Paleontologists don’t go looking for brains, and I’m not surprised. I once got to hold a fresh brain in my hands (it was at a medical school–nothing fishy, I promise), and I can vouch that they are marvelously delicate: a custard for thinking. When any vertebrate with a brain dies, be it human, turtle, or guppy, that fragile greasy clump of neurons is one of the first organs to vanish. Scientists must infer what ancient brains were like very often by examining the case that held it–that is, if they can find a relatively intact braincase.
In recent years, scientists have been able to get important clues about brains by scanning the brain cases. They can create virtual fossils in their computers that reveal a wealth of details. Alan Pradel of the Museum National d’Histoire Naturelle in Paris and his colleagues recently scanned a 300-million-year old fossil of an ancient relative of sharks called Sibyrhynchus denisoni. They recognized many details of the skull. But when they looked closer, they saw something they could not quite believe. They saw something that looked like a fossilized brain.
Even without a brain, Sibyrhynchus is very interesting. It belonged to a group known as iniopterygians, whose closest living relatives are ratfish. While there are few species of ratfishes today, 300 million years ago they enjoyed a much bigger diversity. Iniopterygians were small (6 inches long) and had big eyes and pectoral fins, along with a club on their tail.
Pradel and his colleagues were pleased enough to see the braincase of Sibyrhynchus, but they were stunned to see a chunk of rock deep inside that looked like a very small fish brain (and I do mean small–its length was 7 mm, or a quarter of an inch). Fossils sometimes form strange structures, but Pradel and his colleagues are pretty sure that they’re actually seeing a brain. It has the shape of a ratfish brain, including the various sections of a ratfish brain. And it even has nerves that extend to the right places to connect to the eyes and ears.
You may be struck by how small the brain (yellow) is compared to the braincase (red). If the scientists are right, it’s a cautionary tale for those who would estimate the size and shape of ancient fish brains from their braincases. But perhaps, in the future, researchers will find more actual brains, and will be able to chart the evolution of these delicate organs in greater detail.
Source: Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography.
Movie: Courtesy of Alan Pradel
March 2nd, 2009 at 6:52 pm
Judging it by the size of the brain, it doesn’t look like it was a very intelligent animal.
March 2nd, 2009 at 7:44 pm
I predict there are going to be a lot a fossils getting CT scans, looking for similar features.
>… it doesn’t look like it was a very intelligent animal.
Fish aren’t for their intelligence, despite all that time they spend in “school”.
March 2nd, 2009 at 7:46 pm
The fossil was not CT scanned. It was synchrotron holotomography. What is that?
March 2nd, 2009 at 7:54 pm
Name’s Alan Pradel. You got it right only the last time (he’s a colleague, that’s why I care…). Most amazing feat is that the fossil was in the collection for a long time. It’s time to open all the drawers again!
[Carl: Thanks, Pierre. I made the fix. This week I'm having some sort of name black-out.]
March 2nd, 2009 at 8:06 pm
I’d bet dollars-for-donuts that there was a whole lot of brain shrinkage. Can’t imagine the difference in size between the braincase and the brain. Also, what appears to be the optic nerve is much smaller than its foramen. Possible, but weird.
If it is that much smaller, the question would be, why have such a big braincase? How would the brain maintain its position? What’s the extra space for? I’d be very surprised if there are extant examples of such size differences.
But it sure does look like a brain. nice movie.
(link to PNAS doesn’t work for me. Can’t find the article in the Feb 24 edition of PNAS)
[Carl: John, for whatever reason, PNAS lifts their embargo on papers before their links work. Go figure.]
March 2nd, 2009 at 9:02 pm
I’m with johnk. We know how chondrichthyan brains fit into their crania.
March 3rd, 2009 at 5:46 am
I think it is highly likely that brain size shrank between death and preservation by fossilisation, via dessication, amongst a number of other factors.
I’ve collected skulls from mammals, birds, fish and reptiles for much of my life, and I usually soak them in sodium hypochlorite (Clorox) which is supposed to dissolve everything but the bone, and leave it all white and clean.
But there is always a stubborn little bit of stuff, right inside the cranium, that I have to extract with a dental pick, or a bent bit of coat hanger wire.
I believe, in humans at least, that that stubborn bit is called the ‘soul’.
It might be interesting to ask creationists if 300MY old fish had souls.
March 3rd, 2009 at 10:29 am
whou I can’t imagine that my results will be so quickly in the web! Thanks for your interest!
Your questions are very interesting…
The holotomography is a relatively new approach of Synchrotron X-Ray imaging. It shows the phase contrast (not only the absorption contrast) of the different interphases of the sample with a better resolution than a common Synchrotron X-Ray imaging.
some extant fishes possess a small brain compared to the endocranial cavity (e.g. coelacanths). In iniopts, the nerves perfectly reach their respective foramen without any deformation and the overall morphology of the endocranial cavity is different from the morphology of the brain. Consequently we supposed that the brain did not fill the endocranial cavity. Concerning the optic foramen which is very large, a lot of other thing passed through it (e.g. the efferent pseudobranchial artery) as in extant ratfishes.
Hum good question about the putative soul of iniopterygians hehe…
March 3rd, 2009 at 10:47 am
This is great!
March 3rd, 2009 at 11:01 am
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March 15th, 2009 at 7:13 am
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March 26th, 2009 at 1:26 am
I’m wondering if this was from a Kansas dig I got to go on with my father, Leslie F. Marcus, from La Brea and the American Museum of Natural History.