The Ediacaran period, from 635-541 million years ago, saw a revolution in life. For the first time organisms large enough to be seen without a microscope appeared, some of which grew to 2 meters (7 feet) in size. A study of fossils from Newfoundland has revealed this growth wasn't about competition for food, but to give the next generation the best opportunities.
Rangeomorphs were some of the earliest macroscopic organisms, among the largest living things of their day. They looked like ferns but are considered possible ancestral animals. Their bodies grew into a variety of shapes, often with a long stem attaching themselves to the sea floor, contributing to their unprecedented height.
Noting trees grow so tall to avoid being overshadowed by those around them, Dr Emily Mitchell of the University of Cambridge wondered if the same forces were at play more than half a billion years ago. “Did life on Earth get big as a result of competition?” she asked in a statement.
Mitchell examined the dispersal of rangeomorph fossils at Mistaken Point, Newfoundland, the best-known rangeomorph deposit, and one that appears to have preserved entire communities in remarkable detail. Since rangeomorphs are thought to have been filter-feeders, or perhaps to have taken in nutrients from the surrounding water by osmosis, they would not have been competing for sunlight like photosynthesizing organisms. Nevertheless, advocates of the competition theory proposed having stalks of different lengths allowed individuals to access nutrient-rich waters those around them could not.
However, in Nature Ecology & Evolution, Mitchell reports she found no correlation between the height of rangeomorph fossils and the intensity of food competition in an area. In particular, colonies of rangeomorphs did not have individuals using stems of different lengths to place themselves at heights where they faced less competition from those nearby.
“The oceans at the time were very rich in nutrients, so there wasn’t much competition for resources, and predators did not yet exist,” Mitchell said. “So there must have been another reason why life forms got so big during this period.”
The reason, it turns out, was reproduction. Rangeomorphs expelled propagules, which would settle on the seabed where they fell, eventually growing into a new specimen. The higher an individual sat, the further its offspring could be pushed, and therefore the more territory it could occupy. Colony analysis showed that those with short stems clustered more closely with their offspring, while longer-stemmed species could spread more widely.
As the Cambrian era replaced the Ediacaran, rangeomorphs disappeared, possibly as a result of predation by newly mobile animals. However, if you think this makes rangeomorphs a failure, consider they were among Earth's dominant species for at least 25 million years previously – will we do as well?
