MESOZOIC PLANKTONIC FORAMINIFERA^121
Fig. 7. Distribution of planktonic foraminifera in the mid- to late Albian.
could have taken place. In Wernli's (1995)
assemblage of Praegubkinella there are some
forms with relatively flat apertural faces while P.
racemosa appears much more inflated. Again
there is the possibility that mature individuals
may have been 'planktonic'. The time gap
between the ranges of P. racemosa and the
earliest true Conoglobigerina is significant and
this is one stratigraphic interval that requires
urgent investigation.
The Conoglobigerina fauna is best described
from Hungary (Wernli & Gorog 1999, 2000),
Morocco (Wernli 1987), the former Soviet
Union (Morozova & Moskalenko 1961;
Kuznetsova & Gorbatchik 1980; Kasimova &
Aliyeva 1984) and Greece (Baumgartner 1985).
In the company of A. Gorog and J. Svabo, the
senior author has visited the Somhegy (Som
Hill) section west of Zirc (Hungary). Here
'ammonitico rosso' carbonates are found in
fissures within the local succession. Samples
collected from the Bajocian/Bathonian sedi-
ments have confirmed the presence of abundant
Conoglobigerina, comprising both thick-walled
and thin-walled forms. Whether this is just a
specific difference or whether this variation in
wall thickness indicates genuine dimorphism
requires further investigation. The external
morphology of the thick-walled forms (Wernli &
Gorog 2000) is such that they appear almost
identical to the thin-walled forms and it seems
unlikely, therefore, that they are simply ben-
thonic/planktonic dimorphic pairs. Species of
Conoglobigerina are known from many loca-
tions in central-southern and eastern Europe
and this certainly marks a major expansion of
the planktonic fauna within the Middle Jurassic
(Fig. 3). If Wernli (1995) was correct in his
derivation of Conoglobigerina from Praegub-
kinella racemosa, then an origin from the
Praegubkinella assemblages of the Alpine area
would appear to be quite plausible in palaeo-
geographical terms.
Evolution of the Mesozoic planktonic
foraminifera
Figure 1 shows the stratigraphical distribution of
the early planktonic foraminifera and while, in
many cases, the evolutionary lineages/linkages
are unknown it is clear that both specific and
generic diversity remained low throughout the
Jurassic and early Cretaceous. Some of the
lineages are quite disjointed and it is apparent
that a considerable amount of research remains
to be done on these faunas. One of the principal
problems, however, is the question of preser-
vation. The early planktonic foraminifera are all
thought to have been aragonitic, very much like
the Reinholdella, Oberhauserella and Epistom-
ina faunas with which Jurassic taxa are often
associated in faunal assemblages. Aragonitic
foraminifera are best preserved in dense clay
lithologies and are rarely well preserved in
porous sandstones and siltstones or some
