Universal presence of these scales in all other aci-
penserids and their outgroups suggests that this loss
defines Scaphirhynchini.In adults ofPolyodon,Huso,andAcipenser,ven-
trolateral edges of the basiventrals curl to the mid-
line to complete a haemal arch (Figure 24a, b). This
is a problematic character because it develops late,
only in adults by progressive elongation and merg-
ing of the basiventrals and interventrals (bv,iv,Fig-
ure 24a). In the thoracic region, independent ven-
tral plates also integrate into the haemal arch. Juve-
nile specimens of Acipenserare similar to Scaphi-
rhynchus,but with a slightly more pronounced
basiventral edge (Figure 24a, c). However, while all
speciesofAcipenserexamined close a haemal arch,
even large Scaphirhynchuslack any expansion of
the basiventrals.
Character55.Spikespresentonthe frontals-Pseu-
doscaphirhynchusSpines of the frontals, parietals, and anterior dorsal
rostral bones of Pseudoscuphirhynchus kaufmanni
are large spikes. Spikes are always present in P.
kaufmanni,but are lacking inP.hermanniandP.
fedtschenkoi(Berg 1948a).Pseudoscaphirhynchus
hermannioccasionally has weak spines on anterior
dorsal rostral bones (Berg 1948a), perhaps suggest-
ing that a morphotype possessing spines is plesio-
Character53.Caudal fin filament present -Scaphi- morphic for the genus.IacceptP.kaufmannias rep-
rhynchini resentative for the genus, with spines as a scaphi-
rhynchine character (Character 41) secondarily lost
The caudal fin filament (or cercus) is found inSca- in otherspecies of thegenus.
phirhynchusandPseudoscaphirhynchuskaufman- Spines ofScaphirhynchusare raised ridges simi-
niextending from the tip of the caudal fin(Figure larto, but more shallow than spikesofP.kaufman-
23). It is formed by a thin coreofcartilage sheathed ni. This variation in morphology and presence
by small scales. It originates posterior to the noto- among scaphirhynchines makes it difficult to define
chord, developing distinctly later than the caudal distinct types of spines or spikes. While spikes of
endoskeleton in small juveniles. Pseudoscaphirhynchusare distinct in size from the
All species of Scaphirhynchus,P.kaufmanni,and spines of Scaphirhynchus,they are similar in shape
certain morphotypes of P. fedtschenkoi (Berg and notaccepted as discrete shape characters here.
1948a) possess a cercus. It is not present inP.her- Nevertheless, no spines are present on the frontal in
manni,but presumably due to secondaryloss.The Scaphirhynchus,and presence offrontalspikes is
fin filament is short inScaphirhynchus,rarely ex- distinctive ofPseudoscaphirhynchus.
ceeding five centimeters in length, but can reach 13
centimeters or more in P.kaufmanni.
Noother extant acipenseriforms possess a caudal
fin filament and its scalation would be visible in fos-
sils if present. The caudal endoskeleton of outgroup
acipenseriforms possesses dorsal and ventral sheets
of cartilage that merge and terminate with the no-
tochord (Figure 23a).
Character 56. Lateral extrascapulars enclose the tri-
radiation of thetrunk,occipital, andsupratemporal
canals-PseudoscaphirhynchusThe occipital canal (ocll)is carried by a lateral ex-
trascapular series (excl,Figure 5e) in Pseudosca-
phirhynchusthat includes its origin separating from
the trunk (trll)and supratemporal canals (stll).The
lateralmost lateral extrascapular encompasses the
tri-radiationofthese canals as this series ofbones
(usually three) intrudes centrally over the posttem-
poral (Figure 5e).
In all other acipenserids examined (Figure 5b, c,
d), splitting of these canals occurs within the post-Character54.Lossofpectoral scales- Scaphirhynchini
Pectoral scales with elevated, recurved tips typical-
ly found onthe opercular wall of acipenseriforms
(see Character 23) are absent in scaphirhynchines.