Eosphargis insularis ( Cope 1872 )

Robert & Weems, E., 2014, Paleogene chelonians from Maryland and Virginia, PaleoBios 31 (1), pp. 1-32 : 14-15

Eosphargis insularis ( Cope 1872)

Synonymy — Lembonax insularis Cope 1872 , Allopleuron insularis Weems 1988 , Allopleuron insularis Karl et al. 2012 .

Specimen —USNM 359002, nuchal, first neural, right and left first and second peripherals, and left third peripheral, described in Weems (1988).

Locality, horizon, and age — Found in bluff between Aquia Creek and Potomac Creek, western bank of Potomac River, Stafford County, Virginia; nannofossils in the attached matrix indicate the specimen came from “zone 6,” Piscataway Member of the Aquia Formation ( Clark and Martin 1901); late Paleocene (early Thanetian) .

Remarks — Weems (1988) assigned to Allopleuron insularis the anterior portion of a large but lightly built carapace found in the late Paleocene (early Thanetian) Piscataway Member of the Aquia Formation. This Aquia carapace has a peculiar first peripheral that is sutured to the second peripheral on both its distal and internal sides so that it is completely out of contact with the costoperipheral fontanelle. This condition is characteristic of a species found in the age equivalent late Paleocene (early Thanetian) Vincentown Formation of New Jersey that Cope named Lembonax insularis . Cope included three species in the genus Lembonax Cope 1870 : L. polemicus Cope 1870 which is the type species, L. insularis Cope 1872 and L. propylaeus Cope 1872 . Unfortunately, the type species of Lembonax is a nomen dubium ( Weems 1988) and cannot be adequately defined. Because of this, Weems (1988) chose to refer Lembonax insularis to the similar (but not identical) Late Cretaceous turtle genus Allopleuron Baur 1888 (type species Allopleuron hoffmani ( Gray 1831)) as a new combination ( A. insularis ) and assigned Allopleuron to the Dermochelyidae . At that time, this assignment was in accord with the conclusion of Gaffney and Meylan (1988) and Hiramaya (1992) that Allopleuron was an aberrant protostegid or dermochelyid turtle. Somewhat later, however, Hirayama (1994) concluded that A. hoffmani was an aberrant pancheloniid turtle, and this placement has been followed since ( Lapparent de Broin 2001, Karl 2007, Karl et al. 2012). In recent years two new species of Allopleuron have been named: A. lipsiense Karl 2007 from the early Oligocene of Germany and A. qazaqstanense Karl, Gröning, and Brauckmann 2012 from the early middle Eocene of Kazakhstan. These new records show that in Europe Allopleuron survived at least until the early Oligocene.

Significantly, the three European species assigned to Allopleuron ( A. hoffmani , A. qazaqstanense , and A. lipsiense ) all have a normal progression of peripheral elements away from the nuchal, a nuchal that is anteroposteriorly short, and a first neural that is as long as or longer than wide ( Karl et al. 2012:164). This stands in marked contrast to the condition seen in “ A.” insularis in which the second peripheral makes broad contact with the nuchal beneath the first peripheral so that the first peripheral is nested above the second peripheral-nuchal contact, the nuchal is relatively much narrower and more elongate, and the first neural is very wide. These are striking differences that clearly indicate that (1) “ A.” insularis should not be referred to the European genus Allopleuron and (2) that Allopleuron henceforth should be considered to be an exclusively Eurasian genus.

It still seems likely that the material included in Lembonax represents a dermochelyid rather than a pancheloniid turtle. Referral of the species Lembonax insularis to Eosphargis is proposed because the early Eocene dermochelyid Eosphargis gigas (Owen) Lydekker 1889a also has a nuchal that is strongly indented anteriorly and because Lembonax occurs at a horizon equivalent in age to the oldest specimens of Eosphargis known from Denmark ( Lapparent de Broin 2001). No skull or plastron material referable to Lembonax insularis has been found, so there is no way to perform a rigorous comparison between L. insularis and the two described European species of Eosphargis , E. gigas and E. breineri Nielsen 1959 . In the absence of such material, there is no way to determine with certainty if “ A.” insularis might be identical with E. gigas or E. breineri . Therefore, at least for now it seems most parsimonious to refer the long established and diagnosable species Lembonax insularis to Eosphargis as a third species, Eosphargis insularis .

Eosphargis breineri was described as an Eocene dermochelyid ( Nielsen 1959), but it is now known to be latest Paleocene in age ( Bonde 1987). Therefore this species most probably is age equivalent to the late Thanetian Paspotansa Member of the Aquia and somewhat younger than the early Thanetian source horizon of E. insularis in both New Jersey and Maryland. It remains possible that E. breineri is synonymous with E. insularis . Until such time as this can be tested in a meaningful way, however, they should remain as separate species of slightly different age and distinctly different provenance.