et al. 1997 this volume). Published molecular phylo- netic allometry of paddle growth in Polyodon
genetic research including Acipenseriformes is lim- spathula was concisely described by Thompson
ited to questions concerning higher relationships (1934, also see Grande & Bemis 1991). At the start
among Actinopterygii (e.g., Normark et al. 1991), of the feeding larva period, North American pad-
and no study has yet included all living species of dlefishes have a barely detectable paddle. But soon
sturgeons and paddlefishes. Nothing approaching afterwards, the paddle grows with positive allom-
the comprehensive morphological-molecular-kary- etry to make up more than half of the total body
ological-data sets now available for many groups of length. Later in life, paddle growth shows negative
tetrapods (e.g., plethodontid salamanders, Wake & allometry with respect to total length. Even after
Larson 1987) has been attempted for Acipenseri- Polyodon spathulaachieves reproductive maturity.
formes or indeed for actinopterygians generally. there can be significant qualitative morphological
From this brief history, it is clear that phylogenet- changes, such as the appearance of new ossification
ic studies of Acipenseriformes are still in their in- centers in the necurocranium (Grande & Bemis
fancy. Some barriers to phylogenetic study seem ‘in- 1991). Many acipenseriforms achieve very large siz-
trinsic’ to these fishes. In particular, acipenseri- es at maturity, and may continue to grow for many
forms often exhibit great individual and ontogenet- years thereafter, but most systematic studies and
ic variation. It is critical to betterunderstand and collections are disproportionately weighted to-
distinguish between these types of variation in any wards more easily studied (and stored) juvenile and
comprehensive phylogenetic review, and this in it- ‘sub-adult’ specimens. We have already pointed out
self is a daunting task. Extensive variation confused the necessity of collecting and including large adult
systematists such as Duméril (1870), who proposed specimens in phylogenetic studies (Grande & Be-
more than 40 new species ofAcipenserthat were mis 1991, 1997). In studying acipenseriforms, this
rejected by later workers. Variation is frequently goal is often impractical, if not impossible, due to
noted in other contexts. For example, in a largepop- depletion or extinction of many populations. In par-
ulation study of shortnose sturgeon,Acipenserbre- ticular, members ofdepleted populations of acipcen-
virostrum,Dadswell et al. (^1 , p. 2) noted that speci- seriforms rarely achieve the historically reported
mens ranged ‘... from sharp-plated, rough-skinned maximum sizes of individuals prior to exploitation
individuals to flat-plated, smooth-skinned’in the (e.g., Acipenser transmontanus,Galbreath 1985).
St. John Estuary in New Brunswick. There is also Anotherexampleofanintrinsic barrier to phyloge-
much individual variation in the pattern of skull netic study is the potentially large but unknown role
roofing bones, as illustrated forA. fulvescensby Jol- of natural hybridization (see Birstein et al. 1997 for
lie (1980, perhaps even more extreme than variation review), and varying anthropogenic impacts on hy-
in skull roofing bones reported forAmiaby Jain bridization ranging from creation and release of
1985 and Grande & Bemis 1997). Although it has new hybrids to selective overfishing of one species
not been the subject of formal study, rostral shape in to large scale alterations in river systems. For exam-
Scaphirhynchusis positively allometric during early ple, some workers suggest that the hybridization
life, as shown by the photograph of a growth series frequency of shovelnose and pallid sturgeons (Sca-
in Figure 2. The rostrum provides other well known phirhynchus platorynchusandS. albus) increased
examples of variation. For example, rostral Iength as a result of dredging, damming, and channelizing
and width of the North American species ofAcipen- big-river habitats (Carlson et al. 1985, Phelps & Al-
servaries ontogenetically, geographically and inter- lendorf 1983).
specifically (Vladykov & Greeley 1963). Ontoge- The main reason, however, why phylogenetic
studies of Acipenseriformes are still in their infancy
is that few people have ever concentrated on the
systematics of the group. This is unfortunate, be-
cause systematics offers the only mechanism for
comprehensive comparative studies, and such stud-
(^1) Dadswell, M.J., B.D. Taubert, T.S. Squires, D.Marchette & J.
Buckleye.1984. Synopsis of biological data on shortnose stur-
geon,Acipenser brevirostrumLeSueur 1818. NOAA Technical
Report NMFS 14.