Sunday, April 5, 2015


The external basicranial air sacs of Tyrannosaurus rex (A) and the internal air sacs of Albertosaurus libratus (B), both in left lateral view. The black oval in A represents the caudal tympanic recess, the point of entry from the middle ear cavity into the basicranium; in B I neglected to include a corresponding ellipse for the large paramedian pneumatic foramen, where the basisphenoid recess air sac enters the bony ceiling of the basisphenoid recess. The outline in A is modified after Osborn (1912), and that of B is modified after Russell (1970) and I included information from the A. libratus specimen ROM 1247.
This is another set of images that harken back my Master's degree days. I can't recall if these were done for the same project on paranasal diverticulae that I did for my Comparative Anatomy term paper; given that they did not appear in my thesis, they were either in the course essay or excluded altogether.

As with the antorbital air sacs, I wanted to gain an understanding of the form and function of this air sac system through visualizing it in the round. Published figures of the empty spaces only provided a hole's eye view of the donut, and my goal was to rectify the perspective by showing the balloons that actually dissolved the spaces out of bone. It is notable that in this region of the skull there is more air space than bone. The external and internal air sacs were separated by sheets of bone only millimeters thick.

These images are minimalist, in that they do not show the relationships between the air sacs with the tympanic cavity, the median pharyngeal air sac system, or the cervical air sac system. In fact, the external view may actually correspond mostly to the tympanic cavity. The continuity between the tympanic air sac and the subcondylar air sacs are not shown in B, which is something that I should have included. Also, I did not show the air sacs within the paroccipital process; my primary focus was the basicranium so I excluded what I considered to be extraneous information.

These drawings were based on observation of actual tyrannosaurid braincases, primarily that of ROM 1247. Without the real structure in hand, I am certain that I would not have shown continuity between the subsellar and basisphenoid air sacs; nor would have I been so exact as to show the ventral split in the tympanic air sac and its course around the paroccipital process.

It is worth noting that the tympanic air sac in life would have been bounded above and laterally by the powerful jaw closing muscles and it was crossed by the stapes (ear ossicle), and the subsellar air sac extended into the ventromedial part of the orbital cavity, the space that contained the eye, its extrinsic muscles, and associated glands, vasculature and nerves. The tympanic air sac and the subsellar air sac occupied anatomically sharply separated anatomical domains that housed primary sense organs, the middle ear cavity and the orbital cavity, respectively. Regardless, the air sac system did provide a connection between those regions if the reconstruction shown here is at all accurate.

References Cited

Osborn, H. F. 1912. Crania of Tyrannosaurus and Allosaurus. Memoirs of the American Museum of Natural History 1: 330.

Russell, D. A.  1970.  Tyrannosaurs from the Late Cretaceous of western Canada. National Museum of Natural Science Publications in Palaeontology 1: 1-34.