Evolution: earliest pre-Cambrian animals? (Evolution)

by David Turell , Wednesday, July 28, 2021, 19:34 (1212 days ago) @ David Turell

Were sponges here 890 million years ago? Very possibly:

https://www.newscientist.com/article/2285295-sponge-fossils-suggest-animals-already-exi...

"The origin of animals may have happened 350 million years earlier than thought. Fossils that seem to be sponges, one of the first animals to evolve, have been found in rocks from 890 million years ago.

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"Turner studied rocks from north-west Canada that contained the preserved remains of reefs from 890 million years ago, during the Tonian period. These reefs weren’t made by corals, like modern reefs, as these didn’t exist yet. Instead, they were made by photosynthetic bacteria living in shallow seas. The reefs, known as stromatolites, were many kilometres across and rose to heights of hundreds of metres above the seafloor. “These are spectacular reefs,” says Turner.

"Within the rocks, Turner found the preserved remains of a network of fibres, which branched and joined up in a complex mesh. These are the remains of sponges, she argues, but “not a normal fossil”.

"The bodies of modern sponges contain a mesh made of a protein called spongin, which forms a soft skeleton. Turner’s work suggests that when ancient sponges died, their soft tissues became mineralised, but the tough spongin didn’t. Eventually, though, it decayed, leaving hollow tubes within the rock that later filled with calcite crystals. These networks of calcite (pictured above) are what Turner then found – and the way the network branched looked just like spongin (pictured below).

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"If sponges existed 890 million years ago, then the origin of animals must have occurred much earlier than previous fossils have suggested. “Molecular clock” studies that use modern DNA to estimate when key points of evolution occurred have indicated that animals emerged long before the earliest fossils. However, this approach is often thought to be less reliable when there aren’t any fossils available to calibrate the molecular clock. Turner’s finding “brings the fossil record back into line with the molecular clock estimates”, says Penny.

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"Finally, there is the question of which animal groups were the first to emerge. Palaeontologists have generally assumed that sponges were first, but in the past decade some genetic studies have suggested that comb jellies – which superficially look like jellyfish – actually preceded them. The debate is ongoing: Penny would only say that finding early sponges doesn’t mean there weren’t also comb jellies very early, because such soft-bodied animals are rarely preserved.

Comment: Very simple early animals had to have a start in our past. Doesn't fill teh Cambrian gap.