51813_Sturgeon biodioversity an.PDF

Character6.The rostralcanal archeslaterally–Aci-

penseridae

Rostral sensory canals extend ventromedially un-

der the rostrum, but then curl laterally around the

outer barbels before recurving to parallel the ros-

tral midline (Figure 6). The rostral canal develops

with this lateral arch originally and is not displaced

by the developingbarbels.

In extant polyodontids the rostral canals con-

verge smoothlytoparallel the midline to the ante-

rior tip of the rostrum without disruption. Incom-

plete remainsofrostraltubebones in†Crossopho-

lisare consistent with this condition(Grande &Be-

mis 1991), but are missing in†Paleopsephurus.

Rostralcanals of†Chondrosteus(Hennig1925, and

‘median nuchal’ in†Paleopsephurus,but the speci-
menistoo poorly preserved to identify an autono-
mouselement or note whether posttemporal exten-
sions typicalofpolyodontids expand over this area.
All other polyodontids conclusively lack a median
extrascapular.†Chondrosteus(Traquair 1887) pos-
sesses multiple small bones that cross the posterior
midlineofthe skull roof (Figure 5a), but they are
small bones more comparable to lateral extrascap-
ulars.†Peipiaosteus(Liu&Zhou 1965, Zhou1992)
isnot well described in this area, but no bone sep-
arates the parietals.

Figure8.Dermal pectoral girdle ofAcipenserbrevirostrum:a – Anterolateral view of an intact pectoral girdle showing the medial

opercular wall (Character 10) rising vertically from the cleithrum (clt) and clavicle (clv), the clavicle process (cvp) binding the cleithrum

and clavicle (Character13),the propterygial restraining process (prp; Character 12), and the close contact between the supracleithrum

(scl) and posttemporal (pt). b – Dorsal (left) and ventral (right) views of the supracleithrum. The anterior shelf of the supracleithrum

(Character 9) extends as a flat, unornamented area anterior to the trunk canal (arrows). The supracleithral cartilage (sclc; Character 15) is

bound to the ventral surface of the supracleithrum. c – Ventral view of the pectoral girdle showing the expanded clavicle and cleithrum

forming the cardiac shield (Character 11) and the extension of the propterygial restraining process (Character 12) laterally from the

cleithrum.