1. Introduction

South American Paleogene mammals document the definitive establishment and subsequent modernization of early stocks of therian immigrants, that quickly gave rise to the highly autochthonous Cenozoic continental fauna (Simpson, 1980; Pascual et al., 1996; Pascual and Ortiz-Jaureguizar, 2007; Croft, 2016). Important innovations and biotic dynamics were attained or arose during this period, including the rise in body size disparities within and among clades (Vizcaíno et al., 2012); the diversification of ecological partitioning through marked morphological specializations (Croft et al., 2008; Goin et al., 2012a); and the establishment of the continental biogeographic distinctiveness (Marshall and de Muizon, 1988; Pascual and Ortiz Jaureguizar, 1990; Flynn et al., 2007). These patterns and processes were built and maintained through the interplay of regional-wide diversification dynamics (Buffan et al., 2025), including trans-continental dispersalist events, through episodic migrations (Bond et al., 2015; Goin et al., 2016; Gelfo et al., 2009a, 2019). Additionally, the Paleocene-Eocene also witnessed the final demise of much of the South American endemic Mesozoic non-therian lineages that survived through the K-Pg global extinction event, being the last remnants of a past and still enigmatic Gondwanan radiation (Gelfo and Pascual, 2001; Goin et al., 2012b; Martinelli et al., 2021; Rougier et al., 2021).

While Paleogene fossil mammals are presently recorded across the continent (Marshall et al., 1983; Woodburne et al., 2014; Antoine et al., 2016), most of the current knowledge on the processes of emergence, diversification, and extinction of lineages comes from the superb deposits of central Patagonia in Argentina, especially those located in the provinces of Chubut and northeastern Santa Cruz (Simpson, 1941a, 1967a; Pascual et al., 2002; Carlini et al., 2022). In this area, sedimentary exposures including notable early Paleocene sites like Punta Peligro, type locality of the Peligran South American Land Mammal Age (SALMA) (Bonaparte et al., 1993; Goin et al., 2022), or the late Paleocene to early Eocene Río Chico Group (Simpson, 1935; Raigemborn et al., 2010), documents faunistic arrangements and turnovers, illustrating evolutionary patterns of community assembly during fluctuating tectonic and climatic conditions. Outstanding in this taxonomic, ecologic, and geographic diversity are the thick volcaniclastic exposures occurring south of Lake Colhué Huapi, Chubut Province, Argentina, commonly referred to as Gran Barranca (Ameghino, 1901, 1904; Simpson, 1948, 1967a; Madden et al., 2010a). There, the middle Eocene through late Oligocene superposed fossiliferous levels of the Sarmiento Formation have yielded one of the most remarkable records of terrestrial mammals of the continent, profusely studied since the end of the 19^th century (Ameghino, 1895, 1897, 1902; Simpson, 1948, 1967a; Madden et al., 2010b).

In Chile, Paleogene mammals have been described from late Eocene to late Oligocene volcaniclastic deposits along the Andean range of the central zone of the country (Flynn et al., 2005, 2012; Charrier et al., 2015, 2024), including those allowing for the recognition of the early Oligocene Tinguirirican SALMA (Flynn et al., 2003). This notable faunal assemblage have become a relevant component for the understanding of the evolutionary and ecological transformations of South American mammals during the global-scale Eocene-Oligocene environmental transition (Croft et al., 2008; Goin et al., 2010, 2012b; Buffan et al., 2025). Recently, Bostelmann et al. (2017, 2021) reported the discovery of middle Eocene mammals in the Aysén Region, Patagonia, including at least seven species representing three different orders. This new fossil assemblage, in turn, represents the oldest therians known in Chile, opening a promising research avenue for the study of the austral zoogeography and its relationship with climatic reconfigurations, and biotic responses associated with the Andean orogeny (Pascual et al., 1996; Goin et al., 2012a).

During the last 15 years, paleontological and stratigraphic work carried out in Magallanes, in the Chilean Patagonia, has allowed us to recover hundreds of new fossils, largely expanding our understanding of the evolution in the austral ecosystems (Bostelmann et al., 2013, 2022). Sierra Baguales, located in the northeastern corner of the Última Esperanza Province, next to the Chilean-Argentine border, is one of the most relevant and best-studied areas along this region (Fig. 1). The homonymous mountain range made of eroded Miocene basaltic flows and intercalated volcaniclastic deposits, unconformably overlies a thick stratigraphic succession composed of several lithostratigraphic units, ranging from the Late Cretaceous to the Early Miocene (Gutiérrez et al., 2017; Ugalde et al., 2018), hosting one of the largest, diverse, and best-preserved fossiliferous concentrations in Chile (Otero et al., 2013; Gutiérrez et al., 2019). Previous fieldwork in Neogene outcrops of Sierra Baguales recovered an abundant record of fossil vertebrates, especially mammals (Marshall and Salinas, 1990; Bostelmann et al., 2013). However, Paleogene deposits, composed almost entirely by middle Eocene marginal marine facies (Gutiérrez et al., 2017), deltaic systems (Le Roux et al., 2010; Morales, 2020; Morales et al., 2022), and estuarine complexes (Alarcón, 2020; Alarcón et al., 2023), have only yielded plants, invertebrates, fishes, and sauropsids remains (Otero et al., 2012, 2013; Alarcón et al., 2022). The recent discovery of mammals belonging to both marine and continental taxa (Bostelmann et al., 2022) adds a new Eocene local fossil mastofauna for Chile, contributing to the paleontological knowledge of a period that is barely known on the westernmost flank of the Patagonian foreland.

Fig. 1. Geographic context and study area. A. Map of South America demarking in inset B central and southern Patagonia. B. Detail of the localities (yellow circles) in which Albertogaudrya unica has been previously recorded in central Patagonia. The black square denotes Sierra Baguales that is detailed in inset C. Main cities are denoted by purple circles. C. Geographic map of Sierra Baguales, Magallanes, Chile, and detailed study area (red square) expanded in figure 2. Key cities are shown as purple circles. The hillshade basemap was obtained from the free repository of ArcGIS Pro 3.1, built over the ALOS PALSAR shuttle mission, the GeoEYE Satellite constellation and the USGS database.

In this communication we present a detailed description of the first Paleogene fossil mammal of Magallanes, briefly introduced by Bostelmann et al. (2022). The fossil, consisting of an isolated lower molar, is assigned to Albertogaudrya unica Ameghino 1901, a medium-sized member of the order Astrapotheria, previously recorded in localities of central Patagonia in Argentina and Chile (Ameghino, 1901; Simpson, 1967a; Bostelmann et al., 2021). Astrapotheres were a particular clade of mid-size to gigantic South American native ungulates (SANUs), first recorded during the earliest Eocene Itaboarian/Riochican SALMAs (Paula Couto, 1963; Soria, 1988; Croft et al., 2020) and becoming extinct after the late Middle Miocene Laventan SALMA (Johnson and Madden, 1997; Vallejo-Pareja et al., 2015). Their particular skeleton with graviportal rhino-like proportions in the larger forms, coupled with the development of a small proboscis and hypertrophied canines, undisputedly makes them one of the most bizarre groups of endemic mammals of the continent (Scott, 1928; Riggs, 1935; Croft et al., 2020). By middle Eocene times, astrapotheres attained high diversity, with coexistence of large-sized taxa with derived characters like Astraponotus or Isolophodon (Kramarz et al., 2010; Kramarz and Bond, 2013) and smaller, basal ones, like Trigonostylops and Tetragonostylops (Simpson, 1933; Soria, 1982; Kramarz et al., 2019a). The Patagonian endemic Albertogaudrya unica was one of these primitive species, constituting the largest of the ancestral, brachydont forms, and certainly, an enigmatic taxon from which little is known other than its general dentition (Ameghino, 1901, 1904; Simpson, 1967a).

2. Materials and methods

The fossil was collected as surface material over a deflationary exposure at Loma Tiburón Locality 2, a north-south directed flat area south of the Bandurrias River valley (Fig. 2A). The recovered vertebrate assemblage included a large set of chondrichthyan teeth, fish scales, and other tetrapod remains (Otero et al., 2013; Alarcón et al., 2022). Mammals are represented by a few dental pieces, including a lower molar, which is the focus of this communication, and large vertebral centra of cetaceans, all presently under investigation (Bostelmann et al., 2022).

Fig. 2. A. Panoramic view towards the northeast of Loma Tiburón Locality 2, with eroded outcrops of the Upper Member of the Río Turbio Formation. The white star marks the location where FMHN.PV.850 was collected. Photograph courtesy of José Luis Oyarzún Barría. B. Geologic map of the study area in Sierra Baguales with discussed locations, dates, and geographic references. The Purple bar denotes the Chorrillo Jabón stratigraphic section of Alarcón (2020). Yellow boxes depict sample locations of George et al. (2020) with published detrital zircon U-Pb ages in the area, expressed as weighted maximum depositional ages MDAs.

