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The Birth of Filter-Feeding Giants

The Cambrian seas have a new apex predator with the discovery of one of the earliest arthropods measuring twice as large as the previous heavyweight champion.

Every so often a new fossil turns up that really surprises even hardcore palaeontologists, such as the recent discovery of the world’s largest animal of its time, Aegirocassis benmoulae, in Morocco. The beautifully preserved Ordovician fossil remains, dated at around 470–480 million years old, show that Aegirocassis was a close relative of the previous Cambrian heavyweight, the arthropod Anomalocaris, which reached around 1 metre in length and had been considered the apex predator in the Cambrian seas.

However, Aegirocassis was at least double this size (2 metres) and lived primarily as a filter-feeder, scooping schools of plankton and small shrimp-like creatures into its mouth with large scoop-shaped appendages that look similar to the paired flaps at the mouth of a manta ray. Both structures help direct schools of small creatures into the mouth. The work was published online in Nature by a team from Yale University led by Dr Peter Van Roy.

Anomalocarids used a series of flaps along the sides of their body to swim, and it has long been debated if these are homologous to other arthropod limbs. The new fossils solve this mystery and reveal the truth about the relationships of this enigmatic group of animals.

The new specimen shows that each trunk segment had separate dorsal and ventral flaps, with a series of special “setal” blades attached at the base of the dorsal flaps. Comparisons with other primitive arthropods indicate that the anomalocaridid ventral flaps are homologous to the walking limbs of true arthropods, which have two sets of appendages, while the dorsal flaps and associated setal blades are homologous with the flaps of gilled lobopodian arthropods.

This evidence therefore shows that anomalocaridids represent a stage before the fusion of the two types of limb (exopod and endopod) into the Cambrian biramous limb (two appendages on each limb). This discovery confirms that anomalocarids are indeed very basal in the overall evolution of arthropods.

The large body size and conspicuous filter-feeding apparatus at the mouth of Aegirocassis suggests that early arthropods could diverge into new niches to take advantage of the rising abundance of plankton in the seas dusring what is known as the Great Ordovician Biodiversity Event. But what is the history of giant filter-feeding animals? Have they always been around since Aegirocassis?

Until about 10 years ago we knew of very few giant filter-feeding animals except for living animals like baleen whales, manta rays and the whale shark and basking sharks. Baleen whales seem to have evolved around 30 million years ago, and early forms from Victoria like Janjucetus still retained teeth widely spaced on the jaw. The teeth were probably filled with baleen.

New fossil finds have revealed that giant bony fishes (teleosts) reaching up to 12–15 metres in length persisted right throughtout the Mesozoc Era (tinyurl.com/kfwh3m9). The biggest was Leedsichthys from the late Jurassic, known primarily from articulated remains found in the UK, and Bonnerichthys from the Late Cretaceous.

But did gigantic filter-feeders exist between Aegirocassis (470 mya) and the giants of the Mesozoic Era (250–625 mya)? The answer is yes, and again the evidence is only starting to be pieced together.

The first really large fishes (>2 metres) are from the Early Devonian period about 400 mya. These were armoured placoderms of the homosteid family, some of which had skulls about 60–80 cm long, indicating fishes at least 4 metres in length. Their jawbones were smooth, lacking teeth, which suggests they were filter-feeders. Similarly, another giant Late Devonian placoderm, Titanichthys, possibly reached as large as 8 metres. It also had smooth jaw bones and was likely a filter-feeder.

There are still several gaps in the geological timescale where giant marine filter-feeders are missing. I predict that they will eventually be found, so keep tuned.

John Long is Strategic Professor in Palaeontology at Flinders University and current President of the Society of Vertebrate Paleontology.