Related mammals that nest at the bases of several hoofed clades

The value
of the large reptile tree (LRT, 1013 taxa) and the reconstructions gathered together at ReptileEvolution.com lie in their ability to put faces on names (Figs, 1) sometimes to scale (Fig. 2) to help one recognize patterns that may have gone unrecognized while just looking at names and scores.

These are the mammals that nest with one another
as sisters after deletion of more derived taxa in each of their several clades (listed at right, Fig. 1). Thus they, more or less, represent the basal radiation of hoofed mammals prior to each clade radiation. And to no one’s surprise, they look like each other, despite wide variations in size.

Frankly,
I’m reexamining the traits of these taxa because the LRT had trouble resolving them. There were mistakes in there. Now, after some score corrections, the resolution is complete again, but some Bootstrap scores have risen to just above 50. Not great, but better than below 50. Remember, I don’t have access to these specimens and sometimes work from published drawings.

Figure 1. Skulls of taxa nesting at the bases of several mammal clades starting with mesonychids.

Figure 1. Skulls of taxa nesting at the bases of several mammal clades starting with mesonychids. The differences are harder to see than in derived taxa in each clade (column at right). See figure 2 for skeletons to scale.

Some of these basal taxa
gave rise to baleen whales. Others were ancestral to giraffes, elephants, horses and everything in between. None of these taxa are nearly so famous or interesting to the general public, but it is from these generalized (plesiomorphic) taxa that the few and subtle evolutionary changes that are key to each clade first make an appearance.

Figure 2. Skeletons of taxa basal to various clades derived from basal mesonychids, all to scale.

Figure 2. Skeletons of taxa basal to various clades derived from basal mesonychids, all to scale. Note the presence of phylogenetic miniaturization at the base of the Artiodactyla. Consider this scenario: mesonychids radiated widely, including to create larger and smaller taxa. The larger homalotheres did not radiate greatly, as far as we know. On the other hand, the smaller taxa radiated to become a long list of extinct and exact hoofed taxa. And, of course, the mesonychid clade radiated to include today’s hippos and baleen whales. 

Sometimes a ‘show and tell’ drives a point home
better than just a ‘tell’. Even so, these are not the precise individuals in the direct lineage of known derived taxa, but the close relatives of those perhaps eternally unknown and hypothetical individuals. The suite of traits that lump and separate these taxa can be gleaned from the present MacClade file, continually added to and updated, and available here., which is where you can also see the cladogram from which the above taxa were pulled.

 

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Splitting up the Ungulates

Get ready for a good dose of heresy
brought to you by the large reptile tree (LRT 1003 taxa) a fully resolved online morphological study.

A little backstory
According to Wikipedia: Ungulates are any members of a diverse group of primarily large mammals that includes odd-toed ungulates such as horses and rhinoceroses, and even-toed ungulates such as cattlepigsgiraffescamelsdeer, and hippopotami.”

“However, in 2009 morphological and molecular[ work has found that aardvarks, hyraxes, sea cows, and elephants are more closely related to sengistenrecs, and golden moles than to the perissodactyls and artiodactyls, and form Afrotheria. Elephants, sea cows, and hyraxes are grouped together in the clade Paenungulata, while the aardvark has been considered as either a close relative to them or a close relative to sengis in the clade Afroinsectiphilia. This is a striking example of convergent evolution.

“There is now some dispute as to whether this smaller Ungulata is a cladistic (evolution-based) group, or merely a pheneticgroup (form taxon) or folk taxon (similar, but not necessarily related). Some studies have indeed found the mesaxonianungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the Ferae (the carnivorans and the pangolins) in the clade Fereuungulata or to the bats. Other studies found the two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae in Pegasoferae and others place the artiodactyls as close relatives to bats.” 

About that last hypothesis ~
If I had recovered that relationship I would expect pitchforks and torches.

Today
the LRT presents a hypothesis of mammal relationships in which hippos nest apart from other artiodactyls and perissodactyls nest apart from those two, separated by the elephant/hyrax/manatee clade with the hyrax-like Ectocion at it base (Fig. 1).

Figure 1. Subset of the LRT focusing on ungulates, which split into three clades here.

Figure 1. Subset of the LRT focusing on ungulates, which split into three clades here.

