Asioryctes: Re-restoring a pes, re-nesting a taxon

I should have noticed this pairing earlier.
Evidently it escaped everyone else’s notice, too. Asioryctes nemegetensis (Kielan-Jaworowska 1975, 1984; Figs. 1,2; middle Late Cretaceous, Djadokhta Formation, ~85 mya) is a good match for the living bandicoot, Perameles. Maga and Beck 2017 nested Asioryctes with the coeval Ukhaatherium, and the extant Perameles with another bandicoot, Echymipera.

FIgure 1. Skulls of Asioryctes, Perameles and Macrotis compared.

FIgure 1. Skulls of Asioryctes, Perameles and Macrotis compared. The overall shapes are similar, and so are the teeth, and other details. Historically the feet have been different, and that’s our starting point. 

Figure 2. Left: original restoration of Asioryctes pes. Colors added. Right: New restoration based on phylogenetic proximity to Perameles and other marsupial taxa with vestigial digit 1 and gracile digits 2 and 3 (grooming claws).

Figure 2. Left: original restoration of Asioryctes pes. Colors added. Right: New restoration based on phylogenetic proximity to Perameles and other marsupial taxa with vestigial digit 1 and gracile digits 2 and 3 (grooming claws). 

The first three taxa
are members of the large reptile tree (LRT, 1272 taxa), but the first two don’t nest together. The LRT now nests Asioryctes with Perameles and Macrotis, two extant bandicoots. Ukhaatherium nests with the basalmost members of Theria several nodes earlier.

One of the problems with this
is the original restoration of the Asioryctes pes, based on disarticulated parts (Kielan-Jaworowska 1975; Fig. 2). The REAL problem is no other mammal has gracile lateral metatarsals. Sans the pes, the skull nests with Perameles and Macrotis (Fig. 1), taxa with only a vestige pedal digit 1 and reduced digits 2 and 3.

Hmmm.
That opens up a possibility not foreseen by Kielan-Jaworowska.

A new restoration
of the illustrated elements (Fig. 2) identifies the slender metatarsals as 2 and 3. The tarsal elements are all present (contra Kielan-Jaworowska 1975) just reidentified here in accord with a standard bandicoot foot.

And… so… for the first time
we can see a predecessor taxon demonstrating a transitional morphology to the reduced pedal digits 1–3 seen in bandicoots and kangaroos.

References
Geoffrey Saint-Hilaire E 1803. Note sur les genres Phascolomis et Perameles, nouveaux genres d’animaux à bourse. Bulletin des Sciences par la Société Philomathique de Paris 80, 49–150.
Kielan-Jaworowska Z 1975. 
Preliminary description of two new eutherian genera from the Late Cretaceous of Mongolia. Palaeontologia Polonica 33:5-15.
Kielan-Jaworowska, Z 1984. Evolution of the therian mammals in the Late Cretaceous of Asia. Part VII. Synopsis. Palaeontologia Polonica 4:173-183. online pdf
Maga AM and Beck RMD 2017. Skeleton of an unusual, cat-sized marsupial relative (Metatheria: Marsupialiformes) from the middle Eocene (Lutetian: 44-43 million years ago) of Turkey. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0181712

wiki/Asioryctes
wiki/Perameles
wiki/Macrotis

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Cladogram of the Mammalia (subset of the LRT)

A summary today…
featuring a long cladogram (Fig. 1), a subset from the large reptile tree (LRT, 1259 taxa) focusing on the Mammalia. This is how this LRT subset stands at present. Not much has changed other than the few node changes from the past week.

The transition from Prototheria to Theria (Metatheria)
includes long-snouted taxa, like Ukhaatherium. Nearly all Prototheria are also long-snouted (Cifelliodon is the current sole exception).

The transition from Metatheria to Eutheria (simplified)
includes small omnivorous didelphids arising from the carnivorous/herbivorous split among larger metatherians. Basal Carnivora, the most basal eutherian clade, are also omnivores. Caluromys, the extant wooly opossum, has a pouch, but nests at the base of all placental taxa (the LRT tests only skeletal traits), so it represents the size and shape of the earliest placentals (contra O’Leary et al. 2013)… basically didelphids without pouches, and fewer teeth, generally (but not always).

