Introduction
The life-span of a clade of any organism through the ages is difficult to calculate. Most living beings leave no record of their presence on earth. However, some animals produced hard parts that persist as a witness of the evolution of the whole group. Mollusks and corals are probably among the best known animals regarding the duration of their lineages, based on the records of hard parts they leave behind. The ages based on the survival of a clade are very variable depending on the author and taxa considered. Wallace and Bosellini (Reference Wallace and Bosellini2015) found several Eocene species of Acropora corals living as long as 28 Myr. Raup (Reference Raup1990) cited different authors and groups as examples of the average duration of species in the fossil record (e.g., ammonoids 1–2 Myr, according to Kennedy, Reference Kennedy1977; Cenozoic bivalves 10 Myr, sensu Stanley, Reference Stanley1979) among mollusk groups. Prothero (Reference Prothero2014) cited 3.21 Myr for large mammals.
Different from other groups of invertebrates, gastropods show the larval life in the morphology of the protoconch. This feature was explored by several authors (Shuto, Reference Shuto1974; Hansen, Reference Hansen1980; Jablonski, Reference Jablonski1994; Jablonski and Hunt, Reference Jablonski and Hunt2006, etc.) as a tool that, when linked to geographic distributions, could eventually lead to some conclusions on speciation and lineage duration. In this way, Jablonski (Reference Jablonski1994) pointed out that molluscan species durations are positively correlated with geographic ranges. Gili and Martinell (Reference Gili and Martinell1994), in a thorough study on nassarids from Europe, agreed with results of previous authors that species with planktotrophic larvae have a longer life-span than those with non-planktotrophic larvae. These authors indicated that the main factor influencing duration of a species is the larval ecology that modified its dispersion capacity. Valentine and Moores (Reference Valentine and Moores1970) and Valentine (Reference Valentine, Witman and Roy2009) mentioned environmental stability among factors that regulate species diversity.
Sinistral gastropods are unusual in the southwestern Atlantic malacofauna, where only 14 species belonging to different groups have been reported so far. The genus Triphora Blainville, Reference Blainville1828 is well represented by eight species in Brazil, of which only one reaches Uruguayan waters (Rios, Reference Rios2009). Also from Brazil is a sinistral Conoidean represented by the Borsoninae Borsonia brasiliana Tippet, Reference Tippet1983. The genus Blauneria Shuttleworth, Reference Shuttleworth1854, a sinistral Ellobidae, is represented by only one species in the western Atlantic (Martins, Reference Martins1996). Three sinistral species were described from truly Antarctic waters under the genus Prosipho. The remaining forms belong in Antistreptus Dall, Reference Dall1902, including two nominal species described from Patagonia, mostly from shallow waters (e.g., A. magellanicus Dall, Reference Dall1902, and A. rolani Castellanos, Reference Castellanos1986).
In this paper we review the gastropod genus Antistreptus Dall, Reference Dall1902 that, together with a recent study of bivalves of the genus Cyclochlamys Finlay, Reference Finlay1926, has species with morphologically indistinguishable specimens spanning the unusually long time range of ~20 Myr (i.e., early Miocene–Recent).
Materials and methods
The fossil specimens described herein come from shell-beds at the top of the Punta Entrada Member of the Monte León Formation (Bertels, Reference Bertels1970, Reference Bertels1980). The locality (50°21′25.4″S, 68°53′05.9″W) lies within the boundaries of the Monte León National Park. The shell-beds are included within a loose or very poorly cemented sandstone exposed along the cliff just south of the Monte León beach, which is interpreted as part of the generally regressive sedimentation represented by the Monte León Formation. These sedimentological concentrations are parautochtonous and contain a rich, abundant, and well-preserved megafauna (Ihering, Reference Ihering1907; del Río and Camacho, Reference Del Río and Camacho1998; del Río, Reference Del Río2004a, Reference Del Ríob; Griffin and Pastorino, Reference Griffin and Pastorino2005, Reference Griffin and Pastorino2006; del Río and Martínez, Reference Del Río and Martínez2006, and references therein). A schematic section of the locality is given in Griffin and Pastorino (Reference Griffin and Pastorino2012).
