Nesting twin-horned Arsinoitherium within the Condylarthra

Figure 1. Famous and enigmatic, Arsinoitherium has been known for over a century, and traditional paleontologists still do not know what it is.

Figure 1. Famous and enigmatic, Arsinoitherium has been known for over a century, and traditional paleontologists still do not know what it is.

FIgure 1. Subset of the large reptile tree, the Condylarthra, featuring Astrapotherium. Note the phylogenetic proximity of Astrapotherium and Tapirus.

FIgure 1. Subset of the large reptile tree, the Condylarthra, featuring Astrapotherium. Note the phylogenetic proximity of Astrapotherium and Tapirus.

Traditionally Arsinoitherium zitteli has been hard to classify.
Wikipedia reports, “Arsinoitherium (Beadnell 1902; Eocene-Oligocene, 36-30mya; 3 m in length; Fig. 1) is related to elephants, sirenians, hyraxes and the extinct desmostylians.” That’s a pretty broad gamut of taxa.

And they’re all wrong according to the large reptile tree (now 812 taxa, subset Fig. 2).

And this came as a surprise to me, too
among 811 other taxa, Arsinoitherium nests with Gobiatherium mirificum (Fig. 3; Osborn and Granger 1932; Middle Eocene), which Wikipedia considers, “one of the last uintatheres” of which Uintatherium is the titular and most famous member. Wikipedia goes on to report, “Gobiatherium lacked knob-like horns, or even fang-like tusks. Instead, it had enlarged cheekbones and an almost spherical snout. Because of the noticeable lack of many diagnostic uintathere features (the horns and tusks), the genus is placed within its own subfamily.” Here’s where tradition and the LRT agree… but let’s push this a little further to see where it takes us within the friendly confines of the current LRT taxon list.

Figure 3. Gobiatherium skull in three views. Though not immediately apparent, Gobiatherium is closest to Arsinoitherium in the LRT.

Figure 3. Gobiatherium skull (A. M. 26624) in three views. Though not immediately apparent, Gobiatherium is closest to Arsinoitherium in the LRT. Image from Osborn and Granger 1932.

Among all tested placental taxa, and despite distinct overall appearances
only Arsinoitherium and Gobiatherium:

  1. redevelop the ascending process of the premaxilla, completely enclosing the naris;
  2. produce a wide, elevated set of nasals, further expanding into horns in Arsinoitherium;
  3. only two molars, rare among placentals;
  4. and no other condylarths have a wide flat cranium, usually a crest or a convex cranium is present.

That premaxillary ascending process
looks so normal. But among marsupial and placental mammals it is very rare indeed! Of course, the LRT does not depend on one or several traits, several dozen nest Arsinoitherium with Gobiatherium and their sisters.

Even without Gobiatherium
Arsinoitherium nests with Uintatherium. Coryphodon nests closer to Uintatherium. All descend from a sister to Thomashuxleya (Fig. 4), which we’ll look at soon in greater detail.

Figure 4. Thomashuxleya is basal to uintatheries and arsionoitheres. It is not a notoungulate, an invalid taxon.

Figure 4. Thomashuxleya is basal to uintatheries and arsionoitheres. It is not a notoungulate, an invalid taxon.

We hold as an ideal
a gradual accumulation of derived traits in derived taxa, like Gobiatherium and Asinoitherium. In this clade, unfortunately we don’t have enough taxa to make that gradual accumulation of traits any more gradual than it currently is. This is the best we can do, at present, with available data and the present taxon list.

But it’s a good start!
And closer than anyone figured out before.

References
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.
Lucas SG 2001. Gobiatherium (Mammalia: Dinocerata) from the Middle Eocene of Asia: Taxonomy and biochronological Significance. Paläontologische Zeitschrift 74 (4): 591–600.
Osborn HF and Granger W 1932. Coryphodonts and uintatheres from the Mongolian expedition of 1930. American Museum Novitates 552:1-16.

wiki/Arsinoitherium
wiki/Gobiatherium

2 thoughts on “Nesting twin-horned Arsinoitherium within the Condylarthra

  1. The nasal characters you claim link Arsinoitherium and Gobiatherium – ascending premaxillary process, elevated naris – most certainly evolved via convergence, and bear only superficial resemblance. Although there is controversy over whether Embrithopoda is a valid clade (as certain genera such as Palaeoamasia and Namatherium bear dental similarities to phenacolophid stem-perissodactyls; see von Koenigswald, 2013 & Erdal et al., 2016), it is widely accepted that Arsinoitherium differs substantially from phenacolophids and uintatheres in craniodental morphology, and is most likely an paenugulate afrothere (Cooper et al., 2014; Tabuce, 2014; Gheerbrant et al., 2016).

