51813_Sturgeon biodioversity an.PDF

used for prey processing opposite to the palatal consistent. Scaphirhynchines lack these scales
complex (Character 18). (Character 54).
Hypobranchial one of Psephurus and †Cross- Pectoral scales of fossil and extant polyodontids
opholisis cylindrical with a flattened dorsal surface (Grande & Bemis 1991) are small, round elements
bearing teeth (Figure 13a), but with no expanded with a single, recurved process (Figure 14a). †Pei-
shelf (Grande & Bemis 1991).Polyodonpossesses a piaosteuspossesses pectoral scales with three pro-
small hypobranchial one not comparable to other cesses extending from a narrow tip (Liu & Zhou
acipenseriform taxa. 1965), but processes of these scales fan directly from
Hypobranchials are typically cartilaginous (al- the base and are not elevated, recurved tips that
though a small ossification occurs in large acipense- overhang the scale (Figure 14b). Such scales have
rids and †Crossopholis) and not known from not been described in †Chondrosteus.Individual
†Chondrosteusor †Peipiaosteus. Anterior shelves sections of acipenserid pectoral scales (with a single
are not present in †Mimia(Gardiner 1984a),Pol- tip) are more robust than, but similar to, the pecto-
ypterus, Lepisosteus,orAmia. ral scales of polyodontids.

Character 22.Hypobranchial three makesabicon-

tact joint with basibranchial one-Acipenseridae

Hypobranchial three articulates with basibranchial
one at two sites in all acipenserids examined. The
anterior end is crescentic, with a short dorsal pro-
cess articulating with the posterior tip of basibran-
chial one and a ventral prong that curls anteroven-
trally under basibranchial one (Figure 13c, inset).
Hypobranchial three ofPsephurusand Polyodon
is slender and makes single contact with the basi-
branchial (Grande & Bemis 1991). Hypobranchial
three of †Crossopholis(when preserved) is appar-
ently similar(Grande& Bemis 1991), with a narrow
medial end suggesting a slender tip. It is cartilagi-
nous in adults and not known from †Chondrosteus
or†Peipiasteus.Hypobranchial three of †Mimia
(Gardiner 1984a) and Polypterus possess single
joints with the basibranchial.

Character 24. Commissure of the occipital canals -

Acipenseridae

In all acipenserids examined, the contralateral oc-

cipital canals enter the median extrascapular and

merge as a short common canal extending anterior-

ly (Figure 5).

Occipital canals of Polydon, Psephurus, and

†Crossopholisare borne by tube bones that expand

inadults but do not meet at the midline (Grande &

Bemis 1991). Extrascapulars, and passage of lateral

line canals, of †Paleopsephurusare unknown. A

complete row of extrascapulars is present in †Chon-

drosteus acipenseroides(Traquair 1887), but not in

†C.hindenburgi(Hennig 1925). These extrascapu-

lars are small (Figure 5a), and the commissure

might be variable, but a persistent canal has not

been described in these bones. † Peipiaosteuslacks

extrascapulars in described specimens (Liu & Zhou

1965, Zhou 1992).

Character 23. Pectoral scales are elongate with mul-

tiple toothlike extentsions–Acipenseridae

Pectoral scales studding the inner surface of the

opercular chamber are elongate elements with mul-

tiple recurved tips (Figure 14c, d). Typically, three to

five recurved tips overhang opposing depressions

within the scale. Morphology of these scales varies

inAcipenserandHuso,although the basic pattern is

Character 25. Basitrabecular processes form flat-

tened shelves-Huso

Basitrabecular processes of Huso are elongate

shelves that flare laterally under the orbit (Figure

15a). They are flat ventrally, extending flush from

the base of the neurocranium with little or no ven-

trolateral curvature. The groove carrying the pala-

tine ramus of the facial nerve and segregating the