Introduction
The subfamily Tapetinae is the only venerid group recorded in the Eocene of Antarctica. Cytherea antarctica Sharman and Newton, Reference Sharman and Newton1894 was one of the first fossil invertebrates described and illustrated from the Cenozoic of the Antarctic Peninsula, later included in the first systematic study of the Eocene molluscan Antarctic fauna by Wilckens (Reference Wilckens1911). However, it was not until the late twentieth century that Zinsmeister (Reference Zinsmeister1984) resumed the analysis of this fauna, recognizing eight species of Tapetinae included in the genera Eurhomalea Cossmann, Reference Cossmann1920 (Cytherea antarctica Sharman and Newton, Reference Sharman and Newton1894; Venus newtoni Wilckens, Reference Wilckens1911; E. florentinoi Zinsmeister, Reference Zinsmeister1984; E. carlosi Zinsmeister, Reference Zinsmeister1984; E. inflata Zinsmeister, Reference Zinsmeister1984), Gomphina Mörch, Reference Mörch1853 (G. iheringi Zinsmeister, Reference Zinsmeister1984), and one species doubtfully placed in Cyclorismina Marwick, Reference Marwick1927 (“C.” marwicki Zinsmeister, Reference Zinsmeister1984). Afterwards, Stilwell and Zinsmeister (Reference Stilwell and Zinsmeister1992) added two new Antarctic species, Eumarcia (Eumarcia) austrolissa Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992 and Eumarcia (Atamarcia) robusta Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992.
Later, del Río (Reference del Río1997) included E. antarcticus and E. newtoni in Retrotapes del Río, Reference del Río1997, and Beu (Reference Beu2009) placed all the Antarctic species of Zinsmeister (Reference Zinsmeister1984) into Retrotapes, considering Gomphina iheringi as an indeterminate taxon because of the lack of diagnostic characters since the species was based on a single fragmented and eroded valve. More recently, Alvarez et al. (Reference Alvarez, del Río and Marenssi2014) agreed with these authors in assigning R. antarcticus, R. newtoni, and R. robustus to Retrotapes.
Because E. florentinoi, E. carlosi, E. inflata, and Cockburnia lunulifera were defined on articulated specimens, Cyclorismina marwicki has a hinge plate with very similar characters to those of Retrotapes antarcticus, and Eumarcia austrolissa is known through one partially eroded valve with broken cardinal teeth, it is necessary to carry out a full revision of these species in the light of the new material collected by MJA in the 2014 field season performed by the Instituto Antártico Argentino (IAA; Argentinean Antarctic Institute).
Materials and methods
Studied Tapetinae come from the marine Eocene outcrops of Marambio Island (also known as Seymour Island), Antarctica (Fig. 1). The La Meseta Formation (Fig. 2) is an unconformity-bounded unit (Elliot and Trautman, Reference Elliot, Trautman and Craddock1982; Ivany et al., Reference Ivany, Lohmann, Hasiuk, Blake, Glass, Aronson and Moody2008) ~560 m thick, deposited between the upper Thanetian (ca. 58.4 Ma) and the lower Lutetian (ca. 45.8 Ma) (Marenssi, Reference Marenssi, Francis, Pirrie and Crame2006; Ivany et al., Reference Ivany, Lohmann, Hasiuk, Blake, Glass, Aronson and Moody2008; Montes et al., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013, Reference Montes, Beamud, Nozal and Santillana2019a, Reference Montes, Nozal, Olivero, Gallastegui, Maestro, Santillana, Martín-Serrano, Montes, Nozal and Santillanab), although recent research proposed a late early Eocene (Crame et al., Reference Crame, Beu, Ineson, Francis, Whittle and Bowman2014) or middle Eocene age (Douglas et al., Reference Douglas, Affek, Ivany, Houben, Sijp, Sluijs, Schouten and Pagani2014) at the beginning of deposition, Amenábar et al. (Reference Amenábar, Montes, Nozal and Santillana2019) determined a middle Lutetian to Priabonian age (46.2–36 Ma). This unit includes mudstones and sandstones with interbedded conglomerates, and it is organized into seven allomembers (Marenssi et al., Reference Marenssi, Santillana, Rinaldi and Casadío1998a): Valle de Las Focas, Acantilados I, Acantilados II, Campamento, Cucullaea I, Cucullaea II, and Submeseta. It is interpreted as estuarine and shallow marine environments as part of a tectonically controlled incised valley system (Porębski, Reference Porębski1995; Marenssi et al., Reference Marenssi, Santillana and Rinaldi1998b), in which the allomembers represent different sedimentation stages related to sea level fluctuations (Marenssi et al., Reference Marenssi, Casadío and Santillana2002).
Geographic location of the samples collected during the 2014 expedition to Marambio Island. Details of the localities are given in the Appendix 1, and the list of samples and where they were collected is clarified in the Supplementary Data Set 1. Numbers in bold correspond to localities of Stilwell and Zinsmeister (Reference Stilwell and Zinsmeister1992).
Stratigraphic column of La Meseta and Submeseta formations, showing the new stratigraphic range of the studied species. *Isotopic age in Ma from Marenssi (Reference Marenssi, Francis, Pirrie and Crame2006), **isotopic age in Ma from Ivany et al. (Reference Ivany, Lohmann, Hasiuk, Blake, Glass, Aronson and Moody2008).
The Submeseta Formation was informally named by Montes et al. (Reference Montes, Nozal, Santillana, Marenssi and Olivero2013) (Fig. 2), being the uppermost part of the former La Meseta Formation and is dated 43.4–33.9 Ma according to Montes et al. (Reference Montes, Nozal, Santillana, Marenssi and Olivero2013). However, a new chronologic scheme for the La Meseta Formation (Amenábar et al., Reference Amenábar, Montes, Nozal and Santillana2019) suggests that the Submeseta Formation could be late Eocene to Oligocene in age. This unit corresponds to the Submeseta Allomember of Marenssi et al. (Reference Marenssi, Santillana, Rinaldi and Casadío1998a), and is characterized by a uniform sandy lithology that represents a tidal shelf influenced by storms (Cenizo et al., Reference Cenizo, Acosta Hospitaleche and Reguero2015). The Submeseta Formation is organized into three allomembers: Submeseta I, Submeseta II, and Submeseta III (Montes et al., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013).
During the 2014 expedition to Marambio Island, more than 640 specimens of the studied species were collected at 33 localities (Fig. 1) distributed along all the allomembers defined by Marenssi (Reference Marenssi, Santillana, Rinaldi and Casadío1998a). Precise information about the mentioned localities is available in Appendix 1. We also used the material donated by W. Zinsmeister to the MACN-Pi collection, which was collected during the field seasons 1990–1994. Most of the materials collected by W. Zinsmeister and J. Stilwell are housed at the Paleontological Research Institute (PRI), and were recently studied by CJR, who noted that said materials are currently being reclassified and renumbered. For this reason, we decided to not include these specimens in our analysis, considering that the materials housed at the MACN-Pi collection and the new materials collected in the Field Season 2014 deposited in the IAA are sufficient to perform the present study.
Geographic and stratigraphic distributions of each studied species are summarized in Supplementary Data Set 1, and the materials corresponding to taxa used for comparison and phylogenetic analysis are summarized in Supplementary Data Set 2. The method used for the phylogenetic analysis is described in the corresponding section of this study.
