Introduction
Orbitestellidae Iredale, Reference Iredale1917 is a family of heterobranch marine gastropods characterized by their small, low-spired, and widely umbilicated shells (Ponder, Reference Ponder1990; Simone and Zelaya, Reference Simone and Zelaya2004; Rolán et al., Reference Rolán, Rubio and Letourneux2020, among others). After the study of type materials and soft parts from several species, Ponder (Reference Ponder1990) provided new insight into the knowledge on Orbitestellidae. This study revealed well-known species in this family by Ponder (Reference Ponder1990) and several subsequent workers (e.g., Warén and Bouchet, Reference Warén and Bouchet2001; Simone and Zelaya, Reference Simone and Zelaya2004) in several areas of the world. Currently, Orbitestellidae is considered to be part of Heterobranchia (Bouchet et al., Reference Bouchet, Rocroi, Hausdorf, Kaim, Kano, Nützel, Parkhaev, Schrödl and Strong2017) and composed of the genera Orbitestella Iredale, Reference Iredale1917; Microdiscula Thiele, Reference Thiele1912; Boschitestella Moolenbeek, Reference Moolenbeek1994; Kaiwarella Bandel, Gründel, and Maxwell, Reference Bandel, Gründel and Maxwell2000; Lurifax Warén and Bouchet, Reference Warén and Bouchet2001; and Absonus Rubio and Rolán, Reference Rubio and Rolán2021. Assigned species are mostly living but can be found in the fossil record since the Eocene (e.g., Squires and Goedert, Reference Squires and Goedert1996; Kiel, Reference Kiel2006; Hybertsen and Kiel, Reference Hybertsen and Kiel2018; Chernyshev and Goedert, Reference Chernyshev and Goedert2021).
Two living species—Orbitestella ponderi Linse, Reference Linse2002 and O. patagonica Simone and Zelaya, Reference Simone and Zelaya2004—are reported from the Magellanic Region (Fig. 1). Rehder (Reference Rehder1980) cited O. toreuma Powell, Reference Powell1930 from Eastern Island, which is also the type locality of O. aequicostata Raines, Reference Raines2002. Lima et al. (Reference Lima, Barros, Fransisco and Oliveira2011) reported O. bermudezi (Aguayo and Borro, Reference Aguayo and Borro1946) from the Saint Peter and Saint Paul Archipelago off of Brazil, representing the only record of the group in the temperate waters of the southwestern Atlantic. Microdiscula vanhoeffeni Thiele, Reference Thiele1912 from the South Shetland Islands and Microdiscula subcanaliculata (E. A. Smith, Reference Smith1875) from the South Orkney and South Georgia islands (Ponder, Reference Ponder1983, Reference Ponder1990) are the only species reported from sub-Antarctic areas.
Figure 1. Map showing main areas from the Magellanic Region. BB = Burdwood Bank; BC = opening of the Beagle Chanel; MS = entrance of the Magellan Strait; open square = type locality of Patagorbitestella ponderi (Linse, Reference Linse2002) n. comb.; open circle = type locality of P. patagonica (Simone and Zelaya, Reference Simone and Zelaya2004) n. comb.; filled circle = northernmost record of living Patagorbitestella spp. in the Magellanic Region; star = type locality of P. leonensis n. sp.
In this contribution, we provide new taxonomic and biogeographical insights on Orbitestellidae from the Magellanic Region based on extant and lower Miocene specimens with the description of a new genus and a new species. Previous worldwide reports dealing with Orbitestellidae were compiled in an attempt to understand the scenario in which the findings of the present work should be interpreted.
Geological setting
The fossils 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 shell beds lie within loose or very poorly cemented sandstone, exposed along a cliff just south of the Monte León beach; these lithologies are interpreted as part of the generally regressive sedimentary package represented by the Monte León Formation. These sedimentological concentrations are parautochtonous and contain a species-rich, abundant, and well-preserved macrofauna (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 was given by Griffin and Pastorino (Reference Griffin and Pastorino2012).
