Extinct genus of primates
Chiromyoides is a small plesiadapid primatomorph that is known for its unusually robust upper and lower incisors, deep dentary, and comparatively small cheek teeth. Species of "Chiromyoides" are known from the middle Tiffanian through late Clarkforkian North American Land Mammal Ages (NALMA) of western North America, and from late Paleocene deposits in the Paris Basin, France.
The unique dental morphology of "Chiromyoides" has led several authors to propose a specialized ecological role for the genus. Gingerich (1976) hypothesized that "Chiromyoides" was a specialist on seeds, while Szalay and Delson (1979) and Beard et al. (2020) suggested that it may have consumed wood-boring insects in a manner similar to the aye-aye.
Origins and discovery.
The type species of "Chiromyoides", "Chiromyoides campanicus", was originally described in 1916 from fragmentary craniodental material discovered in Cernay, France, with additional material also later found at Berru. Gingerich (1973) described the first North American species, "Chiromyoides caesor", from two upper incisors found in northern Wyoming. Several years later, he described three additional North American species, "Chiromyoides major", "Chiromyoides minor", and "Chiromyoides potior", from isolated upper incisors found in northern Wyoming, southern Wyoming, and southern Colorado, respectively. Secord (2008) named "Chiromyoides gingerichi" from material found in northern Wyoming and southern Montana, Burger and Honey (2008) named "Chiromyoides gigas" from several incisors found in northern Colorado, and Beard et al. (2020) named "Chiromyoides kesiwah" from material found at several localities in southwestern Wyoming. A second European species, "Chiromyoides mauberti", was named by De Bast et al. (2018) from isolated teeth and several mandibular fragments found near Rivecourt, France.
Evolutionary relationships.
"Chiromyoides" is known only from isolated teeth, mandibular fragments, and maxillary fragments, and its relationships to other plesiadapiforms are not well understood. "Chiromyoides" is generally acknowledged to be a member of the family Plesiadapidae, along with "Plesiadapis", "Platychoerops", "Nannodectes", and "Pronothodectes". Recent phylogenetic analyses suggest that "Chiromyoides" is descended from "Plesiadapis", perhaps most closely related to "Plesiadapis walbeckensis" or "Plesiadapis tricuspidens".
Beard et al. (2020) found that species of "Chiromyoides" separated into two distinct clades: a more southern clade consisting of "Chiromyoides gigas", "C. minor", and "Chiromyoides kesiwah" from southern Wyoming and Colorado, and a northern clade including "Chiromyoides major", "Chiromyoides gingerichi", "Chiromyoides campanicus", and "Chiromyoides mauberti". "Chiromyoides caesor" formed a polytomy with the two main clades. "Chiromyoides potior" was not included in their analysis. The nesting of the European species "C. campanicus" and "C. mauberti" in the northern clade suggests that "Chiromyoides" dispersed into Europe from North America.
Age and biogeography.
The oldest 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 that has been referred to either "C. minor" or an indeterminate species of "Chiromyoides". Both the Chappo locality and the Ray's Bonebed locality of southwest Texas where the edentulous mandible was found are arguably middle Tiffanian (Ti3) in age. "Chiromyoides caesor" and "C. kesiwah" come from slightly younger Tiffanian (Ti4) beds in the Bighorn Basin and Washakie Basin, respectively, while "C. potior", "C. gigas", "C. gingerichi", and "C. major" come from even younger Tiffanian and Clarkforkian deposits in Colorado, Wyoming, and Montana.
"Chiromyoides campanicus" comes from localities in the Paris Basin that appear to correlate with the late Tiffanian of North America, while "C. mauberti" occurs in somewhat younger strata that correlates with the North American Clarkforkian NALMA.
References.
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