Repositories and institutional abbreviations
Material included in the present contribution is housed at: Repositorio Antártico de Colecciones Paleontológicas y Geológicas del Instituto Antártico Argentino, San Martín, Buenos Aires, Argentina (IAA-Pi); División Paleoinvertebrados, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires (MACN-Pi); División Invertebrados, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires, Argentina (MACN-In); Museo de La Plata, Argentina (MLP); Cátedra de Paleontología de la Universidad de Buenos Aires, Buenos Aires, Argentina (CPBA); Colección Paleoinvertebrados, Museo de Historia Natural, Santiago, Chile (SGO.PI); Field Museum of Natural History, Chicago, USA (FMNH); Paleontological Research Institution, Cornell University, Ithaca, New York, USA (PRI); National Museum of Natural History, Smithsonian Institution, Washington D.C., USA (USNM); Natural History Museum Rotterdam, Netherlands (NMR); Samling Paleobiologi, Naturhistoriska Rikmuseet, Stockholm, Sweden (PZ-NRM Mo); Natural History Museum, London, United Kingdom (NHMUK); Natural History Museum of Denmark (Zoology), Copenhagen, Denmark (ZMUC); Auckland Museum, Auckland, New Zealand (AM); South Australian Museum, North Terrace, Adelaide, Australia (SAM).
Systematic paleontology
Family Veneridae Rafinesque, Reference Rafinesque1815
Subfamily Tapetinae Gray, Reference Gray1851
Remarks
Some controversial taxa were erected by Zinsmeister (Reference Zinsmeister1984) from the La Meseta Formation, including the genus Cockburnia. The type species elected for the latter genus was Cytherea lunulifera Wilckens, Reference Wilckens1911 (p. 17, pl. 1, fig. 13a, 13b; PZ-NRM MO 2029), however its internal characters were unknown and its assignment is therefore arguable.
During the 2014 field season several specimens were collected from concretionary facies of Cerro Jonas (Acantilados II Allomember, La Meseta Formation). This new material has a hinge plate with long and thin anterior and posterior cardinal teeth, a very short middle one, and an anterior lateral tooth in the left valve. These features are not coincident with those of Tapetinae, and added to the presence of a sub-central umbo and a very large and wide lunule, suggest it belongs to the Family Corbiculidae; but more studies are being carried out to confirm this assignment.
As previously mentioned, Gomphina iheringi Zinsmeister, Reference Zinsmeister1984 (Fig. 3.10, 3.11) has no internal diagnostic characters because the only known specimen is filled with sedimentary matrix, and Zinsmeister placed it in Gomphina because of its shape. However, this feature is similar to that of some Meretricinae or Corbiculidae species; for that reason, we follow the proposal of Beu (Reference Beu2009) and consider this specimen nomen dubium.
(1–6) Retrotapes antarcticus (Sharman and Newton, Reference Sharman and Newton1894). (1, 3) MACN-Pi 5303a, a left valve: interior and exterior views (Cucullaea I Allomember); (2) MACN-Pi 5413a, a left valve: interior view (Cucullaea I Allomember); (4–6) MACN-Pi 6374, subrounded specimens of Cyclorismina marwicki Zinsmeister, Reference Zinsmeister1984: (4) right interior view; (5, 6) left valve, lateral and interior views (Cucullaea I Allomember). (7–9) Cyclorismina woodsi Marwick, Reference Marwick1927. (7) TM 2564, left hinge plate; (8) TM 2565, right valve and internal cast; (9) TM 2566, right hinge plate (Late Cretaceous, New Zealand). (10, 11) Gomphina iheringi Zinsmeister, Reference Zinsmeister1984, USNM 365531 (holotype), left valve, lateral and interior views (Acantilados II Allomember). Scale bar = 1 cm.
Zinsmeister (Reference Zinsmeister1984, p. 1522, figs. 10A–10C) named the species ‘Cyclorismina’ marwicki (Fig. 3.3–3.6) (Eocene; La Meseta Formation) based on its subcircular shells, adducing that although the generic position is uncertain, the combination of the shell-shape and internal features are similar to those of the genus Cyclorismina Marwick, Reference Marwick1927 (type species C. woodsi Marwick, Reference Marwick1927, figs. 185–187; Late Cretaceous, New Zealand; Fig. 3.7–3.9). Cyclorismina has circular-shaped valves, a small umbo, sunken lunule, unbounded from the rest of the shell, absent escutcheon, almost vertical nymphs, divergent cardinal teeth, entire 1, and long pallial sinus, which is triangular, ascendant, and with a sharp apex. The type material illustrated by Zinsmeister (Reference Zinsmeister1984), as well as those specimens donated by this author to the MACN-Pi collection, do not have any of the features described for Cyclorismina, even the shape of both species is not similar; therefore the Eocene species do not belong to this genus. In the present contribution, ‘C.’ marwicki is synonymized with Retrotapes antarcticus (Fig. 3.1–3.3) because of its similarity to the globoid morphotypes of this species analyzed by Alvarez and Pérez (Reference Alvarez and Pérez2016). The only significant difference is that the contacts between the anterior and posterior margins with the dorsal and ventral ones are slightly more rounded, giving the subcircular shape described by Zinsmeister (Reference Zinsmeister1984); but it is within the range of shapes observed in the globoid morphotypes of R. antarcticus.
Another species from the Eocene of Antarctica included in the genus Eurhomalea is ‘Eurhomalea’ claudiae Stilwell, Reference Stilwell2000, from McMurdo Sound (Ross Sea). Stilwell (Reference Stilwell2000) tentatively included this species in Eurhomalea due to its similarity to the shape of Retrotapes newtoni (Wilckens, Reference Wilckens1911) and Katelysia florentinoi (Zinsmeister, Reference Zinsmeister1984), both previously considered as Eurhomalea by Zinsmeister (Reference Zinsmeister1984) and Stilwell and Zinsmeister (Reference Stilwell and Zinsmeister1992), but mentioned that it might belong to Retrotapes del Río, Reference del Río1997 instead. However, ‘E.’ claudiae has divergent cardinal teeth, which means that it does not belong to Retrotapes. The teeth are very small, the middle one is vertical and the posterior is slightly curved, resembling those of K. florentinoi. However, it is not possible to determine if the middle teeth of both valves are bifid, a diagnostic character of Katelysia, either in the material illustrated or in the description provided by Stilwell (Reference Stilwell2000). In addition, neither muscular characters nor the pallial sinus can be observed. A search of the type material supposedly housed at Smithsonian Museum of Natural History was made, but the specimens were not in the collection, and are presumably lost. Therefore it is possible that ‘Eurhomalea’ claudiae belongs to a new genus, but the lack of key features and any type material makes it impossible to determine.
Genus Katelysia Römer, Reference Römer1857
Type species
Katelysia scalarina (Lamarck, Reference Lamarck1818) Pliocene–Recent; South Australia (by subsequent designation of Dall, Reference Dall1902) (Fig. 4.10–4.14).
(1–9) Katelysia florentinoi (Zinsmeister, Reference Zinsmeister1984). (1, 2) MACN-Pi 5305 right valve, lateral view and longitudinal section showing the hinge configuration (Campamento Allomember); (3, 4) MACN-Pi 5304 articulated specimen, dorsal and anterior views (Campamento Allomember); (5) MACN-Pi 6367 right hinge plate (Campamento Allomember); (6–8) IAA-Pi 74 left valve, interior, lateral and anterior views (Campamento Allomember); (9) IAA-Pi 63 right valve of a Eurhomalea carlosi specimen. (10–14) Katelysia scalarina (Lamarck, Reference Lamarck1818), MACN-In 40943, interior, lateral, dorsal and anterior views (Recent, Victoria, Australia). Scale bars = 1 cm.
Other species
Katelysia florentinoi (Zinsmeister, Reference Zinsmeister1984), La Meseta and Submeseta formations (Ypresian–Priabonian, Antarctica); K. corioensis (Tate, Reference Tate1887), Fyansford Formation (middle Miocene, Australia); K. lunulata Marwick, Reference Marwick1931, Tutamoe Formation (middle Miocene, New Zealand); K. rhytiphora (Lamy, Reference Lamy1935), Ascot Formation (Pliocene, southeastern Australia) and Recent from South Australia; K. peronii (Lamarck, Reference Lamarck1818), Recent, South Australia; K. victoriae (Tenison-Woods, Reference Tenison-Woods1878), Recent, South Australia.
