Dinosaur gastralia; origin, morphology, and function

Abstract

ABSTRACT Gastralia are dermal ossifications situated in the ventral abdominal wall. Gastralia may be plesiomorphic for tetrapods, but are only retained in extant Crocodylia and Sphenodon, and possibly as part of the chelonian plastron. In contrast to previously published reports, a similar structural configuration of the gastralia is shared throughout prosauropods and (non-ornithurine) theropods. Within the Prosauropoda and Theropoda, the gastralial system consists of approximately 8 to 21 metamerie rows. Each row consists of four bones: two lateral and two medial rods. Gastralia of the cranialmost or caudalmost rows may coalesce, forming a median chevron-shaped gastralium. The lateral gastralia articulate in parallel with the medial gastralia in an elongated groove. The medial gastralia imbricate with contralateral gastralia along the ventral midline, creating a series of cranially directed chevrons. Thus all the gastralia are connected to one another, and operate as a single functional unit. The bones recently identified as sauropod gastralia show no morphological similarities with the gastralia of prosauropods and theropods and are probably sternal elements. No gastralia have been recovered in the Ornithischia. In contrast to the reduction of the gastralia in other amniote groups, theropod gastralia show elaborate modification. The anatomy of the gastralial system indicates a more active function than abdominal support or protection. The gastralia may have affected the shape and volume of the trunk in theropods, and may have functioned as an accessory component of the aspiration pump, increasing tidal volume. Moreover, if the caudal region of the lungs in some theropods had differentiated to form abdominal air-sacs, the gastralia might have ventilated them. Gastralial aspiration may have been linked to the generation of small pressure differences between potential cranial and caudal lung diverticula, which may have been important for the evolution of the unidirectional airflow lung of birds.