by tube bones independent of the skull roof Character5.Medianextrascapularbonepresent–
(Grande & Bemis 1991). The infraorbital canal of
†Peipiaosteus (Zhou 1992) is similarly borne by
tube bones rather than a dominant series of ele-
ments and is not comparable with acipenserids. An
anterior process of the jugal has been reconstructed
for †Chondrosteus(e.g., Traquair 1887, Gardiner &
Schaeffer 1989), but it apparently bears the infraor-
bital canal unlike acipenserids.
AcipenseridaeThe median extrascapular (excm) is a triangular
plate present between the posterior ends of the pa-
rietals (pa) and anterior to the first dorsal scute
(ds1,Figure5). It is present in all acipenserids exam-
ined, and bears the commissure of the occipital ca-
nals (see Character 24 below).
The median extrascapular develops extremely
late, first appearing as splints bracketing the occip-
ital canal before expanding to cover the posterior
skull. Several bones may compose the median ex-
trascapular initially, but the ossification is single in
all adult specimens examined. Expansion beyond
the occipital canals and consistent shape suggests
that the median extrascapular has developed phylo-
genetically from an anamestic bone into a perma-
nent bone of the skull.
No similar median element in the posterior skull
roof is known from any other acipenseriform.
Grande & Bemis (1991) provisionally identified aCharacter4.Antorbitalbonepresent- Acipenseridae
The antorbital defines the anterodorsal corner of
the orbit (Findeis 1993) and is present in all acipen-
serids examined. It lies dorsally between the orbit
and olfactory capsule as a small plate that angles
ventrolaterally onto the postnasal wall as ventral
points (Figure4).
The antorbital varies in size and shape, but not
position or presence, in Acipenseridae. Its ventral
points are short or absent in Huso(Figure 4a) and
variably extended in most species of Acipenser
(Figure 4b, c), but antorbitals of scaphirhynchines
(Figure 4d, e) possess elongate ventral processes
(see Character 46). Antorbitals rest alongside the
nasal, but never bear the supraorbital canal.Aswith
the anterior process of the jugal (Character 3), pres-
ence and ventrolateral extension of the antorbital is
consistent with the expanded neurocranium as the
skull is covered locally by appearance of a new
bone.
The antorbital is not present in polyodontids
(Grande & Bemis 1991), and has not been described
in †Chondrosteus(Traquair 1887, Hennig 1925).
Zhou (1992) names an antorbital in †Peipiaosteus,
but it is a tube bone carrying the supraorbital canal
not comparable to acipenserids. The only position-
ally comparable bone in † Mimia(Gardiner 1984a)
and other palaeoniscids is the nasal, but the nasal
ubiquitously bears the supraorbital canal.
Figure6.The rostral canal and rostrum ofHuso huso:The rostral
canalextendsonto theventralsurface of therostrum from the
jugal(see Character3)to converge mediallybeforearching lat-
erallyaround theouterbarbels(Character6).Aftercurling
around the outerbarbels, the rostrumcanalsextend anteriorlyin
parallel beforeconverging at therostraltip.