According to Bertels (Reference Bertels1970, Reference Bertels1975), the Monte León Formation ranges between Chattian and Rupelian, based on its foraminifera content. Also based on foraminifera, Náñez (Reference Náñez1988) suggested a late Oligocene–early Miocene age for the Monte León Formation. Barreda and Palamarczuk (Reference Barreda and Palamarczuk2000) considered it early Miocene based on palynological data. We agree with Parras et al. (Reference Parras, Dix and Griffin2012), who indicated an entirely early Miocene (Aquitanian–early Burdigalian) age for the Monte León Formation based on 87Sr/86Sr ages drawn from shells of oysters, pectinids, and brachiopods; the recorded ages ranging from 22.12 Ma (+0.46, −0.54) at the base to 17.91 Ma (+0.38, −0.4) at the top.
The fossil samples were washed with diluted H2O2 and sieved following the usual procedure for foraminifera and small mollusks, as described in Beu and Maxwell (Reference Beu and Maxwell1990). Specimens were also analyzed and photographed under SEM at the MACN.
New material from extant populations was collected mainly during two expeditions aimed at studying the biodiversity in Burdwood Bank, located ~150 km east of Isla de los Estados (Staten Island, off the eastern tip of Tierra del Fuego) and 200 km south of the Malvinas (Falkland) Islands and from stations in the area by ships en route to the Campaña Antartica de Verano (CAV, Antarctic Summer Fieldwork). The samples were obtained with a Rauschert sledge on board the Argentine R/V Puerto Deseado. The sledge has a 55 x 15 cm mouth-opening, equipped with a 1 x 1 mm mesh-size nylon net. Radulae were taken from preserved specimens, cleaned with commercial bleach (sodium hypochlorite), coated with gold-palladium, and examined using a Philips XL30 SEM at the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN).
Repositories and institutional abbreviations
The fossil and extant material is housed in MACN Invertebrate Paleontology collection (MACN-Pi) and Invertebrate collection (MACN-In), respectively. Additional collections studied were those housed in the MACN (Buenos Aires, Argentina) and the Museo de La Plata, Malacology collection (MLP-Ma), in La Plata, Argentina. Type material used in this research is housed in the MLP; United States National Museum, Smithsonian Institution, Washington, D.C. (USNM), USA, and the Naturhistoriska Riksmuseet, Stockholm (NHR), Sweden.
Systematic paleontology
Class Gastropoda Cuvier, Reference Cuvier1795
Order Neogastropoda Wenz, Reference Wenz1938
Family Buccinidae Rafinesque, Reference Rafinesque1815
Genus Antistreptus Dall, Reference Dall1902
Type species
Antistreptus magellanicus Dall, Reference Dall1902 from Magellan Strait, Chile, 52°38′00″S, 70°10′30″W, in 19.75 fathoms, by original designation.
Other species
Only one species was described in this genus.
Remarks
In the original description of the genus, Dall (Reference Dall1902, p. 532) wrote: “…sinistral with a dextral nucleus;” yet the illustration of the protoconch of the holotype of A. magellanicus showed that, while its axis is slightly inclined with respect to the teleoconch axis, it is always sinistral. When described by Dall (Reference Dall1902), the type species was known to occur in the southwestern Atlantic, and no fossil representatives were then recorded.
Antistreptus magellanicus Dall, Reference Dall1902
Figures 1–5
- Reference Dall1902
Antistreptus magellanicus Dall, p. 532.
- Reference Strebel1908
?Euthria (Glypteuthria) contraria Strebel, p. 29, pl. 1, figs. 4a–c.
- Reference Dall1908
Antistreptus magellanicus Dall, pl. 15, fig. 14.
- Reference Melvill and Standen1912
Antistreptus magellanicus; Melvill and Standen, p. 354.
- Reference Doello-Jurado1918
Antistreptus magellanicus; Doello-Jurado, p. 123.
