SVP abstracts – Ambolestes and the origin of placentals

Bi S-D et al. 2019 discuss Early Cretaceous Ambolestes
(Figs. 1, 2) and the Early Mesozoic marsupial/placental split.

Figure 1. Ambolestes tracing from Bi et al. 2018.

Figure 1. Ambolestes tracing from Bi et al. 2018.

From the abstract:
“Extant placental and marsupial mammals are the dominant vertebrates in many ecosystems, which makes the placental-marsupial dichotomy a significant event in Earth’s history.”

The large reptile tree (LRT, 1592 taxa) splits placentals from marsupials as shown below (Figs. 3, 4). The Early Cretaceous marsupial Bishops splits from the placental outgroup taxon, the extant marsupial Caluromys (Fig. 6). More timely, derived placental multituberculates, like Megaconus (Fig. 5), have been found in Middle Jurassic strata. That means a long line of undiscovered small, arboreal, placentals extends back to the Late Triassic/Earliest Jurassic.

Figure 3. Ambolestes skull reconstructed. Jaw tips restored.

Figure 2. Ambolestes skull reconstructed. Jaw tips restored.

Bi et al. continue:
“Molecular estimates of the divergence of placentals and marsupials (and their broader clades Eutheria and Metatheria) fall primarily in the Jurassic.”

Since Early Jurassic Megazostrodon is the proximal outgroup for all mammals, and Early Triassic Morganucodon is a marsupial, and Middle Jurassic Megaconus the LRT supports a Late Triassic split for placentals and marsupials.

Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

Figure 3. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here. Note the large gaps of time in which fossils are not known.

Bi et al. continue:
“In support, the oldest purported eutherian, Juramaia, is reported to be from the early Late Jurassic (160 million-years ago) of Liaoning Province, northeastern China.”

In the LRT (subset Fig. 1) Juramaia nests as a basal prototherian, an egg laying basal mammal.

“The oldest purported metatherian, Sinodelphys, is 35 million-years younger from the
Early Cretaceous Jehol Biota also in Liaoning Province, northeastern China.”

In the LRT Sinodelphys is another monotreme.

“In 2018, we reported a new eutherian, Ambolestes zhoui, also from the Jehol Biota. The fossil, a nearly complete skeleton, preserves anatomical detail unknown from contemporaneous eutherians including the hyoid apparatus and ectotympanic. The complete hyoid is the first known for any Mesozoic mammaliaform, and the ectotympanic resembles that in some extant didelphid marsupials.”

In the LRT (Fig. 1) Ambolestes (Figs. 3, 4) is a metathere/marsupial close to the extant Virginia opossum, Didelphis.

Figure 1. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

Figure 4. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

Bi et al. continue:
“In our phylogenetic analysis concentrating on the eutherian-metatherian 
dichotomy, the closest relative of Ambolestes was Sinodelphys, and both fell within Eutheria.”

As shown above, the LRT does not confirm that hypothesis of interrelationships.

Figure 1. Subset of the LRT focusing on Glires and subclades within.

Figure 5. Subset of the LRT focusing on Glires and subclades within.

Bi et al. continue:
“With Sinodelphys as a eutherian, postcranial differences formerly thought to indicate different invasions of a scansorial niche by meta and eutherians in Jehol are only variations among the early members of the placental lineage. Additionally, the earliest known metatherians are approximately 15 million years younger than previously thought and their

fossils, isolated teeth and fragmentary jaws, are from North America. Our tree results in a 50 million-year ghost lineage for Metatheria, accepting the 160 million-years age for Juramaia. 

The LRT confirms a 210 mya origin for Metatheria, starting with Morganucodon, so no ghost is necessary.

Figure 8. Caluromys, the largest of the mouse opossums, to scale with its LRT sister, Vulpavus, a basal member of Carnivora.

Figure 6. Caluromys, the largest of the mouse opossums, to scale with its LRT sister, Vulpavus, a basal member of Carnivora and Placentalia.

Bi et al. continue:
“A possibility raised elsewhere is that the age of Juramaia is incorrect; rather than Late Jurassic, perhaps it is from the Early Cretaceous Jehol Biota. In our study, Juramaia is in a clade with Albian/Aptian Prokennalestes and Late Cretaceous eutherians by having a more molariform ultimate upper premolar. In contrast, Ambolestes, as in the outgroups, has a non-molariform ultimate upper premolar. Although resolution of this intriguing debate is not currently possible, our understanding of the issues has been furthered by the discovery of Ambolestes.”

