Cenozoic bryozoans from Borneo

Abstract

Bryozoans are colonial marine invertebrates with an abundant fossil record ranging from Ordovician to Holocene. They are found particularly in shelf sediments deposited at all palaeolatitudes. The Cenozoic bryozoan fauna of Indonesia has been severely neglected in the past. The paucity of previous research has led to the mistaken belief that bryozoans are rare and of low diversity, in contrast with the high diversity of bryozoans living in the same region at the present day. The main research area is located in the Kutai Basin, the largest sedimentary basin in Borneo, which was formed during the Middle to Late Eocene as a consequence of rifting of the Makassar Strait. Early Miocene to Middle Miocene deltas prograded rapidly eastwards, contributing to the infilling of the Kutai Basin. Within this depositional regime, carbonate production took place in proximal delta-front settings contemporaneously with the rapid and near-constant siliciclastic input. Sampled sections are Miocene in age, ranging from late Burdigalian to Messinian. Only 31 bryozoan species had been previously recorded from the Cenozoic of the entire Indonesian Archipelago. The present work has resulted in a remarkable increase in this diversity. Among the 123 bryozoan species found, 65 species have been identified only at genus- or family-level owing to deficient preservation and/or scarcity of available material, 20 species show affinities with Recent taxa from the Indo-Pacific, 2 species show similarities to Recent species recorded circumtropically, and three species were known previously from the Cenozoic fossil record of Australia or the Early Miocene of India. The remaining 33 species are described as new. Furthermore, first and last occurrences are documented for a few genera. Thus far, Cenozoic tropics are undersampled and fossil first and last occurrences are dominated by records from Europe and North America. However, two main considerations reveal that much species diversity remains to be discovered and described: (1) although the bryozoan assemblage is sufficiently well-preserved, 10% of specimens recorded are indeterminate owing to deficiencies in diagnostic features, and (2) the estimated proportion of species with aragonitic skeletons reveals a substantial loss by dissolution. Based on the limited evidence of the ‘Throughflow’ collection, the composition of the Kalimantan bryozoan fauna is characterised by a moderate level of potential endemism with at least three endemic genera; 18% of genera made their earliest appearance in the global fossil record in the Kalimantan sequences, and have an Indo-Pacific or circumtropical distribution at the present day, apart from Pseudidmonea, which interestingly has been recorded living only in Antarctic and the Subantarctic (and the Miocene of New Zealand). All of the remaining genera, with a few exceptions, have been previously recorded as far as back as the Jurassic and are still extant. Approximately 48% of these genera have been cosmopolitan in distribution; 20% of genera are circumtropical or Indo-Pacific in distribution. For a few genera (9%), this study documents their latest fossil record. Bryozoan assemblages from single sites show a high level of potentially localized endemism, which corresponds to patchy species distributions.