overarching trend of giant filter-feeders evolving from predators at the time of a major plankton diversification,” said Peter Van Roy, a palaeontologist with Yale and Ghent universities and lead author of a paper published in Nature. The creature “represents the oldest example of that trend”, he said.
Aegirocassis benmoulae also solves a puzzle for palaeontologists. Unlike other early arthropods, it had the makings of the double-branched limbs of modern arthropods. Each section of its body had two flaps, though they had not yet fused together or become true limbs, as Yale News explains:
The upper flaps were equivalent to the upper limb branch of modern arthropods, while the lower flaps represent modified walking limbs, adapted for swimming. Furthermore, a re-examination of older anomalocaridids showed that these flaps also were present in other species, but had been overlooked. These findings show that anomalocaridids represent a stage before the fusion of the upper and lower branches into the double-branched limb of modern arthopods.
“Until this discovery, we thought early arthropods had only one set of flaps, which was problematic,” said lead researcher Peter Van Roy of Yale University. “But this upper flap we’ve found is the equivalent of the upper branch. It represents a state where you have these multiple branches, but they haven’t yet fused together.”
What’s more, Van Roy and his colleagues were able to find signs of these secondary flaps in older arthropods when they went looking for them. He believes the creature fills the gap in the evolution of today’s bugs and shrimp.
Bill Condie is a science journalist based in Adelaide, Australia.