Ekgmowechashalines: The last North American primates (for a while)

Dentition of Ekgmowechashala and potential relatives. A) Tarkops; B) Tarka; C) Tarkadectes; D) Macrotarsius (omomyid); E) Ekgmowechashala; F) Ellesmene (plagiomenid); G) Plagiomene (plagiomenid); H) Cynocephalus (the living colugo), from Ni et al. (2010).

Dentition of Ekgmowechashala and potential relatives. A) Tarkops; B) Tarka; C) Tarkadectes; D) Macrotarsius (omomyid); E) Ekgmowechashala; F) Ellesmene (plagiomenid); G) Plagiomene (plagiomenid); H) Cynocephalus (the living colugo), from Ni et al. (2010).

Primates and their plesiadapiform relatives, which had been numerically abundant members of mammalian faunas in North America for much of the preceding Paleocene and Eocene epochs, appear to wither and die off before the end of the Chadronian Land Mammal Age*, as climates took a sharp turn from the equitable times of the Eocene toward the “snowball Earth” of the Oligocene. However, a few stragglers appear to have hung on. These latest surviving primates in North America (spare much later dispersals of platyrrhines from South America) belong to the sparsely represented but apparently geographically widespread genus Ekgmowechashala.

Figure 1. Mandible of E. philotau, from Szalay (1979).

Figure 1. Holotype of E. philotau (SDSM N. 5550), from Szalay (1979).

Ekgmowechashala” (James Macdonald transcribes the pronunciation as “Igg-uh-moo Wee-cháh-shah-lah”), comes from the term the Sioux term for “monkey,” which translates to “little cat-man.” Macdonald, who worked extensively in South Dakota on Sioux reservation land, was fond of giving Sioux names to new taxa. He described Ekgmowechashala philotau from partial left and right mandibles from the Early Arikareean* Sharps Formation (28.5-29 Ma) near Wounded Knee, SD (Macdonald 1963). An additional partial maxilla was later described from the John Day formation of Oregon and assigned to Egkmowechashala on the basis of similar dental morphology and an apparently strong fit between the maxillary dentition from Oregon and the mandibular dentition known from the holotype (Rose and Rensberger 1983). It was only resolved to genus as the maxilla couldn’t be directly compared to the two mandibles known for E. philotau. An additional lower molar has since been recovered from John Day but remains unpublished. These specimens appear to date to around 28 Ma. A single lower fourth premolar from Texas was recovered from the Late Arikareean of Texas in 2005 and also classified as Ekgmowechashala sp. It is more difficult to date, but appears to date to around 24 Ma (Albright 2005).

The taxonomic placement of Ekgmowechashala has proven contentious. Macdonald described it as an omomyid, which was followed by Szalay in his 1979 revision of the systematics of the omomyids. He erected the separate subfamily Ekgmowechashalinae for Ekgmowechashala within the Omomyidae. This scheme was also followed by Rose and Rensberger in 1983. Both sources propose a possible link to the enigmatic late Eocene primate Rooneyia, which they also consider a derived omomyid.

Figure 2. The noble colugo, from Wikimedia Commons.

Figure 2. The noble colugo, from Wikimedia Commons.

McKenna (1990) shook things up by placing the ekgmowechashalines with a group of animals known as plagiomenids, which he in turn linked to the living flying lemurs (or colugos) of the order Dermoptera, specialist gliders thought to be related to primates that today live in Southeast Asia. He also added two much older, newly discovered plagiomenid taxa to ekgmowechashalinae, Tarka and Tarkadectes. McKenna was struck by similarities in molar cusp pattern between Ekgmowechashala and plagiomenids. In the main they seem to share extensive crenulation of the molar surface and the presence of small, extra cusps.*

Szalay and Lucas (1996) argued sharply against this notion, calling it “inexplicable.” They argue that Ekgmowechashala‘s apparent similarities to plagiomenids in molar anatomy are convergencies and point to features like the presence of a hypocone on the upper molars of Ekgmowechashala, which plagiomenids like Tarka and Tarkadectes lack. Ni et al. (2010) took a slightly different tack in placing Tarka and Tarkadectes in omomyidae on the basis of additional skeletal material from Mongolia, but agree with Szalay and Lucas in the affinities of Ekgmowechashala.