- Reference Carcelles1944
Antistreptus magellanicus; Carcelles, p. 7, fig. 5.
- Reference Carcelles1950
Antistreptus magellanicus; Carcelles, p. 62, pl. 2, fig. 39.
- Reference Carcelles and Williamson1951
Antistreptus magellanicus; Carcelles and Williamson, p. 298.
- Reference Powell1951
Antistreptus magellanicus; Powell, p. 148.
- Reference Boss, Rosewater and Ruhoff1968
Antistreptus magellanicus; Boss et al., p. 196.
- Reference Castellanos1970
Antistreptus magallanicus; Castellanos, p. 103, pl. 9, fig. 9.
- Reference Castellanos1986
Antistreptus rolani Castellanos, p. 132 (dated 1985, published 1986).
- Reference Castellanos1989
Antistreptus rolani; Castellanos, p. 90, fig. 3 (dated 1 December 1988, published September 1989).
- Reference Castellanos1992
Antistreptus rolani; Castellanos, p. 26, pl. 3, fig. 30.
- Reference Castellanos1992
Antistreptus magellanicus; Castellanos, p. 25, pl. 3, fig. 29.
- Reference Bastida, Roux and Martinez1992
Antistreptus magellanicus; Bastida et al., p. 294.
- Reference Forcelli2000
Antistreptus magellanicus; Forcelli, p. 91, fig. 241.
- Reference Linse2002
Antistreptus magellanicus; Linse, p. 100, pl. 12, figs. 9.1.1, 9.1.2.
- Reference Martín and César2004
Antistreptus rolani; Martín and César, p. 17.
- Reference Forcelli, Narosky and Zaffaroni2015
Antistreptus magellanicus; Forcelli et al., p. 66, fig. 164.
- Reference Signorelli, Urteaga and Teso2015
Antistreptus rolani; Signorelli et al., p. 53, pl. 1, figs. G, H.
Figure 1. Antistreptus magellanicus Dall, Reference Dall1902. (1, 2) Holotype (USNM 96190); (3, 4) holotype of Euthria (Glypteuthria) contraria Strebel, 1902 (NHR Type-1051). Scale bar = 1 mm.
Figure 2. Antistreptus magellanicus Dall, Reference Dall1902. (1, 2) Uncoated SEM views of a probable syntype of Antistreptus rolani Castellanos, Reference Castellanos1986 (MLP-Ma 4692); (3, 4) uncoated SEM views of the protoconch of specimen in (1, 2); (5) uncoated SEM apertural view of another probable syntype of Antistreptus rolani Castellanos, Reference Castellanos1986 (MLP-Ma 4692). Scale bars = 1 mm (1, 2, 5); 500 µm (3, 4).
Figure 3. Antistreptus magellanicus Dall, Reference Dall1902. (1, 2) MACN-In 16244-1 SEM coated; (3–5) three views of the protoconch. Scale bars = 2 mm (1, 2); 500 µm (3–5).
Figure 4. Antistreptus magellanicus Dall, Reference Dall1902, MACN-In 16244-1. (1) Dorsal view of radula; (2) detail of the radula; (3) external and internal view of the operculum. Scale bars = 25 µm (1); 20 µm (2); 500 µm (3).
Figure 5. Antistreptus magellanicus Dall, Reference Dall1902, MACN-Pi 6450 from the Punta Entrada Member of the Monte León Formation, 50°21′25.4″S, 68°53′05.9″W. (1) Apertural view, SEM coated; (2, 3) two views of the protoconch; (4, 5) another specimen, apertural and abapertural views, MACN-Pi 6450; (6, 7) two views of the protoconch of specimen in (4, 5). Scale bars = 2 mm (1, 4, 5); 200 µm (2, 3); 500 µm (6, 7).