Observations and detailed descriptions of the tooth were performed using a Wild M5 Optical magnifier, while all measurements were taken using a digital Ubermann Vernier caliper, with a ±0.02 mm precision. Images of the fossil were taken at the photo studio facilities of the Millennium Nucleus Early Evolutionary Transitions of Mammals (EVOTEM) in Santiago, using a Nikon Z2 camera with a 50 mm Nikkor Z MC lens, mounted in a Kaiser rs1 Fotostand.

Chronostratigraphic units followed Gradstein et al. (2020), and the International Chronostratigraphic Chart on its latest version v2024/12 (updated from Cohen et al., 2013). Aubry et al. (2022) nomenclatural recommendations were applied for the Neogene subseries. Biostratigraphic and biochronologic units were recognized following the reasoning exposed in Lucas (2025). Temporal boundaries, extensions, and formal denominations of the biochronological units (SALMAs) followed the initial proposals of Simpson (1940), Pascual et al. (1965) and Pascual and Odreman Rivas (1971), amended and complemented by the works of Kay et al. (1999), Gelfo et al. (2009b), Tejedor et al. (2009), Madden et al. (2010b), Raigemborn et al. (2010), Ré et al. (2010), Dunn et al. (2013), Woodbourne et al. (2014), Clyde et al. (2014), Krause et al. (2017), and Gosses et al. (2021).

The studied specimen is housed in the vertebrate paleontology collection of the Fundación Museo de Historia Natural de Puerto Natales, at Puerto Natales, Magallanes, Chile, and was collected under the authorization provided by Ord. 4706/2010 of the National Monuments Council (Consejo de Monumentos Nacionales) of Chile, as part of the research activities conducted during the development of the Anillo de Ciencia Antártica ACT-105 project.

2.1. Taxonomic assignments, dental terminology, and phylogenetic placement of Astrapotheria

Detailed morphological descriptions of the dentition, phylogenetic relationships, and current taxonomic assignments at the generic/specific level within Astrapotheria, followed the works of Scott (1928, 1937); Kraglievich (1928), Paula-Couto (1952, 1963, 1978), Simpson (1957, 1967a), Carbajal et al. (1977), Soria (1982, 1984, 1988), Soria and Powell (1982), Soria and Bond (1984), and Johnson and Madden (1997), which were reviewed, emended and/or expanded by Kramarz and Bond (2008, 2009, 2011, 2013), Kramarz (2009), Kramarz et al. (2010, 2017, 2019a,b), Bond et al. (2011), and MacPhee et al. (2021). Nevertheless, dental homologies for structures like crests/cristids and lophs/lophids, used the recommendations of Gelfo (2024). In this sense, the anterior crescent, as mentioned or recognized in previous works (i.e., Bond et al., 2011; Kramarz and Bond, 2009), was here defined as the structure formed by the combination of the paralophid, protoconid, protolophid, and metaconid, while the posterior crescent is understood as the structure formed by the merging of the cristid obliqua, hypoconid, hypoconulid, and the entoconid (Gelfo, 2024). The general orientation of the dental elements followed the proposal of Smith and Dodson (2003), while wear facets were described and named using the terminology of the Modular Wear Facet Nomenclature system proposed by Shultz et al. (2018).

The Welker et al. (2015) definition of Panperissodactyla was followed here, recognizing the possible inclusion of the order Astrapotheria within this presently unranked group, as part of the Boreoeutheria radiation (for a discussion on this topic, see also Kramarz and MacPhee, 2023).

Specimens utilized for comparison were first-hand observed by E. Bostelmann and/or J. Gelfo in different collections of Argentina, Chile, Perú, Brazil, and the United States of America, or studied from figures in selected publications and high-quality photographs provided by colleagues. A detailed list of all the revised specimens is presented in the Appendix.

2.2. Quotation marks

Following international uses and recommendations, quotation marks were employed in this paper to denote informal stratigraphic units, unsolved phylogenetic status, or non-monophyletic taxa.

Institutional Abbreviations: AMNH, American Museum of Natural History, New York, United States of America; DGM, Divisao de Geologia e Mineralogia do Departamento Nacional da Produção Mineral, Brazil; FMHN.PV., colección de paleontología de vertebrados, Fundación Museo de Historia Natural de Última Esperanza, Puerto Natales, Chile; IAA-Pv, repositorio Antártico de Colecciones Paleontológicas y Geológicas, Instituto Antártico Argentino, Colección Paleovertebrados, Argentina; MACN-A and MACN-Pv, Ameghino and Vertebrate Paleontology collections, respectively, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires, Argentina; MHN, Tournouër Collection of the Muséum National d’Histoire Naturelle, Paris, France; MNHN, colección de paleontología del Museo Nacional de Historia Natural, Montevideo, Uruguay; MLP-PV, División Paleontología de Vertebrados, Museo de La Plata, La Plata, Argentina; MPEF PV, palaeovertebrate collection, Museo Paleontológico Egidio Feruglio, Trelew, Argentina; MURAY.PV., colección de paleontología de vertebrados, Museo Regional de Aysén, Coyhaique, Chile; YPM PU, Yale Peabody Museum, Princeton University Collection, New Haven, United States of America.

Anatomical abbreviations: co: cristid obliqua, cus: cuspule, enf: entoflexid, ent: entoconid, hd: hypoconid, hd-db: hypoconid distobuccal wear facet, hd-mb: hypoconid mesiobuccal wear facet, hyf: hypoflexid, lac: labial cingulid, m: lower molar, md-d: metaconid distal wear facet, me: metaconid, pld: paralophid, pld-cr: paralophid lingual cristid, pld-mb: paralophid mesiobuccal wear facet, pr: protoconid, pr-db: protoconid distobuccal wear facet, prld: protolophid, wle: wrinkled lingual enamel.

Other abbreviations: CA-ID-TIMS: Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry. FA: Facies association. Ma: megaannum, million years ago. MDA/s: Maximum depositional age/s. NWA: Northwestern Argentina. LA-ICP-MS: Laser Ablation Inductively Coupled Plasma Mass Spectrometry. PCM: patterning cascade model of tooth morphogenesis. SANU/s: South American native ungulate/s. SALMA/s: South American Land Mammal Age/s. YDZA/s: Youngest detrital zircon age/s.

3. Geological setting

Sierra Baguales contains one of the most extensive and continuous succession of Cenozoic epiclastic deposits in Última Esperanza, making it an important geologic and fossiliferous area in southern Chile (Otero et al., 2013; Bostelmann et al., 2013; Fig. 1). Stratigraphic units that crop out include alternations of shallow marine, deltaic, estuarine, and continental fluvial successions, with a total vertical thickness up to 600 m, across the Baguales and Bandurrias drainage basins. The geological units occur on the eastern flank of a wide regional monocline in the external domain of the Patagonian Andes fold-and-thrust belt (Ghiglione et al., 2009, 2021). Temporally, these units range from the Late Cretaceous (Maastrichtian) marginal marine facies of the Dorotea Formation (Manríquez et al., 2019; Morales, 2020; Alée et al., 2023), through the Early Miocene volcanic and volcaniclastic deposits of the “Sierra Baguales Formation” (Bostelmann et al., 2013; Gutiérrez et al., 2017; Münthener, com. pers., 2025). Early Pleistocene volcanic necks and lava flows (Münthener, com. pers., 2025) here termed “Donoso Basalts”, other subvolcanic bodies of unknown age, and poorly consolidated Quaternary fluvial and fluvio-glacial deposits, unconformably overlie or intrude the whole sedimentary succession.

Paleogene marginal marine conditions are represented by the Lutetian to Priabonian/Chattian? Río Turbio Formation, previously recognized as the Man Aike Formation in the area (Bostelmann et al., 2013; Gutiérrez et al., 2017). This unit was originally named as “Río Turbio Beds” in the homonymous Argentine coal mining district (Feruglio, 1938), and formally defined and described by Hünicken (1955), who gave a modern definition to the unit (Malumián and Caramés, 1997; Malumián et al., 2000). Chilean exposures in Sierra Baguales (Morales, 2020) represent the northern geographical continuation of the type and auxiliary sections exposed in the localities of Río Guillermo, Route 40, and Río Turbio coal mine district (Hünicken, 1955; Rodríguez Raising, 2010).

The Río Turbio Formation is mainly composed of epiclastic deposits and is formally divided into a Lower Member dominated by coarse-grained levels, interpreted as deltaic deposits (Morales, 2020; Morales et al., 2022), and an Upper Member, dominated by fine-grained levels, interpreted as estuarine deposits (Gutiérrez et al., 2017; Alarcón, 2020; Alarcón et al., 2022). Both members reflect tectonic and eustatic controls (Alarcón et al., 2023; Morales et al., 2023) during the middle and late Eocene, through the latest Oligocene. Exposures of the Upper Member of the Río Turbio Formation in Sierra Baguales crop out in a northwest-southeast trend along the Baguales river valley and adjacent tributaries, reaching a maximum thickness of up to 92 m (Fig. 3; Alarcón et al., 2023). The unit presents lateral changes in the facies between the northwestern and southeastern exposures, reaching its maximum thickness southeast of Sierra Baguales, in Argentina (i.e., Cancha Carrera, Río Guillermo, Route 40, and Río Turbio village exposures; Rodríguez Raising, 2010; Pearson et al., 2013; Albano et al., 2023). Facies analysis suggests shallow marine to transitional environments developed in a coastal plain, interrupted by estuary mouths (Rodríguez Raising, 2010; Pearson et al., 2013; Ugalde et al., 2018; Alarcón, 2020).

