Figure 19. Lateral views of the opercular series of four taxa of
Acipenseriformes to show loss of the opercle in Acipenseroidei
(Polyodontidae + Acipenseridae), hypertrophy of the suboper-
cle. and reduction in the number of branchiostegal bones:
† Chondrosteus acipenseroides (from Traquair 1887), †Peipiaos-
teus pani (from Zhou 1992), Psephurus gladius (from Grande &
Bemis 1991), and Huso huso (original, CAS 37541).
could be reliably scored in at least 16 of the 18 taxa
listed in Table 7, but most soft-tissue and cartilagi-
nous characters cannot be scored in the fossil taxa
(see Grande & Bemis 1997 for additional explana-
tions of this approach). Incorporated into this dia-
gram are published interpretations of relationships
for the genera of Polyodontidae (from Grande &
Bemis 1991) and Acipenseridae (from Findeis 1993.
1997). Consult those references for characters with-
in the two families.
The starting points for our analysis are Patterson
(1982), Gardiner & Schaeffer (1989), Grande & Be-
mis (1991), Rieppel (1992) and Findeis (1997). For
those characters that are well-known, and ex-
plained in detail elsewhere, we provide abbreviated
descriptions. For other characters, new information
warrants presentation, and in a few cases, new il-
lustrations to clarify definitions. Each subsection
concludes with notations on other potential charac-
ters, comments on alternative interpretations, and a
statement concerning the robustness of the clade.Characters of †Birgeria + AcipenseriformesWe specify three characters as synapomorphic for
this group.Character 1. Reduction in size of the opercle
This character was noted by Gardiner & Schaeffer
(1989). The largest element of the opercular series
of Acipenseriformes is the subopercle bone (Figure
19). It is the most dorsal element of the opercular
series in acipenserids and polyodontids, but a
smaller, more dorsal opercle bone is present in
†Chondrosteusand †Peipiaosteus. The opercle of
†Birgeriais also small (Nielsen 1949, Gardiner &
Schaeffer 1989). InPolypterus,†Mimia,†Moytho-
masia,Lepisosteus,Amiaand most other actinopte-
rygians (McCallister 1968), the opercle is the largest
element of the opercular series, and we interpret its
reduction to a small bone (as in †Chondrosteusand
†Peipiaosteus) as synapomorphic for Acipenseri-
formes. The opercle is eventually lost in Acipense-
roidei (character 14, below).Character 2. Elongate posterior extension of paras-
phenoid
The parasphenoid of all Acipenseriformes has an
elongate posterior extension. The extension under-
lies a series of vertebral segments partially or com-
pletely fused into the occipital region of the neu-
rocranium. The parasphenoid bones of †Chondros-
teusand †Peipiaosteusare shorter than those of liv-
ing Acipenseriformes, but still much longer than in
any of the outgroups, including †Birgeria, which is
noted for having a more elongate parasphenoid
than other paleonisciforms (Nielsen 1949). In the
outgroup taxa, a vertebral joint close to the neuro-
cranium is probably necessary if head-lift is to play
any role in raising the upper jaw (discussed in final
section of this paper). Because acipenseriforms
project their jaws and do not utilize head-lift when
feeding, the elongate parasphenoid and extension
of the neurocranium present no interference.