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A fossil marine turtle from east central Louisiana.

Abstract. -- A turtle carapace fragment from the well known Tunica Hills Locality, West Feliciana Parish, Louisana is referable to the marine genus Caretta. Attributed to the Citronelle Formation, its best comparisons are to a member of the genus from the Yorktown Formation at Lee Creek Mines, North Carolina (Latest Hemphillian). It indicates close proximity of the Pliocene coastline to the locality, now 150 km inland.

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The numerous fossils from the Tunica Hills of West Feliciana Parish, Louisiana have long attracted the attention of amateur and professional paleontologists (Manning & MacFadden 1989). Despite problems of age interpretation, the thousands of individual specimens are a significant record of Cenozoic life in the Gulf Coastal region. The specimen reported here, a fossil of a marine turtle, is unprecedented, however, and adds a new dimension to interpretations, with possibilities of marine paleoenvironment and/or Tertiary epochs represented. Despite some uncertainties of provenience and interpretation, the significance of these aspects calls for placing the specimen on record.

SYSTEMATIC PALEONTOLOGY

ORDER TESTUDINES Linnaeus

FAMILY CHELONIIDAE Gray

GENUS CARETTA Rafinesque

Caretta sp.

(Figure 1)

Referred specimen. -- CCVC 1553, Centenary College Vertebrate Collection, now in Vertebrate Paleontology Collection at Louisiana State University.

Provenience. -- surface collected along Tunica Bayou; believed to have been derived from Citronelle Formation.

Inferred age. -- Early Pliocene, equivalent to Latest Hemphillian.

[FIGURE 1 OMITTED]

Description. -- Specimen consists of two sequential neurals and portions of adjacent costals, presumably the third and fourth (Figure 1). Profile of curvature low, with projecting prominences along midline, giving appearance of median ridge or keel. The neurals are generally elongate "coffin" shaped (six-sided) in dorsal view, but the posterior margins have convex (caudad) curvature, as do the posterior margins of the intervening sulci. Based on comparisons to more complete carapaces, the entire length of the carapace was originally 700-800 mm. Especially interesting is the passage of one sulcus over the anterior portion of what is believed to be the fourth neural, although such positions of sulci may be subject to considerable individual variation. There is vermiculate sculpture but it is inconspicuous, visible only under magnification.

Identification is based on comparisons to all available genera of marine turtles (and large freshwater genera) of Cretaceous to Recent age, pleurodire and cryptodire. The general shape and proportions of the neurals, however, matches only the turtles of the Family Cheloniidae, which includes all living sea turtles (except the highly specialized Dermochelys coriacea). Detailed comparisons were thus made to the four genera of living cheloniids.

The estimated carapace size of the Louisiana specimen would be unusually great for Eretmochelys or Lepidochelys, reasonable for Chelonia, and probably of subadult dimensions for Caretta, based on comparisons to the extant species. The overall neural shape is similar to all recent taxa of Cheloniidae except Lepidochelys. However the posterior neural curvature (convex; caudad) is found only in Chelonia and Caretta among specimens compared during this study. Knob-like projections on the neural series are distinctive to Lepidochelys and some young specimens of Caretta. The sulcus positions seen in the Lousiana specimen are found only in some subadults of Caretta.

To summarize, the genus Caretta has the greatest resemblance to CCVC1553, more than to any other cheloniid. The shape of the neurals, including the posterior curvature, is very similar. Subadults of the living species are similar in size and at least some of them have sulcus positions like those of this specimen (Zangerl 1958). Young specimens may have knobbed and/or keeled midlines, and one Pliocene species is keeled throughout life. This latter species, to be named and described by G. Zug (pers. comm.), is notably found at the Lee Creek Mines of North Carolina in the Yorktown Formation (Latest Hemphillian).

Other Tertiary records of Caretta lack specimens with knobbed or keeled midlines (Portis 1890; Rothausen 1986). Dodd & Morgan (1992) discussed specimens from the Bone Valley Formation of Florida which may be conspecific with the North Carolina and Louisiana specimens, but reported no carapace material directly comparable.

