PLIENSBACHIAN (LOWER JURASSIC) BIOSTRATIGRAPHY AND ...

Type species. Leptaleoceras leptum Buckman, 1918
(p. 284, PI. 26, fig. la, b) by original designation.
Rem arks. Shells evolute; whorl section compressed,
elliptical. Flanks convex, ventrolateral shoulder
rounded. Venter narrow, carinate; keel flanked by
narrow flat zones that become sulcate on the body
chamber of some species.
Ornamentation consists of dense, slightly sinuous
ribs that arise at or just above the umbilical shoulder,
trend more or less rectiradiately on the flanks, then
fade out at the ventrolateral shoulder where they
project slightly. Innermost whorls, up to diameters of
10 mm, are smooth.
Leptaleoceras is distinguished from Arieticeras by
its greater rib density and the nonsulcate venter
observed on all but the body chamber of some species
of Leptaleoceras [e.g., L. accuratum and L. insigne
(Braga, 1983)].
A g e and distribution. Leptaleoceras is primarily a
Tethyan form but is occasionally found in the Boreal
Realm. In Europe it is found in the middle Upper
Pliensbachian, in association with A rie tic e ra s
algovianum . In western North America, Leptaleoceras
is found associated with Arieticeras and Fanninoceras,
indicating a Late Pliensbachian age (Smith et al.,
1988).
Leptaleoceras sp. aff. L. accuratum (Fucini, 1931)
Plate 13, figures 1-6
aff. *1931 Arieticeras(7) accuratum Fucini, p. 107,
PI. 8, fig. 10.
aff. 1934 A rie tic e ra s p se u d o ra d ia n s (Reynes).
Monestier, p. 63, PI. 8, fig. 62; non
figs. 61, 68.
1964a Leptaleoceras pseudoradians (Reynes).
Frebold, p. 15, PI. 4, non PI. 5, figs. 4, 5.
1970 Leptaleoceras pseudoradians (Reynes).
Frebold, p. 443, PI. 2, fig. 2 (1964
material partly refigured)
aff. 1980 A r i e t i c e r a s a c c u r a t u m (F u c in i).
Wiedenmayer, p. 109, PI. 17, figs. 15-18.
1981 L e p ta le o c e ra s cf. L . p s e u d o r a d ia n s
(Reynes). Imlay, p. 40, PI. 11, figs. 12, 13.
71981 Arieticeras cf. A . domarense (Meneghini).
Imlay, p. 39, PI. 10, fig. 15; non figs. 1, 2,
6-14, 22.
aff. 1983 Leptaleoceras accuratum (Fucini). Braga,
p. 256, PI. 11, figs. 27-29; PI. 12,
figs. 1-10.
1988 Leptaleoceras aff. accuratum (Fucini).
Smith et al., PI. 4, fig. 9.
Material. Eight specimens, poorly to moderately well
preserved as external and internal moulds in calcareous
concretion matrix.
Measurements.
Specim en D UD U W H W H D WW W W D W W W H P R H W
G SC 83106 38.5 17 44 a 13 34 __ _ — 26
G SC 83107 39 19 49 a 13 33 - — - 25
G SC 83109 a25 alO 40 a9 36 - — — a25
G SC 83110 31.5 14 44 11 35 — — - 27
G SC 83111 a70 33 47 a 18 26 a8 — — 22
G SC 84967 a45.5 a l9 42 a l 5 33 a5 - — 27
M ean — _ 44.3 — 32.8 _ _ ___ ___
Si. Dev. — 3.3 — 3.5 — — — —
D escription. Evolute, whorl section compressed,
ogival. Umbilical wall low, shallow; umbilical shoulder
rounded. Flanks slightly convex; ventrolateral shoulder
rounded. Venter bears a distinct keel flanked by two,
narrow, flat zones or slight sulci in some larger
specimens.
Ornamentation varies with growth. The innermost
whorls are smooth to a diameter of 3 to 4 mm. The
middle whorls bear fine, dense, sinuous ribs that have
a slightly rursiradiate overall trend. On the upper part
of the flanks the ribs project strongly adorally. At
diameters greater than about 35 mm (or UD =
13-15 mm) ribbing becomes less dense, more sinuous,
and more rursiradiate.
Discussion. In his discussion of L. accuratum, Braga
(1983) defined a new morphotype, L. accuratum
" p re a ccu ra tu m ". This morphotype, which has the
same stratigraphic range, differs from typical forms of
L. accuratum by its slightly more densely ribbed inner
whorls and more sinuous ribs that maintain their
sinuosity with growth, rather than becoming straighter,
as seen in L. accuratum proper.
The Spatsizi specimens of L. sp. aff. L. accuratum,
together with the other North American material in the
synonomy, compare well with the llpreaccuratum "
material illustrated by Braga (1983), the only difference
being the slightly greater rib density of the North
American specimens (Fig. 16). Previous assignment of
the North American material to L. pseudoradians
(Frebold, 1964a, 1970; Imlay, 1981) is questioned here
due to the significantly greater rib density, maintained
throughout growth, of L. pseudoradians (Fig. 15).
Braga (1983) does note, however, that there is a