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A LEATHERBACK SEA TURTLE FROM THE EOCENE OF ANTARCTICA:IMPLICATIONS FOR ANTIQUITY OF GIGANTOTHERMY INDERMOCHELYIDAEAuthor(s): L. BARRY ALBRIGHT III, MICHAEL O. WOODBURNE, JUDD A. CASE, and DAN S.CHANEYSource: Journal of Vertebrate Paleontology, 23(4):945-949. 2003.Published By: The Society of Vertebrate PaleontologyDOI: http://dx.doi.org/10.1671/1886-19URL: http://www.bioone.org/doi/full/10.1671/1886-19

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Journal of Vertebrate Paleontology 23(4):945–949, December 2003q 2003 by the Society of Vertebrate Paleontology

NOTE

A LEATHERBACK SEA TURTLE FROM THE EOCENE OF ANTARCTICA: IMPLICATIONS FOR ANTIQUITY OFGIGANTOTHERMY IN DERMOCHELYIDAE

L. BARRY ALBRIGHT, III1, MICHAEL O. WOODBURNE2, JUDD A. CASE3, and DAN S. CHANEY4, 1Department of Geology, Museum ofNorthern Arizona, Flagstaff 86001; 2Department of Earth Sciences, University of California, Riverside 92521; 3Department of Biology, St. Mary’sCollege, Moraga, California 94575; 4Department of Paleobiology, Smithsonian Institution, Washington, D. C. 20560

In his discussion of Miocene sea turtles from the Calvert Formationof coastal Maryland, Weems (1974:301) listed the known distributionof the fossil leatherback (dermochelyid) genus Psephophorus Meyer,1847, as ‘‘probably cosmopolitan in Tertiary marine strata except inpolar regions.’’ De la Fuente et al. (1995) recently put to rest this highlatitude exception in describing material they referred to ‘‘cf. Psepho-phorus’’ from Seymour Island (Marambio), off the northeastern tip ofthe Antarctic Peninsula (Fig. 1). Here we report the recovery of addi-tional material of a fossil leatherback sea turtle from Seymour Island;we provisionally refer this material to a recently described Eocene spe-cies, ‘‘Psephophorus’’ terrypratchetti Kohler, 1995, from New Zealand;and we also briefly review paleoceanographic data derived for the Eo-cene southern ocean that bears on the history of thermoregulation indermochelyids.

Despite their widespread geographic distribution and large size, ex-tinct leatherback sea turtles (Dermochelyidae) are typically describedon the basis of ‘‘relatively small and often indeterminate shell frag-ments’’ primarily because of their predominantly pelagic existence andbecause their shells readily disintegrate after death (Gove and Wood,1992). Such was the case for the first record of dermochelyid materialreported from Seymour Island by de la Fuente et al. (1995), whichincluded a few isolated carapace ossicles plus a small fragment of car-apace with four fused ossicles. In contrast, the new material reportedherein consists of several more coherent portions of the carapace (of asingle individual), each with several fused ossicles. This material iscurated in the paleontology collections at the University of California,Riverside, Department of Earth Sciences (UCR).

Although marine reptiles (plesiosaurs and mosasaurs) are known fromthe Late Cretaceous of Seymour Island and the surrounding James RossBasin, no pre-Eocene marine turtles are known. This contrasts with therelatively common occurrence of sea turtles in similarly aged marinedeposits of the northern hemisphere (e.g., Hirayama, 1997). The Eocenespecimen reported herein, together with the material reported by de laFuente et al. (1995), represent the only records of marine turtles knownfrom Antarctica and thereby expand upon the rapidly growing list oftaxa being recovered from the southern continent as geological andpaleontological investigations there continue.

