From the Eocene of West Antarctica J.J
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Antarctic Science 4 (1): 107-108 (1992) Short note An additional record of a placental mammal (Order Astrapotheria) from the Eocene of West Antarctica J.J. HOOKER Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK accepted 28 November 1991 formed part of a strong distally flaring ectoloph as its buccal Introduction wall is slightly concave and the area of dentine exposed by Recently, fossil land mammals have been recorded in the natural wear is of equal width from cusp tip to distal edge. A James Ross Island area, east of the Antarctic Peninsula, from postcingulum and, at a slightly higher occlusal level, a the marine middle-late Eocene strata of Seymour Island. postmetaconule crista are truncated lingually by breakage These include two endemic species of poly dolopid marsupial: (Fig. lb). In distal view, the buccal ectoloph wall tilts Antarctodolops dailyi Woodburne & Zinsmeister 1984, and lingually, making an angle of about 50" to the crown base. In Eurydolops seymourensis Case, Woodburne & Chaney 1988, contrast to its lack of expression on the buccal wall, the and three partly determinate placentals identifiedas a tardigrade metacone isvery salient lingually, with a slope converging on edentate, a sparnotheriodontid litoptern and a trigonostylopid and equivalent to that of the buccal wall. The exposed enamel astrapothere (Bond etal. 1989). Themarsupials are represented of the ectoloph shows prominent transverse ridges reflecting by fragmentary jaw and complete dental material, the litoptern differential wear of decussating Hunter-Schreger bands. The and astrapothere only by tooth fragments. All are recorded near vertical buccolingual fracture of the tooth close to the from Unit TELM 5 of the La Meseta Formation (Sadler 1988), central axis of the metacone exposes the sides of some of the the two ungulates at a lower level than the rest. The subject Hunter-Schreger bands in a series of shallow steps. A fracture of the present note is a second tooth fragment from Seymour at the crown base, parallel with its plane, sections them Island identified as an astrapothere. It is worth recording transversely. The decussation is therefore vertical. because such mammal remains are rare and it is from TELM The only orders of mammals including medium-sized 4, the unit below that yielding the other land mammals. animals that have ectoloph-bearing upper molars are Artiodactyla, Perissodactyla, Litopterna, Notoungulata and Astrapotheria. The first two can reasonably be excluded as Provenance they are unknown outside the Old World and North America The new specimen was collected loose from the surface by the before the Pleistocene. The rest are endemic to South author in January 1989at an outlier of the TELM 4 Cucullaea America, and so are better candidates on geographical grounds. shellbed(BritishAntarcticSurveylocalityDJ.154,64013'51"S, The straight lingual tilt of the buccal ectoloph wall, lingual 56'37'36"W,north Seymour Island) near the transect of Sadler's saliency of the metacone and trend of the postmetaconule (1988, fig. 3) north section. Calcitic matrix, adhering to some crista round the distolingual wall of the metacone towards the of the broken and abraded surfaces, suggests that the tooth was metastyle are most like primitive astrapotheres. Notoungulates already fragmented prior to deposition. Co-occurring penguin usually have more vertical ectoloph buccal walls and their bones likewise show evidence of predepositional breakage, metaconule crests trend buccally or mesiobuccally. Litopterns consistent with erosion and reworking in the shelly lag with an equivalent crown height tend to have more slender conglomerate (Sadler 1988). metacones whose long axes are curved lingually. Among South Americanungulate orders, vertically decussating Hunter- Schreger bands are only recorded in Astrapotheria and Description and identity Pyrotheria, Litopterna and Notoungulata having horizontal The tooth fragment has been deposited in the Department of decussation, (Fortelius 1985). As pyrothere teeth are very Palaeontology, Natural History Museum, London, under the different in gross morphology, BMNH BAS M2584 is judged registration number BMNH BAS M2584. It comprises the to be an astrapothere. distal half of a metacone plus postmetacrista of a left upper Within the Astrapotheria, the closest resemblance appears molar (Fig. la-d). Bevelling on part of the postcingular edge to be with the trigonostylopid Trigonostylops, which has the may represent an interstitial facet. If so, the tooth is a first or appropriate orientation of the postmetaconule crisla and second molar. The height of the worn cusp, measured parallel similar crown height (e.g. see Simpson 1967, p1.43, figs 8-20). with its buccal wall, is 10.5 mm, while the distance from the Albertogaudrya and astrapotheriid astrapotheres are higher approximate midpoint of the metacone (the broken edge) and crowned and have differently orientated metaconule crests. the abraded distal tooth edge is5.6 mm. The metacone clearly When complete, the tooth would have been about twice the 107 Downloaded from https://www.cambridge.org/core. Open University Library, on 19 Jan 2020 at 21:40:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0954102092000154 108 J.J. HOOKER Fig. la-c. Metaconal fragment of left upper molar of Trigonostylopidae indet., (BMNH BAS M2.584) in occlusal a., distal b. and buccal c. views. Specimen coated with ammonium chloride. Each scale division measures lmm. d. Sketch of trigonostylopid tooth in occlusal view, with part represented by specimen indicated by bold outlining. linear dimensions of those of T. wortmani Ameghino (see domination. In fact two leaves, similar in outline and gross Simpson 1967). The only morphological difference seems to venation pattern to those figured by Case (1988) asNofhofagus, be slight concavity of the buccal ectoloph wall, which in were found by M.E. Dinn at DJ.154. However, interpreting T. wortmani is flat or very gently convex. The new specimen the structure of a Nothofagus-dominated vegetation requires is thus tentatively identified as Trigonostylopidae indet. knowledge of relationships to living taxa, leaf impressions being the least reliable guides, (Hill 1991) and/or in situ tree stumps (e.g. Jefferson 1982, gymnosperms). Discussion On the basis both of marsupials, which were then the only Acknowledgements known land mammals from La Meseta Formation, and of a Nothofagus-dominated associated megaflora (Case 1988), I would like to thank the Trans-Antarctic Association and Case et al. (1988, p.517) envisaged closedNothofagusrainforest British Antarctic Survey for financial support for the 1989 across Antarctica during the Eocene. This was invoked as a fieldwork, Mike Dinn and Tim Whitcombe for help in the filter, allowing only arboreal (primitive australidelphian) field, Mike Thomson and the weather for allowing the marsupials into Australia and excluding placentals, thus unscheduled stop at Seymour Island, and Phil Hurst of the explaining the dominant composition of Cainozoic Australian Natural History Museum Photo-unit for the photography. mammal faunas. Their modern model was the dense humid Nothofagus forests of southern Argentina, where medium- to References large-sized mammals are absent and the arboreal australidelphian Dromiciops is restricted to the densest stands BOND,M., PASCUAL, R., REGUERO,M., SANTILUNA,S. & MARENSSI,S. 1989. of trees. Although the La Meseta mammal fauna is still very Los primeros “ungulados” extinguidos sudamericanos de la Antartida. Ameghiniana, 26,240. meagre and may be heavily biased taphonomically by transport CASE,J.A. 1988. Paleogene floras from Seymour Island, Antarctic Peninsula. and reworking, the taxa of placentals (Bond et al. 1989 and Memoir of the Geological Society ofAmerica, 169,523-530. herein) now outnumber those of marsupials. Moreover, at CASE,J.A., WOODBURNE,M.O. & CHANEY,D.S. 1988. A new genus of least two were medium to large, strictly terrestrial, hoofed, polydolopid marsupial from Antarctica. Memoir of the GeologicalSociey browsing herbivores. The trigonostylopid described here ofAmerica, 169,505-521. FORTELIUS,M. 1985. Ungulate cheek teeth: developmental,functional and would probably have been about the size of a tapir. This evolutionary interrelations. Acra Zoologica Fennica, 180, 1-76. suggests that at least by themiddle/late Eocene, thevegetation HILL,R.S. 1991. TertiaryNorhofagus (Fagaceae) macrofossilsfrom Tasmania in the Weddellian province (Antarctic Peninsula area) was and Antarctica and their bearing on the evolution of the genus. Botanical more open than envisaged by Case et al. (1988), perhaps Journal of the Linnean Society, 105,73-112. following the global cooling trend that steepened near the JEFFERSON,T.H. 1982. The Early Cretaceous fossil forests of Alexander Island, Antarctica. Palaeonrology, 25,681-708. Eocene-Oligocene boundary (Shackleton 1986). This does SADLER,P.M. 1988. Geometry and stratification of uppermost Cretaceous not diminish Case et al.’s (1988) dispersal mechanism and Paleogene units on Seymour Island, northern Antarctic Peninsula. hypothesis as far as the early Eocene was concerned. However, Memoir of the Geological Society ofAmerica, 169,303-320. by the time of TELM 4-5 deposition, mammal dispersal to SHACKLETON,N.J. 1986. Paleogene stable isotope events. Palaeogeography, Australia must already have taken place as the two continents Palaeoclimatology,