Tulips in the rocks.
Artist’s conception of what the living creatures would have looked like.
The Burgess Shale fossil beds in the Canadian Rockies are famous for showing us some of the creepiest evolutionary dead-ends to ever grace the planet. They conjure up underwater scenes of many-legged spiky creatures scuttling beneath gigantic spider shrimp, but a recent find in the Burgess Shale suggests a more pastoral landscape: fields of waving tulip-shaped creatures, each about 8 inches high.
These newly discovered filter feeders, named Siphusauctum gregarium by their discoverers, have been found in clumps of over 65, and appear to have fed by sucking water through their bodies and extracting food particles.
About 540 million years ago, things were looking pretty rosy for complex life on Earth. Conditions were favorable, and the diversity of multicellular organisms took off during the so-called Cambrian Explosion. Trilobites frolicked. Brachiopods abounded. And then, things went south.
Between 490 million and 520 million years ago, a swift extinction event wiped out many of the Cambrian lifeforms. Geologists Benjamin Gill and Graham Shields-Zhou thinks they have found the trigger right in the midst of that era. According to their study in this week’s Nature, the ocean’s oxygen level plunged and the sulfur levels rose sharply 499 million years ago, killing off species that could not quickly adapt. That included some, but not all, of the trilobites that ruled the seas of the time.
Gill’s team decided to look at a specific subset of Cambrian extinctions that began 499 million years ago and lasted for 2 million to 4 million years. Other researchers had proposed that low oxygen levels — a condition known as anoxia — could be involved. But no one had marshaled enough evidence to prove that. [Science News]
The key to showing it in this case is in the chemical compositions of the samples the team collected, which hold clues to the ocean conditions of the time.
Gill and his colleagues took samples of 500-million-year-old rock from six locations around the world and measured the amounts of various isotopes of carbon and sulphur. Both were significantly different from the norm, suggesting that enormous amounts of carbon and sulphur were being buried. In modern oceans, this only occurs in low-oxygen waters like the Black Sea. [New Scientist]
The next question is, what drove down the oxygen levels so quickly? To that, Gill doesn’t have an answer. But such cyclical dramatic changes driving extinction is the rule, not the exception—there were several events during the latter Cambrian when many species were wiped out, and anoxia could have been at play in some of those, too.
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Image: Wikimedia Commons
A huge spike in the Earth’s atmospheric oxygen about 800 million years ago, the story goes, paved the way for the Cambrian explosion a couple hundred million years later, and with it the rise of complex life. But a new study out in Nature says that picture is incomplete. Researchers found evidence of substantial oxygen 1.2 billion years ago, meaning that the conditions needed for complex life appeared much earlier than scientists knew, and that perhaps something else was required to set off the explosion of biodiversity.
The geologists led by John Parnell hunted in the Scottish Highlands for clues in ancient rocks, where evidence of ancient bacteria could reveal how much oxygen was around 1.2 billion years ago.
Before there was a useful amount of free oxygen around, these bacteria used to get energy by converting sulfate, a molecule with one sulfur atom and four oxygens, to sulfide, a sulfur atom that is missing two electrons. Geologists can get a glimpse of how efficient the bacteria were by looking at two different sulfur isotopes, versions of the same element that have different atomic masses. Converting sulfate to sulfide leaves the rock with a lot more of the isotope sulfur-32 than would be there without the bacteria’s help. [Wired.com]
The retreat of the ice covering “Snowball Earth” 700 million years ago might have been the key to the Cambrian explosion that seeded our planet with diverse forms of life. But the trigger may not have been the changes to the climate, but rather the release of phosphorus into the ocean.
During this time period, called the Cryogenian or Snowball Earth stage, the entire planet was covered in snow and ice, and the oceans may even have been frozen. Many researchers believe that the ice receded twice during this freezing period, first around 700 million years ago and then again around 635 million years ago. In a paper published in Nature this week, a team of researchers propose that these receding sheets released phosphorus into the oceans.
In the scheme offered by [Noah] Planavsky and his colleagues, the snowball ice sheets would, as their modern counterparts do, grind up continental rock that would release phosphorus when the glaciers retreated. That phosphorus would wash into the ocean, where it would fertilize algal blooms that could drive a surge in the production of organic matter and oxygen. And the added organic matter that settled into the mud on the ocean bottom would leave additional oxygen behind, eventually boosting atmospheric and oceanic oxygen. [ScienceNOW]