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Zonneveld et al.—Bored turtle shell 1


K. lakkos i


K. kulindros ii 2 iii ii i K. fiale


Figure 8. Cross-sectional shape of Karethraichnus and Thatchtelithichnus species. (1) Karethraichnus lakkos (i) comprise shallow, non-penetrative pits with margins oriented obliquely to the external surface of the trace;


Karethraichnus kulindros (ii) are deep, non-penetrative pits with rounded to flattened bases. (2) Karethraichnus fiale (i and ii) are penetrative cylinders that pass fully from the outside surface of the shell to the inside surface. The boring walls may be straight (i) or, more commonly, convex outwards (ii), a morphology that likely results penetration through the dense outer cortex, the porous interior cancellous layer and the dense inner cortex. Rare examples penetrate fully through the primary bone and continue into the secondary bone patch (iii). Thatchtelithichnus holmani consists of a circular to subcircular groove in the outer layer of the turtle bone. These grooves may have sharp (iv) or curved (v) margins.


Diagnosis.—As for the genus. Non-penetrative borings into a bone substrate, having a ring-like profile, and a central raised boss or pedestal.


Description.—Thatchtelithichnus holmani is a shallowly penetrating boring that rarely penetrates more than 0.25–1.5mm into the bone substrate. Most T. holmani penetrate only into the cortical bone without reaching the cancellous bone beneath. The outside wall of the trace is unaffected by the borers activ- ities. The external margin of the trace is sharp. The central enraised portion may consist either of a partially raised, rounded central boss or nubbin (Figs. 9.1–9.5, 9.7–9.9) or, more rarely, a pedestal of unaltered bone in the center of the trace (Fig. 9.6).


Etymology.—After the late Dr. J. Alan Holman, paleontologist and herpetologist who contributed much to our understanding of recent and fossil pathology in reptiles.


Holotype.—The holotype (UA-TF149-1) consists of a circular bi-convex hole through the medial portion of a costal bone (Fig. 9.1). Paratypes designated herein include other specimens on the same turtle plastron (Fig. 9.1–9.5). The type locality consists of low badland hills at the base of Bush Rim, consisting of interstratified pedogenically altered siltstone and mudstone, and sheet-like to channelized sandstone and conglomerate of the Cathedral Bluffs Tongue of the Wasatch Formation, South Pass, Wyoming (Fig. 1.2).


Occurrence.—Early Eocene Cathedral Bluffs Tongue, Wasatch Formation, South Pass, Wyoming.


Remarks.—Thatchtelithichnus holmani is the rarest bone boring taxon observed in the study area. It has, thus far, only been


T. holmani iv


811


observed on the ventral surface of plastron fragments. No con- sistent relationship between position on the plastron and the location of the trace fossil was identified. Of the 33 specimens identified, 28 occur on the ventral surface of the plastron of a single cf. Echmatemys sp. individual. Similar traces were illustrated on dinosaur long bones and


v


ribs from the Jurassic Morrison Formation (Bader et al., 2009). The Morrison material (~1–3mm in diameter) was interpreted as the pupation chambers of dermestid-type beetles (Bader et al., 2009). Dermestids typically feed on desiccated vertebrate carcasses (Timm, 1982). Although the larvae prefer soft tissue, they will bore into bone if other resources are unavailable (Hefti et al., 1980). Ring-shaped or ‘rosette’ traces were interpreted to be intermediate forms with hemispherical pits (Bader et al., 2009). An analogous ichnotaxon, Sedilichnus excavatus occurs on


invertebrate skeletal tests (holasteroid echinoids and bivalves) from marine successions (Donovan and Jagt, 2002; Thomas, 1976; Zonneveld and Gingras, 2014). Sedilichnus excavatus had been interpreted to be attachment or embedment structures of a sessile, filter-feeding animal (Donovan and Jagt, 2002) and has been identified as failed naticid gastropod borings (Thomas, 1976; Zonneveld and Gingras, 2014). As discussed above, ring- shaped traces are rare in the study area and are not interpreted to be a common intermediate or precursor form for species of Karethraichnus.


Discussion


Taxonomic differentiation.—Trace fossil taxa differ from zoological taxa, both fossil and extant in that individual ichno- taxa (particularly ichnospecies) are commonly morphological end-members within a spectrum. We name the clearly differ- entiated states. For example, Ophiomorpha, Thalassinoides, and Spongeliomorpha are ichnogenera named for branching, three-dimensional networks of cylindrical burrows. They differ primarily in the nature of their linings: external pelleted lining in Ophiomorpha, smooth unlined to lightly lined walls in Thalassinoides, and ridged to scratch-marked in Spongeliomorpha (Häntzchel. 1962; 1965; Kennedy, 1967; Müller, 1970). Although they commonly co-occur within a single specimen attempts to lump these ichnotaxa into one or two ichnogenera (e.g. Fürsich, 1973; Bromley and Frey, 1974) have failed and all three ichnotaxa remain in common usage. Similarly, Sedilichnus, a ichnotaxon used for borings in


invertebrate shells, possesses 10 ichnospecies, several of which are clearly end-members on a morphological spectrum (e.g., S. spongiophilus and S. paraboloides) yet all of which have clearly definable and differentiable morphological characteris- tics (Zonneveld and Gingras, 2014). Thus, although an argument could be made that gradations


exist between the three Karethraichnus ichnospecies named herein we contend that sufficient morphological distinctions occur to warrant retention of all three names; one for shallow, surficial pits (K. lakkos), one for deep, narrow pits (K. kulindros) and the last for penetrative holes (K. fiale). It is worth noting that some turtles are exemplified by all three forms; however, many turtles are limited to one form or another. More commonly, K. lakkos co-occurs with either K. kulindros or K. fiale on single


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