Sinclair has had the most complete material on which to work, and with his general grouping I have come to agree. This recognizes three divisions of South American Marsupials, the Didelphidae, representatives of which have not yet been found in the Deseado, though occurring in both the earlier and later formations; the Caenolestidae represented today by Caenolestes, the only survivor of the South American diprotodonts; and the Borhyaenidae (= Thylacynidae of Sinclair this name having been used to indicate a much nearer relationship to the Australian Thylacynus than I feel is warranted), which includes a large range of medium to large sized animals ranging from the Casamayor formation throughout the Santa Cruz beds.
The locality from which these marsupials emigrated to South America and the time of their arrival is not yet agreed upon, and can not be settled until much more complete material is discovered in the Casamayor formation. I feel, however, that the three groups were separate when they entered South America.
Borhyaenidae
Ameghino has grouped in this family a considerable number of genera of powerful, wolf-like carnivorous marsupials, characterized by a dental formula
- 4-3 1 4 3[24]
- ————,
- 3 1 4 3
heavy heads, short limbs with usually five semidigitigrade toes. The genera are mostly distinguished by the relative development of the protocone on the upper molars and the talonid on the lower ones. [Figure 138] gives a typical marsupial upper molar 2 and a lower molar 2 to show the sense in which these terms are used. The Santa Cruz genera are the best known and I therefore use them as a basis for comparison with the less known Deseado forms, of which we found but the one genus Pharsophorus at all abundant. In addition to this, Ameghino has reported a gigantic form designated Proborhyaena. The following table indicates the relationships of the best known genera.
| Age | Formula | Protocone | Upper Molar 3 | Talonid on Lower Molar 3 | Symphysis | |
|---|---|---|---|---|---|---|
| Cladosictis | Santa | 4 1 4 3 | on pm. 4-m. 3 | Protocone | Small | Symphysis |
| Cruz | 3 1 4 3 | Paracone | basin | |||
| vestigal | with one | |||||
| Metacone | post. cusp | |||||
| Ant. ext. | ||||||
| style | ||||||
| Amphiprovivera | Santa | 4 1 4 3 | on pm. 4-m. 3 | Protocone | Basin with | Ligamentous |
| Cruz | 3 1 4 3 | Paracone | two post. | |||
| Ant. ext. | cusps | |||||
| style | ||||||
| Prothylacynus | Santa | 4 1 4 3 | on pm. 4-m. 1 | Protocone | Small | Fused |
| Cruz | 3 1 4 3 | vestigal | basin | |||
| Metacone | with one | |||||
| post. cusp | ||||||
| Borhyaena | Santa | 3 1 4 3 | vestigal | Paracone | No basin | Fused |
| Cruz | 3 1 4 3 | Ant. ext. | One post. | |||
| style | cusp | |||||
| Pharsophorus | Deseado | 1 4 3 | vestigal | Protocone | Very small | Ligamentous |
| 1 4 3 | vestigal | No basin | ||||
| Paracone | One post. | |||||
| Ant. ext. | cusp | |||||
| style | ||||||
| Proborhyaena | Deseado | Fused | ||||
| 1 4 3 |
From the foregoing, it will appear that Pharsophorus approaches Borhyaena and Prothylacynus in the structure of its upper molars, being, however, nearer to the former, and the same is true of the structure of the talonid; but Pharsophorus differs markedly from both in retaining the metaconid as a small cusp on the side of the protoconid on all of the lower molars; also in the extremely small size of the talonid of the lower molars, which in Pharsophorus have no basin and consist of a single cusp; and, lastly, in the symphysis of the lower jaws being ligamentous, whereas in the two preceding genera, it is fused. Pharsophorus is probably ancestral to Borhyaena. In the case of Proborhyaena, only a mandible, with the canine and premolars 3 and 4 intact, has been found. The fourth premolar is more reduced than in other genera, but, until more teeth are known, its affinities can not be at all closely determined.
Fig. 138. Diagram of a generalized upper molar, U.m., and a lower molar, L.m., of Borhynidae; a.s., ant. style; hld., hypoconulid; mt., metacone; mtd., metaconid; pa., paracone; pad., paraconid; pr., protocone; prd., protoconid; td., talonid.