Abstract
Indonesian lepidocyclinids of the Nephrolepidina group have been investigated
on a quantitative basis. The material was derived from 42 samples, all
from North and East Borneo, and from Java and Madura. The analysis was
based on the "classical" parameters of the embryonic-nepionic stage: C, Ai,
B, DII and Dr. In addition, attention was paid
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to features of the other
equatorial chambers and to external morphology.
From the observations inferences could be made on evolutionary trends
because the relative stratigraphic position of most of the individual samples
could be ascertained from either associated planktonic faunas or Miogypsina
assemblages, or from both.
The data obtained on lepidocyclinids in samples from India, Fiji and
Australia, and from published reports on European representatives were
compared with the results from the Indonesian material.
The Nephrolepidina assemblages from India, Fiji and possibly those from
Australia resemble those from Indonesia very well, except one from the
Upper Oligocene of India which shows affinity to the European lepidocyclinids.
In the Indo-Pacific material two groups of Nephrolepidina were found to
occur together in part of the samples. The groups are separated on differences
in size parameters of the embryon. In each sample the specimens with
the smaller embryons are taken together in the so-called group 1, and those
with the larger embryons form group 2. Group 1 is more frequent than
group 2; the latter seems to occur always together with group 1.
In group 1 the parameters C and Ai show a time-dependent change and
they serve to subdivide the assemblages into five successive biometric units,
called species: L. isolepidinoides, L. sumatrensis, L. angulosa, L. martini and
L. rutteni.
In group 2 no sustained change could be found in C and Ai' All assem-
1?lagesare lumped under the name L. transiens. This species is thought to be
polyphyletic, but it cannot be excluded that it represents an agglomerate of
ecophenotypes of group 1.
Considering all data known so far, it seems likely that both the European
and the Indo-Pacific lepidocyclinids derived from an American ancestral
stock. The former Tethys is suggested as the most likely migration route
until the end of the Oligocene. The "European" assemblage from India can
be regarded as the only immigrant from the West. From the Miocene onwards
and into the Pliocene the Indo-Pacific lepidocyclinids are clearly different from those of Europe. They have smaller embryons and progress
farther along the road of embryonic-nepionic acceleration. At comparable
C levels their embryonic Ai is distinctly larger.
Within the Indo-Pacific group 1the lineage is twice interrupted by remarkable
50% drops in embryon size, which do not seem to affect the course
of the correlated embryonic (Ai) and nepionic (C) acceleration.
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