Character 3. Body scaling reduced to tiny isolated
elements or absent
This character was defined by Patterson (1982),
noted by Gardiner & Schaeffer (1989) and accepted
by Grande & Bemis (1991, character 5). The scales
of †Cheirolepisare not typical for stem actinoptery-
gians in that they are small tubercles, but they are
not isolated from each other as in our ingroup taxa.
Acipenseriformes differ from all of the outgroups
we considered except †Birgeria, which has reduced
scalation that Nielsen (1949) regarded as very simi-
lar to that of Polyodon.
Discussion of clade †Birgeria +Acipenseriformes
Like Berg (1948b) and Yakovlev (1977) and in con-
trast to Aldinger (1937) and Nielsen (1949), our
analysis does not support a sister group relationship
between †Birgeria and paddlefishes. However,
†Birgeriashares at least three features with Acipen-
seriformes. These three characters warrant addi-
tional specimen based study in †Birgeriaand other
outgroups not considered here.
Characters of AcipenseriformesWe report eight osteological synapomorphies of
Acipenseriformes keyed to the cladogram in Figure
17.Character 4. Palatoquadrates with an anterior sym-
physis
Woodward (1891) and many others have noted this
feature, but it was more explicitly defined by Pat-
terson (1982). Gardiner & Schaeffer (1989) and
Grande & Bemis (1991, character 1) regarded this
feature as an acipenseriform synapomorphy. The
symphysis is a flexible cartilaginous connection in
all recent Acipenseriformes, and even though the
cartilage is not preserved, the configuration of the
upper jaw suggests its presence in †Chondrosteus
and †Peipiaosteus. (More study of this character in
†Peipiaosteusis needed, however.) The palatoqua-
drate symphysis appears to be absent in †Cheirole-
pis,Polypterus,†Mimia,†Birgeria,Amia, and other
actinopterygians. Gardiner & Schaeffer (1989) in-
cluded as part of their character definition that the
palatoquadrates of Acipenseriformes ‘do not artic-
ulate with the neurocranium’, but we note that the
palatoquadrate is guided by the cartilaginous basi-
trabecular processes during projection, so their ad-
ditional qualification is unclear (see discussion un-
der ‘Putative and problematic characters of Aci-
penseriformes’, below).Character 5. Palatoquadrate with broad autopala-
tine portion, palatoquadrate bridge, and quadrateSchaeffer (1973) and Patterson (1982 ) considered
palatoquadrate shape to be an acipenseriform char-
acter, but did not specify which aspects are synapo-
morphic. An illustration of the palatoquadrate of a
large adultAcipenser brevirostrumis shown in Fig-
ure 18. All acipenseriform palatoquadrates have a
broad autopalatine plate (Figure 18a), a narrow pa-
latoquadrate bridge (pqb), and a quadrate flange
(qf). In comparison, †Cheirolepis,Polypterus,†Mi-
mia, andAmia possess roughly triangular palato-
quadrates with thin anterior tips that extend to con-
tact the ethmoid region of the neurocranium.Character 6. Presence of a triradiate quadratojugal
bone
This character was defined by Patterson (1982) and
accepted by Grande & Bemis (1991, character 6).
The condition of the quadratojugal is unknown in
†Birgeria(Nielsen 1949). Figure 18 shows the quad-
ratojugal ofAcipenser brevirostrum. An example of
the triradiate quadratojugal in a polyodontid is il-
lustrated in Grande & Bemis (1991, fig. 35A,C).Character 7. Gill-arch dentition confined to first two
hypobranchials and upperpart of first arch
This character was discussed by Nelson (1969, p.
257) defined by Patterson (1982), noted by Gardin-
er & Schaeffer (1989) and accepted by Grande &
Bernis (1991, character 2). It is illustrated in Grande
& Bemis (1991, fig. 17D). More information on this
character in †Birgeriais needed.Character 8. Subopercle possesses an anterior pro-
cess
The subopercle possesses an anterior process thatflange