Character 13. Clavicle process interdigitates with the Character 15.SupracleithraI cartilage present–Aci-
cleithrum-Acipenseridae penseridae
The clavicle process is a small wedge that interdig-
itates with the cleithrum in all acipenserids exam-
ined(cvp, Figure 8a, c). The clavicle process ex-
tends along the anteroventral edge of the pectoral
girdle inScaphirhynchus(Findeis 1993), but typical-
lyundercuts the clavicle slightly in other acipense-
rids (Figure 8 forAcipenser).The process interlocks
with a opposing groove in the cleithrum to solidify
the pectoral girdle.
No clavicle process is present in extant polyodon-
lids or †Crossopholis(Grande & Bemis 1991). This
character is a small feature not easily noted from
illustrations or descriptions of † Chondrosteus(Tra-
quair 1887, Hennig 1925) or †Peipiaosteus(Liu &
Zhou 1965), but their slender pectoral girdles are
not suggestive of such an interlocking process.
The supracleithral cartilage lies under the supra-
cleithrum (Figure 8b). It varies in shape among aci-
penserids from an elliptical, tall cartilage in some
species of Acipenserto an elongate, shallow ele-
ment inHusoandScaphirhynchus.
The supracleithral cartilage is absent in extant
polyodontids, polypterids, lepisosteids, and amiids.
It never ossifies and is unknown from any fossil aci-
penseriform. Lacking † Chondrosteusor † Peipiao-
steusas outgroups clouds cladistic analysis of this
character within Acipenseriformes, but its absence
in extant polyodontids suggests it is restricted to
Acipenseridae.Character 16. Propterygial fossa present in scapulo -
coracoid-AcipenseridaeThe propterygial fossa is a broad, semicircular
opening in the scapulocoracoid between the gle-
noid ridge and the propterygium restraining spine
of the cleithrum (ptgf, Figure 9; see Character 12). It
is bounded posteriorly by the propterygial bridge
(ptb) and dorsally by the vertical cleithral arch (cta,
Figure 9a) or cleithral wall (ctw, Figure 9b; see
Character 66). The propterygial fossa opens a space
for the rotating propterygium that articulates with
the propterygial bridge (see Character 2) and is fil-
led with pectoral musculature that pulls the pecto-
ral fin spine forward. The propterygial fossa is a
rounded notch (Figure 9a) in all acipenserids ex-
ceptScaphirhynchus,where it forms a half circle
(Figure 9b; see Character 67).
The glenoid ridge extends completely to the
cleithrum in extant polyodontids. The propterygi-
umof these taxa is small and can articulate with the
lenoid ridge without need for an anterior fossa. Al-
though this character cannot be confirmed in fossil
taxa, lack of a cleithral notch (see Character 12) in
†Chondrosteus (Traquair 1887, Hennig 1925) or
†Peipiaosteus(Liu & Zhou 1965) suggests that there
would be no opposing opening in the scapulocora-
coid. No propterygial fossa is present in † Mimia
(Gardiner 1984a) and other palaeoniscids.Character14.Coracoidshelfspreadsover the car-
diacshield-Acipenseridae
The coracoid wall (cw) extends ventrally from the
middle region of the scapulocoracoid to spread on-
to the cardiac shield as a flat coracoid shelf (csh, Fig-
ure 9; Findeis 1993). The coracoid shelf makes im-
mediate contact with the cleithrum and spreads an-
teromedially almost to the tip of the clavicle in all
acipenserids (Figure 9a) exceptScaphirhynchus.In
Scaphirhynchus,the coracoid shelf spreads laterally
and is restricted to the cleithrum (Figure 9b; see
Character 68).
In extant polyodolitids, the coracoid wall is cy-
lindrical and extends anteromedially separate from
the cleithrum before curving anteroventrally onto
the clavicle. Shape of this coracoid process corre-
sponds to the slender dermal girdle. The scapulo-
coracoid is largely cartilaginous and not scorable in
fossil taxa, but shape of the dermal girdle of †Cross-
opholis and †Paleopsephurus (Grande & Bemis
1991), †Chondrosteus(Traquair 1887), and †Pei-
piaosteus(Liu & Zhou 1965) is similar to extant po-
lyodontids, suggesting that a correspondingly cylin-
drical coracoid process would be consistent in out-
group acipenseriforms.
g