gans than any other groups, with estimates of 70000
organs in an adult Polyodon(Nachtrieb 1912). The
chief location of these organs is the rostrum, where
the organs are surrounded by stellate bones in the
case of paddlefishes or contained in pockets be-
tween the rostral bones of sturgeons. Although dif-
ferent workers use different terminologies (e.g.,
Norris 1925, Nikolskaya 1983), the ampullary or-
gans of Polyodon are distributed on the paddle,
cheek, and opercular flap in a characteristic way
that is constant from individual to individual (Be-
mis & Northcutt personal observation). Ampullary
organs of Polyodonincrease in number throughout
life by subdivision of existing organs. There is one
important unanswered question concerning Polyo-
don: does it use electroreception to detect swarms
of zooplankton, and if so, how? This will be a chal-
lenging behavior tostudy, for it is difficult to mea-
sure or to mimic the electrical field of a plankton
swarm, and it is probably impossible to completely
denervate electroreceptive input (see Kalmijn 1974
for discussion of plankton and electroreception).
Interesting differences in two other sensory sys-
tems of sturgeons and paddlefishes say much about
their different sensory worlds. First, polyodontids
have two small barbels on the ventral surface of the
rostrum in contrast to the four, large, often fimbriat-
ed barbels on the rostrum of acipenserids. In both
families, the surfaces of the barbels are covered
with chemoreceptive organs (taste buds), although
sturgeons have many more chemoreceptive organs
on their barbels than do paddlefishes. This may be
linked to the evolution of benthic habits in Acipen-
Electroreception as thedominantsensory system of seridae in contrast to the mid-water habits of pad-
paddlefishes dlefish. The second basic difference in the sensory
systems of paddlefishes and sturgeons concerns the
Chondrichthyans, sarcopterygians, amphibians, relative sizes of their eyes. Paddlefishes have abso-
and non-neopterygian actinopterygians share a lutely smaller eyes than do comparably sized stur-
common organ system for electroreception (groups geons. The behavioral meaning of this has never
enclosed by dotted outline in Figure 3). The sense been rigorously evaluated, but it suggests that vi-
organs of this system are known variously as ampul- sual information may be less important for adult
lae of Lorenzini (chondrichthyans), rostral organ paddlefishes than for sturgeons. The relative de-
(coelacanths), or ampullary organs (aquatic am- emphasis on chemosensory and visual systems in
phibians, lungfishes, polypterids, sturgeons and paddlefishes is possibly countered bytheir extraor-
paddlefishes; see Jorgenson et al. 1972 and North- dinarily high number of ampullary organs. These
cutt 1986 for review). Among all of these taxa, pad- ideas might be tested by comparing the size of tar-
dlefishes have many more individual ampullary or- get areas in the brain for each sensory system (e.g.,As noted above, recent and fossil paddlefishes
are restricted to freshwater. Although we cannot
conclude this with certainty, †Protopsephurusap-
pears to be the sister taxon of all other paddlefishes
(see discussion of clade Polyodontidae above). If
we add †Protopsephurus to our earlier analysis
(Grande & Bemis 1991), and still assume strict vi-
cariance, then we conclude that North America and
China shared a link at least as early as the Upper
Jurassic. This trans-Pacific affinity is intriguing be-
cause many taxa of fishes from the western United
States share trans-Pacific ties (Grande 1985,1994).
Similar conclusions were reached by Jin (1995) in an
article received after this manuscript was accepted;
see Grande & Bemis (1996) for discussion of Jin
(1995).
We regard †Protoscaphirhynchusas a member of
tribe Scaphirhynchini, and note that no characters
clearly distinguish it fromScaphirhynchus.All re-
cent members of the tribe Scaphirhynchini are con-
fined to large rivers, and generally prefer a strong
current and soft, silty bottoms. Thus their known
distribution is more likely to have resulted from vi-
cariance rather than trans-oceanic dispersal. In this
case, the trans-Pacific link between central Asia and
North America east of the Rocky Mountains must
be at least as early as the late Cretaceous because of
the presence of †Protoscaphirhynchus in eastern
Montana. We predict that scaphirhynchine fossils
will eventually be recovered from late Mesozoic de-
posits of eastern Asia.