ideal for retaining small prey (fishes and/or insect Rostrum and flattening
larvae) that they prefer, as well as to winnow detri- Many benthic fishes are flattened to allow direct,
tus inevitably ingested while feeding in their pre- stable interaction with the substrate. Outgroup aci-
ferred habitats. penseriforms and Husopossess cylindrical heads
These characters organize a feeding morphotype and trunks, but varying degrees of flattening occur
that initially develops prey processing surfaces sup- within Acipenserinae. This is most apparent with
portive of jaw projection in acipenserids and then expansion and flattening of the head, rostrum, and
restricts the jaws into their benthic specific ventral trunk within Acipenseridae that develops a plat-
orientation in acipenserines. form parallel to the substrate.
Appearance of the jugal (Character 3), antorbital
Respiration (Character 4), and median extrascapular (Charac-
Acipenserids possess an accessory respiratory ter 5) all suggest that skull expansion occurs in Aci-
shunt allowing them to pull water into the buccal penseridae. The neurocranium is broader in gener-
cavity from the opercular chamber whenever the al than in polyodontids and these bones reflect this
mouth is occluded (Burggren 1978). This shunt al- expansion dermally.
lows acipenserids to respire when the buccal cavity The rostrum ofHusois thin and cylindrical, but
is blocked by substrate ingested during benthic the rostrum of all acipenserines is expanded by ad-
feeding or large prey that acipenserids preferential- dition of the lateral ethmoid ridges (Character 27),
1yfeedonfill the mouth during ingestion. Juvenile broadening of the dorsal rostral series (Character
sturgeons ingest remarkably large prey requiring 28),and appearance of border rostral bones (Char-
extensive processing before swallowing. The sub- acter 29). While the acipenserine rostrum appears
opercle is bound to the hyomandibula and automat- flatter than inHuso,this is due more to addition of
ically flares laterally duringjaw movements to open the lateral ethmoid ridges that broaden the ventral
the shunt during ingestion or winnowing. face of the rostrum than actual dorsoventral coni-
The respiratory shunt is predicatedona dorsal pression of the head inAcipenser.Rostrum expan-
opening in the operculum (not a character here), sion is likely associated with expansion of electro-
solid pectoral girdle (Characters 9,13), and curved sensory ampullary fields and hydrodynamics of a
gill filaments that direct water ventrally out of the head with ventral jaws. An indicator of skull expan-
buccal cavity (Findeis 1989). The branchiostegals of sion in acipenserines is addition of pineal bones in
acipenserids (Character 7) arch ventrally and then the widely opened pineal fontanelle (Character 32)
medially to support an opercular cavity open to dor- and ventral extension of the antorbital (see Charac-
sal inflow and ventral outflow. The medial opercu- ter 4).
lar wall (Character 10) stabilizes the opercular Scaphirhynchines possess flattened rostra en-
chamber as an open, concave structure promoting Iarged by expansion of the lateral ethmoid shelves
unimpeded water outflow. Such skeletal stabiliza- (Character 39) and dorsal coverage with the ampul-
tion of the girdle is central to opening the dorsal in- lary bones (Character 40). The skull itself is wider
current channel and then directing outflow ventral- and flatter than inAcipenseras indicated by addi-
ly. tional extension of the antorbital (Character 46).
This basic morphology of the pectoral skeleton is Head flattening is not indicated by skull characters
typical of acipenserids, and dimensional changes that simply form a shallower cover of the head, but
among genera are not recognized as discrete char- by dorsoventral compression of the hyomandibula
acters here. The medial opercular wall is larger in (Character 50) and operculum (Characters 47, 62)
acipenserines than inHuso,and the girdle is more beneath the skull.
compact in Scciphirhynchus than other genera. Further, scaphirhynchies are the only acipense-
These changes do not reflect obvious functional rids to extensively flatten their trunk. In contrast to
changes so much as changes in overall body flatten- the cylindrical trunk ofHusoandAcipenser,the
ing (see below). ventral surface of scaphirhynchines is nearly flat be-
martin jones
(Martin Jones)
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