The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Never going back again? On the reversibility of mandibular symphyseal fusion

JUSTIN B. LACK1, JEREMIAH E. SCOTT2, AARON S. HOGUE3 and MATTHEW J. RAVOSA4,5.

1Department of Zoology, Oklahoma State University, 2Department of Biological Sciences, University of Notre Dame, 3Department of Biological Sciences, Salisbury University, 4Departments of Biological Sciences, Aerospace and Mechanical Engineering, and Anthropology, University of Notre Dame, 5Department of Zoology, Mammals Division, Field Museum

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Complete ossification of the mandibular symphysis is a widely recognized synapomorphy of the crown anthropoid radiation and has evolved independently in other primate clades, including multiple times in Eocene adapids and subfossil lemurs. In contrast, decreases in ossification have yet to be documented in primates. The absence of reversals in anthropoids is particularly notable, given the age of this clade, its diverse array of feeding adaptations, and the fact that other primates show diet-related variation in fusion. This pattern suggests that once symphyseal fusion evolves, reversion to an unfused character state is unlikely. If correct, this hypothesis has important implications for functional and phylogenetic analyses. The goal of this study was to test this hypothesis in Chiroptera, a clade where fused and unfused symphyses are both common. Using a sample of 59 species of vespertilionoid and noctilionoid bats, we generated a molecular phylogenetic tree via Bayesian phylogenetic analysis and reconstructed ancestral states using parsimony and likelihood methods. We further evaluated character-state evolution using the BiSSE model in order to account for the effects of character-dependent diversification on ancestral-state reconstructions. Results indicate that reversals from fusion have occurred within the Chiroptera, highlighting the unusualness of the anthropoid pattern and suggesting that decreased joint ossification may be selectively advantageous in some contexts. Possible explanations for the lack of reversions in Anthropoidea include: functional influences unrelated to the initial dietary shift in basal anthropoids, selective neutrality in recent taxa, or developmental constraints related to the early onset of fusion in this clade.

This research was funded by NSF grants BCS-1029149 and BCS-0924592.

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