The Monte León Formation ranges from the Chattian to Rupelian (Oligocene) ages, based on its foraminiferan content (Bertels, Reference Bertels1970, Reference Bertels1975). Also based on foraminiferans, Náñez (Reference Náñez1988) suggested an upper Oligocene–lower Miocene age for the Monte León Formation, whereas Barreda and Palamarczuk (Reference Barreda and Palamarczuk2000) considered it lower Miocene based on palynological data. Parras et al. (Reference Parras, Dix and Griffin2012) indicated an entirely lower Miocene (Aquitanian to lower Burdigalian) age for the Monte León Formation based on 87Sr/86Sr dates drawn from the shells of oysters, pectinids, and brachiopods, with dates ranging from 22.12 Ma (+ 0.46, - 0.54) at the base, to 17.91 Ma (+ 0.38, - 0.4) at the top of the unit.
Materials and methods
Seven bulk fossil samples, each filling a 5-liter container, were collected from the shell beds located at 50°21′25.4″S, 68°53′05.9″W, within the boundaries of the Monte León National Park (Fig. 1). These samples were washed with diluted H2O2 and then sieved following the usual procedure for processing foraminiferans and small mollusks, as described by Beu and Maxwell (Reference Beu and Maxwell1990). Recent material comes from several cruises at the Magellanic Region (Campaña Antártica de Verano CAV) on board the R/V Puerto Deseado (Fig. 1). Bottom samples were obtained with a dredge net (2 mm mesh) and fixed in 5% formalin solution on board, then transferred to 70% ethanol. Both fossil and Recent specimens were sorted under a Leica MZ 95 stereoscopic microscope (X max = 60), then photographed with Zeiss Discovery V20 stereoscopic microscope and/or Philips XL 30 scanning electron microscope (SEM) both at the Museo Argentino de Ciencias Naturales (MACN). Shell morphologies of Orbitestellidae species, based on previous reports and photographs of specimens housed in different institutions, are provided (see Table in Supplemental data).
Abbreviations used in the text are: an. = complete animal; sh. = shell. Diameter of the protoconch (D) indicates the maximum distance between the prototeleoconch limit and other opposite points.
Repositories and institutional abbreviations
Specimens here reported are deposited in the Invertebrate Collection (MACN-In) and in the Paleontological Collection (MACN-Pi) at the MACN, Buenos Aires, Argentina. Type material of previously described species from the area is deposited at Museo de La Plata (MLP), La Plata, Argentina; Museu de Zoologia da Universidade de São Paulo (MZSP), San Pablo, Brazil, and Zoological Museum (ZMH), Hamburg, Germany. Specimens listed (herein and in Table in Supplemental data) are housed at Australian National Museum (AMS), Sydney, Australia; Bailey-Matthews National Shell Museum (BMSM), Sanibel, Florida, USA; Los Angeles County Museum (LACM), California; Muséum National d'Histoire Naturelle (MNHN), Paris; Victoria Museum of Science (SMV), Melbourne, Australia; and Te Papa Tongarewa Museum (NMNZ), Wellington, New Zealand.
Systematic paleontology
Class Gastropoda
Subclass Heterobranchia
Superfamily Orbitestelloidea Iredale, Reference Iredale1917
Family Orbitestellidae Iredale, Reference Iredale1917
Genus Patagorbitestella new genus
Type species
Orbitestella patagonica Simone and Zelaya, Reference Simone and Zelaya2004, here designated.
Diagnosis
Shell very small (Figs. 2.1–2.6, 2.8–2.10, 3.1–3.5), to 1.1 mm (Simone and Zelaya, Reference Simone and Zelaya2004); protoconch with granular nucleus and 10–12 spiral threads toward teleoconch boundary (Figs. 2.7, 2.11, 3.6–3.8) that continue on the teleoconch covering the entire shell surface. Axial sculpture from strongly developed (Fig. 2.1–2.6) to almost obsolete (Fig. 2.8–2.10); macroscopic spiral sculpture rare, but relatively well developed if present (Fig. 3.1–3.5). Central radular teeth with wide pectinate cutting edge; lateral teeth narrow with few cusps; marginal teeth narrow, hook-like without additional cusps. A single jaw plate per row.
