Introduction
The Polonez Cove Formation, cropping out at King George Island (KGI), West Antarctica (Fig. 1), preserves a diverse Cenozoic marine biota that has been subject of numerous papers dealing with coccoliths (Gazdzicka & Gazdzicki Reference Gazdzicka and Gazdzicki1985), foraminifers, ostracods (Blaszyk Reference Blaszyk1987), brachiopods (Bitner & Pisera Reference Bitner and Pisera1984, Bitner & Thomson Reference Bitner and Thomson1999), polychaete worms, bryozoans, gastropods, bivalves (Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984), scaphopods (Pugaczewska Reference Pugaczewska1984), and echinoderms (Jesionek-Szymanska Reference Jesionek-Szymanska1984). The Low Head Member of the Polonez Cove Formation is a richly fossiliferous unit (Bitner & Thomson Reference Bitner and Thomson1999), and crops out at various sites along the eastern coast of KGI. Most of the stratigraphic and palaeontological studies of the Polonez Cove Formation available in the literature refer to the type-section of the formation exposed from Lions Rump to Low Head along the southern coastal of KGI (Fig. 1). Although briefly described by Birkenmajer (Reference Birkenmajer1982), Porebski & Gradzinski (Reference Porebski and Gradzinski1987) and Troedson & Riding (Reference Troedson and Riding2002), Cenozoic sediments cropping out at Vauréal Peak, on the northern margin of Admiralty Bay, had never been palaeontologically studied or correlated with the Polonez Cove Formation, perhaps due to the limited exposure at this site.
Location maps of the Polonez Cove Formation exposures and other related units. a. location of Vauréal Peak, Admiralty Bay, King George Island, Antarctica; the area inside the square corresponds to Cenozoic glacial sediments detailed in b. b. Occurrence of the Polonez Cove Formation and associated stratigraphic units (adapted from Birkenmajer Reference Birkenmajer2001 and Troedson & Smellie Reference Troedson and Smellie2002).
The new taxonomic data here presented, allied to previous studies on fossil invertebrates from Antarctic regions, are essential for establishing the palaeontological affinities of KGI fossils with other Cenozoic faunas from Antarctic peripheral southern continents. These data bear on the identification of migratory routes of marine faunas from northern warmer waters into Antarctica, and on the understanding of palaeobiogeographical patterns established during the glacial history of West Antarctica at the time of its separation from South America at the end of Palaeogene (e.g. Zinsmeister Reference Zinsmeister1982, Crame Reference Crame1999, Briggs Reference Briggs2003).
The present paper is a part of a series aiming to describe the marine invertebrate faunas from Cenozoic deposits of KGI (Anelli et al. Reference Anelli, Rocha-Campos, Dos Santos, Perinotto and Quaglio2006), evaluating their taxonomic affinities in order to recognize dispersal routes that shaped their composition, and interpreting their palaeoenvironment in the context of their depositional history.
Geological setting, age and palaeontology
Fossil invertebrates described herein were collected from outcrop assigned to the Low Head Member of the Polonez Cove Formation at Vauréal Peak, Admiralty Bay, KGI, West Antarctica (Fig. 1), in the summer of 2004, during the Operação Antártica XXII of the Brazilian Antarctic Program (PROANTAR). Other exposures of the Polonez Cove Formation occur on a cliff along the eastern coast of the island, between Lions Rump and Low Head, King George Bay (type-area of the unit, Birkenmajer Reference Birkenmajer1982), at Three Sisters Point, as well as inland at Godwin Cliffs and Magda Nunatak (Porebski & Gradzinski Reference Porebski and Gradzinski1987, Birkenmajer Reference Birkenmajer2001, Troedson & Smellie Reference Troedson and Smellie2002). (Fig. 1b.)
In its type-area, the Polonez Cove Formation is represented by a sequence of diamictites and fossiliferous basaltic conglomerates and sandstones. The main characteristic of the formation is the presence of conglomerate facies containing faceted and striated pebble- to boulder-sized clasts of local and continental Antarctic provenance, assigned to the Krakowiak Glacier Member (Birkenmajer Reference Birkenmajer2001, Reference Birkenmajer2003). A detailed description of the stratigraphy, facies and depositional environment of the Polonez Cove Formation appears in Porebski & Gradzinski (Reference Porebski and Gradzinski1987, Reference Porebski and Gradzinski1990), Santos et al. (Reference Santos, Rocha-Campos, Tompette, Uhlein, Gipp and Simões1990), and Troedson & Smellie (Reference Troedson and Smellie2002).
The Polonez Cove Formation sequence cropping out at Vauréal Peak starts with diamictite, up to 5 m thick, correlated to the Krakowiak Glacier Member, which passes upwards to lenticular beds of matrix or clast-supported, massive gravel and grainy to pebbly sandstone, 6 m thick, of the Low Head Member (Fig. 2).
Stratigraphic column of the Polonez Cove Formation measured at Vauréal Peak.
The Low Head Member facies described in this paper can be correlated to the LH3 facies (sensu Birkenmajer Reference Birkenmajer1994, Reference Birkenmajer1995) and to the L2 facies (sensu Troedson & Smellie Reference Troedson and Smellie2002) of the Low Head Member (Table I).
Sedimentary facies of the Polonez Cove Formation at Vauréal Peak area.
As shown on the 7 m thick stratigraphic section measured at Vauréal Peak (Fig. 2, Table I), at least four gravelly sandstone beds, around 1–1.5 m thick occur, normally graded, with erosive base and lenticular geometry at outcrop scale. Beds are composed, from base to top, of massive matrix (Gmm) or clast-supported (Gmc) gravels, that grade upward into massive sandstones with sparse granules and pebbles (Sm).
Invertebrate fossils occur in the lower portions of the section. None of the specimens was found in life position; all of them occur randomly distributed in the matrix. All brachiopods are articulated, lacking the anterior portion of the shell and most of all bivalves are articulated. Most invertebrate specimens (brachiopods and particularly serpulid tubes, bryozoans and echinoderms) are fragmentary, but with no features of prolonged reworking, such as roundness or selection. These characteristics suggest that these are allochthonous fossil assemblages (Brett & Baird Reference Brett and Baird1986, Kidwell et al. Reference Kidwell, Fürsich and Aigner1986). The occurrence of pyrite framboids in some fossils (Fig. 3) indicates a period of low oxygen conditions (reducing environment) after deposition and none or very low reworking until lithification of the sediments (Brett & Baird Reference Brett and Baird1986). No evidence of ichnofossils was found, that also suggests a probable reducing environment and absence of reworking by bioturbators (Speyer & Brett Reference Speyer, Brett, Allison and Briggs1991).
Pyrite framboids occurring in bryozoan specimen 5352. a. framboids photographed by SEM (scale bar = 2 µm). b. EDS granules composition.
We interpret these deposits as products of episodic sedimentation from cohesive and non-cohesive debris flows, originated by slumping or even from the action of bottom traction currents, followed by aggradational deposition of sediments by deceleration of high-energy episodic flows. This facies association is interpreted as deposited in a fan delta setting, in high- to medium-energy, shallow marine environment (Eyles & Eyles Reference Eyles, Eyles, Walker and James1992). Regionally, this facies association and arrangement conform to available sedimentary models of a marine transgressive phase, following a retreating grounded ice margin (Porebski & Gradzinski Reference Porebski and Gradzinski1987, Santos et al. Reference Santos, Rocha-Campos, Tompette, Uhlein, Gipp and Simões1990, Birkenmajer Reference Birkenmajer2001, Reference Birkenmajer2003).
