The sea bed was thought to lack enough oxygen to sustain them. From ScienceDaily:
The rocks came from an area in the remote Mackenzie Mountains of the Northwest Territories in Canada which Pratt found 35 years ago.
Pratt then digitally enhanced images of the rock surfaces so he could examine them more closely. Only then did the hidden ‘superhighway’ of burrows made by several different sizes and types of prehistoric worm emerge in the rock.
Some were barely a millimetre in size and others as large as a finger. The smaller ones were probably made by simple polychaetes — or bristle worms — but one of the large forms was a predator that attacked unsuspecting arthropods and surface-dwelling worms.
Pratt said he was “surprised” by the unexpected discovery.
“For the first time, we saw evidence of large populations of worms living in the sediment — which was thought to be barren,” he said. “There were cryptic worm tunnels — burrows — in the mud on the continental shelf 500 million years ago, and more animals reworking, or bioturbating, the sea bed than anyone ever thought.”
Now here’s the kicker:
It has always been assumed that the creatures in the Burgess Shale — known for the richness of its fossils — had been preserved so immaculately because the lack of oxygen at the bottom of the sea stopped decay, and because no animals lived in the mud to eat the carcasses.
Pratt’s discovery, with co-author Julien Kimmig, now of the University of Kansas, shows there was enough oxygen to sustain various kinds of worms in the sea bed. Paper. (open access) (paywall) – Brian R. Pratt, Julien Kimmig. Extensive bioturbation in a middle Cambrian Burgess Shale–type fossil Lagerstätte in northwestern Canada. Geology, 2019; 47 (3): 231 DOI: 10.1130/G45551.1 More.
Well, either something else was stopping the decay or oxygen is not as necessary as supposed. See, for example, “Life form’s environment is so extreme it has never been cultivated in a laboratory” and Why did bigger, more complex organisms get started earlier in deep oceans?
See also: Complex Worm Find From Cambrian (541-485 Mya) “Helps Rewrite” Our Understanding Of Annelid Head Evolution
Symbiosis found in Cambrian fossil worms
and
Cambrian explosion ended surprisingly quickly
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Also:
Researchers: Earth’s oxygen rose and fell several times before the Great Oxidation Event 2.2 bya
Photosynthesis pushed back even further. Time to revisit the “Boring Billion” claim
Researchers: Extreme fluctuations in oxygen levels, not gradual rise, sparked Cambrian explosion Explanations of the dramatic Cambrian explosion of life forms (540 million years ago) are a cottage industry, with arguments about oxygen a staple of the discussion. At times, it feels like trying to understand World War II without allowing for the possibility that any intention underlay any of the events. Maybe that’s why the issues can’t be resolved.
Maverick theory: Cambrian animals remade the environment by generating oxygen
Did a low oxygen level delay complex life on Earth?
There was only a small oxygen jump
Animals didn’t “arise” from oxygenation, they created it, researchers say
Theory on how animals evolved challenged: Some need almost no oxygen
New study: Oxygenic photosynthesis goes back three billion years
Enough O2 long before animals?
Life exploded after slow O2 rise?