Eomakhaira molossus, Engelman & Flynn & Wyss & Croft, 2020

Engelman, Russell K., Flynn, John J., Wyss, André R. & Croft, Darin A., 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), pp. 1-76 : 8-10

Eomakhaira molossus , gen. et sp. nov.

Figures 3–4 , 6–16 ; tables 1–2

HOLOTYPE: SGOPV 3490 , a partial rostrum of a senescent individual preserving the right maxilla with C-P3, alveoli and partial roots of M1–2, and part of M3; left maxilla with C-P3, anterior root of M1, and M3–4; left and right horizontal rami of the mandible, including both lower canines and most of the postcanine dentition, as well as parts of the coronoid processes; the entire left and parts of the right nasal; parts of the palatine; and the orbital process of the left lacrimal.

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 .

TYPE LOCALITY: Cachapoal locality, west side of Estero Los Llanos of the upper Río Cachapoal drainage, Libertador General Bernardo O’Higgins Region, central Chile (fig. 2).

STRATIGRAPHIC OCCURRENCE: Abanico Formation. Most specimens from Estero Los Llanos were recovered from talus cones at the SE nose of a roughly N-S running ridge of ~ 1,500 m relief. This ridge roughly parallels the strike of the steeply west-dipping beds. The thickness of the Abanico Formation in the Cachapoal region has not been measured in detail but is on the order of 2000–4000 m. Within this thick succession, the exact horizon that produced SGOPV 3490 is not known, as the specimen was collected from talus. For additional geological context of the Cachpaoal locality see Flynn and Wyss (2004), Hitz et al. (2006), and West et al. (2014) .

AGE: Probably early Oligocene,?Tinguirirican SALMA. Fossils from the Cachapoal locality are likely at least 29.3 ± 0.1 million years old (at least in part), based on an unpublished date for a volcanic tuff that is thought to either correlate with or overlie the fossil-producing horizons at Los Llanos (Charrier et al., 1997; Flynn and Wyss, 2004). It must be cautioned, however, that this date is from ~ 5 km to the south, in the neighboring Las Leñas drainage, and that the units involved have not been traced directly between the two locations due to precipitous intervening topography. The only radioisotopic date for the Cachapoal Valley itself is an 40 Ar/ 39 Ar date of 11.1 ± 1.8 Ma reported by West (2017) from levels far above the fossil-producing strata, which does little to precisely constrain the age of the fossils. The presence of the polydolopid Kramadolops ( Polydolops in Flynn and Wyss, 2004) and the archaeohyracid Archaeotypotherium (Croft et al., 2008a) suggest a pre-Deseadan age, and the presence of the interathere Johnbell hatcheri , otherwise known only from the Tinguirirican type locality (Hitz et al., 2006), suggests that fossils from the Cachapoal locality are probably similar in age to those of the Tinguiririca Fauna (~33–32 Ma; Flynn et al., 2003).

ETYMOLOGY: The name of the genus derives from the Greek root Eo s, 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.

Figures

FIG 3. A, C, Photographs and B, D, CT segmentation of the holotype of Eomakhaira molossus, a partial skull of a senescent individual preserving the rostrum and the anterior portion of the mandible (SGOPV 3490) in left (A, B) and right (C, D) lateral views. In renderings of the CT segmentation, nasal in orange, facial process of the lacrimal in teal, palatine in blue, all other bones of the cranium (maxilla, jugal, frontal, etc.) in purple, teeth in yellow, and dentary in green. Anterior to left in A–B and to right in C–D. Scale = 30 mm.

FIG 4. A, Photograph and B, line drawing of the exposed right upper canine and postcanine dentition of SGOPV 3490, showing their extreme wear. In B, enamel is denoted in dark grey, dentine in brown, and wear surfaces by light grey. The morphology of these teeth is not entirely visible, as much of the occluded upper and lower jaws of the specimen remain encased in matrix (compare with figs. 3 and 9). Scale = 10 mm.

