Acipenseridae is defined by a pervasive array of rinac; Characters 48. 52, 54 for Scaphirhynchini),
peramorphic characters. Scutes (Character 1), the they are generally associated with shape changes
pectoral fin spine (Character 2), the antorbital and that require reduction or loss of elements to facil-
median extrascapular bones (Characters4, 5), the itate the morphological shift to benthic cruising and
supracleithral cartilage (Character15),and palatal are significantly outnumbered by peramorphic
complex (Character 18) all appear without precur- characters. Many of these peramorphic characters
sor in Acipenseridae. Many other characters are pe- are not linked and occur at all major nodes within
ramorphic enlargements of features such as the Acipenserdae, supporting peramorphy, not paedo-
skull (Character 3), pectoral girdle (Characters 9- morphy, as a dominant influenceinacipenserid evo-
14), pelvic girdle (Character 17), hyobranchial skel- lution.
eton (Characters 19–22), pectoral scales (Character
23), and even the occipital canal (Character 24).
include the expanded rostrum (Characters 2–29),
central trabecular process (Character 31), and der- (1)Husoexemplifies a precedent primitive condi-
mopalatine shelf (Character 34) that are all addi- tion within Acipenseridae as a pelagic sturgeon
tions to the plesiomorphic (e.g.,Huso) condition. compared to other acipenserids. However, charac-
Acipenserinesare also the first level within Acipen- ters defining Acipenseridae (including Huso)ap-
seridae with numerous anamestic bones in the skull pear to be linked to benthic behaviors and may in-
roof including pineal bones (Character 32) and dicate importance for benthic orientation for juve-
anamestic lateral extrascapulars. Anamestic bones nile acipenserids. Thus, many features diagnostic of
generally lack consistent organization, filling areas acipenserids may have evolved in support of the ju-
of the skull with variable numbers and sizes of venile period of life history, not for adults.
bones between individuals and species. While they (2) Acipenserines are clearly more benthicallyori-
can define peramorphic characters, they may also ented than Huso. Acipenserines possess ventral
represent general peramorphic processes beyond mouths and a broadened, somewhat flattened ros-
cladistic characterization. trum and head. These features define a morphology
trend with extreme expansion of the rostrum (3)Scaphirhynchusrepresents an ultimate benthic
(Characters 39, 40), appearance of basitrabecular morphology within Acipenseridae. Their increased
cartilages (Character 43), ventral expansion of the dermal armor, highly flattened head and trunk, and
antorbital (Character 46), posterior expansion of pectoral ‘legs’ allow for stable benthic exploration.
hypobranchial one (Character 51), and appearance Pseudoscaphirhynchus is similarly flattened, but
of the caudal fin filament (Character53). this genus lacks features that facilitate the level of
While Pseudoscaphirhynchus is generally de- benthic behaviors characteristic of Scaphirhynchus
fined by characters that are peramorphically neu- (4) In the scenario of benthic cruising, initial oppor-
tral, several peramorphic transformations occur in tunities forHusoto exploit substrate habitats were
Scaphirhynchus. The caudal peduncle iselongate expandedupon by phylogenetically successive aci-
(Character 64), and armored (Character65). This penserid taxa that focused on substrate habitats and
genus possesses a circumorbital series based on ex- prey.
panded antorbital and postrostal bones (Character (5) Peramorphy is a dominant mechanism in aci-
61), an expanded cleithral wall (Character 66), and penserid evolution. Appearance of new elements
fusion of the dermopalatine and ectopterygoid and enlargements of preexisting features are typical
(Character 69). for characters defined in this study.
While putative paedomorphic characters occur
at several nodes of the cladogram (Character 8 for
Acipenseridae; Characters 35, 37, 38 for Acipense-Peramorphic characters defining Acipenserinae ConclusionsScaphirhynchines continue this peramorphic allowing more precise benthic foraging.