EVIDENCE OF ORIBATID MITE DETRITIVORY IN ANTARCTICA DURING THE LATE PALEOZOIC AND MESOZOIC

Journal of Paleontology, Nov 2004 by Kellogg, Derek W, Taylor, Edith L

The coprolites in the Triassic peat were similar in shape (Fig. 1.3), but were found in a much more diverse range of plant organs than those in the Permian. Whereas the Permian borings were only found in stems, the Triassic ones occurred in stems, petioles, and roots. Several of the petioles which had been bored belonged to the fern genus Antarctipteris (Gleicheniaceae?) (Millay and Taylor, 1990) and were nearly identical in the pattern of infestation: the entire inner cortex was eaten away with the exception of the vascular tissue, which consisted of a V-shaped vascular trace with two pairs of bundles at either end (Fig. 1.4). In place of the cortical parenchyma was a dense mass of coprolites. The Triassic borings extended through both parenchyma and xylem tissue. The coprolites within these tunnels had a highly regular outline and were roughly ovoid in shape. They were fairly densely textured with few to no recognizable components. There were also two distinct size classes of coprolites at this locality (Fig. 1.5). One size class (Triassic Group 1 in Table 1) ranged from 50 to 75 �m in length, while the other (Triassic Group 2 in Table 1 ) ranged from 95 to 110 �m in length. Coprolites in both size classes were fairly similar in content. A large number of the Triassic coprolites exhibited a reticulation pattern on their surface, which was not observed in the coprolites from the Permian peat. These reticulations appeared to represent several different levels of development. Some of the coprolites were composed of finely meshed plant material, probably tracheids, and were completely free of reticulations (Fig. 1.3). Other coprolites contained recognizable plant debris with variable amounts of reticulation on the surface of the coprolite (Fig. 1.6). Finally, some coprolites were covered completely by this reticulation pattern and contained no recognizable pieces of plant material (Fig. 2.1).

The tunnels from the Jurassic peat occurred in woody stems, fern rhizomes, and petioles. As was the case in the Triassic, both parenchyma and xylem were consumed. The coprolites found were circular to ovoid in shape and had few to no recognizable elements (Fig. 2.2). The two size classes ranged from 60 to 85 �m (Jurassic Group 1 in Table 1) and 170 to 250 �m (Jurassic Group 2 in Table 1) in length. The coprolites from this locality also possessed a reticulation pattern similar to that of the Triassic specimens. However, no coprolites from the Jurassic were found with recognizable plant tissue fragments, nor were there any with only partial reticulations, which correlates with the relatively poor preservation of the plants from this site.

The size of the tunnels at all three sites was highly variable, with some only 0.35 mm in width and others well over a millimeter. While the smaller classes of coprolites were often in smaller tunnels, there was a large degree of overlap in tunnel diameter among the various size classes. For instance, tunnels containing Triassic Group 2 coprolites (95-110 �m) were often as large as the tunnels containing Jurassic Group 2 coprolites (170-250 �m). There were two types of coprolite arrangement within the tunnels: clustered on one side (Fig. 2.3) or evenly distributed throughout the cross-sectional area (Fig. 2.4). This suggests that either the arthropod excavating the tissue was capable of hollowing out an area larger than itself, or that there were multiple arthropods in one tunnel at some point in time.