Two decades ago, geologist Elizabeth Turner set out to explore ancient coral reefs trapped in the Mackenzie Mountains in northwest Canada.
the main points:
- Branching structures in 890 million-year-old rocks could be ancient sponge fossils
- If confirmed, it could be the oldest known animal fossil
- Some experts say the findings hoist more questions than answers
Her heart was sure to understand how photosynthetic microbes built immense corals millions of years ago.
Instead, the then doctoral student walked away with a pile of rocks, a handful of which had some unusual features.
But it turns out that Dr. Turner may have found the oldest known animal fossils, according to a research paper published in Nature.
The tiny structures embedded in the 890-million-year-old rock look remarkably similar to those of sea sponges, indicating that these simple creatures were thriving in the oceans much earlier than previously thought.
Dr Turner said the structures were too complicated to be due to algae or bacteria.
“The process of exclusion suggests it can’t be these other things,” said Dr. Turner, now at Laurentian University in Ontario.
But some scientists interrogate whether the fossils were sponges or may have been created by a biological process.
Simple animals are firm to find
Sponges are simple animals that have been around lengthy prior the emergence of dinosaurs, which makes them useful for studying how life on Earth evolved from single-celled organisms to the animals we know today, including humans.
To date, the oldest fossilized traces of sponges in ancient rocks date back about 540 million years, placing them at the start of the Cambrian period – a period when evolution took off to a lofty level and produced an extraordinary diversity of animals.
But the genes of modern sponges suggest they could have appeared 400 million years earlier than their fossils suggest.
This indicates that they may have existed prior Dickinonia, a flat, oval-shaped creature that left behind 558 million-year-old fossils, which currently tops the list as the oldest known animal in the geological record.
Finding fossilized sponges that date so far is not an simple task, because their soft bodies do not uphold rocks as well as animals with solid skeletons, making them firm to distinguish from other types of fossils.
It also doesn’t aid that ancient sponges are much simpler than their modern counterparts, said Jochen Brooks, a geologist who specializes in paleo-ecosystems at the Australian National University.
“If you go back further and further, a lot of the complexity disappears and it becomes harder and harder to recognize what you’re looking for,” said Professor Brooks, who was not involved in the study.
“The simpler it is, the more likely it is that something non-biological created it by accident.”
But if the fossils found by Dr. Turner are indeed ancient sponges, that means they were thriving in the oceans 90 million years prior the oxygen abundance on Earth, suggesting that animal life may have begun to evolve prior this event.
Dr Turner said the ancient corals where these sponges lived could be “oxygen factories,” thanks to the cyanobacteria that lived alongside them.
“There may have been oases with higher oxygen levels,” Dr. Turner said.
The results also indicate that some animal life survived the massive ice age that occurred between 720 and 635 million years ago.
“In all likelihood, these glaciers didn’t wipe out life and you didn’t have to start all over again following that,” Dr. Turner said.
Getting to know the ancient sponge
Unbeknownst to her, Dr. Turner discovered the tiny 890-million-year-old fossils when she was looking at hundreds of rock samples she had taken from her field site in the mountains.
While she was more interested in researching traces of reef-building microbes in rocks, a handful of specimens popped up.
Dr. Turner knew she was on to something, but decided to put aside the unusual rock slides for a deeper look at another time.
Twenty years later, Dr. Turner finally got a chance to put samples under a microscope.
The slides contained a complicated pattern of tubular structures that branched out in three dimensions.
“This is a very complicated structure,” said Dr. Turner.
The branching pattern of the fossils also looked remarkably similar to the skeletons of horny sponges living today, and those seen in the fossils of younger sponges.
Professor Brooks says that while more work needs to be done to confirm the fossils are sponges, their age matches estimates of the time they may have first appeared.
Professor Brooks said: “It makes sense. This is the considerate of timing where we think that could have happened.”
Is it a sponge, a microbe, or none of the overhead?
But Professor Brooks is not entirely convinced that the sponge-like structures are a smoke gun.
In fact, “they might not have anything to do with biology at all,” he said, adding that carbonate minerals can also form in the branching structures.
Jim Gehling, a paleontologist at the South Australian Museum, added that the fossils do not show the classic physical features of sponges, such as an opening to filter seawater or expel waste.
This makes it firm to tell whether the net-like fossils are really sponges or colonies of marine algae that have a similar appearance, he said.
“For a fossil to be spongy, it needs to uphold evidence of structural complexity,” said Dr. Gilling, who was not involved in the study.
“Unfortunately, it is very firm to divide beforetime fossil sponges from beforetime bacterial colonies.”