A reassessment of Nemolestes (Mammalia, Metatheria): Systematics and evolutionary implications for Sparassodonta

Abstract

Nemolestes and Procladosictis are Eocene sparassodonts of controversial affinities. Nevertheless, despite the poor preservation of their holotypes, several other specimens have been tentatively assigned to these genera, as exemplified by some isolated teeth from the Itaboraí and Laguna Fría faunas. Here, we revise specimens referred to Nemolestes and Procladosictis from Itaboraí, Laguna Fría and Ameghino’s collection. Nemolestes differs from Patene in its more reduced metaconid, and “carnassialized” m4 (with reduced talonid and talonid cuspids, except the hypoconulid), features that make it possible to identify specimens from Itaboraí and Laguna Fría as Nemolestes. This taxon occurred from the early Eocene (Itaboraian SALMA) to the middle Eocene (Casamayoran SALMA). Nemolestes represents the oldest known hypercarnivorous sparassodont. The inclusion of Nemolestes and Procladosictis in a comprehensive phylogenetic analysis recovered them in a polytomy as the sister taxa of Borhyaenoidea and a new unnamed clade that includes Lycopsis, Dukecynus, Hathliacynidae, Hondadelphys and Stylocynus. Nemolestes shares with these taxa, but not Patene, the presence of a “carnassialized” m4. Nemolestes differs from known borhyaenoids in the absence of a medially positioned protoconid and short anterolabial cingulid. Therefore, Nemolestes is an early lineage of Sparassodonta. “Procladosictis erecta” is a lower premolar of the unnamed clade, as it shows symmetric walls and a small precingulid. "Procladosictis erecta" should be, in fact, referred to Procladosictis. Our phylogenetic analysis indicates that during the early middle Eocene (Laguna Fría and La Barda faunas), the unnamed clade and Borhyaenoidea had already diverged. Therefore, the evolutionary history of these groups is older than previously thought.