ENIGMATIC TRACE FOSSILS FROM THE AEOLIAN LOWER JURASSIC
CLARENS FORMATION, SOUTHERN AFRICA
The Lower Jurassic aeolienites of the Clarens Formation in southern Africa contain unique sedimentary structures that are unlikely to be non-biogenic. They are also unlike any known modern or ancient trace fossils. Here, some enigmatic, horizontal, regularly-oriented sedimentary structures are described, which occur in association with other trace fossils as well as features that were previously interpreted as nests of termites or termite-like ancient social insects. These spectacular structures are exposed in enormous profusion as straight, ~5 mm cylinders with strong compass orientation, in parallel alignment with one another and to ancient horizontal bedding planes. Their fill is identical to that of the host rock: clean, well-sorted, very fine- to fine-grained quartz-arenite. In cross-section, each structure is defined by a subtle, ~0.1 mm thin, concentric gap.
Without comparable modern biogenic structures, the biological origin of the structures is uncertain. Their strong compass orientations are, however, also inconsistent with an inorganic origin, even though they may resemble pipey concretions generated by flowing groundwater. Nonetheless, this paper, based on spatiotemporal distribution patterns of the oriented structures, their locally high abundance and association with obvious trace fossils, as well as other sedimentological and palaeontological lines of evidence, argues that the compass structures may be products of ancient social invertebrates living in a resource-limited, semi-arid to arid environment. Furthermore, the compass structures as well as the accompanying structures of the predominantly aeolian Clarens Formation collectively imply the recurrence of favourable ecological parameters (e.g., moist substrates) related to episodic climate fluctuations in the Early Jurassic of southern Pangaea (i.e., southern Gondwana).
Emese M. Bordy. Rhodes University, Department of Geology, Grahamstown, 6140, South Africa.
KEY WORDS: enigmatic trace fossils; horizontal, regularly-oriented structures; aeolian deposits; Early Jurassic; Karoo
PE Article Number: 11.2.16A
Copyright: Palaeontological Association October 2008
Submission: 14 December 2007. Acceptance: 25 September 2008