acter 15). One to three branchiostegals are present
in Acipenseridae. whereas there is consistently a
single branchiostegal in Polyodontidae (with the
possible exception of † Protopsephurus; see Lu
1994).
Findeis (1993) noted that the jugal bone has a
prominent anterior process that extends beneath
the orbit to contact the rostrum. uniting the dermal
sk ull with the rostrum .Alt hough li ving paddlefish -
es and outgroup actinopterygians lack this anterior
process of the jugal, it is impossible to score it in
†Chondrosteusand † Peipiaosteusbased on current-
ly available descriptions.
In all living sturgeons. the postero-dorsal margin
of the operculum is emarginate. This allows a respi-
ratory current of water to flow into the dorsal por-
tion of the opercular chamber. over the gills. and
then exit the opercular chamber ventrally (Burg-
gren 1978). This character complex includes spe-
cializations in the shapes of the gill arches and fil-
aments (Findeis 1989). Living paddlefishes lack the
special respiratory water-flow and the associated
branchial specializations. but. depending on how it
is defined, paddlefishes can be considered to have a
dorsally emarginate operculum. Moreover, few of
the features comprising this system can be scored
reliably in fossils because they arc soft tissues. Thus
weconsider that the opercular water flow system is
probably synapomorphic for sturgeons. but cannot
assess this with certainty.
Supraneurals are absent in the caudal peduncle
of sturgeons (Findeis 1993). In contrast. polyodon-
tids and † Chondrosteushave a complete row of su-
praneurals in this region of the axial skeleton
(Grande & Bemis 1991). The condition in † Pei-
piaosteusis unclear, and in outgroup taxa such as
Polypterusand † Mimia,it is variable. so we defer
analysis of this character until it is better under-
stood.
Cartilaginous components of the pectoral girdle
may provide characters useful for understanding
relationships within Acipenseridae. but these fea-
tures cannot be scored in † Peipiaosteusand † Chon-
drosteusbased on available descriptions. For exam-
ple, a coracoid shelf. formed by the flattening of the
coracoid wall of the scapulocoracoid along the car-
diac shield. is probably synapomorphic for Acipen-
scridae because its presence is correlated with that
of the cardiac shield (character 32; Findeis 1997 uses
aspects of this character in intrafamilial phyloge-
netic analysis of Acipenseridae). Similarly. the su-
pracleithral cartilage on the ventral surface of the
supracleithrum appears to be unique to acipense-
rids.
A feature unique to Acipenseridae among living
basal actinopterygians is a basipterygial process ex-
tending ventrally from the antero-ventral edge of
the basipterygium. Unfortunately. because the pro-
cess is cartilaginous. the condition is unknown in
†Chondrosieusand † Peipiaosteus.
Among living actinoplerygians, sturgeons
uniquely possess a palatal complex composed of
several plates of cartilage (Findeis 1997). This prob-
ably plays an important role in feeding because it
forms one of the two opposing surfaces used in
crushing food (Findeis 1993). Unfortunately, this
cartilaginous feature cannot be studied in † Pei-For the present analysis, we also disregard sever-
al interesting cartilaginous features of the hyoid
arch and branchial skeleton described by Findeis
(1997).Discussion of clade Acipenseridae
Many characters confirm that this clade is mono-
phyletic. Additional characters relevant to recover-
ing a generic-level phylogeny of living Acipenseri-
dae are given by Findeis (1997).piaosteusand † Chondrosieus.Evolutionary questions and scenariosSonic interesting aspects of acipenseriform evolu-
tion concern the distribution of functional charac-
ters such as feeding systems or reproductive fea-
tures. These features cannot be confirmed in fossils
and have incongruent distributions among living
Osteichthyes. We also are intrigued by the current
biogeographic distribution of paddlefishes and
shovelnosed sturgeons. as well as such issues as the
origin of the rostrum and its extreme hypertrophy
in Polyodontidae. In each of the five cases we dis-
cuss, the cladogram presented in Figure 17 serves to
organize our discussion.