Holotype
Antistreptus magellanicus (USNM 96190) from R/V Albatross sta. 2777, 52°38′00″S, 70°10′30″W, Magellan Strait, Chile, 19.75 fathoms [36.1 m] (Dall, Reference Dall1908, pl. 15, fig. 14). In 1908 Dall added sta. 2773, 52°23′S, 68°11′W, in 10 fathoms [18.3 m]; Euthria (Glypteuthria) contraria, northern Argentina, 37°50′S, 56°11′W in 100 m; Antistreptus rolani, apparently 37°35′S, 56°25′W, 40 fathoms [73.1 m], off Buenos Aires Province, Argentina, but see below.
The holotype (USNM 96190) is photographed for the first time here (Fig. 1.1, 1.2). The material of Euthria (Glypteuthria) contraria from the Swedish Antarctic Expedition is a specimen represented only by the shell housed in the NHR collection as Type-1051; it is illustrated here (Fig. 1.3, 1.4). The type material of A. rolani (MLP-Ma 4692) consists of two specimens, the measurements of which differ from those indicated by the author. After careful reading of the original description, it appears that Castellanos had at least three specimens from three different expeditions: (1) from the research vessel Undine, 37°35′S, 56°25′W, at 40 fathoms depth, of 3.8 x 2.6 mm height and width, respectively, and 2.1 x 0.9 mm of aperture dimensions; (2) from the Spanish fishing vessel Puente Gondomar, 46°S, 60°W, 3.9 x 2.2 mm and aperture 2.1 x 0.9; and (3) “another specimen” 3.2 x 2 mm and aperture 2 x 0.7 mm, perhaps from the research vessel Shinkai Maru from 43°30′S, 59°50′W at 116 m depth. In 1992 Castellanos, in a re-description of A. rolani, mentioned only two specimens: “tipo,” a supposed holotype of 3.8 x 2.6 mm and aperture of 2.1 x 0.9 mm and “paratipo,” a supposed paratype of 3.0 x 2.2 and the same aperture size. She mentioned the type locality as 46°S, 60°W at 600 m depth, which is different from the locality given in the original description. Signorelli et al. (Reference Signorelli, Urteaga and Teso2015) mentioned that both specimens housed under MLP-Ma 4692 are apparently syntypes collected by the Japanese vessel Shinkai Maru, although none of the specimens agrees with the original measurements published by the author (Castellanos, Reference Castellanos1986). In any event, these specimens were identified by the author. Both are illustrated here in Figure 2.1–2.5, with 3.45 and 3 mm of maximum height. Thus, regardless of the uncertain identification of the type material, we can safely assume that they represent accurately the idea that she had of A. rolani. Whichever specimen is finally considered holotype or lectotype, or whether they are deemed to be syntypes, they can all easily be identified as juveniles of A. magellanicus. All the specimens identified as A. rolani have the last whorl undeveloped showing a smaller aperture and an apparently longer siphonal canal than the rest. Adult specimens have a thicker shell, smaller aperture, and the siphonal canal appears somewhat deeper.
Occurrence
Fossil material comes from the Punta Entrada Member of the Monte León Formation (Bertels, Reference Bertels1970, Reference Bertels1980). Extant specimens are from off Buenos Aires Province (~37°S) to Ushuaia, including Malvinas Is. and Burdwood Bank, in 20–262 m. Bastida et al. (Reference Bastida, Roux and Martinez1992) studied material from R/V Shinkai Maru, reportedly from a depth range of 56–169 m.
Description
Shell small, up to 4.9 mm high, sinistral, fusiform, with four slightly convex whorls; protoconch sinistral, with coiling axis sloping ~40–45° with respect to teleoconch coiling axis, of ~1.5 convex, smooth, whorls; transition to teleoconch well defined; suture impressed; aperture oval, elliptic, labrum sharp; siphonal canal rather deep, oblique, and long, inclined ~30° from shell axis; parietal callus thin; growth lines closely spaced all over shell surface; spiral ornamentation of three cords, anterior weaker, all starting right after protoconch end; sometimes a flat cord running over suture line; 12 cords in last whorl, the basal seven whorls narrower and less marked; axial ornamentation of low ribs after the first whorl, 7–8 in the second whorl, 15 in the third, 20 in the last; axial ribs forming nodes at the intersection with spiral cords; periostracum translucent, scaly between axial ribs; color off whitish.
