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Journal of Paleontology 89(5):802–820
surface. This differs slightly from the use of these terms in Pirrone and others (2014), wherein both pit and hole are used for non-penetrative borings.
Systematic paleontology
The focus of this paper is the ichnology of borings in turtle plastron and carapace bones. A formal taxonomic treatment of the host taxa is not provided herein. Several taxa are present in the turtle fauna of the Wasatch Formation at South Pass including at least two trionychids, one baenid, one dermate- mydid, and several emydids (sensu lato). Overwhelmingly (n ≥99%) the borings discussed herein occur on the shells of a new taxon of small emydid turtle closely related to Echmatemys, referred to herein as cf. Echmatemys sp. A few rare specimens were also noted on baenid and dermatemydid shell bones. At several localities in the lower Wasatch Formation at
South Pass, both in the Honeycomb Buttes area and along Bush Rim (Fig. 1.2), bored turtle bone is exceptionally abundant. Thus, although measurements were made of many specimens in the field, only representative samples were obtained and incorporated into collections.
Ichnogenus Karethraichnus new ichnogenus Type ichnospecies.—Karethraichus lakkos new ichnospecies.
Other ichnospecies.—Karethraichus fiale new ichnospecies; Karethraichus kulindros new ichnospecies.
Diagnosis.—Circular to subcircular holes bored into a bone substrate. Holes may penetrate fully though the substrate or terminate within the bone. Penetrative holes may have straight or convex vertical margins. Non-penetrative pits terminate within the substrate as a shallow, bowl-shaped pit or as a deeper shaft with a rounded, blunt, or pointed terminus.
Etymology.—From the greek ‘karethra’ (romanization of
κηρήθρα) meaning ‘honeycomb,’ referring to the type locality of the type species in the Honeycomb Buttes area of South Pass, Wyoming, and ‘ichnus’ meaning trace.
Occurrence.—Eocene of the Green River Basin, Wyoming.
Remarks.—The observed diameter of Karethraichnus ranges from 0.5 to 8.0mm with 1 to 5mm being typical. Karethraichnus bears morphological and size similarities to the ichnogenera Sedilichnus (Müller, 1977), Oichnus (Bromley, 1981), Tremichnus (Brett, 1985), and Fossichnus (Nielsen et al., 2001), which occurs in marine and freshwater invertebrates. Recent taxonomic reassessments have suggested that Tremichnus is a junior synonym of Oichnus (Pickerill and Donovan, 1998) and that Oichnus, Tremichnus, and Fossichnus are junior synonyms of Sedilichnus (Zonneveld and Gingras, 2014). Karethraichnus differs in the plasticity of its overall morphology and the nature of the substrate in which the trace occurs.
The morphological variability exhibited by specimens of Karethraichnus is considerably wider than that displayed by
fcirc = 4 A P2
A = area P = perimeter
fcirc = 1 fcirc = 0.9
Sedilichnus. Individual ichnospecies of Sedilichnus may show considerable variance in size, however, they are remarkably consistent in the circularity of the external burrow opening although this may not be true for the internal burrow opening (i.e., circularity ~0.95–1.0; Fig. 4) and, to a lesser extent, in the evenness/symmetry of their cross-sectional profile. Karethraichnus are considerably more variable in both these criteria. Many specimens, although rounded, have irregular external boundaries (i.e. circularity 0.75–0.95; Fig. 4). Another important distinction between Karethraichnus and
Sedilichnus is the differing substrate in which they occur. Karethraichnus occurs within vertebrate bone, whereas Sedilichnus and its junior synonyms occur within invertebrate skeletal tests such as bivalve shells. Bone provides a unique substrate that is both mineralogically and compositionally distinct from invertebrate shells. Turtle shell, the substrate in which all of the borings discussed herein occur, consists of an external keratinous epidermal layer with a tripartite inner layer of bone (Achrai and Wagner, 2013; Fig. 5). The epidermal scutes consist of external layer of dead, cornified cells in a keratin matrix, overlying layers of keratinocytes in an amorphous keratin matrix that includes melanocytes, pigments, and lipids (Alibardi and Thompson, 2006). The sandwich-like structure of the bone layer consists of dense outer and inner
Figure 4. Circularity in trace fossils. Traces with perfect, equidimensional horizontal symmetry have circularities of 1. Trace fossils with imperfect equidimensional horizontal symmetry have circularities less than 1. Most Sedilichnus species have regular outlines and thus a circularity approximating 1. Karethraichnus have more variance within their horizontal profiles and may have circularities ranging from ~0.75 to 1.
epidermal scute
epidermal sulcus
external cortex
trabecular (cancellous) bone
internal cortex
Figure 5. Layers in living turtle and plastron bone. Adapted from Achrai and Wagner (2013). Note that the boundaries (sulci) between epidermal scutes are not generally coincident with the sutures between individual carapace and plastron elements. The compact outer bone is considerably denser than the inner cancellous bone. All four layers were penetrated by borings in the study interval.
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