ground swell of interest in sturgeon biology (e.g.,
Binkowski & Doroshov 1985, Williot 1991, Gersha-
novich & Smith1995),including their systematics
and evolution (Bemis et al. 1997 this volume).
A better understanding of acipenserid interrela-
tionships is central not only to organizing modern
studies in sturgeon biology, but to examining evolu-
tionary and functional attributes of the lineage. Out
of only four extant clades of non-teleostean acti-
nopterygians (Lauder & Liem 1983), acipenserids
are the most speciose and have the largest bioge-
ographic range. As such, they are pivotal to evolu-
tionary studies of Actinopterygii and more precise
understanding of their morphology and phyloge-
netic position is crucial for cladistic comparison to
other taxa. Further, given their age and unique ad-
aptations, extant acipenseriforms differ markedly
from other actinopterygians, with distinct, parallel
evolutionary solutions to the challenges of aquatic
life faced by all fishes.
The history of acipenserid systematics is long,
with major studies from the 19th century addressing
species and generic recognition (Rafinesque 1820,
Brandt & Ratzeberg 1833, Fitzinger & Heckel 1836,
Brandt 1869, Dúmeril 1867, 3870). Unfortunately,
there has never been a consensus about recognition
of genera or subgenera with current usage accept-
ing the generaHuso, Acipenser,Scaphirhynchus,
andPseudoscaphirhynchus.As many as six subgen-
era (or genera) have been proposed to subdivide
Acipenser, and most attempts subsumedHusointo
Acipenser(Brandt & Ratzeberg 1833, Fitzinger &
Heckel 1836, Dúmeril 1870). While Husois now rec-
ognized as an independent genus (defined by
Brandt 1869), its diagnostic characters (Tatarko
1936, Antoniu-Murgoci 1936a, b) remain untested.
At a higher level, Husohas remained paired with
Acipenser-within the subfamily Acipenserinae (sen-
su Bonaparte1838),and the shovelnose sturgeon
generaScaphirhynchyusandPseudoscaphirhynchus
are universally accepted individually and compos-
ing the subfamily Scaphirhynchinae (Scaphirhyn-
chini of Bonaparte 1846). However, anatomical
characters accepted as defining genera or subfam-
ilies have never been examined rigorously.
In this study, I use comparative osteology and cla-
distics to survey for and test skeletal characters rele-
vant to acipenserid phylogeny. Previous studies of
acipenserid systematics focused on external fea-
tures that are significantly variable and pose prob-
lems for a phylogenetic analysis. The only studies
emphasizing the skeleton were made by Antoniu-
Murgoci (1936a, b, 1942), but her analysis only in-
cluded Rumanian species and only examined por-
tions of the skeleton. Comparative osteology can
develop numerous phylogenetic characters and no-
tably allows use of fossil taxa as outgroups (e.g.,
Nielsen 1949, Patterson 1973,1982,Gardiner 1984).
Cladistic methods focus on defining characters at
specific phylogenetic nodes to determine interrela-
tionships (Hennig 1966, Wiley 1981), but also arrays
character transformations as focal changes defining
taxa. This provides insight into evolutionary pro-
cesses underlying phylogeny and allowing evalua-
tion of performance and behavior. Cladistics and
comparative osteology have dramatically increased
our understanding of actinopterygian evolution,
but acipenserids have been neglected. Only recent-
ly have interrelationships within Acipenseriformes
been convincingly defined (Grande & Bemis 1991,
Findeis 1993, Bemis et al. 1997) recognizing out-
groups necessary for a new examination of acipen-
serid characters (Figure I). These studies accept the
Polyodontidae as the sister group to Acipenseridae
(together composing the Acipenseroidei), with an
unresolved trichotomy among the Acipenseroidei,
†Chondrosteidae, and † Peipiaosteidae with Aci-
penseriformes (but see Grande & Bemis 1996).
Morphological and systematic studies of acipen-
serids pose certain problems making comprehen-
sive phylogenetic studies difficult. Specimens of
many species are not readily available, so taxonom-
ic coverage is difficult. Previous morphological de-
scriptions are not detailed (e.g., Marinelli & Stren-
ger 1973) or comprehensive enough (e.g., Tatarko
1936, Antoniu-Murgoci 1936a, b, 1942) to support
cladistic analyses and new descriptions are needed
(Findeis 1993). Acipenseriforms are largely cartila-
ginous and acipenseriform fossils do not show car-
tilage preservation, leaving incomplete data for
comparison. Because of these problems, this study
focuses on generic interrelationships within Aci-
penseridae.
Although little definitive phylogenetic work has