Paleontologists unearthed a strange sight in Newfoundland in the early 2000s: an ancient fossil bed of frond-shaped organisms, some 571 million years old and up to two meters long.
Researchers had discovered these mysterious extinct creatures—called rangeomorphs—before, but they still do not know exactly what they are. “For 50 years the rangeomorphs have continued to confuse scientists as we try to understand how they lived and where they might have fitted in the ‘tree of life,’” explains Jack Matthews, a geology research fellow at Memorial University of Newfoundland and the University of Oxford. Some believe they may be primitive animals, but there is no consensus.
Nevertheless, scientists believe these bizarre-looking beings could help answer key questions about life on Earth, because they are “the oldest evidence we have for large, architecturally complex, multicellular life,” Matthews wrote in an e-mail to Scientific American. Now a recent study may support the hypothesis that Earth’s geochemistry allowed these organisms to grow into large, diverse shapes.
First identified in Namibia by a German geologist in 1930, rangeomorphs show up in vast numbers in the fossil record around the world, including Russia, Australia and the U.K. They lived during the Ediacaran period, which lasted from about 635 million to 541 million years ago—directly after a massive glacial period and right before the Cambrian period. Scientists know these creatures lived in communities, anchored to the ocean floor. They had complex structures: stemlike bodies that sprouted fractal-like branches and were soft like jellyfish, swayed by currents. Although scientists cannot decide how to classify rangeomorphs, most do not think they were plants because they existed at depths beyond sunlight’s reach, where no photosynthesis could occur. “That rules out…a seaweed or anything like that,” says Jennifer Hoyal Cuthill, a paleobiology research fellow at the University of Cambridge and Tokyo Institute of Technology. “A lot of people think they’re probably an early animal.”
Perhaps most significant of all, scientists think they grew to sizes previously unseen on Earth—up to two meters in length, although some only measured three centimeters or less in width. After rangeomorphs went extinct, however, the planet saw an explosion of diverse, large animal life during the Cambrian. “Rangeomorphs are part of the broader context of what was going on at this time in Earth’s history,” Hoyal Cuthill says. “It was a time of quite dramatic change.” Other scientists have found evidence Earth’s geochemistry was shifting at the time—the organisms’ appearance in the fossil record coincides with an increase in marine oxygen levels. Figuring out how they grew to such great sizes, she adds, could help provide context for understanding how big, diverse animals originated, what the conditions were like when they appeared and whether a connection exists between the two.
To better understand this possible relationship, Hoyal Cuthill and University of Cambridge paleontologist Simon Conway Morris analyzed several rangeomorph fossils. For their study, published earlier this month in Nature Ecology & Evolution, the pair performed a micro-CT scan on one well-preserved fossil of a species called Avalofractus abaculus, found in Spaniard’s Bay, Newfoundland, to examine its three-dimensional structure in fine detail. They also took photographic measurements of two other specimens for comparison.
The researchers examined various aspects of the rangeomorphs’ stems and branches and then used mathematical models to investigate the relationship between their surface areas and volumes. Their models, combined with the fossil observations, revealed key information about how the organisms grew—specifically, their size and shape appeared to be governed by the amount of nutrients available to them. According to Hoyal Cuthill, “Rangeomorph size growth did indeed follow a pattern we would expect if it was strongly dependent on uptake of oxygen or other nutrients.” This may explain why the organisms could grow to such large sizes during a period when Earth’s geochemistry was changing. “The same genetics can produce a number of different sizes and shapes, depending on what the environmental conditions are,” she explains. “This would allow rapid change, where you can have rangeomorphs exploiting increased nutrient levels to reach large size over the scale of a lifetime, rather than requiring mutations.”
But other experts are hesitant to draw a conclusion for all of these creatures. “This is an interesting finding that supports the growing consensus among researchers that rangeomorphs had the potential to grow differently in response to their environment,” Matthews says. However, “it is perhaps premature for this study to apply its finding to all the rangeomorphs, especially when the oldest and largest fossils have yet to be included in the analyses.” Researcher David Jacobs agrees. “It’s a useful study in a lot of ways, but I’m unsure whether you can strongly come to the conclusion that this is all dependent on nutrients,” says Jacobs, a professor of ecology and evolutionary biology at the University of California, Los Angeles. “Further development of such models would be useful.”
If Hoyal Cuthill and Morris’s findings turn out to be correct, though, they could help fill in the picture of a crucial period in the evolution of life on our planet. It could provide an answer, Hoyal Cuthill says, for “what links this amazing appearance of larger organisms in the fossil record with what’s happening on Earth.”