Macraucheniidae

Püschel, Hans P., Shelley, Sarah L., Williamson, Thomas E., Perini, Fernando A., Wible, John R. & Brusatte, Stephen L., 2024, A new dentition-based phylogeny of Litopterna (Mammalia: Placentalia) and ‘ archaic’ South American ungulates, Zoological Journal of the Linnean Society 202 (1), pp. 1-50 : 7-9

Macraucheniidae ( Figs 4A, D–E, 5A–D)

With 18 currently accepted genera, Macraucheniidae is the second most diverse family of litopterns (Supporting information, Table S1 ), being particularly diverse during the Neogene alongside proterotheriids ( Fig. 2B–C). It was first proposed by Gervais (1855) as a member of the order Perissodactyla (‘Ordre des Jumentés’) to include one species, Macrauchenia patachonica Owen, 1838 . This taxon was discovered by Charles Darwin in 1834 ( Keynes 2001) and later described by Owen (1838), being the first litoptern known to the scientific community. Later, Ameghino (1902a), based mostly on the position of the external nares in the cranium, subdivided this family into three subfamilies: Cramaucheniinae represented by Cramauchenia Ameghino 1902a from Deseadean to Colhuehuapian South American Land Mammal Ages [hereafter SALMAs; Stages of Cione and Tonni (1995)], Theosodontinae represented by Theosodon Ameghino, 1887 (Colhuehuapian to Laventan SALMAs), and Macraucheniinae represented by Macrauchenia (Pliocene to Holocene). Simpson (1945) reclassified Macraucheniidae into the subfamilies Macraucheniinae and Adianthinae , without giving an anatomical justification for this change ( Table 1 ). By Simpson’s (1945) definition, Macraucheniinae was composed of the members of the three subfamilies mentioned by Ameghino (1902a) and close Neogene relatives discovered since then (e.g., Promacrauchenia Ameghino, 1904a ), but also included Paramacrauchenia Bordas, 1939 and Victorlemoinea Ameghino, 1901 , the former now considered a proterotheriid and the latter a sparnotheriodontid (Soria 2001; see Sparnotheriodontidae section for more details). The subfamily Adianthinae was composed of adianthids such as Adianthus (see Adianthidae section for more details).

Similarly to Ameghino (1902a), Soria (1981) recognized important anatomical differences between macraucheniids before and after the Chasicoan SALMA (Late Miocene) that justified a subfamilial division, separating Macraucheniidae into two subfamilies: Cramaucheniinae (pre-Chasicoan SALMA) and Macraucheniinae (Chasicoan and post-Chasicoan SALMA or post-Santacrucian SALMA). This separation was based on the fact that Macraucheniinae tends to have more derived features than Cramaucheniinae , possessing a retracted nasal aperture to a more centrodorsal position and fused zeugopodial elements in the forelimbs (ulna-radius) and hind limbs (tibia-fibula; Soria 1981). However, of these two subfamilies only Macraucheniinae is monophyletic according to phylogenetic analyses (Schmidt and Ferrero 2014, Forasiepi et al. 2016, McGrath et al. 2018, Püschel et al. 2023; Table 2 ).

Macraucheniids are distinguished from the two other families, Adianthidae and Proterotheriidae . Among other differences, macraucheniids lack the trilobed m3, which is characteristic of adianthids ( Cifelli and Soria 1983a). Also, macraucheniids are functional tridactyls,unlike proterotheriids that show reduced(or lost) lateral digits II and IV (Soria 2001). However, the origin of Macraucheniidae in the Palaeogene and the affinities of the family within Litopterna are still contentious topics. The oldest uncontroversial members of the family are Coniopternium Ameghino 1894a , Cramauchenia Ameghino 1902a , and Pternoconius Cifelli and Soria 1983b , all with a first appearance datum (FAD) in the Deseadean SALMA (Late Oligocene; Dozo and Vera 2010). These genera show dental and/or postcranial anatomical features that link them closely to later macraucheniids, like the presence of a mesolophid or cristid that connects the cristid obliqua with the entoconid [=entolophid (Soria and Hoffstetter 1985)] in all or some of the lower molars. Before the Oligocene, Polymorphis Roth, 1899 from the Late Eocene of Patagonia has been proposed as the earliest member of Macraucheniidae (Soria 1982, Cifelli 1983a), revalidating a previous proposal of Ameghino (1904b). Cifelli (1983a) even considered it as a member of a different subfamily, Polymorphinae , within Macraucheniidae ( Table 1 ). Previously, Polymorphinae was also considered a distinct family of the Proterotheriidae (Odreman Rivas 1969) . Key features for associating Polymorphis with macraucheniids are related to similarities in the upper molars, in particular, the presence of an oblique crest that connects the protocone with the metacone (Soria 1982; Fig. 4A; see also Polymorphis spp. in Supporting information, File S2). In contrast, the lower molars of Polymorphis spp. show important differences with Deseadean SALMA macraucheniids, such as the complete absence of a mesolophid. The proposal of Polymorphis as a macraucheniid found support in a phylogenetic analysis ( Cifelli 1993; Fig. 1C), and since then most recent studies have considered this taxon as the earliest member of Macraucheniidae without any further testing (e.g., Dozo and Vera 2010, Croft et al. 2020; Table 2 ). Cifelli (1993) also found Macraucheniidae as the sister group of Adianthidae (Macrauchenioidea) , and both families closely related to Proterotheriidae ( Lopholipterna ; Fig. 1C; Table 1 ). Most additional phylogenetic studies with macraucheniids in the taxon sampling have either focused on testing relationships within the family (e.g., Püschel et al. 2023) or determining the interordinal relationships within Placentalia (e.g., O’Leary et al. 2013, Buckley 2015) instead of the interfamilial affinities ( Table 2 ).

Considering the Late Eocene Polymorphis as the first occurrence of the family and the Pliocene to Late Pleistocene Macrauchenia ( Bond 1999, Prado et al. 2015) and the probably Late Pleistocene Xenorhinotherium ( Cartelle and Lessa 1988) as the last occurrences of Macraucheniidae , the temporal interval for this family would be around 39.00–~0.11 Mya ( Fig. 2B; Supporting information, Table S1 , File S2).