Bishops enters the LRT

Figure 1. The dentary of Bishops compared to its Late Cretaceous sister, Asioryctes, which has fewer and larger premolars.

Figure 1. The dentary of Early Cretaceous Bishops (1.5cm long) compared to its Late Cretaceous sister, Asioryctes, which has fewer and larger premolars and one more molar.

The genus Bishops whitmorei
(Rich et al. 2001; Early Cretaceous, Australia; Fig.1) is represented by a small mandible with a high coronoid process, six premolars and only three molars. In the LRT it nests basal to the much larger carnivorous marsupials (= creodonts), starting with the wolf-sized Arctocyon. It is a sister to Asioryctes (Fig. 1) which is basal to the herbivorous marsupials of Australia.

What makes this important?
It is the only tiny creodont known. All others are dog to wolf-sized. Cenozoic descendants of Bishops include the following carnivorous marsupials: Thylacinus, Thylacosmilus, Borhyaena, Hyaenodon and Vincelestes.

References
Rich TH, Flannery TF, Trusler P. Kool L, van Klaveren NA and Vickers-Rich P 2001. A second tribosphenic mammal from the Mesozoic of Australia. Records of the Queen Victoria Museum 110: 1-9.

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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

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

Another taxon shift: Deltatherium moves to Arctocyon

Continuing to clean up a recalcitrant basal Theria…
further reexamination finds overlooked similarities and another taxon shift. Here, the similarities are obvious as a former basal carnivorous placental moves next to a basal carnivorous marsupial.

Deltatherium fundaminis (Cope 1881, middle Paleocene 60 mya, 14 cm skull length,  AMNH 16610, Figs. 1, 2) this former creodont and former tillodont now nests with the most basal carnivores of the Metatheria like Arctocyon (Fig. 1). Wikipedia reports, “its relatives are far from clear.” The large cranial crest anchored large jaw muscles for driving in those long saber teeth. These taxa are derived from the Virginia opossum, Didelphis (Fig. 1).

Figure 1. Arctocyon surrounded by related taxa to scale, including Deltatherium.

Figure 1. Arctocyon surrounded by related taxa to scale, including Deltatherium.

Arctocyon primaevus (Blainville 1841, Gould and Rose 2014; YPM VP 021233; Paleocene, 60 mya) was long and widely considered a primitive plantigrade ungulate condylarth procreodi placental. In the LRT Arctocyon nests with basal carnivorous/omnivorous marsupials. Essentially it is a giant opossum, like Didelphis, but with a few derived traits, more like Thylacinus, a taxon that reduces the epipubes and molar count, hence the earlier traditional confusion.

Figure 2. Deltatherium nests with Arctocyon as a basal carnivorous marsupial.

Figure 2. Deltatherium nests with Arctocyon as a basal carnivorous marsupial.

References
Blainville HM 1841. Osteographie et description iconographique des Mammiferes récentes et fossiles (Carnivores) 1, 2 Paris.
Cope ED 1881. Mammalia of the lower Eocene beds. American Naturalist 15:337-338.
Gould FDH and Rose KD 2014. Gnathic and postcranial skeleton of the largest known arctocyonid ‘condylarth’ Arctocyon mumak (Mammalia, Procreodi) and ecomorphological diversity in Procreodi. Journal of Vertebrate Paleontology 34(5):1180-1202.

wiki/Arctocyon
wiki/Deltatherium

Toxodon: now closer to kangaroos than to wombats

Figure 1. The skulls of Toxodon, Procoptodon and Interatherium resemble one another more than their post-crania might suggest. Now they nest together in the LRT (subset in figure 2).

Figure 1. The skulls of Toxodon, Procoptodon and Interatherium resemble one another more than their post-crania might suggest. Now they nest together in the LRT (subset in figure 2).

Another short one today
just to dash off a progress report as I wrestle with metatherian data. Like everyone else, I’m learning as I go. Toxodon and Eurygenium (Fig. 1) were always close to Interatherium, which was recently nested at the base of all kangaroos with the addition of the short-faced kangaroo, Procoptodon to the large reptile tree (LRT, 1258 taxa). Bulky, quadrupedal Toxodon and Eurgenium previously nested with quadrupedal wombats. 

The loss of pedal digit 1
found in kangaroos, interatheres and
Toxodon + Eurygenium turns out to be not a convergence, but a homoplasy as the Toxodon + Eurygenium node shifts over to the Interatherium node. All three are quadrupeds.

Now basal wombats with five pedal digits,
like the koala, no longer have four-toed taxa, like Eurygenium and Toxodon, separating them from their five-toed ancestors. 

Interatheres are getting to be more interesting. 

Figure 2. Subset of the large reptile tree focusing on the Metatheria. The tree is fully resolved, but many bootstrap scores under 50 indicate that only one or two characters separate those nodes with low scores. Scores higher than 50 are separated by three or more traits.