Diagnosis
Shell medium to small sized, suboval, laterally compressed. Lunule very narrow, bounded by a line. Escutcheon very narrow, wider in the left valve. Hinge plate narrow, with short cardinal teeth that do not exceed the hinge plate margin; 1 and 2a bifids. Pallial sinus very short, with apex rounded. Shell sculptured with low commarginal ribs, wider than the interspaces. Some species have very fine radial ribs.
Occurrence
Ypresian–Recent. West Antarctica, New Zealand, and South Australia.
Remarks
The genus Katelysia was erected by Römer (Reference Römer1857), including Venus scalarina Lamarck, Reference Lamarck1818 and Venus exalbida Dillwyn, Reference Dillwyn1817, without clarifying which was the type species. Dall (Reference Dall1902) reassigned Venus exalbida into the genus Marcia Adams and Adams, Reference Adams and Adams1857 (Retrotapes exalbidus sensu del Río, Reference del Río1997) and selected Venus scalarina as type species of Katelysia.
Katelysia currently occurs in South Australia and its known fossil record goes back to the middle Miocene, represented by K. corioensis (Tate, Reference Tate1887) from Australia and K. lunulata Marwick, Reference Marwick1927 from New Zealand, although the specimens of both species are articulated and the internal characters remain unknown.
The assignment of Eurhomalea florentinoi Zinsmeister, Reference Zinsmeister1984 into the genus Katelysia expands the stratigraphic and geographic ranges of the latter genus to the Eocene of the Antarctic Peninsula. Katelysia differs from Eurhomalea Cossmann, Reference Cossmann1920 (type species E. rufa [Lamarck, Reference Lamarck1818], Pacific Ocean between the central region of Chile and Panama) because of its smaller size, suboval shape, shorter cardinal teeth that do not exceed the hinge plate margin, 1 and 2a teeth bifid, and shorter pallial sinus. Katelysia is distinguishable from Retrotapes del Río, Reference del Río1997 by having a narrower hinge plate, with divergent cardinal teeth, 1 and 2a teeth bifid, lanceolate lunule bounded by a line, and shorter pallial sinus.
Katelysia florentinoi (Zinsmeister, Reference Zinsmeister1984) new combination
Figure 4.1–4.9
- Reference Zinsmeister1984
Eurhomalea florentinoi Zinsmeister, p. 1520, figs. 8 K–M.
- Reference Zinsmeister1984
Eurhomalea carlosi Zinsmeister, p. 1521, figs. 8 Q–S.
- Reference Stilwell and Zinsmeister1992
Eurhomalea florentinoi; Stilwell and Zinsmeister, p. 79, pl. 8, figs. a–c.
- Reference Stilwell and Zinsmeister1992
Eurhomalea carlosi; Stilwell and Zinsmeister, p. 80, pl. 8, figs. d–f.
- Reference Beu2009
Retrotapes florentinoi; Beu, p. 210.
- Reference Beu2009
Retrotapes carlosi; Beu, p. 210.
Holotype
Articulated specimen, USNM 365516, from the Submeseta Formation. Paratype, articulated specimen, USNM 365515, from the Submeseta Formation.
Emended diagnosis
Pallial sinus short (but longer than that observed in extant species). Sculptured with low commarginal ribs, which are closer towards the ventral margin of the disk.
Occurrence
Acantilados II, Campamento, Cucullaea I, Cucullaea II, and Submeseta allomembers (late Ypresian–Priabonian; La Meseta and Submeseta formations).
Description
Shell thin, medium to small sized, suboval to elliptical. Umbo small, placed in anterior third of length. Posterodorsal and anterior margins tend to be dorsally straight and ventrally convex, ventral margin convex. Lunule lanceolate, short, bounded by a line. Escutcheon very narrow and short, wider in the left valve, with commarginal sculpture similar to the rest of the shell. Nymph narrow and smooth. Hinge plate narrow, curved behind the cardinal teeth, which are divergent, short, and do not exceed the ventral margin of the hinge plate. Right hinge with 3a tooth lamellar, sloped forwards; 1 triangular, narrow, bifid, tilted backwards; 3b wide, bifid, sub-horizontal; posterodorsal region of right valve with a groove for the insertion of left valve. Left hinge with 2a triangular, wide, bifid, higher than the other teeth; 2b narrow, bifid; 4b lamellar, sub-horizontal, and separated from the nymph by a groove. Adductor muscle scars isomyarian, the anterior one is deeper; anterior pedal retractor scar placed below the anterior margin of the hinge plate and separated from the adductor muscle scar; posterior pedal retractor scar joined to the posterior adductor muscle scar; and small pedal elevator muscle scars under the hinge plate. Pallial sinus short, triangular, dorsal and ventral margins straight, and apex rounded. Shell sculptured with low and narrow commarginal ribs, which are closer to each other towards ventral margin of the disk, and wider than the interspaces. Some well-preserved specimens have very fine radial ribs.
Material
One hundred and fifty nine specimens, MLP 18303 (1 specimen), SGO.PI 4959 (1 specimen), SGO.PI 4962 (1 specimen), CPBA 16778 (58 specimens), IAA-Pi 63 (7 specimens), IAA-Pi 70 (2 specimens), IAA-Pi 74 (4 specimens), IAA-Pi 85 (6 specimens), IAA-Pi 111 (2 specimens), MACN-Pi 5304 (1 specimen), MACN-Pi 5305 (1 specimen), MACN-Pi 6366 (17 specimens), MACN-Pi 6367 (9 specimens), MACN-Pi 6368 (21 specimens), MACN-Pi 6377 (1 specimen), MACN-Pi 6378 (10 specimens), MACN-Pi 6386 (6 specimens), MACN-Pi 6387 (3 specimens), MACN-Pi 6446 (8 specimens).
Measurements
Holotype USNM 365516: length 41 mm, height 30 mm (Appendix 2).
Remarks
Zinsmeister (Reference Zinsmeister1984) erected Eurhomalea florentinoi describing hinge plate characters, but without providing any internal images, and erected Eurhomalea carlosi based only on articulated specimens, considering the presence of a lunule in E. carlosi as the only difference from E. florentinoi. The revision of specimens of both species herein allows us to synonymize them because they have the same shape, sculpture, position of the umbones, and hinge plate. Regarding the presence or absence of a lunule depends on the grade of erosion of the external surface; with significant erosion, the line that bounded the lunule is not recognizable. The only difference between both species is that E. carlosi is smaller than E. florentinoi. The specimens of E. carlosi (Fig. 4.9) have four to six annual growth lines, whereas specimens of E. florentinoi have more than ten. This fact, added to the morphological evidence discussed before, indicates that the specimens of E. carlosi are probably young specimens of E. florentinoi.
Most of the new specimens of K. florentinoi (Fig. 4.6–4.8) have an initial shell shape that is similar in form and number of annual growth lines to those of the specimens of E. carlosi. However, in the same specimens, the convexity of the shell and the number of annual grow lines increases, developing a globoid shape, similar to that observed in R. antarcticus, which is associated with the great longevity of this Antarctic taxon (Alvarez and Pérez, Reference Alvarez and Pérez2016).
Eurhomalea florentinoi does not belong to the genus Eurhomalea (type species E. rufa) because this genus has a large and subquadrate shell, with sub-central umbo, narrow hinge plate with thin and high cardinal teeth that exceed the hinge plate margin, and a pallial sinus, which is large, triangular, and with an acute apex.
