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.

Dinosaur systematics and ancient biogeography

This type of research can have interesting, if slightly unexpected,
spin-offs. One spin-off that will be considered here links
phylogenetics with the geographic history of the Earth. The Earth
may in fact have exerted a profound influence on the overall pattern
of life.

Unravelling the genealogy of dinosaurs
The geological timescale of the Earth was pieced together through
painstaking analysis of the relative ages of sequences of rocks
exposed at various places on Earth. One important component
that assisted this process was the evidence of the fossils that they
contained: if rocks from different places contained fossils of exactly
the same type, then it could be assumed with reasonable confidence
that the rocks were of the same relative age.

In broadly similar fashion, evidence of the similarity of fossils from
different parts of the world began to suggest that the continents
might not have been as fixed in their positions as they appear to be
today. For example, it had been noted that rocks and the fossils that
they contained seemed to be remarkably similar on either side of
the southern Atlantic Ocean. A small aquatic reptile Mesosaurus
was known to exist in remarkably similar-looking Permian rocks in
Brazil and in South Africa. As long ago as 1620, Francis Bacon had
pointed out that the coastlines of the Americas and Europe and
Africa seemed remarkably similar, (see Figure 32d) to the extent
that it seemed as if they could have fitted together as a pair of
gigantic jigsaw pieces. On the basis of evidence from fossils, rocks,
and general shape correspondence, Alfred Wegener, a German
meteorologist, suggested in 1912 that at times in the past the
continents of the Earth must have occupied different positions to
the ones they are in today, with, for example, the Americas and
Eur-Africa nestled together in the Permian Period. Because he was
not a trained geologist, Wegener’s views were ignored, or dismissed
as irrelevant and unprovable speculations. For all its self-evident
persuasiveness, Wegener’s theory lacked a mechanism: common
sense dictated that it was impossible to move things the size of
continents across the solid surface of the Earth.

However, common sense proved to be deceptive. In the 1950s
and 1960s, a series of observations accumulated that supported
Wegener’s views. Firstly, very detailed models of all the major
continents showed that they did indeed fit together remarkably
neatly and with a correspondence that could not be accounted for
by chance. Secondly, major geological features on separate
continents became continuous when continents were reassembled
jigsaw-like. And finally, palaeomagnetic evidence demonstrated the
phenomenon of sea-floor spreading – that the ocean floors were
moving like huge conveyor belts carrying the continents – and the
historical remnants of magnetism in continental rocks confirmed
that the continents had moved over time. The ‘motor’ that was
driving this motion was in effect the heat at the core and the fluidity
of rocks in the mantle layer inside the Earth. The theory of plate
tectonics that accounts for the movement of continents over the
surface of the Earth over time is now well established and
corroborated.

From a dinosaur evolutionary perspective, the implications of
plate tectonics are extremely interesting. Reconstructions of past
configurations of the continents, largely based on palaeomagnetics
and detailed stratigraphy, indicate that at the time of their origin all
the continents were lying clustered together in a single gigantic
landmass, known as Pangaea (‘all Earth’) (Figure 32a). Dinosaurs at
this time were quite literally capable of walking all over the Earth,
and in reflection of this it appears to be the case that the fossil
remains of rather similar types (theropods and prosauropods) have
been found on nearly all continents.

During subsequent Periods, the Jurassic (Figure 32b) and
Cretaceous (Figure 32c), it is evident that the supercontinent began
to fragment as the immensely powerful tectonic conveyor belts
imperceptibly, but remorselessly, wrenched Pangaea apart. The end
product of this process at the close of the Cretaceous was a world
that, though still different geographically (note particularly the
position of India in Figure 32c), has some very familiar-looking
continents.

The earliest dinosaurs seem to have been able to disperse across
much of Pangaea, judging by their fossils. However, during the
Jurassic and subsequent Cretaceous Periods it was clearly the case that the unified supercontinent became gradually subdivided by
intervening seaways as continent-sized fragments gradually drifted
apart.
An inevitable biological consequence of this intrinsic (Earth-bound)
process of continental sundering is that the once cosmopolitan
population of dinosaurs became progressively subdivided and
isolated. The phenomenon of isolation is one of the keystones of
organismal evolution – once isolated, populations of organisms
tend to undergo evolutionary change in response to local changes to
their immediate environment. In this instance, although we are
dealing with comparatively huge (continent-sized) areas, each of
the continental fragments carried its own population of dinosaurs
(and associated fauna and flora); each of which, with the passing
time, had the opportunity to evolve independently in response to
local changes in environment, stimulated by, for example,
progressive changes in latitude, longitude, adjacent oceanic
currents, and prevailing atmospheric conditions.

Logic dictates that it must clearly have been the case that tectonic
events during the Mesozoic affected the scope and overall pattern of
32(d). The continents as they are today. Close the Atlantic Ocean and
the Americas fit neatly against West Africa.
the evolutionary history of dinosaurs. Indeed, it seems perfectly
reasonable to suppose that the progressive fragmentation of
ancestral populations over time must have done much to accelerate
the diversification of the group as a whole. Just as we can
represent the phylogeny of dinosaurs using cladograms, we could
also represent the geographic history of the Earth through
the Mesozoic Era as a series of branching events as
continental areas separated from the ‘ancestral’ Pangaean Earth.
Of course, this general approach is a simplification of true
Earth history because, on occasion, continental fragments have
coalesced, welding together previously isolated populations.
But at least as a first approximation, this provides a fertile area
for investigating some of the larger-scale events in Earth
history.

If this model of the natural history of dinosaurs were generally true,
we might expect to be able to detect some evidence in its support by
probing the details of the fossil record of dinosaur species, and the
tectonic models of continental distribution through the Mesozoic.
This type of approach has been developed in recent years to probe
for coincident patterns in the evolutionary history of dinosaurs and
whether their evolutionary history is echoed in their geographic
distribution.