Figure 2. Subset of the large reptile tree focusing on the Metatheria. The tree is fully resolved, but many bootstrap scores under 50 indicate that only one or two characters separate those nodes with low scores. Scores higher than 50 are separated by three or more traits.

BTW
loss of pedal digit 1 also occurs alone in the wolf-like marsupial carnivore, the Tasmanian wolf, Thylacinus, by convergence.  By convergence, pedal digit 1 also shrinks in the bandicoot (Perameles) clade.

Geologically
western Gondwana (Africa + South America) separated from eastern Gondwana (Madagascar, India, Australia, Antartica) about 180 mya (Jurassic). That’s when the ancestors of South American Interatherium and Toxodon separated from the Australian ancestors of kangaroos. This is one way to estimate the antiquity of mammal clades.

Final thought for paleontologists and soon-to-be-paleontologists:
Reexamining data, like this, is good science. Making mistakes. like this, goes with the territory. Naiveté and enthusiasm go hand-in-hand. Apologies often follow. Gaining experience is a slog, but worthwhile when the puzzle pieces fit better in the end. More work often brings new insight. 

 

 

A surprising ancestor for kangaroos: Interatherium

Traditionally the short-faced kangaroo,
(genus: Procoptodon; Owen 1870; Pleistocene; Figs. 1, 3) was considered an aberrant taxon with a weirdly shortened face, so unlike that of traditional kangaroos, like Macropus (Fig. 1). However, by adding taxa, like Procoptodon and Dendrolagus (Fig. 1), to the large reptile tree (LRT, 1248 taxa, subset Fig. 4) Interatherium (Figs. 1,2) shifts over to become an ancestral kangaroo, despite lacking hopping legs and diprotodont teeth. The skulls of Interatherium and Procoptodon are incredibly similar, even if the post-crania and dental formula of Procoptodon has evolved.

Figure 1. Skulls of kangaroo ancestors and sisters, including Procoptodon (short-faced kangaroo) alongside Interatherium (ancestral kangaroo) and Dendrolagus (basal kangaroo). Interatherium does not have diprotodont dentition.

Figure 1. Skulls of kangaroo ancestors and sisters, including Procoptodon (short-faced kangaroo) alongside Interatherium (ancestral kangaroo) and Dendrolagus (basal kangaroo). Interatherium does not have diprotodont dentition. Nambaroo nests within Glires, not Metatheria.

Interatherium (Mid-Miocene)
has not been linked to Procoptodon (Pleistocene) before.

And why should it?

  1. Balbaroo (Flannery, Archer and Plane, 1983; Black et al. 2014, Middle Miocene) was hailed as a kangaroo ancestor, but in the LRT it nests with the phalanger, Phalanger
  2. Cookeroo bulwidarri (Butler et al. 2016; Late Oligocene, Early Miocene, 23-18mya)was hailed as a non-hopping kangaroo ancestor. The LRT has not tested it yet, but it looks like Macropus (Fig. 1).
  3. Palaeopotorous priscus (den Boer and Kear 2018; middle Miocene) was hailed as a non-hopping kangaroo ancestor, based on teeth.
  4. Tradition considers Interatheriidae “an extinct family of notoungulate (placental) mammals from South America, known from the Eocene through the Miocene. These animals were principally small-sized, occupying a habitat like hares and marmots.The majority were very small, like rodents.”
  5. Interatherium has four fingers (Fig. 2), lacking a thumb (convergent, it turns out, with Protypotherium, a placental herbivore traditionally considered related). Kangaroos retain five fingers (but I’d like to see a good X-ray or something similar).
Figure 2. Interatherium is the surprising ancestor of kangaroos, with a special affinity to the short-face kangaroo.

Figure 2. Interatherium is the surprising ancestor of kangaroos, with a special affinity to the short-face kangaroo.

Current DNA studies
place a small wallaby, Lagostrophus, at the base of their kangaroo cladogram, but Lagostrophus already has diprodontid teeth. That’s too easy. We’re looking for an earlier, more primitive taxon, without obvious kangaroo traits.

Figure 4. Procoptodon is a basal kangaroo, close to Interatherium (Fig. 3).

Figure 3. Procoptodon is a basal kangaroo, close to Interatherium (Fig. 3). Here longer legs and longer feet differentiate this taxon from Interatherium.

Interatherium (Miocene) represents a late-surviving member
of a much earlier (Late Jurassic) kangaroo radiation, in which the interathere clade lost its thumb. Alternate scenario: perhaps the thumb was never collected in the matrix. The epipubes were likewise somehow overlooked, though I think I see them is an online image of an in situ fossil. More data needed here.