Basal members of most placental clades
are all Caluromys-like taxa, with a rapid radiation in the Late Triassic/Early Jurassic generating most of the major placental clades in the LRT (Fig. 1). Larger members of each of these placental clades appeared in the fossil record only after the K-T extinction event. So hardy where these basal taxa, that many still live to this day.

As shown earlier, higher eutheria are born able to able to walk or swim. They are no longer helpless with arboreal parents (tree-climbing goats the exception). Basal eutherians reproduce more like their metatherian ancestors, with helpless infants.

Figure 1. Subset of the LRT focusing on mammals.

Figure 1. Subset of the LRT focusing on mammals. Extant taxa are colored. Thylacinus is recently extinct.

The latest competing study
(O’Leary et al. 2013, Fig. 2) recovers the highly specialized edentates, aardvarks, elephants and elephant shrews as the most primitive placentals. Carnivora + bats are quite derived in the O’Leary team cladogram, somehow giving rise to ungulates and whales. This is an untenable hypothesis. It doesn’t make sense. Evidently the O’Leary team had faith that smaller didelphid-like ancestors would fill in the enormous phylogenetic gaps in their cladogram. By contrast the LRT has all the operational taxonomic units (OTUs) it needs to produce a series of gradually accumulating derived traits between every taxon in its chart (Fig. 1). The LRT makes sense.

Figure 5. Simplified version of the O'Leary et al 2013 cladogram showing placental relations exploded after the K-T boundary.

Figure 5. Simplified version of the O’Leary et al 2013 cladogram showing placental relations exploded after the K-T boundary.

References
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of  placentals. Science 339:662-667. abstract
Wible JR, Rougier GW, Novacek MJ, Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary Nature 447: 1003-1006

https://pterosaurheresies.wordpress.com/2016/08/31/another-look-at-the-oleary-et-al-hypothetical-ancestor-of-placentals/

https://pterosaurheresies.wordpress.com/2013/02/15/post-k-t-explosion-of-placentals-oleary-et-al-2013/

ArchibaldEtAl.pdf
protungulatum-donnae website

Naked, horned and pocket gophers

Figure 1. Subset of the LRT focusing on the rabbit/rodent and kin clade where gophers nest with hedgehogs.

Figure 1. Subset of the LRT focusing on the rabbit/rodent and kin clade where gophers nest with hedgehogs.

In the large reptile tree (LRT, 1258 taxa, Fig. 1) the naked mole rat (genus: Heterocephalus, Fig. 2) nests with the hedgehog clade, one node off from the mouse/rat/clade. So the naked mole rat should be the  naked mole gopher.

Figure 1. The naked mole rat, Heterocephalus is closer to hedgehogs than to rats.

Figure 2. The naked mole rat, Heterocephalus is closer to hedgehogs than to rats.

Heterocephalus glaber (Rüppell 1842-5; 8-10cm) is the extant naked mole rat. It has a cold-blooded metabolism, lives underground, and can move backwards as fast as forward. Not the claws, but the teeth (protruding outside the lips) are used for digging. Heterocephalus is essentially hairless, lives in a colony dominated by a queen and may live up to 32 years in a low oxygen environment, or several times longer than related taxa.

Figure 2. Naked mole rat (Heterocephalus) skull in several view. The mandibles are disarticulated here, but the glenoid appears to be reduced to absent, providing great mobility to the jaws.

Figure 2. Naked mole rat (Heterocephalus) skull in several view. The mandibles are disarticulated here, but the glenoid appears to be reduced to absent, providing great mobility to the jaws.

Ceratogaulus hatcheri is the extinct horned gopher (Fig. 3). It nests with the naked mole rat in the LRT (Fig. 1).

Figure 3. Ceratogaulus, the extinct horned gopher

Figure 3. Ceratogaulus, the extinct horned gopher

Thomomys bottae (Figs. 4, 5) is the extant pocket gopher, another rodent nesting with hedgehogs.

Figure 4. Skull of Thomomys, the extant pocket gopher.

Figure 4. Skull of Thomomys, the extant pocket gopher. No large retroarticular process here.

These taxa look like rodents
but they nest with hedgehogs. So do we expand our concept of rodents (lumping)? Or make new clades (splitting)?

Figure 5. Skeleton of Thomomys, the pocket gopher.

Figure 5. Skeleton of Thomomys, the pocket gopher.

