Evolutionary escalation in an exceptionally preserved Cambrian biota from the Grand Canyon (Arizona, USA)
By: Giovanni Mussini, James W. Hagadorn, Anne E. Miller, Karl E. Karlstrom, Rhydian Evans, Carol M. Dehler, Salvador Bastien, and Nicholas J. Butterfield
Summarized by: Kayla Adeyemi is a senior biochemistry major at Binghamton University. She plans to attend medical school in pursuit of becoming a physician. In her free time, Kayla likes to ballroom dance and read fantasy novels.
What data were used? 29 shales, or hardened mud, around 15 cm thick, were collected in the Grand Canyon in the Bright Angel Rock Formation in Arizona, USA. The shales contained 1,539 non-biomineralized, or soft-bodied, fossil organisms, from the Cambrian (~500 million years ago). Hard-bodied, or biomineralized, organisms are more likely to be fossilized than soft-bodied organisms since they have hard parts like bones that are more resistant to decay. The fossils found are referred to as small carbonaceous fossils (SCFs), meaning they are under a millimeter (for scale, that’s smaller than a sesame seed) and preserve small details of soft-bodied organisms. Since the fossils were preserved in shale, a rock of small grains, scientists interpreted the environment to be low-energy and not frequently disturbed by strong ocean currents. Trace fossils (fossils that show behavior, rather than the body) previously found in the area were used to estimate the environment: they show evidence of seafloor animal activity through traces left by feeding, burrowing, and resting.
What was the hypothesis being tested? Researchers tested the evolutionary escalation hypothesis. The escalation hypothesis proposes that animal competition and predators influenced evolution (i.e. the biological change in populations over time). Over time, less competitive organisms would be confined to less resourceful environments, while more competitive organisms would inhabit resource-rich environments. The researchers here propose that the escalation hypothesis happens in stable and resource-rich environments. Most Cambrian non-biomineralized fossils are found in environments that would have had limited oxygen and information about their ecosystem. However, this study found non-biomineralized fossils from an environment with oxygen availability and an abundance of animal activity and nutrients, as evidenced by diverse trace fossils. Therefore, they could infer a resource-rich environment, and the escalation hypothesis could be tested with their data. If escalation occurred, researchers would expect to find non-biomineralized Cambrian organisms with adaptations for catching food and similarity in their body shapes that would indicate similar pressures (like how sharks and dolphins have similar bodies due to similar environments, not evolution).
Methods: The researchers collected 29 shales around 15 cm thick, along the Bright Angel Rock Formation in the Grand Canyon. The formation contains 100 m thick rock from shallow marine environments during the Cambrian. The scientists unearthed the fossils carefully by dissolving the shale in hydrofluoric acid and filtering the residue through a tiny strainer. The residue contained the fossils, which were viewed and separated under a microscope.
Results: The researchers found 1,539 well-preserved small carbonaceous fossils, SCFs. They also found bacteria microfossils which were interpreted as photosynthetic; that means this environment must have received at least some sunlight. All of the SCFS were disarticulated organisms, but had some intact body parts. Most of the fossils were of priapulids (the group containing worms), crustaceans (the group containing lobsters, clams, and barnacles), and molluscs (the group containing squid, clams, and snails). The organism parts ranged in size from four µm to three mm long, or the height of two U.S. pennies. They were identified by their feeding structures and similarities to modern-day critters. Priapulids are smooth and round predatory worms. The researchers discovered a new species of priapulid with hundreds of branched teeth in rows. Its teeth were likely too delicate for catching larger prey, but thicker towards the outside, with fine bristles in between for scraping food off sediment. Its feeding structure was notably more complex than other known Cambrian priapulids that lacked branching and bristles. Crustaceans were identified by their lined and scaly crescent-shaped teeth. They showed adaptations for breaking down larger food with long incisors and comb-like teeth structures to filter out particles. Scientists found two types of crustacean teeth: one with uniform scales and short incisors, and another with asymmetric scales and longer incisors. The first type could be useful for eating plankton, and the second type could be useful for predation. This shows how different organisms in the same environment can adapt to utilize different resources and reduce competition. The mollusks the researchers found have distinct boot-shaped radula or “teeth” that vary in shape, as seen in Figure One.
The teeth have either a shovel-shaped form, pictured in Figure One, Image A, B, and L, or a thinner cone-shaped form, as seen in Images C and F. These particular mollusks possessed both sets of specialized teeth to shovel and scrape their food from sediment, as evidenced by the wear shown in Image E in Figure One. This is far more advanced than the teeth of organisms that lived in more resource-limited environments (like Wiwaxia and Odontogriphus). Their resource-limited counterparts had weaker, fewer, and generalized teeth. These fossilized organisms’ dissimilarity to resource-limited Cambrian organisms and similarity with current species led the researchers to believe that escalation occurred in this Cambrian environment. Their environment was oxygenated and nutrient-filled, allowing them to invest their energy in more specialized feeding structures.
Why is this study important? It is rare to find fossils from the Cambrian that are soft-bodied, well-preserved, and from environments better suited for hosting diverse life. The fossils found had unexpectedly well-adapted feeding structures for selective feeding and were comparable to living species. The fossilized organisms existed in a pivotal time during the Cambrian Explosion, where there was a surge in marine animal diversity within the fossil record. This study fills a gap in our fossil record for soft-bodied animals and proposes escalation as a driver of evolution.
Broader Implications beyond this study: The evolution of predation is an active area of study within paleobiology. The arms race theory is a type of escalation theory that claims prey adapt to avoid predators. This can be used to explain the abundance of hard-bodied organisms that evolve for the first time during the Cambrian. However, organism adaptation in this study appeared to be mainly driven by competition and resource acquisition, despite them being soft-bodied and more vulnerable to predators. It suggests that predation was not a primary pressure for organisms in this particular ecosystem. Furthermore, predators may have evolved from these organisms. We see evidence of this from the crustacean fossils researchers found with larger incisors associated with predation. This study can give insight into ways escalation occurred and how modern animals evolved from these Cambrian species.
Citation: Mussini G., Hagadorn J.W., Miller A.E., Evans R., Dehler C.M., Bastien S., and Butterfield N.J. (2025). Evolutionary escalation in an exceptionally preserved Cambrian biota from the Grand Canyon (Arizona, USA). Science Advances, 11(30). https://doi.org/10.1126/sciadv.adv6383