An unassuming brown pebble, found more than a decade ago by
a fossil hunter in Sussex, has been confirmed as the first example of
fossilised brain tissue from a dinosaur.
The fossil, most likely from a species closely related to
Iguanodon, displays distinct similarities to the brains of modern-day
crocodiles and birds. Meninges - the tough tissues surrounding the actual
brain - as well as tiny capillaries and portions of adjacent cortical tissues
have been preserved as mineralised 'ghosts'.
The results are reported in a Special Publication of the
Geological Society of London, published in tribute to Professor Martin Brasier
of the University of Oxford, who died in 2014. Brasier and Dr David Norman from
the University of Cambridge co-ordinated the research into this particular
fossil during the years prior to Brasier's untimely death in a road traffic
accident.
The fossilised brain, found by fossil hunter Jamie Hiscocks
near Bexhill in Sussex in 2004, is most likely from a species similar to
Iguanodon: a large herbivorous dinosaur that lived during the Early Cretaceous
Period, about 133 million years ago.
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| Triceratops skeleton. Source: Allie_Caulfield Derivative: User:MathKnight [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons |
Finding fossilised soft tissue, especially brain tissue, is
very rare, which makes understanding the evolutionary history of such tissue
difficult. "The chances of preserving brain tissue are incredibly small,
so the discovery of this specimen is astonishing," said co-author Dr Alex
Liu of Cambridge's Department of Earth Sciences, who was one of Brasier's PhD
students in Oxford at the time that studies of the fossil began.
According to the researchers, the reason this particular
piece of brain tissue has been so well-preserved is that the dinosaur's brain
was essentially 'pickled' in a highly acidic and low-oxygen body of water - similar to a bog or swamp - shortly after its death. This allowed the soft
tissues to become mineralised before they decayed away completely, so that they
could be preserved.
"What we think happened is that this particular
dinosaur died in or near a body of water, and its head ended up partially
buried in the sediment at the bottom," said Norman. "Since the water
had little oxygen and was very acidic, the soft tissues of the brain were
likely preserved and cast before the rest of its body was buried in the
sediment."
Working with colleagues from the University of Western
Australia, the researchers used scanning electron microscope (SEM) techniques
in order to identify the tough membranes, or meninges, that surrounded the
brain itself, as well as strands of collagen and blood vessels. Structures that
could represent tissues from the brain cortex (its outer layer of neural
tissue), interwoven with delicate capillaries, also appear to be present. The
structure of the fossilised brain, and in particular that of the meninges,
shows similarities with the brains of modern-day descendants of dinosaurs,
namely birds and crocodiles.
In typical reptiles, the brain has the shape of a sausage,
surrounded by a dense region of blood vessels and thin-walled vascular chambers
(sinuses) that serve as a blood drainage system. The brain itself only takes up
about half of the space within the cranial cavity.
In contrast, the tissue in the fossilised brain appears to
have been pressed directly against the skull, raising the possibility that some
dinosaurs had large brains which filled much more of the cranial cavity.
However, the researchers caution against drawing any conclusions about the
intelligence of dinosaurs from this particular fossil, and say that it is most
likely that during death and burial the head of this dinosaur became
overturned, so that as the brain decayed, gravity caused it to collapse and
become pressed against the bony roof of the cavity.
"As we can't see the lobes of the brain itself, we
can't say for sure how big this dinosaur's brain was," said Norman.
"Of course, it's entirely possible that dinosaurs had bigger brains than
we give them credit for, but we can't tell from this specimen alone. What's
truly remarkable is that conditions were just right in order to allow
preservation of the brain tissue - hopefully this is the first of many such
discoveries."
"I have always believed I had something special. I
noticed there was something odd about the preservation, and soft tissue
preservation did go through my mind. Martin realised its potential significance
right at the beginning, but it wasn't until years later that its true
significance came to be realised," said paper co-author Jamie Hiscocks,
the man who discovered the specimen. "In his initial email to me, Martin
asked if I'd ever heard of dinosaur brain cells being preserved in the fossil
record. I knew exactly what he was getting at. I was amazed to hear this coming
from a world renowned expert like him."
The research was funded in part by the Natural Environment
Research Council (NERC) and Christ's College, Cambridge.
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