Anomalocarids And The Early Arthropods

Aegirocassis benmoulae. At seven feet long this anomalocarid was larger than us yet lived as a filter feeder

In the 1960s and 1970s, palaeontologists made a return to the remarkable Cambrian-age fossils collected from the Burgess Shale by Charles Doolittle Walcott nearly 50 years previously. They found life during the Cambrian period was far more complicated than they had assumed. Many of the species, which had been shoe-horned into existing taxa, were found to have far more blurred taxonomic relationships. Some did fit into existing taxa, albeit distantly. Others appeared to lie in entirely new groupings.

One such group were the anomalocarids, the first super predators on the planet. Growing up to two metres in length and possessing powerful eyes, teeth and spiked claw-like appendages, they were formidable yet bizarre. Their morphology is unlike that of any other group of arthropods, their closest relatives, yet the anomalocarids may shed light on the origins of the arthropod group, according to a recent study from Yale University.

The name arthropod means jointed legs, More importantly those legs are described as being biramous, meaning that they have two branches. Depending upon where they lie along the length of the body, each branch serves a different function, such as locomotion, feeding or gas exchange. Understanding how the biramous nature of arthropod limbs evolved is key to understanding where the arthropod group came from in evolutionary terms. The new fossil evidence provides a link between the arthropods, their onychophoran ancestors, and the anomalocarids which lie somewhere between the two.

The 480 million year old fossils of Aegirocassis benmoulae from Morocco

Aegirocassis benmoulae was discovered recently from a locality in Morocco. Dating from the Ordovician 480 million years ago, it is among the youngest of the anomalocarids and is one of the most unusual.

At seven foot in length it was larger than all other members of its group. Instead of the spiked appendages found in most anomalocarids its spines were adapted to filter feeding; its superior size combined with its ecological niche makes it the blue whale of the arthropod family.

Anomalocarids had bodies with a set of flaps lining the ventral (bellyside) edges. These were limbs which had been adapted to propel them through the water. 'It was while cleaning the fossil that I noticed the second, dorsal set of flaps,' said Peter Van Roy, also from Yale. 'It's fair to say I was in shock at the discovery, and its implications. It once and for all resolves the debate on where anomalocaridids belong in the arthropod tree, and clears up one of the most problematic aspects of their anatomy.' Examination of older anomalocarid species shows that they also possessed two sets of flaps.

When viewed in an evolutionary context, these sets of flaps, are a transition between a single set of invertebrate legs and the biramous limbs of arthropods. Genetic analysis demonstrates that the arthropods are descended from a group of invertebrates known as the onychophorans whose members include the velvet worms. Onychophorans have a single set of un-jointed limbs. While Anomalocarids have a precursor to biramous limbs, yet these are also un-jointed. It is for this reason that anomalocarids and many other Cambrian animals are put into a third group, the lobopodians. Although there is some debate as to whether the lobopodians are a group within the larger arthropod family or entirely separate.

In either case what Aegirocassis gives us is a clear evolutionary pathway linking the onychophorans to the arthropods via the lobopods. Arthropods are one of the most diverse and ecologically important groups on the planet. They owe their evolutionary success not just to the morphological flexibility of exoskeletons, but the facility for multiple, specialised adaptations, thanks to those biramous limbs.