Zonneveld et al.—Bored turtle shell
turtle specimens in which case the two taxa are commonly of markedly different sizes. This likely indicates that either the parasite colonization occurred at two distinct times (our preferred interpretation) or that two different parasites were involved.
Timing of emplacement.—Numerous examples have been dis- cussed in the literature wherein invertebrate taxa have bored into the bones of dead continental organisms (e.g., Tappan, 1994; Kaiser, 2000; Britt et al., 2006; Roberts et al., 2007; Cabral et al., 2011; Pirrone et al., 2014). Borings into long bones and ribs by invertebrate macrobiota such as silphid and dermestid beetles or termites can only occur after the death, and at least some level of decay of the host organism. In some cases, necrophagic invertebrates, such as silphid and dermestid bee- tles, are attracted to the carcass by decaying soft tissues and only attack the bone when the soft tissue biomass is insufficient to support the colonizing population of beetles (Britt et al., 2008). In other cases, the organisms colonize the relatively clean bone, either as a means of constructing a domicile in a rigid, protective substrate, or to access residual organics and/or minerals within the bone (Roberts et al., 2007; Britt et al., 2008; Cabral et al., 2010; Pirrone et al., 2014). Several lines of evidence support the contention that
Karethraichnus ichnospecies discussed here were emplaced on the shells of living turtles. Non-penetrative forms (K. lakkos,
813
K. kulindros) occur on both the plastron and carapace, solely in positions that would have been accessible to a parasite while the animal was alive (i.e., external surfaces; lip areas of epiplastra, xiphiplastra, nuchal, and cranial peripherals; Fig. 3). Karethraichnus fiale, the sole ichnotaxon described herein which penetrates fully through the shell elements, are less common than other ichnospecies. These traces commonly exhibit evidence of a physiological response to parasite penetrations. Although many traces do not penetrate through the outer layer (Fig. 10.1, 10.3, and 10.6) where they do, the bone is often altered (Fig. 10.8, 10.4, and 10.6), forming a smooth inner wall compositionally identical to the rest of the turtle bone. In some cases this consists of a raised lip surrounding the burrow that stands out from less than 1.0mm to several mm from the surface of the surrounding bone (Figs. 7.4, 7.10, and 7.11; 10.3; 11.2–11.4; and 12.2). In other cases, the host organism formed a bone patch that completely covered over the penetration of the bone (Figs. 7.1, 7.6, and 7.7; 11.1, 11.5, and 11.6; and 12.1). In the case of the latter, the depth of the boring typically exceeded the average thickness of the shell in the area of the boring (Fig. 7.1, 7.6, and 7.7), indicating that the parasite’s activities occurred simultaneously with the bone- healing response of the host organism. The bone material that forms the healed patch is typically dense and laminar, similar to the external and internal cortical bone that forms the outer surfaces of the turtle shell (Fig. 12). Traces that penetrate fully
Figure 10. SEM images of Karethraichnus (UA-TF148). (1) Tiny K. kulindros at the terminus of a costal bone. The jagged edge at bottom right is the suture at the distal end of the bone. (2) Close-up of the edge of a K. kulindros. Note the smooth wall of the boring and the coelous nature of the bone to the left and right of the boring. (3) Three tiny K. kulindros.(4) Healed margin on the edge of a penetrative K. kulindros.(5) Elongate K. lakkos with latitudinal ridges near the base of the pit. (6) Shallow circular K. lakkos with possible scratch marks at bottom centre of photograph.
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