FIG 6. Cranium of the holotype of Eomakhaira molossus (SGOPV 3490) in palatal view. Anterior to right. Colors of elements in this CT segmentation are the same as in figure 3. Abbreviations: mpps, medial postpalatine spine; pp, palatal pit; ptor, palatine torus; dental abbreviations as in Materials and Methods. Scale = 30 mm.

FIG 7. Posterior palate of the holotype of Eomakhaira molossus (SGOPV 3490), in oblique anterior view. Anterior to lower left. Shows the paired palatine tori and broken border of the minor palatine foramen. Colors of elements in this CT segmentation are the same as in figure 3. Abbreviations: mpf, minor palatine foramen; ptor, palatine tori. Scale = 10 mm.

FIG 8. Left orbital region of the holotype of Eomakhaira molossus (SGOPV 3490) in oblique posterolateral view. Anterior to left. Colors of elements in this CT segmentation are the same as in figure 3. Abbreviations: laf, lacrimal foramen, mpf, minor palatine foramen; spf, sphenopalatine foramen. Scale = 30 mm.

FIG 9. Mandible of the holotype of Eomakhaira molossus (SGOPV 3490). Right dentary in A, labial and C, lingual views. Left dentary in B, labial and D, lingual views. Colors of elements are the same as in figure 3. Abbreviations:cor, coronoid process of dentary; sulc, lingual sulcus of the lower canine; menf, mental foramina; symph, mandibular symphysis. Scale = 50 mm.

FIG 10. A, Photograph and B, C, CT images of the right upper canine of the holotype of Eomakhaira molossus (SGOPV 3490). A, Lateral view, showing bluntness of canine apex and the absence of enamel, longitudinal ridges, and labial median canine sulcus. B, Transverse section of canine, slightly below alveolar border (actual point at alveolar border obscured by a crack), showing posterior keel. Anterior root of P1 (aP1) marked to show that the longitudinal ridge is not an artifact of postmortem damage. C, Transverse section of canine, at level of tooth row, showing presence of median labial keel and lingual median sulcus. Approximate location of sections in B, C denoted by arrows on A. Anterior to right in all images, and lingual to top in B, C. Abbreviations: aP1, anterior root of P1; mk, labial median keel; ms, lingual median sulcus; pr, posterior ridge. Scale = 10 mm (A); 5 mm (B, C).

FIG 11. Morphology of the left lower canine root in the holotype of Eomakhaira molossus (SGOPV 3490). A, CT reconstruction in oblique lateral view, showing prominent depression on proximal end of canine and its connection to the pulp cavity. Connection to pulp cavity has been artificially darkened for contrast.B, Oblique lateral CT image slice of SGOPV 3490, showing flared, smooth distal end of left lower canine (denoted by arrow). Scale = 10 mm.

FIG 12. Oblique lateral CT image slice of SGOPV 3490 along the postcanine tooth row, showing position of lower molars and depth of horizontal ramus. Note how worn surfaces of P3 and m1 closely match one another. Scale = 30 mm.

FIG. 13. Left M3–4 of SGOPV 3490 in A, labial, B, occlusal, and C, lingual views (C rotated upside down for easier comparison with A, B). Anterior to left in all CT segmentation images. Scale = 3 mm.

FIG. 14. Cross-sectional CT image of left M4 of SGOPV 3490 in A, oblique occlusal and B, posterior views. Shows presence of three roots. Scale bars = 5 mm.

FIG. 15. Lower right dentition of SGOPV 3490 in A, labial, B, occlusal, and C, lingual views. Colors of elements in this CT segmentation are the same as in figure 3. Anterior to right in A, B and to left in C. Scale = 30 mm.

FIG. 16. Results of parsimony phylogenetic analysis under equal weights, showing the strict consensus of 12 most parsimonious trees (MPTs).Eomakhaira molossus in bold. Numbers to upper left of each node represent Bremer supports/decay indices, numbers to lower left of each node represent bootstrap values. Support values not given for Dasyuromorphia, as this node was constrained a priori (see text).