51813_Sturgeon biodioversity an.PDF

other Acipenseriformes, is present in all Acipense-

riformes (character 8, above), but the fan of rod-

like ossifications in combination with it are unique

to Polyodontidae. This feature is not well under-

stood in †Protopsephurus(Lu 1994).

Character 22. Elongate anterior and posterior divi-

sions of the fenestra longitudinalis in the skull roof

This character was defined by Grande & Bemis

(1991, character 18) as a synapomorphy of Polyo-

dontidae. This feature is not well understood in

†Protopsephurus(Lu 1994).

Character 23. Posttemporal with elongate anterio r

arm suturing into the dermosphenotic

This character was defined by Grande & Bemis

(1991, character 19) as a synapomorphy of Polyo-

dontidae. It appears to be present in †Protopsephu-

rus(Lu 1994).

Character 24. Single branchiostegal with branched
posterior edge
This character was defined by Grande & Bemis
(1991, character 20) as a synapomorphy of Polyo-
dontidae (also see Nielsen 1949, p. 302). Lu (1994)
noted that several branchiostegals occur in †Pro-
topsephurus. It is important to confirm this in addi-
tional specimens, because if true, then it would help
to place †Protopsephuruswithin Polyodontidae.

Putative and problematic characters of Polyodonti-

dae

Findeis (1993) noted that inPsephurusandPolyo-

don, the incurrent arid excurrent nares, as well as

the bulk of the olfractory capsule, lie dorsal to a line

drawn through the longitudinal axis of the eye. In

sturgeons, the nares and olfactory capsule lie ven-

tral to a line drawn through the eye. It is difficult to

polarize this character, because of variation within

outgroup genera such asPolypterus, and impossible

to score it reliably in fossils. Nevertheless, the dor-

sal location of the olfactory system in paddlefishes

may be synapomorphic for Polyodontidae.

In living and fossil paddlefishes, the supracleith-

rum attaches by ligaments to the cleithrum, unlike

the condition in living sturgeons and †Chondros-

teus, which have an immobile sutural connection

between these two bones. Based on our outgroup

comparisons, the ligamentous attachment is prob-

ably a synapomorphy of Polyodontidae, although

we do not yet know the condition in †Peipiaosteus.

In the pectoral fin skeleton of living polyodon-

tids, a single radial caudal to the propterygium ar-

ticulates with the scapulocoracoid (Grande & Be-

mis 1991). This differs from the condition in stur-

geons, and most of our outgroup taxa, which have

three independent radials articulating with the sca-

pulocoracoid (Findeis 1993). This cartilaginous

character cannot he scored in fossil acipenseriforms

because it is not preserved.

Discussion of clade Polyodontidae

There is little doubt of the monophyly of Polyodon-

tidae. With the exception of the new genus †Pro-

topsephurus, intergeneric relationships within the

family are well understood (Grande & Bemis 1991).

I t seems probable that †Protopsephurus will

emerge as the sister group of all other Polyodonti-

dae, for it appears to share most of the polyodontid

characters except for the single branchiostegal

(character 24, above).

Characters of Acipenseridae

Characters of Acipenseridae listed here derive

from Grande & Bemis (1991) and Findeis (1997 this

volume). Osteological synapomorphies of Acipen-

seridae are listed in Table 6, coded in Table 7, and

keyed to the cladogram in Figure 17. Figure 17 re-

ports the same branching arrangement within Aci-

penseridae proposed by Findeis (1997).

Character 25. Five scute rows along trunk

This character of sturgeons was originally men-

tioned Bemis Linnaeus (1758) and has been noted by

many workers since. Grandc & Bemis (1991, charac-

ter 11) and Findeis (1997) regard this as a synapo-

morphy of Acipenseridae. Gardiner & Schaeffer

(1989), however, suggested that up to six rows of

scutes may be present in acipenserids, but we are

unaware of any examples of six scute rows in fossil

or extant sturgeons.