 

Rodentia is characterized by a single pair of continuously growing incisors in each of the jaws, as opposed to rabbits, which have two incisors.

Glires (Latin glīrēsdormice) is a clade consisting of rodents and lagomorphs (rabbits, hares, and pikas). In the LRT many more clades of small mammals nest with rabbits and rodents.

Euarchontoglires (synonymous with Supraprimates) is a clade of mammals, the living members of which belong to one of the five following groups: rodentslagomorphstreeshrewscolugos and primates. In the LRT rodents nest with primates, but not colugos.

References
Rüppell E 1842-5. Säugethiere aus der Ordnung der Nager, beobachtet im nordöstlichen Africa. Museum Senckenbergianum: Abhandlungen aus dem Gebiete der beschreibenden Naturgeschichte. 3: 99–101.

wiki/Hedgehogs
wiki/Erinaceus
wiki/Echinops
wiki/Naked_mole-rat

https://blogs.scientificamerican.com/tetrapod-zoology/african-mole-rats-so-much-more-than-just-the-naked-mole-rat/

Molecules vs morphology in mammals: Beck and Baillie 2018

Some published thoughts
on traits vs. molecules just out in the last week.

Beck and Baillie 2018 titled their paper: 
“Improvements in the fossil record may largely resolve the conflict between morphological and molecular estimates of mammal phylogeny.” No. Just the opposite. But you can see exactly where they put their faith… not in what they can see and measure.

From the abstract (annotated):
“Morphological phylogenies of mammals continue to show major conflicts with the robust molecular consensus view of their relationships.” True.

“This raises doubts as to whether current morphological character sets are able to accurately resolve mammal relationships, particularly for fossil taxa for which, in most cases, molecular data is unlikely to ever become available.” Just the opposite. Doubts should have been raised about molecular data, which can be influenced by local viruses. Only physical traits, i. e. the expression of activated molecules, resolves relationships, as the large reptile tree (LRT, 1255 taxa) attests. 

“We tested this under a hypothetical ‘best case scenario’ by using ancestral state reconstruction (under both maximum parsimony and maximum likelihood) to infer the morphologies of fossil ancestors for all clades present in a recent comprehensive molecular phylogeny of mammals, and then seeing what effect inclusion of these predicted ancestors had on unconstrained analyses of morphological data. We found that this resulted in topologies that are highly congruent with the molecular consensus, even when simulating the effect of incomplete fossilisation. Most strikingly, several analyses recovered monophyly of clades that have never been found in previous morphology-only studies, such as Afrotheria and Laurasiatheria.” In other words, we used our imaginations to make molecule phylogenies work, rather than considering the possibility that molecular phylogenies did not work. 

“Our results suggest that, at least in principle, improvements in the fossil record may be sufficient to largely reconcile morphological and molecular phylogenies of mammals, even with current morphological character sets.” They used far too few taxa. And they used suprageneric taxa. They avoided fossil taxa. This is omitting available data. 

This is not the way science is supposed to work.
So why was this published?

References
Beck RMD and Baillie C 2018. Improvements in the fossil record may largely resolve the conflict between morphological and molecular estimates of mammal phylogeny. bioRxiv doi:10.1101/373191. First posted online July 20, 2018.
http://www.biorxiv.org/content/biorxiv/early/2018/07/20/373191.full.pdf

Think of aardvarks and sloths as naked and hairy glyptodonts respectively

Because
that’s what they really are… aardvarks are naked and sloths are hairy glyptodonts. And, yes, that comes as a surprise, it breaks a paradigm, it spins your head around, it’s heretical… and it’s exactly where the data takes us.

The Edentata is an odd clade
in which the basalmost taxa, like Barylambda, Glyptodon and Holmesina are very large. On the other hand, terminal extant and derived taxa, like Peltephilus and Cyclopesare much smaller, just the opposite of most mammal clades (in which smaller usually lead to larger, following Cope’s Rule.)

Figure 1. Subset of the LRT focusing on edentates and their outgroup, Barylambda. Here two glypotodonts nest at the bases of the two major clades.

Figure 1. Subset of the LRT focusing on edentates and their outgroup, Barylambda. Here two glypotodonts nest at the bases of the two major clades.

According to Wikipedia,
“Glyptodontinae (glyptodonts or glyptodontines) are an extinct subfamily of large, heavily armored armadillos which developed in South America and spread to North America.”

