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
spread across the northern hemisphere pretty much unhindered
and replaced iguanodonts wherever they came into contact.
While the pattern of replacement of iguanodonts by hadrosaurs in
Late Cretaceous times appeared to be reasonably uniform, there
were one or two puzzling anomalies that needed to be investigated.
There were reports, written at the turn of the 20th century, of
iguanodonts from Europe (primarily France and Romania) in
rocks of very latest Cretaceous age. From the analysis above, these
would not have been expected to have survived into Late Cretaceous
times because everywhere else the pattern was one of hadrosaurs
replacing iguanodonts. In the early 1990s, the best-preserved
material came from Transylvania, a region of Romania. However,
the phylogenetic analysis prompted expeditions to reinvestigate
these discoveries. Fresh study proved that this dinosaur was not a
close relative of Iguanodon, but represented an unusually
long-lasting (relict) member of a more primitive group of
ornithopods. An entirely new name was created for this dinosaur:
Zalmoxes. So, one of the outcomes of the preliminary analysis was a
great deal of new information about an old, but apparently not so
well understood, dinosaur.

A report published in the 1950s suggested that a very Iguanodonlike
dinosaur lived in Mongolia in Early Cretaceous times. This
tantalizing report also needed to be investigated further to check
whether its anomalous geographic range – in Asia in Early
Cretaceous times – was real or, as in the Romanian example,
another case of mistaken identity. The material, though
fragmentary, was stored in the Russian Palaeontological Museum
in Moscow, and had to be re-examined. What emerged was again
not as expected. This time the earlier reports proved correct, the
genus Iguanodon itself seemed to be present in Mongolia in Early
Cretaceous times, and the pieces recovered were indistinguishable
from the very well known European Iguanodon.
This second discovery did not fit at all comfortably with the
evolutionary and geographic hypothesis that had been created in
the 1984 analysis. Indeed, in more recent years a suite of very
interesting Iguanodon-like ornithopods have emerged in Asia,
as well as North America, in what can best be described as
‘middle’ Cretaceous times. Much of this very recent, and steadily
accumulating, evidence suggests that the original evolutionary and
geographic model had a number of fundamental flaws that
continued investigation and new discoveries were able to expose.