Adianthidae

Adianthidae (Figs 4F–H, 5E–G) Adianthidae currently includes six genera, being considerably less diverse than proterotheriids and macraucheniids during the Neogene (Fig. 2; Supporting information, Table S1). Cifelli and Soria (1983a) presented a detailed revision of this family, so to avoid unnecessary repetition, only the most relevant taxonomic aspects are discussed here, along with the main advances since their revision. Known as the pygmy litopterns because of their reduced size, the family Adianthidae was named by Ameghino (1891a) to place the Early Miocene Adianthus bucatus Ameghino 1891a from the Santa Cruz Formation, Argentina (Cifelli and Soria 1983a). Among other distinct anatomical features, adianthids present a trilobed m3 and enamel fossettes in the P4–M3 formed by hypertrophied conular cristae (Cifelli and Soria 1983a). Following new fossil discoveries, Ameghino (1906) added three additional genera to the family Adianthidae: Proadiantus Ameghino, 1897 and Tricoelodus Ameghino, 1897 from the Sarmiento Formation (Woodburne et al. 2014a), and Pseudadiantus Ameghino, 1901. The last was later excluded from the family and formally synonymized with the notopithecine interatheriid Antepithecus by Simpson (1967). Bordas (1939) added Proheptaconus Bordas, 1936 from the Colhue-Huapi Member of the Sarmiento Formation (Soria 1981) to what he considered the subfamily Adianthinae, a subfamily of Macraucheniidae, which was followed by other authors (Patterson 1940, Simpson and Minoprio 1949). However, Simpson et al. (1962) later restored the family Adianthidae, but now including a new taxon, Adiantoides leali Simpson & Minoprio, 1949 from the Divisadero Largo Formation, Argentina (age uncertain; Cerdeño et al. 2008, López 2010, Woodburne et al. 2014a). Bond and Vucetich (1983) added to Adianthidae Indalecia grandensis Bond & Vucetich, 1983 from the Lumbrera Formation, Argentina (Early Eocene; Fernicola et al. 2021), based on unique shared anatomical features with Adiantoides leali, such as the absence of a postprotocrista connecting the protocone with the metaconule and a strong parastyle in M1– M2. These features are not present in other adianthids, so they grouped them in the new subfamily Indaleciinae, keeping both species tentatively within the family Adianthidae, which was later followed with doubts by Cifelli and Soria (1983a). In the same work, Cifelli and Soria (1983a) added the genus Thadanius Cifelli & Soria, 1983 from La Salla-Luribay Basin, Bolivia to the family Adianthidae. Cifelli (1983a) revisited the members of the family, and included Proectocion Ameghino 1904b [previously considered a didolodontid by Simpson (1948)] as the earliest member of Adianthidae based on dental similarities. In addition, he proposed a new superfamily called Macrauchenioidea, to group adianthids and macraucheniids based on dental similarities, such as an absent paraconid and the presence of a paralophid in the lower molars (Table 1). Soria (1984a), based on dental similarities with ‘amilnedwarsids’, elevated the rank of the subfamily Indaleciinae to the family Indaleciidae, and later Soria (1989b) classified them as a family of the order Notopterna (see sections of Amilnedwardsiidae and the order Notopterna, and the family Indaleciidae). Adianthidae has also been considered by some authors as part of the suborder Lopholipterna (Cifelli 1983a, Soria 2001), which includes the families Macraucheniidae and Proterotheriidae (Table 1), a hypothesis that later found phylogenetic support (Cifelli 1993; Fig. 1C). In the same study, Macrauchenioidea also found support, but only when indaleciids are excluded (Cifelli 1993). Strikingly, there has been not a single study examining the phylogenetic relationships within Adianthidae using modern phylogenetic methods, and most recent phylogenetic studies have included adianthids as outgroups for phylogenies of Macraucheniidae (e.g., Forasiepi et al. 2016; Table 2). If we follow Cifelli (1983a) and consider Proectocion from the Barranca south of Lago Colhue-Huapi of Argentina (Middle Eocene; Kramarz and Bond 2013, Woodburne et al. 2014a) as the earliest member of Adianthidae and considering Adianthus godoyi from the Río Frías Formation at Alto Río Cisnes, Chile as the last member of this family (Cifelli 1991, de la Cruz and Cortés 2011), the family Adianthidae encompasses a temporal interval of 42–14.8 Mya (Fig. 2B; Supporting information, Table S1).