For the first time ever, paleontologists have identified fossilized soft brain tissue from a dinosaur, providing an unprecedented glimpse inside the heads of these iconic Mesozoic animals.
According to a Special Publication of the Geological Society of London released Thursday, an innocuous rock found near Bexhill-on-Sea in southern England contains traces of capillaries, partial cortical tissues, and meninges—protective membranes that envelop the brain—which belonged to an Iguanodon-like dinosaur some 133 million years ago.
It's hard to overstate the sheer implausibility of this find. Examples of fossilized soft tissues are extremely rare because these organs deteriorate much faster than sturdier skeletal structures. Aquatic animals are more likely to experience some degree of tissue preservation because underwater sedimentary beds offer prime conditions for fossilization. But it is pretty much unheard of to find soft tissues of long-extinct terrestrial animals, let alone brain matter.
"Soft tissues usually need to be in direct contact with either sediments, or nearby fluids, very soon after death if they are to be preserved as either impressions or mineralized tissues," University of Cambridge paleobiologist Alex Liu, a co-author on the new study, told me via email.
"Since brain tissues (particularly of land-living organisms) are usually shielded from sediments and fluids by being trapped within a bony skull and braincase, they are very poor candidates for fossil preservation," Liu continued. "This specimen reveals that despite their location, brain tissues can be preserved, and I am hopeful that more will be discovered in the coming years."
Only a freak stroke of luck can explain how the brain matter of this iguanodontid came to be entombed in Cretaceous rock. The new research suggests that the dinosaur might have died with its head positioned upside-down in a deoxygenated and highly acidic bog or swamp, causing its skull to be essentially "pickled," resulting in the exceptional condition of the endocast (the cranial vault upon which the animal's neural imprints are etched).
"These are fluke preservational conditions," David Norman, a University of Cambridge paleontologist who also co-authored the paper, told me. "We are simply amazed that we have any structures present given how delicate these things are. [T]his is, after all, 133 million years old."
The specimen has been the subject of speculation since it was first discovered by fossil hunter Jamie Hiscocks in 2004. Hiscocks passed it along to the prolific University of Oxford paleobiologist and astrobiologist Martin Brasier, who helped spearhead a sophisticated analysis of the endocast. Using scanning electron microscopy (SEM) and computed tomography (CT) scanning, Brasier's team confirmed that this is, indeed, the first soft brain matter found from a dinosaur.
Sadly, Brasier died in a road accident in 2014; the new paper is a tribute to his life and work, and lists him as the lead author. "Martin realised [the fossil's] potential significance right at the beginning, but it wasn't until years later that its true significance came to be realised." Hiscocks, another co-author on the new study, commented in a statement.
"In his initial email to me, Martin asked if I'd ever heard of dinosaur brain cells being preserved in the fossil record," Hiscocks said. "I knew exactly what he was getting at. I was amazed to hear this coming from a world renowned expert like him."
The ghostly tissues confirm that dinosaurs had similar neural features to their close relatives, crocodiles and birds, and that this individual's brain had been pushed directly into the cranium. Whether this was because its living brain was somehow pressed flush against the braincase or because the skull collapsed after the animal's death remains up for debate, along with many other key details that the extraordinary fossil cannot answer at this time.
"We have taken this study pretty much as far as it is possible at present," Norman told me. "The acidic conditions would have denatured any of the biochemical signals that might have been present at one point, so there is not going to be anything that can be pursued on that front."
"In terms of this endocast, use of other tools or techniques will not tell us a huge amount more about the specimen," Liu agreed. "We have imaged it to a high resolution with SEM and determined the chemistry of the minerals that preserve the tissues, and mapped out the interior of the specimen using CT."
"A synchrotron might be able to provide finer resolution of the 3D structure of the mineralized region, and therefore of the original soft tissues," Liu added. "That is probably the technique I would most like to try to extract further information on this dinosaur from the specimen."
For now, however, the singular nature of this discovery should provide more than enough excitement for dinosaur enthusiasts to geek out over. The fact that researchers have recognized neural structures that once animated an Early Cretaceous creature is a genuine mind-boggler.
"[T]his find will encourage researchers to look again at their specimens and see if there are other traces of brain tissue preservation in other dinosaur endocasts," Liu told me. "It is difficult to say how common this sort of preservation will turn out to be, but I think people will be surprised that brain tissue can be preserved at all."
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