In the large reptile tree (LRT, 1252, edentate subset Fig. 1) the glyptodont, Glyptodon, nests between the massive Barylambda and giant sloths, followed by smaller tree sloths and small extinct horned armadillos, like Peltephilus and Fruitafossor. On another branch (Fig. 1) another large glyptodont, Holmesina, nests between the massive Barylambda and the much smaller aardvark, Orycteropus, the armadillo, Dasypus, and the anteaters, Tamandua and Cyclopes.

Such a big-to-small phylogenetic pattern,
is known as phylogenetic miniaturization or the Lilliput Effect and is often the product of neotony (adults retaining juvenile traits, including juvenile size).

Figure 2. Holmesina, the glyptodont ancestor to aardvarks, anteaters and armadillos.

Figure 2. Holmesina, the glyptodont ancestor to aardvarks, anteaters and armadillos. Those are aardvark hands (Fig. 3), glyptodont feet.

Holmesina (Fig. 2) is added to the LRT today.
Basically it is a longer-snouted glyptodont, basal to the longer snouted above-mentioned aardvarks, armadillos and anteaters.

Following a reader comment,
(suggesting ‘taxon exclusion’ was the issue that did not unite glyptodonts with armadillos) I was looking for a transitional taxon to more closely nest glyptodonts with armadillos, rather than sloths. I did so and the tree topology did not change when Holmesina was added. Armadillos are still one taxon removed from glyptodonts, but at least now we have a glyptodont on the long-nosed clade of aardvarks, etc.. As before, aardvarks nest between glyptodonts and armadillos. Looking at all the edentate taxa in detail and overall. I think this nesting and this tree topology seem very reasonable (= it produces a gradual accumulation of derived traits at all nodes and between all taxa).

Figure 3. Orycterpus, the extant aardvark, is a living sister to Barylambda from the Paleocene.

Figure 3. Orycterpus, the extant aardvark, is a living sister to Barylambda from the Paleocene. Aardvarks traditionally nest alone, but in the LRT they are edentates without armor… or hair.

Other workers, like Fernicola, Vizcaíno and Fariña 2008,
described the phylogeny of glyptodonts by putting taxa like Holmesina at the base while omitting Barylambda. Thus such studies do not present the full picture due to taxon exclusion. Everyone seems to omit Barylambda and all the other edentate outgroups back to Devonian tetrapods… but not the LRT.

Goodbye ‘Xenarthra’. Goodbye ‘Pilosa’. Goodbye ‘Cingulata’.
According to Wikipedia, “The order Pilosa is a group of placental mammals, extant today only in the Americas. It includes the anteaters and sloths, including the extinct ground sloths, which became extinct about 10,000 years ago.” According to Wikipedia, Cingulata, part of the superorder Xenarthra, is an order of armored New World placental mammals.” In the LRT ‘Xenarthra’ (Cope 1889) is a junior synonym for ‘Pilosa’ (Flower 1883) and that is a junior synonym for Edentata (Darwin 1859).

References
Darwin C 1859. On the origin of species.
Fernicola JC, Vizcaíno SF and Fariña RA 2008.
The evolution of armored xenarthrans and a phylogeny of the glyptodonts. Chapter 7 in: The Biology of the Xenarthra, Eds: Vizcaíno SF and Loughry WJ. University Press of Florida.
Gaudin TJ and Croft DA 2015. Paleogene Xenarthra and the evolution of South American mammals. Journal of Mammalogy 96 (4): 622–634. https://doi.org/10.1093/jmammal/gyv073

http://www.finedictionary.com/Edentata.html

 

 

 

 

Ukhaatherium: a late-surviving Morganucodon in the LRT

Ukhaatherium nessovi (Late Cretaceous, Novacek et al. 1997) has traditionally been nested with the proto-placental, Asioryctes. Here in the large reptile tree (LRT, 1243 taxa), Ukhaatherium nests as a basal therian in the large reptile tree. Morganucodon (Latest Triassic/Earliest Jurassic, Küehne 1949) nests with Ukhaatherium, with virtually identical scores and are similar in size.

Figure 1. Morganucodon skull in several views. Compare to Ukhaatherium in figure 2.