Fig. 3. Stratigraphic section of the Upper Member of the Río Turbio Formation at Chorrillo Jabón locality. The section lies 2 kilometers to the northwest of Loma Tiburón Locality 2. Black silhouettes mark the approximate stratigraphic position of fossil vertebrates collected as surface materials. Colors in the stratigraphic logs represent the fresh-rock original tones. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.

Fossiliferous exposures in Sierra Baguales occur in abrupt cliffs and gullies, interrupted by flat morphologies and ledges, that create deflation planes, where abundant surface-exposed vertebrate fossils are found. The fossil material here described was collected at Loma Tiburón Locality 2 (50°43’41” S; 72°28’17” W), one of these active erosive surfaces forming a north-oriented valley that widens to the northeast, generating a slightly inclined plane, following the dip of the regional monocline (Fig. 2A, B). The erosional nature of this surface limits the precise placement of the tooth in the local stratigraphic scheme. However, the approximate stratigraphic position was constrained by direct correlation with two fossiliferous sections immediately north and south of Loma Tiburón Locality 2; the Chorrillo Jabón (50°42’32’’ S; 72°28’28” W) and El Encierro (50°44’6” S; 72°28’24” W) stratigraphic logs (Fig. 2B). In Chorrillo Jabón, sixteen lithofacies were described and grouped into eight facies associations (FA1-8; Alarcón, 2020), interpreted as marginal-marine gravel bars and sandbars (FA1 and FA4), tidal channels (FA3 and FA6) and creeks (FA2), intertidal flats (FA5), fluvial braided deposits alternating with overbank fines (FA7) and foreshore deposits (FA8), originated on a mainly tide-dominated estuary (Alarcón, 2020; Alarcón et al., 2022, 2023). Fossil vertebrates were collected in the central portion of the succession, exclusively from massive, fine-grained sandstones and siltstones, grading to medium-grained heterolithic sandstones with subordinated cross-bedded sandstones to the top, with a maximum thickness of 32 m (Fig. 3). These beds were interpreted as tidal channels and creeks of the FA2 and FA3 (Alarcón, 2020; Alarcón et al., 2022; Bostelmann et al., 2022). The associated fossil content includes a diverse array of chondrichthyans, consisting of thousands isolated shark and ray teeth assigned to 19 species (Otero et al.,  2013); incomplete plates and postcranial fragments of large sized Testudines; and teeth and steoderms assigned to Mesoeucrocodylia indet. (Otero et al., 2012). In nearby localities, FA’s 2 and 3 also bear bony fish scale agglomerates, disarticulated vertebrae, and bone fragments of at least two different species of Osteichthyes; incomplete bone remains of a large member of Sphenisciformes; and robust cetacean vertebral centra, among others (Bostelmann et al., 2022; Fig. 3). Biochronological constraints, based on the chondrichthyo-fauna from Loma Tiburón localities 1, 2, and Chorrillo Jabón, support a middle to late Eocene age (Bartonian-Priabonian) for the fossiliferous beds (Otero et al., 2013; Alarcón et al., 2022).

Geochronologic control of the Upper Member of the Río Turbio Formation is also provided by several U-Pb ages on detrital zircons in the fossiliferous exposures and at the base and top of the unit in neighboring localities (Fig. 2B). George et al. (2020) provide a U-Pb detrital zircon maximum depositional age (MDA) of 40.47±0.26 Ma, from a medium to coarse-grained sandstone ~25 m below the contact between the lower and upper members of the Río Turbio Formation, ~400 m west from the El Encierro site. Ages for the base of the Lower Member were also provided by George et al. (2020), from samples collected at the Tetas de Las Chinas hill and the El Encierro site (Fig. 2B). Further south, at the Río Guillermo Valley (Argentina), Fosdick et al. (2020) obtained, by the same method, MDAs of 36.6±0.3 and 35.4±0.2 Ma for the upper third of the Upper Member, coincident with a middle to late Eocene biozonation suggested by microfossils, as described in Malumián et al. (2000), Guerstein et al. (2014), González Estebenet et al. (2017), and Amenábar et al. (2022). Recently, Albano et al. (2023) also obtained a middle Eocene age for the top of the Lower Member at Arroyo Picana, near the international border, based on a U-Pb detrital zircon sample with an MDA of 41.0±3.0 Ma (YDZA of 40.0±1.0 Ma). The sum of all the available information suggests that the Upper Member of the Río Turbio Formation started its deposition immediately after 40 Ma, during the Bartonian age.

4. Results

Systematic paleontology
Mammalia Linnaeus, 1758
Eutheria Huxley, 1880
?Panperissodactyla Welker et al., 2015
Astrapotheria Lydekker, 1894
Albertogaudrya Ameghino, 1901

Type species: Albertogaudrya unica Ameghino, 1901.

Referred species: The type, and A.? carahuasensis Carbajal et al., 1977.

Comments: The known record of Albertogaudrya is restricted to continental deposits from the middle Eocene of Patagonia, in the Chubut Province of Argentina (Ameghino, 1904, 1906; Simpson, 1967a; Cifelli, 1985; Marshall et al., 1986; Kramarz et al., 2019a; Folino et al., 2024), and the Aysén (Bostelmann et al., 2021) and Magallanes regions of Chile (this work). Simpson's monographic review presented a detailed account of the many taxonomic entities (species and genus) erected by Florentino Ameghino and Santiago Roth that were synonymized by him under A. unica, at that time the sole species of the genus (Simpson, 1967a). After this comprehensive work, only a few mentions of this species have been presented in the literature (i.e., Cifelli, 1985; Marshall et al., 1986; Kramarz et al., 2019a), most of them devoted to anatomical comparisons or zoogeographic analyses. Cifelli (1985) listed Albertogaudrya in his account of the faunistic assemblage of Cañadón Vaca (Cañadón Vaca Member of the Sarmiento Formation, Vacan subage of the Casamayoran SALMA) in Chubut Province, Argentina, but Kramarz et al. (2019a) expressed doubts on the validity of this assignment based on the uncertainty regarding the remains that would have sustained the identification.

On the other hand, A.? carahuasensis is known solely from its holotype specimen (CNS-PV 10000), composed of a poorly preserved mandible with associated incomplete dentition, collected from middle Eocene continental levels of the Lower Lumbrera Formation in Salta Province, NWA region (Carbajal et al., 1977). Although the fragmentary condition of the type specimen limits comparisons, important differences in the lower dentition (p4-m1) are discernable, including the absence of cingulids in lower molars, a straighter crista obliqua and hypolophid, a narrow and elongated trigonid in m1, and less-marked flexids in the molars, suggesting that this taxon may not be congeneric with the Patagonian form, as previously discussed by others (Carbajal et al., 1977; Kramarz et al., 2019a). Although these differences may support the designation of a new genus for the type material of A? carahuasensis, the advanced tooth wear in CNS-PV 10000, the absence of better-preserved materials with informative dental characters, and our lack of a direct examination of the type material, make the recognition of a new generic entity inadvisable. López (1997) and Fernández et al. (2021) reported indeterminate Eocene astrapotherid dental remains from the NWA region that might resemble the size of Albertogaudrya, but their fragmentary nature and/or poor preservation prevent any generic assignment.

Albertogaudrya unica Ameghino, 1901
(Figs. 4-5).

Geographic distribution, stratigraphic provenance, and age: Unknown localities, Chubut Province, Argentina (lectotype MACN-A 12000 and MACN-A 12001; Simpson, 1967a). Tapera de López, localities III and VI (=Rinconada de Lopez of Simpson, 1967a, or Tapera de Lopez of Cifelli, 1985), Chubut Province, Argentina. Sarmiento Formation, middle Eocene, Bartonian age, Barrancan subage of the Casamayoran SALMA (Simpson, 1967a; Marshall et al., 1986). Gran Barranca (including localities traditionally recognized as “Colhué-Huapi”, “Barranca Sud of Lake Colhué-Huapí”, and “Cerro Negro” of Tournouër, 1903 collections, see Simpson, 1964), Chubut Province, Argentina (Ameghino, 1902; Simpson, 1967a; Marshall et al., 1986, Fig. 1). Gran Barranca Member of the Sarmiento Formation, middle Eocene, Bartonian age, Barrancan subage of the Casamayoran SALMA. Cerro Blanco and Cerro del Humo (=“Colhué-Huapí Norte” of  Ameghino, 1901, and “Cretáceo Superior Lago Muster” of Roth, 1904, collections), Chubut Province, Argentina. Sarmiento Formation, middle or late Eocene, Casamayoran or Mustersan? SALMAs (Simpson, 1967b; Bond and Deschamps, 2010). Cañadón Pelado, Chubut Province, Argentina. “Mustersan levels”, middle or late Eocene, Priabonian? age, Mustersan SALMA (Folino et al., 2024). Alto Río Simpson, Estancia La Frontera, Aysén Region, Chile. “Estancia La Frontera beds”, middle Eocene, Bartonian age, Barrancan subage of the Casamayoran SALMA (Bostelmann et al., 2021, 2024).

