PLIENSBACHIAN (LOWER JURASSIC) BIOSTRATIGRAPHY AND ...

Pliensbachian sections in the accreted terranes that
constitute the western Cordillera of North America but
is absent in time equivalent sections on the North
American craton. In South America, D. freb o id i is
found in Lower Pliensbachian rocks in Chile
(Hillebrandt, 1981a). Although D. freboldi is unknown
in the Mediterranean region, the closely related species
D. dubari is present.
In the Spatsizi area, D. freb o ld i is very abundant in
Lower Pliensbachian rocks that are approximately
equivalent to the Ibex and Davoei zones of northwest
Europe. In its lowest position, it is associated with
U p to n ia ? sp., D a y ic e r a s sp., M e ta d e r o c e r a s
talkeetnaense, and Dubariceras silviesi. Higher in the
sequence it occurs with Aveyroniceras sp. A and B,
and Reynesocoeloceras sp. cf. R. incertum.
The Spatsizi specimens were found at localities
3-13, 15, 16, 25, 26, 28, 35-41, 46, 52-54, 56, 59, 68,
69, 76, 78, 80-82, 84, 85, 89-91, and 97.
A ge. Early Pliensbachian (Freboldi Zone).
Dubariceras silviesi (Hertlein, 1925)
Plate 10, figures 2-5
*1925 Uptonia silviesi Hertlein, p. 39, PI. 3, figs. 1,
2, 5.
1981 Crucilobiceras cf. C. subm uticum (Oppel).
Imlay, p. 33, PI. 7, figs. 1-3.
? 1981 a U p to n ia cf. U. o b s o le ta (S im p so n ).
Hillebrandt, p. 509, PI. 5, figs. 1, 2, 5.
1988 Dubariceras silviesi (Hertlein). Smith et al., PI.
2, fig. 5.
Material. Twenty-three specimens preserved as external
moulds with rare internal moulds in siltstone.
M easurements.
Specimen D UD U WH W H D PRHV
GSC 83083 al50 a75 50 a43 28.7 —
GSC 83083 al06 a53 50 a22 20.8 21
GSC 83084 a70 30 43 al9 27.0 22
GSC 83085 a80 39 49 a23 28.8 23
G SC 84963 37 18 49 alO 27.0 al4
Mean — — 48.2 — 26.5 —
St. Dev. — — 2.9 — 3.3 —
D escription. Shell evolute; whorl section appears to be
rectangular but cannot be directly observed in the
Spatsizi collections because of poor preservation. The
umbilical wall is low, steep; umbilical shoulder
rounded. Flanks slightly convex; ventrolateral shoulder
abruptly rounded. Venter low, slightly convex, and
fairly broad.
Ornament consists of dense, simple, straight to
slightly sinuous ribs. The ribs arise on the umbilical
wall where they appear to trend slightly rursiradiately,
then subsequently become prorsiradiate and continue
virtually straight to the ventrolateral shoulder. At the
ventrolateral shoulder each rib bears a prominent
tubercle. The ribs then project forward and continue
onto the venter from the tubercles but decrease in
strength. On larger whorls the ribs form ventral
chevrons, although the apices of the chevrons tend to
be faint.
D isc u ssio n . D u b a ricera s silv ie si appears to be
morphologically intermediate between the genera
M etaderoceras and D ubariceras (Mouterde, pers.
comm., 1985). Its dense, relatively finely ribbed inner
whorls are reminiscent of Dubariceras, whereas the
coarser, robustly tuberculate and less dense ribs of the
middle and outer whorls are characteristic of
M etaderoceras (Figs. 14, 15). The ribs on the outer
whorls of D. silviesi tend to become slightly sinuous,
but again, their coarseness, lower density, and
tuberculation set them apart from the ribbing of D.
freboldi.
In the ventral region of the larger specimens of D.
silviesi (PI. 10, fig. 3), the ribs project beyond the
ventrolateral tubercles and form chevrons. According
to Mouterde (pers. comm., 1985), ventral chevrons are
an adult characteristic of Metaderoceras, but they also
occur faintly on the outermost whorls of some
specimens of D ubariceras dubari from Italy. As
pointed out in the description of Dubariceras freboldi,
weak ventral chevrons are also found on the outer
whorls of some of the Canadian specimens (Frebold,
1970; this study).
O ccurrence. D u b a ricera s silv ie si appears to be
restricted to the eastern Pacific region; it has been
reported from western South America (Hillebrandt,
1981a), western United States (Hertlein, 1925) and
Canada, and from southern Alaska (Imlay, 1981).
In Spatsizi, it was found just below and in the lower
part of the range of Dubariceras freboldi at localities 3,
6, 8-10, 13, 25, 26, 46, 51, 55, 67-70, 81, 82, 84-86,
and 90.
Age. Early Pliensbachian (Whiteavesi and Freboldi
zones).