The so called “pecten conglomerate” of the Low Head Member (Adie Reference Adie1962, Barton Reference Barton1965, discussed in Birkenmajer & Gazdzicki Reference Birkenmajer and Gazdzicki1986, and Birkenmajer Reference Birkenmajer2001) in the type-area, is the most fossiliferous component of the Polonez Cove Formation. It contains marine macro- and microfossils, including coccoliths, diatoms, foraminifers, polychaete worms, bryozoans, brachiopods, gastropods, scaphopods and echinoderms (Bitner & Pisera Reference Bitner and Pisera1984, Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984, Jesionek-Szymanska Reference Jesionek-Szymanska1984). Some taxa described herein are comparable with fossils previously described from the “pecten conglomerate”; others are for the first time described for the Polonez Cove Formation or even for KGI.
The age of the Low Head Member is still controversial. Available K–Ar dates, Sr-isotope dating and palaeontological data from the type-area indicate a latest early Oligocene age (late Rupelian) for this member (Birkenmajer & Gazdzicki Reference Birkenmajer and Gazdzicki1986, Dingle et al. Reference Dingle, Mcarthur and Vroon1997, Dingle & Lavelle Reference Dingle and Lavelle1998, see Fig. 4). So the “pecten conglomerate” of KGI is probably a unit distinct from the Pliocene “pecten conglomerate” (Cockburn Island Formation; Jonkers Reference Jonkers1998, Reference Jonkers2003) originally described from Cockburn Island, in the western Weddell Sea.
Stratigraphic position of Cenozoic units at Vauréal Peak (plotted ages after studies in other locations in which these units also occur). 1 = K–Ar dating of basaltic lava at Polonez Cove (Birkenmajer & Gazdzicki Reference Birkenmajer and Gazdzicki1986), 2 = K–Ar dating of andesitic lava at Turret Point (Birkenmajer et al. Reference Birkenmajer, Soliani and Kawashita1989), 3 = K–Ar dating of andesitic lava at Lions Rump (Smellie et al. Reference Smellie, Pankhurst, Thomson and Davies1984), 4 = Strontium-isotope dating of bivalve shells from Krakowiak Glacier Member tillites at Polonez Cove (Dingle & Lavelle Reference Dingle and Lavelle1998), 5 = Strontium-isotope dating of bivalve shells from Low Head Member conglomerates at Low Head, Polonez Cove and Lions Rump (Dingle et al. Reference Dingle, Mcarthur and Vroon1997), 6 = Strontium-isotope dating of bivalve and brachiopod shells from basal tillites at Magda Nunatak (Dingle & Lavelle Reference Dingle and Lavelle1998), 7 = K–Ar minimum age from andesite–dacite lavas at Polonez Cove and Boy Point (Birkenmajer & Gazdzicki Reference Birkenmajer and Gazdzicki1986).
Taxonomy
Material and methods
Brachiopods were measured with reference to the anterior-posterior axis of the shell (Fig. 5a & b), according to the “landmarks 1 and 9” of Krause (Reference Krause2004, p. 462). Bivalves were measured with reference to the hinge line of the shell. The length, height and width of the bivalve shell correspond to the greatest measured lines parallel, perpendicular and orthogonal to the hinge line, respectively (Fig. 5c & d). Elongation and obesity indexes were calculated according to Stanley (Reference Stanley1970). Symbols in tables are as follows: D = dorsal valve, V = ventral valve, R = right valve, L = left valve, D/V or R/L = articulated valves, * = specimen incomplete, AOL = length of the anterior outer ligament, POL = length of the posterior outer ligament, AVH = anterior dorsal valve height, PVH = posterior dorsal valve height.
Orientation and characters measured. a–b. Brachiopod, c–d. bivalve. a. lateral view, b. dorsal view, c. anterior view, d. lateral view of RV. ADM = anterior dorsal margin, AM = anterior margin, AOL = length of the anterior outer ligament, AVH = anterior dorsal valve height, AVM = anterior ventral margin, DM = dorsal margin, H = height, HL = hinge line, L = length, LM = lateral margin, PDM = posterior dorsal margin, PM = posterior margin, POL = length of the posterior outer ligament, PVH = posterior dorsal valve height, PVM = posterior ventral margin, T = thickness, UA = umbonal angle, VM = ventral margin, W = width.
Taxonomy above the genus level applied to the brachiopods is based on the classification of Williams et al. (Reference Williams, Brunton, Carlson, Baker, Carter, Curry, Dagys, Gourvennec, Hong-Fei, Yu-Gan, Johnson, Lee, Mackinnon, Racheboeuf, Smirnova, Dong-Li and Seldon2006); suprageneric names utilized for the bivalves are based on the synoptical classification proposed by Amler (Reference Amler1999); systematics of serpulids, bryozoans and echinoderms follows the classification of Howell (Reference Howell and Moore1962), Boardman et al. (Reference Boardman, Cheetham, Cook and Boardman1983) and Durham et al. (Reference Durham, Fell, Fischer, Kier, Melville, Pawson, Wagner and Moore1966), respectively.
The stratigraphic distribution of taxa is shown on Fig. 2. All material is from a single locality at Vauréal Peak, and, therefore, data on locality and stratigraphy are not repeated below.
All specimens are housed in the scientific collection of the Laboratório de Paleontologia Sistemática of the Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São Paulo, under prefix GP/1E.
Systematic palaeontology by Quaglio & Anelli
Type-species
Terebratula uva Broderip
Remarks
The specimen found at Vauréal Peak shows some of diagnostic characters of the genus Liothyrella pointed out by Lee & Smirnova (Reference Lee, Smirnova and Seldon2006, p. 2056–2057), which are: shell large to very large, elongate oval to subcircular; foramen usually large; teeth narrow; low myophragm; outer hinge plates attached near dorsal edge of crural bases [here called crural plates].
Dimensions in millimetres and character values of specimen of Liothyrella sp.
Material
Internal mould of articulated valves, with anterior part missing (5465).
Description
Shell of medium size, lateral margins rounded; ventral valve apparently more convex than dorsal valve; lateral commissures straight, anterior not preserved. Beak short and erect; foramen large, circular; crural plates triangular, extending along margin of outer hinge plates; hinge teeth narrow, sulcus constricted. Adductor scars of dorsal valve very broad, with myophragma evident; elongate furrows posteriorly in dorsal valve, close to the umbonal region, are possibly related to the muscular field; two sets of several marks of circular shape laterally to the adductors in the posterior region of both valves, possibly correspond to genital scars.
Comparison
Liothyrella sp. from Vauréal Peak differs from Liothyrella sp. from Cape Melville Formation (Lower Miocene) at Melville Peninsula (Bitner & Crame Reference Bitner and Crame2002) in its greater size, more circular shape and higher convexity of the dorsal valve. It is impossible to compare other features of the two taxa because only external characters of the Miocene Liothyrella sp. are known, while the Oligocene Liothyrella sp. is known only as an internal mould. The Liothyrella specimens from the Eocene La Meseta Formation (Bitner Reference Bitner1996) are larger and more elongate. Although Bitner (Reference Bitner and Glowacki1997) illustrated a specimen of Liothyrella sp. from the Low Head Member of the Polonez Cove Formation at Mazurek Point (p. 26, fig. 4A–C), it is not described or commented on in a taxonomic approach. The shell from Mazurek Point is smaller and more elongate than Liothyrella sp. from Vauréal Peak, but it is apparently distorted (compressed) and poorly preserved.
