Figure 20. Scanning electron micrographs of Polyodon spathula larva: a -Lateral view of the head. The infraorbital lateral line canal is still
a groove at this point in development and can be seen continuing onto the rostral region. The olfactory pit has not yet completely
subdivided into anterior and posterior nares. Many clusters of ampullary electroreceptors are visible on the cheek region dorsal to the
upperjaw (see Bemis & Grande 1992). b-Oral view of the upper jaw of a similar specimen. The teeth of the upperjaw are eruptingin two
series. Additional erupting teeth can be seen at the leading edge of infrapharyngobranchial 2 (see Grande & Bemis 1991 fig. 17B).overlaps the hyomandibula and attaches the sub-
opercle to the hyoid arch (Figure 19). The process is
present, but smaller in †Chondrosteusand †Pei-
piaosteusthan in acipenserids and polyodontids. It
was included as part of a polyodontid character by
Grande & Bemis (1991, character 17) but redefined
by Findeis (1993) as an acipenseriform character.
Nielsen (1949, p. 302, #13) considered the unusual
subopercle of †Birgeria‘lobate’ and similar to that
ofPolyodon. This appears to be a superficial simi-
larity, and in any event †Birgerialacks the anterior
process of the subopercle. †Cheirolepis,Polypterus,
†Mimia,Lepisosteus, andAmiapossess oblong or
circular subopercles lacking the anterior processes.
Character 9. Preopercular canal in a series of ossi-
cles, mandibular canal short or absent
This character was discussed by Jollie (1980), de-
fined by Patterson (1982) and redefined in modified
form by Gardiner & Schaeffer (1989) and Grande &
Bemis (1991, character 4). It may be linked to the
next character.Character 10. Infraorbital sensory canal in a series of
ossicles
In recent acipenseriforms, †Chondrosteusand †Pei-
piaosteus, the infraorbital sensory canal is carried
by a series of small canal bones. This contrasts with
the condition in †Cheirolepis,Polypterus,†Mimia,
†Birgeria,Amiaand other outgroups in which large
circumorbital bones carry the infraorbital sensory
canal.Character 11. Loss of premaxillary and maxillary
bones
This interpretation derives from observations of a
developmental series ofScaphirhynchus made by
Findeis (1991). In contrast to the solid bony cheek
found in outgroups such as †Cheirolepis,Polypte-
rus,†Mimia,†BirgeriaandAmia, the cheek of re-
cent sturgeons and paddlefishes is composed large-
ly of skin, and none of the tooth bearing bones of
the upper jaw are exposed on the surface of the
cheek. There is no bone in the upper jaw of acipen-
seriforms homologous to the premaxilla of the out-
group osteichthyans surveyed. Most historic and
contemporary authors (e.g., Traquair 1887, Parker
1882, Woodward 1891, Sewertzoff 1928, Jollie 1980,