ton (Traquair 1887, Woodward 1891, I895a,b,c).
Many workers equated these changes with ‘degene-
racy’ of the skeleton (Goodrich 1909), which is
merely another way of emphasizing that this is a
secondary condition. Some workers speculate that
paedomorphosis was the driving force behind many
changes in acipenseriform anatomy. Paraphrasing
Yakovlev (1977, p. 141), paedomorphosis (‘fetaliza-
tion’) played a decisive role in destroying the ar-
chaic structure of the paleonisciform type of jaw, a
critical step leading to formation of the acipenseri-
form jaw (see also Tsessarsky 1992).
Paedomorphosis is of both theoretical and practi-
cal interest in systematics (Nelson & Platnick 1981,
Fink 1982), and many groups of fishes. such as lung- Pacificbiogeography of Polyodontidae and Sca-
fishes (Dipnoi), provide clear examples of paedo- phirhynchinae
morphic loss of skeletal elements and degree of os-
sification (Bemis 1984). There are only two types of An interesting aspect of acipenseriform biology
evidence available to support hypotheses of paedo- concerns links between Asian and North American
morphic change. The first is ‘relatively direct evi- freshwater taxa (Grande & Bemis 1991). Yakovlev
dence’, such as that provided by Grande & Bemis (1977) asserted that Acipenseriformes originated in
(1991) for paddlefishes. Large adult paddlefishes freshwaters of northeastern Asia in the Triassic and
continue to ossify new endochondral bones after subsequently dispersed throughout the Holarctic.
onset of reproductive maturity. We note here that His interpretation is based on the occurrence of taxa
this phenomenon also occurs in sturgeons, such as a such as †Stichopterusin the Jurassic of Northeastern
series of adultAcipenser brevirostrumthat demon- Asia. A problem with his interpretation is that the
strates progressive ossification of at least four neu- location of the earliest fossils is not a reliable clue to
rocranial bones (Figure 24). Additional observa- identifying a place of origin for a group (Grande
tions of largeA. breviostrumconfirm delayed ossi- 1985). Also, Yakovlev (1 977) did not consider sister
fication of several hyobranchial elements. ‘Less di- group relationships of Acipenseriformes (see Bemis
rect evidence’, such as that provided for dipnoans & Kynard 1997 lor such analysis, which suggests Eu-
by Bemis (1984) derives from a combination of phy- rope as the place of origin for the group).
logenetic analysis and developmental data from A basic question about the biogeography of Aci-
other taxa. For example, if the ontogeny of a struc- penseriformes is to explain the presence in North
ture is studied in an outgroup andfound to proceed America of †Crossopholis, Polyodon, †Protosca-
from state A to state B to state C during develop- phirhynchus,andScaphirhynchus,which have
ment, whereas the ontogeny of the same structure Asian sister taxa, †Protosephurus, Psephurusand
in the ingroup always stops at state B, then pacdo- Pseudoscaphirhynchus. Patterson (1981) clearly
morphosis can be hypothesized to have occurred outlined the criteria for analyzing the biogeography
(Figure 25). Thus, recognition of paedomorphosis of primary freshwater fishes. His central point is the
by less direct means relies solely on the relative con- need to resolve three taxon statements for endemic
gruence (or lack thereof) of developmental pat- taxa to identify vicariant distributions. Our phylo-
terns within a phylogenetic framework. genetic analyses allow some progress on this both
Apart from the direct evidence of paedomorpho- for Polyodontidae and Scaphirhynchini, although
sis presented by Grande & Bemis (1991), no study to the placement of the two taxa of greatest interest
date has provided adequate analysis of the role of (†Protopsephurus and †Protopsephurus)
paedomorphosis in acipenseriform evolution. This needs additional study.
is because without a cladogram to specify the ar-
rangement of taxa, it is not possible to organize the
developmental data within a phylogenetic context.
Moreover, the analysis shown in Figure 17 confirms
that many of the most relevant outgroups are fossil
taxa, which are unlikely to provide the necessary
developmental information for a formal analysis.
Finally, we note that only recently have ontogenetic
data become available for the skeleton in taxa such
as Polypterus (e.g., Bartsch & Gemballa 1992)
which are necessary for any study of paedomorpho-
sis in Acipenseriformes.