Note:
Hippos do indeed nest with whales, but only the mysticete whales. And desmostylians separate hippos from mysticetes. Odontocetes, like Orcinus, had a separate ancestry as we learned earlier here. The even-toed artiodactyls nest together with pig-types toward the base and deer-types a little later. The odd-toed perissodactyls nest together.

There are no
tenrecs, aardvarks or golden moles in this sector of the LRT.

Ungulata is not a monophyletic cladistic group,
unless it also includes the elephants and whales with Mesonyx as a basal member, (a taxon close to the last common ancestor). I’m not the first one to come to this conclusion. Probably best just to drop this name from usage and realize that hooves appeared several times in mammal evolution.

A little extra study
cleared up the earlier Pappaceras issue. Now the nesting is more in accord with consensus with regard to the indricotheres, but they’re still closer to 3-toed horses than to 3-toed rhinos.

Also note:
the <50 Bootstrap scores surrounding Hyracotherum, a basal horse/rhino. I could only separate sister taxa by 2 steps. It takes at least 3 steps to bounce the Boostrap score over 50. Small steps. Small steps.

 

Maybe Paraceratherium is really a giant horse.

Figure 1. Subset of the large reptile tree focusing on horses and their kin.

Figure 1. Subset of the large reptile tree focusing on ungulates and their kin.

Today’s heresy began when several ungulate taxa
were added to the large reptile tree (LRT, Fig. 1, now 907 taxa, completely resolved with 228 traits). Equus, the horse; Paraceratherium, the giant hornless ‘rhino’; Ceratotherium the white rhinoceros and Embolotherium, a  Mongolian brontothere.

It is widely accepted
and supported by the LRT that horses and rhinos share a common ancestor (Fig. 1). In this case, the LRT recovered either 1) an overlooked relationship, or 2) a case of convergence between Paraceratherium and Equus (Fig. 2). Wikipedia  notes that rhinos are more closely related to tapirs. In the LRT tapirs are basal to a sister clade to the rhino/horse clade. Hyracotherium and Heptodon are basal to the rhino/horse clade.

Figure 1. Equus and Paraceratherium nest together on the LRT.

Figure 2. Equus and Paraceratherium currently nest together on the LRT. Additional taxa will, no doubt, change that, but at present, Paraceratherium shares more traits with Equus than Ceratotherium, the white rhino (Fig. 2). The long neck and premaxillay teeth, along with other traits, separate these two from extant rhinos.

Equus ferus (Linneaus 1758; Figs. 2,3) includes several extant horses, mules and zebras. Compared to Hyracotherium the Equus preorbital region is longer, the frontal produces a postorbital bar that contacts the squamosal. The premolars are molarized. All four limbs end in a single toe, digit 3.

Figure 1. Equus the extant horse.

Figure 3. Equus the extant horse has a postorbital bar, an elongate rostrum and a single toe on each limb.

Hyracotherium leporinum (Owen 1841; 78 cm long; Eocene, 55-45 mya; BMNH C21361nests basal to the horse/rhino clade, EquusHeptodon (Fig. 5) is an Eocene sister. Canines remained. A diastema separated the canine from the premolars. The manus has four hoofed toes. The pes has three hoofed toes. The premolars were becoming molarized. Compare the skull of dog-size Hyracotherium to the similar skull of Paraceratherium, one of the largest land mammals of all time and Equus, the horse.

Figure 2. Hyracotherium is an Eocene horse sister in the LRT. Skull bones are colorized here.

Figure 4. Hyracotherium is an Eocene horse sister in the LRT. Skull bones are colorized here.

Heptodon (Pachynolophus) posticus (Cope 1882; Eocene, 50 mya; 1m in length) was derived from a sister to Tapirus and was itself a sister to Hyracotherium and the base of brototheres like Embolotherium (below). Heptadon had reduced canines and developed a diastema (lack of teeth) posterior to them. The posterior cranium was slightly elevated. The manus had four digits. The pes had three. 

Figure 4. Heptodon originally nested with Tapirus, but with the addition of Equus, Hyracotherium and Embolotherium it shifted to nest with Embolotherium.

Figure 5. Heptodon earlier nested with Tapirus, but with the addition of Equus, Hyracotherium, Paraceratherium and Embolotherium it nested closer to them.

