In Journey to Chandara, James Gurney states that Dinotopia no longer appears on modern maps. The manner in which he says so implies that the island no longer exists, except that he says that Dinotopia no longer appears on modern maps. He never states that it doesn't appear under another name; it could have been renamed over the years to keep it hidden; someone in modern day — like in Jurassic Park, for example — would go right in there at the mention of "dinosaur" and a "Utopia" to retrieve or enslave a dinosaur, so it may have just been renamed for its own safety.
All the dinosaurs are referred to by the names given to them by the mainstream scientific community (despite the island being completely isolated from that community)... including the ones that haven't yet been discovered and named at the time the books are set in.
Not the only terms either; words such as "taxi" and "helium" didn't exist yet either.
The Dinotopians are probably using whatever words the specific species of animals are using for themselves, so we could probably consider this a Translation Convention.
The fact that the Latin translation of Tyrannosaurus rex is a minor plot point in The World Beneath rules out Translation Convention. Denison also expresses fear of them — in 1862, four decades before the species was named.
It gets even more confusing in the prequel novel First Flight. In it, we're introduced to "Northies", or Quetzalcoatlus northrop. This species was named after Jack Northrop, meaning that the Dinotopians somehow know the name of an animal named after a person who wasn't even born yet!
Dinotopians use leathers, skins, furs, and otheranimal products were seen in use by the Dinotopians. In Journey to Chandara it's explained when Arthur Dennison is given a new journal bound in the skin of an Intellectual Animal "whose dying wish was to donate his body to science".
The dinosaurs are depicted as having found some path to peace, to the point that even with the issue of Carnivore Confusion addressed, the carnivores and herbivores can get along, but this doesn't explain how the dinosaurs found this path and why they chose it. Though Carnivore Confusion is addressed, this story relies heavily on the Herbivores Are Friendly trope. The carnivores are said to be "hungry by nature and have no stomach for civilization" so it doesn't make sense that they would agree this path. Second, herbivores themselves can be nasty (as discussed on the trope page for Herbivores Are Friendly) so it doesn't explain how they all get along, especially when they would go through food very quickly and how they resolve disagreements such as a Love Triangle or human traits such as greed, pride, envy and deceit. Third, this society is dangerously vulnerable to a military invasion force (the closest Dinotopia has to an army are the non-weapon wielding Skybax and the natural weapons of the dinosaurs, assuming they don't call in help from the carnivores...), and they'd better pray to God that if an army manages to arrive that they doesn't have weapons (I don't think even the Posedian robots had weapons on them).
In regards to the first, it is plausible that the Carnivores simply chose to split off from civilization of their own free will. The description of them indicates that they're intelligent, but aware of their own nature and less than fond of the idea of "civilization", and so simply decided to go their own way to avoid conflict (so that they don't cause undue harm in a heated moment and aren't confined to towns and cities). To the second point, it seems like a cultural acceptance of a way of life and central philosophy that guides the lives of all Dinotopians. Think of it like a monastic order, where each day is dedicated to meditation on a central philosophy. They may feel those emotions (though it's possible that Saurian species don't feel them in the same way as humanity does ((EG, there's less overconsumption because, when the situation calls for it, the "herd" either moves on or splits off, greed and pride may be semi-foreign concepts, etc))). And to the third, the general society seems to rely more on nature keeping them isolated and protected, rather than force. Similar to other fictional cultures where their safety is simply in being hidden from the world (Hobbits, for example).
