Australasian Science Magazine

Spanning 1 metre in length, the Cambrian super-predator Anomalocaris patrolled the world’s oceans more than half a billion years ago. Credit: Katrina Kenny

By Allison Daley & John Paterson

The discovery of the world’s oldest apex predators in the oceans more than half a billion years ago is a puzzling story that began well over a century ago. We now have a much clearer picture of these spectacular animals, but the debate about their feeding habits continues.

The classic Australian cliché to “throw another shrimp on the barbie” may well have an origin that dates back much further than expected – to the Cambrian Period over 500 million years ago.

If humans had existed in the Cambrian, walking into a fish market would have been a very different experience. Firstly, it would be called an arthropod market because it was the ancestors of today’s crabs, lobsters and prawns that dominated the oceans more than half a billion years ago. While the earliest fish had made their appearance in the fossil record, it would be tens of millions of years before they would really have their time in the sunlit waters.

Many of the creatures in the ice-filled trays of a Cambrian arthropod market would have been a sight to behold – from spiny trilobites the size of dinner plates to various prawn-like beasts armed with elaborate clasping appendages on their heads.

But perhaps the strangest of all was an animal that would take up an entire tray: Anomalocaris. Considered the “Great White Shark” of the Cambrian oceans, this fellow could probably grow up to a metre in length, with its body consisting of a formidable pair of large, spinose limbs at the front of the head, a circular mouth of plates lined with teeth-like serrations, a pair of stalked eyes, a series of swimming flaps along the length of its body, and tail flukes at the back end.

Despite its age, the Cambrian arthropod market is something we can describe quite well from the fossil record. All of these strange and wonderful arthropods are known to us from special fossil sites that are exceptionally preserved. The majority of fossil remains consist of hard parts such as shells, teeth and bones, but these special Cambrian sites have also preserved soft tissues such as skin, eyes, gills and even guts – sometimes with remnants of the animal’s last meal. This gives us a window to the past through which we can see what animals looked like during their earliest burst of evolution during the Cambrian Period.

Anomalocaris was the very first animal described from one of these fossil sites, and its story is a long and complicated one.

Palaeontologists spend a lot of time piecing together ancient animals based on whatever fossils can be found, like putting together a big jigsaw puzzle. The obvious example is a dinosaur skeleton – even though it may have been found with all of the pieces jumbled up, palaeontologists can put this boney puzzle back together relatively easily because animals with similar bones are still alive today for comparison – most notably their feathery descendants.

But imagine trying to put together a jigsaw puzzle without having the picture on the box and, not only that, the picture is of something totally unlike anything you’ve ever seen before. This was the problem faced by the palaeontologists who first tried to make sense of the unusual creatures of the Cambrian– especially Anomalocaris.

Like a jigsaw puzzle, the body of this strange beast was made up of several pieces, and these had a tendency to break apart. The frontal limbs, mouthparts, head shield and body of Anomalocaris were all discovered and interpreted in isolation before it was realised they belonged to the same animal.

The frontal limbs were the first piece of the Anomalocaris puzzle to be discovered. These fossils were first described by Joseph Whiteaves in 1892 based on about 50 specimens from the Trilobite Beds on Mount Stephen in the Canadian Rocky Mountains. At the time, these strange fossils were thought to be a new type of shrimp consisting of a segmented body with pairs of short, spiny leg-like projections, but unfortunately the specimens seemed to be missing the head. The leg-like appendages also lacked joints – a defining characteristic of arthropods – so Whiteaves coined the name Anomalocaris meaning “anomalous shrimp”.

A few years later on Mount Field, just across the valley from Mount Stephen, Charles Walcott stumbled across the first fossils from the Burgess Shale and began excavating a quarry that would produce a rich treasure trove of Cambrian oddities. Between 1911 and 1914, Walcott described dozens of new fossils, most of which were totally unknown to science, with several turning out to be pieces of the Anomalocaris jigsaw puzzle.

A circlet of plates with sharp teeth-like spines pointing inwards was thought to be a jellyfish that resembled a pineapple ring (with teeth) floating through the Cambrian seas. This “jellyfish” was named Peytoia, and we now know it to be the mouthparts of Anomalocaris and other related species.

Another fossil called Laggania was almost schizophrenic in how often it changed personalities. It was originally interpreted to be a sea cucumber, then a polychaete worm, then a sponge. It is now known to be the body of one of the cousins of Anomalocaris.

Perhaps out of sheer frustration or desperation when trying to identify and reconstruct these bizarre creatures, some palaeontologists even began creating “Frankenstein animals” by piecing together Anomalocaris frontal limbs and other totally unrelated but enigmatic fossils. They could not have been further from the truth, which turned out to be much more interesting and weird.

The key to revealing the true identity of Anomalocaris relied on the fact that fossils from the Burgess Shale and other special Cambrian sites are so well-preserved that they can actually be dissected. The animals that make up these fossils were flattened to a width of only a few millimetres, but it wasn’t like the splat of squashing a bug on the windscreen of a car. Instead, the bodies were carefully flattened without any lateral expansion so all the body parts remained intact and in their relative locations – just very flat (like blossoms in a flower press).

Like a surgeon, any palaeontologist with a steady hand can use a sharp needle-like tool to chip away layers of the rock, moving progressively from the outer skin though muscle, internal organs and then out the other side of the body. Unlike surgery, however, this process is entirely destructive since the pieces of rock cannot be put back in place.

