Chiromyoides is a small plesiadapid primatomorph (a member of the clade Primatomorpha, which includes primates and their close relatives) known from the late Paleocene epoch. It is notable for its unusually robust upper and lower incisors, deep dentary (jawbone), and comparatively small cheek teeth, suggesting a highly specialized feeding ecology.
Temporal Range
Chiromyoides lived from the Thanetian through Ypresian stages (roughly 56.8–48.6 million years ago), corresponding to the middle Tiffanian through late Clarkforkian North American Land Mammal Ages (NALMA) and the late Paleocene of Europe.
Discovery and Taxonomy
The type species, †Chiromyoides campanicus, was originally described in 1916 by Stehlin from fragmentary craniodental material discovered at Cernay, France, with additional material later found at Berru. The genus name Chiromyoides references its resemblance to the modern aye-aye (Daubentonia madagascariensis, formerly placed in the genus Chiromys).
Species
Eight species are currently recognized:
| Species | Author | Year | Region |
|---|---|---|---|
| †Chiromyoides campanicus | Stehlin | 1916 | Paris Basin, France |
| †Chiromyoides caesor | Gingerich | 1973 | Wyoming, USA |
| †Chiromyoides major | Gingerich | 1975 | Wyoming, USA |
| †Chiromyoides minor | Gingerich | 1975 | Wyoming/Colorado, USA |
| †Chiromyoides potior | Gingerich | 1975 | Colorado, USA |
| †Chiromyoides gigas | Burger & Honey | 2008 | Colorado, USA |
| †Chiromyoides gingerichi | Secord | 2008 | Wyoming/Montana, USA |
| †Chiromyoides kesiwah | Beard et al. | 2020 | Wyoming, USA |
| †Chiromyoides mauberti | De Bast et al. | 2018 | Paris Basin, France |
Anatomy and Morphology
Chiromyoides is characterized by:
- Robust, chisel-like incisors — both upper and lower incisors are unusually large and strong relative to body size.
- Deep, thick mandibular corpus — the lower jaw is notably deep and robust, providing structural support for powerful gnawing.
- Reduced cheek teeth — the premolars and molars are comparatively small, indicating a reduced role in chewing relative to the incisors.
This unique dental morphology has led to comparisons with both seed-eating rodents and the aye-aye (Daubentonia madagascariensis), a modern lemur that uses its rodent-like incisors to gnaw into wood and extract grubs.
Paleobiology and Ecology
The specialized dentition of Chiromyoides has prompted two main ecological hypotheses:
- Seed specialization — Gingerich (1976) proposed that Chiromyoides was a specialist on hard seeds, using its powerful incisors to crack open seed shells.
- Extractive foraging ("mammalian woodpecker") — Szalay and Delson (1979) and Beard et al. (2020) suggested that Chiromyoides may have consumed wood-boring insects in a manner analogous to the modern aye-aye, gnawing into bark and wood to extract larvae.
Recent analysis of dentary metrics (Boyer et al., 2021) comparing Chiromyoides with seven other plesiadapids and 22 extant primates, dermopterans, and scandentians supports the interpretation that Chiromyoides was a Daubentonia-like extractive forager that evolved from ancestors whose diets emphasized exudates (gums and resins).
Evolutionary Relationships
Chiromyoides is a member of the family Plesiadapidae, alongside Plesiadapis, Platychoerops, Nannodectes, and Pronothodectes. Phylogenetic analyses suggest that Chiromyoides is descended from Plesiadapis, most closely related to either Plesiadapis walbeckensis or Plesiadapis tricuspidens.
Beard et al. (2020) identified two distinct clades within the genus:
- Southern clade — C. gigas, C. minor, and C. kesiwah from southern Wyoming and Colorado.
- Northern clade — C. major, C. gingerichi, C. campanicus, and C. mauberti from northern Wyoming, Montana, and Europe.
C. caesor formed a polytomy with the two main clades. The nesting of the European species C. campanicus and C. mauberti within the northern clade indicates that Chiromyoides dispersed from North America into Europe during the late Paleocene.
Biogeography
The oldest known specimens of Chiromyoides are C. minor from the Chappo Type Locality in Lincoln County, Wyoming, and an edentulous mandible from the Black Peaks region of southwest Texas, both arguably middle Tiffanian (Ti3) in age. Younger species are found across the Bighorn Basin (Wyoming), Washakie Basin (Wyoming), Piceance Creek Basin (Colorado), and Montana.
In Europe, C. campanicus comes from localities in the Paris Basin correlating with the late Tiffanian of North America, while C. mauberti occurs in slightly younger strata correlating with the North American Clarkforkian NALMA.
References
- Gingerich, P.D. (1976). "Cranial Anatomy and Evolution of Early Tertiary Plesiadapidae (Mammalia, Primates)". Papers on Paleontology. No. 15.
- Szalay, F.S. & Delson, E. (1979). Evolutionary history of the primates. Academic Press.
- Beard, K.C.; Jones, M.F.; Thurber, N.A.; Sanisidro, O. (2020). "Systematics and paleobiology of Chiromyoides...". Journal of Vertebrate Paleontology. 39 (6): e1730389.
- Stehlin, H.G. (1916). "Die Säugetiere des schweizerischen Eocaens". Abhandlungen der schweizerischen paläontologischen Gesellschaft. 38: 1165–1298.
- Gingerich, P.D. (1973). "First record of the Palaeocene primate Chiromyoides from North America". Nature. 244 (5417): 517–8.
- Secord, R. (2008). "The Tiffanian Land-Mammal Age...". Papers on Paleontology. No. 35.
- Burger, B.J. & Honey, J.G. (2008). "Plesiadapidae from the late Paleocene Fort Union Formation...". Journal of Vertebrate Paleontology. 28 (3): 816–825.
- De Bast, E.; Gagnaison, C.; Smith, T. (2018). "Plesiadapid mammals from the latest Paleocene of France...". Journal of Vertebrate Paleontology. 38 (3): e1460602.
- Boyer, D.M.; Scott, C.S.; Fox, R.C. (2012). "New craniodental material of Pronothodectes gaoi...". American Journal of Physical Anthropology. 147 (4): 511–50.
- Jehle, M. et al. (2019). "Evolution of plesiadapid mammals in Europe across the Paleocene/Eocene boundary". Palaeobiodiversity and Palaeoenvironments. 99 (2): 293–351.
