face of the neurocranium without a cartilage cover
(Figure 17a, b). These taxa also lack an ethmoid-or-
bital angle as the ventral neurocranium curves
smoothly into the orbit inHuso(Figure 20b),Pse-
phurus, and †Paleopsephurus (Grande & Bemis
1991), or is straight in the derived polyodontidsPo-
lyodon(Figure 20a) and † Crossopholis.The central
trabecular process correlates phylogenetically with
ventral displacement of the jaws of acipenserines
and possibly deflects them ventrally during projec-present inHuso(Figure 18b). Stellate bones sup-
porting the lateral rostrum in polyodontids are pre-
sent lateral to the endochondral rostrum only with-
in the skin of the paddle (Grande & Bemis 1991).
Border rostral bones lie directly on the rostrum
posterolaterally, while polyodontid stellate bones
are isolated anterolaterally.
Character 30.Firstventral rostral bone in single-
Acipenserinae tion.
The first ventral rostral bone of acipenserines (vrb1,
Figure 19c,d,e) is a single bone that contacts the
parasphenoid as the most posterior bone of the ven-
tral rostral series (Findeis 1993). It is only modestly
larger than other ventral rostral bones in all species
ofAcipenserexamined, but is elongate in scaphi-
rhynchines (Character42).
InHusoand all polyodontids (Figure 19a, b), the
ventral rostral series possesses paired bones con-
tacting the parasphenoid. These bones were named
vomers by Grande & Bemis (1991), but they devel-
op on the rostrum well anterior to the parasphenoid
and are not present in † Chondrosteus (Traquair
1887, Hennig 1925) or † Peipiaosteus(Liu & Zhou
1965), suggesting that they are novel bones of the
rostrum and rostral series of acipenseroids.Character 32. Pineal bones present-Acipenserinae
Pineal bones develop within the pineal fontanelle
between the frontals (pf, Figure 18b, c) in juveniles
and typically expand to fill the space in adults (pb,
Figure 18c, d). Number of pineal bones varies, but
they are consistently present in all acipenserines ex-
amined.
Pineal bones are anamestic bones that are devel-
opmentally constrained by bones surrounding the
pineal fontanelle. Pineal bones fill the pineal fonta-
nelle completely inScaphirhynchusand mostspe-
cies ofAcipenser,butA. oxyrinchusandA. sturio
are often recognized by an open fontanelle (Figure
18c; Vladykov & Greeley 1963, Magnin 1963, Mag-
nin & Beaulieau 1963). However, pineal bones ap-
pear even in these species, but do not enlarge to fill
Character 31. Central trabecular process present- the fontanelle except in very large adults.
Acipenserinae The pineal fontanelle occasionally closesbyex-
pansion of the frontals. Pseudoscaphirhynchus
The central trabecular process is a ventral cartilagi- kaufmanni shows both morphologies, with the
nous wedge between the ethmoid and orbital re- frontals separated by pineal bones (illustrated by
gions of the neurocranium in all acipenserines ex- Sewertzoff 1926a) or also exhibiting frontal closure
amined (ctp, Figure 20c, d). Thisprocess protrudes posteriorly. Specimens of Acipenser mediostris and
posterior to the trabecular ridge (tr, Figure 17) be- A. oxyrinchusalso partially close the pineal fonta-
fore rising sharply under the orbit as the ethmoid- nelle by frontal expansion. While pineal bones ap-
orbital angle (Findeis 1993) to open a ventral space
for the jaws (Figure 20c, d). The central trabecular
process covers the posterior end of the first ventral
rostral bone and anterior extension of the para-
sphenoid (shown in Figure 17c, d).
InHusoand extant polyodontids (Grande & Be-
mis 1991), the paired first ventral rostral bones (see
Character 30) contact the parasphenoid on the sur-pear universally in small juveniles, frontal expan-
sion occurs only in adults and all specimens showing
frontal expansion also possess pineal bones.
No pineal bones are present inHusoor any po-
lyodontid (Grande & Bemis 1991). The pinealfon-
tanelle of these taxa (when present) is narrow be-
tween frontals (Figure 18a, b).Thefontanelle is re-
duced or absent in large juveniles ofHuso,but no