The extant white rhinoceros
Ceratotherium simus (extant, Figs. 6, 7)) shares little in common with Paraceratherium, but shares a long list of traits with Embolotherium, including an oddly elevated pair of nasals and the near complete loss of the lumbar region. Both had smaller ancestors, so direct comparisons yield several possible convergences that currently nest as homologs. That’s what happens with taxon exclusion.

Figure 6. Ceratotherium, simum, the white rhinoceros with keratinous horns in dark brown. Note the elevated nasals and convex dentary ventral margin.

Figure 6. Ceratotherium, simum, the white rhinoceros with keratinous horns in dark brown. Note the elevated nasals and convex dentary ventral margin.

Figure 7. Ceratotherium (white rhino) skeleton, distinct from the long-legged Paraceratherium.

Figure 7. Ceratotherium (white rhino) skeleton, distinct from the long-legged Paraceratherium.

The last taxon to be added is
Embolotherium andrewsi (Osborn 1929; Late Eocene; 2.5m tall at the shoulder; Mongolia; Figs. 4, 5). It nests as a sister to the rhino, Ceratotherium, but this is likely to be overturned on convergence when additional taxa are added. This highly derived brontothere (= titanothere) has forked ‘horns’ (= rams). The rams are elevated nasal bones, hollow and fragile. These are in contrast to the solid rams of North American brontotheres. Click here to see the original AMNH illustration of the Embolotherium skull that portrays the area beneath the elevated nasal as a thick fleshy area. Alternatively Wikipedia reports, “the bony nasal cavity extends to the peak of the ram, thus implying that the nasal chamber was greatly elevated, possibly creating a resonating chamber.”

Figure 2. Brontotherium, a sister to Embolotherium.

Figure 8. Brontotherium, a sister to Embolotherium, which is known from skulls, but no complete post-crania.

The skull of Embolotherium was 2x wider than tall at the orbits. The molars were much larger than the premolars, which were themselves molarized. The canines were vestiges. The posterior skull was greatly elevated. The dorsal spines were greatly elevated (Fig. 4). The lumbar region was reduced. The ilia were transverse. Four fingers are retained by the manus, indicating an early divergence from three-fingered horses and rhinos.

Figure 4. Embolotherium andrewsi modified from the AMNH website to show a possible inflatable narial area.

Figure 9. Embolotherium andrewsi animation modified from the AMNH website (see link above) to show a possible inflatable narial area, contra the original restoration with a fleshy, immobile sub-nasal area.

Figure 8. Paraceratherium pes. Note the reduced lateral and medial toes (2 and 4). As in Equus, and distinct from Ceratotherium, the central toe is much larger.

Figure 10. Paraceratherium pes. Note the reduced lateral and medial toes (2 and 4). As in Equus, and distinct from Ceratotherium, the central toe is much larger.

These are all perissodactyls
(odd-toed ungulates)
despite the fact that the manus has four digits. The pes (with toes) has an odd-number of digits (three or one). Wikipedia nests Desmostylia and Anthracobunidae among the perissodactyls, but they nest with hippos and mesonychids in the LRT.

Hyracodon
is not included here, but nests in the LRT between Hyracotherium and Heptodon, far from Paraceratherium. Apparently Equus and Paraceratherium have not been tested together in phylogenetic analysis under the assumption that one was a horse and the other a rhino.

Finally
note the large third digit of the pes of the giant three-toed horse, Paraceratherium (Fig. 10), as in Equus and distinct from Ceratotherium, which has three toes, but sub equal in size.

The above was dashed off
a little more quickly than usual. Please bring to my attention any typos or dangling hypotheses.

 

References
Cope ED 1882. Paleontological Bulletin 34:187.
Froehlich DJ 2002. Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla). Zoological Journal of the Linnean Society. 134 (2): 141–256.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Osborn HF 1929. Embolotherium, gen. nov., of the Ulan Gochu, Mongolia. American Museum novitates; no. 353.
Owen R 1841. Description of the Fossil Remains of a Mammal (Hyracotherium leporinum) and of a Bird (Lithornis vulturinus) from the London Clay. Transactions of the Geological Society of London, Series 2, VI: 203-208.

wiki/Equus
wiki/Hyracotherium
wiki/Embolotherium
wiki/Paraceratherium
wiki/Heptodon

Pigs and whales? No.