LOCALITY AND STRATIGRAPHY

In Cenozoic coastal plain deposits, derivation, reworking and redeposition are possibilities. As with many such coastal plain fossils, notably those found in streams rather than in situ, there may be doubts concerning the specimen's provenance. Fortunately, the matrix adhering to the specimen, the identity of the species, and the age determinations for other fauna from the locality are quite consistent. Although the Tunica Hills localities are well known, and have been described often, the uniqueness of the specimen justifies review in order to allay suspicions regarding its origins.

The matrix adhering to the specimen is a white quartz sand with calcareous cementation, which has weathered to a rust red color in more exposed portions. Although only moderately indurated it appears to have completely encased the specimen quite well, assuring its durability despite reworking. This matrix is consistent with lithologies given for the Citronelle Formation, beginning with its original description by Matson (1916), although more calcareous than is usually the case. It is speculated that the fossil was encased in a concretionary mass of sand in the Citronelle Formation cemented by carbonate, which assured its durability despite reworking.

DISCUSSION

The morphology of the specimen suggests age correlation to the Yorktown Formation at Lee Creek Mine, latest Hemphillian of North Carolina. This is totally consistent with other Pliocene fossils from Tunica Bayou, the very age-diagnostic horses. Manning & MacFadden (1989), noted that all Pliocene vertebrates from the parish were from a twenty meter distance along Tunica Bayou. The turtle described here was within thirty meters distance from the Pliocene horse specimens.

The fauna of the Tunica Hills localities is otherwise essentially devoid of marine-adapted taxa of any kind, and supposedly represents terrestrial environments, of the late Cenozoic. This one specimen, however, indicates that sometime during the latest Hemphillian a coastal plain environment with closer marine proximity may have been the case.

ACKNOWLEDGMENTS

We thank Drs. Donald Baird (Princeton University), Linda Ford and Carl Mehling (American Museum of Natural History), and George Zug, Kenneth Tighe and David Bohaska (Smithsonian Institution, National Museum of Natural History) for useful discussions and access to comparative material. Dr. Ernest Williams (Harvard University) assisted with reference publications. Drs. Judith Schiebout, Julia Sankey and Ting Su Lin (Louisiana State University) assisted with loan of the specimen.

LITERATURE CITED

Dodd, C. K. & G. S. Morgan. 1992. Fossil Sea turtles from the early Pliocene Bone Valley Formation, Central Florida. J. of Herpetol., 26:1-8.

Manning, E. M. & B. J. MacFadden. 1989. Pliocene three-toed horses from Louisiana, with comments on the Citronelle Formation. Tulane Stud. Geol. and Paleont., 22(5):35-46.

Matson, G. G. 1916. The Pliocene Citronelle Formation of the Gulf Coastal Plain, U. S. Geol. Surv. Prof. Pap. 98-L: 167-208.

Portis, A. 1890. I rettili pliocenici del Valdarno superiore e di alcune altre localita plioceniche di Toscana, Florence, 32 pp.

Rothausen, K. 1986. Marine Tetrapoden im tertiaren Nordsee-Becken. 1. Nord-und mitteldeutscher Raum ausschliesslich Niederrheinische Bucht. Pp. 510-557 in Nordwestdeutschland im Tertiar (Tobien, Heinz, editor), 763 pp.

Zangerl, R. 1958. Die Oligozane-Meerschildkroten von Glarus. Schweiz. Palaont. Abhandl., 73:1-56

David C. Parris, James L. Dobie and A. Bradley McPherson

Bureau of Natural History, New Jersey State Museum

PO Box 530, Trenton, New Jersey 08625-0530

211 South Spanish Oak Trail

Kerrville, Texas 78028-2026 and

Department of Biology, Louisiana Centenary College

Shreveport, Louisiana 71104

DCP at: dparris@museum.sos.state.nj.us
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Author:Parris, David C.; Dobie, James L.; McPherson, A. Bradley
Publication:The Texas Journal of Science
Geographic Code:1U7LA
Date:Aug 1, 2000
Words:1215
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