Acipenseridae. Some species ofAcipenserregress Character 2. DermaIboneforms a pectoral fin spine
the ventral scutes (e.g.,A. fulvescens)almost com- -Acipenseridae
pletely (Dúmeril 1867), but the row is always pre-
sent in juveniles and most adults. Dorsal scute mor- The pectoral fin of all acipenserids is supported
phology has been used as a phylogenetic character along its anterior edge by a thick pectoral fin spine.
by some workers (Antoniu-Murgoci 1936a, 1942), The fin spine is composed of dermal bone extending
but no clear evidence of cladistically useful charac- from the Ieading edge of the propterygium to typ-
ter states based on scute shape has been demon- ically sheath two fin rays(Ipt1, lpt2,Figure 2a). One
strated. ray is included in the weak spine ofScaphirhynchus
In addition to scutes, acipenserids possess three platorynchusand rarely three are included in taxa
novel and discrete series of other scales: (1) median with strong fin spines (occasionally inAcipenser ox-
predorsal and preanal scales at the anterior bases of yrinchus).
these fins, (2) a variable assemblage of scales ante- The base of the pectoral fin spine is an expanded
rior to the anal fin, and (3) scales on the caudal pe- propterygium(ptg) articulating with the scapuloc-
duncle (Findeis 1993). Rhombic caudal scales sup- oracoid (Findeis 1993). This enlarged propterygium
porting the caudal fin are plesiomorphically present is also diagnostic of acipenserids and included with-
in all acipenseriforms except † Peipiaosteus(Liu & in this character as an associated feature of the fin
Zhou 1965) and are typical for palaeoniscids. spine. Dermal bone molds to the propterygium and
Scutes and other large scales develop as bony anterior fin rays to unify the spine with its proptery-
plates with their approximate adult morphology gial base (Figure 2a). Fusion of dermal bone with
distinct from smaller scales of the skin. Sewertzoff the fin rays is extensive and segmentation ofthe Iep-
(1926b) suggested that scutes develop as compos- idotrichia is lost as they merge into the spine in
ites of these trunk scales, but I find no scale assem- adults (comparelpt1andlpt2in Figure 2a). In large
blages coalescing in early ontogeny of scutes or (= old) specimens, the dermal spine fuses with a
other named scales. Scutes are the earliest scales to perichondral ossification or the propterygium (Fig-
appear, and develop as series distinct from other ure 2b).
scales. Other named scales or scale groups appear Dermal bone and their incorporated fin rays ap-
later in ontogeny, but also as discrete units. pear separately in ontogeny. Most authors have as-
No large trunk scales are present in any other aci- sumed that fin rays fuse into the fin spine (e.g., Vla-
penseriform taxa. Psephurus bears small scales dykov & Greeley 1963, Jollie 1980, Grande & Bemis
studding the skin of the trunk andPolyodon pos- 1991), but my observations suggest that lepidotri-
sesses denticular scales restricted to the anterior chia are simply covered by the expanding dermal
baseofthe median fins (Grande & Beinis 1991). bone (Figure 2b). Recognition of a dermal compo-
†Crossopholis and †Paleopsephurus possess nent in the pectoral fin spine defines this character.
fringed scales infiltrating the skin of the trunk Polyodontids (Grande & Bemis 1991), †Chon-
(Grande & Bemis 1991), but they are still small, drosteus(Traquair 1887). and †Peipiaosteus(Liu &
scattered scales. Scales in polyodontids are compa- Zhou 1965) lack a pectoral fin spine. No accessory
rable in size and distribution to the isolated trunk dermal bone and no enlarged propterygium is pre-
scales present in all acipenserids examined, but not sent inPolydonorPsephurus.Individual fin rays
scutes or other named scales. †Chondrosteus(Tra- at the leading edge of the pectoral fin are recogniz-
quair 1887) possesses no known trunk scales. †Pei- able in †Chondrosteus(Traquair 1887, Hennig 1925)
piaosteus(Liu & Zhou 1965) has thin, paired scales and†Peipiaosteus(Liu & Zhou1965,Zhou 1992), so
bracketing the putative trunk canal, but no other no dermal sheath could be present in these out-
trunk scales. groups.
martin jones
(Martin Jones)
#1