Operculum, pale yellowish, ovate-elliptical, nucleus subterminal, attachment area small, somewhat indefinite.
Radulae rachi glossated with very small rachidian teeth, subrectangular with a very thin base, with three very short, blunt, obsolete cusps. Laterals triangular, fan-shaped, basal plate with long, narrow Prosipho-like “handle,” with three short, slightly curved cusps pointing towards the center of the ribbon and two larger curved cusps, the outer one bifid. Eyes present.
Materials
One specimen, MACN-In 16244-1, 37°35′S, 56°25′W in 40 fathoms [73.1 m]; one shell, MACN-In 25779, 54°57′S, 64°42′5″W in 20 fathoms [36.5 m]; one shell, MACN-In 25780, 54°46′S, 64°36′8″W in 20 fathoms [36.5 m]; one sp., MACN-In 16158, 37°31′S, 56°23′W in 70 m; one shell, MACN-In 22087, Puerto Cook, Isla de los Estados in 20 fathoms; one sp. MACN-In 42257, Burdwood Bank Expedition, collected on April, 2017, St. 39 Cr. 79, 54°13.934′S, 66°30.997W in 53 m, bottom net trawl; two shells, MACN-In 42258, Campaña Antartica de Verano CAV2014, St. 8, Cr. 26, collected 30 March 2014, 55°4.297′S, 66°1.857′W in 207 m; two shells, MACN-In 42259, Burdwood Bank Expedition, collected April 2017, St. 26, Cr. 316, 54°5.501′S, 60°41.978′W in 122 m, modified Agassiz dredge; four shells, MACN-In 42260, R/V “Aldebaran” St. 9506, Cr. 56, 13 June 1995, 37°46.42′S, 55°03.07′W, 253 m to 37°44.2′S, 55°01.34′W, in 262 m, collected from the root of kelp Macrocystis pyrifera (Linnaeus, Reference Linnaeus1771) (described in Scarabino and Ortega, Reference Scarabino and Ortega2004); 14 shells, MLP-Ma 14697, Ushuaia ?, Tierra del Fuego, Argentina.
Fossil material
Five shells, MACN-Pi 6450, shell-beds at the top of the Punta Entrada Member of the Monte León Formation, 50°21′25.4″S, 68°53′05.9″W.
Remarks
Dall described the new genus and new species in Reference Dall1902 without illustrating it, which he did in Reference Dall1908 (Dall, Reference Dall1908, pl. 15, fig. 14). In this same year, Strebel (Reference Strebel1908) described Euthria (Glypteuthria) contraria, making no reference to Dall's species, of which he had no knowledge at that time. However, Dall included the locality of Strebel in his Reference Dall1908 paper. The illustration and the holotype of E. contraria, illustrated here for the first time (Fig. 1.3, 1.4), clearly suggest they are synonyms.
Thiele (Reference Thiele1929) acknowledged a shell similarity between his Anomacme smithi Strebel, Reference Strebel1905 and Antistreptus magellanicus, including both species in Antistreptus, with two sections: Antistreptus s. s. and Anomacme, in which shell coiling direction (dextral in Anomacme and sinistral in Antistreptus) was the sole difference. He also described the radula of his Antistreptus s. l. based on the radula of Anomacme smithi, illustrated previously (Thiele, Reference Thiele1912, pl. 16, fig. 14), which was the only one known at that time. However, Powell (Reference Powell1951, p. 148) disagreed with this combination of both genera because the radula of A. magellanicus was unknown.
Hain (Reference Hain1990) discussed three sinistral species as belonging in the genus Prosipho (e.g., P. contrarius Thiele, Reference Thiele1912; P. perversus Powell, Reference Powell1951; P. reversa Powell, Reference Powell1958), considering contrarius and perversus as synonyms. He illustrated the radula of P. contrarius. This radula with absent rachidian and only two lateral teeth with similar-sized cusps is completely different from that of A. magellanicus. This rules out Antistreptus or Prosipho for these species with the sinistral shell as a unique similarity. Unaware of this radular difference, Hain (Reference Hain1990, p. 59) compared the rachidian-less radula of Prosipho contrarius with Antistreptus (sensu Thiele, Reference Thiele1929); however he was actually comparing Anomacme instead, which does have a central tooth as well.