Fig. 4. Albertogaudrya unica Ameghino, 1901. FMHN.PV.850, incomplete left m1 or m2. A. Occlusal view. B. Labial view. C. Antero-lateral view. D. Lingual view. Abbreviations: co: cristid obliqua, enf: entoflexid, ent: entoconid, hd: hypoconid, hd-mb: hypoconid mesiobuccal wear facet, hd-db: hypoconid distobuccal wear facet, hyf: hypoflexid,  lac: labial cingulid, me: metaconid, md-d: metaconid distal wear facet, pld: paralophid, pld-cr: paralophid lingual crest, pld-mb: paralophid mesiobuccal wear facet, pr: protoconid, pr-db: protoconid distobuccal wear facet, prld: protolophid. Scale bar: 10 mm.

Comments: Although most researchers consider A. unica a representative taxon of the Barrancan subage of the Casamayoran SALMA (Simpson 1967a; Kramarz et al., 2019a), it is difficult to rule out its potential occurrence in younger, Mustersan SALMA deposits. This ambiguity is related to the uncertainties on the provenance of historical remains collected at the end of the 19^th century, in localities with imprecise geographic and lithostratigraphic information. These include Cerro del Humo (=“Colhué-Huapí Norte”) and probably Cerro Blanco, near Gran Barranca, where levels carrying both Barrancan and Mustersan faunal assemblages have been recognized (Simpson, 1967b; Bond and Deschamps, 2010). New collections and a detailed study of the fossilization and preservation of the historical specimens (i.e., MACN-A 12002, 12014) could help clarify whether Albertogaudrya unica should also be recognized as present in the Mustersan SALMA.

Referred specimen: FMHN.PV.850; left m1 or m2 with complete trigonid and partial talonid (Fig. 4A-D).

Locality and stratigraphic occurrence: western slope of Loma Tiburón Locality 2, Sierra Baguales, Última Esperanza Province, Magallanes, Chile. Upper Member of the Río Turbio Formation, middle Eocene, Bartonian age.

Description and comparisons: The tooth is brachydont (low crowned) and bi-crescentic, preserving the complete trigonid and half of the talonid (Fig. 4). In size and general outlook it matches the right m1 and m2 of MACN-A 12001, Albertogaudrya unica (Table 1), being larger than the homologous tooth of Eoastrapostylops, Antarctodon, Trigonostylops, Tetragonostylops, Maddenia, and Comahuetherium (inferred); and smaller and less hypsodont than the molars of Scaglia (inferred), Astraponotus, Isolophodon, Parastrapotherium, and all known astrapotheriine and uruguaytheriine astrapotheres (Johnson and Madden, 1997; Kramarz and Bond,  2008, 2013, 2019a, 2021; Gelfo, 2024). The occlusal surface shows advanced wear, with wide and well-formed lophids and flattened cusps, attesting to its adult but non-senile condition. The trigonid is almost complete and the talonid lacks its posterior portion, preserving just half of both the entoconid and hypoconid (Fig. 4). The roots are missing, although their contact with the crown is discernible.

The general contour of the preserved portion of the tooth is sub-rectangular with its base constricted by the labial hipoflexid and the lingual flexid, which separates the trigonid from a larger talonid (Fig. 4A). The hypoflexid forms a deep vertical depression posterior to the protoconid and mesially to the level of the metaconid, while the lingual flexid is more pronounced, running straight from the lingual side of the tooth and then turning obliquely in anterolabial direction (Fig. 4A). This configuration separates the distal wall of the metaconid from the mesial border of the entoconid, forming a widely open entoflexid (Figs. 4). As in Albertogaudrya and Maddenia, a marked labial cingulid is present at the base of the tooth, running uninterruptedly along the border and reaching at least the distal end of the hypoconid (Fig. 4B). The cingulid rises at the mesial edge, bordering the labial wall of the paralophid, (Fig. 4B, C). Similar to Albertogaudrya, Astraponotus, and Maddenia, a mesiolingual cristid (pld-cr; Fig. 4) is present, enclosing the trigonid basin as an extension of the paralophid. The lingual side of the tooth is uniform, lacking an evident lingual cingulid as in Albertogaudrya, Antarctodon, Tetragonostylops, and Trigonostylops, and distinct from Astraponotus, Isolophodon, Maddenia, and more advanced astrapotherines, which have well-developed lingual cingulids (Simpson, 1967a; Kramarz and Bond, 2009, 2013).

As is common in astrapotheriids, the posterior crescent was probably large and distally extended, while the anterior crescent is less developed, bearing a reduced paralophid, as in Astraponotus. In lower molars of Trigonostylops (MACN-A 10627, 12505), Tetragonostylops (DGM 309-M, 263-M), Maddenia (MPEF PV 7738)and Isolophodon (MPEF PV 7475, MLP-PV 12-2139), the paralophid is shorter or absent (Simpson, 1967a; Kramarz and Bond, 2009, 2013), while in Antarctodon (MLP-PV 67-II-27-168, IAA Pv 826), Parastrapotherium (MACN-A 52-503, 52-506, 52-604), Astrapotherium (MLP-PV 12-94, MACN-A 3207, 3209, 3210, 3274-3278), Astrapothericulus (MACN-A 52-410, 52-411, 52-605), and the uruguaytheriines it is larger (Bond et al., 2011; Kramarz and Bond, 2008; Kramarz et al., 2019b; Gelfo, 2024). Parallel Hunter-Schreger vertical bands are visible at the occlusal surface of the enamel labial and lingual walls, as recognized in other members of Astrapotheria (Fig. 4A-C).

The trigonid is short and wide, and higher than the preserved portion of the talonid. The metaconid is massive, forming a wide conical cusp that is higher and posteriorly positioned with respect to the protoconid. A broad, oblique, and distolingually directed protolophid connects these two cusps (Fig. 4A), as in Tetragonostylops, Albertogaudrya, Astraponotus, Isolophodon, Astrapotherium, and Astrapothericulus; and distinct from Antarctodon, Trigonostylops, Maddenia, and Parastrapotherium, in which the protolophid is less oblique. The protoconid is also massive and conical, although less robust than the metaconid, with labial walls that form an acute angle that gives a marked V-shape to the anterior crescent, similar to the observed condition in Antarctodon (IAA Pv 826) and some specimens of Tetragonostylops (i.e., DCM 309-M). Compared with Trigonostylops, Tetragonostylops, and Antarctodon, the combination of the extended paralophid and the labial angularity of the protoconid wall forms a particular morphology exclusive to Albertogaudrya unica (Figs. 4A), differing from the remaining members of the order which show protoconids with a rounded labial wall in their molars. The trigonid basin is well-defined and shallow, mostly enclosed by the protoconid and the transversally directed anterior crescent formed by the paralophid and protolophid. On the mesial face of the trigonid, the enamel is broken, exposing a portion of dentin from the paralophid, revealing the area of contact with the distal border of the anterior tooth. The basin opens lingually between the metaconid and the paralophid crest, distinct from homologous known molars of Trigonostylops, Tetragonostylops, Albertogaudrya, and Islopohodon, which present mesiolingually directed open basin as a result of the lack or reduced paralophid and an abbreviated cristid (but see discussion).

Wear facets are present on the labial wall of the protoconid and paralophid, similar to the ones described by Kramarz and Bond (2009) for Maddenia. An obliquely oriented mesiolabial facet (pld-mb) is extended mesial to the protoconid, affecting the labial wall of the paralophid and obliterating the upper border of the labial cingulid (Fig. 4B). A second wear facet distal to the protoconid cusp (pr-db), forms a small but deep distolabially directed notch. The development of these facets, particularly the pld-mb, certainly accentuates the angular shape of the protoconid wall (Fig. 4C). On the lingual side of the tooth, a wear facet occurs in the metaconid distal wall (md-d), affecting both the enamel band and dentine. It runs obliquely through the distal wall of the protolophid, simultaneously obliterating and smoothing the internal (lingual) wall of the cristid obliqua (Figs. 4A, C). This wear facet also extends into the enamel layer of the flexid, putting it in contact with the distal edge of the protolophid (Fig. 4C).