This species is included in the genus Katelysia because of its medium-sized shell, suboval, laterally compressed shape, with very narrow lunule, which is lanceolate and bounded by a line. The escutcheon is very narrow, but wider in the left valve. The hinge plate is narrow, with short cardinal teeth that do not exceed the hinge plate margin and bifid 1 and 2a teeth. The pallial sinus is very short, with rounded apex. The shell is sculptured with low commarginal ribs, wider than the interspaces. Some well-preserved specimens have very fine radial ribs.
Beu (Reference Beu2009) assigned K. florentinoi to the genus Retrotapes del Río, Reference del Río1997 and commented that the lack of lunule would support its inclusion in the genus Frigichione Fletcher, Reference Fletcher1938. The differences between Katelysia and Retrotapes are discussed above. Still, Katelysia florentinoi cannot be assigned to Frigichione because of its smaller and thinner shells, suboval shape, with narrow escutcheon and thinner teeth.
Katelysia florentinoi is distinguishable from the Miocene Australian and New Zealand species (K. corioensis and K. lunulata) by its suboval shape, larger shell, and sculpture of low commarginal ribs. Katelysia florentinoi differs from the extant species of the genus by its larger shell, longer pallial sinus, and sculpture of low commarginal ribs. However, its low sculpture is closer to that of K. florentinoi polita Nielsen, Reference Nielsen1963 (p. 223, pl. 1, figs. 4–6). The flat morphotype of K. florentinoi is less inflated than the extant species, which are similar in convexity to the globoid morphotype (Fig. 4.8, 4.14).
Marciachlys new genus
Type species
Eurhomalea inflata Zinsmeister, Reference Zinsmeister1984, Bartonian–Priabonian, Marambio Island, Antarctica. By monotype (Fig. 5.1–5.7).
(1–7) Marciachlys inflata (Zinsmeister, Reference Zinsmeister1984) n. comb.. (1, 4–6) MACN-Pi 6379, longitudinal section showing the hinge configuration and lateral, dorsal and anterior views (Submeseta III Allomember); (2, 3, 7) IAA-Pi 109 interior of a right valve, left hinge plate and internal cast showing muscle scars and pallial line (Submeseta III Allomember). (8) Marciachlys? indet. MACN-Pi 2533 right valve, internal cast (Paleocene, Tolhuin, Tierra del Fuego Province). Scale bars = 1 cm.
Diagnosis
Shell very inflated, umbo prominent and rounded. Hinge plate narrow with divergent cardinal teeth, short and straight, the anterior ones are angled forward, 1 tooth almost vertical, 2b entire. Pallial sinus short (shorter than K. florentinoi), ascendant, with apex rounded.
Occurrence
Bartonian–Priabonian. Submeseta Formation, Marambio Island, Antarctica.
Etymology
Marcia of the mist. The name refers to Marcia because the external similarity to this genus and Achlys refers to the Greek word for mist, because the new specimens of this new genus were found at the northern end of the Marambio Base airstrip in a day of thick mist.
Remarks
Marciachlys n. gen. differs from Retrotapes in having a lunule bounded by a line, divergent cardinal teeth, with the anterior ones very much tilted forward, entire 2b, and ascendant pallial sinus. Tooth 3b is horizontal in Retrotapes, but is ventrally oriented in Marciachlys n. gen. This genus is distinguishable from Katelysia in having a more convex shell, prominent and rounded umbo, wider lunule, larger hinge plate and teeth, entire 2b, and longer pallial sinus.
Marciachlys n. gen. differs from the New Zealand Eumarcia and Atamarcia by its smaller shell, prominent and rounded umbo, entire 2b, and shorter and ascendant pallial sinus. The specimens MACN-Pi 2531–2533 (Fig. 5.8) from Tolhuin in the vicinity of Kaiken Hostel (Tierra del Fuego Province, Argentina), of Paleocene age, could be assigned to Marciachlys n. gen. since they have similar shape, convexity, umbo, and sculpture. However, these specimens are larger and no inner characters are known.
Marciachlys inflata (Zinsmesiter, Reference Zinsmeister1984), new combination
Figure 5.1–5.7
- Reference Zinsmeister1984
Eurhomalea inflata Zinsmeister, p. 1521, figs. 9L–9P.
- Reference Stilwell and Zinsmeister1992
Eurhomalea inflata; Stilwell and Zinsmeister, p. 80, pl. 8, figs. g–k.
- Reference Beu2009
Retrotapes inflata; Beu, p. 210.
Holotype
Articulated specimen, USNM 365524, from the Submeseta Formation. Paratypes, an articulated specimen USNM 365525, a left valve USNM 365526, an articulated specimen USNM 365527, a right valve USNM 365528, from the Submeseta Formation.
Diagnosis
Same as for genus, by monotypy.
Description
Shell thick, medium sized, suboval, very inflated. Umbo prominent, rounded, placed at anterior quarter of length. Posterodorsal margin slightly convex, anterior margin tends to be dorsally straight and ventrally convex, ventral margin convex with straight central area. Lunule lanceolate, long, bounded by a line. Escutcheon very narrow and short, with commarginal sculpture similar to that of rest of the shell. Nymph narrow and smooth. Hinge plate narrow, curved behind the cardinal teeth, which are divergent, short, and do not exceed the ventral margin of the hinge plate. Right hinge with 3a tooth thick, triangular, sloped forward; 1 triangular, narrow, entire, sub-vertical; 3b wide, bifid, sub-horizontal. Left hinge with 2a triangular, wide, entire, and higher than the other teeth; 2b narrow and bifid; 4b lamellar, straight, and separated from the nymph by a groove. Adductor muscle scars deep; anterior pedal retractor scar placed below the anterior margin of the hinge plate and separated from the adductor muscle scar; posterior pedal retractor scar joined to the posterior adductor muscle scar; pedal elevator muscle scars small and deep, placed under the hinge plate. Pallial sinus short, triangular, ascendant, with apex wide and rounded. Shell sculpture smooth with annual growth lines.
Material
One hundred specimens, SGO.PI 4958 (1 specimen), IAA-Pi 109 (74 specimens), IAA-Pi 113 (4 specimens), MACN-Pi 6379 (21 specimens).
Measurements
Holotype USNM 365524: length 37 mm, height 33 mm (Appendix 2).
Remarks
Zinsmeister (Reference Zinsmeister1984, p. 1521, fig. 9L–9P) included Marciachlys inflata n. comb. in the genus Eurhomalea Cossmann, Reference Cossmann1920 (type species E. rufa [Lamarck, Reference Lamarck1818]), without knowing all its internal characters; new material collected during the 2014 field season, showed these features. Therefore, this species does not belong in Eurhomalea because of its suboval shape, prominent and rounded umbo, thicker cardinal teeth that do not exceed the hinge plate margin, and a short, ascendant, and round-ended pallial sinus.
Adelfia new genus
Type species
Eumarcia (Eumarcia) austrolissa Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992, late Ypresian–Lutetian, Marambio Island, Antarctica.
Other species
Adelfia arenosa (Ortmann, Reference Ortmann1899) n. comb. (Loreto Formation, late Eocene, Punta Arenas, Chile); Adelfia omega n. gen. n. sp. (Submeseta Formation, Priabonian, Marambio Island).
Diagnosis
Shell with lunule flat, narrow, and bounded by a line. Cardinal teeth narrow, divergent; 1 entire; 4b short and curved. Pallial sinus short, triangular, with the dorsal margin subhorizontal and the apex slightly truncated. Sculpture of broad, low, and flat ribs, closer to each other towards the ventral margin, separated by interspaces of equal depth throughout the valve.
Occurrence
Late Ypresian–Priabonian. The La Meseta and Submeseta formations (Marambio Island, Antarctica) and the Loreto Formation (Punta Arenas, Chile).
