form a broad shield expanding anterior to the olfac-
tory cavity (drb, Figure 18c, d). The dorsal rostral
shield broadens in concert with expansion of the en-
dochondral rostrum of acipenserines (see Charac-
ter 27).
Husoand polyodontids (Grande & Bemis 1991)
possess a narrow dorsal rostral series with consis-
tent breadth along the rostrum (Figuse 18a, b).
Lacking rostral expansion anterior to the olfactory
bulb, there is no space for an expanded rostral
shield in the outgroups. The expanded dorsal ros-
tral series is likely linked with the lateral ethmoid
ridges as the bones occupy the expanded rostral sur-
face in acipenserines. Initially developing bones of
the dorsal rostral series form a median row similar
toHusoand polyodontids that ontogenetically and
phylogenetically expands by addition of novel
bones (Findeis 1993).Character 27.Presenceoflateralethmoid ridges-
Acipenserinae
Lateral ethmoid ridges form the lateral edges of the
endochondral rostrum in scaphirhynchines (les,
Figure 17d) and all species ofAcipenserexamined
(ler, Figure 17c). They are ventrolaterally expanded
bands of cartilage lateral to the grooves housing the
rostral nerves (grn; Findeis 1993). These ridges ex-
tend seamlessly from the median rostrum and eth-
moid region of the neurocranium without obvious
boundaries. In acipenserines, the lateral ethmoid
ridges support a ‘hard’ rostrum of cartilage and der-
mal bone. Lateral ethmoid ridges of Acipenser
curve ventrally as thin ridges parallel to the median
trabecular ridge (tr). These ridges expand into
broad lateral ethmoid shelves in scaphirhynchines
(see Character 39).
Huso and extant polyodontids possess endo-
chondral rostra that indent anterior to the olfactory
bulbs (Figure 17a, b). InHuso,the ethmoid region
possesses expanded olfactory bulbs contributing to
the postnasal wall as in acipenserines (see Charac-
ters3,4),but no lateral ethmoid ridges extend ante-
riorly (Figure 17b) as the rostral grooves (grn) pass
under the olfactory capsule (oc) and open anterior-
ly alongside the trabecular ridge (tr, Figure 17b)
without cartilage bracketing them laterally. The
rostrum correspondingly tapers rather than re-
maining expanded as a shield as in acipenserines.
PsephurusandPolyodonlack a broad ethmoid re-
gion, with a bare olfactory capsule exposed along
the thin endochondral rostrum (Figure 17a). In
Husoand polyodontids, the rostral surface expands
laterally beyond the endochondral rostrum as a
‘soft’ rostrum supported by skin. The rostrum con-
verges anteriorlyinHusoand the rostral skin is not
supposted by any bones. The rostrum expands as a
paddle in polyodontids supported peripherally by
an extensive series of stellate bones (Grande & Be-
mis 1991).
Character. Presence of border rostralbones- Aci-
penserinaeBorder rostral bones (brb) are a thin series (one or
two bones wide) of small bones along the postero-
lateral edge of the rostrum anterior to the jugal (j,
Figure 18c, d). They are positionally separate and
ontogenetically distinct from the dorsal rostral se-
ries as a novel group ofrostral bones in scaphirhyn-
chines (Findeis 1993) and all species ofAcipenser
examined (Figure 18c, d). Although variable in
number, border rostral bones form a consistent as-
semblage.
Spatial separation of dorsal and border rostral se-
ries is typical inAcipenseroxyrinchus,A. transmon-
tanus,andA. ruthenus,with an open corridor hous-
ing ampullary organs of the dorsal rostrum (Figure
18c). Only in large adults ofA. brevirostrum(among
species ofAcipenserexamined) did this gap partial-
lyclose to make identification of the groups diffi
cult. In scaphirhynchines, bones entirely cover the
rostrum (Figure 15, 16), but by addition of ampul-
lary bones between these series (Character 40). The
multiplicity of rostral bone groups confuses charac-
ter recognition in the dermal rostrum, but each se-
ries is ontogenetically discrete.
No bones lateral to the dorsal rostral series areCharacter 28.Dorsalrostralseries forms an expand-
edshield-Acipenserinae
Dorsal rostralbones in all acipenserines examined