51813_Sturgeon biodioversity an.PDF

Character49.Dorsal head of the hyomandibula is

cylindrical-Scaphirhynchini

The dorsal head of the hyomandibula of scaphi-

rhynchines is cylindrical and ends in a small ellipti-

cal tip (Figure 11d) that articulates with the neuro-

cranium as a loose joint.

Psephurus(Grande & Bemis 1991), Huso,andall

species ofAcipenserexamined possess hyomandib-

ulae with the dorsal head expanded frontally (Fig-

ure 11c). In extant polyodontids, the dorsal head is

rectangular. InHusoandAcipenser,the head is el-

liptical with an expanded bony shaft at its base (Fig-

ure 11c). The frontally expanded hyomandibula

forms a restrictive joint allowing pivoting in only

(Figure 4a). All species ofAcipenserexamined pos-
sess a ventral point of varying size (Figure 4b, c; see
Character 4), but none are as elongate as in scaphir-
hynchines.

Character47.Subopercle is distinctively shaped -

Scaphirhynchini

The subopercle is distinctive in scaphirhynchines by

being elongate with a prominent anterior process,

flattened dorsal edge, and abrupt anteroventral

notch that angles ventrally (Figure 7e, f). This shape

is typical ofScaphirhynchus, Pseudoscaphirhyn-

chusand also†Protoscaphirhynchus(Wilimovsky

1956). one plane.

Subopercle shape varies within Acipenseridae.

Husoand several species of Acipenser(e.g.,A.oxy-

rinchus,A.ruthenus)possess a triangular suboper-

cle (Figure 7c), while other species (e.g.,A.brevi-

rostrum,A.gueldenstaedtii)possess circular plates

with blunt anterior and ventral processes (Figure

7d).Nospecies ofAcipenserexamined possesses a

subopercle as dorsoventrally compressed as sca-

phirhynchines. The scaphirhynchine subopercle

correlates with the flattened head limiting space for

the operculum. As a shape character, it is difficult to

segregate subopercle areas as characters, but over-

all bone shape is unique.

Character50. Ventral edge of the hyomandibula is

flat-Scaphirhynchini

The ventral head of the hyomandibula of scaphi-

rhynchines ends in a flat edge roughly perpendic-

ular to the long axis of the element (Figure 11d). The

interhyal joint opens on the anteroventral face, but

is not isolated from the ventral edge.

The hyomandibula ofPsephurus(Grande & Be-

mis 1991),Huso,and all species ofAcipenserexam-

ined taper ventrally into narrow anteroventral tips

(Figure 11a, b, c) that articulate with the interhyal.

Ventral flattening of the hyomandibula of scaphi-

rhynchines correlates with overall flattening of the

head and consequent restricted dorsoventral space

for the hyoid skeleton.

Character48. Lossof prearticular bones-Scaphi-

rhynchini

Prearticular bones are generally present on the oral

surface of Meckel’s cartilage (see Character 35),

but are absent in scaphirhynchines.

Prearticular bones are present inPsephurus(Fig-

ure 21a) and†Crossopholis(Grande & Bemis 1991),

†Chondrosteus(Traquair 1887, Watson 1925),Huso

(Figure 21b), and all species ofAcipenserexamined

(Figure 21c). Prearticulars are unknown in †Paleo-

psephurusand†Peipiaosteus(Liu & Zhou 1965),

but otherwise ubiquitous distribution of the bone in

outgroups suggests it is plesiomorphically present.

Character 51.Hypobranchial one contacts hypo-

branchial two -Scaphirhynchini

In scaphirhynchines, the posterior edge of hypo-

branchial one forms a wedge contacting hypobran-

chialtwo(Figure 13). Contact is slight, but liga-

ments consolidate the elements as a unit.

InPsephurus(Grande & Bemis 1991), hypobran-

chials are roughly cylindrical, without expansion of

the oral surface (Figure 13a).Husoand all species of

Acipenserexamined possess rectangular first hypo-