Etymology
From the Greek Adelphos (brother) and ia (action or will), in recognition of the spirit of brotherhood of the GeoMarambio camp, composed of Argentinean and Spanish geologists and paleontologists; located over sediments of the Campamento Allomember (La Meseta Formation), from which new material of the type species of this genus was collected during the 2014 field trip.
Remarks
Stilwell and Zinsmeister (Reference Stilwell and Zinsmeister1992) included the type species of Adelfia n. gen. in the genus Eumarcia Iredale, Reference Iredale1924 (type species E. fumigata [Sowerby, Reference Sowerby1853; Recent, South Australia], but this species does not belong to Eumarcia because it does not have a posteriorly lengthened and sharpened shell shape as Eumarcia has, and because of the presence of a larger umbo, well-defined escutcheon, tooth 4b separated from the nymph by a groove, entire 2a, thinner tooth 1, narrow and subhorizontal pallial sinus, and sculpture of broad, low, and flat ribs, closer to each other towards the ventral margin, separated by interspaces of equal depth throughout the valve.
Beu (Reference Beu2009) reassigned Eumarcia austrolissa as Atamarcia Marwick, Reference Marwick1927 (type species A. sulcifera Marwick, Reference Marwick1927; Miocene, New Zealand). Both species have similar shell shapes, but Adelfia austrolissa n. comb. does not belong in Atamarcia because of its flat lunule bounded by a line in its entire extension, shorter 4b, bifid 2b with a shallower groove, narrower 3b, smooth 1, and triangular, shorter, and subhorizontal pallial sinus. In addition, the interspaces of the sculpture have the same depth throughout the valve and are closer to each other towards the ventral margin in Adelfia n. gen., while in Atamarcia the interspaces are deeper in the anterior margin of the valve and are closer to each other towards the posterior one.
Ortmann (Reference Ortmann1899) erected Venus arenosa (PRI 72690; Fig. 6.10, 6.11) (Loreto Formation, late Eocene, Punta Arenas, Chile) based on three right valves embedded in sedimentary matrix. This taxon is included in Adelfia n. gen. because it shows a similar shell shape, with a hinge plate with cardinal teeth similar to those of Adelfia austrolissa n. comb., but with a margin more curved behind the teeth, and the sculpture when preserved is similar to that of Adelfia n. gen. The lunule is lanceolate and slightly concave, and this is the only difference from species of Adelfia n. gen. The scarcity of observable characters in the eroded and matrix-embedded right valves, as well as the complete absence of left valves, plus the fact that the differences with Adelfia austrolissa n. comb. are very little, make it difficult to differentiate both species. Based on these characters and due to the geographic and stratigraphic distance between both taxa, they are recognized here as valid species. If new evidence on Adelfia arenosa n. comb. would appear, allowing us to synonymize them, the specific name arenosa would have priority over austrolissa n. comb.
(1–9) Adelfia austrolissa (Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992) n. comb. (1–4) USNM 441638 holotype, right valve, interior, anterior, lateral, and dorsal views (Cucullaea I Allomember); (5, 6) MACN-Pi 6381 left valve, interior and lateral views (unknown allomember); (7–9) IAA-Pi 81 articulated specimen, lateral, dorsal, and anterior views (Campamento Allomember). (10, 11) Adelfia arenosa (Ortmann, Reference Ortmann1899) n. comb., right valve, interior and lateral views (late Eocene, Loreto Formation, Punta Arenas, Chile). (12–15) Adelfia omega n. gen. n. sp. (12) IAA-Pi 416 holotype, right valve, interior view (Submeseta III Allomember); (13–15) IAA-Pi 417 paratype, articulated specimen, lateral, anterior and dorsal views (Submeseta III Allomember). Scale bar = 1 cm.
Adelfia n. gen. differs from Retrotapes by its flat lunule bounded by a line, divergent cardinal teeth, entire 1, and narrower 3b. It is distinguishable from Katelysia by its larger and more convex shell, wider escutcheon, higher hinge plate with longer teeth, entire 1, and longer and subhorizontal pallial sinus. Adelfia n. gen. differs from Marciachlys n. gen. by its less-convex shell, less-prominent umbo, narrower lunule, less-tilted forwards anterior cardinal teeth, backward sloping 1, subhorizontal 3b, and longer and subhorizontal pallial sinus. The sculpture of Adelfia n. gen. differs from that observed in the three compared taxa, and consists of broad, low, and flat ribs.
Adelfia austrolissa (Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992) new combination
Figure 6.1–6.9
- Reference Stilwell and Zinsmeister1992
Euamrcia (Eumarcia) austrolissa Stilwell and Zinsmeister, p. 82, pl. 8., figs. r, s.
- Reference Beu2009
Atamarcia austrolissa; Beu, p. 210.
Holotype
One right valve, USNM 441638, from Cucullaea I Allomember, La Meseta Formation (Fig. 6.1–6.4).
Emended Diagnosis
Shell medium to large sized. Up to 10 pedal elevator muscle scars. Pallial sinus with dorsal margin subhorizontal, ventral margin curved, and apex small and truncated.
Occurrence
Acantilados II, Campamento, Cucullaea I, and Cucullaea II allomembers (late Ypresian–Lutetian, La Meseta Formation).
Description
Shell thin, large size, suboval. Umbo small, placed at anterior quarter of length. Dorsal margin slightly convex; ventral, anterior, and posterior margins convex. Lunule narrow, flat, and bounded by a line. Escutcheon narrow, wider in the left valve. Nymph narrow and smooth. Hinge plate wide, slightly curved behind the cardinal teeth, which are divergent, narrow, and do not exceed the ventral margin of the hinge plate. Right hinge with 3a tooth lamellar, straight, sloped forward, and shorter and lower than the 1, which is thin, entire, and tilted backwards; 3b triangular, bifid, and sub-horizontal; posterodorsal region of right valve with a groove for the insertion of left valve. Left hinge with 2a triangular, straight; 2b triangular, bifid, and higher than 2a; 4b lamellar, curved, sub-horizontal, and separated from the nymph by a groove. Adductor muscle scars deep; anterior pedal retractor scar placed below the anterior margin of the hinge plate and separated from the adductor muscle scar; posterior pedal retractor scar joined to the posterior adductor muscle scar; up to 10 pedal elevator muscle scars small, deep. Pallial sinus short, triangular, with dorsal margin straight and subhorizontal, and ventral margin curved, and apex small and truncated. Sculpture of broad, low, and flat ribs, closer to each other towards the ventral margin, separated by interspaces of equal depth throughout the valve.
Material
Twenty-eight specimens, IAA-Pi 64 (2 specimens), IAA-Pi 81 (5 specimens), IAA-Pi 86 (2 specimens), IAA-Pi 100 (1 specimen), IAA-Pi 106 (1 specimen), MACN-Pi 6370 (7 specimens), MACN-Pi 6371 (7 specimens), MACN-Pi 6381 (3 specimens).
Measurements
Holotype USNM 441638: length 60.5 mm; height 46.5 mm (Appendix 2).
Remarks
Adelfia austrolissa (Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992) n. comb. was erected based on only one specimen found in the Cucullaea I Allomember (middle Eocene). During the 2014 field season, new articulated material was collected at the cliff near Campamento Point (64°13′45.6″S; 56°39′55.9″W), from the Campamento Allomember, extending the stratigraphic range of this species. Later, based on this new material, other specimens of this species were recognized in the collection of MACN-Pi from Acantilados II and Cucullaea II allomembers. The external surface of the holotype is almost smooth, and its name is derived from this fact, but the surfaces of the new material are not smooth, revealing that the holotype is partially eroded and the outer layer of the shell is almost absent.
