51813_Sturgeon biodioversity an.PDF

Character21.Anterior shelf protrudes from hypo-

branchial one –Acipenseridae

An anterodorsal shelf expands from hypobranchial

one(hbl)as a wedge overhanging the anterior cer-

atohyal and hypohyal (hh)in all acipenserids exam-

ined (arrow in Figure 13b, c). The shelf is concave

medially, opening a gap where the hypohyal ex-

tends dorsally to articulate with basibranchial one

(Figure 13b, c). The anterior shelf, in concert with

the exposed hypohyal and anterior basibranchial,

forms a flat anterior edge of the ventral branchial

skeleton. This edge bears connective tissue ridges

quair 1887), † Gyrosteus(Woodward l889), and
†Peipiaosteus(Liu & Zhou 1965) all expand ven-
trally as in all other acipenseriforms, but since this
cartilaginous character is unpreserved in fossils
they do not assist in assigning polarity. More distant
outgroups lack the hyostylic jaw suspension of aci-
penseriforms and are useless in assessing this char-
acter. I accept Psephurusas representing the plesio-
morphiccondition.

Character 20. Anterior ceratohyalis flat and rectan-

gular, with a mandibulo-hyoid process and asym-

metric hypohyal joint -Acipenseridae

Anterior ceratohyals of all acipenserids examined

are dorsoventrally flattened and rectangular in oral

view (Figure 12). The lateral head is centrally

grooved with a rising posterior ridge articulating

with the posterior ceratohyal and anterolateral pro-

cess that is the origin of the mandibulo-hyoidliga-

ment (arrows, Figure 12c, d, e). The medial end is

blunt, with an asymmetric, anteriorly displaced fac-

et for the hypohyal (hh, displaced arrows of Figure

12c, d, e).

Anterior ceratohyals of Psephurusarecylindri-

cal,hourglass-shaped elements lacking distinct pro-

cesses and possessing a central hypohyal facet on

the medial head (Figure 12a). The lateral end lacks

a distinct mandibulo-hyoid ligament process or cen-

tral groove and the posterolateral head is a blunt tip

articulating with the posterior ceratohyal. The ante-

rior ceratohyal of † Paleopsephurusis unknown, but

that of † Crossopholisis also cylindrical based on

shape of its ossified center (Grande & Bemis 1991).

Anterior ceratohyals of Polyodonare flattened and

not comparable to other taxa (Grande & Bemis

1991). Cylindrical morphologies are inferred for

†Chondrosteus(Traquair 1887) and † Gyrosteus

(Figure 12b; Woodward 1889) based on their cylin-

drical core. The hyobranchial skeleton of † Pei-

piaosteusis unpreserved (Liu & Zhou 1965).

Figure12.Anterior ceratohyal ofPsephurus,†Gyrosteus,and

representatives of three genera of Acipenseridae: The anterior

ceratohyal is narrow and cylindrical in acipenserid outgroups (a,

b; †Gyrosteusredrawn from Woodward 1889), but expanded

centrally as a flattened rectangular element (thin arrows in c, d,

e; Character20).The acipenserid anterior ceratohyal also pos-

sesses an anterolateral process supporting the mandibulo-hyoid

ligament (heavy arrows in c, d, e) and an asymmetric facet for the

hypohyal on the medial tip (hhand bent arrowsinc, d, e).