This Late Jurassic kangaroo genesis
is based on the Early Cretaceous appearance of Anebodon, a kangaroo cousin more closely related to the extant marsupial mole, Notoryctes. These burrowers, in turn, have more kangaroo-like sister taxa, today represented by the bandicoot Perameles and the biliby, Macrotis, which combine long hind limbs and digging front limbs.

Note, the front dentary teeth of Interatherium
(Fig. 1). The change to diprotodonty (two anterior fangs) has not happened yet in Interatherium, but the canines are on the way out and the squamosals are very tall.

Figure 4. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Figure 4. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Interatherium rodens (Ameghino 1887, 1894; Middle Miocene; 50cm in length) the Interatheridae and Interatherium were long considered members of the Notoungulata, a clade that has broken up in the LRT. Here (Fig. 4) Interatherium nests at the base of the kangaroos, derived from the more basal marsupials like Eomaia. Interatherium retains several small incisors, but apparently has lost its thumb, unlike kangaroos.

Note
that Interatherium, nesting at the base of the kangaroo clade (Fig. 4), is also the sister to the Toxodon + the wombat (genus: Vombatus) clade. There the diprotodont dental pattern appears by convergence because, like Interatherium, basal taxa (genus: Eurygenium, late Oligocene, and Toxodon) lack a diprotodont dental pattern.

Goodbye, Diprotodontia.
The clade Diprotodontia is no longer monophyletic (Fig. 4) and can no longer be exclusively defined by the diprotodont dental pattern, which now appears twice within the Metatheria. Please test this heresy and let me know what you get. Taxon exclusion is once again the problem here.

References
Ameghino F 1887. Observaciones generales sobre el orden de mamíferos estinguidos sud-americanos llamados toxodontes (Toxodontia) y sinopsis de los géneros y especies hasta ahora conocidos. Anales del Museo de La Plata 1:1-66.
Ameghino F 1894. Enumeration synoptique des especes de mammifères fossiles des formations éocènes de Patagonie. Boletin de la Academia Nacional de Ciencias en Cordoba (Republica Argentina) 13:259-452.
Black KH et al. 2014. A New Species of the Basal “Kangaroo” Balbaroo and a Re-Evaluation of Stem Macropodiform Interrelationships. PloseOne https://doi.org/10.1371/journal.pone.0112705
den Boer W and Kear BP 2018. Is the fossil rat-kangaroo Palaeopotorous priscus the most basally branching stem macropodiform? Journal of Vertebrate Paleontology; e1428196 DOI: 10.1080/02724634.2017.1428196
Butler K, Travouillon KJ,Price GJ, Archer M and Hand SJ 2016. Cookeroo, a new genus of fossil kangaroo (Marsupialia, Macropodidae) from the Oligo-Miocene of Riversleigh, northwestern Queensland, Australia. Journal of Vertebrate Paleontology. doi:10.1080/02724634.2016.1083029.
Cooke BN 2000. Cranial remains of a new species of balbarine kangaroo (Marsupalia: Macropodoidea) from the Oligo-Miocene freshwater limestone deposits of Riversleigh World Heritage Area, Northern Australia. Journal of Paleontology 74(2) 317-26.
Flannery TF, Archer M and Plane MD 1983. Middle Miocene kangaroos (Macropodoidea: Marsupialia) from three localities in northern Australia, with a description of two new subfamilies. Bureau of Mineral Resources, Journal of Australian Geology and Geophysics 7: 287–302.
Owen R 1873. Procoptodon goliah, Owen. Proceedings of the Royal Society of London 21, 387.

wiki/Interatherium
wiki/Lagostrophus
wiki/Procoptodon
http://www.abc.net.au/news/2016-02-19/extinct-non-hopping-species-may-be-ancestors-of-kangaroo/7185650
Palaeopotorous PR: https://www.sciencedaily.com/releases/2018/04/180411111019.htm

Metathere (aka: marsupial) issues

Metatherians
(aka marsupials) can be a difficult clade to understand phylogenetically. Taxon exclusion, as usual, causes problems, here and elsewhere. Case in point: recently adding taxa to the large reptile tree (LRT, 1247 taxa) shifted and clarified some prior interrelationships in which certain untenable dental patterns appeared. This was cause for concern and re-study. I’m pleased to report that the herbivorous metathere subset of the tree topology did change to a more logical and gradual pattern after adding several taxa. Now the dental atavisms no longer appear. But this new topology comes at the cost of recovering a dual and parallel origin for the diprotodont form of dentition (Fig. 1) in which the anteriormost dentary teeth are larger than typical canines and jut out anteriorly.

Figure 1. Marsupial mandibles. Traditional diprodonts have two large anterior dentary teeth. These arose twice in the LRT.

Figure 1. Marsupial mandibles. Traditional diprodonts have two large anterior dentary teeth. These arose twice in the LRT with the present list of taxa, once with kangaroos, and again with wombats. See figure 3.

