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SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
1 Ornithischia
FIGURE 39. Facial trends in Ornithopoda. In the course of ornithopod evolution, the external antorbital fenestra becomes reduced and eventually
closed at the level of Hadrosauridae. The antorbital cavity also becomes reduced and eventually completely internalized, as in crocodylomorphs.
These trends are associated with restriction of the main paranasal air sinus in connection with the elaboration of the masticatory apparatus and
enlargement of the nasal vestibule. Skull drawings modified from Galton (1974), Norman (1986), Weishampel and Horner (1990), Weishampel
and Witmer (1990b), and Sereno (1991a).
independently show the same basic trends, namely, like crocodylomorphs,
reduction of the antorbital cavity and closure of
the external antorbital fenestra (Fig. 39). The primitive ornithischian
condition, manifested by Lesothosaurus diagnosticus,
is to have a small internal antorbital fenestra (essentially the
ostium of the antorbital paranasal air sinus) and a relatively
large external antorbital fenestra. Basal ornithopods, such as
Heterodontosaurus tucki, display the first signs of the trend in
that lateral laminae from the maxilla and lacrimal constrict the
external antorbital fenestra. These laminae are even more extensive
in hypsilophodontids, and the external antorbital fenestra
is relatively small. In basal iguanodontians, the antorbital
cavity and external fenestra are further reduced and displaced
caudally. Finally, in hadrosaurids, the external fenestra is completely
closed (sometimes partly covered by the jugal), and the
antorbital cavity is internalized and relatively small.
An important concurrent trend here is the expansion of the
feeding apparatus, in particular, the dentition and its bony buttresses
(see Weishampel, 1984, 1993 and references therein).
Ornithopods show functional innovations indicative of exten-
sive oral processing in association with herbivory, such as a
transverse power stroke (achieved independently through differelit
mechanisms in heterodontosaurids and euornithopods).
Aspects of this masticatory trend are increases in the number
of teeth (but a decrease in the relative size of each tooth), their
packing in the jaws, and the relative size of the maxilla, culminating
in the characteristic dental batteries of hadrosaurids.
Another trend worth noting takes place in Iguanodontia, and
this relates to expansion of the nasal vestibule with enlargement
of the naris and resultant caudal displacement of the antorbital
cavity. (There is also a marked trend for size increase, but such
allometric effects are too complex to merit laboring the discussion
here.)
Therefore, the relationship between the trends in the antorbital
cavity (and its enclosed sinus) and in other anatomical
systems is relatively straightforward. As the relative volume of
the dentition and its buttresses increases, the relative volume of
the antorbital paranasal air sinus and its bony cavity decreases;
and, in iguanodontians, as the nasal vestibule expands, the antorbital
cavity becomes further reduced. As in crocodylo-