defining Acipenseroidei, which we modify and sup-
plement here.
Character 14. Loss of opercle
This character was defined by Grande & Bemis
(1991, character 7) as a synapomorphy of Acipense-
roidei. As defined in character 1 above, the sub-
opercle replaces the opercle as the largest bone of
the opercular series in Acipenseriformes. However,
†Chondrosteusand †Peipiaosteusretain small oper-
cles, which are entirely missing in Polyodontidae
and Acipenseridae.Character 15. Reduction in number of branchioste-
gals supporting gill cover
This character was defined as an acipenseroid syn-
apomorphy by Grande & Bemis (1991, character 8).
†Chondrosteuspossesses at least eight branchioste-
gals in a continuous series along the lateral and ven-
tral faces of the operculum. †Peipiaosteuspossesses
at least seven branchiostegals. Acipenserids typi-
cally possess two (occasionally three branchioste-
gals are present; see Findeis 1997 this volume). The
branchiostegals are elongate inHusoand some spe-
cies ofAcipenser(e.g.,A. oxyrinchus), shorter in
other species ofAcipenser(e.g.,A. brevirostrum),
and much shorter in scaphirhynchines. With the
possible exception of †Protopsephurus, polyodon-
tids possess only a single branchiostegal separate
from the subopercle (character 24, below).Character 16. Endocranium with extensive rostrum
Grande & Bemis (1991, character 9) defined this
character. This extension of the ethmoid region of
the endocranium ranges from an elongate, thin
sword or paddle shaped structure in polyodontids
to a broad, flattened shovel shape in scaphirhyn-
chines. It is absent in †Peipiaosteus,†Chondrosteus,
†Birgeria,†Cheirolepis,†Mimia, Polypterus and
Amia.†Saurichthysis sometimes regarded as a po-
tential sister group of Acipenseriformes (e.g., Gar-
diner & Schaeffer 1989, Rieppel 1992), partly be-
cause of its rostral shape. However, the rostrum of
†Saurichthysis based upon extended jaws, super-
ficially similar to those ofLepisosteus, so its rostrum
is not homologous with that of acipenseroids.Character 17. Dorsal and ventral rostral bones
Grande & Bemis (1991, character 10) defined this
character. In our initial formulation, only the ven-
tral keel of rostral bones was emphasized, but the
presence of dorsal rostral bones should be included
as well. Dorsal and ventral rostral series are always
present in Polyodontidae and Acipenseridae. Ros-
tral bones of polyodontids are so constant that ho-
mologues are readily recognizable throughout the
family, but acipenserids lack this constancy. Some
variation in the rostral bones of sturgeons corre-
lates with the underlying shape of the rostrum, but
the number and arrangement also vary between in-
dividuals with comparably shaped rostra (e.g., Jollie
1980). Neither †Chondrosteus nor †Peipiaosteus
possesses extensive series of bones anterior to the
frontals and parasphenoid, much less entire series
of bones, and, based on outgroup comparison with
†Cheirolepis,Polypterus, and other basal actinopte-
rygians, the absence of rostral bones is plesiomor-
phic.Character 18. Ventral process of posttemporal bone
The posttemporal bones bear prominent ventral
processes that closely articulate with the neurocra-
nium. Among sturgeons and paddlefishes, these
ventral processes of the posttemporal bone are al-
ways present, although most strikingly developed in
Polyodon (e.g., Grande & Bemis 1991, fig. 10B). The
extreme condition inPolyodonwas noted as a puta-
tive synapomorphy of this genus (Grande & Bemis
1991, character 39). The posttemporal bones of
†Chondrosteuslack a ventral process and are sep-
arated from the dermal skull roof by the lateral ex-
trascapular bones. Similarly, the posttemporal bone
of †Peipiaosteusis not integrated into the skull roof,
so that there was not a close association between its
dermal skull roof and neurocranium in this region.
No similar feature of the posttemporal bone occurs
inPolypterus,†Mimia, or other actinopterygians.Putative and problematic characters of Acipenseroi-
dei
Some skeletal features at the level of Acipenseroi-
dei are parts of character complexes. For example,
the endochondral rostrum and rostral bones (char-
acters 16 and 17) are presumably coordinated neo-