Remarks
Liothyrella sp. from Vauréal Peak is the oldest record of the genus from KGI together with the specimen from the Low Head Member at Mazurek Point (illustrated by Bitner Reference Bitner and Glowacki1997, p. 26, fig. 4A–C).
Type-species
Terebratula lenticularis Deshayes, 1839
Discussion
Characters attributed to the genus Neothyris by MacKinnon & Lee (Reference MacKinnon, Lee and Seldon2006, p. 2231, 2233) shared by specimens from the Polonez Cove Formation at Vauréal Peak are shell large, smooth; with beak incurved; strong posterior shell thickening; median septum short; crura rather short; loop teloform.
Dimensions in millimetres and character values of representative specimens of Neothyris sp.
Material
Articulated valves lacking anterior margin (5374c, 5594a); posterior region of articulated valves (5374e, 5438f, 5438g, 5438i, 5464); dorsal valve (5462); posterior region of dorsal valve (5461, 5594b).
Description
Shell of medium size, trigonal to ovoid in outline, ventral valve more convex than dorsal, lateral margins rounded, with greatest width at anterior mid-length; lateral commissure straight, anterior region apparently rounded (in specimen 5594a), posterior region of ventral valve very thick. Shell surface with well defined growth lines. Beak curved, well developed; foramen not observable. Crura and septum reaching around 30% of shell length; loop teloform (see Fig. 8). Adductor scars of dorsal valve broad, with myophragm weakly developed; myophragm of ventral valve well developed. Shell microstructure composed of thin primary layer, mosaic structure observed in the second layer; punctae large.
Liothyrella sp. a–d. specimen 5465, internal mould of articulated pair; a. ventral view, b. dorsal view, c. posterior view, d. lateral view. e. latex cast of anterior region of 5465, dotted line marks the limit between ventral (upper) and dorsal (lower) valves (all bars = 5 mm).
Neothyris sp. a–c. Specimen 5374c, articulated pair; a. posterior view, b. lateral view, c. ventral view. d–e. Specimen 5374e, articulated pair lacking anterior part; d. posterior view, e. ventral view. f–m. Specimen 5594a–b, posterior part of shell and internal mould of articulated valves of same individual; f. ventral view of internal mould, g. the same with posterior part added, h. dorsal view of internal mould, i. the same with posterior part added, j. lateral view, k. posterior view of internal mould, l. the same with posterior part added, m. internal view of posterior part (all bars = 5 mm). n–q. Microstructure photographed by SEM, n–p. specimen 5300, n. primary (p) and secondary (s) calcitic layers (scale bar = 200 µm), o. detail of n, showing secondary layer only (scale bar = 100 µm), p. mosaic arrangement of secondary layer (scale bar = 10 µm), q. specimen 5438, showing punctae (scale bar = 100 µm).
Serial sections of Neothyris sp. Numbers represent distance (mm) from ventral beak; dorsal valve is upward; filled areas correspond to calcitic shell; first appearance of some structures is indicated. Length of specimen around 25 mm (scale bar = 5 mm).
Comparison
Specimens from Vauréal Peak locality are similar to Neothyris sp. identified from Lions Rump by Bitner & Pisera (Reference Bitner and Pisera1984), both from Low Head Member of the Polonez Cove Formation. They differ in the smaller size, more ovoid shape and more developed beak of the Vauréal Peak specimens. Neothyris sp. from Vauréal Peak resembles Neothyris sp. from the Destruction Bay Formation at Melville Peninsula (Biernat et al. Reference Biernat, Birkenmajer and Popiel-Barczyk1985) in general shape and beak, but is smaller. The differences could be due to ecological factors, but internal characters of the specimens cannot be compared.
Remarks
Neall (Reference Neall1972) pointed out that the anterior region of the Neothyris shell is thin and fragile, and the posterior part is the most commonly preserved. All of our specimens preserved only the posterior region of the shell. The lack of preservation of the foramen presents the greater difficulty for the identification of our specimens. However, other characters above described point to the genus Neothyris. Besides, the occurrence of Neothyris at Vauréal Peak is possible because this genus is common in other Cenozoic deposits of KGI (Bitner & Pisera Reference Bitner and Pisera1984, Biernat et al. Reference Biernat, Birkenmajer and Popiel-Barczyk1985, Bitner Reference Bitner and Glowacki1997, Bitner & Crame Reference Bitner and Crame2002).
Type-species
Pecten colbecki Smith, 1902
Remarks
Jonkers (Reference Jonkers2003, p. 67) proposed several diagnostic characters of the genus Adamussium. Among these, the material from Vauréal Peak presents: shell smooth or with low costae formed by simple crenulation of the disc; very wide umbonal angle; hinge straight or with dorsal projections; anterior auricle of the left valve curved outward, anterior auricle of the right valve with a shallow arcuate to relatively deep and acute byssal notch; and a functional ctenolium may be present in adults.
Dimensions in millimetres and character values of representative specimens of Adamussium auristriatum sp. nov.
Material
Holotype, internal mould of articulated valves (5457b), external mould of right valve (5457a). Paratypes, internal moulds of articulated valves (5458b); internal moulds of right valve (5301a, 5361, 5405); external moulds of right valve (5301b, 5394, 5458a); external mould of left valve (5428).
Etymology
From Latin auris = ear and striatum = striated, referring to the radial costae on the anterior auricle.
Diagnosis
Umbonal angle around 130°; commarginal sculpture of lirae, weakly marked, almost equally and widely spaced down the entire shell; RV auricles almost symmetrical, posterior and anterior with well defined commarginal lines, anterior auricle with radial costae.
Description
Shell varying from moderate to small in size, subcircular; dorsal margin short, ventral margin wide and rounded; equant; very compressed, with moderately convex valves; beaks orthogyrate; umbonal angle wide (around 130°); valves slightly opisthocline (mean AVH/PVH = 1.10); radial sculpture of around 10–15 main plicae, intercalated with lower and less developed plicae; commarginal sculpture of lirae, weakly marked, almost equally and widely spaced down the entire shell; microsculpture of narrow antimarginal ridgelets; hinge line almost straight; RV auricles highly symmetrical (mean AOL/POL = 1.04), posterior and anterior with well defined commarginal lines, anterior auricle rounded, with 4–6 radial costae; byssal notch acute, ctenolium with six byssal teeth (in 5457b).