Figure 1. Morganucodon skull in several views. Compare to Ukhaatherium in figure 2.

When you see them together,
(Figs. 1,2) the similarities between Morganucodon and Ukhaatherium are strikingly obvious.

Figure 2. Skull and dentition of Ukhaatherium. Compare to Morganucodon in figure 1.

Figure 2. Skull and dentition of Ukhaatherium. Compare to Morganucodon in figure 1.

The long snout of prototheres
is retained in basal therians like Morganucodon and Ukhaatherium. The epipubic bones found in Ukhaatherium are no surprise in the LRT, where it nests in the middle of other taxa that share this trait.

Figure 3. Ukhaatherium in situ.

Figure 3. Ukhaatherium in situ.

Rowe 1988 defined Mammaliaformes
as Morganucondontidae + Mammalia. Here Morganucodontidae nest within the Mammalia, so Mammaliaformes is a junior synonym of Mammalia.

Figure 3. Subset of the LRT with the addition of Perameles and Morganucodon.

Figure 4. Subset of the LRT with the addition of Perameles and Morganucodon. Sharp-eyed readers with good memories will not subtle changes in this tree near protorotheres.

And, by the way,
Ukhaatherium + Morganucodon is still close to Asioryctes in the LRT. If we let a little time go by from the Latest Triassic to the Latest Cretaceous, then that’s enough time for the tiny posterior jaw bones in Morganucodon to evolve into the tiny ear bones of Ukhaatherium, convergent and parallel to those of other therian and prototherian mammals (which already show distinct patterns in this regard).

References
Kuehne WG 1949. On a triconodont tooth of a new pattern from a fissure-filling in South Glamorgan. Proceedings of the Zoological Society of London 119:345-350
Novacek MJ, Rogier GW, Wible JR, McKenna MC, Dashzev g D and Horovitz I 1997. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia Nature 389: 483-486.
Rowe TS 1988. Definition, diagnosis and origin of Mammalia. Journal of Vertebrate Paleontology 8(3):241–264.

wiki/Ukhaatherium
wiki/Morganucodon

New(?) mammal at the Eutheria-Metatheria split

Bi et al. 2018 bring us a small Virginia opossum from the early Cretaceous
“Molecular estimates of the divergence of placental and marsupial mammals and their broader clades (Eutheria and Metatheria, respectively) fall primarily in the Jurassic period. Supporting these estimates, Juramaia, the oldest purported eutherian is from the early Late Jurassic (160 million years ago) of northeastern China. Sinodelphys, the oldest purported metatherianâis from the same geographic area but is 35 million years younger, from the Jehol biota. Here we report a new Jehol eutherian, Ambolestes zhoui, with a nearly complete skeleton that preserves anatomical details that are unknown from contemporaneous mammals, including the ectotympanic and hyoid apparatus. This new fossil demonstrates that Sinodelphys is a eutherian, and that postcranial differences between Sinodelphys and the Jehol eutherian Eomaia, previously thought to indicate separate invasions of a scansorial niche by eutherians and metatherians, are instead variations among the early members of the placental lineage. The oldest known metatherians are now not from eastern Asia but are 110 million years old from western North America, which produces a 50-million-year ghost lineage for Metatheria.”

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

Figure 1. Ambolestes tracing from Bi et al. 2018. plate and counter plate. Note the scale bar. This taxon is one third the size of the extant opossum. Apparently and oddly no unguals were preserved.

In the large reptile tree (LRT, 1131 taxa) the new fossil at the Metatheria/ Eutheria split is phylogenetically identical to and therefore congeneric with Didelphis, the Virginia opossum. So, this is pre-marsupial in a clade at the base of the marsupial/placental split. There are also a series of pre-placentals, some of which are extant and retain a reduced pouch, like Monodelphis.

Figure 2. Ambolestes skull in situ with DGS applied.

Figure 2. Ambolestes skull in situ with DGS applied.

Ambolestes zhoui gen. & sp. nov. (Bi et al. 2016, 126 mya; 25cm in length) is is one third the size of the extant opossum, but otherwise nearly identical based on tested traits. In the LRT Didelphis nests with Ambolestes and they nest with Eomaia and Agilodocodon as the last common ancestors to Metatheria and Eutheria. Bi said in an interview, “Ambolestes zhoui is an early member of the placental lineage. It also carries mixed features both placentals and marsupials”. In the LRT, Ambolestes is exactly as much in the placental lineage (Fig. 4) as Didelphis is… and Didelphis has a pouch. Both have prepubic (epipubic) bones.

