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.

Teeth in the shrew/rodent/rabbit/multituberculate clade

The problem:
Always ready for a review, I noticed in the rat/rabbit clade of the large reptile tree (LRT, 1272 taxa) canine teeth (and sometimes nearby others) were lost creating a diastema in seven subclades (Fig. 1). The biggest worry was the apparent reappearance of a full arcade of teeth in highly derived taxa, like Paulchaffotia and Carpolestes, after a several clades without a full arcade (including rodents and the aye-aye). Generally, that’s not supposed to happen. So I reviewed all the data and made a helpful image (Fig. 2).

Figure 1. Subset of the LRT focusing on the clade of rodents, shrews, rabbits and multituberculates. White taxa have a small or large tooth gap between the incisors and premolars.

Figure 1. Subset of the LRT focusing on the clade of rodents, shrews, rabbits and multituberculates. White taxa have a small or large tooth gap between the incisors and premolars.

The solution:
After trying and failing to force all taxa with a diastema together, the LRT recovered a cladogram in which canine teeth disappeared creating a diastema seven times by convergence in the rabbit/rodent clade (Fig. 1). Apparently unknown taxa with small canines linked the last taxa with canines (hedgehogs) with the first taxa with canines beyond rodents (multituberculates).

You might remember
that marsupials and large placental ungulates also produced taxa with a similar diastema. So it is a common convergent trait.

When charts don’t help, sometimes pictures  do.
Here (Fig. 2) are several taxa from the the subset cladogram above (Fig. 1) so you can see for yourself how evolution works in tiny steps that slowly add up to large changes. Particularly interesting here is the central place of hedgehogs (with a full arcade of teeth) basal to higher clades with a full arcade of teeth alongside yet another clade or two with lost canines (diastema).

Figure 2. A selection of taxa from figure 1 more or less to scale and in phylogenetic order (pink arrows). Hope this helps with the concept of a gradual accumulation of traits. The hedgehogs Erinaceus and Echinops are transitional to the higher taxa with teeth and without.

Figure 2. A selection of taxa from figure 1 more or less to scale and in phylogenetic order (pink arrows). Hope this helps with the concept of a gradual accumulation of traits. The hedgehogs Erinaceus and Echinops are transitional to the higher taxa with teeth and without.

Note:
The rodent-like ‘primates’ Ignacius, Plesiadapis and Daubentonia (Figs. 1, 2) are more closely related to rodents in the LRT (contra Gunnell et al. 2018.) That’s heresy, still waiting to be confirmed or refuted by testing by other workers. Note how similar Ignacius is to the hedgehog, Erinaceus (Fig. 3).

Figure 3. The hedgehog, Erinanceus, compared to Ignacius from the Paleocene.

Figure 3. The hedgehog, Erinanceus, compared to Ignacius from the Paleocene. Note the reduction to loss of the canine in the latter.

References
Gunnell GF et al. (9 co-authors) 2018. Fossil lemurs from Egypt and Kenya suggest an African origin for Madagacar’s  aye-aye. Nature Communications 9(3193).

 

Who ever said the giant otter shrew was a tenrec??

Everybody did and does.
Every reference (e.g. Wikipedia, EOL) you can find lists the giant otter shrew Potamogale velox (Du Chaillu 1860, Nicoll 1985) as a tenrec… and I don’t know why. But I’m willing to learn, if anyone can offer up an explanation based on phylogenetic analysis.

Until then…

Adding Potamogale to
the large reptile tree, nests it with Scutisorex the shrew (Fig. 2), not Tenrec the tenrec (Fig. 3).

Figure 1. Potamogale velox, the giant otter shrew nests with Scutisorex (Fig. 2), the hero shrew, in the large reptile tree.

Figure 1. Potamogale velox, the giant otter shrew nests with Scutisorex (Fig. 2), the hero shrew, in the large reptile tree.

Both shrews have
canine-like medial incisors and lack canine-like canines. Both have a skull shorter than half the presacral length. Both have a relatively small scapula. And a long list of minor traits. Shifting Potamogale to Tenrec adds 30 steps to the LRT.

Figure 2. Scutisorex (below) and Crocidura (above) are extant shrews.

Figure 2. Scutisorex (below) and Crocidura (above) are extant shrews.

Based on the shape of the more plesiomorphic dentition
Potamogale (Fig. 1) is the more primitive taxon compared to Scutisorex (Fig. 2). Neither is very much like Tenrec (Fig. 3).

Figure 3. Tenrec skeleton

Figure 3. Tenrec skeleton, not much in common with Potamogale (Fig. 1). 

 

References
Du Chaillu P 1860. Descriptions of mammals from equatorial Africa. Proceedings of the Boston Society of Natural History, 7, 358–369.
Nicoll M 1985. The biology of the giant otter shrew *Potamogale velox*. National Geographic Society Research Reports, 21: 331-337.

wiki/Giant_otter_shrew

Newsflash: Shrews evolved from Apatemyidae! (or vice versa)

Wikipedia reports,
“A shrew (Figs. 2, 3) or shrew mouse (family Soricidae) is a small mole-like mammal classified in the order Eulipotyphla.”

