TERLINGUACHELYS FISCHBECKI, A NEW GENUS AND SPECIES OF SEA TURTLE (CHELONIOIDEA: PROTOSTEGIDAE) FROM THE UPPER CRETACEOUS OF TEXAS

Journal of Paleontology, Nov 2004 by Lehman, Thomas M, Tomlinson, Susan L

ABSTRACT-

Remains of a large sea turtle, Terlinguachelys fischbecki n. gen. and sp., were recovered from paralic deposits of the Upper Cretaceous (Campanian) Aguja Formation in Big Bend National Park, Texas. T. fischbecki is a primitive protostegid that retains a constricted humerus, well ossified plastron and costals, prominent retroarticular process on the lower jaw, and long slender femora; however, it has some features, such as a prominent tubercle at the base of the scapular acromion process, found elsewhere only in derived leatherback sea turtles. The unique combination of primitive and derived traits in T. fischbecki illustrates further diversity among Cretaceous sea turtles and another case of parallelism common in sea turtle phylogeny.

INTRODUCTION

SEA TURTLES (superfamily Chelonioidea) were diverse and abundant during Late Cretaceous time, and likely reached their peak diversity during the Campanian stage (Weems, 1988; Hirayama, 1997). They were the only major group of marine rep tiles to survive the terminal Cretaceous mass extinction relatively unscathed. Remains of sea turtles are commonly preserved in the pelagic marine shale and chalk deposits that accumulated in the Late Cretaceous interior epeiric sea of North America. Most of these remains pertain to the extinct family Protostegidae. It is therefore of interest to report an unusual and previously unknown sea turtle from paralic Upper Cretaceous (Campanian) strata that accumulated along the western shore of the epeiric sea in Texas. The single and as yet only known specimen of this turtle is here designated the type of Terlinguachelys fischbecki n. gen. and sp. Recognition of this new species further increases the known diversity of Cretaceous sea turtles, and is of interest in examining the phylogeny of sea turtles. T. fischbecki is likely a primitive protostegid but has several features generally thought to be synapomorphic for more derived lineages, making its systematic placement uncertain. A brief description of this specimen was given previously by Tomlinson (1997).

Together, most marine turtles comprise a natural group (super-family Chelonioidea). Of the six extant genera of sea turtles, five (Caretta, Chelonia, Eretmochelys, Lepidochelys, Natator) are allied together in family Cheloniidae, and one (Dermochelys) is the sole living representative of family Dennochelyidae. The extinct family Protostegidae is known only from Cretaceous strata. These three families of sea turtles are recognized by most authors and each comprises a relatively distinct and well-defined clade. However, some extinct taxa are hypothesized to represent near relatives or sister groups of one or several of these three families, and authors have interpreted such taxa alternatively as either constituting separate families (e.g., Toxochelyidae, Osteopygidae, Desmatochelyidae) or have included some or all within three more expansively defined conventional families. Retention of plesiomorphic features among their early representatives and parallel evolution in the separate lineages presents a problem of recognizing "grades" rather than clades, particularly among the least derived sea turtles. Nevertheless, in the following description and discussion, comparisons are made with other sea turtles, using the terms cheloniid, dermochelyid, and protostegid generally in a manner consistent with the phylogenetic hypothesis of Hirayama (1998), recognizing, however, that taxa included in each family vary among authors ( see discussion of sea turtle relationships below).

GEOLOGIC SETTING

Stratigraphy.-The sea turtle specimen described in the present study was collected from the Aguja Formation, near the western border of Big Bend National Park in Brewster County, Texas. The Aguja Formation comprises an eastward-thinning wedge of paralic and marine sandstones inlerbedded to the west with mudstone and lignite deposited in coastal plain and fluvial environments (Fig. 1). The Aguja Formation is underlain by, and to the east intertongues with, marine shale of the Pen Formation. Two depositional sequences are represented in these deposits, the lower of which is present only in the western Big Bend region.

Lehman (1985) informally subdivided the Aguja Formation into several members. The basal sandstone member consists of progradational deltaic and littoral faciès. It is overlain by the lower shale member, which consists of interbedded carbonaceous shale and lignite that accumulated in coastal marsh and swamps landward of the shoreline. A thin transgressive marine sandstone, the Rattlesnake Mountain sandstone member, overlies the lower shale disconformably and was deposited by landward retreat of a barrier island system and later submerged inner shelf shoals. The specimen described herein was collected from these paralic facies. A westward-thinning marine shale overlies the Rattlesnake Mountain sandstone. This marine shale unit, interposed within the Aguja, is informally referred to as the McKinney Springs tongue of the Pen Formation. Overlying this marine shale is the second depositional sequence of the Aguja, extensive over the entire Big Bend region. A progradational deltaic unit, the Terlingua Creek sandstone member, is overlain by the upper shale member which consists of mudstone, carbonaceous shale, and lenticular sandstone deposited in coastal plain and fluvial environments. Most of the known vertebrate fauna of the Aguja Formation has been collected from the upper shale member (Lehman, 1989; Rowe el al., 1992). The Aguja Formation is overlain by fluvial deposits of the Javelina Formation.