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The Eternal Question: Lips or Skin?

Apart from the interpretation of the maxillary and dentary foraminina in 
determining whether dinosaurs had fleshy coverings over the teeth, there is 
another very practical problem that affects this issue: conservation of body 
moisture and exclusion to some degree of ectoparasites. Crocs (excluding 
extinct terrestrial forms) don't have to deal with the potential dessication 
of lip and mouth mucosa that other tetrapods do, since they're either in the 
water or close to it. A "baretoothed" croc-like mouth in dinosaurs would be 
subject to this, as well as a having a continuing problem in keeping flies 
and other flying or crawling ectoparisitic forms from reaching these soft 
tissues. Lips (I'm not using the word in the mammalian sense of muscular 
outer tooth coverings) would be effective in reducing this even if the teeth 
protruded, as I think they must have in big ceratosaurid and tyrannosaurid 
theropods. More tightly closing ones were probable in sauropods, 
ornithischians and segnosaurs, among others. Since early dinosaurs, like 
other tetrapods may have evolved in arid ecosystems, I think this was a 
feature that developed early on....

Mark Hallett