allowing mobile benthic foraging. This proposed Feeding
behavioral styleIterm benthic cruising is unique to As benthic cruisers, acipenserines forage by feeding
Acipenseridae and incorporates a medley of ben- focally from the substrate. Hyostylic jaw suspension
thic and non-benthic features. Benthic cruising will and resulting jaw projection is characteristic of the
be addressed according to features that dominate Acipenseriformes, but nonacipenserid acipenseri-
acipenserid evolution including: (1) feeding, (2) re- forms universally possess jaws that lace anteriorly.
spiration,(3) rostral expansion and head flattening, Husoalso possesses large, anterior jaws that typi-
(4) scalation, and (5) locomotion. Characters are cally extend onto the lateral surface of the head.
examined at several phylogenetic levels to show in- These jaws are positioned to open anteriorly and
creasing specialization within Acipenseridae. are not obviously linked to benthic feeding.
The linchpin to this phylogeny is definingHusoas Nevertheless, acipenserids, includingHuso,have
the sister group to a redefined subfamily Acipense- modified the jaws and skull to exploit benthic prey.
rinae.Husois distinct from all other acipenserids Acipenserids possess ‘internal’ jaws based on the
morphologically by lacking 12 acipenserine synapo- novel palatal complex which extends the oral sur-
morphies and ecologically as both species ofHuso face of the upper jaw posterodorsally (Character
maintain a life history style distinct compared to 18) and the sharply defined tongue pad with biting
other acipenserids, but comparable to the polyo- ridges on the expanded first hypobranchials
dontid Psephurus. Additionally, although Huso (Chracter 21). Double articulation of hypobran-
shares 24 synapomorphies with acipenserids, many chial three with basibranchial one (Character 22)
skeletal features plesiomorphically resemble those further consolidates the ventral hyobranchial skele-
ofPsephurusin shape of the rostrum and associated ton within the tongue pad. Functionally, the dorsal
bones, lack of several bone groups in the dermal palatal complex shears across the tongue pad as the
skull, and possessing anterior racing jaws compared upper jaw is projected and retracted to hold prey.
to acipenserines. While these character states are This mechanism acts to retain prey during winnow-
plesiomorphic, similarities in life history among ing of ingested substrate and is a putative benthic
HusoandPsephurusprovide a backdrop to inter- specialization.
preting morphological and behavioral changes in Within Acipenserinae, acipenserines possess
acipenserid evolution. ventral jaws restricted beneath the orbit. The jaws
LivingHuso(e.g., Aritipa 1933, Berg 1948a) and are sequestered behind the central trabecular pro-
Psephurus(Liu & Zeng 1988) prey dominantly on cess (Character 31) and expanded postnasal wall
fishes, and fossil †Crossopholisoccur with fishes en- (see Character 3) that form a barrier anterior to the
closedinits remains (Crande & Bemis 1991),sug- orbit. At rest, the jaws are held entirely within the
gesting that acipenseroids were originally piscivo- confines of the head. The jaws are short and trans-
rous predators. Piscivory in itself does not contra- verse to fit ventrally, and the lower jaw is straight-
diet a benthic feeding pattern, but midwater species ened (see Characters 33,35) opposite the upper jaw.
included in the diet of adultHuso(Berg 1948a, Piro- During feeding, these jaws open obligately ventral-
govskii et al. 1989), and fishes enclosed in †Cross- ly in an ideal position for benthic reeding. Posterior
opholis,confirm that they are pelagic predators. In displacement of the interhyal-posterior ceratohyal
contrast, most species ofAcipenserand all scaphi- joint (Character 36) facilitates projection by spa-
rhynchines focus on benthic prey such as molluscs, tially separating the jaws from the non-projectile
crustaceans, and substrate orientedfishes. Second- anterior ceratohyal.
arycharacteristics such as the cylindrical body and Scaphirhynchines have essentially identical jaws,
head shape of Husopolyodontids, †Chondrosteus but the tongue pad expands onto hypobranchial
and Peipiaosteus futher bolster suggestions that two as it is included into the tongue pad by posterior
nonacipenserine acipenseriforms are neither ben- expansion of hypobranchial one (Character51).Al-
thic in morphology nor in life history. though their rakers differ (Characters 58, 63), both
scaphirhyncline genera possess complex rakers
martin jones
(Martin Jones)
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