Sometimes when you add a common extant taxon
to the LRT, there can be more here than meets the eye. That was the case with Sus, the extant pig (Fig. 1).

Figure 1. Skeleton of Sus, the pig, a taxon commonly used as an outgroup for whales. In the LRT it is a sister to other even-toed ungulates, like Giraffa, not Odontoceti nor Mysticeti.

Figure 1. Skeleton of Sus, the pig, a taxon commonly used as an outgroup for whales. In the LRT it is a sister to other even-toed ungulates, like Giraffa, not Odontoceti nor Mysticeti.

Backstory
I was looking at a list of outgroup taxa for whales in Bianucci and Gingerich 2011 and comparing to to the outgroup taxa for whales in Geisler et al. 2011 and other workers:

  1. Gingerich 2011 listed: Elomeryx and illustrated a cow (Bos).
  2. Bianucci and Gingerich 2011 listed: Sinonyx, Mesonyx, Hippopotamus and Sus in that order toward Cetacea.
  3. Geisler et al. listed: Sus, Bos and Hippopotamidae in that order toward Cetacea.
  4. O’Leary and Gatesy 2008 listed: Eoconodon, Sinonyx and Hapalodectes [all considered Mesonychia by them]
  5. O’Leary et al. 2013 listed: Sus, Bos and Hippopotamus in that order toward Cetacea.

Note that
Sus, the pig; Bos, the cow; and Hippopotamus, the obvious, somehow makes it to three lists as outgroup taxa for whales in general. Believe it or not, these three earned their status after testing by traditional paleontologists. Despite having very few traits in common with whales, creating a great leap of phylogenetic faith to connect them all. If you’re bothered by that, I join you!

Figure 1. Ancodus nests as a more derived sister to Sus and it retains digit 1 on the manus and pes.

Figure 2. Ancodus nests as a more derived sister to Sus and it retains digit 1 on the manus and pes. Is this the same taxon as Elomeryx? If not, they appear to be quite close.

Sidenote:
Elomeryx (see above) is said to be widespread and common, but apparently has been confused online with Ancodus (Fig. 1). Are they the same? If different, how different? I’m confused and could use some clarity.

When we add Sus to the LRT
Sus nests much more reasonably between Tapirus and Ancodus (Fig. 2), two pig-like taxa with hooves. Notably extant Sus (Fig. 2) loses digit 1 on both the manus and pes while extinct Ancodus retains those digits indicating a convergent loss of these digits in the ancestors of pigs and in the ancestors of deer + giraffes.

By contrast O’Leary and Gatesy 2008 report, 
“Cetacea was the extant sister taxon of Hippopotamidae, followed successively by Ruminantia, Suina and Camelidae. The wholly extinct Mesonychia was more closely related to Cetacea than was any ‘‘artiodactylan. The osteological–dental data alone, however, did not support inclusion of cetaceans within crown ‘‘Artiodactyla.’ Recently discovered ankle bones from fossil whales reinforced the monophyly of Cetartiodactyla but provided no particular evidence of derived similarities between hippopotamids and fossil cetaceans that were not shared with other ‘‘artiodactylans’’. Can you sense their lack of resolution? Based on present evidence, O’Leary and Gatesy were suffering from taxon exclusion.

No such problem
with the LRT where whales are not related to pigs, cows or camels. Odontocete whales arise from tenrecs. Mysticete whales arise from desmostylians, as we learned earlier here.