Comparison
Adamussium auristriatum sp. nov. differs from the Pleistocene–Recent A. colbecki colbecki in its smaller size, its equally convex valves, its narrower umbonal angle, its more equally spaced and weaker commarginal sculpture, and the presence of radial costae on the RV anterior auricle. Adamussium auristriatum sp. nov. differs from A. colbecki cockburnensis from the Late Pliocene Cockburn Island Formation (Jonkers Reference Jonkers2003) in its opisthocline valves, narrower umbonal angle, more symmetrical auricles, weaker commarginal sculpture and the presence of radial costae on the RV anterior auricle. The auricles and umbonal angle of A. alanbeui Jonkers, Reference Jonkers2003 are very distinct from those of A. auristriatum sp. nov., which contains commarginal sculpture and symmetric auricles. However, the pattern of antimarginal microsculpture is very similar in the two. The specimens of A. alanbeui from the type-area of Polonez Cove Formation (initially identified as Eburneopecten sp. by Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984) are poorly preserved, but the sculpture pattern is very unlike A. auristriatum sp. nov. Comparison with other Adamussium species is presented in the Table V.
Comparative features of Adamussium colbecki colbecki, A. colbecki cockburnensis, A. alanbeui and A. auristriatum sp. nov.
Notes: 1according to Jonkers Reference Jonkers2003, 2specimens from the Low Head Member of the Polonez Cove Formation at Vauréal Peak, - = not observable.
Remarks
Adamussium colbecki colbecki is known from Pliocene and Pleistocene deposits of West Antarctica, and from the Recent of the Southern Ocean (Jonkers Reference Jonkers2003). Adamussium colbecki cockburnensis is recorded only from Late Pliocene Cockburn Island Formation (Jonkers Reference Jonkers2003). One specimen of A. alanbeui from the Polonez Cove Formation, presumably from the Low Head Member, at Godwin Cliffs, Lions Rump area, was identified and illustrated by Jonkers (Reference Jonkers2003). This species differs from other species of Adamussium in lacking radial costation and commarginal lirae (Jonkers Reference Jonkers2003). Together with A. alanbeui, A. auristriatum sp. nov. is the oldest record of the genus and probably represents a closer relative to the Recent A. c. colbecki than A. colbecki cockburnensis and A. alanbeui do. Despite the resemblance of the Oligocene A. auristriatum sp. nov. to the Recent A. colbecki colbecki and the Pliocene A. colbecki cockburnensis, it is reasonable that they comprise three distinct, but closely related taxa.
Dimensions in millimetres and character values of specimen of ?Adamussium cf. A. alanbeui.
Material
Internal mould of articulated valves (5354b); fragment of external mould of left valve (5354a).
Description
Shell small; equant; very compressed; valves orthocline, moderately convex; external sculpture of very thin commarginal growth lines and very faint antimarginal riblets. In right valve, dorsal margin of posterior auricle short; anterior auricle apparently fragmented.
Remarks
Jonkers (Reference Jonkers2003) pointed out that A. alanbeui is distinguished from A. colbecki by its smaller size, its lower convexity, and the lack of radial costation and commarginal lirae. A. alanbeui (previously identified as Eburneopecten sp. by Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984) occurs in Oligocene strata of McMurdo Sound region, in the type-area of the Polonez Cove Formation, KGI, and in Oligocene–Miocene Cape Melville Formation at Melville Peninsula, KGI (Jonkers Reference Jonkers2003).
Comparison
The specimen collected from Vauréal Peak resembles A. alanbeui erected by Jonkers (Reference Jonkers2003) mainly in the absence of radial macrosculpture, presence of narrow antimarginal sculpture, and shape and size of the anterior right valve auricle (see Table V). Although the specimen from Vauréal Peak is poorly preserved, it is a plausible record because A. alanbeui is recorded from the Low Head Member at Lions Rump (Jonkers Reference Jonkers2003, p. 70).
Type-species
Pecten subauriculata Montagu, 1808
Remarks
Fleming (Reference Fleming1978, p. 17) characterized Limatula as having “tall, narrow, cylindrical and equilateral shell, almost circular in cross-section when the paired valves are closed, radial ornament, sometimes restricted or best developed in a median zone, flanked by almost smooth anterior and posterior submargins, and having a median structure consisting of a central riblet slightly more prominent than its fellows, reflected internally by a central groove flanked by costellae that are the most prominent internal structure”. However, Fleming (Reference Fleming1978) stated that the pattern of radial sculpture is variable among Limatula species, with some having radial costellae restricted to an ornamented band in the median part of the disc, others with the ribs persisting on the auricles or persisting but becoming weaker toward the auricles area, and some species having riblets on the ears. Some Recent (Dell Reference Dell1990, Narchi et al. Reference Narchi, Domaneschi and Passos2002) and fossil species of Limatula (Fleming Reference Fleming1978, Beu & Maxwell Reference Beu and Maxwell1990) have a more trigonal than ovoid shape. All characters proposed by Fleming (Reference Fleming1978) as diagnostic of Limatula are present in the specimens described here.
Type-species
Lima (Limatula) hodgsoni Smith, 1907
Remarks
Fleming (Reference Fleming1978) erected the subgenus Squamilima just after Habe (Reference Habe1977) erected Antarctolima; both authors based it on the same type-species. Thus, we use the name Antarctolima here, despite the fact that Fleming (Reference Fleming1978, p. 81) have described the subgenus more accurately. This author pointed out that this Limatula group has “medium-sized shell of trigonal to broadly pyriform outline, height about 1.25 times length, with weak auricular sinuses and weak median structure, represented internally by a square-cut sulcus bordered by costellae slightly more prominent than the rest; ornament of regularly spaced, numerous, crowded ribs separated by narrow deep sulci, decussated by incremental lirae or rugae that may form prominent cusp-like scales on the ribs; radials continuing to the auricles or fading on submargins”. All these features are observed in Limatula specimens from Vauréal Peak.
Dimensions in millimetres and character values of representative specimens of Limatula ferraziana sp. nov.
Material
Holotype, internal and external moulds (53322, 5324). Paratypes, internal moulds (5360a, 5451, 5452, 5453), external moulds (5309, 5346, 5360b).
Etymology
Referring to the Brazilian Antarctic Station Comandante Ferraz, located at Admiralty Bay, King George Island, West Antarctica.
Diagnosis
Shell pyriform, umbones prominent, sharp; radial sculpture almost reaching auricle area; median ridge more prominent than others, reflected internally as a groove, adjacent ridges to the median more constricted, acute in transverse section; commarginal sculpture more conspicuous when intersecting radial sculpture, with scaly appearance weaker than in L. (A.) hodgsoni; auricles small, almost symmetrical, posterior auricle slightly larger than anterior.
Description
Shell varying in height from 12.5 to 27.4 mm; equilateral in smaller specimens, slightly inequilateral in larger specimens; pyriform, equant, moderately inflated; umbones prominent, sharp; radial sculpture of 28–30 well developed ribs, almost reaching auricle area, and reflected internally, median ridge (14–15th) more prominent than others, reflected internally as a groove, adjacent ridges more constricted, with acute margin when observed in transverse section; commarginal sculpture of numerous regularly spaced growth lines, extending to auricle area, raised into scale-like ridges when intersecting radial sculpture; dorsal margin short; ventral margin longer, rounded and slightly crenulated; hinge line almost straight, hinge edentulous; auricles small, almost symmetrical, posterior auricle slightly larger than anterior.