As shown above, the LRT does not confirm the Bi et al. hypothesis of interrelationships.


References
Bi S-D et al. 2019. The Early Cretaceous eutherian Ambolestes and its implications for the Eutherian/Metatherian dichotomy. Journal of Vertebrate Paleontology abstracts.

Morganucodon and Kuehneotherium are mammals, not stem-mammals

Newham et al. 2019 report,
“Surprisingly long lifespans and low femoral blood flow suggest reptile-like physiology in key Early Jurassic stem-mammals.

Abstract:
“There is uncertainty regarding the timing and fossil 5 species in which mammalian endothermy arose, with few studies of stem-mammals on key aspects of endothermy such as basal or maximum metabolic rates, or placing them in the context of living vertebrate metabolic ranges. Synchrotron X-ray imaging of incremental tooth cementum shows two Early Jurassic stem-mammals, Morganucodon and Kuehneotherium, had lifespans (a basal metabolic rate 10 proxy) considerably longer than comparably sized living mammals, but similar to reptiles, and that Morganucodon had femoral blood flow rates (a maximum metabolic rate proxy) intermediate between living mammals and reptiles. This shows maximum metabolic rates increased evolutionarily before basal rates, and that contrary to previous suggestions of a Triassic origin, Early Jurassic stem-mammals lacked the endothermic metabolism of living mammals.”

That conclusion would be true
if their cladogram was correct. Unfortunatley, it was not.

Figure 1. Subset of the LRT focusing on Basal Mammalia including Creodonta.

Figure 1. Subset of the LRT from 2018 focusing on Basal Mammalia including Morganucodon and Kuehneotherium.

According to
the large reptile tree (LRT, 1579 taxa; subset Fig. 1), Kuehneotherium (Fig. 2) is a basal protothere mammal (= monotreme) in the lineage of echidnas and platypuses. Morganucodon is a very basal metathere mammal (= marsupial). The Virginia opossum, Didelphis, is the most closely related extant taxon in the LRT.

Figure 1. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Figure 2. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Here’s a data point of interest:
Newham et al. report, “Only the short-beaked echidna Tachyglossus aculeatus, a monotreme with long lifespan and low metabolic rate, exceeds the Kuehneotherium, but not Morganucodon, distance above the mammalian mean.” And THAT is reflected in the LRT. I also note the platypus, Ornithorhynchus, is not mentioned in the text, only in the citations. Same with Didelphis.

So what does that do to the results?
Seems like the Newham et al. study is suffering from taxon exclusion and an invalid traditional understanding of basal mammal interrelations. Unfortunately Professor MJ Benton is a co-author, infamous for taxon exclusion and guiding his students and any protégé to do the same.

Please tell Elis Newham et al.
to add the platypus and opossum to their study and get back to us! Don’t let this work become another waste of time due to taxon exclusion.


References
Newham E et al. (19 co-authors) 2019. Reptile-like physiology in Early Jurassic stem-mammals. bioRxiv preprint http://dx.doi.org/10.1101/785360

Revisiting the predatory metathere clade, Sparassodonta

Figure 1. A traditional selection of sparassodont metatheres.

Figure 1. A traditional selection of sparassodont metatheres from Forasieppi, MacPhee and del Pino 2019

In their study of the cranium
of the South American sparassodont sabertooth, Thylacosmilus, Forasieppi, MacPhee and del Pino 2019 report, “Sparassodonta is the group that includes the common ancestor of Patene and all its descendants. Undisputed records of Sparassodonta, including ones for Patene simpsoni, begin in the Early Eocene (Itaboraian) and extend through to the Pliocene (Chapadmalalan), when the last of them disappeared.”

FIgure 1. Cladogram of the traditional Sparassodonta from Babot and Forasiepi 2016. Taxa also found in the LRT are colored.

FIgure 2. Cladogram of the traditional Sparassodonta from Babot and Forasiepi 2016. Taxa also found in the LRT are colored. Compare to Figure 2. The Babot and Forasiepi 2016 cladogram includes tooth only and mandible only taxa.

A recent cladogram of Sparassodonta and its outgroups
(Fig. 1) was published in Babot and Forasiepi 2016 (Fig. 2). This cladogram is distinct from the large reptile tree (LRT, 1530 taxa), so no confirmation here.

FIgure 2. Subset of the LRT focusing on the clade Metatheria (Marsupialia). Taxa shared with Babot and Forasiepi 2016 are colored. Compared to Figure 1.

FIgure 3. Subset of the LRT focusing on the clade Metatheria (Marsupialia). Taxa shared with Babot and Forasiepi 2016 are colored. Compared to Figure 1. This cladogram includes relatively well-known and complete taxa.

