The first signs of complex animal life begin in the Ediacaran Period, which started more than 600 million years ago. But it's difficult to understand how those organisms relate to the life we see around us today. Part of this issue is that those fossils are rare, as many rocks of that period appear to have been wiped off the Earth by a globe-spanning glaciation. But another problem is that the organisms we do see from this period aren't clearly related to anything that came after them.
With the arrival of the Cambrian Period about 550 million years ago, all of that changed. In fossil beds like the famed Burgess Shale, we can see organisms that clearly have features of the major groups of life that have persisted to this day. As more collections of fossils become available, we can even watch groups diversify as the Cambrian progressed. But there's still considerable debate over whether these changes represent a true, multi-million-year "explosion" and what environmental changes might have driven this diversification.
We may be on the verge of some big help in answering these questions, as scientists are announcing the discovery of a spectacular deposit of Cambrian fossils from South China. The fossils include dozens of species, half of which we've never seen before, and appear to represent a previously upsampled ecological zone. The preservation is such that soft-bodied creatures like jellyfish, and the softer body parts of creatures with shells, can easily be made out in the rocks. Best yet, the researchers who uncovered the samples suggest that rocks from the same formation are widespread in China.
Saved for a half-billion years
A number of elements had to come together to preserve these fossils in the Changyang area of China. The animals found in the deposits, collectively termed the Qingjiang biota, required sufficient oxygen in the ocean waters to flourish. But oxygen also feeds organisms that would decompose the bodies of these creatures once they died or dig through the sediments after the bodies were buried in them. Here, the researchers suggest that the organisms flourished in water on the relatively shallow continental shelf but were pulled by currents into deeper, oxygen-poor waters once they died. Those same currents also ensured a steady flow of sediment that quickly buried the bodies of soft creatures while they were still intact.
Once compressed into sedimentary rock, the material had to avoid being exposed to any geological processes, like volcanism, that would heat or distort the rocks and all they contained. The result of this combination of rare conditions is what's technically termed a Lagerstätte, a rich fossil bed in sedimentary rock. In this case, the Lagerstätte lies in Hubei Province along the Qingjiang River, visible as a series of alternating light and dark layers of rock, with the total assembly as thick as 50 meters in some places.
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Some of the fossils come from a site along the Qingjiang River.Dong King Fu
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Preserved jellyfish look much like those of today.Dongjing Fu et. al.
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Some of the deposit has been opened up by erosion, but other sections remain as large slabs of rock.Dong King Fu
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A sea anemone with its mouth visible.Dongjing Fu et. al.
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Prying apart sections of the rock slab can reveal fossils within.Dong King Fu
This formation drew the interest of Chinese researchers because the rocks appear to have been deposited at the same time as fossil beds in Chengjiang, about 518 million years ago, or slightly earlier than the Burgess Shale.
Their interest paid off in the collection of a staggering amount of fossils from the Cambrian, with more than 100 animal species represented. Even though these come from the same time as the Chengjiang fossil deposits, more than half of the species at Qingjiang had not been described previously, and only eight are shared between the two sites. To the authors, this suggests that Qingjiang represents a distinct ecosystem from a community in deeper waters.
Well-preserved
The preservation of these fossils is absolutely stunning. Soft-bodied creatures like sponges, jellyfish, and anemones have details like tentacles and mouths preserved. In many cases, internal organs can also be identified. "Unexpectedly, submillimeter- to millimeter-sized, delicate, larval or juvenile forms are abundant on some bedding surfaces of the Qingjiang assemblage," the authors write. Details of the algae that they shared their environment with can also be made out.
Even without formal descriptions of the new species, the fossils are answering questions. Ctenophores, also called comb jellies, are one of the earliest branches of the animal evolutionary tree. In their present form, they have tentacles, but there had been no early fossil with these appendages, leading to the suggestion that they evolved relatively recently. But there's a fully tentacled comb jelly in Qingjiang, so that debate appears to be over.
All of which suggests that there's a lot to come from the four seasons of field work the team has already put in at Qingjiang. But the revelations may only be starting. To begin with, the Qingjiang haven't been heated or experienced any weathering; that's not true of the Burgess Shale or Chengjiang formations. The preservation of soft fossil materials may be even better than at those sites.
The authors also call Cambrian-age deposits in the Qingjiang area of China "widespread," and initial samplings have suggested that similar fossils can be found at a number of other locations in the region. Depending on how distant they are, there's a chance that these other sites could preserve additional ecosystems. If that turns out to be the case, these rocks could give us a clearer picture of the diversity of ecosystems available to some of the Earth's first animals and show how the conditions in each shaped the species there.
Science, 2019. DOI: 10.1126/science.aau8800 (About DOIs).
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