Really, aren’t we ALL didelphids?

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.

Figure 3. A selection of Therian skulls leading to placentals. This is the 'gradual accumulation of traits' recovered by the LRT.

Figure 1. A selection of basal mammal skulls leading to placentals. This is the ‘gradual accumulation of traits’ recovered by the LRT. A third of these are traditional didelphids. Or two-thirds of these are cladistic didelphids. And, if so, then we humans are also didelphids. Haplodectes (IVPP V5235) nests as the basal primate in the LRT.

Traditionally
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.

Figure 1. Subset of the LRT focusing on basal Mammalia after the addition of several marsupials.

Figure 1. Subset of the LRT focusing on basal Mammalia after the addition of several marsupials. Red taxa are represented by only a few bones, like mandibles with teeth. Note the proximity of traditional creodonts to the basal placental clade, Carnivora, basal members of which are small, arboreal and opossum-like.

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.

Wikipedia reports,
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.”

Figure 1. Crowned as the largest didelphid (by not much actually) Thylophorops nests between leopard-like Oxyaena and wolf-like Thylacinus in the LRT.

Figure 3. Crowned as the largest didelphid (by not much actually, but it is a juvenile) Thylophorops lorenzini nests between leopard-like Oxyaena and wolf-like Thylacinus in the LRT. All are shown to scale here.

Unfortunately
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.

References
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.

wiki/Thylophorops

 

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