    Uintatheres (Dinocerata) are no longer considered condylarths; they nest with xenungulates and pyrotheres within a clade of Meridungulata, the Uintatheriamorpha (Lorente, 2015; Muizon et al., 2000), and appear to have been the only group of panameriungulates to not colonized South America. Condylarths sensu stricto
    comprises only of Procreodi, Pantodonta [= Tillodonta?], and Periptychoidea, and apparently left no descendants (Halliday et al., 2013 & 2015 in press).

    Also, “only two molars” counts as a character? Really? Sure, dental formulas are important, but why pay attention only to tooth count? The morphology of molars (i.e. position, shape of cusps and ) is much more valuable. That’s how Ocepeiidae got kicked out of Condylartha; pantodont-like bunoselenodonty proved only superficially similar to barylambdids, with significant differences in lingual cusp orientation (Gheerbrant et al., 2014 & 2016).

    REFERENCES
    Cooper, L. N., Seiffert, E. R., Clementz, M., Madar, S. I., Bajpai, S., Hussain, S. T., & Thewissen, J. G. (2014). Anthracobunids from the middle Eocene of India and Pakistan are stem perissodactyls. PLOS ONE, 9(10), e109232.

    Erdal, O., Antoine, P. O., & Sen, S. (2016). New material of Palaeoamasia kansui (Embrithopoda, Mammalia) from the Eocene of Turkey and a phylogenetic analysis of Embrithopoda at the species level. Palaeontology, 59(5), 631-655.

    Gheerbrant, E., Amaghzaz, M., Bouya, B., Goussard, F., & Letenneur, C. (2014). Ocepeia (Middle Paleocene of Morocco): The oldest skull of an afrotherian mammal. PLOS ONE, 9(2), e89739.

    Gheerbrant, E., Filippo, A., & Schmitt, A. (2016). Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco. PLOS ONE, 11(7), e0157556.

    Halliday T., Unchurch P., & Goswami A. (2013). A phylogenetic analysis of palaeocene mammals. Society of Vertebrate Paleontology abstracts 2013.

    Halliday, T. J., Upchurch, P., & Goswami, A. (2015, in press). Resolving the relationships of Paleocene placental mammals. Biological Reviews. doi: 10.1111/brv.12242.

    Lorente, M. (2015). Desarrollo de modelos de asociación y clasificaciones de restos postcraneanos aislados de ungulados nativos del Paleoceno-Eoceno de América del Sur (Doctoral dissertation, Facultad de Ciencias Naturales y Museo).

    Muizon, C. D., & Cifelli, R. L. (2000). The “condylarths”(archaic Ungulata, Mammalia) from the early Palaeocene of Tiupampa (Bolivia): implications on the origin of the South American ungulates. Geodiversitas, 22(1), 47-150.

    Tabuce, R. (2014). Evolution des caractères crâniens et endocrâniens chez les Afrotheria (Mammalia) et phylogénie du groupe (Doctoral dissertation, Université de Montpellier II).

    von Koenigswald, W. (2013). Unique differentiation of radial enamel in Arsinoitherium (Embrithopoda, Tethytheria). Historical Biology, 25(2), 183-192.

    • Good job, Jason. Very thoughtful.

      re: nasal traits… you may be correct, but at present, with the dataset I have, those are the results. More taxa will solve this problem, but as you know, we don’t have the pertinent taxa.

      If you are a long time reader, then you know I have problems with suprageneric taxa (clades). So many are invalid as determined by the LRT. If you do find other sisters for currently included taxa, please suggest specimens or species to test, nothing like a larger clade. With the current list of taxa, which will ALWAYS be incomplete,. this is the way the computer lumps and separates them. And they make sense!

      To your point on molar shapes… I’m coming to the conclusion that sometimes those plastic shapes may be more convergent than previously realized. I grant that too often only teeth are known. That’s why I’m using more or less complete specimens whenever possible.

      There really is a revolution brewing here. I resurrected Condylarthra just as I resurrected Enaliosauria and created Triotosauria and Fenestrasauria, because those clades are important parts of the cladogram that has been developing here.

      Finally, you’re not taking my word for it, and that’s a good thing. Whenever possible, don’t parrot the literature either. Test for yourself and let me know what you find.

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