Study of Lepidocyclinidae from Southeast Asia, particularly from Java and Borneo

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 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.