and dorsoventrally flattened with a rectangular
cross-section. Elongation occurs suddenly and rap-
idly in small juveniles to achieve adult dimensions
by elongation of vertebral cartilages, with individu-
al vertebral segments over twice the length of typ-
ical vertebral segments (Findeis 1993). Total num-
ber of segments composing the peduncle is consis-
tent with other acipenserids. †Protoscaphirhynchus
squamosus(Wilimovsky 1956 possesses an elon-
gate peduncle linking this fossil species toScaphi-
rhynchus.
The caudal peduncle of all other acipenserids is
short, with a cylindrical cross-section. Vertebral
cartilages within the peduncle of the outgroups are
unlengthened and retain a recognizable segmental
organization.InAcipenserandHusobranchiostegal one is typ-
ically rectangular and hourglass-shaped (Figure 7c,
d) as it drops vertically before angling ventrome-
dially under the operculum. All scaphirhynchines
possess a flattened head with restricted vertical
space in the operculum, but address these restric-
tions differently. Pseudoscaphirhynchushas a rect-
angular branchiostegal, but it is short and indents
medially immediately from the subopercle. Bran-
chioslegal one ofScaphirhynchus is vertical, but
dorsoventrally compressed inshapeto define this
character. The independent branchiostegal of po-
lyodontids (Grande & Bemis 1991, Findeis 1993)
and branchiostegal series of †Chondrosteus (Tra-
quair 1887, Hennig 1925) and†Peipiaosteus(Liu &
Zhou1965, Zhou 1992) are not comparable (see
Character 7).
Character65.Caudal peduncle and preanal area ar-
Character 63. Gill rakers are crenelated–Scaphi- mored-Scaphirhynchus
rhynchus
Six groups of peduncle scales make mutual contact
Gill rakers ofScaphirhynchuspossess short double- with the Flank scutes and anal scale series to com-
branched nubs at regular and close intervals along pletely armor the posterior trunk from the vent and
the oral edge (Figure 25c). Basic raker shape is a dorsal fin to the caudal scales of the caudal fin
right triangle, with a vertical posterior edge and (Findeis 1993). †Protoscaphirhynchuspossesses ar-
gradually descending anterior edge bearing the rays of peduncle scales similar toScaphirhynchus,
nubs.Scaphirhynchus albuspossesses slender rak- but the specimen is too weathered to make detailed
ers with a single pair of nubs on the first arch (Bailey comparisons.
& Cross 1954), but rakers of other arches are similar Similar peduncle and preanal scale rows are
toS. platorynchus found in Pseudoscaphirhynchus, Huso, and many
These rakers are distinctive compared to most species ofAcipenser,but they are not organized in-
acipenserids, but apparently similar to the to plates as numerous, large, or with precise inter-
branched rakers ofAcipenser baerii(Sokolov & Va- digitation asScaphirhynchus. Acipenser oxrinchus
sil’ev 1989). They differ from the pronged rakers of is the best scaled species ofAcipenserexamined,
Pseudoscaphirhynchus(see Character 58) in pos- possessing paired rows of scales preanally and on
sessing blunt nubs and being generally triangular. the peduncle, but they remain separate rows with-
The lanceolate rakers ofHusoand several spcecies out mutual contact. Pseudoscaphirhynchus Kauf
ofAcipenserare elongate (Figure 25a) and the tri- mannipossesses robust preanal scales and paired
angular rakers of other speciesof Acipenser(Figure scales on the dorsal peduncle, but ventral peduncle
25b) are symmetric and lack nubs. scales are not organized. More typically in other
acipenserids, peduncle and anal scales form single
rows of variable size and separation.
Character 64. Caudalpeduncle is flattened and elon-
gate-Scaph irhyn ch us
The caudal peduncle ofScaphirhynchusis elongate