Comparison
Gazdzicki & Pugaczewska (Reference Gazdzicki and Pugaczewska1984) identified Limopsis (Pectunculina) cf. insolita from the Low Head Member (Polonez Cove Formation) at Lions Rump. Their shells are quite similar to the material from Vauréal Peak and we consider that both represent the same species. The size, general shape, ornamentation and auricles are quite similar in both materials. The description by Gazdzicki & Pugaczewska (Reference Gazdzicki and Pugaczewska1984) included features that disagree with the diagnosis provided by Newell (Reference Newell and Moore1969, p. 248) for the Order Arcoida (which Limopsidae belongs to), and we consider that Gazdzicki & Pugaczewska (Reference Gazdzicki and Pugaczewska1984) misidentified their specimens.
Limatula (Antarctolima) ferraziana sp. nov. is similar to the Recent species Limatula (Antarctolima) hodgsoni, which is common in continental and peninsular Antarctica (Fleming Reference Fleming1978, Dell Reference Dell1990, Absher & Feijó Reference Absher and Feijó1998, Narchi et al. Reference Narchi, Domaneschi and Passos2002). It differs from the Recent species in being smaller, and in having smaller auricles and weaker scaly sculpture on the radial ornamentation. Page & Linse (Reference Page and Linse2002) considered L. (A.) hogsoni to be the basal group of Antarctic Limatula (which includes L. hodgsoni, L. ovalis and L. pygmaea) and estimated the minimal speciation time of L. (A.) pygmea and L. (A) ovalis to be around 19 Ma. Hence, as the age of the Low Head Member is estimated at 29.8 Ma (Dingle et al. Reference Dingle, Mcarthur and Vroon1997), it is not surprising that L. (A.) ferraziana sp. nov. is more closely related to L. (A.) hodgsoni than to the other Antarctic Limatula (Antarctolima) species.
Material
Several fragmentary calcareous tubes (5288a–d, 5316, 5344, 5349a–c).
Description
The tubes are calcareous, cylindrical, thin, straight, or with some curved in to “L” and “C” shapes; size varying from 3 to 35 mm long and 1 to 2 mm wide (diameter), most tubes having the same width along their entire length, but some tapering at the tips (possibly due to preservation conditions). Transverse section cylindrical. External ornamentation of 2–3 primary and 6–8 secondary transverse ridges per mm; smooth internally.
Comparison. Tubes from Vauréal Peak are quite similar to those found in Miocene deposits of CRP-1 (Jonkers & Taviani Reference Jonkers and Taviani1998, Cape Roberts Science Team 1998) and early Oligocene CRP-3 (Taviani & Beu Reference Taviani and Beu2001) cores at Cape Roberts, Victoria Land Basin. The external ornamentation of tubes from Vauréal Peak is also similar to that reported from the type-area of the Low Head Member (Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984).
Remarks
The elongate form, cylindrical transverse section, smooth inner surface and calcareous composition are characteristic features of serpulid tubes (see Schweitzer et al. Reference Schweitzer, Feldmann, Marenssi and Waugh2005 for features of this and other Polychaeta families). Besides, there are several records of polychaete tubes from Palaeogene and Neogene deposits of Antarctica (Jonkers & Taviani Reference Jonkers and Taviani1998, Taviani & Beu Reference Taviani and Beu2001, Schweitzer et al. Reference Schweitzer, Feldmann, Marenssi and Waugh2005), including the type-area of Low Head Member of Polonez Cove Formation at KGI (Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984). Also, Recent polychaetes are common in several regions of Antarctic (e.g. Cantone Reference Cantone1995, Ramos & San Martín Reference Ramos and San Martín1999). All these points make the presence of polychaete tubes at Low Head Member of the Polonez Cove Formation at Vauréal Peak plausible.
Material
Several fragments (5300, 5310, 5318a–d, 5318f, 5337, 5349, 5352, 5293).
Remarks
Several taxa of bryozoans are recorded from Oligocene deposits of KGI, including the type-area of the Polonez Cove Formation (Gazdzicki & Pugaczewska Reference Gazdzicki and Pugaczewska1984). Specimens from Low Head Member at Vauréal Peak are fragments, lacking critical features for taxonomic analysis (e.g. zooids), and some with recrystallized calcite, preventing refined taxonomic identification within the phylum.
Material
Internal mould of interambulacrum and external mould of ambulacral plate fragment (5328a), fragment of primary spine (5339).
Description
Specimen 5328a (Fig. 12j–n) corresponds to an interambulacrum and an ambulacral plate. The interambulacrum is 9.1 mm × 3.6 mm, composed of two columns of six alternating plates; primary tubercles not preserved, instead, the locations of each tubercle next to the adradial suture; miliary tubercles are preserved on the fragment of an ambulacral plate. Specimen 5339 (Fig. 12o & p) corresponds to a spine 27 mm long and 0.8 mm in diameter; thickness is the same along the length, enlarging at one tip which possibly corresponds to the shaft; external ornamentation of fine longitudinal striations; transverse section cylindrical, internal microstructure composed of narrow medulla; radiating layer occupying most of the area of the section; cortex layer relatively thick.
Adamussium auristriatum sp. nov. a–h. Holotype 5457a–b, a–b. (5457a), external mould of RV, b. detail of auricles, c–h. (5457b), internal mould of articulated pair; c. right view, d. left view, e. anterior view, f. posterior view, g. dorsal view, h. detail of ctenolium, showing teeth sockets (arrows). i. Latex cast of ctenolium of 5457b, showing teeth (arrows). j–k. Paratype 5394–5405, RV, j (5405), internal mould, k. (5394) external mould. l. Latex cast of 5394. m–n. Paratype 5458a–b, m. (5458a) RV auricles of external mould (all bars = 5 mm), n. (5458b) right view of internal mould of articulated pair. o–q. Paratype 5301b, commarginal frills (c) and antimarginal microsculpture (a) in SEM micrographs of central region of external mould of RV (umbo to the left) (scale bars of o = 1 mm, p = 300 µm and q = 100 µm).
?Adamussium cf. A. alanbeui. a–b. Specimen 5354a–b, a. internal mould of articulated pair, right view, b. fragment of external mould of LV. c–d. latex mould of external mould of RV, d. detail of auricles of c, dotted line indicates dorsal margin (all bars = 5 mm).
Limatula (Antarctolima) ferraziana. a–b. Holotype 5322/5324, LV, a. (5324), external mould, b. (5322), internal mould. c–d. Oaratype 5360a–b, LV, c. internal mould, d. fragment of external mould. e–f. Paratype 5451/5309, RV, e. internal mould, f. latex cast of external mould. g. Paratype 5346, LV, latex cast. Arrows indicate “median structure” of the shells (all bars = 5 mm).
Miscellaneous. a–d. Serpulidae, a–c. specimen 5349c, a. two fragmented tubes, b–c. the same specimens magnified, showing the external ornamentation (scale bars = 5 mm), d. specimen 5316, smooth aspect of the tube internally (scale bar = 500 µm). e–i. Bryozoa, e. specimen 5318b, general aspect of a fragment (scale bar = 5 mm), f–g. specimen 5349, f. general aspect of a fragment (scale bar = 1 mm), g. the same specimen magnified (scale bar = 1 mm), h. specimen 5352b, showing poorly preserved zooids (arrow) (scale bar = 300 µm), i. specimen 5352a (scale bar = 500 µm). j–p. Echinoidea, j–n. specimen 5328a, j. internal mould of interambulacrum (scale bar = 1 mm), k. the same specimen magnified (scale bar = 300 µm), showing the location of tubercules (t) and the adradial suture (s), l. external mould of a fragment of ambulacral plate (scale bar = 1 mm), m. the same specimen magnified (scale bar = 300 µm), n. miliary tubercule (scale bar = 300 µm), o–p. specimen 5339, fragment of primary spine, o. external aspect (scale bar = 100 µm), p. transverse section showing medulla (m), radiating (r) and cortex (c) layers (scale bar = 20 µm).