Figure 3. The strange dentition of Egkmowechashala, from Szalay (1979).

Figure 3. The strange lower dentition of Egkmowechashala, from Szalay (1979).

Much of the confusion emerges from the highly derived nature of the known Ekgmowechashala material. It’s teeth have become stongly crenulated and complex, but also relatively low crowned, indicating a specialization on fruits (Szalay 1979). Ekgmowechashala exhibited the most strongly molarized premolars of any omomyid and its molars decrease in size along the tooth row, contra most primates. Ekgmowechashala appears to have inhabited relatively dry, scrubby habitats in both South Dakota and Oregon relative to the lush surroundings of its Eocene forebears, although Toledo Bend in Texas was apparently much wetter (Albright 2005). If body mass estimates from molar dimensions are to be believed, it was also remarkably large bodied, weighing in at around 1.8 kg, the size of a small monkey. This constellation of traits presumably contributed to a unique ecology that allowed Ekgmowechashala to survive the trying conditions of the Oligocene. More material is badly needed to better understand how this taxon may have fit in to North American Oligocene ecosystems and how it might have lived.

References

Albright LB. 2005. Ekmowechashala (Mammalia, ?Primates) from the Gulf Coastal Plain. Bull Fla Nat Hist 45(4): 355-361.

Macdonald JR. (1963). The Miocene faunas from the Wounded Knee area of Western South Dakota. Bull Am Mus Nat Hist 125(3): 139-238.

McKenna MC. (1990). Plagiomenids (Mammailia: ?Dermoptera) from the Oligocene of Oregon, Montana, and South Dakota, and Middle Eocene of northwestern Wyoming. In Dawn of the age of mammals in the northern part of the Rocky Mountain interior, North America, vol. 243 (eds TM Bown & KD Rose), pp 211-234. Boulder, CO: Geological Society of America.

Ni X, Meng J, Beard KC, Gebo DL, Wang Y, Li C. (2010). A new tarkadectine primate from the Eocene of Inner Mongolia, China: Phylogenetic and biogeographic implications. Proc R Soc B 277: 247-256.

Rose KD, Rensberger JM. (1983). Upper dentition of Ekgmowechashala (Omomyid Primate) from the John Day Formation, Oligo-Miocene of Oregon. Folia Primatol 41: 102-111.

Szalay FS. (1979). Systematics of the Omomyidae (Tarsiiformes, Primates): Taxonomy, phylogeny, and adaptations. Bull Am Mus Nat Hist 156(3): 157-450.

Szalay FS, Lucas SG. (1996). The postcranial morphology of Paleocene Chriacus and Mixodectes and the phylogenetic relationships of archontan mammals. Bull NM Mus Nat Hist Sci 7: 1-47.

* North American Land Mammal Ages, or NALMAs, are biostratigraphic units into which paleontologists divide the Cenozoic of North America, parallel with the lithostratigraphic epochs (Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene). There are 19 in the Cenozoic, the most important for our purposes being the Chadronian (38-33.9 Ma), which corresponds to the end of the Eocene and very beginning of the Oligocene, by which time most primates had gone extinct in North America; and the Arikareean (30.6-20.8 Ma), which overlaps the end of the Oligocene and beginning of the Miocene.

* Macdonald described the Sharps Formation as Miocene, as the whole Arikareean was believed to fall in the Miocene at that time. Since then it has been revised to straddle the Oligocene-Miocene border, with the Sharps Formation falling in the Oligocene. This has lead to some confusion in the secondary literature as to the actual age of E. philotau, with some sources citing the Miocene and others the Oligocene. The Texas Ekgmowechashala material is late Arikareean, however, which is considered Miocene, so the point is now somewhat moot as concerns the genus.

*The National Park Service seems to still be on this particular train, as their official brochures on fossils in the national parks depict Egkmowechashala as a colugo-like gliding creature. I promise I got a copy of one of these at Agate Fossil Beds, but I can’t seem to find an online image of it.

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