Prosauropods such as Plateosaurus (Figs. 5, 65, 83e) and Massospondylus (Figs. 65,83f; Sect. 7.3) had lightly built skulls and the jaws were articulated below the line of the teeth. Consequently, the teeth of the upper and lower jaws could be almost parallel as they approached each other when the mouth was closed. In addition, the biting force would have been distributed more evenly than it is when rows of teeth approach each other in a scissor-like fashion (King 1996). Galton (1985) demonstrated convincingly that they were herbivores. As noted by John Attridge and his colleagues in 1985, Massospondylus appears to have possessed a gastric mill. They suggested that comparatively large stomach stones or gastroliths were probably lodged in the muscles of the stomach walls in a region which functioned like an avian gizzard to grind the food. Unlike many ornithischian dinosaurs, the prosauropods did not have complicated teeth adapted for chewing: they were gut processors. To maintain their large bulk, these animals must have had to spend much of their time feeding, so their gastroliths would have become rounded and polished in a comparatively short space of time. They would then have been regurgitated and fresh, sharp-edged stones swallowed in their place. The lack of wear on the teeth of most prosauropods suggests that the teeth of the upper and lower jaws did not come into contact with each other. Oral processing probably involved a piercing and puncturing action of the teeth before the food was ingested and the gastric mill took over (Sect. 11.2).
Stomach stones were also swallowed by sauropods but, in the absence of fossil evidence regarding the soft tissues, the function of these, like those of prosauropods, cannot necessarily be equated with that of grit in the gizzards of birds, as is often presumed to be the case. Indeed, Gillette (1994) questioned the notion of any direct participation of dinosaur gastroliths in grinding vegetable food. From his study of the stomach stones associated with Seismosaurus (Fig. 108), he argued at some length that they probably served to stir the digestive juices of the animal. Although usually rounded, some were more polished than others, suggesting that they had been swallowed somewhat earlier. None was angular with sharp edges. This indicated that some gastroliths remained in the digestive tract for long periods - even years - until they were passed out in the faeces with the remnants of digested food. A problem remains: how were the polished stones selected by the gut? Maybe when stomach stones, both worn and angular, were excreted by chance, they were replaced only by gastro-liths with sharp edges. This would account for some very worn stones being found in the alimentary canal.
The gastroliths of Seismosaurus were found in two groups, one large cluster in the forward part of the alimentary canal, and a smaller one in the region of the pelvic bones. These were insufficient in number to have served as a grinding device for pulverising food in a crop gizzard, but might have helped fermentation by mixing digestive juices with the plant material that had been swallowed. The second cluster of gastroliths probably served the same function at the lower end of the digestive tract (Gillette 1994).
Tiffney (1997) summarised the various opinions that have been expressed. Gastroliths could have functioned to aid digestion either by muscular activity in the stomach causing them to rub against each other and thereby crushing and grinding the food; or they might have served to mix the contents of the alimentary canal, thus ensuring more complete digestion. Although there is no evidence for the presence of a gizzard, muscular crop or muscular stomach in dinosaurs, King (1996) argued that there is some indirect evidence for one. She quoted the review of Farlow (1987) who presented five scenarios to account for how sauropods, ankylosaurs and stegosaurs, in particular, could have managed to obtain sufficient energy by gut processing. First, they might have had lower standard metabolic rates than dinosaurs such as hadrosaurs and ceratopsians and, consequently, did not require the batteries of sophisticated teeth found in those families. Alternatively, they had lower metabolic requirements because they led less active lives. Thirdly, they might have fed on low-fi bre, highly nutritious food. Fourthly, the enormous size and relatively lower metabolic rates of sauropods may have reduced rates of turnover so that the food could be digested more slowly and, finally, sauropods, ankylosaurs and stegosaurs might have had lower digestive efficiency but higher rates of turnover. Whatever the case, herds of feeding dinosaurs, to use the words of Benton (2004) "must have rattled, grunted and burped furiously as their rough plant diet was reduced to a digestible state!" This subject has also been discussed by Norman (1985, 1991), Dodson (1990, 1996) and Fastovsky and Weishamspel (1996), among others.
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