Remarks
There are several records of echinoderms from Palaeogene and Neogene deposits of KGI (Jesionek-Szymanska Reference Jesionek-Szymanska1984, Reference Jesionek-Szymanska1987, Meyer Reference Meyer and Oji1993, Blake & Aronson Reference Blake and Aronson1998); the commonest records, including those from the type-area of the Low Head Member (Jesionek-Szymanska Reference Jesionek-Szymanska1984), are assigned to the Order Cidaroida. Representatives of this order have enlarged primary tubercles and conspicuous aureolae. Our specimens, however, comprise two minor fragments (whose primary tubercles are not preserved) and a fragment of spine, which is too thin comparatively to cidaroid spines. These conditions do not allow their assignment to lower taxonomic categories.
Comparative (palaeo)ecology
All genera from the Polonez Cove Formation at Vauréal Peak have modern representatives whose environmental preferences and geographical distribution can be used in order to interpret a hypothetical palaeoenvironmental scenario for the fossil assemblage in the context of the depositional pattern of the formation.
Liothyrella is a pedunculate, suspension feeder brachiopod occurring from Central America to Antarctica and New Zealand (Foster Reference Foster1989). The species Liothyrella uva Broderip, very common in southern South America and Antarctica, is found at depths up to 1500 m (Foster Reference Foster1974, Reference Foster1989, Bitner Reference Bitner1996), in clumps attached to hard substrata and in such cryptic environments as cracks, crevices, and fjords, where turbulence is reduced (Barnes & Clarke Reference Barnes and Clarke1995, Peck Reference Peck1996, Peck et al. Reference Peck, Brockington and Brey1997). Specimens of Liothyrella neozelanica Thomson from New Zealand, collected at depths of 9–22 m, have thicker and larger shells than specimens from depths of 101 m (Foster Reference Foster1989). Regardless of the fact that physiological adaptation of the brachiopod L. uva to polar Antarctic conditions is not well corroborated (Peck & Robinson Reference Peck and Robinson1994, Peck Reference Peck1996, Peck et al. Reference Peck, Brockington and Brey1997), it is only found in cool waters. So, Liothyrella sp. from the Polonez Cove Formation may have lived in cool shallow waters, somewhat protected from wave currents, as its modern South American and Antarctic representatives.
Recent Neothyris is a free-lying, suspension feeder brachiopod, commonly found in Southern Hemisphere cool shelf waters, mostly in sub-Antarctic islands and New Zealand (Neall Reference Neall1970). The species N. lenticularis occurs in subtropical waters from New Zealand to the external boundary of the Circum-Antarctic Current, being absent in Antarctica (Neall Reference Neall1970). According to this author (Neall Reference Neall1970) the genus is indicative of mean annual surface water temperature of 8–11°C. Similarly to Liothyrella sp., the presence of the genus Neothyris in the Polonez Cove Formation corroborates the idea that cool temperate conditions were present in West Antarctica around early Oligocene (Zachos et al. Reference Zachos, Pagani, Sloan, Thomas and Billups1994, Dmitrenko Reference Dmitrenko2004). Neothyris larvae attach at shell fragments or granules, loosing totally or partially their pedicles during adult phase (Neall Reference Neall1970). This may be the case of Neothyris from the Polonez Cove Formation, as indicated by the apparently absence of the foramen. Modern forms are found in sandy, shelly and/or pebbly sediments and may tolerate some turbulence (Neall Reference Neall1970), which also agrees with the depositional framework proposed to the Polonez Cove Formation (Troedson & Smellie Reference Troedson and Smellie2002; this work).
The Antarctic endemic Adamussium colbecki is a suspension feeder bivalve with circumpolar distribution (Chiantore et al. Reference Chiantore, Cattaneo-Vietti, Povero, Albertelli, Faranda, Guglielmo and Ianora2000). During its earlier stages, the species is found attached by byssus to living adult shells or to hard substrata, while adults are commonly found free-lying (Nicol Reference Nicol1966). Abundant populations are found living at depths above 100 m (Chiantore et al. Reference Chiantore, Cattaneo-Vietti, Povero, Albertelli, Faranda, Guglielmo and Ianora2000), but isolated specimens are also reported from depths of 594 m (Hedley Reference Hedley1916) or even 1500 m (Dell Reference Dell1990). It seems that the water energy is more critical than the nature of the substrate, as A. colbecki is found also in hard, gravel, fine sand or silty substrates (Jonkers Reference Jonkers2003). Nigro (Reference Nigro1993) suggested that strong wave action occurring after sea-ice melting, and the prevalence of rocky substrate might inhibit the presence of A. colbecki at depths of 10–15 m in Terra Nova Bay. Berkman et al. (Reference Berkman, Cattaneo-Vietti, Chiantore and Howard-Williams2004) concluded that A. colbecki prefers low-energy deep water environments or coastal areas under sea ice, while Nigro (Reference Nigro1993) supposed that first growth stages may occur in deeper areas and that adults move later towards shallow waters. However, as some adults may live continuously attached by byssus, some water current typical of shallow waters may be tolerable. The presence of deep byssal notch and asymmetric auricles in modern species corroborates this life style (Jonkers Reference Jonkers2003). Besides, considering the morphology of the fossil species A. auristriatum, it is acceptable that, during the deposition of the Polonez Cove Formation, it might live attached by byssus in pebbles, below wave base environments, or even protected under the sea ice.
Limatula is a free-lying suspension feeder bivalve with worldwide distribution (Allen Reference Allen2004), including tropical and polar regions, living from shallow to deep waters (Fleming Reference Fleming1978). Living representatives of Antarctolima subgenus occur at depths of 1180 m, but higher densities of this brachiopod were found in the Ross Sea from 6–695 m (but often in quite shallow water), on hard substrata, gravel or even on the surface mat of sponge spicules (Nicol Reference Nicol1966, Dell Reference Dell1990). Accordingly, the Oligocene L. (Antarctolima) ferraziana sp. nov. may have preferred shallow depths and gravel substrata conditions, as indicated by the inferred depositional pattern of the Polonez Cove Formation.
Palaeobiogeography
Figure 13 shows the occurrences of genera Liothyrella, Neothyris, Adamussium and Limatula in the latest Cretaceous, late Eocene, early Oligocene, early Miocene and Recent. In spite of the restrict number and diversification, known spatial and stratigraphical distributions of these taxa allow some interesting views on their possible dispersal pattern during the Cenozoic.
