New dentaries of Chiromyoides (Primatomorpha, Plesiadapidae) and a reassessment of the “mammalian woodpecker” ecological niche

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Based on their relatively large, chisel-like incisors and robust dentaries, species of the Paleocene plesiadapid mammal Chiromyoides have been described as potential ecological analogues of either seed-eating rodents or the unusually specialized lemur Daubentonia madagascariensis. Here, we analyze the most complete dentaries of Chiromyoides currently known in order to illuminate jaw form and function in this taxon. Principal Component Analysis shows that Chiromyoides campanicus and Daubentonia are uniquely similar in select dentary proportions when compared with a sample including seven other fossil plesiadapid taxa as well as 22 extant primates, dermopterans and scandentians. Comparative allometric analyses indicate that in both Daubentonia and Chiromyoides, the unique jaw proportions are likely achieved through hypertrophy of masseteric fossa length and dentary depth, rather than simple reduction of tooth row length. Consistent with these dentary features indicative of powerful gnawing, we show that incisor apex morphology became increasingly chisel-like in certain younger species of Chiromyoides. Importantly, slight reduction in molar area relative to jaw length and body mass appears to characterize all species of Chiromyoides in which molar proportions can be estimated. Notably, this pattern occurs in one of the oldest known specimens of Chiromyoides, an edentulous but relatively complete dentary from the middle Tiffanian of Texas, which differs from other Chiromyoides specimens in having a relatively shallower corpus. Taken together, this evidence suggests that Chiromyoides was a Daubentonia-like extractive forager that evolved from taxa whose diets emphasized exudates.