Patene is known
from a tiny partial maxilla and mandible. I have not added it to the LRT. Mayulestes has just been downloaded, awaiting testing.

Figure 4. Thylacosmilus compared to Vincelestes separated by tens of millions of years. The both have maxillae conjoined dorsally to house the large canines.

Figure 4. Thylacosmilus compared to Vincelestes separated by tens of millions of years. The both have maxillae conjoined dorsally to house the large canines.

According to the LRT,
taxa missing from the Babot and Forasiepi tree include Vincelestes (a sister taxon to Thylacosmilus in the LRT) and a long list of other carnivorous marsupials. Hadrocodium is not included in Babot and Forasiepi. It attracts the other sabertooth, Patagosmilus (Fig. 5), as we learned earlier here.

Figure 1. Patagosmilus to scale alongside Hadrocodium. These sabetooth taxa are not directly related to Thylacosmilus in the LRT.

Figure 5. Patagosmilus to scale alongside Hadrocodium. These sabetooth taxa are not directly related to Thylacosmilus in the LRT.

FIgure 2b. Borhyaena skull cracked and angled to match the glenoid to the jaw joint, distinct from the original illustration (above).

FIgure 6. Borhyaena skull cracked and angled to match the glenoid to the jaw joint, distinct from the original illustration (above).

Late note: added the same evening as the original post:
Mayulestes ferox (Fig. 1) was just now added to the LRT, and it nests at the base of the Masrasector + Borhyaena clade. Nothing else changed. Thylacosmilus is still not related to  the dasyurids, including the creodonts and borhyaenids.


References
Babot J and Forasiepi AM 2016. Mamíferos predadores nativos del Cenozoico sudamericano: evidencias filogenéticas y paleoecológicas. Contributions del MACN 6 Historia evolutiva y paleobiogeografica de los vertebros de America del Sur. Agnolin FL et al. editors.
Forasiepi AM, MacPhee RDE and Hernandez del Pino 2019. Caudal cranium of Thylacosmilus atrox (Mammalia, Metatheria, Sparassodonta, a South American predaceous sabertooth. Bulletin of the American Museum of Natural History 433:1–64.

Hegetotherium and Pachyrukhos: not ungulates and not notoungulates

Hegetotherium and Pachyrukhos
were recently redescribed by Seoane and Cerdeño 2019 who considered them to be members of the Notoungulata, an invalidated polyphyletic clade with former members now nesting in various hooved marsupial and hooved placental clades.

Traditionally notoungulates are considered placentals and ungulates.
Seoane and Cerdeño report, “Hegetotheriidae is one of the most derived clades in the Order Notoungulata, the most abundant and diverse group of South American native ungulates.” The key word in there is ‘diverse’. Notungulates are SO diverse some of them are not related to the others.

Figure 1. Image from Cassini 2013. Pink taxa are marsupials. Others are placentals.

Figure 1. Image from Cassini 2013. Pink taxa are marsupials. Others are placentals.

We looked at Hegetotherium,
earlier. In the large reptile tree (LRT, 1517 taxa) Hegetotherium nests with Mesotherium and Interatherium at the base of the marsupial Toxodon clade, derived from the wombat (Vombatus) clade. All those are derived from the Paedotherium clade (e.g. Paedotherium, Phalanger, Petaurus and Thylacoleo). In Seoane and Cerdeño, Paedotherium is a taxon nesting close to Pachyrukhos, a taxon not included in the LRT.

The Seoane and Cerdeño taxon list also includes
PaedotheriumMesotherium and Hegetotherium, but not InteratheriumToxodon, Phalanger, Petaurus and Thylacoleo. So taxon exclusion and lacking a wide gamut viewappear to be twin problems here. Seoane and Cerdeño did not realize the taxa in their study were marsupials close to phalangers and wombats. They assumed, by tradition, they were dealing with placental ungulates, close to cows and deer.


References
Cassini G 2013. Skull Geometric Morphometrics and Paleoecology of Santacrucian (Late Early Miocene; Patagonia) Native Ungulates (Astrapotheria, Litopterna, and Notoungulata). Ameghiniana 50 (2):193–216. DOI: 10.5710/AMGH.7.04.2013.606
Seoane FD and Cerdeño E 2019. Systematic revision of Hegetotherium and Pachyrukhos (Hegetotheriidae, Notoungulata) and a new phylogenetic analysis of Hegetotheriidae. Journal of Systematic Palaeontology http://dx.doi.org/10.1080/14772019.2018.1545146

Ever hear of Palaeothentes?

