When plants and animals live together in long-term interaction, it's called symbiosis. It might be mutually beneficial, or one of the organisms may be a parasite on the other. There are many examples in nature today, from the algae that live inside corals, to fungi that grow on the roots of plants (mutualistic relationships), to many types of parasites such as worms and bugs preying on their hosts.

There is evidence that dinosaurs like Tyrannosaurus rex were affected by parasites, as some fossil bones are pitted with lesions that resemble those caused by protozoan parasites in modern-day birds.

Evidence for mutualistic symbiosis between dinosaurs and other species is not so clear, as such evidence is unlikely to fossilise.

But just like birds that help pick the teeth of crocodiles today, it is easy to imagine that dinosaurs must have had similar relationships, perhaps with their bird descendants, in the past.

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Asked by: Freddie Williams, London

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Steve is a professor and palaeontologist at the University of Edinburgh and the author of the book The Rise And Reign Of The Mammals (£20, Picador), a 325-million-year odyssey of mammalian evolution and the people who study mammal fossils.