Figure 2. Recent species of Patagorbitestella n. gen. from the Magellanic Region: (1–7) Patagorbitestella ponderi (Linse, Reference Linse2002) n. comb. (MACN-In 44058: 54°15′48.3″S, 59°59′2.52″W, 103 m): (1–3) dorsal, ventral, and lateral views of a specimen; (4–6) lateral, ventral, and dorsal views of another specimen; (7) protoconch, with arrows delimiting protoconch diameter (D); (8–12) Patagorbitestella patagonica (Simone and Zelaya, Reference Simone and Zelaya2004) n. comb. (MACN-In 44060: 50°30′40″S, 68°02′33″W, 62 m): (8–10) lateral, dorsal, and ventral views of a specimen; (11) protoconch, with arrows delimiting protoconch diameter (D); (12) detail of the teleoconch spiral threads and commarginal growth lines. Scale bars = 200 μm (1–6, 8–10); 50 μm (7, 11, 12).
Figure 3. Patagorbitestella leonensis n. sp. from Monte León Formation (early Miocene, 50°21′25.4″S, 68°53′05.9″W): (1–3) dorsal, lateral, and ventral views of the holotype (MACN-Pi 6502); (4, 5) dorsal and lateral views of a paratype (MACN-Pi 6503a); (6, 7) setails of the firsts whorls in dorsal and ventral positions from two paratypes (MACN-Pi 6503c, b), with arrows on 3.6 delimiting protoconch diameter (D); (8) detail of the protoconch in dorsal view from a paratype (MACN-Pi 6503c); (9) detail of the periphery of the holotype (MACN-Pi 6502) in lateral view; (10) detail of the dorsal sculpture of the holotype (MACN-Pi 6502); (11, 12) details of the teleoconch spiral threads and commarginal growth lines: (11) dorsal view of a paratype (MACN-Pi 6503a); (12) ventral view of the holotype (MACN-Pi 6502). Scale bars = 200 μm (1–5); 50 μm. (6, 7, 9, 10); 20 μm (8, 11, 12).
Occurrence
Recent: southern Chile (Linse, Reference Linse2002); Beagle Channel (Linse, Reference Linse2002; Simone and Zelaya, Reference Simone and Zelaya2004); Burdwood Bank (Di Luca and Zelaya, Reference Di Luca and Zelaya2019; this work); and off of the mouth of the Santa Cruz River (this work), 0–300 m depth. Lower Miocene: 50°21′25.4″S, 68°53′05.9″W, Punta Entrada Member of the Monte León Formation within the boundaries of the Monte León National Park, Argentina (this work) (Fig. 1).
Etymology
The name is a conjunction of ‘Patagonia,’ a term commonly employed to refer the southern area of South America, and Orbitestella.
Remarks
Patagorbitestella n. gen. includes P. ponderi n. comb., P. patagonica n. comb., and P. leonensis n. sp. According to Simone and Zelaya (Reference Simone and Zelaya2004), radulae and jaws are similar between P. patagonica n. comb. and P. ponderi n. comb., and jaws also differ in the number of rows reported (five in P. patagonica vs. six to eight in P. ponderi). Patagorbitestella n. gen. is a distinctive group exclusively inhabiting the Magellanic Region (Fig. 1); its presence in the area can be traced to at least the early Neogene.
Patagorbitestella ponderi (Linse, Reference Linse2002) new combination
Figure 2.1–2.7
- Reference Ponder1990
Orbitestella sp.; Ponder, p. 516.
- Reference Linse1999
Orbitestella sp.; Linse, p. 401.
- Reference Linse2002
Orbitestella ponderi Linse, p. 82, pl. 9a, figs. 9.1.1 65–68, pl. 9b, figs. 9.1.1 69–72.
- Reference Simone and Zelaya2004
Orbitestella ponderi; Simone and Zelaya, p. 165.
- Reference Di Luca and Zelaya2019
Orbitestella ponderi; Di Luca and Zelaya, p. 62, fig. 5A.
Type specimens
Holotype from Isla Picton, Beagle Channel, Chile: 55°06.89′S, 66°39.95′W, 63 m (ZMH 2826); 53 paratypes from the holotype locality (ZMH 2827); 42 paratypes from Beagle Channel: 55°28.8′S, 66°03.5′W, 1,279 m (ZMH 2828); and 17 paratypes from Isla Picton: 55°07.30′S, 66°52.78′W, 25 m (ZMH 2829).
Diagnosis
Shell to 1.04 mm D; protoconch somewhat bulging toward the prototeleoconch boundary; spire low; teleoconch whorls convex; axial sculpture of strong ribs, forming granules at the dorsal side.
