The spreading of the Central Atlantic is a complex series of
events that include a ridge jump to the east and a ridge jump to the west at c. 170Ma and c. 162Ma respectively. These jumps were influenced by plate interactions as Laurasia separated from Gondwana, although the second jump may have been influenced by spreading in the Gulf of Mexico, which occurred at about the same time. Dinosaur evolution and diversity were influenced by the spreading of the Central Atlantic that split Laurasia from Gondwana. The presence of dinosaurs, from the same taxonomic groups existed in the two regions from the Triassic until the Early
Cretaceous, suggests that there continued to be at least some mixing between Laurasian and Gondwanan faunas. This may
have been due to the proximity of the continents during the early stages of opening of the Atlantic, island hopping, or other means, enabling limited contact between Laurasia and Gondwana throughout the Mesozoic. Dinosaur faunas did, however, become more isolated and diverse later, in the Cretaceous, due to higher sea levels and seafloor spreading, but some elements of the faunas were still common to both continents. Similarities in faunas between North America and Asia, suggest that there were still links between these regions until the Maastrichtian at the end of the Late Cretaceous. In Laurasia some apparently endemic dinosaur groups (ceratopsians, pachycephalosaurs, therizinosaurs
and others) developed as a result of the break up of the continents and do not seem to have had any contact with Gondwana.
The diversity of dinosaurs has some differences between
Laurasia and Gondwana from the Late Jurassic onwards. During
the Cretaceous, Laurasian hadrosaurs and ceratopsians dominated the herbivores. Both these groups of ornithischian
dinosaurs were able to chew their food, which aided their ability to digest tough vegetation making it possible for them to evolve to be much smaller in size than their Gondwanan counterparts, the titanosaurid sauropods — a surviving group of the giant saurischian hebivores characterised by their inability to chew and by their enormous size.
There is also always the possibility that the apparent differences between the faunas are due to collecting bias and to differences in outcrop exposure in the present-day remnants of these once vast continents. There are certainly a lot fewer localities in the land masses that made up Gondwana as compared to Laurasia
