Grand Canyon was a ‘Goldilocks zone’ for the evolution of early animals

A treasure trove of exceptionally preserved early animals from more than half a billion years ago has been discovered in the Grand Canyon, one of the natural world’s most iconic sites.

The rich fossil discovery – the first such find in the Grand Canyon – includes tiny rock-scraping molluscs, filter-feeding crustaceans, spiky-toothed worms, and even fragments of the food they likely ate.

By dissolving the rocks these animals were fossilised in and examining them under high-powered microscopes, researchers led by the University of Cambridge were able to get a highly detailed picture of a unique period in the evolution of life on Earth.

The fossilised animals date from between 507 and 502 million years ago, during a period of rapid evolutionary development known as the Cambrian explosion, when most major animal groups first appear in the fossil record.

In some areas during this period, nutrient-rich waters powered an evolutionary arms race, with animals evolving a wide variety of exotic adaptations for food, movement or reproduction.

Most animal fossils from the Cambrian are of hard-shelled creatures, but in a handful of locations around the world, such as Canada’s Burgess Shale formation and China’s Maotianshan Shales, conditions are such that softer body parts could be preserved before they decayed.

So far, however, fossils of non-skeletonised Cambrian animals had been known mostly from oxygen and resource-poor environments, unlikely to kickstart the most complex innovations that shaped early animal evolution.

Now, the Grand Canyon has revealed the first soft-bodied, or non-mineralised, Cambrian fossils from an evolutionary ‘Goldilocks zone’ that would have provided rich resources for the evolution of early animals to accelerate. The results are reported in the journal Science Advances.

“These rare fossils give us a fuller picture of what life was like during the Cambrian period,” said first author Giovanni Mussini, a PhD student in Cambridge’s Department of Earth Sciences. “By combining these fossils with traces of their burrowing, walking, and feeding – which are found all over the Grand Canyon – we’re able to piece together at an entire ancient ecosystem.”

Mussini and colleagues from the US located the fossils during a 2023 expedition along the Colorado River, which began carving the Grand Canyon in what is now Arizona between five and six million years ago.

“Surprisingly, we haven’t had much of a Cambrian fossil record of this kind from the Grand Canyon before – there have been finds of things like trilobites and biomineralised fragments, but not much in the way of soft-bodied creatures,” said Mussini. “But the geology of the Grand Canyon, which contains lots of fine-grained and easily split mud rocks, suggested to us that it might be just the sort of place where we might be able to find some of these fossils.”

The researchers collected several samples of rock and returned them to Cambridge. The fist-sized rocks were first dissolved in a solution of hydrofluoric acid, and the sediment was passed through multiple sieves, releasing thousands of tiny fossils within. None of the animals were preserved in their entirety, but many recognisable structures helped the researchers identify which groups the animals belonged to.

Further examination of the fossils revealed some of the most complex ways animals were evolving during the Cambrian to capture and eat their food. “These were cutting-edge ‘technologies’ for their time, integrating multiple anatomical parts into high-powered feeding systems,” said Mussini.

Many of these fossils are of crustaceans, likely belonging to the group that includes brine shrimp, recognisable by their molar teeth. These tiny creatures had hair-like extensions on triangular grooves around their mouths, and used their hairy limbs to sweep up passing food particles like a conveyer belt. Tiny grooves on their teeth could then grind up their food. The detail on the fossils is such that several plankton-like particles can be seen near the crustaceans’ mouths.

Other modern-looking animals from the Cambrian of the Grand Canyon include slug-like molluscs. These animals already had belts or chains of teeth not dissimilar to modern garden snails, which likely helped them scrape algae or bacteria from rocks.

The most unusual creature identified by the researchers is a new species of priapulids, also known as penis or cactus worms, which were widespread during the Cambrian but are nearly extinct today. The Grand Canyon priapulid had hundreds of complex branching teeth, which helped it sweep food particles into its extensible mouth. Due to the size of the fossil and its exotic rows of teeth, the researchers named this new animal Kraytdraco spectatus, after the krayt dragon, a fictional creature from the Star Wars universe.

“We can see from these fossils that Cambrian animals had wide variety of feeding styles used to process their food, some which have modern counterparts, and some that are more exotic,” said Mussini.

During the Cambrian, the Grand Canyon was much closer to the equator than it is today, and conditions were perfect for supporting a wide range of life. The depth of the oxygen-rich water, neither too deep or too shallow, allowed a balance between maximising nutrients or oxygen and reducing wave damage and exposure to UV radiation from the Sun.

This optimum environment made it a great place for evolutionary experimentation. Since food was abundant, animals could afford to take more evolutionary risks to stay ahead of the competition, accelerating the overall pace of evolution and driving the assembly of ecological innovations that still shape the modern biosphere.

“Animals needed to keep ahead of the competition through complex, costly innovations, but the environment allowed them to do that,” said Mussini. “In a more resource-starved environment, animals can’t afford to make that sort of physiological investment. It’s got certain parallels with economics: invest and take risks in times of abundance; save and be conservative in times of scarcity. There’s a lot we can learn from tiny animals burrowing in the sea floor 500 million years ago.”

The research was supported in part by the US National Science Foundation and the UK Natural and Environment Research Council (NERC), part of UK Research and Innovation (UKRI). Giovanni Mussini is a Bye-Fellow of Magdalene College, Cambridge.