Copyright © 1997-2009 by Mike Everhart
(ABOVE) A closer view of the mid-portion of the left side of
the skull, showing the location of the sclerotic ring. The narrow finger of bone
projecting forward over the sclerotic ring is the anterior extension of the
postorbitalfrontal (POF). This is one of the key characteristics that makes this specimen
a Tylosaurus proriger. In most mosasaurs, the POF barely contacts the prefrontal.
In some genera, such as Clidastes, the POF and prefrontal are widely separated,
and the frontal itself forms the bony ridge over the eye socket. In Tylosaurus
proriger, the POF overlaps the prefrontal and may have provided additional
strengthening of the skull
Also shown is the flexible joint in the middle of the lower jaw
(dentary). This structure allowed the lower jaw to flex outward and enabled the animal to
hold and swallow large prey in a manner similar to snakes..
(ABOVE) The business end of a mosasaur, showing the extension
of the premaxilla over the most anterior teeth which is also typical of Tylosaurs. In some
fossil remains, the premaxilla shows evidence of being used as a 'battering ram'. The
joint between the premaxilla and maxilla is shown between the second and third teeth on
the upper jaw. Pits shown in the upper and lower jaws are probably openings for blood
vessels and nerves.
(ABOVE) Another shot of the anterior end of the skull, showing
the left nasal opening (nares). In this specimen, the nares are located slightly forward
of the location typical in later examples (and the type specimen) of Tylosaurus
proriger. This may be an indication that this specimen has transitional characters
linking it to an earlier species of Tylosaurus.
(ABOVE) The top of the skull, showing the location of the
frontal and parietal bones. Apparently, after the animal died, the skull came to rest in
an upright position on the sea floor. After some period of time (oysters had time enough
to colonize what would become the lower side of the fossil!), the skull came apart and
slumped over on it's left side where the top of the skull was preserved in the same plane
as the left side.
(ABOVE) The back of the skull showing the connection of the
quadrate (center) with the squamosal bone of the skull and the articular of the lower jaw.
Small bone to the right of the quadrate is part of the atlas vertebra, most of which is
located behind the squamosal. Small bones to the left of the quadrate are apparently from
one of the front paddles.
(ABOVE) A close up of the quadrate showing evidence of soft
tissue preservation (probably the tympanic membrane / ear drum). Although completely at
home in the ocean, mosasaurs had evolved from terrestrial lizards and certainly had a
sense of hearing.
Credits: The opening picture on this page is from a photograph of a nearly
complete, 28 foot long Tylosaurus proriger specimen on display at the
American Museum of Natural History. This specimen is about the same size as our material,
but is probably somewhat younger.