Fossil alouattines and the origins of alouatta: Craniodental diversity and interrelationships

Abstract

The howler monkey clade includes species of Alouatta and four extinct genera, Stirtonia, Paralouatta, Protopithecus, and probably Solimoea as well. Contrary to expectations, this radiation may have originated as a largely frugivorous group; advanced, Alouatta-like leaf-eating is a novelty well-developed in the Alouatta-Stirtonia sublineage only. Revised body mass estimates place Stirtonia and Paralouatta within the size range exhibited by the living forms and confirm the place of Protopithecus in a larger, baboon-like size range. While their dentitions are more primitive than the Alouatta-Stirtonia pattern, the cranial anatomy of Protopithecus and Paralouatta is distinctly similar to living howler monkeys in highly derived features relating to enlargement of the subbasal space in the neck and in head carriage, suggesting that ancestral alouattines may have had an enlarged hyolaryngeal apparatus. All alouattines also have relatively small brains, including Protopithecus, a genus that was probably quite frugivorous. The successful origins of the alouattine clade may owe more to key adaptations involving communication and energetics than dental or locomotor breakthroughs. While the fossil record confirms aspects of previous character-analysis reconstructions based on the living forms, alouattines experienced a complexity of adaptive shifts whose history cannot be recoverable without a more complete fossil record.