Grande & Bemis 1991) have regarded the bone on
the leading edge of the upper jaw of Acipenseri-
formes as homologous to the maxilla of other os-
teichthyans. Instead, Findeis (1991) interpreted this
bone as a dermopalatine, one of a series of investing
bones of the palatoquadrate present in many os-
teicthyans (e.g., dipnoans, Rosen et al. 1981). A new
illustration labeled with this interpretation is shown
in Figure 18. If the upper jaw of Acipenserifornies is
a composite of only the palatoquadrate and its in-
vesting bones, then both the maxilla and premaxilla
must have been lost.
Putative and problematic characters of Acipenser-
formes
This section describes some additional features
which either cannot be surveyed in all acipenseri-
form taxa or which we cannot easily define. For ex-
ample, sensory barbels were probably present at
this node, but we cannot evaluate their condition in
fossils nor are they present in living outgroups, so
we do not know whether 2 barbels (as in living Po-
lyodontidae) or 4 barbels (as in living Acipenseri-
dae) is the plesiomorphic condition. Similarly,
alone among living actinopterygians, embryonic
paddlefishes and sturgeons share a hairpin-loop
shaped pronephros (Ballard & Needham 1964, Be-
mis & Grande 1992).
Three cranial character complexes provide most
of the characters of Acipenserifornies: the cheek re-
gion, the jaws and the opercular series. It is possible
that some of the characters within these complexes
are linked. For example, paedomorphosis is
thought to cause ‘global’ changes in morphology
(Bemis 1984). so that several seemingly distinct
skeletal characters actually change as a unit. In Aci-
penseriformes, the apparent loss of cheek bones
and presence instead of small ossicles to carry the
sensory canals might be linked to such a common
underlying process. Evaluation of this putative
character complex, however, requires far more in-
formation about the development of the skeleton
than is currently available.
The cartilaginous basitrabecular processes on the
midventral surface of the neurocranium of recent
Acipenseriformes serve as ‘pivot points’ guiding
projection of the palatoquadrates. These processes
are present in all Acipenseridae and Psephurus
The loss of these processes inPolyodonis a second-
ary condition associated with immobility of the up-
per jaw. Sewertzoff (1928) described ligaments con-
necting the basitrabccular processes and the palato-
quadrate, but Findeis (1993) found no discrete liga-
ments in this position in any of the nine species of
acipenserids he surveyed for this character. Be-
cause they also possess hyostylic jaw suspensions,
we infer (but did not feel certain about scoring) that
basitrabecular processes were present in †Chon-
drosteusand †Peipiaosteus.†Cheirolepis,Polypte-
rus,†Mimia,Lepisosteus, andAmia lack basitra-
becular processes.
Gardiner & Schaeffer (1989) regarded a large,
blade shaped hyomandibula as a synapomorphy of
Acipenserifornies. This may be true, but is difficult
to define in an unambiguous way. It is difficult to
decide, Tor example, whether the hyomandibula of
†Mimia(which is integrated into a more typical ac-
tinopterygian suspensorium) is ‘blade shaped’. Al-
so, in all recent Acipenseriformes, the interhyal is
hypertrophied. Interhyal hypertrophy was regard-
ed by Gardiner & Schaeffer (1 989) and Grande &
Bemis (1991, character 3) as a synapomorphy of
Acipenseriformes. Although this is probably cor-
rect, it is impossible to evaluate this character based
only on published descriptions of the fossil taxa sur-
veyed here because the interhyal is cartilaginous.
Gardiner & Schaeffer (1989) noted that the ba-
sisphenoid of Acipenseriformes lacks a parabasal
canal; as we understand their description, this ap-
pears to be plesiomorphic for the group. Similarly,
the polarity of change in myodome characters
noted at this level by Gardiner & Schaeffer (1989) is
not obvious to us. The myodomes present in Aci-
penseridae are small and separated, but similar to
those of †Mimia.
We do not understand a putative acipenseriform
character noted by Gardiner & Schaeffer (1989, p.
176): ‘rostral bones reduced, numerous’, for rostral
bones are not particularly large or stable features in
any of the stem actinopterygians known to us.
Many workers assert that absence of jaw teeth
characterizes Acipenseriformes or some clade
within the order (e.g., Gardiner 1984b, character
12). However, as stated, this character is problem-