Most reptiles are carnivorous and have rather simple and not overly long guts, meat being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting about the fact that they can not divide and masticate their food like mammals do, and their lower metabolism. Being cold blooded their energy requirement is about a 5th to a 10th of that of a mammal of the same size. Large reptiles like crocodiles and the large constrictors can basically live from a single large meal for months, digesting it slowly.
While modern reptiles are predominately carnivorous, this has not always been so. During the early history of reptiles, several groups produced big-bodied herbivorous megafauna, in the Paleozoic the Pareiasaurs and the synapsid Dicynodonts, and in the Mesozoic several lines of Dinosaurs. Today the turtles are the only predominantly herbivorous reptile group, but several lines of agams and iguanas have developed to live wholly or partly from plants.
Herbivorous reptiles face the same problems of mastication as herbivorous mammals, but lacking the complex mammal teeth, quite a few species swallow rocks and pebbles to aid in digestion, so called gastrolithes. The rocks are washed around in the stomach helping to grind up plant matter. Fossil gastrolithes has also been found associated with sauropods. Sea turtles, crocodiles and marine iguanas also use the gastrolithes as ballast, helping them to dive.
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