The talonid presents a wide, short, and rounded crescent as in Tetragonostylops, Albertogaudrya, Maddenia, and Astraponotus, and more extended than in Antarctodon (Bond et al., 2011; Gelfo, 2024). The tooth is broken through the entoconid and hypoconid, leaving only the anterior half of both cusps and completely missing the distal wall. Despite this, it is possible to infer that the entoconid probably formed a robust and wide cusp, slightly lower than the hypoconid, as in Albertogaudrya and Astraponotus. The mesial flank of the entoconid is somewhat straighter than that observed in the m1 and m2 of other specimens of Albertogaudrya (MACN-A 12001), likely due to more pronounced wear that significantly affects the talonid basin. The anterior part of the cristid obliqua meets the posterior edge of the protolophid, close to the inner side of the protoconid, as in most Astrapotheria. The talonid basin seems to be wide and shallow and opens lingually in a distal direction (Figs. 4A, C). Similar to the trigonid area, the most noticeable wear facets occur on the labial wall of the tooth. A mesiolabial and a distolabial wear facet are identified on the hypolophid. The first one (hd-mb; Fig. 4C) mainly affects a portion of the hypoconid and the cristid obliqua. In contrast to pld-mb, it does not reach the labial cingulid ending before its contact, closing mesially and following the invagination of the hypoflexid. The second facet, hd-db, is distally interrupted by the tooth fracture but the preserved extension reveals it as shallower than hd-mb, although it seems to widen distally (Figs. 4B, C).

5. Discussion

Although Magellanic Cenozoic fossil mammals were first discovered in 1895 (Roth 1899; Martinic, 1996), their study has been highly discontinuous and heterogeneous, with knowledge still far from being complete or even satisfactory. Currently almost all published records refer to late Pleistocene species and faunal associations in archaeo-paleontological contexts (Massone, 2004; Borrero, 2009), or brief reports of Early Miocene specimens (Hemmer, 1935; Simpson, 1941b; Marshall and Salinas, 1990; see Bostelmann et al., 2013 for a more ample faunistic description). The discovery of Albertogaudrya unica in Sierra Baguales partially fills a temporal gap between the Neogene and younger Magellanic mastofaunas and the recently discovered latest Cretaceous Las Chinas non-therian mammals (Goin et al., 2020; Martinelli et al., 2021; Püschel et al., 2025), bringing opportunities for the regional correlation during important geobiotic changes. Together with the Aysén discoveries (Bostelmann et al., 2021), the recognition of A. unica in Sierra Baguales also represents the oldest occurrence of the order Astrapotheria in Chile, raising the known Chilean diversity of SANUs and contributing to the knowledge on the zoogeography of the austral portion of the continent. South American Paleogene mammals are indeed poorly known south of 48° S, being the present record the first extended description of a terrestrial mammal in the Eocene shallow marine deposits of southern Patagonia and Tierra del Fuego (Malumián and Náñez, 2011; Pascual et al., 2002 see also Bostelmann et al., 2022, and Kaempfe et al., 2024). Although scarce, the occurrence of other dental elements belonging to a different species, currently under study, demonstrates that the estuarine facies of the Upper Member of the Río Turbio Formation in Sierra Baguales still holds unexplored potential for the search and recovery of continental mammals. However, the environmental and taphonomic settings involved in the formation of these deposits allow us to assume that, if present, these fossils will be scarce and fragmentary.

5.1. Taphonomic attributes of the fossil terrestrial vertebrates in shallow-marine and estuarine contexts

Detailed stratigraphic analyses and modes of preservation of the fossil materials need to be considered as fundamental elements when prospecting for inland mammals in marginal marine settings. Unlike previous records of Albertogaudrya recovered from fully continental environments, the Magellanic specimen was exhumed from marine transitional levels, interpreted as tidal creeks and channels belonging to an estuarine system (Alarcón et al., 2023). Other fossil vertebrates collected from these exposures in Sierra Baguales include a vast amount of fragmentary specimens. Preserved skeletal elements were found disarticulated, mostly formed by dental pieces and incomplete postcranial bones. Aquatic taxa are dominant, with fully marine and estuarine forms including thousands of chondrichthyan teeth, representing over 19 species (Otero et al., 2013; Garrido et al., 2024), scales and vertebral centra of bony fishes, fragmented teeth and miscellaneous bones of crocodilians, and few postcranial elements of turtles, birds, and cetaceans (Alarcón et al., 2022; Bostelmann et al., 2022). While almost all these fossils have been recovered as surface elements in wind-deflection plains lacking a defined lithological context, the occurrence of few in situ materials suggests that the bearing beds were part of high to medium energy settings, grouped in the FA3 (Fig. 3). Continental vertebrates are extremely infrequent in the assemblage, represented only by few isolated mammal teeth and maxillary fragments, being the one of A. unica here presented the first to be described in detail.

From the perspective of preservational attributes, the low degree of rounding and abrasion, the absence of scratches, and the good general condition of the dental crown, suggest a relatively nearby continental source for the origin of the fossil. Subsequent disarticulation and secondary transport occurred during a relatively short period, likely from marginal fluvial systems adjacent to the estuarine complex. The absence of other bone remains assignable to the same animal suggests that the disarticulation process occurred before the final transport and deposition of the remains. In this context, the presence of a continental mammal in this coastal unit can be interpreted as an allochthonous or parautochthonous component, potentially originated at a short distance from the other vertebrate remains. The greater resistance of the dental enamel to transport and abrasion would have favored the displacement of this tooth from the continental to the transitional environment. Similar situations have been reported in other localities with shallow marine-coastal systems, where continental mammals are equally scarce and represented mostly by isolated incomplete dental pieces or mandibles as in the Argentine deposits of the Banco Negro Inferior bed of the Salamanca Formation at Punta Peligro (Bonaparte et al., 1993; Comer et al., 2015), the Danian levels of the Lefipán Formation at Paso del Sapo (Goin et al., 2006), or the Cucullaea 1 Member of the La Meseta Formation on Seymour Island, Antarctica (Gelfo et al., 2009a, 2019).

5.2. Dental variability and accessory structures in the lower molars of Albertogaudrya unica

Dental elements usually form the bulk of the mammalian fossil record, and astrapotheres are not the exception. Nevertheless, lower molars of Albertogaudrya unica are certainly infrequent, and inadequately published or figured in the past scientific literature. The detailed description of FMHN.PV.850 allows a preliminary analysis of the variability observed in the other, few well-preserved lower molariforms assigned to this genus, which includes a handful of isolated teeth, and a pair of more complete and associated premolar-molar series (Simpson, 1967a; Appendix).

MACN-A 12001 represents the best-preserved known lower cheek teeth of A. unica, and includes both horizontal rami with left p4 and m3, and right p2-m3 (Fig. 5). FMHN.PV.850 shows the general size and proportions of the right m1 and m2 (Fig. 5B, E), but is lower-crowned due to an advanced occlusal wear, characterized by less prominent cusps (especially the protoconid), and an extended paralophid (Fig. 5A, B, C). The wear pattern of the Chilean tooth is indeed intermediary between the m1 and m2 of MACN-A 12001 (Figs. 5D, E), being closer to the condition observed in the p4 (Fig. 5C). The conspicuous pr-db notch of FMHN.PV.850 is nevertheless absent in all other lower teeth known of the genus (Fig. 5B-E, Appendix).

Fig. 5. Compared teeth of Albertogaudrya unica in occlusal view. A. FMHN.PV.850, left incomplete molar, Sierra Baguales, Magallanes. B. MACN-A 12001 (lectotype), inverted right m2. C. MACN-A 12001 (lectotype), inverted right p4. D. MACN-A 12001 (lectotype), lingual view of the right molar series, m1-m3. E. MACN-A 12001 (lectotype), inverted occlusal view of the right molar series, m1-m3. Anatomical abbreviations as in figure 4, and cus: cuspule, wle: wrinkled lingual enamel. Scale bars: 10 mm.

FMHN.PV.850 bears a cristid obliqua with continuity of the dentine to the protolophid. This connection is narrow, giving an acute angle termination to the cristid. In contrast, the m2 of MACN-A 12001 exhibits the anterior part of the cristid obliqua with extensive wear at the contact with the distal wall of the trigonid, although without a continuous connection between the dentin of the cristid obliqua and the protolophid (Fig. 5B). This, on the contrary, is observed in the m1 of the same specimen. These differences can be explained not only by differential wear among the different parts of the tooth but also by variations in the relative height of the protolophid and the cristid obliqua itself.

