There has been a traditional disconnect
in mammalian paleontology regarding the two transitions between the egg-laying Prototheria, the pouched Metatheria, and the pouch-less Eutheria. So far as I can tell, only the large reptile tree (LRT, 1334 taxa; Fig. 2) has documented how and which species form transitional links in this chain of mammal evolution (Fig. 1). At present, and for the foreseeable future, didelphids, like Didelphis (the Virginia opossum), Monodelphis (the gray short-tailed opossum) and Caluromys (the wooly opossum) occupy basal nodes at large radiations of metatherians and eutherians in the LRT…hence the title of this post.
Didelphidae has been a clade restricted to the opossums without any insight to their eventual descendants… the rest of the marsupials and us placentals. That’s why several mousy and not-so-mousy ‘possums have been added to the LRT recently, to more precisely recover evolutionary patterns in deep time. Amazing that our more or less direct ancestors are still with us today, sometimes hidden in Amazon forests, other times raiding our backyard trashcans and tentatively crossing our highways and byways.
A new taxon in the LRT is Thylophorops
considered by Goin et al. 2009 to be the largest didelphid. Unfortunately, in the LRT, Thylophorops does not nest with Didelphis, but with Oxyaena and Thylacinus (Fig. 2)… themselves descendants of Didelphis with cat-like and wolf-like traits respectively.
“Thylophorops species (as well as several other contemporary opossum genera) show a high degree of speciation towards carnivory compared to the still living didelphines. Their premolar and molar teeth were proportionally larger than those of living opossums and their grinding facets imply a more dedicated shearing action; these have been interpreted as “omnivory leading towards carnivory” in Goin et al. 2009.”
there is no reference in Goin et al. to either Oxyaena or Thylacinus. So… taxon exclusion is still an issue with the Goin et al. taxon list. Such problems are largely resolved in the LRT, which tests all possible candidates, and even dozens of fringe candidates that no one else considers, recovering a fully resolved tree based on traits and taxa that extend back to Devonian tetrapods, ultimately relating all descendants to one another.
Ameghino F 1908. Las formaciones sedimentarias de la región litoral de Mar del Plata y Chapadmalal part 2
Goin FJ, Zimicz N, de los Reyes M, Soibelzon L 2009. A new large didelphid of the genus Thylophorops (Mammalia: Didelphimorphia: Didelphidae), from the late Tertiary of the Pampean Region (Argentina). Zootaxa. 2005: 35–46.