51813_Sturgeon biodioversity an.PDF

posterior tips. This condition also occurs in †Sti-
chopterus. Specimen based study is needed before
proposing it as a synapomorphy of †Peipiaosteidae
and Polyodontidae.

Discussion of †Peipiaosteidae

Too little information is currently available to dem-

onstrate the monophyly of this group. Indeed, only

with additional specimen study will we be able to

determine whether †Stichopterusand †Peipiaosteus

are congeneric, as proposed by Yakovlev (1977) and

rebutted by Zhou (1992). Also, Zhou (1992) consid-

ered †Peipiaosteidae as the sister group of Acipen-

seridae. As discussed under clade Acipenseroidei

(below), we disagree with Zhou’s (1992) analysis

(see Grande & Bemis 1996 for new information on

†Peipiaosteidae).

Characters of † Chondrosteidae

Osteological apomorphies of †Chondrosteidae are

listed in Table 6, coded in Table 7, and keyed to the

cladogram in Figure 17.

Character 12. Anterior part of palatoptevygoid club-

shaped

In contrast to all other Acipenseriformes, the palate

of †Chondrosteushas a complete sheathing of bone

on its oral surface. This is because the anterior end

of the palatopterygoid is club-shaped, rather than

deeply notched and having a distinct ectopterygoid

process (see Figure 18 for palatoquadrate ofAci-

penser; additional illustration in Grande & Bemis

1991 fig. 79 ). We do not interpret this condition as a

fusion of an autopalatine ossification with the pala-

topterygoid, although the effect on the shape of the

bone would be similar.

Character 13. Complete loss of trunk scalation

Liu & Zhou (1965) noted this as a feature distin-

guishing †Chondrosteusfrom all other Acipenseri-

formes. This character warrants additional speci-

men based study of well preserved †Chondrosteus,

especially because the scales present in †Peipiaos-

teus, Polyodontidae and Acipenseridae are very

Phylogeny of Acipenseriforrnes from Zhou (1992)

with our interpretation ofZhou's characters Z1-Z9

Z1.Z2.Quadratojugal sickle shaped Infraorbital canal "L"shaped

Z3.Z4Mandibular canal lost Palatoplerygoidshortand broad

Z5.Z6. <7 Suprangular absent bones in opercular series

Z7.Opercularbones branched

Z8.Z9.Broad opercular- pectoral gap Juveniles have hook-like rostral bone

Anc Aci? 1

0 1? 1

1 1 0 1

0 1 0 1

????

Stl 1

(^11)
(^11)
(^11)
??
Pel 1
(^11)
(^11)
(^11)
?
?
Cho 1
11
1
(^00)
(^0)?
?
Pal 0
(^01)
(^0)?
(^11)
?
?
Pol 0
(^01)
(^0)?
(^11)
??
Figure 21. Tree proposed by Zhou (1992). A table showing our
interpretation of his nine relevant characters (characters Z1-Z9)
is shown. This tree is two steps longer than the one we propose in
Figure 17 (treelength 9 versus treelength 7 for the same 6 taxa).
In text, see Discussion of clade Acipenseroidei. Anc =‘ancestor’.
small and potentially difficult to detect in anything
but the best fossils.
Discussion of clade †Chondrosteidae
A modern specimen based study of all species of
this group, including further preparation of previ-
ously described materials, is needed.
Characters of Acipenseroidei
Grande & Bemis (1991) concluded that sturgeons
and paddlefishes are sister groups, which they unit-
ed in the suborder Acipenseroidei. This opinion
contrasts with the interpretation of Schaeffer (1973)
and others, who regarded †Chondrosteidae as the
sister group of Acipenseridae. It also differs from
ideas of Aldinger (1937) and Nielsen (1949).
Grande & Bemis (1991) discovered four characters