The biogeographic history of dinosaurs has been difficult to decipher. Much of the problem lies in the incomplete fossil record of dinosaurs, which includes large gaps that span significant periods in Earth history. More importantly, each continent lacks fossils from one or more periods of geologic time, so that the same time period cannot be compared for every location. As a result, it is impossible to determine the true distributions of all dinosaurs for any single period during the Mesozoic.
In spite of this, several very general patterns are apparent. Because all dinosaurs are descended from a single common ancestor, the earliest dinosaurs must have existed in one single area. Dinosaurs appeared during the latest Middle or earliest Late Triassic, at a time when all of the
Earth's continents remained connected as the supercontinent Pangaea. It would have been relatively simple for early dinosaurs to spread across most regions of Pangaea at this time.
The distributions of Late Triassic and Early Jurassic dinosaurs suggest that this may have occurred. Primitive theropods (such as herrerasaurids and coelophysids), primitive ornithischians, and prosauropods are found in Triassic-age deposits worldwide. Although different species are present in each region, the general types of dinosaurs are quite similar across the globe.
The Middle Jurassic terrestrial record is scarce, and dinosaur distributions during this time period are correspondingly poorly known. But well-studied Late Jurassic deposits in Europe, North America, and Africa still show great similarities among the dinosaur species. Allosaur, ceratosaur, and megalosaur theropods, along with stegosaurs, dryosaurid and camptosaurid ornithopods, and numerous sauropods (including brachiosaurids and several diplodocoid species) occur on all these continents. Although Pangaea had begun to separate into distinct northern (Laurasia) and southern (Gondwana) continents, this was not yet reflected in dinosaur distributions.
This situation began to change during the Early Cretaceous. As Laurasia and Gondwana continued to separate, northern and southern faunas became more distinct. In the south, ceratosaur theropods began to dominate over coelurosaurs, titanosaur sauropods were common, and armored dinosaurs were rare. In the north, coelurosaurs were common, along with ankylosaurs and derived ornithopods.
By the Late Cretaceous, broad differences between northern and southern dinosaur faunas are apparent, but numerous dispersal events may also have occurred. Ankylosaurs and hadrosaurs were present in South America and Antarctica, and titanosaurs in Europe. Pachycephalosaurs and ceratopsians dispersed between North America and Asia. This last world of the dinosaurs was in many ways the opposite of the first; whereas the Late Triassic was a world of connections, the Late Cretaceous was one of separation.
Some dinosaurs show unique distributions that hint at a complex biogeographic history. For example, carcharodontosaurid theropods were present in the Early Cretaceous of North America, South America, and Africa. This may be the result of vicariance—the group could have dispersed across these lands when they were still connected, during the Late Jurassic. They would have been separated by continental drift during the Early Cretaceous. Alternatively, these species could have dispersed from an initial distribution (in North America, for example) after these continents
began to separate.
Dinosaur distributions and geologic data can also be mutually informative. South America, Madagascar, and India have several closely related Late Cretaceous theropod species, suggesting that these three areas remained connected at that time. Similar animals are absent from Laurasian continents. If these theropods are also present in the Late Cretaceous of Africa, it may indicate that Africa also remained connected to Gondwana in the late Mesozoic. Alternatively, their absence would suggest an early separation of Africa from the other southern continents.