Shrew opossums (caenolestids) are supposed to be marsupials

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.

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.

The trouble is
tested caenolestids, Caenolestes (Fig. 1) and Rhyncholestes (Fig. 2), do not have a pouch. Nor do they nest with marsupials in the large reptile tree (LRT, 1412 taxa, subset Fig. 3). But female caenolestids do have a marsupial-like double vagina (see below).

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.”

Contra Dr. Naish’s amusing musings,
shrew opossums nest with placental shrews alongside the otherwise extinct Apatamys (Fig. 3) + Trogosus (Fig. 4) in the Glires clade. All are derived from a tree shrew taxon close to Tupaia. It’s unfortunate that Dr. Naish did not test these taxa while they were on his mind in 2015. That’s how initial errors become perpetuated as long-standing traditions.

Figure 1. Skull of Rhyncholestes along with in vivo photo.

Figure 2. Skull of Rhyncholestes along with in vivo photo.

Rhyncholestes raphanurus (Osgood, 1924; long-nosed shrew-opossum, Chilean shrew opossum, extant; snout-vent length 20cm), nests in the large reptile tree between the shrew-mole, Uropsilus, and the tree shrew, Tupaia at the base of the Apatemys clade. 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).

Figure 2. Apatemys nests as a proximal sister to bats in the Halliday et al. tree. But it shares very few traits with bats. Note the very odd dentition.

Figure 3. Apatemys nests as a proximal sister to bats in the Halliday et al. tree. But it shares very few traits with bats. Note the shrew-opposum/rodent-like dentition.

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 the splitting of eutherian mammals from the phytometatherians and carnimetatherians.  Even so, we’re talking about deep time here. Don’t trust genes. Test traits.

Figure 3. Subset of the LRT focusing on primates and basal glires, including the caenolestids, Caenolestes and Rhyncholestes.

Figure 4. Subset of the LRT focusing on primates and basal glires, including the caenolestids, Caenolestes and Rhyncholestes.

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.”

Unfortunately
the LRT tests only skeletal material, not for ‘number of uteri and vaginas’. While Larry Martin and Darren Naish might wave this trait about in support of a marsupial affinity, the LRT documents the emergence of placentals from marsupials. So the reappearance of a long-lost trait, like a long tail, a sixth digit, or double vaginas is well within the realm of possibilities in placentals.

As a matter of fact,
a double vagina sometimes occurs in humans.

Here, as elsewhere in paleontology,
maximum parsimony is the only yardstick. PAUP is free to nest taxa wherever 231 unbiased scores indicate it should. Moving the two caenolestids to the Metatheria adds 12 steps to the MPT.

The Apatamyidae is a clade that was long considered extinct.
Now it joins several other clades that are no longer extinct, thanks to the LRT.

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)

Apatemys chardini (Marsh 1872, Eocene, 50-33 mya) was a squirrel-lke arboreal herbivore with a massive skull. Here it nests with Trogosus and Tupaia, a tree shrew. It had long slender fingers, a long flexible lumbar region, and a long gracile tail.


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

Click here for Glires skulls compared.

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