Traditional paleontology considers
Ectocion osbornianus [osbornianum] (Cope 1882a, Granger 1915, Thewissen 1990; Fig. 1; Paleocene 55 mya) a member of the Condylarthra or Phenacodontidae, alongside Phenacodus. Cope 1882a originally named the type, Oligotomus osbornianus from maxillary and dentary fragments with lophodont teeth. The specimen was renamed by Cope 1882e when he realized he had already given a fossil horse than generic name. The genus Ectocion is known from more than 500 jaws and hundreds of loose teeth. Skulls, evidently, are rare.
While searching online I found
two distinct skulls referred to Ectocion (Fig. 1). Neither is the holotype which was described from teeth and fragments. The Ectocion on the left is from Thewissen 1990. The one on the right is from Wikipedia. See the difference?
Neither nests with Phenacodus
or any other tested phenacodontid in the LRT. Instead the LRT nests the Ectocion on the left with Protictis and other basal Carnivora. That dipping transverse premaxilla and sabertooth canine are two traits shared by members of the Carnivora.
The LRT nests the Ectocion on the right between Procavia, the rock hyrax and Elephas and Dusisiren all with incisors transforming into proto-tusks and completely lacking a large canine or a dipping transverse premaxilla.
Both of these appear to represent novel hypotheses of relationships.
When shifted to phenacodontids
the one on the left adds 14 steps The one on the right adds 11 steps.
the two skulls do share an odd sort of overall similarity. But there’s one issue we should consider:
Lophodont dentition on a carnivore?
Protictis has carnassial teeth. Sister carnivore Ectocion (Fig. 1 left) does not, but has lophodont teeth in their place. If one scores tooth shapes, Ectocion might not nest with Protictis, but that’s an experiment waiting to happen. At present, the LRT gives both Ectocion specimens the opportunity to nest with several phenacodontids and neither does that.
So, given that Carnivora is near the base of all placentals,
and that Proticitis nests near the base of all Carnivora, and that placentals had their origin in the Late Triassic/Early Jurassic and that the outgroup taxon, Nandinia, has carnassial teeth, it is possible that some members of the Carnivora had time to evolve lophodont teeth. After all, many other derived mammals did so.
Like the DNA problem
these nestings call into question the practice of using teeth alone in phylogenetic analysis. Teeth need to be part of any solution, but not the whole answer.
Cope ED 1882a. Contributions to the history of the Vertebrata of the lower Eocene of Wyoming and New Mexico, made during 1881. Proceedings of the American Philosophical Society: 139-197.
Cope ED 1882e. Note on Eocene Mammalia. American Naturalist 16:522.
Granger W 1915. A revision of the lower Wasatch and Wind River faunas, Part III: Order Condylarthra, families Phenacodontidae and Meniscotheriidae. Bulletin of the American Museum of Natural History 34:329-361.
Mac Intyre GT 1962. Simpsonictis, a new genus of viverravine miacid (Mammalia, Carnivora). American Museum Novitates 2118: 1-4.
Mac Intyre GT 1966. The Miacidae (Mammalia, Carnivora) Part 1. The systematics of Ictidopappus and Protictis. Bulletin of the American Museum of Natural History 131(2):115-210.
Matthew WD 1937. Paleocene faunas of the San Juan basin, New Mexico. Transactions of the American Philosophical Society, new series 30: 1-510.
Simpson GG 1935. New Paleocene mammals from the Fort Union of Montana. Proceedings of the U. S. National Musem 83: 221-244.
Thewissen JGM 1990. Evolution of Paleocene and Eocene phenacodontidae (Mammalia, Condylarthra). University of Michigan Papers on Paleontology 29:1-107.