ter populations (e.g., A. fulvescens Vladykov &
Greeley 1963, A.baeriiin Lake Baikal, Ruban 1997
this volume). while stocks of other species spend a
great percentage of time at sea(e.g.,A. medirostris).
Findeis (1997) studied seven species ofAcipenser
(A. brevirostrum,A.oxyrinchus.A.transmontanus
A.fulvescens. A. medirostrisandA. ruthenus), but
failed to find any osteological synapomorphies of
thegenus.Acipensermay be paraphyletic, and it
as a first approximation (Figure 14, courtesy of E.
Artyukhin, see Artyukhin 1994, 1995a,b; also see
Birstein et al. 1997). Meaningful biological general-
izations are difficult. for species ofAcipenserrange
in size from the tiny sterlet (A.ruthenus) and short-
nose sturgeon (A. brevirostrum) to the immense
white sturgeon (A. transmontanus), Atlantic stur-
geon (A. oxyrinchus) and common sturgeon (A.stu-
rio).Several species have some cxclusivcly freshwa-
Table 6.Character key. Characters for acipenseriform outgroups taken from sources listed in Table 1.
Synapomorphies of †Birgeria+ Acipenseriformes
- Reduction of the opercle
- Elongate posterior extension of parasphenoid
- Bodyscaling reduced to tiny isolated elements or absent
- Palatoquadrates with an anterior symphysis
- Palatoquadrate with broad autopalatine portion, palatoquadrate bridge, and quadrate flange
- Presence of a triradiate quadratojugal bone
- Gill-arch dentition confined to first two hypobranchials and upper part of first arch
- Subopercle possesses an anterior process
- Preopercular canal in a series of ossicles. mandibular canal short or absent
- Infraorbitalsensorycanal in a series of ossicles
- Loss of premaxillary and maxillary bones
Synapomorphies of †Peipiaosteidae
None here; see Grande & Bemis (1996, written after this paperwas accepted
Synapomorphies of Chondrosteidae
- Anterior part of palatopterygoid club-shaped
- Complete loss of trunk scalation
Synapomorphies of Acipenseroidei
- Reductioninnumber of branchiostegals supporting gill cover
- Endocranium with extensive rostrum
- Dorsal and ventral rostral bones
- Ventral process of posttemporal bone
Synapomorphies of Polyodontidae
- Many small stellate bones make up lateral supports for the paddle
- Series of very elongate dorsal and ventral medial rostral bones, with cylindrical cross-sections
- Uniqueshapeof subopercle
- Elongate anterior and posterior divisions of the fenestra longitudinalis in the skullroof
- Posttemporal with elongate anterior arm suturing into the dermosphenotic
- Single branchiostegal with branched posterior edge
Synapomorphiesof Acipenseridae
- Five scute rows along trunk
Synapomorphies ofAcipenseriformes
- Loss of opercle
- Pectoral fin spine
- Antorbital bone
Commissure of occipital canals in median extrascapular bone (see
Rostral canals curve lateral to barbels
Supracleithrum tightly joined to dermalskullroof
Opercular wall formed by cleithrum and clavicle
Cardiac shield formed byclavicle
Cleithral process limits mobility of pectoral fin spine
Grande&Bemis 1996 for correction