Adelfia austrolissa n. comb. differs from the other Tapetinae from Acantilados II to Cucullaea II allomembers (Retrotapes antarcticus and R. robustus), by its suboval shape and characteristic sculpture. The articulated specimens of A. austrolissa n. comb. and R. newtoni have the same shape and they can be easily confused with each other. The difference between them is that R. newtoni has a concave lunule, bounded by a deep groove, whereas A. austrolissa n. comb. has a flat lunule, bounded by a line, which is not visible in eroded specimens.
Adelfia omega new species
Figure 6.12–6.15
Holotype
One right valve, IAA-Pi 416, from Submeseta III Allomember, Submeseta Formation. Paratypes: IAA-Pi 417, an articulated specimen; IAA-Pi 418, a right hinge plate; IAA-Pi 419, an articulated specimen with some visible cardinal teeth; IAA-Pi 420, a right valve with a broken left hinge plate, so the six cardinal teeth are articulated; IAA-Pi 421, an internally polished right valve; IAA-Pi 422, an internal mold with muscle scars, and pallial line and sinus; from Submeseta III Allomember, Submeseta Formation.
Diagnosis
Shell small sized. Pallial sinus with dorsal and ventral margins straight, ascendant, and apex sharpened. Elements of the sculpture lower than those of Adelfia austrolissa n. comb.
Occurrence
Priabonian. Submeseta III Allomember, Submeseta Formation, Marambio Island, Antarctica.
Description
Shell thin, small sized, suboval. Umbo small, placed at anterior quarter of length. Dorsal margin slightly convex and ventral, anterior and posterior margin convex. Lunule narrow, flat, and bounded by a line. Escutcheon narrow, wider in the left valve. Nymph narrow and smooth. Hinge plate narrow, slightly curved behind the cardinal teeth, which are divergent, narrow, and do not exceed the ventral margin of the hinge plate. Right hinge with 3a tooth lamellar, straight, sloped forward, and shorter and lower than the 1, which is thin, entire, and tilted backwards; 3b triangular, bifid, and sub-horizontal; posterodorsal region of right valve with a groove for the insertion of left valve. Left hinge with 2a triangular, straight; 2b triangular, bifid and higher than 2a; 4b lamellar, curved, sub-horizontal, and separated from the nymph by a groove. Adductor muscle scars deep; posterior pedal retractor scar joined to the posterior adductor muscle scar. Pallial sinus short, triangular, dorsal and ventral margins straight and ascendant, and apex sharpened. Sculpture of broad, low, and flat ribs, closer to each other towards the ventral margin, separated by interspaces of equal depth throughout the valve.
Etymology
Omega is the last letter of the Greek alphabet, and the name refers to some of the specimens of this taxon collected in the last bed with veneroids of the Submeseta Formation, found just below the Weddell Sea Formation (Pliocene) outcrops.
Material
Two hundred fifty four specimens, IAA-Pi 107 (37 specimens), IAA-Pi 110 (113 specimens), IAA-Pi 114 (53 specimens), IAA-Pi 416–422 (7 specimens), MACN-Pi 6380 (36 specimens), MACN-Pi 6382 (8 specimens).
Measurements
Holotype IAA-Pi 416: length 43.3 mm; height 33.25 mm (Appendix 2).
Remarks
During the 2014 expedition several specimens of this new species were found in the beds with veneroids at the top of the Submeseta Formation. They have the same shape, umbo, lunule, and similar sculpture and hinge plate to those of Adelfia austrolissa n. comb., which is why this species is included in Adelfia n. gen.
All specimens are articulated, so it is not possible to observe internal features. However, some cracked shells were broken to have access to the inner cast and these showed some internal characters. This demonstrated that there are some differences between the pallial sinuses of both species, which are used as diagnostic characters. In addition, the elements of the sculpture of the new species are lower than those of Adelfia austrolissa n. comb.
The most conspicuous difference is the size, with A. austrolissa n. comb. being larger than A. omega n. gen. n. sp. (Appendix 2). This could indicate that the new species is a juvenile of the type species, but counting of the annual growth ribs observed in eroded specimens of A. omega n. gen. n. sp. revealed that some specimens are more than 30 years old.
Specimens of Marciachlys inflata n. comb. and Retrotapes newtoni were also collected from the beds where A. omega n. gen. n. sp. was found. These three species have a suboval shape and most of their specimens are eroded. Although it is very hard to identify them, there are some distinguishing features. Marciachlys inflata n. comb. has a larger lunule, prominent umbo, and is wider than the other two species. Retrotapes newtoni has a concave lunule bounded by a deep groove, and A. omega n. gen. n. sp. has a smaller and flatter lunule and the disk is sculptured with low, broad, and flat ribs.
Phylogenetic analysis
Characters
A matrix of 80 characters was developed (Appendix 3), describing the whole shell morphology of extant and fossil taxa, including: shape (13), hinge (30), umbo (1), lunule (6), nymph (3), escutcheon (5), pallial sinus (7), muscles scars (7), and sculpture (8) (Alvarez, Reference Alvarez2019).
In order to minimize the loss of information, most of the reviewed material was studied first hand. Only 3.57% of the entries are missing in the data matrix. Characters from 18 to 23 are lineal measurements (not ratios), therefore, in order to compare the measurements, these were rescaled to the average size of Tapes literatus (Linnaeus, Reference Linnaeus1758) (average height value of 45.9 mm). The ratios of the other continuous characters were logarithmized (log10) following Mongiardino Koch et al. (Reference Mongiardino Koch, Soto and Ramírez2015).
Ingroup
Previously, Alvarez (Reference Alvarez2019) tested the monophyly of the genus Retrotapes. The results allowed him to argue about the possibility of the existence of an Austral Tapetinae clade that includes the genera Retrotapes, Atamarcia, Paleomarcia, and Katelysia. With the objective of evaluating the relationships of the new genera described herein within this Austral clade, the selected terminals of the ingroup settled in Alvarez (Reference Alvarez2019) were resampled. Only four of the 13 known species of the genus Retrotapes (Alvarez et al., Reference Alvarez, del Río and Marenssi2014) were included in the matrix: the three Eocene Antarctic species (R. antarcticus, R. robustus, R. newtoni) and the type species, R. ninfasiensis del Río, Reference del Río1997, the type species of the genera Adelfia (A. austrolissa n. comb.) and A. omega n. gen. n. sp., as well as the type species of Marciachlys n. gen. (M. inflata n. comb.) and the type of Katelysia (K. scalarina) and K. florentinoi. Other austral taxa that share some features with Retrotapes were also included: Atamarcia sulcifera (Marwick, Reference Marwick1927) (type species of Atamarcia), Eumarcia fumigata (Sowerby, Reference Sowerby1853) (type species of Eumarcia), and Paleomarcia tatei (Fletcher, Reference Fletcher1938).
Outgroup
Several genera of the subfamily Tapetinae were included in the matrix, using in most cases only the type species, namely: Neotapes undulata (Born, Reference Born1778), Polititapes aureus (Gmelin, Reference Gmelin and Gmelin1791), P. virgineus (Linnaeus, Reference Linnaeus1767), Venerupis corrugata (Gmelin, Reference Gmelin and Gmelin1791), Ruditapes philippinarum (Adams and Reeve, Reference Adams, Reeve and Adams1850), R. decussatus (Linnaeus, Reference Linnaeus1758), Protapes gallus (Gmelin, Reference Gmelin and Gmelin1791), Marcia opima (Gmelin, Reference Gmelin and Gmelin1791), Paphia rotundata (Linnaeus, Reference Linnaeus1758), Tapes literatus (Linnaeus, Reference Linnaeus1758), Notopaphia elegans (Deshayes, Reference Deshayes1854), Irus carditoides (Lamarck, Reference Lamarck1818), and Eurhomalea rufa (Lamarck, Reference Lamarck1818). Two other genera previously considered as Tapetinae were included: Frigichione permagna (Tate, Reference Tate1900) and Gomphina undulosa (Lamarck, Reference Lamarck1818). Dosinia concentrica (Born, Reference Born1778) was used to root the tree.
