Hadrosaurids have been making headlines more often than
usual in the past few weeks. Starting with the discovery of hadrosaur tails in
both Alberta, Canada and Coahuila, Mexico, new discoveries of the dominant
herbivores of the Cretaceous are popping up quite regularly. Since the two
tails were uncovered, the youngest-known specimen of Parasaurolophus sp. was discovered and affectionately nicknamed
“Joe.” Joe reveals much about lambeosaurine ontogeny in a tribe of
lambeosaurines which are pretty poorly understood in terms of how ontogeny and
gender affect the development of their signature tube-like crests (Farke et. al, 2013).
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| Amurosaurus riabinini. Reconstruction by Sergey Krasovskiy. |
Perhaps the most important news relating to hadrosaurs is
the recent publication of cranial endocasts of the lambeosaurine Amurosaurus riabinini of Russia. These
endocasts allow paleontologists to examine the structure of the brain’s
outermost regions, allowing conclusions to be drawn my correlating the size of
a given region to its importance to the animal. This, subsequently, allows us
to speculate further on the behavior of these animals.
It has been theorized that the volume of the dinosaurian
brain took up about half of the available space within the braincase. The
recovered Amurosaurus, however,
reveals that its brain took up around 60% of the braincase. Being an
herbivorous reptile, this does not indicate a significantly higher level of
intelligence than any standard reptile; however, this larger brain does
indicate that, despite being huge, stolid animals, hadrosaurs were not dull.
Not only were they not dull, but compared to other
herbivorous dinosaurs, hadrosaurids had among the highest brain-to-body ratios!
The Amurosaurus brain reveals that
its Reptilian Encephalization Quotient (REQ), or the ratio of a reptile’s
actual brain-to-body mass to its expected mass, was higher than those of
ceratopsians and sauropods, but lower than those of even some of the earliest
theropods. This latter discovery is not surprising, as the instincts and
brainpower needed to hunt are far greater than those needed to graze or browse.
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| The cranial endocast of A. riabinini. Scale bar represents 2cm for A, B, and C, and 10cm for D. From Lauters et. al, 2013. |
What makes the brain of Amurosaurus particularly important, and what does this show us
about hadrosaurs? The detail of the endocast reveals an interesting aspect of hadrosaur brain physiology: the pituitary
gland is enlarged, possibly explaining one part of just how hadrosaurs attained
such massive sizes in a relatively short period of time. A hadrosaur could grow
from a five-foot-long hatchling to a thirty-foot-long adult in just 12 years,
reaching their maximum size in just half the time it took contemporary
predators to reach adulthood. The astonishing growth rate of hadrosaurs is part
of the reason they prevailed in the late Cretaceous; reaching sexual maturity
at just three years old, hadrosaurs could produce many offspring while being
too large for most predators to tackle. The larger-than-expected size of the
lambeosaurine brain is consistent with the notion that hadrosaurs, in general,
were animals with relatively complex social interactions (Lauters et. al, 2013). The need for intraspecies
communication between herd members is essential for maintaining herd structure.
An enlarged brain processes more auditory and visual signals around it, giving
credence to the theory that the large and elaborate crests in lambeosaurines
were used for communication as well as courtship displays.
It is a common misconception that dinosaurs were truly
dumb animals. Compared to their body size, it is true, their brains are much
smaller than a mammal of the same dimensions. However, this does not exclude
them from having engaged in complex behaviors which are comparable to those of
modern animals. We now know from cranial endocasts of serveral taxa spanning
various families that dinosaurs, in general, were much more complex, active,
and interesting than previously thought.
References
Cranial Endocast of the Lambeosaurine Hadrosaurid Amurosaurus riabinini from the Amur Region, Russia
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