indications from Gardiner & Schaeffer 1989) are 1991) support the caudal fin, leaving a gap in the pe-
apparently also straight. †Peipiaosteuspossesses a duncle.
rostral canal that extends anteriorly onto thesnout Polyodontids (Grande & Bemis 1991) and
and bends dorsally to terminate in a cluster of bones †Chondrosteus (reconstruction from Woodward
(Zhou 1992). In †Mimia(Gardiner 1984a) andPol- 1889) possess supraneurals throughout the axial
ypterus, the rostral canal is straight and no other skeleton. †Peipiaosteusapparently lacks supraneu-
outgroups possess an abrupt lateral arch. rals in the peduncle (Zhou 1992), but †Peipiaosteus
is small and supraneurals are difficult to identify
and may have never ossified. Among other out-
groups, distribution of supraneurals is variable.Pol-
ypterusand †Pteronisculus(and several palaeonis-
cids; Gardiner 1984a) possess a complete series of
supraneurals, but †Mimia (Gardiner 1984a), Lep-
isosteus, Amia,and neopterygians typically possess
incomplete series. Inconsistency among outgroups
clouds analysis of this character, but a complete su-
praneural series in polyodontids suggests that a pe-
duncle gap is characteristic of Acipenseridae. This
correlates with the flattened peduncle of acipense-
rids that presumably puts spatial constraints on the
vertically oriented supraneurals unlike other aci-
penseriform taxa.
Character 7. The opercular series includes a serial
pair of branchiostegals –Acipenseridae
The dominant opercular bone of acipenseroids is
the subopercle, but acipenserids typically possess
two branchiostegals that extend ventrally and then
medially in support of the operculum (Figure 7).
Branchiostegal one (bsg1) forms an elongate verti-
tal support before branchiostegal two (bsg2) angles
medially to undercut the head. Branchiostegal one
is rectangular and variably elongate in Husoand all
species of Acipenserexamined (Figure 7c, d). and
Pseudoscaphirhynchus(Figure 7f), but is stubby
and triangular in Scaphirhynchus(Figure 7e; Char-
acter 46). Occasionally three branchiostegals are
present, but the third was diminutive and did not
significantly enlarge the opercular series when pre-
sent.
Polyodontids possess a single branchiostegal sep-
arate from the subopercle (Findeis 1993, Bemis et
al. 1997). The opercular series of † Chondrosteus
(Traquair 1887, Hennig 1925) and † Peipiaosteus
(Liu & Zhou 1965, Zhou 1992) include multiple, se-
rial branchiostegals not vertically elongate (Figure
7a, b). This condition is plesiomorphically similar to
most palaeoniscids and suggests that reduction to
two branchiostegals is diagnostic of Acipenseridae.Character 9. Supracleithral shelf undercuts the post-
temporal—AcipenseridaeThe supracleithrum (scl) expands anteriorly as a
broad shelf undercutting the posttemporal (pt)in
all acipenserids examined (Figure 8a, b). This shelf
broadly contacts the underside of the posttemporal
and the bones are bound tightly to unify the pecto-
ral girdle and skull roof. This shelf does not bear the
trunk canal that passes from the posttemporal to
the supracleithrum at their surface suture (arrows
in Figure 8b).
Extant polyodontids possess a mobile supra-
cleithral-posttemporal articulation (Grande & Be-
Character 8. Loss of supraneurals in the caudal pe- mis 1991).Polyodon, Psephurus,and tCrossopholis
duncle-Acipenseridae all show slight supracleithral undercutting of the
posttemporal with a short portion of the supra-
Supraneurals of all acipenserids examined are seri- cleithrum extending anterior to the exit of the trunk
ally present above neural arches in the thoracic canal. However, length of this process is not equiv-
trunk between the neurocraniumand dorsal fin, but alent to acipenserids. The supracleithrum of
absent beneath the dorsal fin and in the caudal pe- †Chondrosteus (Traquair 1887) broadly contacts
duncle. Supraneural homologs (Grande & Bemis the posttemporal. but without an anterior process.