The first skeletons evolved multiple times independently because of unusually chalky seas, later becoming essential for survival even when chalk became scarce.
Calcium-based skeletons appeared suddenly in the fossil record around 550 million years ago, fundamentally changing the global carbon cycle and introducing a wealth of new predatory strategies to the sea.
Building a skeleton requires organisms to combine minerals with organic matter in a process known as biomineralisation, and the earliest skeletons were probably quite simple, formed from readily available chemicals such as aragonite, magnesium and calcium. A new study suggests that the rapid emergence of many skeletal animals during the Late Ediacaran (560 – 540 mya) was due to changes in ocean chemistry that provided the raw materials for biomineralisation. A shift from magnesium-dominated ocean chemistry to chalky water full of calcium carbonate (a much better material for making skeletons) meant that the first skeletons formed easily. Once these materials were abundant, skeletons evolved more than 25 times independently, across distantly related marine organisms.
They note that the four oldest skeleton-bearing organisms – disc-shaped Suvorovella, funnel-shaped Cloudina, tube-shaped Sinotubulites, and vase-shaped Protolagena – all had soft-bodied counterparts that were nearly identical to them, with the exception of their mineral skeleton. But while soft-bodied fossils were found in silicate rocks, their skeletal cousins were found exclusively in calcium-rich rocks. The authors suggest that ocean waters saturated with carbonates such as calcium and magnesium paved the way for the earliest skeletons to evolve.
The appearance of hard skeletons allowed organisms to evolve new predatory appendages, and in turn prey needed hard shells to protect them. This represented a step-change in evolution, opening up a huge array of new hunting and hiding strategies. By 540 million years ago, the high-carbonate conditions had subsided, but organisms continued to produce a wide variety of carbonate skeletons, which were now essential for both predators and prey.
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- Wood R, Ivantsov AYu, Zhuravlev AYu. 2017 First macrobiota biomineralization was environmentally triggered. Proc. R. Soc. B 20170059.