Character 26. Pectoral fin spine
The presence of a pectoral fin spine was accepted by
Grandc & Bemis (1991, character 13) as a synapo-
morphy of Acipenseridae. A redefinition of this
character was made by Findeis (1997), who found
that independent fin rays do not actually fuse to
form the spine (see Jollie 1980) but are instead en-
cased in a sheath of dermal bone.
Character 27. Antorbital bone
One of the last bones to form in the dermal skull
roof of all acipeiiserids surveyed is the discrete an-
torbital bone lying on tlie postnasal wall between
the orbit and olfactory bulb (Findeis 1997). It is ab-
sent in polyodontids, †Chondrosteusand †Peipiaos-
teus. This bone does not carry any portion of the
supraorbital lateral line canal, and so is not homolo-
gous to the nasal bone in our actinopterygian out-
groups.Character 28. Commissure of occipital canals in
median extrascupular bone
In all acipenserids, the median extrascapular bone
is present and prominent in the posterior part of the
skull roof because it bears the commissure of the
occipital canals. The bone develops late in ontoge-
ny, but unlike the surrounding anamestic bones, has
an unusually constant shape (triangular) through-
out Acipenseridae (Findeis 1997). No other acipen-
seriforms possess a median triangular bone in this
region of the skull (see Grande & Bemis 1991 on the
nuchal bone of †Paleopsephurus, which does not
carry lateral line canals). This character is defined
by Findeis (1997 this volume).Character 29. Rostral canals curve lateral to barbels
The sensory canals extending into the rostrum of
paddlefishes and sturgeons are continuous with the
infraorbital canal. In sturgeons, the canals curve lat-
erally around the outer pair of barbels, and then
converge toward the ventral midline of the rostrum.
In paddlefishes, the canal leading into the rostrum
is straight. No other acipenseriforms or outgroup
taxa surveyed exhibit the laterally curved condi-
tion. This character is defined by Findeis (1997).Character 30. Supracleithrum tightly joined to der-An elongate dorsal process of the supracleithrum is
tightly bound by connective tissue to the inner sur-
lace of the posttemporal bone. This strong connec-
tion of the pectoral girdle to the skull roof is unique
to acipenserids among acipenseriforms and basal
actinopterygians generally. This character is de-
fined by Findeis (1997 this volume).Character 31. Opercular wall formed by cleithrum
and clavicle
In all sturgeons, a vertical wall of bone defines the
posterior wall of the opercular chamber. The oper-
cular wall is composed of thin medial laminae of the
cleithrum and clavicle. It is unique to acipenserids
among all taxa surveyed. Also see Findeis (1997).Charcter 32. Cardiac shield formed by clavicle
The cardiac shield is formed by laminar extensions
of the clavicle, which cxtcnd toward the ventral
midline to form a dermal ‘shield’ overlying the per-
icardial cavity. No other actinopterygians surveyed
possess this feature. It is illustrated by Findeis
(1997).Character 33. Cleithral process limits mobility of
pectoral fin spine
A process of the cleithrum extciids laterally to
brace the fin spine. It is present in all acipenserids,
but absent in all other taxa surveyed. It is probably
part of a character complex including the presence
of the fin spine (character 26, above). See Findeis
(1997) for illustration and further discussion.Putative and problematic characters of Acipenseri-
dae
Synapomorphies of Acipenseridae derive princi-
pally from the skull roof, the pectoral girdle, and
dermal ossifications such as scutes. Many other
characters which could be included at this level of
analysis are cartilaginous. and thus difficult or im-
possible to score in fossils. They are, however, help-
ful in studying relationships among recent Acipen-
seridae (Findeis 1997).
We noted the reduction in the number of bran-
chiostegals as a character of Acipenseroidei (char-mal skull roof