Mesotheriid (Mammalia, Notoungulata) remains from the Colhuehuapian beds (Early Miocene) of Chichinales Formation, Río Negro Province, Argentina

E. RODRIGO PAZ, E., KRAMARZ, A. AND M. BOND

AMEGHINIANA

ASOCIACION PALEONTOLOGICA ARGENTINA

Lugar: Buenos Aires; Año: 2011 p. 264 - 269

0002-7014

The Mesotheriidae represent one of several clades of theendemic South American ungulate Order ?Notoungulata.This clade has some rodent-like adaptations such as hypertrophiedupper and lower incisors, highly hypsodont cheekteeth, reduced dental formula (in younger forms), simplifiedocclusal morphology, and specialized zygomasseteric structure,among other features (Patterson, 1934; Billet et al.,2008). Traditionally, the Mesotheriidae have been dividedinto two subfamilies: Trachytheriinae and Mesotheriinae(Simpson, 1945). Recent phylogenetic analyses supportedthe monophyly of Mesotheriinae, but indicated that Trachytheriinaeis a paraphyletic assemblage of basal mesotheriids(Cerdeño and Montalvo, 2001; Croft et al., 2004; Regueroand Castro, 2004; Billet et al., 2008).The Mesotheriidae comprise younger forms persistinguntil the Ensenadan SALMA (early?middle Pleistocene)(Ameghino, 1906). Until recently, the Mesotheriinae weresupposed to have appeared in the middle Miocene (Friasiansensu lato), being absent in the early Miocene Colhuehuapianand Santacrucian SALMAs (Simpson, 1940; Marshallet al., 1983; Bond, 1986). The recently discovered ChucalFauna from Northern Chile (Flynn et al., 2002; Croft et al.,2004) containing three species of mesotheriines agrees witha Santacrucian age (Croft et al., 2004).Other basal mesotheriines were reported from other earlyto middle Miocene faunas. The probably early Miocenemesotheriid from the Mariño Formation, Mendoza Province(Cerdeño et al., 2006; Cerdeño, 2007) was found tobe similar to the Chucal species Altitypotherium chucalensis(Cerdeño, 2007). Additionally, ?Trachytherus? mendocensis?(Simpson and Minoprio, 1949; Simpson et al., 1962) ?originallydescribed as a trachytheriine from the Divisadero LargoFormation, Mendoza? was re-interpreted as a primitive mesotheriinemore probably coming from the overlying earlyMiocene Mariño Formation (Cerdeño et al., 2006; 2008).López et al. (2009) reported a new mesotheriine genus andthe presence of ?Altitypotherium from the early MioceneChinches Formation in San Juan Province, but this newtaxon remains formally undescribed.Herein are described the first mesotheriid remains fromthe Chichinales Formation in the surroundings of PasoCórdova, on the southern bank of the Río Negro near thecity of General Roca, Río Negro Province (Fig. 1). Fossilswere collected from lower deposits of this 2.10 m thick unit(Barrio et al., 1989; Kramarz et al., 2004). The fossil faunais Colhuehuapian in age (early Miocene) as indicated bythe presence of Colpodon Burmeister, 1885, CramaucheniaAmeghino, 1902, Willedewu Vucetich and Verzi, 1991, AustraloproctaKramarz, 1998, and Cochilius volvens Ameghino,1901 (Windhausen, 1922; Barrio et al., 1989; Kramarz etal,, 2004). The geological context and stratigraphic profileof the Chichinales Formation is fully detailed in Barrio etal. (1989).