Occurrence
Southern Chile (Linse, Reference Linse2002); Beagle Channel (Linse, Reference Linse2002; Simone and Zelaya, Reference Simone and Zelaya2004); and Burdwood Bank (Di Luca and Zelaya, Reference Di Luca and Zelaya2019; this work) (Fig. 1), 0–300 m depth. Empty shells were reported to 1280 m depth by Linse (Reference Linse2002).
Materials
54°27′18″, 60°57′10.35″W, 100 m (MACN-In 44056: 5 sh.); 54°25′30″S, 58°22′19″W, 300 m (MACN-In 44057: 2 an., 40 sh.); 54°15′48.3″S, 59°59′2.52″W, 103 m (MACN-In 44058: 6 an., 16 sh.); 54°15′24.9″S, 60°34′5.22″W, 113 m (MACN-In 44059: 2 sh.); 54°15′S, 60°00′W, 97–101 m (MACN-In 40697: 1 an., 13 sh.).
Remarks
This species was properly described and illustrated. Specimens here studied were recognized by having strong axial sculpture and a relatively high spire. Some specimens have more spiral threads on the last whorl (i.e., 50 vs. 40–45) than those reported by Linse (Reference Linse2002); this feature is considered as intraspecific variability. Animals deposited under MACN-In 44057 are the deepest known living specimens (300 m) among species of Patagorbitestella n. gen.
Patagorbitestella patagonica (Simone and Zelaya, Reference Simone and Zelaya2004) new combination
Figure 2.8–2.12
- Reference Simone and Zelaya2004
Orbitestella patagonica Simone and Zelaya, p. 161, figs. 2–18
Type specimens
Holotype from Isla H, Beagle Channel, Argentina: 54°52′S, 68°12′W (MLP 6367); 9 paratypes from the same locality (MLP 6368); and 5 paratypes and sections of 2 specimens from the same locality (MZSP 38708).
Diagnosis
Shell to 1.1 mm D; protoconch not bulging toward the prototeleoconch boundary; spire very low; teleoconch whorls convex; axial sculpture almost obsolete; recognizable by wavy shell, mainly on dorsal surface.
Occurrence
Beagle Channel (Simone and Zelaya, Reference Simone and Zelaya2004) and off of the mouth of the Santa Cruz River (this work) (Fig. 1), 0–62 m.
Materials
50°30′40″S, 68°02′33″W, 62 m (MACN-In 44060: 61 an., 18 sh.).
Remarks
This species was properly described and illustrated. Specimens studied herein have the axial sculpture comparatively more developed and more spiral threads on the last whorl (i.e., 42 vs. 35–37) than those reported by Simone and Zelaya (Reference Simone and Zelaya2004). This is the second record of P. patagonica n. comb. and the northernmost record of Recent species of Patagorbitestella n. gen. in the Magellanic Region (Fig. 1).
Patagorbitestella leonensis new species
Figure 3
Type specimens
Holotype (MACN-Pi 6502) and 3 paratypes (MACN-Pi 6503a–c), all from 50°21′25.4″S, 68°53′05.9″W, within the boundaries of the Monte León National Park, Argentina (Fig. 1); Punta Entrada Member of the Monte León Formation.
Diagnosis
Shell to 840 μm D; protoconch somewhat bulging toward prototeleoconch boundary; spire low, teleoconch whorls subangled; sculpture of moderately strong axial ribs, more or less bulging at the dorsal end.
Occurrence
Known only from the type locality, Monte León National Park, Argentina.
Description
Shell (Fig. 3.1–3.5) very small (to 840 μm D), discoidal, widely umbilicated, low-spired. Protoconch (Fig. 3.6–3.8) of ~1⅕ whorls, ~175 μm D, sculptured with 11 or 12 spiral threads; nucleus somewhat sunken, sculptured with granules. Teleoconch to 2¼ whorls, weakly angled ventrally, dorsally, and at the periphery (Fig. 3.2, 3.5, 3.9, 3.10). Sutures somewhat grooved. Axial sculpture of 17–19 ribs per whorl. Ribs stronger ventrally and dorsally toward periphery where they produce more or less pointed bulges; ribs weaker toward aperture. Commarginal growth marks present (Fig. 3.9–3.12). Spiral sculpture of a single peripheral cord (Fig. 3.2, 3.5, 3.9, 3.12), moderately developed (cf. Fig. 3.2, 3.9 vs. 3.5); 10 or 11 spiral threads (Fig. 3.6) on dorsal side of first whorl, 44–49 on last whorl running over entire shell surface (Fig. 3.7, 3.9–3.12). Umbilicus (Fig. 3.3, 3.7) wide (~35% of D) and deep (shell nucleus visible). Aperture somewhat angled to almost circular; columellar callus very narrow.