Engl (Reference Engl2012) justified the inclusion of all sinistral Antarctic Prosipho species in Antistreptus because Hain (Reference Hain1990) stated that the missing central tooth in the radula of Prosipho contrarius also was a feature of Antistreptus. Hain (Reference Hain1990) showed that the radula of P. contrarius is different from the other known species of Prosipho and suggested that all Antarctic sinistral species belong in a genus different from Prosipho. However, he mentioned the radula of Antistreptus, which had not been actually described or illustrated at that time. It seems that he understood Thiele's inclusion of Anomacme smithi and Antistreptus magellanicus in Antistreptus as an indication of similar radular features. The radula, shown here for the first time (Fig. 4.1, 4.2, dissected from the specimen in Fig. 3), is clearly different and suggests that Antistreptus is a valid genus different from Anomacme and Prosipho. Nevertheless, it should probably be treated as a genus of Prosiphinae. Dell (Reference Dell1990) considered all three Antarctic sinistral species of Prosipho as valid and he did not compare them with the Magellanic genus Antistreptus.
Linse (Reference Linse2002) described a new genus and species, Crenatosipho beaglensis, and illustrated a radula extremely similar to the one we described here for A. magellanicus Dall, Reference Dall1902. In addition, the shell of C. beaglensis Linse, Reference Linse2002 is puzzling, similar to Meteuthria martensi (Strebel, Reference Strebel1905), despite the fact that the radula of the latter species points towards a different genus (Thiele, Reference Thiele1912; Pastorino, Reference Pastorino2016).
Discussion
Together with the recently described pectinoid bivalve Cyclochlamys argentina Pastorino and Griffin, Reference Pastorino and Griffin2018, Antistreptus magellanicus Dall, Reference Dall1902 represents long-lasting lineages of mollusks showing no changes (at least in shell morphology, see Fig. 5) since at least the early Miocene from the same deposits.. These two species are unrelated, but share three features in common: (1) their small size (<5 mm), (2) their preference for shelf or upper slope environments, and (3) their habit of living (if not exclusively) in association with the southern kelp Macrocystis pirifera. Such association with Macrocystis could well be a factor contributing to their distribution because Cyclochlamys Finlay, Reference Finlay1926 species are byssate during some period of their lifespan and A. magellanicus Dall, Reference Dall1902 has been documented living among the “roots” of drifting masses of M. pirifera (Scarabino and Ortega, Reference Scarabino and Ortega2004) as far north as the Uruguayan coast.
Yet none of these factors appears to explain such a long duration for these species (~17–20 Myr). The duration or life of a species has been dealt with on many occasions and for different groups of organisms such as, among others, microfossils (Liow et al., Reference Liow, Skaug, Ergon and Schweder2010), graptolites (Rickards, Reference Rickards1977; Cooper et al., Reference Cooper, Sadler, Munnecke and Crampton2010), bivalves (Stanley, Reference Stanley1979; Hoffman and Szubzda-Studencka, Reference Hoffman and Szubsda-Studencka1982), gastropods (Hansen, Reference Hansen1980; Gili and Martinell, Reference Gili and Martinell1994), ammonites and bivalves (Kennedy, Reference Kennedy1977; Hallam, Reference Hallam1987), and mammals (Prothero, Reference Prothero2014). Longevity of species varies and could be correlated to factors such as environmental differences, taxonomic lineage, geographic range, larval type, or other causes (Crampton et al., Reference Crampton, Cooper, Beu, Foote and Marshall2010).
