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(Image: University of Cambridge)
Is it a tree? Is it a fern? No, it's a rangeomorph, one of the first complex organisms to evolve on Earth. A new analysis of their fossils suggests that rangeomorphs' strange bodies evolved to absorb as much food as possible from the surrounding water.
Rangeomorphs ruled the oceans for around 40 million years, beginning 575 million years ago, in a period called the Ediacaran. Before them, life was microscopic. They grew on the sea bed, far too deep to harvest sunlight for photosynthesis. Up to 2 metres long, they had no organs, mouths or means of moving, so they had to passively absorb nutrients from the surrounding water.
"Geometrically, they were perfectly organised for doing that, creating the greatest possible surface area for absorption in whatever space they occupied," says Jennifer Hoyal Cuthill of the University of Cambridge. WithSimon Conway Morris, she studied how the anatomy of 11 types of rangeomorph evolved, using fossils to create computer replicas of each one.
Hoyal Cuthill found three main types. Some were tall and slender, like fir trees, projecting fronds at regular intervals from a central stem. Others had longer fronds that stuck out more to the side, like many deciduous trees. The last group were sponge-like, sprawling over the sea floor.
Each rangeomorph body plan was a fractal, so it looked the same at different scales. That maximised their outer surface area, boosting food absorption. One of the sponge-like rangeomorphs had a surface area of 58 square metres, almost the same as the interior of a human lung.
Rangeomorphs vanish from the fossil record around 542 million years ago during the Cambrian explosion, a sudden profusion of new life forms that could move and hunt, and had exoskeletons. The rangeomorphs were outmatched. "They were both being deprived of food and probably becoming food themselves," says Hoyal Cuthill.
Journal reference: PNAS, DOI: 10.1073/pnas.1408542111