Occurrences and inferred dispersal patterns of brachiopods and bivalves genera studied in this work for the interval latest Late Cretaceous to Recent. a. Late Cretaceous, b. late Eocene, c. early Oligocene, d. early Miocene, e. Recent, f. dispersal routes suggested by fossil record of Liothyrella, Neothyris, Adamussium and Limatula genera in a general paleogeographical base; arrows represent dispersal routes for each taxon. Note dispersal of Liothyrella and Limatula after the opening of the Tasmanian Gateway, from the end of Late Cretaceous until early Oligocene, and dispersal of Liothyrella, Adamussium and Limatula only after the opening of Drake Passage after early Oligocene (Fossil record according to Ihering Reference Ihering1907, Neall Reference Neall1972, Buonaiuto Reference Buonaiuto1977, Fleming Reference Fleming1978, Owen Reference Owen1980, Bitner & Pisera Reference Bitner and Pisera1984, Biernat et al. Reference Biernat, Birkenmajer and Popiel-Barczyk1985, Beu & Dell Reference Beu, Dell and Barret1989, Foster Reference Foster1989, Beu & Maxwell Reference Beu and Maxwell1990, Bitner Reference Bitner and Glowacki1997, Frassinetti Reference Frassinetti1998, Craig Reference Craig2000, Bitner & Crame Reference Bitner and Crame2002, Jonkers Reference Jonkers1998, Reference Jonkers2003, this work. Maps modified from paleogeographical reconstructions kindly provided by Dr Lisa M. Gahagan, from University of Texas, Austin).
Liothyrella, recorded from the Maastrichtian to the Recent, is restricted to the Southern Hemisphere and is more common in Palaeogene than in Neogene deposits (Owen Reference Owen1980, Craig Reference Craig2000, Bitner & Crame Reference Bitner and Crame2002), mainly in Australia and New Zealand. In Antarctica, Liothyrella has been recorded from the Eocene La Meseta Formation, Seymour Island, Western Weddell Sea (Bitner Reference Bitner1996). Another record is from the Low Head Member of the Polonez Cove Formation in Mazurek Point (Bitner Reference Bitner and Glowacki1997). According to the palaeontological record (Fig. 13a–e), the genus originated during the Maastrichtian in south-western margin of Australia and possibly migrated during Palaeogene along the opening sea way between Australia and Antarctica (Tasmanian Gateway) towards the Antarctic Peninsula (Craig Reference Craig2000) and southern South America (Fig. 13f). After the middle Eocene Liothyrella reached the western edge of Antarctica and southern South America. This is suggested by the presence of Recent L. uva in those regions, with five subspecies described by Foster (Reference Foster1989) as evolved clines from South America towards Antarctica.
Neothyris is recorded from the Neogene to the Recent (MacKinnon & Lee Reference MacKinnon, Lee and Seldon2006) and living species are restricted to New Zealand, Australian (Craig Reference Craig1999) and sub-Antarctic (Neall Reference Neall1972) waters. Fossil records in Antarctica were previously recognized in the Low Head Member of the Polonez Cove Formation at Lions Rump (Bitner & Pisera Reference Bitner and Pisera1984), and in the late Oligocene Destruction Bay Formation at Melville Peninsula (Biernat et al. Reference Biernat, Birkenmajer and Popiel-Barczyk1985). Cohen et al. (Reference Cohen, Gawthrop and Cavalier-Smith1998) estimated the divergence time of Neothyris from its stem lineage as around 47–63 Ma (early Paleocene to early Eocene). There is no record of Neothyris prior to latest early Oligocene (Fig. 13c–e), which does not exclude the possibility of Neothyris occurring in West Antarctica during the Eocene. The modern genera closely related to Neothyris (according to Cohen et al. Reference Cohen, Gawthrop and Cavalier-Smith1998) are widely distributed in New Zealand and adjacent waters.
Until recently, Adamussium was considered as a genus restricted to the Holocene, being represented by the Recent circum-Antarctic A. colbecki (Smith 1902). Fossil records of this genus were firstly reported from Oligocene strata of McMurdo Sound region (Beu & Dell Reference Beu, Dell and Barret1989). Further records include specimens from Oligocene/Miocene Cape Melville Formation at Melville Peninsula, KGI (Jonkers Reference Jonkers2003); middle Miocene Battye Glacier Formation, East Antarctica (Stilwell et al. Reference Stilwell, Harwood and Whitehead2002); late Pliocene Cockburn Island Formation at Cockburn Island, and Holocene Taylor Formation at New Harbour and Minna Bluff (Jonkers Reference Jonkers1998) (see Fig. 13c–e). The origin of Adamussium is still unclear, as its hypothetical ancestor seems apparently missing (Dell & Fleming Reference Dell and Fleming1975, Canapa et al. Reference Canapa, Barucca, Marinelli and Olmo2000, Jonkers Reference Jonkers2003, Barucca et al. Reference Barucca, Olmo, Capriglione, Odierna, Canapa, Luporini and Morbidoni2005). This is possibly due to the unique set of characters of the genus and the relatively unknown phylogenetic relationships among the Pectinidae species (Canapa et al. Reference Canapa, Barucca, Marinelli and Olmo2000, Barucca et al. Reference Barucca, Olmo, Capriglione, Odierna, Canapa, Luporini and Morbidoni2005). Jonkers (Reference Jonkers2003) listed some “primitive” characters that Adamussium shares with the Oligocene–Recent Austrochlamys. It is possible that the evolutionary histories of both genera are restricted to the Southern Ocean - as fossil and living Adamussium are so far found only in West Antarctica - with both genera originated from an ancient pectinid lineage which was already established in West Antarctica prior to the Cenozoic. Once Adamussium arose in Antarctic Peninsula, it probably dispersed along the eastern margin of the continent during the Neogene (Fig. 13f).
Limatula is recorded from Middle Jurassic to Recent faunas and has worldwide distribution (Fleming Reference Fleming1978, Allen Reference Allen2004). In the South Pacific, it occurs from the Cretaceous onwards in New Zealand (Beu & Maxwell Reference Beu and Maxwell1990), from the Eocene in Australia (Buonaiuto Reference Buonaiuto1977), from the Neogene in southern South America (Ihering Reference Ihering1907, Fleming Reference Fleming1978, Frassinetti Reference Frassinetti1998), and from the Pleistocene in Victoria Land Basin, Antarctica (Taviani et al. Reference Taviani, Beu and Lombardo1998; Fig. 13b–e). Fleming (Reference Fleming1978) stated that Limatula (represented by the group of Limatula corallina) originated in the Jurassic in Europe, and occupied Africa and India during Mesozoic. At the end of the Late Cretaceous this group would evolved and possibly occupied the Pacific coast of Gondwana. However, the Recent Antarctic Limatula would derivate from stocks of L. crebresquamata, from upper Oligocene of Victoria, and entered the Southern Ocean from Australia, a scenario considered by Fleming (Reference Fleming1978, p. 86) as “an oversimplification of inadequate data”. Indeed, L. (Antarctolima) ferraziana seems more closely related to Recent Antarctic and sub-Antarctic Limatula species (mainly L. hodgsoni), than L. crebresquamata or any southern South America fossil Limatula. Hence L. (Antarctolima) ferraziana is the most adequate group to represent the ancestral stock of Antarticolima group. Considering palaeogeographic reconstructions and morphological affinities of Southern Ocean Limatula species, it would be more plausible to assume that, in the case L. (Antarctolima), ancient lineages were already present in the Pacific margin of Gondwana during the early Cenozoic.
