Updated July 18, 2022
with a shifting of two members of the Paucituberculata, shrew-like Caenolestes and Rhyncholestes, nesting with the marsupial, Acristotherium, in the LRT.
According to Wikipedia,
“The family Caenolestidae contains the seven surviving species of shrew opossum: small, shrew-like marsupials that are confined to the Andes mountains of South America.”
Figure 1. Caenolestes skull and in vivo. It sure looks more like a shrew than an opossum. Skull images from Digimorph.org and used with permission. Colors added. See figure 2 for updated image of skull.
The trouble is
tested caenolestids, Caenolestes (Fig. 1) and Rhyncholestes (Fig. 2), do not have a pouch. Even so, they nest with the marsupial, Acristotherium, in the LRT.
On the traditional side,
Dr. Darren Naish reported online for Tetrapod Zoology/Scientific American in 2015, “Incidentally, the most frequently used name for the group – shrew-opossums – might not be a particularly good one, seeing as they don’t look much like shrews, don’t live like shrews, and don’t act like shrews. And they’re not technically opossums, either, but perhaps we can let that go.”
Figure 2. Shrew-like shrew opossums, Caenolestes and Rhyncholestes, nest with the marsupial, Acristotherium, in the LRT.
Rhyncholestes raphanurus (Osgood, 1924; long-nosed shrew-opossum, Chilean shrew opossum, extant; snout-vent length 20cm).
Genetically
Wikipedia reports. “Genetic studies indicate that they are the second most basal order of marsupials, after the didelphimorphs” (Nilsson et al. 2010). That’s exactly where the LRT documents them. Even so, don’t trust genes. Test traits.
According to AnimalDiversity.org, “In general, members of family Caenolestidae can be distinguished from other marsupial groups by their unique dentition. Their lower middle incisors are large and have a forward slope; likewise, they have a reduced number of incisors. The dental formula for genus Caenolestes is: I 4/3, C 1/1, P 3/3, M 4/4, 46 teeth total. Shrew opossums have short robust limbs, each containing 5 digits; their middle 3 digits are shorter than the outside two. Their humeri are extremely heavy; in comparison, their femurs are relatively slender. Members of family Caenolestidae have unusual lip flaps, they may function as a method of preventing debris from interfering with their whiskers or they may help prevent ingestion of unwanted debris. Similar to other marsupials, Caenolestid females have 2 uteri and 2 vaginas. Members of genus Caenolestes lack a pouch but do have 4 mammae, 2 on either side of their abdomen.”
As a matter of fact, a double vagina sometimes occurs in humans.
Rhyncholestes raphanurus (Osgood, 1924; long-nosed shrew-opossum, Chilean shrew opossum, extant; snout-vent length 20cm), nests in the LRT between the shrew-mole, Uropsilus, and a large living shrew, Scutisorex, all within the placental clade, Glires. Wikipedia and other sources consider this shrew-like South American mammal a marsupial, but Wiki also notes that Rhyncholestes lacks a marsupium (pouch).
Caenolestes fuliginosus (originally Hyracodon fuliginosus Tomes 1863).
References
Marsh OC 1872. Preliminary description of new Tertiary mammals. Part II. American Journal of Science 4(21):202-224.
Nilsson MA, et al. (6 co-authors) 2010. Tracking Marsupial Evolution Using Archaic Genomic Retroposon Insertions”. PLoS Biology. 8 (7): e1000436. doi:10.1371/journal.pbio.1000436
Osgood WH 1924. Field Mus. Nat. Hist. Publ., Zool. Ser. 14:170.
tetrapod-zoology/you-never-hear-much-about-shrew-opossums/
wiki/Shrew_opossum = Caenolestidae
animaldiversity.org/accounts/Caenolestes_fuliginosus/
wiki/Apatemyidae
wiki/Rhyncholestes
wiki/Caenolestes
wiki/Paucituberculata
wiki/Uterus_didelphys
The Pterosaur Heresies 