scapulocoracoid with the cardiac shield supporting
the expanded coracoid shelf (see Character 14).
The clavicle and cleithrum of polyodontids
(Grande & Bemis 1991), †Chondrosteus(Traquair
Character 10. Presence of a medial opercular wall— 1887), and †Peipiaosteus(Liu & Zhou 1965) are
Acipenseridae slender bones lacking shelves. The clavicles ofPse-
phurus and † Crossopholis possess short posterior
The medial opercular wall is a vertical shelf defining wedges smaller than the cardiac shield, but they do
the internal surface of the opercular chamber not flare posteromedially or meet medially
(Findeis 1993). It comprises expanded laminae (Grande & Bemis 1991). No similar cardiac shields
curving dorsally from the cleithrum and clavicle arepresent in Polypterus, Amia, or other distant
(clt, clv,Figure 8a). The medial opercular wall is outgroups.
present in all acipenserids examined. It is not as ex-
pansive inHusoas in acipenserines, but the corre-
sponding opercular chamber is more slender in
Huso. ing process—Acipenseridae
Polyodontids possess slender pectoral girdle
bones with laterally exposed faces that never angle
internally or form expanded laminae (Grande &
Bemis 1991). Clavicles and cleithra of †Chondros-
teus (Traquair 1887) and †Peipiaosteus(Liu & Zhou
1965) arc also slender and lack internal expansion.
No expanded opercular wall is present in Polypte-
rus, Lepisosteus, Amia, or †Mimia (Gardiner thefin in all acipenserids examined.
1984a) and other palaeoniscids.
†Peipiaosteus lacks tight contact between these
bones (Zhou 1992).
Character 12. Presence of a propterygium restrain-The propterygium restraining process extends from
the lateral edge of the cleithrum (prp, Figure 8a, c)
to wrap anterolateral to the propterygial fossa of
the scapulocoracoid (see Character 16) and its artic-
ulating propterygium. It brackets the pectoral fin
spine anteriorly to support and limit movement ofThe propterygium restraining process is a promi-
nent, posteriorly curved process in Scaphirhynchus
and extended ridge in Pseudoscaphirhynchus,but is
Character II. Presence of a cardiac shield less prominent in HusoandAcipenser(Figure 8a).
—Acipenseridae It is well developed in juveniles, but is allometrical-
ly reduced in adults as the cleithrum thickens with
The cardiac shield is a ventral shelf formed by ex- dermal ornament obscuring the shallow ridge.
pansion of the clavicle and cleithrum. The shield ex- When a distinct process is absent, this region is en-
pands posteromedially as a flat, exposedplate that larged as a robust cleithral notch enfolding the base
meets its contralateral partner at the midline (Fig- of the fin spine. This character tacitly includes such
ure 8c). The cardiac shield covers the pericardial a cleithral notch that acts as a pectoral fin spine con-
cavity and is found in all acipenserids examined. straint when the process is less obvious.
The cardiac shield is almost completely flat in No similar process is present in any other acipen-
Scaphirhynchus(Findeis 1993), but angles centrally seriform group. Outgroups possess no pectoral fin
through the anteroposterior axis in other genera, spine (see Character 2) or, among extant polyodon-
with a frontally flat medial region and dorsally an- tids, a propterygial fossa (Character 16) allowing
gled lateral face (Figure 8c). This central angle is frontal pivoting of the propterygium. the † Crossopho-
acute (up to 20°) in species with cylindrical body lis possesses a shallow notch in the cleithrum
shapes (all species of Acipenserexamined and aligned with the fin (Grande & Bemis 1991), but it is
Huso),but shallow in the flatterPseudoscaphirhyn- an isolated condition within Polyodontidae.
chus. The cardiac shield and opercular wall of aci-
penserids meet in a sharply angled anteroventral
edge (approximately 160°). This angle brackets the