Figure 13f synthesizes the fossil record of Liothyrella, Adamussium and Limatula represented in Fig. 13a–e. The dispersal pattern roughly fits the onset of the Circum-Antarctic Current - that started close to the Eocene/Oligocene boundary and was completely shaped during the late Oligocene (Pfuhl & McCave Reference Pfuhl and McCave2005) or later, during middle Miocene (Barker & Thomas Reference Barker and Thomas2004) - at two steps. In the first step the oldest taxa (Liothyrella and Limatula) dispersed after Tasmanian Gateway opening (~33 Ma, Exon et al. Reference Exon, Kennett, Malone, Brinkhuis, Chaproniere, Ennyu, Fothergill, Fuller, Grauert, Hill, Janecek, Kelly, Latimer, Nees, Ninnemann, Nuernberg, Pekar, Pellaton, Pfuhl, Robert, Roessig, Roehl, Schellenberg, Shevenell, Stickley, Suzuki, Touchard, Wei and White2001), during early Cenozoic, from Australia and New Zealand towards the Antarctic Peninsula, through cool currents that represent the western portion of Circum-Antarctic Current (Lazarus & Caulet Reference Lazarus and Caulet1993). While the fossil record shows that Liothyrella may have dispersed previously than Tasmanian Gateway opening (Fig. 13), Beu et al. (Reference Beu, Griffin and Maxwell1997) considered larvae dispersal unlike to have occurred before the development of the Circum-Antarctic Current. Although this cool current initiated later than Eocene/Oligocene boundary, a warmer current flowing from south-east Australia towards southern South America existed during Palaeocene to Eocene (Lazarus & Caulet Reference Lazarus and Caulet1993, p. 165, fig. 18), and may have been responsible for the dispersal of some taxa, as Liothyrella, during this interval. The first step of dispersal is in accordance with the context of Weddellian Province proposed by Zinsmeister (Reference Zinsmeister, Gray and Boucot1979), as we are considering the faunal distribution regardless proximity of landmasses. In addition, Neothyris and Limatula dispersal routes extend to the Palaeogene the hypothesis of larvae transporting from New Zealand/Chatham Rise to Antarctic Peninsula proposed by Stilwell (Reference Stilwell1997).
The second step was accomplished after the Drake Passage opening initiated at 28 Ma or earlier (Barker & Thomas Reference Barker and Thomas2004), following development of eastern part of the Circum-Antarctic Current (~24 Ma, Pfuhl & McCave Reference Pfuhl and McCave2005). At the end of Palaeogene, both genera reached South America. At the beginning of the Neogene, intensification of the Circum-Antarctic Current around the eastern margin of Antarctica allowed the dispersal of Adamussium and Limatula around Antarctica. Considering that the dispersal pattern implies a clockwise route, the presence of Limatula in New Zealand at the end of Cretaceous foresees that this genus may be present also in Australia during this time. Neothyris lacks a clear dispersal pattern, probably due to its incomplete record. Two possible routes can be assigned to this genus: through currents that would have flowed in a shallow sea that existed between West and East Antarctica during early Oligocene to middle Miocene (Lazarus & Caulet Reference Lazarus and Caulet1993), as indicated in Fig. 13f; or the genus would have followed the second stage of dispersal over eastern margin of Antarctica at the end of Palaeogene.
Tasmanian Gateway and Drake Passage openings are considered “key deep sea ocean gateways” (Crame Reference Crame1999, p. 4) of the Antarctic geological history and, not surprisingly, affected biotic composition and distribution in Southern Ocean (Beu et al. Reference Beu, Griffin and Maxwell1997) by changing oceanic circulation during the Cenozoic.
Concluding remarks
Palaeontological results presented here provide new taxonomic information about the diversity of Palaeogene invertebrates in West Antarctica, including new occurrences of brachiopod and bivalve genera - Neothyris sp., Adamussium auristriatum sp. nov. and Limatula (Antarctolima) ferraziana sp. nov. - that may represent some of the oldest lineages of these genera so far recorded in KGI and Antarctica. This new information raises some questions about the palaeobiogeographical evolution in Southern Ocean, as how the thermal isolation of Antarctica affected evolving biota from Southern Ocean areas. Even though such major questions are not the main scope of this work, some interesting aspects bearing on the theme can be achieved.
Taxa from the Polonez Cove Formation at Vauréal Peak (Neothyris sp., Liothyrella sp., Adamussium auristriatum, Limatula ferraziana, serpulid tubes and echinoid fragments) have modern representatives. As discussed above, brachiopods and bivalves have wide bathymetric range. Indeed most Recent bivalves have significantly wide depth ranges in Antarctica (Brey et al. Reference Brey, Dahm, Gorny, Klages, Stiller and Arntz1996), that is interpreted by Barnes et al. (Reference Barnes, Hodgson, Convey, Allen and Clarke2006, p. 124) as a suggestion of the presence of a “past shelf refugia”. Species of wide bathymetric ranges would be favoured after events of expansion and recession of ice sheet grounding line (Barnes et al. Reference Barnes, Hodgson, Convey, Allen and Clarke2006). Where ice offered some protection these species would be able to persist in shelf areas. So, it is possible that at times of the deposition of Polonez Cove Formation, presence of sea ice offered some protection to the fauna, as also indicated by the environmental preferences of the modern Adamussium colbecki.
Considering the depositional model of the Polonez Cove Formation, it is plausible to say that invertebrates at Vauréal Peak lived in a shallow marine environment. This is also indicated by the substrate preferences of modern species, except for Adamussium colbecki which may lives in virtually all kinds of substrate. Moreover, apart from the echinoid, all of them are suspension feeders, which means that some water movement for filtering was required. From the taphonomic data it is also probable that the fossil fauna lived below the fair weather wave base, as no reworking features are observed in specimens.
Regardless the fact that we are considering only four taxa from only one locality, the proposed dispersal pattern seems consistent with available models of oceanic circulation during fragmentation of Gondwana, when Australia/Antarctica and Antarctic Peninsula/South America separated (Lazarus & Caulet Reference Lazarus and Caulet1993, Exon et al. Reference Exon, Kennett, Malone, Brinkhuis, Chaproniere, Ennyu, Fothergill, Fuller, Grauert, Hill, Janecek, Kelly, Latimer, Nees, Ninnemann, Nuernberg, Pekar, Pellaton, Pfuhl, Robert, Roessig, Roehl, Schellenberg, Shevenell, Stickley, Suzuki, Touchard, Wei and White2001). For this reason, the dispersal pattern here proposed is considered as a first approach.
Almost all modern representatives of the Vauréal Peak fauna live in Antarctica, including A. colbecki, that is distributed only in areas inside the Antarctic Convergence. The exception is Neothyris, that occurs in cool temperate water of New Zealand. This suggests that at least part of the modern Antarctic faunal configuration was outlined during latest early Oligocene, even though the cool polar climate as observed today was not present during that period.
Acknowledgements
We are grateful to A. Beu and M.A. Bitner for providing bibliography, revising the original manuscript and for suggestions that improved it; to H.A. Jonkers for providing bibliography and for discussions on pectinid taxonomy; to L.M. Gahagan for allowing the use of her palaeogeographic reconstructions; and to L.V. Warren and R.P. Almeida for assistance with geological descriptions. This paper is a contribution to PROANTAR - CNPq project 55.0352/2002–3, Mudanças climáticas durante o Cenozóico: o registro geológico terrestre.
