Evolution of wood-cutting in castorids.

摘要

A fossil locality in the Canadian High Arctic has yielded remains of the castorid Dipoides in association with an assemblage of cut wood, providing the first definitive evidence of wood-cutting by an extinct castorid. The only other known wood-cutting castorid is the extant Castor. Here I use an integrative approach to investigate the evolutionary history and functional morphology of wood-cutting in castorids.; To investigate the evolution of wood-cutting I conducted a series of phylogenetic analyses of Castoridae using morphological data. I also undertook a comparative analysis of modern rodents in order to diagnose fossoriality. Many castorids have been suggested to have been fossorial specialists. Rigorous diagnosis of fossoriality is important because it is associated with unforested habitats, suggesting that fossorial castorids were not wood-cutters. I then mapped the behaviors, fossoriality and wood-cutting, onto the most parsimonious trees. The results suggest (1) that the fossorial taxa are basal, and (2) that Castor and Dipoides, along with several other taxa (including Hystricops and Castoroides), form a derived clade that may have acquired wood-cutting from an ancestor that lived over 20 million years ago.; Behavioral investigations, including analysis of behavioral artifacts, reveal that Castor and Dipoides both cut wood using a single lower incisor at a time. However, they differ in wood-cutting efficiency. Compared to that of Castor, stick-cutting by Dipoides appears to have involved more cuts. I also investigated the jaw adductor system of Castor, compared it to the reconstructed musculature of Dipoides, and found that Dipoides appears to have been less specialized than Castor for gnawing.; Dipoides and Castor also differ in the shape of their lower incisors. The cutting edge of the Dipoides incisor is rounded in anterior view, whereas that of Castor is straight. I studied the effect of this shape difference experimentally, using physical models. The results suggest that compared to a Castor -like, straight-edged incisor, a Dipoides-like, curve-edged incisor requires less force to cut wood. However, there is also some evidence of performance trade-offs, as the maximum cut width produced by a Dipoides-type incisor was usually found to be less than that of the Castor-type incisor.