PalZ https://doi.org/10.1007/s12542-019-00498-3 SHORT COMMUNICATION The (truly) frst fossil freshwater molluscs from Antarctica Sergio Martínez1 · Roberto A. Scasso2 · Andrés Elgorriaga3 · Ignacio Capelli2 · Rodolfo del Valle4 · Pablo Puerta3 · Juan Manuel Lirio4 · Cecilia Rodríguez Amenábar4 Received: 19 December 2018 / Accepted: 24 October 2019 © Paläontologische Gesellschaft 2019 Abstract New collection work in Mount Flora, Hope Bay, Antarctic Peninsula, has confrmed the presence of bivalve moulds in the Jurassic Mount Flora Formation. In contrast to a twenty-frst century article claiming to report the frst discovery of fossil freshwater molluscs in Antarctica, evidence of these molluscs was frst reported for Antarctica more than 110 years ago by J. Gunnar Andersson. The bivalve was formally described and named some 50 years later, by Camacho, as Antediplodon esperanzaensis. The species was subsequently transferred to the genus Diplodon for unknown reasons. Here, we illustrate specimens from the same locality for the frst time with photographs, considering them Unionida incertae saedis, in view of the absence of diagnostic characters. In fact, two morphotypes are present, one elongated, very similar to the nominal species A. esperanzaensis, and another one subelliptical in shape. Keywords Bivalvia · Unionida · Jurassic · Mount Flora · Antarctica Introduction Transantarctic Mountains. Thus far, this claim has remained unquestioned. The objective of this study was to resurrect a largely forgot- However, the fact is that Andersson (1906: 27) first ten fossil fnding, which led to an erroneous attribution of mentioned freshwater molluscs from Antarctica in a brief priority. account of the fossils of Mount Flora, Hope Bay, Antarc- Ashworth and Preece (2003) reported the supposedly tic Peninsula. Halle (1913) cited this fnding, and Seward frst fnding of fossil freshwater molluscs from Antarctica. (1913), in a literature comment about the former publica- They corresponded to an indeterminate gastropod species tions, took note of it too. However, in the next few decades, belonging to the superfamily Lymnaeoidea and to a bivalve the reference to the bivalves was forgotten. assigned to the genus Pisidium (Sphaeridae), both found in Half a century later, Camacho (1957: 30) described and the late Neogene Meyer Desert Formation (Sirius Group), named for the frst time a species of an Antarctic freshwa- ter mollusc, Antediplodon esperanzaensis, on the basis of two moulds collected from the same area as that studied by Andersson (1906): the dark shales with fossil plants of the Handling Editor: Simon Schneider. Jurassic Mount Flora Formation (Fig. 1). The species was * Sergio Martínez later illustrated by a drawing in Herbst and Camacho (1970: [email protected] fg. 3, redrawn here in Fig. 2) as Diplodon esperanzaensis without any justifcation about the genus transfer. Subse- 1 Dpto. Paleontología, Facultad de Ciencias, Universidad de la República, Iguá 4225, 11400 Montevideo, Uruguay quently, this fnding was only mentioned by Martínez and Figueiras (1991), Martínez et al. (1993), Morton and Herbst 2 IGeBA, Departamento de Cs. Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos (2001), Watters (2001), and Parras and Grifn (2013). Aires, Int. Guiraldes 2160, Ciudad Universitaria, Pab. II, New collections from the type locality confrmed the C1428EHA Buenos Aires, Argentina presence of bivalves in Mount Flora and reminded us of 3 Museo Paleontológico Egidio Feruglio, Av. Fontana 140, the fact that the frst freshwater molluscs of Antarctica were 9100 Trelew, Chubut, Argentina reported almost 100 years before the most recent report. The 4 Instituto Antártico Argentino, Av. 25 de Mayo 1147, collected material is deposited in the Repositorio Antártico Villa Lynch, Provincia de Buenos Aires, Argentina Vol.:(0123456789)1 3 S. Martínez et al. Fig. 1 a Location map of the Hope Bay region in the Antarctic Penin- The orange arrows indicate the advance of the Flora Glacier and the sula. b Geological map of the region of Hope Bay (after Montes et al. area where fossil-rich blocks are abundant. c Main geological units in 2005). The uppermost, fossiliferous interval of the Mount Flora For- the Hope Bay region mation is marked, as well as its inferred trace below the Flora Glacier. de Colecciones Paleontólogicas y Geológicas of the Insti- 300-m-thick terrestrial clastic sedimentary unit of Jurassic tuto Antártico Argentino under the acronym and number age (see Rees 1993 for a discussion on the age of the unit), IAA-Pi 397. separated by an angular unconformity from the underly- ing Hope Bay Formation of Late Palaeozoic/Triassic age. It is covered by the Kenney Glacier Formation (Antarctic Geological setting Peninsula Volcanic Group) of probable Late Jurassic-Early Cretaceous age. The Mount Flora Formation (Caminos and Massabie 1980) The Mount Flora is a fning-upward succession depos- was discovered by Andersson (1906) and later studied in ited as an alluvial fan in a fault-controlled basin margin. detail by Bibby (1966), Elliot and Gracanin (1983), Farqu- Detailed facies analyses (Elliot and Gracanin 1983; Birken- harson (1984), Birkenmajer (1993), Montes et al. (2005) majer 1993) indicate a palaeoenvironment changing upwards and Birkenmajer and Ociepa (2008). It is an approximately from alluvial to fuvial, fnally capped by lacustrine beds. A 1 3 The (truly) frst fossil freshwater molluscs from Antarctica (Holocene–Recent) and probably in former times. However, before the last glacier retreat the Flora Glacier and the rest of the NE-advancing glaciers in the area mostly delivered the blocks into the Antarctic Strait. Systematic palaeontology Classifcation follows Bieler et al. (2010). Class Bivalvia Linnaeus, 1758 Subclass Palaeoheterodonta Newell, 1965 Fig. 2 Diplodon esperanzaensis. Redrawn from Herbst and Camacho Order Unionida Gray, 1854 (1970: fg. 3), with permission from the Asociación Paleontológica Argentina) incertae sedis, morphotype 1 Figure 3a–c large number of fossil plants, together with rare bivalves, 1957 Antediplodon esperanzaensis Camacho: p. 30. beetles and fsh remains (Halle 1913), were recovered from 1970 Diplodon esperanzaensis (Camacho)—Herbst and the lacustrine, uppermost part of the unit, which crops out in Camacho: fg. 3. the Mount Flora (Fig. 1) and from many large blocks spread by the glaciers all over a large area (Fig. 1). Description. Shell moderately infated. Umbo not prominent, The bivalves were exclusively found in loose, black-shale, situated in the anterior third. Dorsal margin short in its ante- moraine blocks in two close localities (63° 24′ 42.3″ S, 57° rior part; posteriorly it is straight, inclined downwards, and 00′ 18.7″ W and 63° 24′ 43.9″ S 57°00′ 21.0″ W; see Fig. 1). being truncated in its posterior portion. Anterior margin very The blocks lay on top of an outcrop of the Lower Member of convex. Posterior margin short, nearly straight. Ventral mar- the Mount Flora Formation. Their lithology is similar to that gin gently convex. From the umbo to the upper half of the of the lacustrine beds currently exposed in the north face of posterior margin there is an angulation of the shell. Imprints the Mount Flora, which also contains similar plant fossils. of growth lines present. The absence of bivalves in the outcrops can be attributed to minor, lateral paleoenvironmental changes. Shale beds not Dimensions. Length: x = 3.8 cm. Height: x = 2.1 cm l/h = 1.8. thicker than 0.5 m are interbedded with sandstones in the fossiliferous interval of the uppermost Mount Flora Forma- Original description. ‘Valva derecha representada por un tion, whereas thicker, well-laminated shale beds are repre- molde interno ovalado con el eje mayor orientado en la sented in the loose blocks of the glacial drift. This is prob- dirección ántero - posterior; extremidad anterior redondeada; ably related to lateral facies changes, expressed by thicker extremidad posterior subaguda; borde dorsal posterior rec- shale beds to the S, probably recording deeper lake deposits. tilíneo inclinado hacia atrás y formando un ángulo obtuso The blocks were eroded from the NE face of the Mount con el borde posterior que es corto; borde ventral suave- Flora Formation and delivered to their fnal location by the mente convexo, formando una concavidad casi imperceptible Flora Glacier, which spread most of the fossil-rich blocks en su unión con el borde posterior; borde anterior circular; along its NE pathway (Fig. 1). Holocene retreat of the gla- superfcie recorrida por líneas concéntricas de crecimiento, cier left exposed a thin cover of glacial detritus (lodgement con insinuación de algunas estrías radiales sobre el umbón. till) bounded by some prominent moraines, the latter being Longitud 4.4 cm; altura 2 cm.’ the result of retreat pulses of the Flora glacier during the Holocene. The fossiliferous blocks are part of the lodgement English translation. Right valve represented by an oval inter- till and moraine deposits. Their abundance decreases mark- nal mould, with the major axis oriented in the anteropos- edly out of the area marked with orange arrows in Fig. 1, terior direction; anterior margin rounded; posterior margin revealing their local provenance. They come from the ero- subacute; posterior dorsal margin straight, inclined back- sion of the beds in the uppermost part of the Mount Flora wards and forming an obtuse angle with the posterior mar- column which can be extrapolated into the Flora Glacier gin, which is short; ventral margin gently convex, forming an valley according to their strike. These beds were at the base almost imperceptible concavity when joining the posterior of the glacier during the last stages of the glacier evolution margin; anterior margin circular; surface with concentric 1 3 S. Martínez et al. Fig. 3 Selected specimens (Unionida inc. sed.) from lot IAA-Pi 397. a–c Morphotype 1 (elongated). d–f Morphotype 2 (subelliptical) growth lines, with an insinuation of some radial grooves in Dimensions. Length: x = 3.1 cm. Height: x = 2.2 cm. the umbo. Length 4.4 cm; height 2 cm.