“Apatemyidae is an extinct family of placental mammals that took part in the first placental evolutionary radiation together with other early mammals such as the leptictids.” 

Figure 3. Apatemys skull in situ and reconstructed shares several similar traits with the extant striped opossum, Dactylopsila, including a squirrel-like size, elongate fingers and similar teeth.

Figure 1a. Apatemys skull in situ and reconstructed shares several similar traits with the extant striped opossum, Dactylopsila, including a squirrel-like size, elongate fingers and similar teeth. Note the stronger similarities recovered by the LRT with the shrew, Scutisorex (figure 2).

Figure 1. Apatemys, only complete fossil skeleton of an apatemyid, turns out to be a basal shrew. So this clade is not extinct.

Figure 1b. Apatemys, only complete fossil skeleton of an apatemyid, turns out to be an Eocene sister to the shrew. So this clade is not completely extinct.

As it turns out
shrews are less like moles in the large reptile tree and more like members of the Apatemyidae, like Apatemys (Marsh 1872; Fig. 1),  thought to have originated in the Paleocene and disappeared in the Eocene. Both are derived. At present they nest as sisters. Neither can be shown, at present, to be more primitive than the other.

Apatemys chardini (Eocene, 50-33 mya; Fig. 2) was a squirrel-lke arboreal herbivore with a massive skull. It had long slender fingers, a long flexible lumbar region, and a long gracile tail. From head to tail, the matrix scores for apatemyids and shrews are quite similar. It’s not just the dentition, which remains amazingly conservative in living shrews.

By contrast
moles have a set of small procumbent anterior dentary teeth and the posterolateral premaxillary teeth are large and canine-like while the medial premaxillary teeth are small like typical incisors. Moles nest closer to Solenodon and rodents [and I need to update that webpage].

Figure 1. The shrew Scutisorex compared to the apatemyid, Labidolemur from the early Eocene. Despite the difference in time, the teeth are still quite comparable.

Figure 2. The shrew Scutisorex compared to the apatemyid, Labidolemur from the early Eocene. Despite the difference in time, the teeth are still quite comparable and unique to themselves. Note sure how this was overlooked. The resemblance is remarkable.

Wikipedia reports,
“Like most Paleocene mammals, the Apatemyids were small and presumably insectivorous. Size ranged from that of a dormouse to a large rat. The toes were slender and well clawed, and the family were probably mainly arboreal.The skull was fairly massive compared to the otherwise slender skeleton, and the front teeth were long and hooked, resembling those of the modern aye-aye and marsupial Dactylopsila, both whom make their living by gnawing off bark with their front teeth to get at grubs and maggots beneath.”

Evidently others have missed the shrew connection. Let me know of any literature that predates this blogpost. I’d like to promote it, if it’s out there.

Figure 2. Scutisorex (below) and Crocidura (above) are extant shrews.

Figure 3. Scutisorex (below) and Crocidura (above) are extant shrews.

The book ‘Evolution of Shrews’ reports,
“Shrews are among the most ancient of all living mammals. They are small and have rather unspecialized body plans, retained almost unchanged since they evolved about 45 million years ago. They appear to be extremely successful as a group in comparison to all other families of Insectivora: the living species of shrews represent approximately 80% of all insectivorans, which in turn make up some 10% of all mammmalian species extant today.”

The apatemyid connection described here
pushes the origin of shrews back another 5-10 million years. True shrews are not to be confused with elephant shrews. They are more closely related now, then what was recovered earlier, but more work needs to be done to figure this out precisely. Then I’ll update the cladogram.

Afterthought
Earlier I posted on Hyopsodus, the pre-dog, former condylarth. One the images appeared in editing, but did not appear in publication. I was not aware of this until today. The JPEG had a ‘bad marker’, whatever that is. Please let me know if you see a technical problem here. That Hyopsodus image has been repaired and is visible now.

References
Churchfield S 1990. The Natural History of Shrews online. Comstock Publishing; 1st edition (January 1990) Amazon link.
Multiple Authors 1998. Evolution of Shrews. Edited By: JM Wójcik and M Wolsan. Cornell Paperbacks. 458 pp.  online.
Gingerich PD and Rose KD 1982. 1. Dentition of Clarkforkian Labidolemur kayiGingerich PD 1982. 2. Labidolemur and Apatemys from the early Wasatchian of the Clark’s Fork Basin, Wyoming. Studies on Paleocene and Early Eocene Apatemyidae (Mammalia, Insectivora). Contributions from the Museum of Paleontology. The University of Michigan. 26(4):49-69.
v. Koenigswald W, Schierning H-P 1987. The ecological niche of an extinct group of mammals, the early Tertiary apatemyids. Nature. 326 (6113): 595–597. doi:10.1038/326595a0.
Marsh OC 1872. 
Preliminary description of new Tertiary mammals. Part II. American Journal of Science 4(21):202-224.

wiki/Apatemyidae