Beginning to Grasp Things (Taxon of the Week: Euprimateformes)

Skull of Plesiadapis tricuspidens, from Gingerich (1976).

The name Euprimateformes was coined fairly recently by Bloch et al. (2007) for a clade uniting crown-group Primates and the extinct plesiadapoids (the exact definition was "the clade stemming from the most recent common ancestor of Carpolestes simpsoni and Homo sapiens), excluding even more basal stem Primates such as paromomyids. The plesiadapoids include the taxa Plesiadapidae, Carpolestidae, Saxonella and Chronolestes and were found in North America and Eurasia from the late Early Palaeocene to the end of the Early Eocene (a possible plesiadapoid has also been described from Africa).


Reconstruction of Carpolestes from here.

Plesiadapoids would have been not dissimilar to squirrels or modern tree shrews in size and appearance. They possessed large, forward-pointing lower incisors and originally fairly long skulls. Few plesiadapoids are known from extensive postcranial remains but what we do know indicates a fair amount of ecological divergence (Bloch et al., 2007). Carpolestes possessed a nail rather than a claw on its hallux (opposible big toe) and shorter claws overall, indicating that it was a grasping climber (wrapping its digits around branches) like modern primates rather than a clinging climber (hanging onto branches with its claws) like squirrels. Plesiadapis, on the other hand, had long narrow claws and was probably more of a clinging climber. Some authors have suggested a more terrestrial lifestyle for Plesiadapis; such interpretations are not currently popular (Kirk et al., 2008) but it would be interesting if postcrania were available for the largest and one of the latest of the plesiadapids, the European Platychoerops, which was comparable in size to a groundhog (Gingerich, 1976). Also notable among plesiadapids was Chiromyoides which appears to have been a specialised seed-eater with a short and deep jaw (and presumably skull) and massive incisors.


Lower jaw of Carpolestes simpsoni from Bloch & Gingerich (1998).

Carpolestids also showed a trend towards reduction in the length of the skull, but the really neat thing is what they did with their teeth, for which Carpolestes simpsoni can be taken as one of the best-known examples (Bloch & Gingerich, 1998). In the lower jaw, the anterior teeth were greatly reduced except the large first incisors which were followed by two teeth reduced to mere nubbins (identified as the second incisor and the canine). Only a single premolar remained (identified as the fourth) but to make up for it that tooth was huge, a massive blade-like multi-cusped structure that ground against the similar obscenely complex third and fourth premolars in the upper jaw (another derived carpolestid, Carpomegodon jepseni, retained a small third premolar in the lower jaw - Bloch et al., 2001). In comparison to all this, the three molars that followed were rather pedestrian. Biknevicius (1986) interpreted this tooth arrangement (comparable only to the multituberculates among other mammals) as indicative of a diet of foods with a tough exterior that would have been pierced by the premolars but a soft interior that did not require a huge amount of molar processing. Carpolestids were probably omnivorous, feeding on insects, seeds and fruit, with later forms becoming increasingly frugivorous.


Upper dentition of Carpolestes simpsoni from Bloch & Gingerich (1998).

REFERENCES

Biknevicius, A. R. 1986. Dental function and diet in the Carpolestidae (Primates, Plesiadapiformes). American Journal of Physical Anthropology 71 (2): 157-171.

Bloch, J. I., D. C. Fisher, K. D. Rose & P. D. Gingerich. 2001. Stratocladistic analysis of Paleocene Carpolestidae (Mammalia, Plesiadapiformes) with description of a new Late Tiffanian genus. Journal of Vertebrate Paleontology 21 (1): 119-131.

Bloch, J. I., & P. D. Gingerich. 1998. Carpolestes simpsoni, new species (Mammalia, Proprimates) from the Late Paleocene of the Clarks Fork Basin, Wyoming. Contributions from the Museum of Paleontology, The University of Michigan 30 (4): 131-162.

Bloch, J. I., M. T. Silcox, D. M. Boyer & E. J. Sargis. 2007. New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates. Proceedings of the National Academy of Sciences of the USA 104 (4): 1159-1164.

Gingerich, P. D. 1976. Cranial anatomy and evolution of Early Tertiary Plesiadapidae (Mammalia, Primates). The Museum of Paleontology, University of Michigan, Papers on Paleontology 15: 1-141.

Kirk, E. C., P. Lemelin, M. W. Hamrick, D. M. Boyer & J. I. Bloch. 2008. Intrinsic hand proportions of euarchontans and other mammals: implications for the locomotor behavior of plesiadapiforms. Journal of Human Evolution 55: 278-299.


This has nothing to do with the above post but as soon as I found it (here) I thought it was too fantastic not to share. From a German book published in the early 1900s, illustrated by one F. John, comes perhaps the creepiest reconstruction of the giant lemur Megaladapis ever.