References
Bianucci G and Gingerich PD 2011. Aegyptocetus tarfa, n. gen. et sp. (Mammalia, Cetacea), from the middle Eocene of Egypt: clinorhynchy, olfaction, and hearing in a protocetid whale. Journal of Vertebrate Paleontology. 31 (6): 1173–88.
Demere TA, McGowen MR, Berta A & Gatesy J. 2008.
 Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales, Systematic Biology, 57 (1) 15-37. DOI: 10.1080/10635150701884632\
Geisler JH, McGowen MR, Yang G and Gatesy J 2011. A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea. Evolutionary Biology 11:112.
Gingerich PD 2005. Aquatic Adaptation and Swimming Mode Inferred from Skeletal Proportions in the Miocene Desmostylian Desmostylus. Journal of Mammalian Evolution, Vol. 12, Nos. 1/2, June 2005.
Gingerich PD 2011. Evolution of whales from land to sea. online here.
Marx FG and Fordyce RE 2015. Baleen boom and bust: a synthesis of mysticete phylogeny, diversity and disparity, Royal Society open Science 2:14034.
Marx FG, Hocking DP, Park T, Ziegler T, Evans AR and Fitzgerald EMG 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria 75:71-82.
O’Leary et al. 2013 The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals. Science 339. 662 SuppData.
O’Leary MA and Gatesy J 2008. Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossils. Cladistics 24:397-442.

 

More notoungulates leave this extinct clade.

Some of the taxa I met this weekend, and you’ll meet shortly,
I’ve never heard of before. Even so, here’s another paleo story waiting to be told.

When I added
Protypotherium (Fig. 1) and Miocochilius (Fig. 2) to the large reptile tree (now 828 taxa, but not yet updated), I learned that they were both considered members of the Interatheridae, which Wikipedia considers,  “an extinct family of notoungulate mammals from South America.”

Figure 1. Protypotherium nests with Miocochilius and Homalodotherium in the LRT between mesonychids and paenungulates.

Figure 1. Protypotherium nests with Miocochilius and Homalodotherium in the LRT between mesonychids and paenungulates.

Earlier we learned
that all tested members of the Notoungulata do nest not with each other, but with a variety of established clades within the Theria. Some members of the invalid Notoungulata nested with wombats, others with various placentals. And today this issue keeps dragging on…

Figure 2. Miocochilius has reduced it imprint to just two functional toes.

Figure 2. Miocochilius has reduced it imprint to just two functional fingers and toes. Look at those long horse-like teeth exposed by erosion of the bone.

So where there’s an Interatheridae…
there should be an Interatherium. And there is one (Fig. 3). Trouble is, the Interatherium skull with that crooked diastema did not look like the straight jawed full set of teeth found in Protypotherium and Miocochilius. So I added Interatherium to the LRT to find out.

Turns out, like many notoungulates before, 
that Interatherium nests with wombats, between Vombatus and Toxodon even though Interatherium is much smaller, longer, and leaner than its sisters. It also lacks digit 1 on all four extremities. Apparently there is a wide range of wombat/marsupial morphologies that we’re just now beginning to appreciate and identify. For instance, that notch and descending maxillary palate in Interatherium (Fig. 3) resemble those of the bizarre marsupial beaver-mimic wombat, Vintana, which also nests nearby.

Figure 3. Interatherium does not nest with notoungulates or other purported interotheres. Rather cat-sized Interatherium nests with wombats, between Vombatus and the giant Toxodon.

Figure 3. Interatherium does not nest with notoungulates or other purported interotheres. Rather cat-sized Interatherium nests with wombats, between Vombatus and the giant Toxodon.

If Interatherium is a marsupial, 
the two placentals, Protypotherium and Miocochilius, cannot be interatheres. Or if they were defined as interatheres, that definition should be modified. Best to just call Protypotherium and Miocochilius typotheres — unless Typotherium turns out to be unrelated to these two, too.

Now,
as I look up “Typotherium,” I see it is better known by its senior synonym, Mesotherium, originally named, according to Wikipedia, in the belief that it was a transitional taxon between rodents and pachyderms. I have not tested Mesotherium (Fig. 4) yet, but with that diastema, it looks more like Interatherium, the wombat, than Protypotherium and Miocochilius, the condylarths. Only testing will tell, though…

Figure 4. Mesotherium. What is it, if not the long lost link between rodents and elephants? : )

Figure 4. Mesotherium. What is it, if not the long lost link between rodents and elephants?  Just kidding! I don’t see scale bars for this taxon, but Wiki reports it was the size of a small sheep at 55 kilograms (121 pounds. So it probably had a skull the size of a small sheep.

