Airhead Alioramus: Why Theropods Were Light-headed
Dinosaurs got big. Whilst some members of the group did remain quite small in size, many species grew to stupendous dimensions, the like of which we have not seen again since their extinction. As dinosaurs only lived on land, they didn’t have the benefits that whales do of letting water support their massive bulk. Dinosaurs had to contend with gravity, which is rather troublesome when you’ve got a great big skeleton to lug around. So how did these leviathans manage to deal with this problem? One way of doing it is to reduce the overall weight of the skeleton via pneumatisation.
What’s that I hear you ask? Essentially this is where bone becomes in-filled by soft tissues that were filled with air. That way you reduce the weight of the skeleton but still retain its structural strength, allowing you to move around more efficiently. These air filled soft tissues are known as sinuses and in theropod dinosaurs these are separated into several major systems. These are the paranasal sinuses (which are diversions from the nasal passage), the suborbital sinuses (these are closely related to the paranasal sinuses) and the tympanic sinuses (which come from the inner ear). Other functions in addition to reducing the weight of the animal are thought to include playing roles in respiration, hearing and balance. The extent and morphology of these sinuses are also very variable, even between closely related taxa, making them especially useful for palaeontologists as they can then be used to distinguish one species from another in their phylogenetic analyses. Despite this incentive however, very few studies have looked these sinuses in detail.
A new paper published this week in the journal American Museum Novitates has set about rectifying this problem (you can download the paper by clicking here). The study, by Maria Gold, Stephen Brusatte and Mark Norell, compared the cranial sinuses of the tyrannosaurid Alioramus altai with other theropods. This species, found in Late Cretaceous rocks in Mongolia in 2001, was found to have a much higher degree of cranial pneumaticity than other theropods.
But how did they view these sinuses if they are inside the bone? One way to do it would be cut the bones open, but this is obviously a not a preferred option as it destroys the specimen. Fortunately there is a non-invasive technique known as CT scanning that allows palaeontologists to see the internal structure of bones and this was the technique used in this paper. This technique essentially takes x-rays of the bones in small slices. These slices are then reconstructed using special computer software, giving palaeontologists a shiny new digital model of the bone that they can play with to their hearts content.
Using CT scans, the sinuses in the spectacularly preserved disarticulated skull elements of A. altai were identified and then compared to those of other theropod dinosaurs. Gold and colleagues found that this particular taxon has taken this cranial pneumaticity to an unprecedented degree, with its sinuses invading the maxilla, lacrimal, jugal, squamosal, quadrate, palatine, ectopterygoid, and surangular (these are the technical names for some of the bones in the skull, you can also read it as ‘skull bones’ if it makes it easier for you). Alioramus has relatively longer snout compared to other theropods and the sinuses have evolved to fill this long snout.
So why does Alioramus have a much more pneumatised skull than other theropods? Gold and colleagues propose several potential explanations. One is that the holotype specimen is a juvenile and that the extent of the sinuses may be different in more mature specimens. A second idea is that Alioramus may have fed in a different manner than other theropods, in a manner that would have required a lighter skull. A third possibility is that there was no particular causal factor and that these sinuses merely formed via passive processes. Whatever the reason, one thing is sure; you still wouldn’t call Alioramus an airhead to its face!
Maria Eugenia Leone Gold, Stephen L. Brusatte, & Mark A. Norell (2013) The Cranial Pneumatic Sinuses of the Tyrannosaurid Alioramus (Dinosauria: Theropoda) and the Evolution of Cranial Pneumaticity in Theropod Dinosaurs. American Museum Novitates, (3790):1-46.
Stephen L. Brusatte, Thomas D. Carr, Gregory M. Erickson, Gabe S. Bever, and Mark A. Norell. A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia PNAS 2009 : 0906911106v1-pnas.0906911106.