Since Ambulestes is congeneric with Didelphis,
you heard it hear first when the LRT nested Didelphis as the last common ancestor of Metatheria and Eutheria. Good to see confirmation.

Figure 3. Ambolestes skull reconstructed. Jaw tips restored.

Figure 3. Ambolestes skull reconstructed. Jaw tips restored. Lower last molar appears to be just erupting. No retroartcular process is apparent here, which sometimes happens with Didelphis (Mohamed 2018)

The Pittsburgh Post-Gazette reported,
“The well-preserved new mammal, an ancient furry creature most similar to a modern tree shrew, is named Ambolestes zhoui.” Actually Ambolestes was a little less exotic than that.

“John Wible, curator of mammals at the Carnegie Museum of Natural History, became involved in the project about two years ago.As soon as I saw the photographs of the fossil I was like, ‘Oh my God this is amazing,’ ” he said. “It was amazingly complete. Right off the bat I saw there were skeletal parts of the body that were not known of other animals of that time period.”

But if Dr. Wible happened upon a certain type of roadkill
or went out after midnight with a flashlight he would have seen a living version of the Early Cretaceous fossil. Rather than, “this is amazing” he could have said, “we could have predicted this.”

Figure 7. Subset of the LRT focusing on basal live-bearing mammals.

Figure 4. Subset of the LRT focusing on basal live-bearing mammals.

The Post-Gazette also reported, 
“The fossil was not allowed to leave China, said Mr. Wible, noting that this is the first paper he’s published where he’s been unable to actually hold the fossil, though he hopes to see it in person in the next few years. Instead, he relied on detailed photographs and scanned images.”

Figure 4. Didelphis, the extant opossum, a sister to the smaller Ambolestes

Figure 5. Didelphis, the extant opossum, a larger sister to the smaller and 126 million years older Ambolestes.

Now it’s important to remember 
that tooth traits can converge and reverse. Think about archaeocetes (pre-whales), which have three cusps all in a row, like cynodont. Consider odontocetes, which have simple cones, like basal reptiles. Thus, tooth only taxa must be treated separately from skeletal taxa and cladograms must be based on skeletal traits, not tooth traits, which can be dangerous based on the issues that arise from the Bi et al. cladgoram (Fig. 6).

Take another look at taxa listed in Bi et al. 2018
Maelestes, when tested with more taxa, nests at the base of the tenrec/odontoete clade. Necrolestes nests with the golden mole, Chrysochloris, a basal member of Glires. Zhangeotherium is a basal pangolin. They (Bi et al. and other basal mammal workers) are going to have to expand their taxon lists to include at least all the mammals that don’t have hooves, and that includes a few that do have quasi-hooves, like the descendants of Maelestes.

FIgure 8. Cladogram published in Bi et al. 2018 with colors added to show taxa appearing elsewhere in the LRT. As you can see, this is a mess, likely created by too much emphasis on teeth traits, which converge and reverse.

FIgure 6. Cladogram published in Bi et al. 2018 with colors added to show taxa appearing elsewhere in the LRT. As you can see, this is a mess, likely created by too much emphasis on teeth traits, which converge and reverse. Treeshrew-like Maelestes, when tested with more taxa, nests at the base of the tenrec/odontoete clade. Necrolestes nests with the golden mole, Chrysochloris, a basal member of Glires. Zhangeotherium is a basal pangolin.

References
Bi S-B, Zheng X-T, X, Wang X-L, Cignetti N-E, Yang SL and Wible JR 2018.
An Early Cretaceous eutherian and the placental/marsupial dichotomy. Nature (advance online publication) DOI: https://doi.org/10.1038/s41586-018-0210-3
https://www.nature.com/articles/s41586-018-0210-3
Mohamed R. 2018. Anatomical and radiographic study on the skull and mandible of the common opossum (Didelphis marsupialis Linneaus, 1758 (in the Caribbean). Veterinary Sciences 5(44) 10 pp. doi:10.3390/vetsci5020044

https://carnegiemnh.org/press/new-mammal-fossil-provides-insights-on-early-placental-mammal-evolution/