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Journal of Paleontology 89(5):802–820
responses of the affected organisms and are not trace fossils (see Bertling et al., 2006; and Donovan, 2014 for further discussion of bioclaustration features and ichnotaxobases). These features are indeed borings and compound boring/
bioclaustration structures rather than solely bioclaustrations structures that resulted from a parasite introduced during the turtle’s embryonic stages. First, several of the borings were observed to preserve horizontal to subhorizontal markings that may reflect scratch marks or etch marks. Second, these traces occur on adult specimens; juvenile cf. Echmatemys sp. shells observed in this study did not exhibit borings. Finally, in many specimens the borings followed the sulci between epidermal scutes (a weakness in the epidermal armor), clearly indicating that the parasite colonization occurred after the animals hatched, likely as adults.
Parasite identification.—Modern chelonians play host to a wide variety of parasites, both internal (endoparasites) and external (ectoparasites). However, despite the diversity and prevalence of parasites on turtles and tortoises, attention has focused primarily on the affects that these parasites have on the health of the host organism (e.g., Esch and Gibbons, 1967; Ernst and Ernst, 1977; Readel et al., 2008; Bielecki et al., 2012). Minimal attention has been given to the effect that ectoparasites have on the integrity of the shells of these organisms. Below we discuss parasites that affect turtles in a myriad of settings including lake, river, terrestrial, marine, and marginal marine environments. Although the chelonian host taxa of the borings discussed herein occur in an interior fluvial-lacustrine depositional basin, isolated from any possibility of marine influences, these trace fossils are remarkably similar to borings we have observed in the shells of sea turtles and thus we include a brief discussion of marine turtle parasites. Note also that several common boring taxa such as
snails and bivalves are not considered likely candidates. Rock and wood-boring bivalves are limited to marine settings, and although there is significant size variability in bivalve borings, the shape is invariably clavate or tear-shaped (Bromley, 2004), thus differing significantly from the traces discussed herein. Shell-boring gastropods are unlikely candidates as well. Saxicavous gastropods are able to bore into carbonate lithic substrates through acid dissolution (Stanton, 1986; Quattier, 2002, 2011). Although shell-boring gastropods do occur in continental settings, typically preying upon other gastropods (Boycott, 1921, 1934; Ellis, 1969; Mordan, 1977), there is no record of these bivalves pursuing other prey types such as turtles.
Parasites in modern sea turtles.—Modern sea turtles play host to anwide variety of epibionts including cnidarians (hydrozoans and anthozoans), poriferans, molluscs (gastropods and bivalves), arthropods (amphipods, isopods, brachyurans, tanaids and cirripedes), platyhelminths, annelids, bryozoans, urochordates, and algae, (Caine, 1986; Frick et al., 1998, 2000; Scaravelli et al., 2001; Schärer, 2003; Pfaller et al., 2008). Of these, cirripedes (barnacles) have the most pronounced effect on the dermal bone. The attachment locales for barnacles commonly result in pitting on the surface of the carapace and plastron. These pits are approximately circular and, in our experience, have rounded
bases that penetrate through the outer dense bone layer into the more coelous bone beneath. Although attachment sites may occur anywhere on the turtle, there appears to be prevalence on the sulci between epidermal scutes (when present). These parasites commonly penetrate into the bone, thus occupying an intermediate position between ectoparasites and endoparasites, and thus may be considered to be mesoparasites. Although barnacles may excavate shallow, non-penetrative pits similar to Karethraichnus lakkos pits observed on some specimens discussed above, they are unlikely tracemakers of the turtle shells discussed herein because the Honeycomb Buttes turtles were collected from a fluvial-lacustrine basin ~1200 km from the nearest Eocene marine setting.
Parasites in modern marginal marine turtles.—Several modern freshwater turtle taxa are tolerant of brackish conditions. This has allowed interactions of quintessentially freshwater vertebrate species with characteristically marine invertebrate species. Barnacles and, less commonly’ oysters have been observed to colonize the shells of Diamondback Terrapins (Malaclemys terrapin) in coastal lagoons and estuaries (Allen and Neill, 1952; Jackson and Ross, 1971a;Ross and Jackson, 1972; Seigel, 1983). Rare examples of barnacles have also been noted on the alligator snapping turtle (Macroclemys temminicki;Jackson andRoss, 1971b), the red-bellied cooter (Pseudemys alabamensis; Jackson and Ross, 1972), and the American red-bellied turtle (P. rubiventris) (Ross and Jackson, 1972; Arndt, 1975; Seigel, 1983). Barnacle infestations may be severe enough to interfere
with nesting and mating behavior, cause significant shell erosion, and in rare cases death (Jackson and Ross, 1971b; Seigel, 1983). Although the carapace is the most common settling site, barnacles occur on the plastron as well (Seigel, 1983). Barnacles may cause significant shell erosion and may, in rare cases, penetrate through the shell causing development of significant scar tissue (Seigel, 1983).
Macroparasites in modern freshwater turtles.—Parasites, particularly leeches, are common ectoparasties on aquatic turtles in temperate and subtropical rivers, bogs, ponds, and lakes (eg. Esch and Gibbons, 1967; Hulse, 1976; McCoy et al., 2007; Oceguera-Figueroa and Siddall, 2008; Readel et al., 2008). Seasonality plays a strong role (Esch and Gibbons, 1967; MacCulloch, 1981; Readel et al., 2008) as does environmental setting. Turtles in turbid, slow-moving, or still water are also characterized by more abundant ectoparasites than those in clear or moving waters (Readel et al., 2008). In Wintergreen Lake near Kalamazoo, Michigan, the
diversity and number of parasites (nematodes and tremadodes) on the midland painted turtle (Chrysemys picta marginata) increased in individuals from their first to their fourth growing seasons and decrease in abundance thereafter (Esch and Gibbons, 1967). In contrast, analyses of multiple taxa from lakes and ponds in Colorado, North Carolina, and South Carolina have shown a simpler relationship, with larger turtles possessing a higher parasite load than smaller turtles both within and between taxa (McCoy et al., 2007; Readel et al., 2008). Bottom-dwelling forms, such as snapping turtles and mud
turtles, commonly have higher ectoparasite loads than aerial baskers, in part because of the effects of desiccation on
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