This confluence provides good supplementary evidence of close proximity between these herbivores and their proposed foodstuffs. As mentioned earlier, no toothmarks, stomach contents, or coprolites are known from marginocephalians, although gastroliths are reported in more than one specimen of the Early Cretaceous cer-atopsian Psittacosaurus of Mongolia. As a result, paleontologists interested in marginocephalian diets must content themselves with inferences based on fossil plants contemporaneous with marginocephalians, as well as the functional morphology of skeletal adaptations to feeding.
One idea regarding ceratopsid feeding habits that has broader implications in evolutionary theory is that ceratopsid browsing, over geologically significant time intervals, was partially responsible for the evolution of flowering plants. Their cooccurrence with herbivorous hadrosaurids and abundant fossils of flowering plants during the Late Cretaceous, coupled with the demise of other low-level browsing herbivores such as stegosaurs (Chapter 12), are used as evidence for this hypothesis. In such a scenario, ceratopsids migrating as hungry herds would have caused selection pressure in favor of faster-growing flowering plants. Co-evolution also would have occurred when deposition of ceratopsid feces dispersed undigested seeds of these flowering plants. This form of evolutionary bribery is still present today, in which indigestible seeds are covered by delicious fruits. No undoubted ceratopsid coprolites or even stomach contents have been found, so this idea, although reasonable, is still speculative.
One problem with ceratopsids and hadrosaurs driving the evolution of flowering plants is the endemism of Late Cretaceous ceratopsids: how did the evolution of flowering plants take place on continents where these large herbivores were either absent or uncommon? In the case of ceratopsids and flowering plants, much more evidence is needed, as well as increased cooperation between paleobotanists and vertebrate paleontologists, before such questions can be answered.
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