Currently there is no Wikipedia page for this taxon.
Even so, I found it to be far more important at filling gaps and shaking up paradigms than it seemed at first. The small, dull-looking taxa tend to be like that, as readers now know.

You could find this rat-sized Miocene taxon
(Fig. 1) in Carroll’s 1988 book, Vertebrate Paleontology, now well-worn and in pieces due to constant page flipping and scanning. Today’s research has revealed several more precise and more recent resources.

Figure 1. Not a marsupial, and not a shrew opossum, Palaeothentes nests in the LRT at the base of the Apatemys + Trogosus clade nest to the clade of living shrew opossums within Glires.

Figure 1. Not a marsupial, and not a shrew opossum, Palaeothentes nests in the LRT at the base of the Apatemys + Trogosus clade next to the clade of living shrew opossums within Glires.

According to Abello and Candella 2010,
Palaeothentes minutes (Ameghino 1887) is a paucituberculatan (details below) from the Santa Cruz Formation The results indicate that Palaeothentes would have been an agile cursorial dweller, with leaping ability, similar to the extant paucituberculatan Caenolestes fuliginosus and the didelphid Metachirus nudicaudatus.”

Okay, so now we have a problem.
In the large reptile tree (LRT, 1445 taxa) Caenolestes is not a marsupial. It nests with Rhyncholestes and more distantly Apatemys and more distantly, the extant tree shrew, Tupaia and the extant shrew, Scutisorex. As noted earlier, shrew opossums are placental shrews, not marsupial opossums in the LRT.

Wikipedia reports,
“Like several other marsupials, they do not have a pouch, and it appears that females do not carry the young constantly, possibly leaving them in the burrow.”
That’s describes most rodent/rabbit/tree shrew mothers and their young.

Wikipedia reports,
“Paucituberculata is an order of South American marsupials. Although currently represented only by the eight living species of shrew opossums, this order was formerly much more diverse, with more than 60 extinct species named from the fossil record, particularly from the late Oligocene to early Miocene epochs.”

Let’s solve that problem
by adding Palaeothentes to the LRT. Doing so recovers this taxon at the base of the Apatemys + Trogosus clade, next to the clade that includes Caenolestes, within Glires, far from Marupialia.

I suspect taxon exclusion
is the cause for the present lack of confirmation for traditional consensus. Many PhDs over several decades have followed tradition in nesting and testing shrew opossums with marsupials without testing them against apatemyids apparently. That’s why the LRT is here, to test taxa that have never been tested together before.

But wait! There’s a novel twist here~~~~~~!
Carroll 1988 reports, “Caenolestids have long been recognized as being very distinct from other South American marsupials, but they share with them a highly distinctive pattern of the spermatozoa, which become paired within the epididymis. Paired sperm are not known in any placental groups or among the Australian marsupials.” 

Sorry.
Physical traits have to trump genes and sperm. It just has to be that way because the LRT includes fossil taxa, which never preserve sperm. There have to be rules that all participants abide by. Interesting that the gene for paired spermatozoa is localized to one continent, just as genes separate other placentals into afrotheres and laurasiatheres. By the way, “The data show that paired spermatozoa exhibit a significant motility advantage over single spermatozoa in a viscous medium” according to Moore and Taggart 1995, who tested Monodelphis, a South American opossum.

Finally
we have a last common ancestor for arboreal Apatemys (Eocene, North America) and terrestrial Trogosus (Eocene, North America), two former enigma taxa with little to no relationship with other better known mammal clades. All members of Glires had their genesis sometime in the Jurassic, based on the presence of highly derived multituberculates (clade: Glires) in the Jurassic.

Wikipedia considers
apatemyids and trogosinae (Tillodontia) to be members of the Cimolesta, “an extinct order of non-placental eutherian mammals.” This bungling of the mammal family tree is due to taxon exclusion and the lack of a phenomic (trait-based) wide gamut cladogram that includes all the taxa present in the LRT. Paleontology needs to toss off a wide range of useless tradition with a reptile revolution led by someone out there confirming (or refuting) the widest gamut cladogram presently available: the LRT.


Palaeothentes lemoinei (Ameghino 1887, MPM-PV 3566; Miocene) was considered a prehistoric shrew opossoum (clade: Paucituberculata) but here nests next to shrew opossums, at the base of the Apatemys + Trogosus clade within Glires. The skull is 2x wider than tall, the canines are still large, the last premolar is large with a flat occlusal surface and the nasals split to form a zigzag suture with the frontals.