A notable morphological difference, which cannot be attributed either to an ontogenetic stage or differential occlusal wear pattern, involves the extension of the paralophid and its descending anterolingual crest. In the Chilean specimen, these structures reach the lingual wall of the tooth, flanking and closing off the mesial border of the trigonid basin (Fig. 5A). In the molars of MACN-A 12001, the lophids are shorter (especially on m2 and m3, Fig. 5B, E), and have an internally curved crest that descends into the mid anterior border of the teeth, allowing the opening of a wider, antero-lingually oriented, trigonid basin (Fig. 5B, C, E). This morphology is common in basal astrapotheres, which lack or have reduced paralophids, like Trigonostylops, Tetragonostylops, and Antarctodon (Simpson, 1967; Soria, 1988; Gelfo, 2024). Short paralophids have also been described in more derived forms, like Maddenia and Isolophodon (Kramarz and Bond, 2009, 2013). The extended paralophid of FNHM.PV.850 mirrors the condition present in Astraponotus and later, post-Oligocene forms, in which the structure runs along the complete anterior margin of the molars (Kramarz and Bond, 2008; Kramarz, 2009). These differences are interpreted as part of the intraspecific variability within Albertogaudrya unica, given that similar variations in the extent of the paralophid have also been documented in other members of the group (Simpson, 1967a).

The short paralophid of basal astrapotheres is sometimes accompanied by a small conical anterior cuspule. This structure is visible in the p4-m2 of MACN-A 12001, rising from the base of the paralophid crest, and facing the lingual wall of the tooth (Fig. 5B, C, D). In the Chilean specimen, this cusp seems to be absent (Fig. 5A), and no evidence of an early obliteration of it can be noted. Indeed, the uniform thickness and regular dimensions of the base at the lingual wall of the paralophid suggest the lack of this cuspule in FNHM.PV.850. Anterolingual cuspules have also been reported in some specimens of Trigonostylops and Tetragonostylops (Simpson, 1933; Paula Couto, 1963; Soria,1984), and more recently in Antarctodon (Bond et al., 2011; Gelfo, 2024). Simpson (1933) wondered if this rather unusual dental structure in Trigonostylops could represent the paraconid, a conjecture also suggested by Paula Couto (1952) for mesiolingual cuspules in lower molars of Tetragonostylops apthomasi. Using a non-cladistic approach, Soria (1988) considered this cuspule to be a neomorph structure, proposing the name “neoparaconid”, but Gelfo (2024) discussed this concept within a phylogenetic framework in Antarctodon sobrali, dismissing Soria´s (1988) suggestion. In MACN-A 12001, these small cuspules seem to be placed in the area in which a paraconid could be expected (anteriorly facing the metaconid, Figs. 5 B, C), supporting Simpson's (1933) and Paula Couto's (1952) conjectures for basal astrapotheres. This stands in accordance with the reported evolvability of mammalian dental cusps and development models of tooth morphogenesis (Hunter and Jernvall, 1995; Jernvall, 2000). An alternative explanation for the origin of this structure, which warrants further exploration using a larger sample, may involve a rudimentary, incipient, or aborted expression of the lingual cingulid. This hypothesis may be further supported by recognizing the paralophid crest and the anterior cuspule as marginal extensions of the precingulid; with both elements exhibiting symmetry relative to the position of the opposing mesiolabial cingulid. The latter, in turn, displays a slightly crenulated surface formed by irregular bony projections, which are also visible at the posterior portion of the lingual side in specimen MACN-A 12001, forming an incipient cingulid on the m2, and specially the m3 (wrinkled lingual enamel, Fig. 5D). This interpretation suggests that the developmental pathway of this anterior cuspule may parallel that of other accessory external structures like the lingual shelf or Carabelli's cusps in upper molars, extensively studied in hominid dental development (van Reenen and Reid, 1995; Ortiz et al., 2012, 2018).

Accessory dental structures like cuspules, crests, and tubercles have been described in lower and upper definitive molars of basal and derived astrapotheres, and upper deciduous premolars in Parastrapotherium, Astrapothericulus, and Astrapotherium (Kramarz, 2009). Individuals in these genera also display lower molariforms with tall bunoid accessory dental columns attached to the posterior wall of the protolophid, forming what Scott (1937) called “pillars” (Kramarz and Bond, 2008; Kramarz, 2009; Gelfo, 2024). Some upper deciduous premolars (and infrequently molars) in Astrapothericulus and Astrapotherium display accessory cuspules in the form of a small tubercle attached to the central valley or the labial wall of the crista, which can also be associated with a small crest (Kramarz, 2009). These accessory structures achieved taxonomic value in the past (Ameghino, 1904; Scott, 1937) and even Kramarz (2009), in his detailed revision of the genus Astrapothericulus, avoided the synonymy of Astrapothericulus emarginatus Ameghino 1904, based on the existence of the accessory tubercle in the type upper molar. He also suggested that the occurrence of the small ridge aligned with the protocone, metacone, and accessory cusp in the type material of A. peninsulatus (junior synonym of A. emarginatus sensu Kramarz, 2009) and other Astrapotheriidae, such as A. iheringi, could represent vestigial elements of the postprotocrista.

While differentially positioned, we speculate that the small cuspules observed in specimens of Albertogaudrya (and Trigonostylops and Tetragonostylops) may share a similar origin with other accessory cuspules, tubercles, and “pillars” observed in derived lineages. The expression of these accessory structures could represent vestigial elements formed following a general tooth development program, inhibited during most of the evolution of the group. As stated by others (Jernvall, 1995, 2000; Zanesco et al., 2019), we suggest that the formation and disposition of these structures likely represent an example of the patterning cascade model of tooth morphogenesis (PCM), associated with the initiation and inhibition of primary and secondary enamel knots, the spacing of cusps, and the timing of crown growth (Skinner et al., 2008; Ortiz et al., 2018). The expression of these accessory cusps could bring mechanical support for the expansion of the lophs and lophids, increasing the occlusal surface area while assisting in greater mechanical improvement in the contact with antagonistic teeth. Although intriguing, the mechanisms that produce accessory dental elements in astrapotheres and other SANUs, together with their functionality, homologies, and even recurrence within populations, are far from being properly understood. At the current state of knowledge, we can only hypothesize on the correspondence of these structures with dental development models mostly studied on mice, seals, primates, and fossil hominids (Ortiz et al., 2018; Chapple et al., 2024). In Astrapotheria, these accessory structures should be presently interpreted as intraspecific phenotypic expressions within lineages, with significant variability across individual dental loci. They may reflect dental morphogenesis at the enamel-dentine junction following the PCM, as suggested for other groups of mammals (Skinner et al., 2008; Salazar-Ciudad and Jernvall, 2010; Zanesco et al., 2019), facilitating modular responses for mechanical adaptation. Potential advantages of such responses might include enhanced occlusal surface area, improved masticatory efficiency, and increased dental resistance, as has been suggested for other structures like cingulids, and the Carabelli’s trait of Homo upper molars (Anderson et al., 2011; Fiorenza et al., 2020). Alternatively, accessory cusp development could represent deviations from the PCM, including the involvement of crest patterning during cusp morphogenesis, independent ameloblast responses during enamel deposition, and/or genetic alterations or developmental disruptions (Riga et al., 2014; Ortiz et al., 2017; Chapple and Skinner, 2023). All these models need to be addressed and further investigated in astrapotheres and other SANUs. Finally, the potential homoplastic nature of accessory cusps must be considered when utilizing them in phylogenetic inferences and taxonomic characterizations (Ortiz et al., 2017, 2018), and their inclusion in such analyses as independent characters should probably be reconsidered or directly avoided.

5.3. Wear facets and masticatory attributes of Albertogaudrya unica

The marked lateral development of the wear facets of the molar FMHN.PV.850, especially those located on the labial side of the paralophid, protoconid, and protolophid, represents something unusual in Astrapotheria, which demands the discussed comparison with the few known molars of Albertogaudrya unica and other astrapotheres.

The development of wear facets involves significant components derived from the morphology of the upper teeth during occlusion. This allows inferences about the upper dentition, based on the understanding that these facets constitute part of the same evolutionary module (Schultz et al., 2018, 2020). In the Chilean specimen of Albertogaudrya unica, there are several wear facets, particularly well-developed on the labial side. The paralophid wear facet (pld-mb, Fig. 5A) is commonly associated with wear generated by the distal end of the metacone in the upper preceding tooth, plus the mesial side of the paracone of the upper occlusal tooth. In the case of the specimen AMNH 28639 of Albertogaudrya unica used for comparison(right P4-M1; see Simpson, 1967a), the ectoloph extends only a short distance from the metacone fold towards the distal side of the tooth but continues as a functional parastylar fold on the adjacent molar, and probably at least the mesial portion of their paracone. This functional zone in the upper teeth may account for the extensive development of the observed pld-mb facet (Fig. 5).

In FMHN.PV.850, the distolabial wear facet in the protoconid (pr-db, Fig. 4) is likely the result of the action of the protoloph and parastyle fold from the upper molar. Notably, pr-db is much smaller than the facet mentioned on the paralophid (pld-mb). The md-d wear and the pr-db, are probably the remnants of the protolophid wear facet on its distal side, formed during the initial stages of wear.

