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THE UNIVERSITY OF CALGARY Eric Snively A ... - Ohio University

THE UNIVERSITY OF CALGARY Eric Snively A ... - Ohio University

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Completing the system in fossil animals: inference of unpreserved structures<br />

Two fruitful approaches facilitate inference of unpreserved soft tissues in<br />

vertebrat5 fossils. 60th methods require anatomical comparisons, and<br />

assessments of correlates to soft tissue occumng on bones of extant vertebrates.<br />

One method involves inference from phylogeny, and the other approach entails<br />

extrapolation from histologieal and kinctional relationships beiween soft and hard<br />

tissues.<br />

Recently the method of phylogenetic inference has been formalized (Bryant<br />

and Russell 1992, Witmer 1995), invoking cornparison with living or well-<br />

preserved extinct relatives of the fossil taon. Phylogenetic inference mandates<br />

detailed anatomical examination of modem forms, and the framework of explicit<br />

cladistic hypotheses.<br />

Soft tissue organs in fossil taxa are robustly inferable when derived and<br />

primitive modem outgroups (the extant phylogenetic bracket) possess<br />

osteological correlates to the structure also present in the extinct clade (Witmer<br />

1995). For example, heads of extant dinosaurs (birds) and the nearest modem<br />

relatives of dinosaurs (mcodilians) have pneumatic diverticula or air sacs in the<br />

region anterior to the eyes, associated with fenestration or foramina in the skull<br />

bones. Extinct dinosaurs had these openings in their skulls; hence it is quite likely<br />

that these dinosaurs also had cranial air sacs (Witmer 1997).<br />

Phylogenetic inference becomes Iess certain in taxa without the both<br />

elements of the extant bracket (Bryant and Russell 19Ç2, Witrner 1995), or if<br />

correlates are ambiguous in any of the examined taxa (Nicholls and Russell

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