First and second,
let’s take a look at two previously published metatherian tree topologies: Horovitz and Sánchez-Villagra 2003 (which covers many living and some extinct taxa) and Williamson, Brusatte and Wilson 2014 (in which taxa are all Cretaceous or earlier and most are known from isolated teeth). The LRT includes no tooth-only taxa… and that’s a good thing.

Horovitz and Sánchez-Villagra 2003 (Fig. 2) employed bones and soft tissue.
From their abstract: “We… assembl[ed] a morphological data matrix consisting of a new suite of 149 postcranial characters and incorporated a series of previously published data on the craniodental (76 characters) and soft tissue (5 characters) anatomy. Twenty-one marsupial terminal taxa representing all the major radiations of marsupials and 10 outgroups… were investigated. All currently accepted marsupial orders were recovered by the analysis.”

Figure 1. A cladogram of metatherian mammals based on skeletal and soft traits by Horovitz and Sánchez-Villagra 2003.

Figure 2. A cladogram of metatherian mammals based on skeletal and soft traits by Horovitz and Sánchez-Villagra 2003. This cladogram lacks a large number of carnivorous metatherians, a large number of basal prototheres and a large number of basal eutherians. On the other hand, the LRT is missing the uncolored taxa. Colors correspond to the metathere subset of the LRT (Fig. 3). Horovitz and Sánchez-Villagra 2003 recovered a monophyletic Diprotodontia in contrast to the LRT.

More recently,
Williams et al. 2014 reported, “Our understanding of this group has increased greatly over the past 20 years, with the discovery of new specimens and the application of new analytical tools. Here we provide a review of the phylogenetic relationships of metatherians with respect to other mammals, discuss the taxonomic definition and diagnosis of Metatheria, outline the Cretaceous history of major metatherian clades, describe the paleobiology, biogeography, and macroevolution of Cretaceous metatherians, and provide a physical and climatic background of Cretaceous metatherian faunas.” They built their study on Williams 2012, which focused on teeth. They report, “As in the previous analysis of Williamson et al. (2012), homoplasy is rampant.” Hmm. That’s a phrase I used to describe character distribution in the LRT!

Williams et al. 2014 reported, 
“The oldest confidently identified therian fossil is the eutherian Juramaia from the early Late Jurassic (ca. 160 million years old) of China (Luo et al. 2011).” The LRT nests Juramaia as a basalmost prototherian. They considered Sinodelphys (Early Cretaceous, 120 mya) to be the oldest known marsupial. In 2015 (a year after Williams et al.) Agilodocodon (Middle Jurassic, 170 mya) was announced as a docodont (but nests with Eomaia, Early Cretaceous, 125 mya) in the LRT.

Williams et al. 2014 reported, 
“Deltatheroidans were long regarded as eutherians (Gregory and Simpson 1926; Van Valen 1966) or stem boreosphenidan species (Fox 1974; Kielan-Jaworowska et al. 1979), but are now generally accepted as basal metatherians (Butler and Kielan-Jaworowska 1973; Kielan-Jaworowska and Nessov 1990; Rougier et al. 1998).” The LRT confirms a nesting within the Metatheria for Deltatheridium, not as a sister.

Williams et al. 2014 reported,
“The interrelationships of most major metatherian subclades are unresolved.” This is due to taxon exclusion and using too many tooth-only taxa. On the other hand, the metatherian taxa and clades within the LRT are fully resolved. The two studies share only 4 taxa in common so the Wiliams et al. cladogram will not be shown. Despite the availability of museum specimens, no extant taxa were used in Williams et al. 2014 study, which concentrated on Cretaceous taxa to the detriment of the study.

Maybe it would have been better
for Williams et al. 2014 to first establish relationships using extinct and extant skeletons of a wide gamut of mammals, as the LRT does, and then see where the tooth-only taxa fit in.

Figure 1. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Figure 3. Subset of the LRT focusing on the Metatheria (= Marsupials). Here the diprotodont dentition evolved twice.

 

 

More on metatherians soon…

References
Horovitz I and Sánchez-Villagra MR 2003. A morphological analysis of marsupial mammal higher-level relationships. Cladistics 19(3):181 – 212.
DOI: 10.1111/j.1096-0031.2003.tb00363.x
Myers P, Espinosa R, Parr CS, Jones T, Hammond GS and Dewey TA 2018. The Animal Diversity Web (online). Accessed at https://animaldiversity.org.
info@tree-kangaroo.net
Williamson TE, Brusatte SL and Wilson GP 2014. The origin and early evolution of metatherian mammals: the Cretaceous record. ZooKeys 465:1–76.

doi: 10.3897/zookeys.465.8178
http://zookeys.pensoft.net