Etymology
The name refers to the type locality of this species.
Materials
Only the type material.
Remarks
The specimens show some degree of variability in the height of the spire, the strength of the peripheral spiral cord, and the development of spiral threads. The holotype (Fig. 3.1–3.3, 3.9, 3.10, 3.12) shows the highest spire and the strongest peripheral spiral cord observed. In contrast, spiral threads are comparatively less developed (or preserved) than on one of the paratypes illustrated (Fig. 3.4, 3.5, 3.11). A weakly angled teleoconch whorl outline, with the dorsal part more depressed, and the presence of a peripheral spiral cord are the distinctive characters of P. leonensis n. sp., contrasting with the convex outline present in P. ponderi n. comb. and P. patagonica n. comb., which also lack the macroscopic spiral sculpture. The bulging protoconch and the moderately strong axial sculpture of P. leonensis n. sp. resemble those of P. ponderi n. comb. However, the ribs are comparatively more developed on the periphery of the latter species. The numerous spiral threads on the protoconch and teleoconch of P. leonensis n. sp. render it similar to P. ponderi n. comb. and P. patagonica n. comb., allowing the identification of a distinctive group of species among Orbitestellidae.
Discussion
Patagorbitestella leonensis n. sp. (Fig. 3) from the Monte León Formation (Aquitanian to lower Burdigalian) constitutes the first record of fossil Orbitestellidae from South America. Shell characters of this species are remarkably similar to those of the extant P. ponderi n. comb. and P. patagonica n. comb. (Fig. 2). The three species are very small (usually < 1 mm) and have protoconchs of ~ 1⅕ whorls with granular nuclei ornamented with 10–12 spiral threads (Figs. 2.7, 2.11, 3.6–3.8) and teleoconchs to 2¼ whorls, sculptured with numerous spiral threads, continuous with the protoconch threads, over the entire shell surface (9–12 on the dorsal side of the first whorl, 35–50 on the last whorl) always interrupted by commarginal growth lines (Figs. 2.12, 3.9–3.12). Patagorbitestella ponderi n. comb. and P. leonensis n. sp. share the feature of a bulging protoconch toward the teleoconch boundary, correlated with strong axial sculpture. In contrast, P. patagonica n. comb. has almost obsolete axial sculpture and non-bulging protoconch.
Among species known from areas surrounding the Magellanic Region, Orbitestella toreuma, with New Zealand as its type locality, was mentioned from Easter Island (Rehder, Reference Rehder1980). It has a slight keel on its protoconch, two peripheral keels, and spiral threads only between the axial ribs (Powell, Reference Powell1930, pl. 88, figs. 16, 17). Also from Easter Island, O. aequicostata has a shell outline comparable to O. toreuma although it was described with a smooth protoconch. Orbitestella bermudezi was described from the Tertiary of Cuba; however, it was also mentioned as living in several localities worldwide (see Rolán et al., Reference Rolán, Rubio and Letourneux2020; Rubio and Rolán, Reference Rubio and Rolán2021) including Saint Peter and Saint Paul Archipelago off of Brazil (Lima et al., Reference Lima, Barros, Fransisco and Oliveira2011). The specimens from this last locality (as far as it can be seen in the illustration; Lima et al., Reference Lima, Barros, Fransisco and Oliveira2011, figs. 4A–G) have a shell outline similar to those of O. aequicostata and O. toreuma, and a protoconch sculptured with a single keel. Microdiscula vanhoeffeni and Microdiscula subcanaliculata, with type localities in the Kerguelen Islands, were mentioned from South Orkney and South Georgia islands by Ponder (Reference Ponder1983, Reference Ponder1990) and Dell (Reference Dell1990). Microdiscula has convex teleoconch whorls with deeper sutures and generally sculptured only by growth lines, sometimes developed as axial threads (Ponder, Reference Ponder1967; Reference Ponder1990, fig. 2). There are also morphological differences in the soft parts, according to the reports of Linse (Reference Linse2002) and Simone and Zelaya (Reference Simone and Zelaya2004); Patagorbitestella n. gen. has species with single jaw plates per row whereas Microdiscula has four jaw plates per row (Ponder, Reference Ponder1990).