Distal to the hypoflexid of FMHN.PV.850, there are two main facets associated with the hypoconid: the hd-mb and hd-db. In more bunoid dentition, these correspond to the facets described by Shultz et al. (2018) in the upper teeth, located in the distal edge of the paracone (PA-d) and the mesial zone of the metacone (ME-m). The well-developed ectoloph in Albertogaudrya, where the recognition of these cusps is limited to the labial enamel folds of the paracone and metacone, suggests that the wear facets hd-mb and hd-db are linked to the more occlusal edge of enamel. In the case of the paracone fold in the M1 of the specimen AMNH 28639, the development is much more pronounced than in the metacone fold, which may indicate the greater depth and extent of hd-mb compared to hd-db in the specimen under study. This wear is similar to what is observed in the lower molars of Maddenia lapidaria,where wear facets labial to the protoconid were described (Kramarz and Bond, 2009), and likely corresponds to those here described as pr-bd and pld-mb. Other basal taxa, like Trigonostylops, or more derived as Astrapothericulus, also exhibit a similar development of wear facets on the labial side. In the latter case, a similar development of the labial cingulid is also present. These facets are particularly evident in the m1 of T. gegenbauri (MLP-PV 121736) and in the m2 of A. iheringi (MACN-A 52-410), where the pr-db, as well as part of what would correspond to pld-mb, are also present, although not as deep as those described for FMHN.PV.850. They also show wear facets in the labial side of the hypolophid; however, in contrast to their extension in the Albertogaudrya specimen described here, on these taxa, they are restricted to the more mesial part. In T. gegenbauri, the only evident facet is the hd-mb likely due to the proportionally less mesiodistal extension of the talonid. In A. iheringi,where the hypolophid is proportionally longer, the exclusive presence of this facet could be related to slight differences in occlusion during the masticatory process.

In lateral view, the labial cingulid of T. gegenbauri and A. iheringi is concave in the talonid, ascending both on the distal side and towards the hypoflexid. The somewhat straighter labial cingulid in the talonid of the specimen of Albertogaudrya FMHN.PV.850 (Fig. 4B) appears to be an artifact since the distal portion of the tooth is missing. The mesial portion of the labial cingulid in Albertogaudrya differs from the aforementioned taxa, as it ascends directly from the inflection at the hypoflexid and is mesially obliterated by the pld-mb facet. These labial wear facets do not seem to be present in Astrapotherium, in which the labial cingulid is also reduced or even absent in many cases.

 These differences in the development of wear facets not only reflect variations in occlusal relationships with the upper antagonistic teeth but may also be influenced by the development of higher crowns and increased hypsodonty. These features are particularly evident in Albertogaudrya, Maddenia,and Astrapothericulus.

5.4. Albertogaudrya unica as a biochronological marker for the middle to early late Eocene in Patagonia

Astrapotheres were common in Patagonia during the Paleogene (Simpson, 1967a; Cladera et al., 2004; Kramarz et al., 2019a, 2022), also present during the early Eocene in the Antarctic Peninsula (Bond et al., 2011; Gelfo, 2024). This frequency makes the group useful for biochronological, biostratigraphic, and biogeographic regional correlation, especially considering the numerous specimens collected from localities or exposures with controlled stratigraphic position and associated isotopic ages (Madden et al., 2010b; Dunn et al., 2013; Kramarz et al., 2022).

Albertogaudrya unica constitutes a conspicuous late Casamayoran (Barrancan subage) through Mustersan(?) species of extra-Andean Patagonia, mostly known from remains collected in the Chubut Province, Argentina. Although somewhat ambiguous, the majority of these specimens come from middle Eocene deposits of the Sarmiento Formation, with older records linked to the Gran Barranca Member at Gran Barranca (i.e., Barranca south of Lake Colhué Huapi, Simpson, 1967b), type locality of ​​the Barrancan subage of the Casamayoran SALMA (Cifelli, 1985; Madden et al., 2010a). Dunn et al. (2013) provided a U-Pb (CA-ID-TIMS) single zircon age for a tuff from this member of 39.861±0.037 Ma, while Ré et al. (2010) integrated ⁴⁰Ar/³⁹Ar dating and magnetochronology, suggesting a depositional age (corrected after Ogg, 2020) between Chron 19r (41.180-42.196 Ma) and Chron C18n.1n (38.398-39.582 Ma). Both studies then constrained the Barrancan subage of the Casamayoran SALMA in Gran Barranca to the lapse between 42.2-38.4 Ma. It has been discussed that Albertogaudrya unica also occurs in Mustersan age fossil assemblages (Simpson, 1967a, b; Folino et al., 2024), which at Gran Barranca are represented by the Rosado and the Lower Puesto Almendra members of the Sarmiento Formation. Dunn et al. (2013) conservatively constrained the age of the Mustersan SALMA at Gran Barranca between 38.2-38.0 Ma, while paleomagnetic data (Ré et al., 2010) suggested an age (corrected after Ogg, 2020) between Chron C18n.1n and the top of Chron C17n.1n, namely 38.398-37.385 Ma. As such, the biochron of A. unica would expand between 42.2 and 37.4 Ma, middle to early late Eocene, late Lutetian to early Priabonian ages.

Bostelmann et al. (2021) were the first to record the occurrence of Albertogaudrya unica in Chilean rocks, based on isolated dental elements collected from volcaniclastic deposits informally named “Estancia La Frontera beds” at the Alto Río Simpson, in the Aysén Region, Patagonia. The detailed lithological, stratigraphic, and paleontological analysis of these rocks favors a direct correlation with the Gran Barranca Member of the Sarmiento Formation, in Argentina (Bostelmann et al., 2024). A middle Eocene age for similar exposures was presented by Gianni et al. (2017), who obtained a U-Pb (LA-ICP-MS) zircon age of 39.9±0.6 Ma from a tuff sample collected in outcrops near El Pedregoso, in the Meseta del Chalía (Argentina), 20 km north of the Alto Río Simpson fossiliferous outcrops at Estancia La Frontera (Bostelmann et al., 2021, 2024).

The discovery of Albertogaudrya unica in Magallanes (Fig. 6), provides a biochronological datum for the fossiliferous beds of the Upper Member of the Río Turbio Formation in Sierra Baguales, which presently lacks direct isotopic dating, but features an important and consistent marine fossil record. The geological and biochronological context previously summarized, supports a middle Eocene age for A. unica in Sierra Baguales, consistent with the suggested Bartonian to early Priabonian age of the Upper Member of the Río Turbio Formation based on its diverse chondrichthyo-fauna (Otero et al., 2013; Garrido et al., 2024), and known sedimentological, stratigraphic, and isotopic information recorded in Chile and nearby Argentina (Casadío et al., 2009; Fosdick et al., 2020; George et al., 2020; Alarcón et al., 2022, 2023; Morales et al., 2022, 2023). George et al. (2020) obtained a U-Pb detrital zircon age with an MDA of 40.47±0.26 Ma for the top of the Lower Member of the Río Turbio Formation near El Encierro creek, 30 m stratigraphically below the lowest Loma Tiburón fossiliferous levels. The available evidence therefore suggests that a 40-37 Ma, Bartonian to Priabonian age, could be confidently assigned for the exposures at Loma Tiburón Locality 2. However, the lack of more complete and diverse mammal remains currently hinders a precise assignment of this Paleogene local fauna to either the Barrancan subage or the Mustersan SALMA.

Fig. 6. Idealized artistic reconstruction of Albertogaudrya unica at the Eocene coastal plain in Sierra Baguales, Última Esperanza Province, Magallanes, Chile. Illustration by Jorge Blanco.

5.5. Establishment of the Patagonian biogeographic provincialism

The taxonomic affinities of the Alto Río Simpson fossil mammals at Estancia La Frontera in Aysén (Bostelmann et al., 2021, 2024) demonstrate that typical Casamayoran age faunas were present westward, reaching the foothills of the Andes, well beyond their known core localities in the central Patagonian plateau and the Atlantic coast (Cifelli, 1985; Pascual et al., 2002; Carlini et al., 2022). This taxonomic similarity provides evidence of a well-established regional biogeographic provincialism across Patagonian faunas, which by the middle Eocene extended at least from latitudes 41° (González Ruiz and Vera, 2017; Vera et al., 2020) to 46° S (Bostelmann et al., 2021). The present record of Albertogaudrya unica in Magallanes, extends the known geographic distribution of this species by 400 kilometers to the south, from central Patagonia to the southern extreme of the continent. This also gives support for a southward extension of this biogeographic pattern, up to 51° S,  reaching the northwestern limits of the Paleogene epicontinental marine embayment, which by that time covered most of eastern Magallanes and Tierra del Fuego (Malumián and Náñez, 2011; Morales et al., 2023). The southern extension of this faunal provincialism has also been recently sustained with the preliminary recognition of a notoungulate with affinities to Puelia sigma in estuarine facies of the Loreto Formation at Río de las Minas, near the city of Punta Arenas, 53° S (Kaempfe et al., 2024). Puelia sigma is another Barrancan?-Mustersan species, previously recorded at different localities of northern Chubut and Río Negro provinces in central Patagonia, Argentina (Simpson, 1967a; Martínez, 2018; Kramarz et al., 2022). Although scant, these new findings suggest that by middle Eocene times, a regional-wide faunistic component was common along the southern cone of South America, constituting assemblages characterized by low local endemism and high ecological disparity (Simpson, 1967a; Kramarz et al., 2022; Bostelmann et al., 2021, 2022; Bostelmann, 2024).