References
Abello MA and Candela AM 2010. Postcranial skeleton of the Miocene Marsupial Palaeothentes(Paucituberculata, Palaeothentidae): Paleobiology and Phylogeny. Journal of Vertebrate Paleontology 30(5):1515-1527.
Ameghino F 1887. Enumeracions sistematicad e las especies de mamiferos
fosiles coleccionados por Carlos Ameghino en los terranos eocenos de la Patagonia austral y depositados en el Museo La Plata. Boletin Museo de La Plata, 1:1-26.
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.
Forasiepi AMSánchez-Villagra MR, Schmelzle T,  Ladevèze S and Kay RF 2014. An exceptionally well-preserved skeleton of Palaeothentes from the Early Miocene of Patagonia, Argentina: new insights into the anatomy of extinct paucituberculatan marsupials. Swiss Journal of Palaeontology, 133(1):1-21.
Moore HD and Taggart DA 1995. Sperm pairing in the opossum increases the efficiency of sperm movement in a viscous environment. Biol. Reprod. 52(4):947-53.
Osgood WH 1921. A monographic study of the American marsupial, Caenolestes. Field Museum of Natural History, Zoological series 14:1–156.

wiki/Apatemyidae
wiki/Paucituberculata
https://en.wikipedia.org/wiki/Shrew_opossum
wiki/Vertebrate_Paleontology_and_Evolution

Paucituberculata -Trouessart 1898, Ameghino 1894


There was some news
about Palaeothentes recently (see below). Note, the experts consulted here consider this genus a marsupial.

New Bolivian Marsupials from the Middle Miocene

Dactylopsila, the striped possum, enters the LRT

Dactylopsila trivirgata (Gray 1858) is the extant striped possum (Fig. 1), closely related to the sugar glider, Petaurus and the marsupial lion, Thylacoleo (below), according to the large reptile tree (LRT, 1412 taxa). Dactylopsila is an arboreal marsupial with a prehensile tail the size and proportions of a placental squirrel. The fourth finger is elongated and used to extract beetles and caterpillars from tree bark, analogous to the extant aye-aye, Daubentonia. Dactylopsila, also eats leaves, fruit and small vertebrates.

By convergence
Dactylopsila has similar teeth and overall proportions to the extinct arboreal placental Apatemys (Fig. 2).

Figure 1. Dactylopsila skull in 3 views, plus in vivo. Comparisons to the extinct arboreal placental Apatemys (figure 2) are intriguing, showing convergence.

Figure 1. Dactylopsila skull in 3 views, plus in vivo. Comparisons to the extinct arboreal placental Apatemys (figure 2) are intriguing, showing convergence.

For comparison, we recently looked at Apatemys
here as it relates to the extant shrew opossums Caenolestes and Rhyncholestes, now nesting as apatemyid placentals in the LRT, rather than as traditional didelphid marsupials. The convergence is powerful here. Despite the phylogenetic distance, only 12 extra steps are needed to nest caenolestids with basal didelphids.

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 2. 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.

The nesting of Dactylopsila
close to Petaurus (Fig. 3) is not controversial.

Figure 1. Subset of the LRT showing the nesting of Dactylopsila, the striped opossum.

Figure 3. Subset of the LRT showing the nesting of Dactylopsila, the striped opossum, with Petaurus the sugar glider and Thylacoleo, the marsupial lion.

The problem continues to be
the traditional nesting of the marsupial lion, Thylacoleo (Fig. 4), as a member of the wombats (Vombatiiformes), rather than the Phalangeriformes and Petauroidea, as recovered by the LRT (Fig. 3), which points to a bigger problem…

Nowhere in traditional taxon lists
will you find interatheres, toxodontids and creodonts. All these taxa need to be tested in traditional metathere trees because the LRT has tested them and they nest with metatheres. It’s a good time for a confirmation or a refutation. PhD students… are you looking for a good subject to write about for your dissertation?

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Figure 4. Large Thylacoleo skeleton compared to small Petaurus skeleton to scale. Dactylopsila is similar in size to Petaurus.

Here, again,
is where tradition, opinion and bias have, so far, trumped testing. Taxon exclusion needs to be tested with taxon inclusion. The list of taxa needing testing is provided by the LRT.


References
Gray JE 1858. List of species of Mammalia sent from the Aru Islands by Mr A.R. Wallace to the British Museum. Proceedings of the Zoological Society of London. 26: 106–113.

wiki/Striped_possum – Dactylopsia trivirgata

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.