Species inhabiting New Zealand and Australian waters are of particular interest because the early Miocene is related to the development of the Antarctic Circumpolar Current (from west to east since opening of the Drake Passage), which could have effects on the distribution of some taxa across these areas and the Magellanic Region (Beu et al., Reference Beu, Griffin and Maxwell1997; Casadio et al., Reference Casadio, Campbell, Taylor, Griffin and Gordon2010). Moreover, Cyclostrema bastowi Gatliff, Reference Gatliff1906 (the type species of Orbitestella) is an Australian species that, together with other species from the area (e.g., O. decorata Laseron, Reference Laseron1954), allowed Ponder (Reference Ponder1990) to recognize the presence of Orbitestella in the Magellanic Region. Anatomical similarities among the Magellanic species of Patagorbitestella and Orbitestella from Australia and New Zealand have been pointed out by Ponder (Reference Ponder1990), Linse (Reference Linse2002), and Simone and Zelaya (Reference Simone and Zelaya2004).
Beu and Maxwell (Reference Beu and Maxwell1990, pl. 54, figs. N, O) and Ponder (Reference Ponder1990, fig. 1A–E) illustrated Orbitestella praetoreuma Laws, Reference Laws1939, a Miocene species, and Recent O. decorata from New Zealand and Australian waters. Both species have protoconchs of ~1¼ whorls, sculptured with a spiral ridge and several minor spiral elements crossed by numerous axial ribs that produce a pitted pattern. This protoconch morphology contrasts with the unique spiral threads of Patagorbitestella n. gen. (Figs. 2.7, 2.11, 3.6–3.8). In addition, macroscopic spiral elements are very common in most Orbitestella spp. (e.g., Bandel, Reference Bandel1988, pl. 4, figs. 5, 6, pl. 5, fig. 3; Beu and Maxwell, Reference Beu and Maxwell1990, pl. 54, figs. L–N; Ponder, Reference Ponder1990, fig. 1; Ortega y Gofas, Reference Ortega and Gofas2019, fig. 25D–F; Rubio and Rolán, Reference Rubio and Rolán2021, figs. 2–9), whereas in Patagorbitestella n. gen., a single, relatively well developed peripheral cord is only present in P. leonensis n. sp. (Fig. 3.2, 3.5, 3.9, 3.12). According to Simone and Zelaya (Reference Simone and Zelaya2004), P. patagonica n. comb. has a smaller stomach and a somewhat different radula than Orbitestella spp. studied by Ponder (Reference Ponder1990). Radulae of species of Patagorbitestella n. gen. have a central tooth with comparatively wider, less triangular, pectinate cutting edge, narrower lateral teeth with fewer cusps, and marginal teeth that are also narrower, hook-like, and without additional lateral cusps.
Boschitestella encompasses very small shells mostly covered by microscopic spiral threads, protoconchs split into two distinctly sculptured sections, and a sharp peripheral keel on the teleoconch (e.g., Moolenbeek, Reference Moolenbeek1994, figs. 9–16; Rolán et al., Reference Rolán, Rubio and Letourneux2020, figs. 1–3; Rubio and Rolán, Reference Rubio and Rolán2021, fig. 10C–F). Absonus has a similar protoconch and keel to those of Boschitestella, however, the shells are larger (~2 mm) and almost without sculpture (Rubio and Rolán, Reference Rubio and Rolán2021, figs. 10A, B, 11A–D, 12, 13). Lurifax has a multispiral smooth protoconch and encompasses relatively large species (to 2.8 mm) related to Orbitestellidae by the anatomy of the soft parts (e.g., Warén and Bouchet, Reference Warén and Bouchet2001, figs. 37C, D, 44E–G, 46C, D, 47A, B; Sasaki and Okutani, Reference Sasaki and Okutani2005, fig. 1; Kiel, Reference Kiel2006, fig. 10.1–10.6). Finally, Kaiwarella is only known from the Jurassic; its type species (Bandel et al., Reference Bandel, Gründel and Maxwell2000, pl. 10, figs. 4–7) has a markedly angled shell with a strong peripheral keel.