GEOLOGICAL SETTING AND AGE

The specimen, UCR 22372, was recovered during the 1994–1995field season from a shell bed near the top of unit Telm4—one of sevendisconformity-based units mapped by Sadler (1988) in the Eocene LaMeseta Formation (Fig. 2). The La Meseta Formation (Elliot and Traut-man, 1982) is located east of the prominent physiographic feature thatessentially divides Seymour Island known as Cross Valley (Fig. 1). Thelocality, RV 9501, has approximate coordinates 648 149 450 S, 568 419W. The lower units of the La Meseta Formation represent a near-shore,tidal-influenced, delta-front environment, and in turn are overlain by atransgressive-regressive depositional stack that aggrades upwardthrough estuary-mouth, then inner-estuary facies (Wrenn and Hart,1988; Porebski, 2000). Deltaic development resulted from sedimentsbeing shed eastward from Antarctic Peninsula uplift and erosion (Elliot,1988). Unit Telm4 is characterized by conglomeratic Cucullaea shellbeds with abundant sharks’ teeth in a matrix of coarse glauconitic sand(Sadler, 1988; Porebski, 2000). Porebski (2000) interpreted Telm4 as

having developed in a main estuary channel during a transgressive cy-cle. Marenssi and Santillana (1994) included Telm4 and Telm5 togetheras an unconformity-bounded ‘‘Cucullaea I Allomember’’ that representsa tide-dominated and wave-influenced estuary where remains of bothterrestrial and marine organisms could be concentrated. The UCR spec-imen was found in association with material of fossil sharks, rays, andpenguins. De la Fuente et al. (1995) reported dermochelyid fossils fromboth Telm4 and Telm5, the latter unit also being that from which thefirst land mammal, a polydolopid marsupial, was recovered from Ant-arctica (Woodburne and Zinsmeister, 1984). Since that discovery, 11additional mammalian taxa have been recovered from Telm4 and 5, allof which represent very small insectivores or frugivores of Patagonianaffinity and 75% of which are marsupials (Goin et al., 1999). Case etal. (1987) reported the fossilized beak of a giant phorusrhacoid bird andfossil penguin remains from the stratigraphically higher unit Telm6, andan unusual, exceptionally high diversity of penguins was reported byCase (1992) from the still higher Telm7. Teeth reported from Telm5and 6 that Gasparini (1980) referred to crocodilians have since beendetermined by one of us (JAC) to more likely belong to gadiform fish.

The age of unit Telm4 is somewhat equivocal. Wrenn and Hart(1988), on the basis of marine palynofloral assemblages, divided the LaMeseta Formation into a lower Interval A of late early Eocene age, andan upper Interval B of late middle to late Eocene age. Although theydid not correlate intervals A and B with the Telm divisions of Sadler(1988), Telm4 should fall within Interval A based on stratigraphic thick-ness measurements reported in both publications. Thus, Telm4 wouldbe considered late early Eocene in age. However, according to Porebski(2000), a sequence boundary at the base of Telm4 may record the majorsea-level low stand (Type 1 sequence boundary) that occurred slightlybefore the early-middle Eocene boundary (49.5 Ma; Berggren et al.,1995; Hardenbol et al., 1998:charts 1 and 2), the consequences of whichresulted in a major hiatus spanning a portion of this time interval. Thisscenario supports Wrenn and Hart’s (1988) conclusions of a missingearly middle Eocene section which, in turn, suggests that unit Telm4may be no older than late middle Eocene (late Lutetian).

DESCRIPTION

That UCR 22372 represents the fossil remains from a member of theDermochelyidae is not in doubt. Particularly diagnostic of the specimenare the dense, irregularly arranged circular to polygonal bony ossiclesthat characterize the carapace of the group (e.g., Weems, 1974; Woodet al., 1996). The material consists primarily of numerous, small, frag-mentary, ‘‘hand-sized’’ carapace fragments, rather than individual os-sicles, firmly embedded in a very coarse-grained, poorly sorted sand-stone with abundant mollusc shell fragments. Additionally, embeddedin this sandstone only centimeters below the carapace fragments, areindeterminate bone fragments likely representing rib pieces. Some ofthe carapace fragments appear to be strongly fused to this underlyingbone, but this may be an artifact of preservation. The largest intact,although highly fragmented, carapace fragment measures about 36.8 cmby 15.5 cm. The most diagnostic specimens, however, are smaller piecesand are shown in Figure 3A–C. The ossicles making up these variouscarapace fragments range between 0.7 and 1.2 cm thick and range insize from about 1.3 cm in diameter to over 3.5 cm (the largest is over4.0 cm, but is broken and would have been slightly larger). Some, but

