In reality, Ankylosaurus fossils are only known from the very end of the Cretaceous, 66-65 million years ago, about 20 million years after the time Terra Nova is set. Other types of Ankylosaurs did live earlier, but Malcolm Wallace specifically referred to it as an "Ankylosaurus", not just an "ankylosaur". So one would have to suppose that in the world of the show, Ankylosaurus evolved considerably earlier than any of the fossils that have been found to date. (but fossils do only contain about 10% of what lived 85mya, Ankylosaurus fossils may not have survived from this time period or may have to be found)
Also, although other types of ankylosaurs lived in South America (see here), Ankylosaurus fossils are only known from North America while Carnotaurus fossils are only known from South America, and the two continents had separated from each other in the Jurassic. One way to rationalize them coexisting in the same area would be to point to what paleontologists call a "faunal interchange", where in the late Cretaceous some animals previously unique to the Northern Hemisphere began to appear in South America, suggesting that a land bridge (similar to modern Central America) had formed between the two continents (see the discussion starting at the bottom of p. 322 here, or p. 399 here, or the right column on p. 243 here). This faunal interchange is only thought to have happened in the Campanian and Maastrichtian ages (the last two ages of the Cretaceous), from 83.5 million years ago to 65.5 million years ago, but this is only slightly after the time of Terra Nova 85 million years ago, so one could imagine the interchange began a little before it's known from the fossil record. In this scenario it could be that Terra Nova is in North America and Carnotaurus had migrated North, or that it's in South America and Ankylosaurus had migrated South (though again, there is no actual evidence that these two dinosaurs crossed the land bridge).