benthic prey fauna continued with evolution of
preferential benthic cruising acipenserines.
Many species ofAcipenserdepend on benthic
abilities. Ventral displacement of the mouth allows
for benthic foraging, but restricts their options to
capture midwater prey. Unlike the piscivorous Hu-
soandPsephurus,all species ofAcipenserand Sca-
phirhynchini feed wholly or partially on benthic
prey. Several species become piscivorous as adults
(e.g.,A. transmontanus A. oxyrinchus), but these
large species still never prey consistently on pelagic
prey to such an cxtent as Huso.Farmorecommonly,
species of Acipenser are molluscivores, insecti-
vores, or generalized benthic predators. While the
focus here is on feeding, the other suites of charac-
ters are critical to allowing acipenserines to interact
with the substrate and together compose a success-
ful benthic cruiser.
Scaphirhynchines are more obligate benthivores
with the flattened trunk and head promoting stabil-
ity and mobility along the substrate. Scaphirhyn-
chusis the most derived acipenserid genus in each
category and exemplifies character acquisitions
pointing to sequential entry into benthic habitats
during evolution of the Acipenseridace
cate that loss of endochondral ossification, dermal
cheek bones, and trunk scalation (the most obvious
paedomorphic characters) are synapomorphies of
Acipenseriformes and plesiomorphic to Acipenser-
ridae. Thus, while reduction of the skeleton exeni-
plifying paedomorphic events are valid phylogenet-
ic characters and evolutionary events. they charac-
terize the origin of Acipenseriformes and define a
morphotype underlying Acipenseridae.
Paedomorphosis is also accepted as the primary
process driving evolution in Acipenseriformes and
Acipenseridae. However, ideas that paedomorphy
dictates morphological change in acipenserifoms
conflicts with the suggestion that selective forces for
performance might drive evolutionary patterns
themselves and only result in paedomorphy sec-
ondarily. For instance, evolution of projectile jaws
in acipenserifoms requires loss of the dermal
cheek to free the mobile palatoquadrate. Invoking
paedomorphosis as a preliminary route removing
the cheek originally ignores the possibility of prece-
dent selection for jaw projection.
In contrast to loss and reduction characters con-
sistent with paedomorphosis, numerous characters
defined in this study entail addition or enlargement
of skeletal elements at all levels with Acipenseridae
and suggest a progressive role for peramorphosis
Peramorphy as a mechanism in acipenserid (gerontomorphosis, Balon 1983, 1985, 1989) in aci-
evolution penserid evolution. For the purposes of this discus-
sion, peramorphic characters are simply defined as
A persistent theme overshadowing studies of the transformations that result in addition of new struc-
Acipenseridae is the putative role of paedomor- tures or enlargement of preexisting structures corn-
phosis dominating their evolutionary change. Pae- pared to outgroups. A peramorphic pattern of char-
domorphosis has been invoked to generally explain acter acquisition contrasts sharply with ideas of
loss of ossification of the endoskeleton and loss of paedomorphy in evolution of Acipenseridae. In
dermal elements in Acipenseridae compared to fact, basal to Acipenseridae, the Acipenseroidei
other fossil and extant actinopterygians. Paedo- (Polyodontidae + Acipenseridae) is defined by a
morphic themes are so pervasive that they have character complex including both paedomorphic
possibly promoted or capitalized on the idea that (e.g., loss of the opercle, reduced branchiostegal
scaphirhynchines are plesiomorphic sturgeons, number) and peramorphic(e.g.,presence of a ros-
with the more lightly armored species ofAcipenser trum) characters (see Findeis 1993, Bemis et al.
andHusobeing natural evolutionary destinations 1997). This suggests that a shift away from dom-
within the family. inant paedomorphic mechanisms toward peramor-
However, paedomorphy has focused on the well phy begins prior to evolution of Acipenseridae.
known Acipenseridae, not the larger assemblage However, the most dominant peramorphic charac-
Acipenseriformes. Our current understanding of ters in number and morphological results appear
polyodontids and fossil acipenseriforms now indi- within Acipenseridae.