TextFig-37.jpg (113136 bytes)

"If I only had a brain...."

A primer on the brain and cranial anatomy of mosasaurs

(With apologies to a certain Scarecrow from the Land of Oz)

 

Copyright 2008-2010 by Mike Everhart

  Page created 09/08/2008; Last updated 01/01/2010

 

 

 

LEFT: The skull of Platecarpus ictericus in right lateral view, with the location of the brain in red. Adapted from Russell (1967, Text-fig. 37),  with kudos to Mike Polcyn for his knowledgable assistance.

Mosasaurs obviously had a brain...maybe not a very big one by mammalian standards, but certainly large enough to allow them to rule as top predators in the Cretaceous seas for millions of years.

The most difficult part of the mosasaur skull for me to understand (and visualize) is the back one-third, including the braincase. With rare exceptions, mosasaur skulls are crushed fairly flat when collected and many of the structural relationships between the individual bones are lost or not readily apparent. While this not a major problem with understanding the tooth bearing structures in front, the three dimensional aspect of the back of the skull is difficult to reconstruct. Note that in many reptiles, including mosasaurs, the braincase is cartilagenous and does not preserve as a fossil (with possible rare exceptions). In addition, there is a thin sheet of cartilage, called the interorbital septum, that separates the eyeballs. The pineal foramen is the mosasaur brain's window to the outside world... often called a "third eye" and is associated with the pineal gland.  The olfactory lobes run between and above the eyes and nest under the frontal bone where they are in contact with the nasal passages.

I've adapted a series of figures from the mosasaur bible (Russell, 1967) that help you understand which bones make up the braincase or are connected it. The large open spaces between the bones, especially those areas on either side of the parietal and behind the squamosals were filled with strong muscles that attached to the lower and provided the means for quickly and powerfully closing the jaws.

TextFig-16a.jpg (28067 bytes) LEFT: Starting with the general shape of the brain (from an endocast).... and location of the cranial nerves - the spinal cord (and vertebrae) would be to the left. The olfactory lobe (sense of smell) was located along the underside of the frontal, between the large eyeballs  It was relatively small in size, not unusual for marine animals. The eyes were quite large, probably useful for seeing in dimly lit or deeper waters, and indicate that mosasaurs probably were sight hunters. However, since the eyes were located on the side of the head, they did not have stereoscopic vision.  Like most animals, including humans, the mosasaur brain has twelve pairs of cranial nerves. Cranial nerves III and IV (not shown) control the movements of the eyes. Cranial nerve VIII is responsible for hearing and balance.
TextFig-10a.jpg (26738 bytes) LEFT: The floor of the mosasaur braincase is composed of two main bones: the basioccipital, which connects to the atlas/axis vertebrae, and the basisplenoid. Most of the blood supply for the brain (basilar and internal carotid arteries) comes through these two bones. These and a cartilageenous structure enclose the brain in something more like a tunnel than the rounded cranium that most of us are familiar with in our own skull.
TextFig-13a.jpg (19016 bytes) LEFT: Next we have a medial view of the braincase, showing an interior view of the bones that enclose the mosasaur brain. Basioccipital and basisplenoid below, paired opisthotic and prootic above. These bones are penetrated by or enclose openings (foramina) for the cranial nerves. 
TextFig-12a.jpg (16654 bytes) LEFT: Then we have the exterior of the braincase in right lateral view, showing the bones that connect the braincase to the rest of the skull. A projections from the prootic, and the paraoccipital processes of the opisthotic combine with the surpratemporals to form two strong, strut-like bones that connect to the squamosals to form the quadratic suspensorium... or the connection for the quadrate bone that serves as the hinge point for the lower jaw. In addition an anterior, upward extension of the supraoccipital meets a downward extension of the parietal along the midline.
TextFig-17a.jpg (25308 bytes) LEFT: From here, we go to the back of the skull and show the relative position of the braincase as it is suspended within the skull:
TextFig-83a.jpg (18700 bytes) LEFT: ... then finish with a dorsal view of the posterior portion of the skull. Note that the rearward projecting wings of the parietal join with the paroccipital process of the opisthotic to reinforce the quadratic suspensorium.
TextFig-20-25a.jpg (23228 bytes) LEFT: Adding to the complexity of this part of the skull is the somewhat flexible attachment of the quadratic suspensorium to the quadrate.  The quadrate supports the ear drum (tympanum) and also serves as the hinge point for the lower jaw:
TextFig-32a.jpg (25501 bytes) LEFT: The ventral portion of quadrate is the hinge point for the lower jaw, seated in the glenoid fossa of the articular.
Note that much of Russell's information on the mosasaur brain goes back to Camp (1942). Here are two scans of Camp's Fig. 24:

Fig. 24. A. Braincast of Platecarpus; and B. Reconstruction of mosasaur brain based on this cast (C not shown)

Fig. 24. D. Dorsal view of the brain of the Nile monitor (Varanus niloticus); E. Cast of the brain of Clidastes; and, F, G. Reconstruction of the brain of Platecarpus in dorsal and ventral views.

TextFig-36a.jpg (14804 bytes) No discussion of the adaptations of the skull in mosasaurs is complete without mentioning cranial kinesis and streptostyly (a special hinge joint between the dorsal portion of the quadrate and squamosal that allows limited movement. I will try to enlarge upon this subject in future additions to this page. In the meantime, the reader is referred to Russell (1967) for a more detailed discussion.

LEFT: A diagrammatic view of the rotation of the quadrate in mosasaurs (From Russell, 1967), showing the forward position of the lower jaw (protracted) and the rear position (retracted). The position of the lower jaw is somewhat exaggerated for clarity.

RIGHT: The skull of Platecarpus ictericus showing mouth opening, with the lower jaw thrust forward (quadrate rocked forward) and with the lower jaw retracting during closure (quadrate returning to a vertical position).

TextFig-37-38a.jpg (17976 bytes)

REFERENCES:

Camp, C. L. 1942. California Mosasaurs. University of California Press, 67 pp.

Russell, D. A., 1967. Systematics and morphology of American mosasaurs. Peabody Museum of Natural History, Yale University, Bulletin 23.


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