51813_Sturgeon biodioversity an.PDF

Chavacter 66. Cleithralwallpresent

• Scaphirhynchus cardiacshield.

The cleithral wall of the scapulocoracoid is a trian-
gular sheet that spreads over the cleithrum in Sca-
phirhynchus(ctw,Figure 9b). It spans the anterior
process of the middle region (anp),the dorsal tip of
the mesocoracoid arch (msc),and the anterior edge
of the propterygial bridge (ptb,Figure 9b).
The scapulocoracoid of all other acipenserids ex-
amined possesses a thin cleithral arch (cta)extend-
ing dorsally, but not expanded medially (Figure 9a).
The anterior edge of the middle region isisolated as
a horizontal spar extending medially from base of
the cleithral arch (cta)to the anterior process (anp,
Figure 9a). Thus, the anterior face of the scapulo-
coracoid of outgroups is L-shaped, with a vertical
cleithral arm and horizontally extended anterior
process.

converging posteriorly and symmetrically on the

The coracoid shelf of Pseudoscaphirhynchus,

Acipenser,andHusospreads anteromedially (Fig-

ure 9a) onto the clavicle as an elongate sheet, but

without lateral expansion (see Character 14). The

scapulocoracoid of polyodontids extends similarly

as a thick cylindrical process. This matches the ante-

riorly extended pectoral girdles of the outgroups

compared to the compact pectoral girdle of Sca-

phirhynchus.Thecoracoid shelf corresponds to the

dermal girdle and implicitly includes a pyramidal

scapulocoracoid shape as a character of Scaphi-

rhynchus.

Character 69.Dermopalatine andectopterygoid

bonesfuse-Scaphirhynchus

The dermopalatine and ectopterygoid develop as

separate bones that fuse in adult Scaphirhynchus

(Findeis 1991,1993). All adult specimens of S.plato-

rynchusexamined exhibited complete fusion, while

all small juveniles retain separate elements, sug-

gesting that fusion occurs in moderately sized juve-

niles.

In†Paleopsephurus(Grande&Bemis 1991), an

elongate process positionally similar to the acipen-

serid ectopterygoid is probably an independent

bone (Figure 22a), but the short process ofPsephu-

rusandPolyodonsuggests that the ectopterygoid is

lost in these genera. In Huso,Acipenser,andPseu-

doscaphirhynchus,the ectopterygoid is independ-

ent and closely contacts the dermopalatine (Figure

22b, c), but even in large specimens of A.bvevivos-

trumandA.oxyrinchusthere was never fusion be-

tween these bones. A small ectopterygoid process is

present in †Peipiaosteus(Zhou 1992), but it is un-

clear if it isa separate bone or fused. †Chondrosteus

has an independent ectopterygoid (=pterygoid of

Traquair1887).

Character 67.Athin processencircles theproptery-

gialfossa-Scaphirhynchus

The scapulocoracoid of Scaphirhynchuspossesses a

thin process extending under the propterygium re-

straining process of the cleithrum (prp,Figure8a).

This process curls anteroventrally as a half-circle

around the propterygial fossa (ptgp,Figure 9b).

This process isnot present in any other acipense-

rid (Figure 9a) and the propterygial fossa is open

primitively anteriorly against the cleithrum.

Character 68.Coracoidshelfrestricted to thecleith-

rum-Scaphirhynchus

The coracoid shelf (csh)ofthe scapulocoracoid in

Scaphirhynchus spreads from the coracoid wall

(cw)medially and laterally onto the cleithrum (ar-

rows in Figure 9b). The largest portion spreads an-

terolaterally into the angle between the opercular

wall and cardiac shields (Findeis 1993). The medial

side slightly overlaps the clavicle, but ends in a small

footprint. In ventral view, the coracoid shelf is

roughly triangular, with the broad anterior edge

Genericlevel phylogenyofAcipenseridae

The character distribution from this analysis sup-

ports a new cladogram of genera of Acipenseridae