But wait, there’s more…
There was, indeed, a larger member of the Protypotherium and Miocochilius clade. Seems that the  clawed browser, Homalodotherium (Fig. 5), which I earlier nested with the clawed ungulate, Chalicotherium, now nests with Protypotherium and Miocochilius given the opportunity to do so (taxon exclusion rises again!). These three form a clade nesting between mesonychids (including hippos and Paleoparadoxia) and paenungulates (elephants, hyraxes and sea cows) where big front teeth rule, but tusks are not yet present in these three (yet-to-be-named clade) taxa. Maybe we should call this clade of three (so far) the new ‘Notoungulata’. Does that work for you? If someone can send me the reference for the origin of this term, we’ll see if that will fit or not.

Figure 6. Bear-sized Homalodotherium nests with cat-sized Protypotherium and Miocochilius. They all have a full arcade (11x4) of relatively flat teeth.

Figure 6. Bear-sized Homalodotherium nests with cat-sized Protypotherium and Miocochilius. They all have a full arcade (11×4) of relatively flat teeth. Note the resemblance to Chalicotherium here, by convergence.

I’m catching up
but still behind in updating the pertinent web page at ReptileEvolution.com. It’s not easy, but I’m getting there.

References
Ameghino F 1882. Ungulata, Typotheria, Interatheridae. Catalog de la Prov. de Buenos Aires en la Expedicion Cont. Sud-America. March 1882. Boletino Instituto Geologia Argentino, June 1882. Cont. Conocimenient Mamif. Fosil. Repub. Argentina, in Accd. Nac. Cien., Cordoba 6:474-480. 1889.
Croft DA 2007. The Middle Miocene (Laventan) Quebrada Honda wildlife, southern Bolivia and a description of ITS Notoungulates. Palaeontology 50(1):277-303.
Huxley TH 1870. Anniversary address of the President of the Geological Society. Quarterly Journal of the Geological Society of London 26:42-64.
Stirton RA 1953. A new genus of interatheres from the Miocene of Colombia. University of California Publications in Geological Sciences 29: 265-348

wiki/Homalodotherium
wiki/Protypotherium
wiki/Miocochilius

 

 

Macrauchenia: a South American perissodactyl

Figure 1. Macrauchenia museum mount.

Figure 1. Macrauchenia museum mount.

Figure x. Macrauchenia cladogtam. Tapir and Chalicotherium are perissodactyls.

Figure x. Macrauchenia cladogtam. Tapir and Chalicotherium are perissodactyls.

Famous for its assumed
elephant-like proboscis, arising from a dorsal narial opening (Fig 2), Macrauchenia was a long-legged grazing ungulate with three toes on each manus and pes. In the large reptile tree it nests with Chalicotherium, which, in turn, nests with Tapirus, (the tapir) an extant perissodactyl with a short flexible trunk. A recent analysis of collagen sequences (Welker et al. 2015) found the same relationship. Not sure why this needed resolution… tapirs also have a trunk, dorsal narial opening, three hooves per foot AND some still live in South America. I guess that used to be considered ‘convergence.’ Here the LRT calls it ‘homology.’

Discovered by Charles Darwin in 1834,
and published by Richard Owen in 1836, Macrauchenia patachonica (Pliocene 7mya to Pleistocene .02mya; 3m length) was otherwise similar to a camel in proportions with a horse-like skull. Macrauchenia was an herbivore with a full arcade of short teeth in its jaws (Fig. 2). The last premolar looks like a molar, but, like other premolars, it is slightly larger than the other teeth and all sister taxa have 3 molars per side.

Figure 2. Macrauchenia skull in several view (from Owen 1836?) with bones colorized here. Note the dorsal extension of the premaxilla.

Figure 2. Macrauchenia skull in several view (from Owen 1836?) with bones colorized here. Note the dorsal extension of the premaxilla. The fossa posterior to the naris could anchor large proboscis muscles. 6 premolars and 3 molars appear to have been present. Not sure about the palatine here.

Wikpedia reports, “Macrauchenia was a long-necked and long-limbed, three-toed South American ungulate mammal, typifying the order Litopterna.  Early forms are near the condylarths, to such an extent that the litopterns might be considered merely as surviving and diversely specialized condylarths.” The LRT did not nest Macrauchenia with the basal Condylartha, but that is still a monophyletic clade that now includes all hoofed and edentate mammals — along with all the original basal condylarths.

Thanks to reader SBJ
for suggesting a number of South American mammals to add to the LRT. This is number one of several to come.

