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About 6 years ago, paleontologists working on fossils from China announced that they had discovered fossilized brain and nervous system tissue in a tiny arthropod that lived about 520 million years ago. Later, they found fossilized blood vessels.

And these discoveries were a huge deal because soft tissues don't usually fossilize well, so they allowed paleontologists to make hypotheses about how the nervous system and other organs evolved. But a new study in the Proceedings of the Royal Society B argues that those researchers were wrong. The bulbous and spiny structures they saw in these fossils were not brains or parts of the cardiovascular system, but rather fossilized sheets of microbes called biofilms.

And if that’s the case, all their ideas about how brains and veins evolved in arthropods might be wrong too. The debated fossils were found in the famous Chéngjiāng fossil beds in southern China. These beds are considered a Konservat-Lagerstätten, a term paleontologists use for a deposit of sedimentary rock that contains lots of exceptionally well-preserved fossils.

Just basically like a paleontologist's happy place. So if we’re going to find fossilized soft tissues, it would be in a place like that. Still, the new team, led by Jianni Liu of Northwest University China, was skeptical.

So they looked at about 800 fossils of the same ancient critters from the same area, specifically hunting for the bulbous and spiny structures previously identified as brains and blood vessels. And they found them, but they also found that their shapes were more variable than originally described. In addition, they found that the brain-like structures often connected directly to structures thought to be part of the gut, which doesn’t make a lot of sense because brains aren’t generally connected to the digestive tract.

And when they looked elsewhere in the animal’s body, they found brain-like structures where they didn’t belong, like in the mid-gut. So Liu and colleagues think these structures are actually formed by microbes that digested the arthropod’s internal organs after it died, and then were fossilized along with its exoskeleton. And that would make sense, given what scientists know about how animals decompose.

A previous study essentially watched how brine shrimp decayed. These shrimp are a modern relative similar in size to the ancient arthropods. And those researchers saw that after death, microbes from the gut start breaking down the brine shrimp's internal organs and form biofilms, eventually making their way to the head and consuming the neural tissue there.

The biofilms often formed bulbous structures and spiny projections, much like the fossilized structures the previous researchers described as brains and blood vessels. And comparing the modern experiment with the fossils made a pretty convincing case that the structures seen are actually biofilms. If they’re right, then paleontologists may have to go back to the drawing board with their ideas about the evolution of brains and blood vessels in ancient arthropods, or back to the fossil beds to find yet better fossils.

And in the future, they have to be much more careful to make sure what they think are traces of organs really are. It seems like it’s a good week to point out potential paleontological errors. Because the next study we’re talking about also aims to set the record straight, this time on ichthyosaurs.

The new paper, published in PLoS ONE, claims that ichthyosaurs could be much bigger than we thought, potentially unseating whales as the largest animals ever. Ichthyosaurs were large marine reptiles that lived a little over 200 million years ago. Basically, the reptilian versions of modern whales and dolphins, just with an extra set of flippers.

But in 2016, a fossil collector discovered a 205 million year old chunk of jaw bone on a beach in the U. K. that appears to belong to one of the largest animals ever. Since there was just a fragment of bone, paleontologists can’t be sure about the animal’s total length, but they made an educated estimate using a technique called scaling.

The scientists compared the size of this jaw bone to jaw bones of other ichthyosaur species where there were more complete skeletons, then assumed the proportions stay similar if the animal were larger. It’s not a perfect method since proportions can vary between individuals, let alone species, but it’s the best you can do when you don’t have much of a skeleton to work with. This jaw bone was about 25% bigger than the same bone from the largest ichthyosaur known beforehand, which is about 21 meters long.

So the researchers think that it belonged to an ichthyosaur that was around 26 meters long. That is the same length as a blue whale, the current record holder for the largest animal to exist ever. But the comparison fossil wasn’t terribly complete either.

So they compared it to a second, more complete but smaller ichthyosaur. And that gave them a slightly smaller estimate of 22 meters. But the blue whale might still have cause for concern, since this bone fragment is causing paleontologists to reconsider five large bone fragments discovered nearby in the 19th century and mid-20th century.

These fragments were originally thought to belong to giant land-dwelling dinosaurs, but the researchers say they more closely resemble the new bone fragment and probably also belong to giant ichthyosaurs. And if so, they might have come from animals that were more than 30% bigger. Which could mean that the blue whale isn’t the biggest animal ever, maybe not even close.

Until paleontologists find a better preserved specimen, though, it’s hard to tell. But even if the blue whale holds its record, this new bone fragment, and the older fragments that are now being reconsidered, still belong to the largest ichthyosaurs ever found. And it’s just kind of awesome that even though paleontologists study things that are millions of years old, they’re still making new discoveries every day.

Recently we launched new finds the SciShow team and I have found on SciShowFinds.com. We’re super happy that so many of you have loved the finds as much as we did and we’re almost out! You can still get a few special artifacts of the world, though, like the bee heart lapel pin, or the pocket lab.

We’re on the hunt for new finds to restock too, and we’ll be sure to tell you about those when we, find them. And if you want to suggest things, that’s also great. I’ve got some ideas though, I’m excited! [♪ OUTRO].