Monday, February 26, 2018

[Paleontology • 2018] Trachemys haugrudi • A New Slider Turtle (Testudines: Emydidae: Deirochelyinae) from the late Hemphillian (late Miocene/early Pliocene) of eastern Tennessee and the Evolution of the Deirochelyines

Trachemys haugrudi  Jasinski, 2018

Artwork by Mary P. Williams

Trachemys (Testudines: Emydidae) represents one of the most well-known turtle genera today. The evolution of Trachemys, while being heavily documented with fossil representatives, is not well understood. Numerous fossils from the late Hemphillian Gray Fossil Site (GFS) in northeastern Tennessee help to elucidate its evolution. The fossil Trachemys at the GFS represent a new species. The new taxon, Trachemys haugrudi, is described, and currently represents the most thoroughly described fossil emydid species known. A phylogenetic analysis, including 31 species, focusing on the subfamily Deirochelyinae is performed that includes the new fossil species, along with numerous other modern and fossil deirochelyine species, representing the first phylogenetic analysis published that includes several fossil deirochelyines. The phylogenetic analysis, utilizing morphological evidence, provides monophyletic clades of all modern deirochelyines, including Chrysemys, Deirochelys, Pseudemys, Malaclemys, Graptemys, and Trachemys. A strict consensus tree finds the recently described fossil species Graptemys kerneri to be part of a clade of Graptemys + Malaclemys. Three fossil taxa, including one previously referred to Pseudemys (Pseudemys caelata) and two to Deirochelys (Deirochelys carri and Deirochelys floridana) are found to form a clade with modern Deirochelys reticularia reticularia, with D. floridana sister to the other members of the clade. Chrysemys is found to be part of a basal polytomy with Deirochelys in relation to other deirochelyine taxa. Two fossil taxa previously referred to Chrysemys (Chrysemys timida and Chrysemys williamsi) form a paraphyly with the modern Chrysemys picta picta and Deirochelys, and may be referable to distinct genera. Additionally, fossil taxa previously attributed to Trachemys (Trachemys hillii, Trachemys idahoensis, Trachemys inflata, and Trachemys platymarginata) and T. haugrudi are found to form a clade separate from clades of northern and southern Trachemys species, potentially suggesting a distinct lineage of Trachemys with no modern survivors. Hypotheses of phylogenetic relationships mostly agree between the present study and previous ones, although the inclusion of fossil taxa provides further clues to the evolution of parts of the Deirochelyinae. The inclusion of more fossil taxa and characters may help resolve the placement of some taxa, and further elucidate the evolution of these New World turtles.

Figure 7: Trachemys haugrudi, paratype skull (ETMNH–3562) in dorsal view. (A) Dorsal portion in dorsal view; (B) ventral portion in dorsal view; (C) reconstruction of skull in dorsal view. Area shaded gray is not preserved and has been reconstructed. Dotted lines represent sutures that were not clear in the specimen. Skull has been reconstructed in the slightly deformed state the specimen is in in real life.

 cs, crista supraoccipitalis; den, dentary; ?ex, ?exoccipital; fr, frontal; ?ju, ?jugal; mx, maxilla; ?op, ?opisthotic; pa, parietal; pf, prefrontal; pm, premaxilla; po, postorbital; ?pr, prootic; ?qj, ?quadratojugal; ?qu, ?quadrate; so, supraoccipital; ?sq, ?squamosal. Scale bars are 1 cm. 

Figure 3: Trachemys haugrudi, holotype shell (ETMNH–8549). (A) Dorsal view of carapace; (B) line drawing of carapace in dorsal view, with bones outlined in black and scutes outlined in gray; and (C) with scutes outlined in black and bones outlined in gray. Missing portions are shaded in gray. Scale bar is 10 cm.

Figure 4: Trachemys haugrudi, holotype shell (ETMNH–8549). (A) Ventral view of plastron; (B) line drawing of plastron in dorsal view, with bones outlined in black and scutes outlined in gray; and (C) with scutes outlined in black and bones outlined in gray. Scale bar is 10 cm.

Systematic Paleontology

Class Reptilia Laurenti, 1768
Order Testudines Linnaeus, 1758
Suborder Cryptodira Cope, 1868

Superfamily Testudinoidea sensu Gaffney & Meylan, 1988
Family Emydidae Bell, 1825

Trachemys Agassiz, 1857

Trachemys haugrudi n. sp.

Type horizon and age: Late Miocene–early Pliocene (late Hemphillian LMA, 7.0–4.5 Ma). This range means the fossil locality, and T. haugrudi, lies somewhere within Hh3–Hh4 (see Tedford et al., 2004 for discussion of substages).

Etymology: The specific name honors Shawn Haugrud, preparator at the GFS who spent countless hours working on many of the specimens cited within and who helped piece this ancient turtle back together.

Diagnosis: Trachemys haugrudi is placed in the Emydidae due to the absence of musk ducts, inframarginals reduced to two, normal hexagonal neurals 2–8 (also occurs in a few batagurids (=geoemydids); e.g., Mauremys), and costal-inguinal buttress confined to C5. It is placed in the Deirochelyinae due to distinct lack of pectoral overlap of the entoplastron and lack of a hingable plastral lobe with ligamentous bridge connection (also present in some emydines). Diagnosed as a member of the genus Trachemys by features discussed by Seidel & Jackson (1990), including the combination of: a posteriorly strongly serrated oval carapace; a vertebral keel; low longitudinal ridges (mainly on pleurals (and costals)); alternating seams of the vertebral and pleural scutes that do not align; ......

Figure 13: Life reconstruction of Trachemys haugrudi during the late Hemphillian at the Gray Fossil Site in eastern Tennessee.
 Several taxa that would have lived alongside T. haugrudi are also shown, including Caudata indeterminate, Tapirus polkensis, cf. Machairodus sp., and Pristinailurus bristoli (Wallace & Wang, 2004; Boardman & Schubert, 2011b; Schubert, 2011; Jasinski, 2013a). Artwork by Mary P. Williams, with permission.  

Steven E. Jasinski. 2018.  A New Slider Turtle (Testudines: Emydidae: Deirochelyinae: Trachemys) from the late Hemphillian (late Miocene/early Pliocene) of eastern Tennessee and the Evolution of the Deirochelyines. PeerJ. 6:e4338.  DOI: 10.7717/peerj.4338

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