The origin of this biogeographic pattern can be traced back to the early Eocene or even the Paleocene, at least in the extra-Andean territory and the Atlantic coastal exposures of Chubut. There, Peligran, “Sapoan” and Itaboraian/Riochican faunas, provide an early indication of the composite nature of the mammal assemblages, mostly formed by well-diversified typical “South American groups”, like marsupials, notoungulates, litopterns, astrapotheres, and dasypodids, among others (Simpson, 1935; Tejedor et al., 2009; Goin et al., 2016, 2022), and relic elements of the latest Late Cretaceous non-tribosphenic mammals (monotremes, meridiolestidans, and gondwanatheres). Unfortunately, the lack of Paleocene or early Eocene fossil mammals south of 44° S makes the generalization of this hypothesis highly speculative. Most of the typical “South American” therian lineages likely originated and flourished in the middle and low latitudes of the Neotropical Region during the latest Cretaceous-Paleocene, derived from earlier Laurasian (North American) stocks. These lineages subsequently dispersed towards the Austral region, following hyperthermal conditions associated with the climatic optima of the early and late Paleocene, and early Eocene (Pascual et al., 1996; Bowen et al., 2015; Carneiro and Oliveira, 2022; The Cenozoic CO₂ Proxy Integration Project [CenCO₂PIP] Consortium, 2023). After their establishment and diversification, they built the homogeneous and cohesive character of the middle to late Eocene Patagonian faunas (Woodbourne et al., 2014; Goin et al., 2016; Goin, 2022).

The auspicious future identification and prospection of new Paleocene and early Eocene fossiliferous deposits in the Chilean Patagonia (Fosdick et al., 2020; Garrido et al., 2022, 2024), will enable us to test the real timing and extent of the proposed regional provincialism pattern. This will help to develop a robust model of the processes and ways in which this biogeographic homogenization was achieved. A pattern that, despite multiple instances of species turnover, displacement, and replacement associated with global climatic shifts during the late Eocene to early Oligocene, and the late Oligocene to Early Miocene (Goin et al., 2010, 2012; Buffan et al., 2025), was repited with similar coherence across Patagonia during the onset of the Neogene (Flynn et al., 2002; Vizcaíno et al., 2022; McGrath et al., 2020, 2023; Bostelmann, 2024).

6. Conclusion

We document the first Paleogene mammal from the Magallanes Region, Chile, a relevant area for understanding early biogeographic patterns in Patagonia. The new record represents the first mention of a continental Paleogene mammal south of 46° S, and also the first occurrence of a fully terrestrial vertebrate in the extensive marginal marine deposits of the middle Eocene Magallanes Basin. The fossil is composed of a medium-sized, isolated left m1 or m2, assigned to the primitive astrapothere Albertogaudrya unica, representing the second mention of this species in Chile, and extending its known geographic distribution more than 400 kilometers towards the south.

The described tooth is similar to other lower molars of Albertogaudrya unica, although some notable differences are evident. These include an extended paralophid that contacts the lingual wall, and the development of unusual wear facets: namely, an extended and lateralized paralophid mesiobuccal wear facet (pld-mb), and a notch-like protoconid distobuccal wear facet (pr-db). While this seems to differ from the common pattern observed in known lower molars of Albertogaudrya unica, we interpreted it as part of an intraspecific variability, reflecting a particular occlusal contact between the trigonid and the opposing upper molar. Accessory dental structures present in the upper and lower teeth of astrapotherids, such as isolated cusps and bunoid structures, are analyzed in the light of morpho-developmental teeth models, questioning their utility as valuable and independent phylogenetic or taxonomic characters.

The presence of A. unica in northern Magallanes allows us to infer an age between 40-37 Ma for the fossiliferous levels of the Upper Member of the Río Turbio Formation present at Loma Tiburón Locality 2, in Sierra Baguales. This age is concordant with published geochronological information of the unit in Chile and nearby Argentina, and the rich and highly informative fossil chondrichthyan assemblage. From a biochronological and biogeographic perspective, Albertogaudrya unica can be considered a characteristic species of the Barrancan subage and also probably from the Mustersan SALMA of Patagonia (late Lutetian to early Priabonian age), supporting the establishment of a biogeographic provincialism in Patagonia, as early as the beginning of the middle Eocene.

Acknowledgements
We express our deepest gratitude to N. Reffer and A. von Bischhoffshausen, who kindly provided constant support during all these years to our work in Sierra Baguales, and the Mac-Lean family for authorizing access to the estancias La Cumbre and Los Baguales. E. Martinic García collected the fossil at Loma Tiburón Locality 2. L. Chornogubsky and A. Martinelli, M. Reguero and S. Bargo, and J. P. Varela, facilitated access to scientific collections under their care in Buenos Aires, La Plata, and Coyhaique, respectively. A. Kramarz kindly shared papers, photographs, and opinions on diverse anatomical aspects of SANUs, which were important for our comparisons. S. Soto Acuña, H. Püschel, A. Martinelli, B. Quaggia, and the Museo Regional de Aysén provided photographic assistance. V. Aquino, in Salta, kindly shared pictures of A.? carahuasensis. We would like to express our special appreciation to R. Alée, who prepared the final versions of the schematic figures. J. Blanco made the reconstruction of Albertogaudrya unica. Finally, we acknowledge the suggestions and recommendations made by the reviewers, M. Armella and D. Croft, and the editor, D. Bertín, which greatly improved the quality of the manuscript. Financial support came from the Antarctic Science and Technology Ring Project (ACT-105), the ANID national doctoral scholarship to J.E. Bostelmann, the Núcleo Milenio EVOTEM-NCN2023_025 project, and various resources provided by the private sector, all of them enormously acknowledged.

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Appendix

List of analyzed specimens

Antarctodon sobrali: MLP-PV 08-XI-30-1 (holotype); IAA Pv 826.

Trigonostylops wortmani: AMNH 28700; MACN-A 10627; MACN-A 10653; MACN-A 12505; MACN-PV CH 1211; MPEF PV 5482.

Trygonostylops gegenbauri: MLP-PV 12-1736 (type).

Trygonostylops sp.: MLP-PV 56-XII-18-43-54; MLP-PV 59-II-24-216-219; MLP-PV 59-II-24-286-287; MLP-PV 59-II-24-409; MLP-PV 59-II-24-518-520; MLP-PV 59-II-24-701; MLP-PV 59-II-24-482-484; MLP-PV 66-V-5-96; MLP-PV 69-III-27-9; MLP-PV 69-III-26-1.

Tetragonostylops apthomasi: DGM 355-M; AMNH 49831, 49832, 49836, 49854, 49838, 49839, 49861, 49862, 49863; MLP-PV 69-I-9-8.

Tetragonostylops cf. T. apthomasi: MPEF PV 5479.

Albertogaudrya unica: MACN-A 10635, 12000, 12001, 12002, 12004 (type of Albertogaudrya separata), 12005 (type of Scabellia cyclogona), 12007 (type of Albertogaudrya oxygona), 12008, 12014 (type of Albertogaudrya regia); AMNH 28639, 28640, 28641, 28947; MHN Tournouër collection N°10; MURAY.PV.004.

Albertogaudrya? carahuasensis: CNS-PV 10000 (1, 2) (holotype).

Scaglia cf. S. kraglievichorum: MPEF PV 5478.

Astraponotus spp.: MACN-A 10971, MLP-PV 12-1471, 12-2217, 67-II-27-28, 67-II-27-379, 67-II-27-167, 83-III-3-1, 82-V-7-2, 69-III-24-295; MPEF PV 1084, 1279, 1296.

Maddenia lapidaria: MPEF PV 6113, 7696a, 7709, 7732, 7738, 7848.

Isolophodon aplanatus: MLP-PV 12-2139, MPEF PV 7475.

cf. Isolophodon cingulosus: MLP-PV 61-VIII-3-387.

Parastrapotherium holmbergi: MACN-A 52-503 (syntype of P. trouessarti), 52-506 (syntype of P. trouessarti).

Parastrapotherium martiale: MACN-A 52-604 (holotype).

Astrapothericulus iheringi: MACN-A 52-410, 52-411, 52-417, 52-419, 52-605.

Astrapotherium magnum: MACN-A 3207, 3209, 3210.

Astrapotherium burmeisteri: MLP-PV 12-94 (type), MACN-A 3274-3278 (type of A. giganteum).

Astrapotherium burmeisteri?: YPM PU 13168.

Uruguaytherium beaulieui: MNHN 213 (holotype).