Figure 24. Direct evidence for paedomorphosis in the neurocranium of Acipenser brevirostrum. In a few cases we can directly observe
post-reproductive changes in morphology. such as the neurocranium of this large shortnose sturgeon, which exhibits four ossification
centers within the neurocranium. Only the very largest (=oldest) individuals exhibit these bones. These neurocranial ossifications serve
no obvious function, and they are highly variable from one individual to the nest These four neurocranial bones are readily homologized
to those of other basal osteichthyans. and we regard their variable presence as direct evidence of paedomorphosis. fII =foramen for optic
nerve; fIII=foramen for oculomotor nerve; fIV=foramen for trochlear nerve; fX = foramen for vagal nerve; fsp = foramen for spinal
nerve; tfc=trigeminofacialis chamber. Total length of specimen: 870 mm. UMA 24-116-2-47.
nial specialization in acipenseriforms. Related is-
sues we consider essential are fresh re-examin-
ations of traditional homologies using extensive de-
velopmental material. This may provide alternative
interpretation for certain structures, such as Find-
eis’ (1991) conclusion that the so-called maxilla of
Acipenseriformes is actually a dermopalatine (see
discussion of character 11 above). This reinterpreta-
tion is pivotal to understanding the loss of many
bones from the cheek region of Acipenseriformes.
Ram ventilation and the origin of filter feedingA characteristic observation aboutPolyodon spath-
ulain aquaria or rearing ponds is that they never stop
swimming, and if confined in a container too small to
allow swimming, then they sink to the bottom. Sink-
ing occurs because the volume of air in the swim
bladder is not sufficient to achieve neutral buoyancy.
Constant swimming by paddlefishes also generates a
constant flow of water over the gills which allows for
ram ventilation.Polyodondepends on ram ventila-
tion, as evidenced by the absence of a buccal valveand the inability to completely close either the oper-
cular chamber or the mouth. As further evidence of
the critical role of ram ventilation, Burggren & Be-
mis (1992) found that juvenile paddlefishes are near-
ly as aerobic as cruising bluefin tuna,Thunnus thun-
nus. Speculating on the connection between ram
ventilation and the evolution of filter feeding within
Polyodontidae, they linked the potential to evolve
filter feeding to the prior evolution of ram ventila-
tion. According to this reasoning, many of the spe-
cializations associated with filter feeding inPolyo-
don(flattened gill arches, elongate gill rakers, and
secondary fixation of the upper jaw to the neurocra-
nium) were possible only because buccal-opercular
pumping and branchial arch mediated respiratory
movements were unnecessary.
Unlike Polyodon, sturgeons such as Acipenser
andScaphirhynchuscan buccal pump to generate
respiratory flow (e.g., Burggren 1978), which allows
them to be more sedentary than paddlefishes. What
is missing, however, are basic data on the possible
coupling of respiration with locomotion inHuso
and Psephurus, which our current phylogenetic
analysis indicates as critical for understanding the