Thylacosmiline sparassodonts (previously recognized as thylacosmilids) are among the most iconic groups of endemic South American Cenozoic mammals due to their distinctive morphology and convergent resemblance to saber-toothed placental carnivores. However, the early evolution of this group and its relationship to other sparassodonts remains poorly understood, primarily because only highly specialized Neogene taxa such as Thylacosmilus, Anachlysictis, and Patagosmilus are well known. Here, we describe a new Paleogene sparassodont, Eomakhaira molossus, from the Cachapoal locality of central Chile, the first sparassodont reported from early Oligocene strata of the Abanico Formation. Eomakhaira shares features with both Neogene thylacosmilines and Paleogene “proborhyaenids,” and phylogenetic analyses recover this taxon as sister to the clade of Patagosmilus + Thylacosmilus. This broader clade, in turn, is nested within the group conventionally termed Proborhyaenidae. Our analyses support prior hypotheses of a close relationship between thylacosmilines and traditionally recognized proborhyaenids and provide the strongest evidence to date that thylacosmilines are proborhyaenids (i.e, the latter name as conventionally used refers to a paraphyletic group). To reflect the internestedness of these taxa, we propose use of Riggs' (1933) original name Thylacosmilinae for the less inclusive grouping and Proborhyaenidae for the more inclusive one. Saber teeth arose just once among metatherians (among thylacosmilines), perhaps reflecting a developmental constraint related to nonreplacement of canines in metatherians; hypselodonty may have relaxed this potential constraint in thylacosmilines. The occurrence of Eomakhaira in strata of early Oligocene age from the Chilean Andes demonstrates that the stratigraphic range of thylacosmilines spanned almost 30 million years, far surpassing those of saber-toothed placental lineages.

MAMMALIA Linnaeus, 1754 

METATHERIA Huxley, 1880 

SPARASSODONTA Ameghino, 1894 

BORHYAENOIDEA Simpson, 1930 

PROBORHYAENIDAE Ameghino, 1897 

Phylogenetic Definition: Proborhyaenidae refers to all sparassodonts more closely related to Proborhyaena gigantea than to Borhyaena tuberata, Prothylacynus patagonicus, Lycopsis torresi, Cladosictis patagonica, or Sipalocyon gracilis. This is a stem-based definition (de Queiroz and Gauthier, 1990). 

THYLACOSMILINAE Riggs, 1933

Phylogenetic Definition: Thylacosmilinae refers to all sparassodonts more closely related to Thylacosmilus atrox than to Proborhyaena gigantea, Borhyaena tuberata, Prothyla­cynus patagonicus, Lycopsis torresi, Cladosictis patagonica, or Sipalocyon gracilis. This is a stembased definition (de Queiroz and Gauthier, 1990).

Etymology: The name of the genus derives from the Greek root Eos, meaning “dawn,” and makhaira, a type of short sword or large knife (often translated as “carving knife”), in reference to the bladelike canines of thylacosmilines. The specific epithet comes from the Greek molossus, a term used to refer to short-snouted, robust-skulled dog breeds such as mastiffs and bulldogs and refers to the short, robust snout of this species. Gender is masculine.

Diagnosis: A member of Borhyaenoidea based on its short, robust rostrum, presence of lingual median canine sulci, extremely small protocone, small and unicuspid talonid on m4. Differs from all other borhyaenoid sparassodonts in the following combination of features: small size (smaller than most other borhyaenoids; length of m1–4 = 37.3 mm, comparable to Fredszalaya hunteri or the extant dasyuromorphian Sarcophilus harrisii); maxilla very deep and maxillary “cheeks” absent; mandibular symphysis unfused and anteroposteriorly narrow; two mental foramina present; length/width ratio of palate >1.5; palate extending to level of M4; presence of postpalatine tori (shared only with Arminiheringia and possibly Callistoe among borhyaenoids); absence of postpalatine torus foramen; sphenorbital foramen opening dorsal to M4; large canines; absence of longitudinal striations on the canine roots (shared only with other thylacosmilines and possibly Lycopsis viverensis); median keel on the labial face of upper canines; medial sulcus on lingual face of upper and lower canines; short lower canine roots; presence of three premolars with no diastemata between them; premolars large and robust but not globular; asymmetric protoconid of P1 (shared only with Arminiheringia and Callistoe); P3 significantly longer than p3 (possibly autapomorphic for this taxon); bulbous roots only on p3; preparacingulum absent; M3 with narrow stylar shelf and prominent ectoflexus; M4 extremely narrow anteroposteriorly (only comparable to Patagosmilus among borhyaenoids), subequal or greater in width to M3, and with three roots; protocone vestigial (at least on M4); absence of an anteriorly projecting ventral keel of paraconid (which only occurs in proborhyaenids among sparassodonts); protoconid of m4 posteriorly salient; metaconid absent on m4 and probably m2–3; posterolabial cingulid present; talonid of m4 almost absent; and p1–3 short relative to m1–4 (shared with Paraborhyaena among borhyaenoids with three premolars). Canines more mediolaterally compressed than in borhyaenoids other than Patagosmilus, Thylacosmilus, and possibly Proborhyaena. P/p3 labiolingually narrower than in Fredszalaya, Plesiofelis, Acrocyon, Arctodictis, Australohyaena, Borhyaena, and Callistoe, but wider than in Prothylacynus and some individuals of Pharsophorus, comparable in relative proportions to Arminiheringia, Paraborhyaena, and Proborhyaena. 

Russell K. Engelman, John J. Flynn, André R. Wyss and Darin A. Croft. 2020. Eomakhaira molossus, A New Saber-Toothed Sparassodont (Metatheria: Thylacosmilinae) from the Early Oligocene (?Tinguirirican) Cachapoal Locality, Andean Main Range, Chile. American Museum Novitates. 2020(3957); 1-75. DOI: 10.1206/3957.1