four generaHuso, Acipenser,Scaphirhynchus,and
Pseudoscaphirhynchus. At higher levels, the taxa
recognized in this study are the Husinae (compris-
ing onlyHusoand usually referred to here with only
the generic name), Acipenserinae (comprising Aci-
penser, Pseudoscaphirhynchus, and Scaphirhyn-
chus),and Scaphirhynchini (comprisingPseudosca-
phirhynchusandScaphirhynchus).cialized. Pseudoscaphirhynchus fedtschenkoi has
been reported to be morphologically variable (Berg
1948a) with distinct morphotypes that differ in var-
ying degrees compared to P. kaufmanni.Acipenser
I examined several species of Acipenserto assess
morphological variation within the genus to allow
comparisons with other genera. Four adultA. brevi- Charactersandphylogenetic comparisons
rostrum,two large juvenileA. medirostris,two large
juvenileA. oxyrinchus,and the head of an adultA. I define 69 characters used to analyze interrelation-
oxyrinchus(2.4 m total length) were hand cleaned. ships of genera within Acipenseridae. All charac-
I cleared and double stained several small speci- ters described and analyzed here are skeletal to al-
mens ofA. brevirostrum, A. transmontanus, and low outgroup comparison with the fossil acipenseri-
heads ofA. ruthenus. Abbreviated developmental form taxa †Chondrosteus(Traquair1887, Hennig
series of A. brevirostrum and A. transmontanus 1925, Watson 1925, 1928), †Peipiaosteus(Liu &
were cleared and double stained. Marinelli & Stren- Zhou 1965, Bai 1983, Zhou 1992), and the fossil po-
ger (1973) describedA. ruthenusand several Eu- lyodontids †Paleopsephurus and †Crossopholis
ropean species ofAcipenserare described in Se- (Grande & Bemis 1991), as well as extant polyodon-
wertzoff (1926a, b, 1925), Tatarko (1936), and Anto- tids. Several soft tissue characters such as presence
niu-Murgoci (1936a, b, 1942). of four barbels, the spiracle, and the ligaments of
the jaws and hyoid arches may characterize specific
nodes within Acipenseridae, but are not included
here. Soft tissue characters are problematic in a cla-
distic analysis of Acipenseridae because Polyodon-
tidae is the only relevant extant outgroup (Bemis et
al. 1997), leaving any putative familial level charac-
ters as two taxon statements (Polyodontidae vs.
Acipenseridae).
HusoHuso husois the only species of the genus examined
in this study, but is similar in morphology to H.
dauricusby all accounts (Berg 1948a). Of three
small juveniles, one was cleared and double stained,
one sectioned, and one examined intact (all CAS
37541). Two moderate sized juveniles (FMNH
96852, 96853; each approximately one meter total
length) were examined externally and by dissection
to confirm character states between small and
larger sizes.Character 1. Trunk bracketed with five rows of scutes- Acipenseridae
Five scute rows are present along the trunk in aci-
penserids (shown in Husoin Figure 1). The five
rows are distinguishable as three groups: (1) the
dorsal scute row extending from the dermal skull to
the predorsal scale of the dorsal fin; (2) paired Clank
scute rows bearing the trunk canal from the supra-
cleithrum into the base of the caudal fin; and (3)
paired ventral scute rows spanning the pectoral and
pelvic fins. Shape, number, and size of individual
scutes varies dramatically, but their position and
presence as complete groups is consistent withinTerminology of acipenserid taxaIn the description of characters, characters are
grouped according to the levels of acipenserid taxa
that they define. Accordingly, character descrip-
tions often refer to higher level taxa defined in this
study that are not consistent with historic usage.
Taxa recognized within Acipenseridae include the