946 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 23, NO. 4, 2003

FIGURE 1. Map of Seymour Island showing generalized geology and location (RV-9501) of UCR dermochelyid specimen (modified fromWoodburne and Zinsmeister, 1984).

not all, of the ossicles, particularly those that in one specimen appearto have occurred near the margin of the carapace, show a subtle radi-ating sculptured pattern, and also have one to four, small, more or lesscentrally located pits on the external surface (Fig. 3B, C). In additionto pieces of the carapace, there are also several indeterminate, relativelylarge bone fragments that likely represent part of a limb, and one bonefragment that may represent a partial vertebra. Although we do make aprovisional taxonomic assignment to a recently described new speciesfrom New Zealand (see below), the nature of the recovered materialmakes positive identification equivocal.

DISCUSSION

Taxonomy

As noted above, nearly all fossil dermochelyid material reported priorto the phylogenetic study of Wood et al. (1996), particularly that ofEocene to Pliocene age, was typically assigned to the largely waste-basket genus Psephophorus (Weems, 1974; Wood et al., 1996). Woodet al. (1996) concluded, however, that most taxa historically referred toPsephophorus do not actually belong to that genus, and that the Der-mochelyidae have a complex evolutionary history with several distinctlineages. Wood et al. (1996) also determined that Psephophorus wasnot ancestral to Dermochelys, in contrast to Kohler (1995:371, 376)who, in describing what he considered a new species of the genus fromSouth Island, New Zealand (see below), referred to Psephophorus as‘‘the predecessor of the extant leatherback turtle, Dermochelys cori-acea’’ and to D. coriacea as ‘‘the extant taxon of Psephophorus.’’

The fragments reported here are not extensive enough to show wheth-er this taxon had the longitudinal carapace ridges, or keels, characteristicof the fossil genus Psephophorus (sensu stricto—see Wood et al., 1996)and the extant Dermochelys coriacea. Nor can it be determined if UCR22372 belongs to a lineage that lacked keels but had a distinctive ‘‘sun-flower’’ pattern of ossicles, such as that reported in Natemys peruvianusand ‘‘Psephophorus’’ rupeliensis from the Oligocene of Peru and Bel-gium, respectively, by Wood et al. (1996). On the other hand, the por-tion of UCR 22372 shown in Figure 3A strongly resembles a part ofthe carapace of the type specimen of Psephophorus polygonus, the ge-noholotype, from the Miocene of Europe, figured in Wood et al. (1996:

fig. 16). Some, but not all, of the ossicles of the UCR specimen alsoshow the radiating sculptured pattern present on many of the carapaceossicles of P. polygonus (Fig. 3C). First noted by Seeley (1880), thenby Wood et al. (1996), this radiating pattern was also noted on two ofthe Seymour Island specimens described by de la Fuente et al. (1995).De la Fuente et al. did not note, however, whether this morphology mayhave played a role in their referral of the Seymour Island material to‘‘cf. Psephophorus.’’ It has not yet been determined if this pattern isspecifically characteristic of P. polygonus, but the presence of this mor-phology on some of the UCR specimens suggests that all of the cur-rently known leatherback material from Seymour Island may representa single taxon. Cosmochelys dolloi Andrews (1919) from the Eoceneof Africa, also has ‘‘linear wrinkles radiating outward from the centerof each ossicle,’’ but Wood et al. (1996:279) noted that they are muchmore prominent in this taxon than in all other dermochelyids. AnotherAfrican taxon, and perhaps the most primitive dermochelyid, the latePaleocene/early Eocene Arabemys crassiscutata, also has ossicles thatshow a ‘‘deeply sculptured’’ texture ‘‘with wrinkles and tubercles ra-diating outward from the centre of the ossicle,’’ but differs from Cos-mochelys primarily in having a carapace with no longitudinal keels(Tong et al., 1999:914).

