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.