INTERPRETATION OF THE HORIZONTAL, REGULARLY-ORIENTED STRUCTURES
Biogenic or Abiogenic
The orientation of the primary physical sedimentary structures (e.g., azimuth of the cross-beds) and structural features (e.g., local faults, joints) in the study areas do not resemble the orientation of the enigmatic horizontal and preferentially aligned structures. Most of the primary sedimentary and tectonic structures display east-west orientations especially in the basins north of the main Karoo (Geological Survey 1984). At this stage, no known inorganic sedimentary or structural process could plausibly explain the generation of these horizontal, regularly-oriented structures, or the differential weathering associated with them. Although the structures may superficially resemble the appearance of elongate concretions, the diversity of orientation of the enigmatic structures at a single outcrop is inconsistent with that of elongate concretions which form diagenetically in groundwater that migrates preferentially along flow paths with highest host rock permeability, and are usually subparallel to palaeocurrent directions (cf.
Mozley and Davis 1996). In relative terms, however, there are clear morphological characteristics associated with these structures that are more consistent with a biologic origin. In particular, the delicate ~0.1 mm concentric gap surrounding each uniform-diameter tube; the recurrence of these structures with respect to their identical shapes, sizes and orientations in geographically separated regions; their regular co-occurrence with other organism-produced (e.g., ribbed tubes) traces support the contention that these features are biogenic sedimentary structures. The differential weathering associated with these structures is also consistent with differential cementation recording permeability variations between the host material and the fill of the biogenic structures (cf.
Pemberton et al. 2001:63).
The delicate gap that surrounds the horizontal, regularly-oriented structures is interpreted here as a mechanical artifact produced by an Early Jurassic organism. It is envisaged that this organism purposefully constructed the horizontal structures as discrete, three-dimensional and horizontal tunnels. While direct evidence for the organic linings of these tunnels was not found, the presence of such organic material could be inferred, based on the small circular opening surrounding the tubes (cf.
Johnston et al. 1996:515). This small gap, which possibly represents the weathered remnant of the original, likely clay lining of the tunnels, together with the massive infilling, most likely increased the preservation potential of the structures. The original lining may have been instrumental in the peculiar and preferential preservation of the structures, by locally altering the permeability patterns. Whether these tunnels were constructed above or excavated below the surface is not obvious from the preserved biological and sedimentological evidence, however, given that the structures form up to half a meter thick units, it seems very likely that they were excavated below the surface. Otherwise, in order to generate the thick accumulation of burrows, very thin increments of new sediment would have to be individually added and then promptly modified by the tunnel-makers before the arrival of next thin increment of sediment. This sedimentation rate would be at odds with aeolian systems where layers typically accumulate as thicker increments (cf.
What Could Have Made Them?
Horizontal, preferentially-oriented trace fossils are rare in continental deposits and have been attributed to fresh-water molluscs (Pryor 1967), arthropods (notostracan branchiopods) (Ekdale et al. 1984), and crane fly larvae (Ahlbrandt et al. 1978;
Ekdale and Picard 1985). The traces of these organisms are, however, significantly different from the horizontal, regularly-oriented structures of the Clarens Formation. Molluscs leave behind surface trails rather than burrowed tunnels; the tiny notostracan crustaceans have diminutive burrows, and the crane fly larvae burrows are meniscate and commonly irregular. None of these organisms are, therefore, plausible tracemakers of the horizontal, regularly-oriented structures observed in the Clarens Formation.
Without known ancient or modern counterparts, the identity of the original tracemaker of these peculiar structures remains elusive. The integration of all sedimentological and palaeontological evidence encountered from these deposits does permit, however, the following argument to be made. The large number of these geometrically extremely regular structures argues in favour of a great density of coeval, behaviourally sophisticated organisms, which burrowed in an orchestrated manner and were capable of profoundly impacting the Clarens sediments. This interpretation leads to the possibility that the tracemakers were, in fact, social organisms that followed a straight, linear foraging pattern, probably to prevent crossing their own path, and ensuring maximum foraging efficiency. Given that arid and semi-arid environments are characterized by episodic abundance irregularly distributed food resources, systematic linear foraging strategies during the Early Jurassic could have ensured reliable access to nutrients (cf.
Faulkes et al. 1997;
Bordy et al. (2004) tentatively assigned these regular burrows to Early Jurassic ants. This interpretation considered, first of all, that at one locality (Tuli Basin) the horizontal, regularly-oriented structures are superimposed on randomly bioturbated surfaces which were attributed to termites, and secondly, that modern ants are predators of termites. These assignments were subsequently disputed (Genise et al. 2005;
Bordy et al. 2005), mainly based on the age discrepancy between the interpreted insect trace fossils in the Clarens Formation and the earliest known body fossils of social insects. Although ants were suggested as tracemakers of Late Jurassic ichnofossils (Hasiotis 2002;
2004) and recent phylogenetic and molecular clock analyses of ant DNA showed that ant phylogeny commenced much earlier (in the Late Cretaceous) than previously thought (Moreau et al. 2006), it is known that ants less commonly alter their environments to the extent seen in these rocks. Given the foregoing, the most likely social tracemakers were either extinct unknown social organisms without any body fossil record or possibly termites, which are known to be the greatest ecosystem engineers of all sediment reworking insects (cf.
Jones et al. 1994;
Hutchins et al. 2004;
Jouquet et al. 2006).
Spatiotemporal Orientation Differences
Given the biogenic origin of the horizontal, regularly-oriented structures, their orientation variations within and between the different localities may signal behavioural adjustments in repose to changes in the physico-chemical conditions of an otherwise stable ecological niche.
As demonstrated through field- and laboratory-based experimental studies, as well as detection of internal biomineralized magnetite, certain extant termite genera are capable of receiving geomagnetic information, able to utilize it for navigational purposes and even generate structures aligned with the present geomagnetic field (Rickli and Leuthold 1988;
Jacklyn and Munro 2002;
Alves et al. 2004;
Esquivela et al. 2004). Consequently, it may be
hypothesized that if the tracemakers were termites, whom were guided during foraging by the orientation of the geomagnetic field, the different burrow orientations in successive strata might represent adjustments in burrowing direction triggered by changes in the geomagnetic field orientation during the Early Jurassic. Such perturbances of the Earth's natural magnetic field take place during geologically "brief" geomagnetic excursions or polarity reversals, which occur at highly variable frequencies (from <1 to >10 reversals per Ma) (Pavlov and Gallet 2001;
Constable and Korte 2006).