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Cambrian apex predator probably preferred soft-bodied prey
The largest predator in the oceans half a billion years ago was likely snacking on soft prey.
New research has ruled out the charismatic ancient marine invertebrate Anomalocaris as the predator of trilobites, suggesting they were being crunched by other large predators instead.
An ancient marine predator preferred a diet of soft-bodied prey.
The 500-million-year-old invertebrate Anomalocaris was one of the biggest predators of its time. But despite its fearsome reputation in the Cambrian seas, new research is showing that it probably had to forego hard-shelled prey for softer dinners.
By intimately studying the fossils of these early animals, a team of international researchers were able to build a three-dimensional model of its spiked arms which it used to capture prey and determined that Anomalocaris had evolved to feed on fast, soft-bodied prey. This is in contrast to previous theories which suggested the animal swam along the seafloor on the hunt for hard-shelled trilobites which it then crushed up with its jaws.
Dr Imran Rahman, a Principal Researcher of palaeontology at the Museum helped to test the 3D models of the extinct animal.
‘This paints a picture of an animal that was probably not feeding on things like trilobites scurrying along the bottom,’ explains Imran, ‘but perhaps fed on soft bodied animals that swam around the water column.’
‘This is a more nuanced picture of the palaeobiology of this extinct iconic predator.’
The invertebrate would have used it keen sense of vision to track and catch fast-moving prey ©Allison Daley
It seems more likely that the large predators were snapping up smaller, shrimp-like animals which were actively swimming just above the surface of the seafloor, while other large predators were cruising along the bottom to hunt down and crush the trilobites. The results have been published in the journal Proceedings of the Royal Society B.
A 500-million-year-old murder mystery
Living during the Cambrian Period, Anomalocaris was one of the biggest animals and top predators at the time. At this point in the history of life on Earth, there was a sudden, rapid diversification of animal life. Over a period of around 20 million years all the major animal body plans evolved in a spurt of innovation during an event known as the Cambrian explosion.
But this was also a period of great environmental and ecological change too. Out of seemingly nowhere, hard-shelled trilobites suddenly appeared and proliferated, spreading right around the world’s oceans within a remarkably short amount of time. Trilobites would go on to become one of the most successful groups of animals this planet has seen, but what caused this sudden shift is still not fully understood. It most likely had something to do with changes in the chemistry of the seas making the building of hard shells easier for the arthropods.
Regardless of how it occurred, these changes to the oceans helped to precipitate ever more complex food chains. One example of this is the crunched up remains of trilobite shells that can be found in fossilised poops known as coprolites, while other fossil trilobites show evidence of having been attacked. Clearly there was some large, active predator swimming in the Cambrian seas picking off and crushing the ancient creatures.
Its two forward-facing arms were lined with spikes that would have made it difficult for food to escape ©Allison Daley
This is where Anomalocaris comes in. While at around 60 centimetres long Anomalocaris was small by today’s standard, when it was swimming the oceans it was relatively massive. The active predator would have cruised the waters looking for prey, before snatching anything too slow with a pair of long, spiked arm-like structures and feeding the food into a crushing mouth.
Because Anomalocaris is one of the biggest and best-known predators of the Cambrian, it has long been assumed that the crushed shells of the trilobites must have been its leftovers. But for postdoctoral researcher Russell Bicknell from the American Museum of Natural History, this theory seemed a bit fishy.
‘That didn’t sit right with me,’ says Russell, ‘because trilobites have a very strong exoskeleton, which they essentially make out of rock, while this animal [Anomalocaris] would have mostly been soft and squishy.’
Dr Greg Edgecombe is an expert on ancient marine arthropods, particularly those that appeared during the Cambrian. ‘It has been a fairly standard assumption that as the largest animal and almost certainly a predator, Anomalocaris might have been doing this,’ says Greg. ‘So we wanted to assess the stress and strain on different parts of the paired appendage to determine whether or not that is consistent with being able to feed on hard organisms like trilobites.’
To test this, researchers have created a detailed 3D model of the creature using some of the best-preserved fossils of the invertebrate.
Cannibalistic tendencies
Building up a detailed picture of Anomalocaris has taken its time. When it died, the relatively soft-bodied animal would have easily fallen apart as it settled on the ocean floor. This meant that separate aspects of the animal, namely its body, mouth parts and paired arms, were often separated in the fossilisation process.
These three different parts were initially described by palaeontologists as three different animals, until in 1985 they were realised to belong to the same creature. But even then, figuring out what it would have looked like in life has been tricky as the fossils are entirely flattened. By comparing many fossils preserved in multiple different planes, the researchers could start to build a three dimensional model to then test.
By studying numerous fossils the team were able to create a 3D model of Anomalocaris' arm-like appendages ©Bicknell et al. 2023
‘We used three different approaches to explore the function of the appendage,’ says Imran. ‘One approach was to look at the range of motion to see how much this appendage could flex. The second was a method commonly used in engineering to look at the distribution of stress and strain across the appendages.’
‘And the third analysis was using a method called computational fluid dynamics to look at how the long-extinct animals might have moved through the water.’
They found that the arms were quite flexible, but that the little spines on their underside were quite fragile. This means it would have been unlikely to have been feeding on hard-shelled creatures that may have broken the spikes. They also found that the most efficient way for Anomalocaris to swim would have been with its arms outstretched, suggesting a more open water-living existence.
This still, however, leaves the mystery of what was chomping on the trilobites on the seafloor. Was it some huge, as yet unknown predator that lurked in the deep? Greg thinks it is more straight forward than that.
‘Other big trilobites,’ explains Greg. ‘There were some very big trilobites in these ecosystems, and they had robust crushing appendages that likely had the capacity to crush hard parts, so we’ve argued that it was probably a durophagus predator. And because we find broken pieces of the same trilobite species in the coprolites, it was probably a cannibal too.’
