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Re: Queries



This replies to the following query:

>At 10:15 AM 3/22/96 -0500, Graeme Worth wrote: And for the
>anatomists - can someone provide a brief explanation for the
>following:

Jugals - The "cheek bones" forming the left and right sides of the skull
behind the muzzle. In dinosaurs, the jugal usually has three or four branches
(_rami_: singular, _ramus_), which contact other bones of the skull. The
anterior ramus (forward branch) contacts the rear of the maxilla (upper
tooth-bearing jawbone); the first superior ramus (forward upper branch)
contacts the lacrimal; the second superior ramus (rearward upper branch)
contacts the postorbital; and the posterior ramus (rearward branch) contacts
the quadratojugal. The space between the anterior and first superior rami
usually forms part of the edge of the antorbital fenestra (in those dinosaurs
that had them); the space between the first and second superior rami forms
the bottom edge of the eye socket (orbit); and the space between the second
superior and posterior rami forms part of the bottom edge of the
infratemporal fenestra. The jaw muscles passed between the jugal and the rear
palatal bones on their way from the top of the skull to the mandible. In
life, the jugal was covered externally with muscular and dermal tissue that
also covered the back of the lower jaw.

Tibiotarsus - In birds, the hind limb bone formed by the fusion of the bottom
of the tibia, or calf bone, with the astragalus, the largest ankle bone of
dinosaurs and birds. In dinosaurs, the astragalus was firmly attached to the
tibia but not fused with it, a feature distinguishing advanced birds from
dinosaurs.

Prefrontal - A small skull bone that forms part of the skull roof; there are
left and right prefrontals. It is bounded in back by the frontal (hence its
name), at the side by the lacrimal, and in front by the nasal. In some
dinosaurs it is either lost or (in adults) fused with one or another of its
neighboring bones--it is one of those skull bones whose degree of fusion to
its neighbors supposedly signals the maturity of the animal. Molnar (1980)
noted that the prefrontal of _Dinotyrannus_ differed in shape from that of
_Tyrannosaurus_, helping to distinguish the two genera.

Osteoderm - Any bone (_osteon_) imbedded in the skin (_dermis_). Osteoderms
include the scutes of crocodiles, the keeled scutes, spines, and tail-club
bones of ankylosaurs, and the plates, spines, and gular ossicles of
stegosaurs. Their function was usually for protection and display.

Sternal plate - One of the two halves of the sternum, or chest. These bones
are usually imbedded in cartilage and are sometimes poorly ossified. In
dinosaurs, they generally do not articulate directly with the shoulder girdle
(at the coracoids) but "float free" in the chest cartilage. They serve to
anchor the tips of the thoracic ribs and the front of the abdominal ribcage,
and they act as braces and spacers for the lower shoulder bones (coracoids).
In flying birds, the sternal plates are fused solidly together into a
relatively huge, keeled breastbone that serves to anchor the pectoral muscles
that work the wings.

Basicranial - Having to do with the basicranium.

Basicranium - The underside of the skull not including the palate; the base
of the braincase and the back of the throat. This region is made up of
irregularly shaped bones whose names end in "sphenoid" and which are
generally fused together into a solid unit.

Parietal bone (Is this what makes the frill in ceratopians?) - A substantial
skull bone that forms the rear of the skull roof and the upper rear of the
braincase; there are left and right parietals, which meet along the midline
of the skull. In ceratopians, the rear edges of the parietals and their
neighboring bones, the squamosals, extend strongly backward to make up the
frill. Besides contacting each other, the parietals contact the frontals
anteriorly (toward the front), the squamosals laterally (along the sides),
and the supraoccipitals posteroventrally (at the back and below). They may
also contact the postorbitals laterally. Each parietal forms part of the rear
edge of the supratemporal fenestra, and along with the back of the frontals
they were the attachment site for the adductor jaw muscles.