Fronds with benefits: what led to the rise of giant rangeomorphs

550 million years ago, the rangeomorph was a giant in a world of micro-organisms.

An artist’s impression of rangeomorphs.
An artist’s impression of rangeomorphs.
Jennifer Hoyal Cuthill

They lived in the ocean and looked like plants but are believed to have been animals. In a world of micro-organisms they grew to become giants. We call them rangeomorphs, and they dominated their environment for 30 million years, up until about 541 million years ago, when the rest of evolution overtook them during the proliferation of complex plant and animal forms known as the Cambrian Explosion.

Understanding what gave rangeomorphs a jump on the rest of creation, super-sizing while most other life remained microscopic, is something scientists believe may help to explain the subsequent evolution of large animals such as whales and dinosaurs. Rangeomorphs varied greatly in size: while some were only a few centimetres in height, the largest grew to 2 metres.

The answer has to do with an ability known as ‘ecophenotypic plasticity’, according to Jennifer Hoyal Cuthill and Simon Conway Morris, of Cambridge University's Department of Earth Sciences, whose research on the rangeomorphs has been published in Nature Ecology and Evolution.

Ecophenotypic plasticity refers to changes in an organism’s form (or phenotype) wrought by environmental influences. In the case of the rangeomorphs, the researchers suggest, the pivotal environmental influence was the presence of nutrients in the water, and the change that dictated for the rangeomorphs’ ecological success was the ability to shape-shift.

Rangeomorphs looked a lot like modern ferns, with branching fronds composed of four fractal levels. This essential structure enabled them, for example, to grow into a long, tapered shape to take advantage of seawater above containing elevated levels of oxygen.

In the period preceding the Cambrian, known as the Ediacaran period, between 635 and 541 million years ago, there seem to have been major changes in the Earth’s oceans, Hoyal Cuthill says. This might have triggered the growth that led to life forms getting much bigger. “It’s probably too early to conclude exactly which geochemical changes in the Ediacaran oceans were responsible for the shift to large body sizes,” she says, “but there are strong contenders, especially increased oxygen, which animals need for respiration.”

Changing conditions would privilege the rangeomorphs for only a short time, geologically speaking. With the Cambrian Explosion came the appearance of most of the major animal groups we know today, and the hybrid advantages of the rangeomorphs’ niche between plants and animals would disappear.

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Cosmos reporter is a contributor to Cosmos Magazine
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