Architectonica karsteni Rutsch, Reference Rutsch1934, an architectonicid gastropod reported by DeVries (Reference DeVries1985) and Nielsen and Frassinetti (Reference Nielsen and Frassinetti2007) from Miocene–Pliocene deposits in Central and South American Pacific coast (Peru and Chile) and extant from Central America Pacific coast, is another example of a similar lifespan for a Neogene mollusk. In addition, in a recent work on the southwestern Atlantic scorched mussels of the genus Brachidontes Swainson, Reference Swainson1840, Trovant et al. (Reference Trovant, Márquez, Del Río, Ruzzante, Martínez and Orensanz2018) showed that B. lepida (Philippi, Reference Philippi1893) from North Argentine deposits of the Paraná Formation of Miocene age is morphologically closer to the extant species B. rodriguezii (d'Orbigny, Reference Orbigny and Bertrand1846) than to the other extant and fossil species of the same genus. However, in both cases, the sizes of the specimens are considerably larger than those described here.
The reasons for the temporal persistence of Antistreptus Dall, Reference Dall1902 and eventually other species nowadays living on the wide shelf along the Atlantic coast of southernmost South America and found fossil in Neogene stratigraphic units in the area remain obscure. However, the fact that the paleoceanographic, tectonic, and paleogeographic conditions on the shelf have remained fairly stable throughout the Neogene (with the obvious paleoclimatic changes affecting adjacent land environments being much more pronounced) may have potentially favored relatively long-lived ecological structures of the different communities inhabiting the shelf and consequently relatively slow evolutionary rates in at least some of the lineages of marine invertebrates that formed them.
It should be noted that while examples of large mollusk species, such as muricids and naticids (Griffin and Pastorino Reference Griffin and Pastorino2005, Reference Griffin and Pastorino2013), appear to have a short stratigraphic records in the area (with very few species or none in common with the recent faunas), some small species seem to have survived for longer—since at least the early Miocene (Pastorino and Griffin, Reference Pastorino and Griffin2018). A comprehensive revision of the fossil fauna is still wanting, but a similar pattern can be observed at a generic rank, with many genera of small species found in Neogene units surviving nowadays on the shelf (Casadio et al., Reference Casadio, Pastorino and Griffin2009; Griffin and Pastorino, Reference Griffin and Pastorino2012; Pérez et al., Reference Pérez, Griffin, Pastorino, López-Gappa and Manceñido2015). The significance of size is not clear, but may be related to the relatively stable paleoecological conditions.
The Atlantic coast of Patagonia lies along a passive continental margin, which has remained relatively stable throughout the Cenozoic, as opposed to the Pacific (Kiel and Nielsen, Reference Kiel and Nielsen2010). While the final opening of the Drake Passage and the ensuing definitive separation of Antarctica—and the subsequent establishment of the circum-Antarctic Current—produced paleoceanographic and paleoclimatic changes that, together with the uplift of the Andes along the eastern margin of the South American plate, strongly affected the continental environments in this part of the continent, the effects on marine environments along the shelf were probably less marked (Lawver and Gahagan, Reference Lawver and Gahagan2003; Lyle et al., Reference Lyle, Gibbs, Moore and Rea2007; Le Roux, Reference Le Roux2012). Contrarily, marine environments along the western seaboard of the South American plate were strongly affected by tectonic activity. Consequently, the diversity in habitats was bound to increase significantly. At the same time, the ongoing tectonic activity entails highly unstable physical environments, with the consequent changes in paleocological structure of the communities living there (Blisniuk et al., Reference Blisniuk, Stern, Chamberlain, Idleman and Zeitler2005).
Acknowledgments
The following persons generously sent material or pictures of types from the collections under their care: A. Tablado (MACN); M. Forshage (NHR); E. Strong (USNM) and C. Damborenea and G. Darrigran (MLP). R. Bieler (Field Museum, Chicago) generously helped with books and German translations of some Thiele's descriptions. S. Nielsen (Universidad Austral, Chile) and an anonymous reviewer considerably improved the manuscript with helpful suggestions.
This work was supported by PICT 2016/1309 from Agencia Nacional de Promoción Científica y Técnológica. We acknowledge funding by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina, to which G.P. and M.G. belong as members of the Carrera del Investigador Científico.