Particularly germane to this discussion is the report by Kohler (1995)of dermochelyid material from upper Lutetian marine sediments (about41.5–45 Ma based on Berggren et al., 1995) of South Island, NewZealand. Kohler erected a new species for the New Zealand material,Psephophorus terrypratchetti, on the basis of an abundance of materialfrom various localities, including large ‘‘platelet-fields,’’ ribs, vertebrae,a partial humerus, and a possible scapular fragment. Although we ques-tion Kohler’s referral of the New Zealand taxon to Psephophorus, thecarapace ossicle pattern shown in his figure 3 strongly resembles thatof UCR 22372 to the extent that both specimens very likely representthe same species. The absence or paucity of ossicle external sculpturing,the lack of keels, and their similar age (Lutetian) supports this, and we,therefore, provisionally assign UCR 22372 to ‘‘Psephophorus’’ terry-pratchetti, the quotes indicating our hesitation regarding Kohler’s ge-neric referral.

947NOTES

FIGURE 2. Generalized stratigraphic section of La Meseta Formation,Seymour Island, noting level at which UCR dermochelyid specimenwas found (modified from Sadler, 1988; de la Fuente et al., 1995; Goinet al., 1999).

Temperature Considerations

The appearance of a leatherback turtle in Eocene estuarine sedimentsof Seymour Island, although surprising given the fact that the extantleatherback, Dermochelys coriacea, is a pelagic rather than coastal spe-cies, is not wholly anomalous. According to Pritchard (1980), Dermo-chelys coriacea ranges from as far north as British Columbia, New-foundland, and the British Isles, to as far south as Australia, Argentina,and the Cape of Good Hope. To the extent that all of the above localitieslie within 608 north and south latitudes, the Seymour Island records, ata little greater than 648 S, thus represent the most poleward occurrencecurrently known for any marine turtle. (Note: using a reference framewhereby the southern tip of South America is fixed, Cunningham et al.,1995, calculated that the northern tip of the Antarctic Peninsula between40 and 50 Ma would have been at ;588 S; Seymour Island, therefore,would still have been located slightly south of 608 S). Dermochelys is

evidently able to tolerate colder waters due to its ability to maintain abody temperature as great as 18 to 218C above that of the surroundingwater through a combination of large body size, insulative tissue, lowmetabolic rate, and changes in blood flow—a condition of temperatureregulation termed ‘‘gigantothermy’’ by Paladino et al. (1990; also seeMrosovsky and Pritchard, 1971; Frair et al., 1972; Greer et al., 1973).

Although it cannot be determined with certainty if the Seymour Islandtaxon had a cool-tolerant physiology similar to that found in the extantDermochelys, paleoceanographic data reviewed below suggests the pos-sibility of such. To address this possibility it is first necessary to determinejust how cold Antarctic waters were at the time of deposition of Telm4.The answer is strongly dependent on the age of that unit which, as notedabove, is not known with certainty, but is thought to be late middleEocene. The early Eocene (the early Eocene Climatic Optimum, 52–50Ma) was the warmest interval of the Paleogene, in fact, of the past 65million years, with southern ocean sea surface temperatures (SSTs) ex-ceeding 148 to 168C (Zachos et al., 1994, 2001). By the early middleEocene, the interval spanning the hiatus recorded between Telm3 andTelm4 that purportedly resulted from the major sea-level low stand nearthe early-middle Eocene boundary (Porebski, 2000), southern ocean SSTshad declined to about 108 to 118C (about 558F), heralding a trend thatwould result in late Eocene SSTs of a relatively cold 38 to 68C (Zachoset al., 1994). Southern ocean SSTs during Telm4 time, therefore, wouldhave been between 68 and 108 C. Several lines of evidence, including thatderived from planktonic, oxygen isotopic, molluscan, palynomorph, andplant megafossil data from Seymour Island and the surrounding JamesRoss Basin, support a drop in temperature over this time interval (seeCase, 1992, and references therein).

