find evidence for them. This is a characteristic shared with the Australian thylacine, and historically argued as a synapomorphy, though nowadays it is considered to have developed independently for poorly understood reasons. As with thylacines, it is very likely that they possessed long cartilaginous elements instead. Teeth The dental formula of sparassodonts varies considerably. In borhyaenids, it is , with three upper and lower incisors, one upper and lower canine, three upper and lower premolars, and four upper and lower molars in each half of either jaw. Proborhyaenids usually only have two lower incisors instead of three, except for Callistoe. Thylacosmylids have at least two upper and only two lower incisors (the uppers grew into elongated sabers), and two upper and lower premolars. Some specimens of Borhyaena and Arctodictis are also missing the last upper molar, showing that the presence of this tooth was variable in these species. Sparassodonta is characterized by dental synapomorphies that distinguish the group from other closely related mammals. Unequivocal traits uniting the earliest Sparassodonts include: a snout that forms a pronounced bulge around the canine teeth when viewed from above a ridge on the upper molar (preparacrista) oriented anterobuccally (towards the cheek) with respect to the long axis of the tooth. a pronounced keel near the base of the front of the paraconid ridges on lower molars (postprotocristid-metacristid) parallel or oblique with respect to lower jaw axis. a very tall protoconid (>90% tooth length, secondarily lost in Hondadelphys and Stylocynus) that bulges to the side and is wider at its midpoint than its base talonid (crushing end) of lower molar narrow in relation to trigonid (shearing end). In borhyaenids, only the third premolar was ever replaced in the animal's lifetime, similar to other metatherians. In thylacosmilids, only the lower third premolar was replaced. The cusps of the sparassodont molar correlate to a cutting function rather than a crushing one. In the upper molars, the paracone (on the lingual, or tongueward, side) is reduced and fused to the metacone (distal, towards the back of the mouth), inflating the postmetacrista (the lingual border of the metacone); and they almost always lack the stylar shelf (on the buccal, or cheekward, side) and associated stylar cusps. In the lower molars, the trigonids (the buccal shearing side) have an inflated paracristid and marginalized or absent metaconid; and the talonid (the distal, or backendwards, crushing side) is either reduced or gone.