Search
A phylogenetic analysis was performed following the maximum parsimony criterion using the TNT 1.5 software (Goloboff et al., Reference Goloboff, Farris and Nixon2008) through a heuristic search of 100 replicates of Wagner trees (with addition of random sequences) followed by TBR branch swapping algorithm holding 10 trees per replicate. Characters 1–25 were considered as continuous. The methodology of character weighting was implied weighting (Goloboff, Reference Goloboff1993), performing 100 searches for k values between 1 and 100, because bivalves and mollusks in general are a homoplastic group. However, an exploratory search without implied weighting was also performed. Support values were estimated by resampling using frequency differences under Bootstrap (BS) (Felsenstein, Reference Felsenstein1985) and Jackknife (JK) (Farris et al., Reference Farris, Albert, Källersjö, Lipscomb and Kluge1996), with a p = 8 (equivalent to removing 10% of the characters) (Goloboff et al., Reference Goloboff, Farris, Källersjö, Oxelman, Ramírez and Szumik2003), and performing 1,000 pseudo-replicates.
Results
Each search performed with a different k value (k between 1 and 100) resulted in a single topology, obtaining ranges of k where the recovered topologies are similar to each other. The ranges of trees that have similar topologies are 6–28 (Fig. 7.2) and 29–100 (Fig. 7.3). The BS and JK values were calculated and informed on each topology (Fig. 7). The tree obtained for the k value 29 is the most abundant (71 of 100 trees), and is similar to the one obtained in an exploratory search performed without implied weighting; it also has the best BS and JK values, and the discussion is based on it.
(1) Topology recovered with a search performed with equal weighting. (2, 3) Topologies recovered at different k values: (2) k = 6–28, (3) k = 29–100. Only values of BS/JK over 50 are informed.
In all the performed searches, Adelfia n. gen., Katelysia, and Retrotapes (closely related to Paleomarcia and Atamarcia) are monophyletic groups. Marciachlys n. gen. is the sister taxon of the clade Marcia + Protapes, and in the search performed with equal weighting (Fig. 7.1), which was a k value of 29, it is possible to recognize a large clade of Austral or sub-Antarctic taxa that was previously unknown.
In the topology with k values ranging from 29 to 100, the Austral clade is supported by eight synapomorphies: (character 15 [c15]) lunule long (35–36% of total height); (c20) width of the tooth 4b (0.679–0.803 mm); (c21) width of the tooth 3a (0.961–0.979 mm); (c51) well-marked escutcheon; (c52) sculpture of the escutcheon similar to the rest of the shell; (c53) slightly narrow escutcheon; (c54) wider escutcheon in the left valve; and (c55) long escutcheon, reaching halfway down the posterior side of the muscle adductor scar.
The inclusion of K. florentinoi in the genus Katelysia is supported by high JK values in all searches. This clade has 13 synapomorphies in the topology with k values ranging from 29 to 100: (c4) inclination of tooth 3a of 69.5°; (c5) inclination of tooth 1 of 121°; (c7) width of the pallial sinus 25% of the total height; (c9) tooth 2a thin (89% of 2b width); (c14) umbo anterior (placed at 75% of total length); (c15) lunule short (32% of total height); (c17) tooth 3a long, occupying 88% of the hinge plate area in its position; (c18) width of the 2a tooth (1.776 mm); (c24) slope of the dorsal margin of 164°; (c26) very shallow pallial sinus; (c44) pedal retractor muscle scar placed below the anterior cardinal teeth; (c50) elements of the commarginal sculpture closer towards the umbo and spaced towards the ventral margin of the disk; and (c61) adults small sized.
Adelfia n. gen. is supported by high JK values in all searches, and has four synapomorphies on the topology with k values ranging from 29 to 100: (c7) width of the pallial sinus 17% of the total height; (c30) high hinge plate; (c39) smooth tooth 1; and (c70) presence of growth commarginal ribs with thin ribs interspaced. Marciachlys n. gen. is recovered as the sister taxon of the clade Marcia + Protapes with low values of support in all searches and has only three synapomorphies: (c5) inclination of tooth 1 of 107–110°; (c10) tooth 4b wide (52% of 2b width); and (c63) smooth angle between the posterior and dorsal margins.
The clade Paleomarcia + Atamarcia + Retrotapes is recovered in all searches performed with implied weighting, and is supported by nine synapomorphies: (c3) inclination of the posterior muscle scar of 93°; (c6) inclination of tooth 3b of 163°; (c7) width of the pallial sinus 22% of the total height; (c12) tooth 3b 1.40–1.94 times wider than tooth 1; (c17) tooth 3a short, occupying 73% of the hinge plate area in its position; (c23) width of tooth 3b (1.931–2.429 mm); (c24) slope of the dorsal margin of 159–160°; (c51) well-marked escutcheon; and (c69) commarginal sculpture of low and thin ribs.
Discussion
In all the performed searches, subtropical taxa are recovered as basal for the clade Tapetinae (e.g., Eumarcia, Pahia, and Neotapes for the searches with equal weighting and with k values of 29–100, and the clade Polititapes + Eurhomalea + Venerupis + Ruditapes in searches performed with k values of 6–28), which suggests a possible subtropical origin for the subfamily Tapetinae. On the other hand, for the searches with equal weighting and with k values of 29–100, a clade of Austral taxa with clear affinities with Eocene Antarctic genera is recovered. Within this group, Adelfia n. gen. and Katelysia (including its type and extant species from the South Australia, K. scalarina) are recovered basal to a clade that includes Paleomarcia (Miocene of Kerguelen Island) and Atamarcia (Miocene of New Zealand) as a sister taxa of Retrotapes. Concerning the clade Retrotapes, the Eocene Antarctic species are recovered basal to the type species of the genus, R. ninfasiensis (late Miocene, Patagonia). In all searches, Marciachlys n. gen. is basal to a clade that includes the genus Marcia and Protapes. These last two genera have a subtropical distribution nowadays and this result raises a possible evolutionary history connected with taxa from the Eocene of Antarctica.
In view of the results previously discussed, a possible scenario for the evolution of Tapetinae is that this subfamily, as a whole, had a subtropical origin and later migrated to higher latitudes, in this case to the south. Once in Antarctica, the group would have diversified during the Eocene, to migrate once again, but this time towards lower latitudes, which led in part to the conformation of the Neogene Austral faunas.
As Beu (Reference Beu2009) argued, it may be too risky to claim an Antarctic origin for the aforementioned fauna due to the scarcity of early Cenozoic record in the southern hemisphere. But the phylogenetic results obtained here can shed light on the origin and distribution of part of the modern fauna from the Austral seas, which without a doubt has a close link with the Eocene fauna of Antarctica.
