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64 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
duced in relative size (increasing the cross-sectional area of
cortical bone), yet the pneumatic recesses within are extensive,
yielding-probably incidentally and automatically-"optimally
designed" tubular bars.
Summary
The discovery of the function of the bony antorbital cavity
of archosaurs allowed the asking of the next logical question:
What is the function of the enclosed structure? In other words,
what is the role of the antorbital paranasal air sinus? A survey
of paranasal sinus function in pneumatic amniotes shows that
no clear, widely applicable function presents itself. Widening
the search to include paratympanic and pulmonary pneumaticity
reveals the same situation: a diversity of suggestions but no
consensus. The position is advanced here that the wrong perspective
has hindered our understanding of pneumatic function.
Rather than attempting to ascribe function to the empty space
enclosed within the bony sinuses, focusing on the pneumatic
epithelium (i.e., the air sac itself) might be more rewarding.
Under this new perspective, pneumatic diverticula are viewed
simply as opportunistic pneumatizing machines, resorbing as
much bone as possible within the constraints imposed by local
biomechanical loading regimes. Thus, this "struggle" between
the tendency to pneumatize and the tendency to deposit bone
secondarily provides a dynamic mechanism for controlling
skull architecture, producing "optimal" structures without necessarily
requiring the action of natural selection.
In light of this hypothesis, crocodylomorphs and ornithopods
both show trends toward reduction and enclosure of the antorbital
cavity not because they share any particular paleobiological
attributes, but rather because both clades independently
(and for different reasons) show an apomorphic change in skull
biomechanics dictating deposition of bone rather than pneumatically
induced resorption of bone. In theropods, the opposite
trend occurs, and in some cases it seems as if the expansion of
the air sacs is simply opportunistic, and is, in a sense, biomechanically
"tolerated" or "unchecked."
SUMMARY
Complete understanding of the structure of extinct organisms
cannot be gained by recourse to bones alone; soft tissues matter.
In fact, soft-tissue components often direct the ontogenetic development,
structure, and arrangement of bony elements, such
that evolutionary studies involving only bones may not have
the appropriate focus, especially if elucidating process (e.g.,
adaptation, selection regimes) is a goal. Soft-tissue relations often
are the foundation of accurate paleobiological inference, and
hypotheses regarding adaptation or the action of natural selection
may require soft-tissue information if the hypotheses are
to be tested adequately. Initial mistakes in soft-tissue assessments
are compounded up the ecological hierarchy. Speculation
in inferring soft tissues in fossils can be identified and minimized
by approaching the problem phylogenetically, basing inferences
on the osteological correlates of the soft tissues observed
in the extant outgroups of a fossil taxon. A major result
of this study is that a surprisingly large amount of sophisticated
soft-tissue information can be teased out of fossil specimens.
Many, if not most, soft tissues have predictable relationships
with osteological structures, which is not unexpected given the
integration of anatomical systems. Even if a soft-tissue component
lacks reliable osteological correlates, information about
its size, position, and conformation can often be recovered by
reference to those surrounding soft tissues with known bony
signatures, which, in effect, limit the realm of possible structures
of the unknown component.
Careful application of this extant phylogenetic bracket approach
has resolved the status of the antorbital cavity of Ar-
chosauria, and, in the process, allowed reconstruction of much
of the facial structure of many taxa within diverse clades. For
virtually all archosaurs, the inference of an air-filled diverticulum
of the nasal cavity housed within the antorbital cavity requires
almost no speculation (a level I inference sensu Witmer,
1995a). Such a paranasal air sinus is found in the extant phylogenetic
bracket (i.e., both birds and crocodilians) of any clade
of fossil archosaurs, and the osteological correlates of this sinus
are ubiquitous in the extinct taxa. Alternative soft-tissue explanations
for observed osteological features also must be sought.
In the present example, the EPB approach demonstrated that
there is good evidence that both the nasal gland and the dorsal
pterygoideus muscle were among the contents of the antorbital
cavity. However, the osteological correlates of these anatomical
systems indicate that neither is associated with the antorbital
fenestrae or fossae.
It is tempting to believe that much of the controversy stems
historically from the traditional treatment of archosaurs as a
paraphyletic group. If at the outset the antorbital cavity of extinct
archosaurs had been recognized as homologous to that of
birds, perhaps the glandular and muscular hypotheses would
have never been proposed. Both the glandular and muscular
hypotheses fail the tests provided by this method and accepting
them requires excessive homoplasy. The pneumatic hypothesis,
arguing that the antorbital fenestra and cavity of virtually all
archosaurs housed a paranasal air sac, survives the tests, requires
little or no speculation, and is applicable to all archosaurs.
In cases where the soft-tissue assessment is equivocal
because an extant outgroup lacks relevant attributes, compelling
morphological evidence still may point to a particular aspect of
soft anatomy, although this necessarily requires more speculation.
Such is the case with the accessory cavities in the bones
surrounding the antorbital cavity of many archosaurs, in that
these bony cavities strongly support the notion of paranasal
pneumaticity. Such is also the case with basal archosauriforms,
where it seems likely that, like Archosauria, the antorbital cavity
lodged an air sac. If in the latter case one regards the level
of inference as acceptable, then it becomes likely that the origin
of the antorbital fenestra and cavity is causally linked to the
origin of the epithelial air sinus. Given this soft-tissue inference,
more accurate reconstructions of archosaurian craniofacial
structure provide a firmer foundation for interpreting the evolving
lifestyles of extinct archosaurs.
Finally, with a firm idea of the function of the bony antorbital
cavity in hand, it then becomes possible to investigate the function
of the structure housed within the cavity. The function of
paranasal pneumaticity-in fact, of any of the air-filled sinuses-has
long been a mystery. This problem stems primarily
from viewing it from what is probably the wrong perspective.
Traditionally, functional explanations have been sought for the
empty spaces (i.e., the enclosed volume of air) within the bony
sinuses. However, a new perspective, focusing instead on the
epithelial lining of the recesses, is much more promising. Under
this hypothesis, pneumatic sinuses are seen as essentially without
a positive function (unless natural selection secondarily co-
-opts a sinus for some novel role). Rather, sinuses result from
the intrinsic capacity of the air sacs to expand and pneumatize
bone in an invasive and opportunistic fashion until constrained
by the biomechanical requirements for maintaining sufficiently
strong structures. Thus, a "struggle" may be envisioned between
the conflicting tendencies for expansive pneumatization
on the one hand and mechanically mediated bone deposition on
the other. This struggle secondarily and automatically provides
a mechanism for producing "optimally designed" structures of
maximal strength and requiring minimal materials without the
necessity of invoking active natural selection for these attributes.
This new hypothesis is borne out in the evolution of the facial