References
Owen R 1838. Description of Parts of the Skeleton of Macrauchenia patachonica. In Darwin, C. R. Fossil Mammalia Part 1 No. 1. The zoology of the voyage of H.M.S. Beagle. London: Smith Elder and Co.
Welker F et al. 2015. Ancient proteins resolve the evolutionary history of Darwin’s South American ungulates. Nature. doi:10.1038/nature14249. ISSN 0028-0836.
wiki/Macrauchenia

Arsinoitherium nests with Uintatherium

There are a few extinct mammals
that are so distinctly different from all others that, like ‘the cheese,’ they seem to stand alone. But that’s not possible. This is, after all, evolution where every taxon is related, whether closely or distantly, to every other taxon. That means all the answers can be found in phylogenetic analysis. Here we’ll use the large reptile tree which keeps on working, now at 775 taxa.

Figure 1. Arsinoitherium in several views. This is one specimen that splits the horns down to their bases. See figure 2 for other specimens.

Figure 1. Arsinoitherium in several views. This is one specimen that splits the horns down to their bases. See figure 2 for other specimens that merge the twin horns medially.

So it is with
Arsinoitherium zitteli (Beadnell 1902, Figs. 1, 2; late Eocene, early Oligocene, 36-30 mya) which Wikipedia describes as, an extinct genus of paenungulate mammal belonging to the extinct order Embrithopoda. It is related to elephants, sirenians, hyraxes and the extinct desmostylians.” None of these candidates have any hint of the huge nasal and tiny frontal horns that make Arsinoitherium distinct. Another trait that makes Arsinoitherium distinct is its nearly complete arcade of teeth, none of them tusks, in which the canine is absent (or at least indistinct) and the medial incisor is missing. The premaxilla also redevelops an ascending process of the premaxilla, a trait not seen in mammals since their origin in the Triassic.

Figure 2. Two other specimens of Arsinoitherium where the nasal horns do not separate except at the top.

Figure 2. Two other specimens of Arsinoitherium where the nasal horns do not separate except at the top. Images from Andrews 1906.

So if you add
Arsinoitherium to the large reptile tree (Subset Fig. 2) it does not nest with elephants, sirenians and hyraxes all of which have premaxillary tusks. Instead Arsinoitherium nests with Uintatherium (Figs. 3, 4), a similarly sized and built herbivore with horns all over its skull, including medium-sized nasal horns and larger frontal horns. The tooth arcade is not complete in Uintatherium and the canine enlarges to become a sabertooth.

Figure 5. Uintatherium skull with a different set of horns and a different sort of tooth row with large canines and only vestigial inicisors.

Figure 5. Uintatherium skull with a different set of horns and a different sort of tooth row with large canines and only vestigial inicisors.

The postcrania
of Uintatherium and Arsinoitherium are practically interchangeable. Both have five short fingers and toes, robust limbs, wide hips and a short neck. Hornless Phenacorus was the last common ancestor among tested taxa (Fig. 5).

Figure 6. Uintatherium overall. Note the postcranial similarities with Arsinoitherium.

Figure 4. Uintatherium overall. Note the postcranial similarities with Arsinoitherium. Both have five fingers and toes.

Figure 5. Arsinoitherium nests with Uintatherium in the large reptile tree.

Figure 5. Arsinoitherium nests with Uintatherium in the large reptile tree.

So why was the Arsinoitherium/ Unitatherium relationship not recognized earlier?
Not sure why this was overlooked. The skulls are distinctly different in several respects. In the end, PAUP finds the path of least resistance, otherwise known as maximum parsimony, from the choices (taxa) it is given. In this case, until more closely related taxa can be added, Arsinoitherium nests with Uintatherium, with which it shares more traits than it does with elephants, sirenians and hyraxes, which nest together in a nearby clade outside the Ungulata (Fig. 5).

References
Andrews CW 1906. A descriptive catalogue of the Tertiary Vertebrata of the Fayum, Egypt. Trustees of the British Museum. London, Taylor and Francis, 324pp.
Beadnell HGC 1902. A preliminary note on Arsinoitherium zitteli, Beadnell, from the Upper Eocene strata of Egypt. Public Works Ministry, National Printing Department. Cairo: 1–4.