Although it is not the objective of the present contribution, it is important to discuss the phylogenetic position of the genus Atamarcia. This fossil genus was erected by Marwick (Reference Marwick1927) as a subgenus of the extant Eumarcia. Later, Stilwell and Zinsmeister (Reference Stilwell and Zinsmeister1992) and Beu (Reference Beu2009) included some species from the Eocene of Antarctica within this genus, including Atamarcia austrolissa, which we considered as the type species of the new genus Adelfia. Eumarcia and Atamarcia were not grouped together in any of our phylogenetic results. Eumarcia is part of the tropical clade of the Tapetinae, whereas Atamarcia is recovered as an Austral Tapetinae. The genera Adelfia n. sp. and Atamarcia were never recovered as sister taxa. Atamarcia is more closely related to Retrotapes than to Adelfia n. sp., in agreement with the morphological differences recognized between the type species of Adelfia n. sp. and Atamarcia discussed in the systematic section, which in consequence led us to name the new genus for the Eocene Antarctic species. We use only the type species of Atamarcia for both the systematic comparison and the phylogenetic study because a preliminary search of the taxa included in Atamarcia allowed us to conclude that a major systematic revision of this genus is needed. There is a big morphological disparity among the species originally assigned by Marwick (Reference Marwick1927) as Atamarcia. Considering only some hinge characters as an example, A. sulcifera has slightly curved cardinal teeth, whereas A. benhami (Hutton, Reference Hutton1874) and A. crassatelliformis Marwick, Reference Marwick1927 have straight cardinal teeth, and A. crassa Marwick, Reference Marwick1927 has a straight right anterior cardinal tooth, joined to the lunule and strongly curved middle and posterior ones.
Conclusions
Only six species of Tapetinae are recognized as valid from the original pool of 10 species described by Zinsmeister (Reference Zinsmeister1984) and Stilwell and Zinsmeister (Reference Stilwell and Zinsmeister1992). Gomphina iheringi Zinsmeister, Reference Zinsmeister1984 is considered as a nomen dubium. Cockburnia lunulifera (Wilckens, Reference Wilckens1911) is not a Tapetinae, but probably a Corbiculidae. Eurhomalea carlosi Zinsmeister, Reference Zinsmeister1984 and ‘Cyclorismina’ marwicki Zinsmeister, Reference Zinsmeister1984 are synonymized with Katelysia florentinoi (Zinsmeister, Reference Zinsmeister1984) and Retrotapes antarcticus (Sharman and Newton, Reference Sharman and Newton1894), respectively.
The presence of K. florentinoi (Zinsmeister, Reference Zinsmeister1984) in the Eocene of Antarctica considerably extends the stratigraphic and geographic occurrences of Katelysia Römer, Reference Römer1857, which nowadays inhabit the marine coast of the Southern Australia.
Two new genera are named: Marciachlys n. gen., which is represented by M. inflata (Zinsmeister, Reference Zinsmeister1984) n. comb., and Adelfia n. gen., which is represented by A. austrolissa (Stilwell and Zinsmeister, Reference Stilwell and Zinsmeister1992) n. comb., A. omega n. gen. n. sp., and the late Eocene taxon from Punta Arenas (Chile), A. arenosa (Ortmann, 1902) n. comb.
Retrotapes, Katelysia, Adelfia n. gen., and Marciachlys n. gen. are grouped together in a clade with other Austral taxa, such as Paleomarcia and Atamarcia, and with two subtropical taxa, Marcia and Protapes, a relationship previously ignored. This reinforces the importance of Antarctica as a center of origin and distribution of fauna during the Cenozoic. The basal position of the clade is occupied by Adelfia n. gen. and Katelysia, represented by the Eocene Antarctic K. florentinoi and the extant K. scalarina (Southern Australia). Marciachlys n. gen. is basal to the clade, formed by the extant genera Marcia and Protapes. The fossil genera Paleomarcia (Miocene, Kerguelen Island) and Atamarcia (Miocene, New Zealand) are the sister groups of Retrotapes. The latter is represented here by its Eocene Antarctic taxa and the Miocene Patagonian R. ninfasiensis, being the most ancient species of the genus, and R. newtoni, which is basal to the rest of the species.
Acknowledgments
The authors especially thank S.A. Marenssi and S. Santillana, as well as the team of Geomarambio, for their assistance during the organization and subsequent realization of the 2014 field trip dispensed to MJA. The authors are indebted to the curators who facilitated access to paleontological and biological collections: M. Longobucco (MACN-Pi and exCIRGEO-PI), A. Tablado and M. Romanelli (MACN-In), M. Tanuz (CPBA), C. Amenábar (IAA-Pi), A. Riccardi (MLP), and C. Salazar and S. Soto (SGO.PI). We also thank everyone who sent us photographs: A. Salvador (NHMUK), G. Dietl (PRI), C. Franzén-Bengtson and J. Hagström (PZ-NRM Mo), S. Hannam (AM), M. Binnie (SAM), J. Gerber (FMNH), T. Schiøtte (ZMUC-BIV), and J. Trausel and F. Slieker (NMR). The authors would like to thank R. Pujana (MACN), C. Greppi (MACN), and M. Miñana (MACN) for their assistance in making the cuts of the new materials, O. Lehmann (MACN) for the script for multiple implied weighting searching, and M.B. von Baczko and L. Tanoni for the improvement of language. We especially thank A. Beu and an anonymous reviewer for their helpful comments and suggestions as reviewers, and to J. Jin for his suggestions as editor that improved this work. The use of TNT software is facilitated by the Willi Hennig Society. CONICET is acknowledged for the post-graduate grant given to MJA. The participation of MJA in the 2014 field trip was financially supported by the Instituto Antártico Argentino (IAA). This research was also supported by ANPCyT-PICT 57.
Accessibility of supplemental data
Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.cfxpnvx2s.
Appendix 1
Explored localities of the La Meseta and Submeseta formations. during the field season 2014 in Marambio Island.
Appendix 2
Measures of the studied species collected during the Field Season 2014 by MJA housed at the Instituto Antártico Argentino, and donated material of Stilwell and Zinsmeister to the Museo Argentino de Ciencias Naturales from previous field trips. All the measured specimens are adults and were randomly selected, except in the case of type materials.
Continuous characters:
Discrete characters:
Dosinia concentrica
3021100101011012121112100223231001100200100001000000000
Marcia opima
3120110121110312011004100100121001010011100000012012000
Ruditapes decussatus
2121010020110402021000131223231030011010202001002001020
Ruditapes philippinarum
2121010020111402001000131223231030010010202001002001020
Polititapes aureus
3021000130011301001004100223231030010200200200002021001
Polititapes virgineus
1120000130010402011013100002101030011010200210002011001
Neotapes undulata
0120000120010112001013100223231020010210200201011000001
Irus carditoides
2020000?2001?302011000120002001130011111202400002111201
Protapes gallus
3122000131010412101114100100120021011001200301012000000
Tapes literatus
1120010130010402001000100001000030001012200200002011001
Retrotapes antarcticus
1022100131112412021001102000003021011011100110002001000
Retrotapes robustus
1122111131010412021004102000023021001200100110010001000
Retrotapes newtoni
1121110131010412021013102000023021010200200110010001000
Retrotapes ninfasiensis
1122100131010412021001102000023022001001000110000000000
Eurhomalea rufa
2022000120111202020003102223231010000010200010001001001
Katelysia scalarina
0121000131010412020013103001020020020200200301012001101
Katelysia florentinoi
0121000131110412020003103102001020020210200010012000001
Frigichione permagna
1121101?0101??120210?4102223231020000200000000012000000
Paleomarcia tatei
10210?1???0??41?0?00?3100001?21020000200100100010000000
Eumarcia fumigata
1121000?2110?3120??0?3100223230020010200200000011011001
Gomphina undulosa
1110100?0110?0110?10?4102223231020021200200000112010000
Notopaphia elegans
2021011?3000?3021?11?0120001?11133021112211400002100201
Atamarcia sulcifera
202100012101?111???0?3100001022021010200100100002011000
Paphia rotundata
2221010?1110?111??10?5100001101030000200200201011000001
Venerupis corrugata
2121010?2001?4020?10?0120223231030010110201111002000010
Marciachlys inflata n. comb.
112101012101?411001003102102001020010000100000002000000
Adelfia austrolissa n. comb.
1121100?31011012011003100100021020000200100210002000000
Adelfia omega n. gen. n. sp.
10211001300110120010?3100101001020010200200210002000001
