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PubMed1997

The expanded mandibular condyle of the Megaladapidae.

The Megaladapidae have a posterior expansion of the articular surface of the mandibular condyle. Several other strepsirhine species exhibit a similar condylar surface. In this study, I propose two behavioral scenarios in which the posterior articular expansion might function: 1) contact with the postglenoid process and resistance to joint stress during browsing, and 2) movement against the postglenoid process during the fast closing and power strokes of mastication, as a consequence of large transverse jaw movements and associated with a strong mandibular symphysis. These models are evaluated through dissection of the TMJ in Lepilemur and from comparative anatomical observations on strepsirhines and ungulates. In Lepilemur the mandibular symphysis is unfused, but compared to the unfused symphyses of other strepsirhines is strengthened by interlocking bony projections (Beecher [1977] Am. J. Phys. Anthropol. 47:325-336). An accessory articular meniscus is found between the posterior articular expansion and the postglenoid process in Lepilemur, suggesting that significant movement occurs in this part of the TMJ. The symphysis is fused in adult specimens of Megaladapis. A posterior articular expansion is common among ungulates, and its presence is associated not with browsing but with symphyseal fusion. This supports the second model and suggests that the posterior articular expansion functions as a movement surface during mastication. Schwartz and Tattersall ([1987] J. Hum. Evol. 16:23-40) cite the posterior articular expansion as a synapomorphy uniting an Adapis-Leptadapis clade with a Megaladapidae-Daubentonia-Indridae clade. The comparative evidence suggests that the posterior articular expansion has evolved convergently in adapines, notharctines, megaladapids, hapalemurids, and indrids as part of a functional complex related to herbivory. However, close morphological similarity of the posterior articular expansion among genera within these strepsirhine subfamilies and families indicates that it is probably a reliable synapomorphy at lower taxonomic levels.

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PubMed2000

SINE insertions: powerful tools for molecular systematics.

Short interspersed repetitive elements, or SINEs, are tRNA-derived retroposons that are dispersed throughout eukaryotic genomes and can be present in well over 10(4) total copies. The enormous volume of SINE amplifications per organism makes them important evolutionary agents for shaping the diversity of genomes, and the irreversible, independent nature of their insertion allows them to be used for diagnosing common ancestry among host taxa with extreme confidence. As such, they represent a powerful new tool for systematic biology that can be strategically integrated with other conventional phylogenetic characters, most notably morphology and DNA sequences. This review covers the basic aspects of SINE evolution that are especially relevant to their use as systematic characters and describes the practical methods of characterizing SINEs for cladogram construction. It also discusses the limits of their systematic utility, clarifies some recently published misunderstandings, and illustrates the effective application of SINEs for vertebrate phylogenetics with results from selected case studies. BioEssays 22:148-160, 2000.

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