In the early 1980s, two scientists at the universities of Cambridge and London, Harry Whittington and Derek Briggs, decided to take on the risk of this destructive process and dissect some mysterious unidentified specimens of Burgess Shale fossils that seemed to have a small portion of an appendage hidden by overlying and somewhat indistinct fossil material. As the rock was slowly chipped away, a truly magical moment occurred when this exposed a pair of Anomalocaris “shrimp bodies” as limbs attached to the front of a head, with the “jellyfish” Peytoia between them forming the mouth of a large arthropod-like creature.

Nearly 100 years after the first specimens were found, the true identity of Anomalocaris was finally revealed. With this key insight, Whittington and Briggs were able to identify several specimens of Anomalocaris and piece together a second animal called Laggania that would have been the cousin of Anomalocaris.

Once the basic body plan of Anomalocaris was known, animals similar to it were identified all over the world, but with variation in the details of their armoury, such as appendages, mouthparts, head shields and tail fans. A true menagerie of these creatures is now known, and collectively they are referred to as the anomalocaridids. At the Burgess Shale alone there are at least seven different species of anomalocaridids, with others making an appearance in the Chengjiang biota in China, several sites in the USA, and the Sirius Passet biota in North Greenland.

Anomalocaris and its close relatives terrorised the Cambrian seas on a global scale, and even Australia was not out of reach of its spiny grasp. Lapped by the azure waters of Investigator Strait, the rocks exposed near Emu Bay on the north-east coast of Kangaroo Island in South Australia have revealed a Cambrian assemblage of fossils unlike any found elsewhere in the world.

The Emu Bay Shale is home to its very own species of Anomalocaris, distinct from other species in having very long, delicate spines bearing tiny barbs on the frontal limbs. But not only this, our recent discoveries show that the Emu Bay Shale uniquely preserves the eyes of Anomalocaris in exquisite detail, revealing a morphology like the compound eye of a modern insect, with a visual surface covered in thousands of tiny lenses.

Over 515 million years ago, Anomalocaris could see as well as or better than many animals living in the oceans today. Such incredible visual acuity, when coupled with its formidable armoury of spiny appendages and mouthparts, large size and streamlined body, adds evidence to a theory that has been toyed with since Anomalocaris’ true morphology was revealed – that this animal was an active swimming predator, stalking the Cambrian oceans for its next meal. In this and other ways, the Emu Bay Shale fossils have played an important role in perpetuating and refining Anomalocaris’ image as the world’s first super-predator.

Soon after the great unveiling of Anomalocaris by Whittington and Briggs, palaeontologists were eager to make it the “poster animal” of an apex predator in the Cambrian and, perhaps in the most ancient of cold cases, make it the number one suspect responsible for the so-called bite marks on the exoskeletons of contemporaneous trilobites.

While this suggestion is entirely reasonable, it is not without controversy. Many have counter-argued that the damage patterns on trilobites could not have been made by the frontal limbs or mouthparts of Anomalocaris.

The added complication is that Anomalocaris and kin do not appear to possess mineralised or even strongly reinforced feeding apparatus capable of crushing the comparatively hard calcite exoskeleton of trilobites. However, in 1999 Chris Nedin, then of the University of Adelaide, documented injuries in both trilobites and non-mineralised arthropods from the Emu Bay Shale, and proposed that Anomalocaris was equipped to inflict such wounds by using the frontal limbs to grasp an individual and flex it back and forth until it broke – like trying to snap a credit card with your hands.

This idea was reinforced with additional evidence from the Emu Bay Shale in the form of large coprolites (or fossilised faeces) containing the crushed remains of the dinner-plate-sized trilobite Redlichia. Anomalocaris is assumed to be responsible for these faeces because nothing else in the Cambrian ocean was large enough to produce faeces of that size – except, of course, for Redlichia itself.

Despite the work of Nedin, the debate about the trilobite-chomping habits of anomalocaridids marches on. In a recent but as-yet unpublished study on the mouthparts of Anomalocaris, James Hagadorn of the Denver Museum of Nature and Science used computer modelling to demonstrate that the mouthparts of Anomalocaris would have been too soft and pliable to crack the mineralised exoskeleton of a trilobite without incurring considerable damage to themselves, but no evidence of such breakage or wear has been found on the mouthpart plates or their teeth-like serrations.

Hagadorn’s models suggest that the mouthparts were better suited to suction feeding, which concurs with a recent study by Jan Bergström of the Swedish Natural History Museum and one of us (AD) on the functional morphology of Anomalocaris’ mouthparts showing them to have a highly variable number of plates that were soft and prone to folding. So there remains considerable doubt about whether Anomalocaris fed on trilobites.

Discounting Anomalocaris as a trilobite-specific hunter does not preclude the notion that it was the top predator of its time. The Cambrian seas were teeming with a veritable smorgasbord of easier nuts to crack, including various worm-like creatures and a plethora of soft-shelled arthropods.

The impressive array of anomalocaridid limbs, and their highly varied morphology among species, is most certainly reflective of diverse diets. At many sites, several anomalocaridid species were living in the same community, so they would have needed to exploit different prey items to reduce competition. For example, limbs with long but delicate barbed spines may have been better adapted to sifting out prey from the sediment or creating a net-like structure to capture its victims. In contrast, limbs with short, stout spines had a dexterity that allowed them to target, grab and manipulate prey.

Even if they weren’t living up to their reputation as trilobite crunchers, there is no doubt that Anomalocaris and kin ruled the oceans over half a billion years ago.