The climatic deterioration that was highly evident by the late middleEocene (post 41 Ma) was likely the consequence of major changes inocean circulation, including the closure of the Tethyan seaway and es-tablishment of deep water circulation between Australia and Antarctica(Lawver et al., 1992; Lawver and Gahagan, 1998). Another conse-quence of these changes was the onset of the Eocene-Oligocene tran-sition (see Prothero and Berggren, 1992) with small ephemeral icesheets first appearing in Antarctica in the early late Eocene (Zachos etal., 1994, 2001; Woodburne and Case, 1996). Drake Passage, betweenthe Antarctic Peninsula and South America, would not open to marinecirculation until the latest Eocene–early Oligocene, between about 30–34 Ma, at which time larger, more permanent ice sheets appeared andbuilt up on the Antarctic craton as a partial consequence of thermalisolation and development of the Antarctic circumpolar current (Lawveret al., 1992; Zachos et al., 1994; Lawver and Gahagan, 1998; Wilsonet al., 1998). Another possible result of these large-scale oceanographicevents may have been the onset of upwelling east of the Antarctic Pen-insula as hypothesized by Case (1992) on the basis of an unusual abun-dance and diversity of shark and penguin species as recorded in Sey-mour Island units Telm6 and 7. Thus, the paleoclimate of the northernAntarctic Peninsula during the middle Eocene would likely have beensimilar to present-day cool-humid-temperate climates in Tasmania, NewZealand, and southern South America (Woodburne and Zinsmeister,1984:935; see also Case, 1988, 1992; Goin et al., 1999).

Given the above, together with even the most liberal interpretationof the timing of deposition of Telm4 (perhaps as old as early middleEocene, but more likely late middle), it seems likely that the SeymourIsland dermochelyid taxon, or taxa, must have had some physiologicalmeans of maintaining a body temperature conducive to a relatively ac-tive metabolism in cold waters. Even if Telm4 was deposited during theglobally warm early Eocene, southern ocean SSTs of 148 to 168C (about608F) would require the maintenance of such a metabolism. To theextent that the SSTs calculated by Zachos et al. (1994) provide estimatesof maximum (summer) temperatures, coastal or estuarine environmentssuch as those in which UCR 22372 was found may have been somewhatwarmer than open ocean (pelagic) environments during summer months.However, by the late middle Eocene, temperatures were considerablycooler such that some mechanism for maintenance of an active metab-olism would very likely have been required. This becomes even morelikely in consideration of the recovery of dermochelyid material fromthe stratigraphically higher Telm5 reported by de la Fuente et al. (1995).Thus it appears that ‘‘gigantothermy’’ in the Dermochelyidae can betraced back at least to the late middle Eocene.

Acknowledgments Thanks are extended to our U.S.A. and Argen-tinian colleagues on the 1994–1995 expeditions and to personnel ofBase Marambio for their hospitality and logistical support. Thanks alsogo to reviewers R. Reisz, P. Holroyd, J. F. Parham, G. Gaffney, and one

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FIGURE 3. Carapace fragments of UCR 22372. A, carapace fragment showing no texture or sculpturing. B, fragment likely representing edgeof carapace. C, same fragment showing ossicles with radiating, sculptured pattern. Scale bars equal 1.5 cm.

949NOTES

reviewer whose signature we could not decipher (!) for suggestions thatimproved the paper. Funding for the expedition was provided by NSFOffice of Polar Program grant 9315831 to M. O. Woodburne.

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Received 23 March 2002; accepted 20 December 2002.