UTRECHT MICROPALEONTOLOGICAL BUllETINS

The oistodiform elements are highly variable. Forms described as Oistodus
basiovalis SERGEEV A occur together with forms described as Oistodus
forceps LINDSTROM and intermediate forms. From below upwards in the
stratigraphic sequence, there is a gradual increase of the relative amount of
Oistodus basiovalis forms per fauna.
The acodiform elements seem to make out a greater part of the multielement
assemblages in the lower portion of the sequence, than higher up.
This change in the composition of the multi-element assemblage is very
gradual.
According to Lindstrom (1971), the species Drepanoistodus basiovalis
evolved from Drepanoistodus forceps. The former species would have
Oistodus basiovalis SERGEEVA as oistodiform element; it would lack acodiform
elements but the drepanodiform elements would be comparable to
those of Drepanoistodus forceps. However, differentiation between these
two species is thought to be possible only on the basis of biometrical data
and statistical methods. As long as these are not available, separation can not
be made between these species.
Regional occurrence: At Kopingsklint this species was found from member
Kk 2 up to the top of the Horns Udde Formation inclusive. In section 6 from
member Kii-d1 up to the top of the sequence. Near Horns Udde from
member Kk 2 up to and including the top of the Horns Udde Formation.
Biostratigraphic range: Drepanoistodus forceps was found from the Drepanoistodus
inconstans Assemblage Zone up to and including the Triangulodus
brevibasis Assemblage Zone (fig. 15).
Drepanoistodus
inaequalis (PANDER), emend. VAN WAMEL, herein
(PI. 2, figs. 7-13)
Oistodus inaequalis PANDER, 1856, p. 27, PI. 2, fig. 37. LINDSTROM, 1955a, p. 576, PI. 3, figs. 52,
55,56,not53,54,57.
Scandodus varanguensis VIIRA, 1970, p. 230, PI. 1, figs. 11,12; Text-figs. 8, 9.
Machairodus (sic) ensiformis PANDER, 1856, p. 23, PI. 2, fig. 36, not PI. 1, figs. 25-28.
Drepanodus deltifer LINDSTROM, 1955a, p. 562, PI. 2, figs. 42, 43.
Paltodus deltifer (LINDSTROM), emend. LINDSTROM, 1971, p. 44, figs. 7,8.
Drepanodus d. subarcuatus FURNISH. LINDSTROM, 1955a, p. 568, PI. 2, figs, 41, 44, 50.
Drepanodus bisymmetricus VIIRA, 1970, p. 226, PI. 1, figs. 1-5; Text-figs. 3,4.
Drepanodus pristinus VIIRA, 1970, p. 227, PI. 1, figs. 7,8; Text-figs. 5, 6.
Acodus tetrahedron LINDSTROM, 1955a, p. 546, PI. 4, figs. 1,2.
Remarks: There is a wide form variation in the drepanodiform elements, due
to differences in curvature and cross section of the cusp and in the shape of
the base (see PI. 2, figs. 9, 10). In one assemblage, drepanodiform elements
with unkeeled and rounded cusps may occur together with keeled ones and