Ornithopod evolution

The earliest work in this field of research, carried out in 1984, concerned a group of dinosaurs that are quite closely related to the familiar Iguanodon. Generally, these types of dinosaur are known as ornithopods (bird feet' - this comes from a passing, trivial resemblance in the structure of the feet of these dinosaurs to those of modern birds). Comparing in some detail the anatomy of a number of the then known ornithopods, a cladogram was constructed. To convert this into a genuine phylogeny it was necessary to chart on to the cladogam the known distribution of this group through time and their geographic distributions.

Some surprising patterns in the history of these ornithopod dinosaurs emerged from this analysis. First it seemed to demonstrate that the forms most closely related to Iguanodon (that is to say, members of the group known as iguanodonts) and their closest relatives (members of the hadrosaur family) probably originated as a result of continental separation during Late Jurassic times. The ancestral population from which both groups may have evolved became subdivided by a seaway at this time. Following this isolation, one population evolved into the hadrosaurs in Asia, while iguanodonts evolved elsewhere. These two groups appear to have evolved distinct from one another through the Late Jurassic and Early Cretaceous period. However, during the latter half of the Cretaceous, Asia became reconnected to the rest of the northern hemisphere continents and its hadrosaurs were apparently able to r spread across the northern hemisphere pretty much unhindered | and replaced iguanodonts wherever they came into contact. J'

While the pattern of replacement of iguanodonts by hadrosaurs in £ Late Cretaceous times appeared to be reasonably uniform, there °

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