Body cavity size in fossil chewing herbivores, non-chewing herbivores, and carnivores: Implications for GIT volume and gigantism.

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In herbivores, digestion of plant material by symbiotic gut microbes is a function of both ingesta retention (time available for fermentation) and particle size. Because smaller particle size accelerates digestion rate, retention time and particle size reduction can compensate for each other. Thus, animals lacking a particle size reduction mechanism (non-chewing dinosaurs such as sauropods, stegosaurs, ankylosaurs) should have adopted long ingesta retention times, facilitated by comparatively large (voluminous) guts. Although body shape has been suggested as an indicator for the volume of the coelomic cavity and hence the gut, quantitative investigations are lacking. Using digital capture of mounted skeletons of extinct herbivores and live animals, body shape reconstructions and volume calculations, we will test the hypotheses that a) carnivorous dinosaurs, reptiles, and mammals have a less voluminous coelomic/abdominal cavity than their herbivore counterparts, and in particular that b) non-chewing herbivorous dinosaurs and more primitve fossil reptiles have a more voluminous coelomic cavity than chewing herbivorous dinosaurs and mammals. We predict that the volume of the body cavity is an important variate in herbivore digestive morphophysiological adaptation, with dinosaurs – where both chewing and non-chewing forms exist – being the ideal model organisms for this principle, displaying more variability in this parameter than mammals. In particular, we predict that a disproportionately large body cavity is a decisive characteristic of sauropods (that facilitated a high food intake and metabolism in spite of a lack of mastication).