Microscopic spiral sculpture densely developed on the entire surface of the teleoconch is known from several Orbitestellidae (e.g., Orbitestella wareni Ponder, Reference Ponder1990; O. decorata; O. praetoreuma; O. nova Rolán, Rubio, and Letourneux, Reference Rolán, Rubio and Letourneux2020; O. papuensis Rubio and Rolán, Reference Rubio and Rolán2021; Boschitestella spp. as described by Moolenbeek, Reference Moolenbeek1994). Patagorbitestella n. gen. has microscopic spiral threads continuous from the protoconch to the teleoconch in all species (Figs. 2.7, 2.11, 3.6–3.8), a characteristic feature that does not occur in any other Orbitestellidae species.
Patagorbitestella n. gen. is a distinctive genus group among Orbitestellidae that can be morphologically distinguished from any other genera currently known. It encompasses two Recent species (i.e., P. ponderi n. comb.; P. patagonica n. comb.) and one fossil species from the lower Miocene (P. leonensis n. sp.), all known only from the Magellanic Region (Fig. 1). Patagorbitestella n. gen. is a consistent generic lineage of gastropods exclusively inhabiting the Magellanic Region, with a presence certainly extending to the lower Miocene, as appears to occur with other mollusks in the area. Pastorino and Griffin (Reference Pastorino and Griffin2018, Reference Pastorino and Griffin2019) described Cyclochlamys argentina Pastorino and Griffin, Reference Pastorino and Griffin2018 (Pectinoidea) and reported specimens of Antistreptus magellanicus Dall, Reference Dall1902 (Prosiphonidae) from the same region and deposits as species of Patagorbitestella n. gen. studied here, suggesting the presence of long-lasting lineages with no changes since the early Miocene. Pastorino and Griffin (Reference Pastorino and Griffin2019) highlighted that both species share a small shell (< 5 mm) and inhabit shelf and upper-slope environments associated with the alga Macrocystis pyrifera (Linnaeus, Reference Linnaeus1771) C. Agardh, Reference Agardh1820. The concept of a long-lasting lineage is also applicable to Patagorbitestella n. gen. and probably to other groups of mollusks. There are also other records of small invertebrates with genera recognizable both from the early Miocene (or late Oligocene) and Recent faunas in this area (e.g., 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 presence of these lineages could be related to a set of tectonic and biogeographical conditions that appears to be constant in the Magellanic Region. Pastorino and Griffin (Reference Pastorino and Griffin2019) mentioned the fact that the SW Atlantic coast is disposed on a passive continental margin that remained relatively stable during the Cenozoic. This could have had effects on the stability of environments in the area, providing ecological conditions suitable for the development of several taxa around this time.
Species of Patagorbitestella n. gen. here studied have few-whorled, large protoconchs (~1⅕ whorls, 160–175 μm D) that suggest direct development (Shuto, Reference Shuto1974; Di Luca et al., Reference Di Luca, Penchaszadeh and Pastorino2020; Rolán et al., Reference Rolán, Rubio and Letourneux2020). Such species without free larvae have limited potential for dispersal into other areas, a fact that contributes to explaining the unique characters of Patagorbitestella n. gen. from the Magellanic Region.
Acknowledgments
I. Chiesa (CADIC, Ushuaia) and B. Dotti (FCEyN, Buenos Aires) collected the samples from which the Recent specimens studied herein were separated; D. Rocatagliata (FCEyN) and collaborators allowed study of this material. J. Signorelli (Centro Nacional Patagónico, Puerto Madryn) and D. Urteaga (MACN) helped collect the bulk of the material from which the fossil specimens were found. F. Tricarico helped with the SEM images at MACN. D. Geiger (LACM), V. Heros (MNHN), P. LaFollette (LACM) and A. Peñas provided hard-to-find references. D. Beechey (AMS), M. Reid (AMS), and D. Staples (SMV) provided images of type material and specimens. The manuscript benefited from the thorough reviews of S. Nielsen (Universidad Austral de Chile, Valdivia) and an anonymous reviewer. This work was supported by PICT 2016/1309 from Agencia Nacional de Promoción Científica y Tecnológica. We acknowledge funding by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina, to which GP and MG belong as members and JD as a fellow of the Carrera del Investigador Científico.
Declaration of competing interests
The authors declare none.
Data availability statement
Data available from the Zenodo Digital Repository: https://doi.org/10.5281/zenodo.7544475.
