Dinosaurs in perspective

The fossilized remains of dinosaurs (with the notable exception of
their lineal descendants the birds – see Chapter 6) have been found
in rocks identified as belonging to the Mesozoic Era. Mesozoic rocks
range in age from 245 to 65 million years ago (abbreviated to Ma
from now on). In order to put the time during which dinosaurs lived
into context, since such numbers are so large as to be quite literally
unimaginable, it is simpler to refer the reader to the geological
timescale.

During the 19th and a considerable part of the 20th centuries, the
age of the Earth, and the relative ages of the different rocks of which
it is composed, had been the subject of intense scrutiny. During the
early part of the 19th century it was becoming recognized (though
not without dispute) that the rocks of the Earth, and the fossils that
they contained, could be divided into qualitatively different types.
There were rocks that appeared to contain no fossils (often referred
to as igneous, or ‘basement’). Positioned above these apparently
lifeless basement rocks was a sequence of four types of rocks that
signified four ages of the Earth. During much of the 19th century
these were named Primary, Secondary, Tertiary, and Quaternary –
quite literally the first, second, third, and fourth ages. The ones that
contained traces of ancient shelled and simple fish-like creatures
were ‘Primary’ (now more commonly called Palaeozoic, literally
indicative of ‘ancient life’). Above the palaeozoics was a sequence of
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rocks that contained a combination of shells, fish, and land-living
saurians (or ‘crawlers’, which today would include amphibians
and reptiles); these rocks were designated broadly as ‘Secondary’
(nowadays Mesozoic, ‘middle life’). Above the mesozoics were found
rocks that contain creatures more similar to those living today,
notably because they include mammals and birds; these were
named ‘Tertiary’ (now also called Cenozoic, ‘recent life’). And finally,
there was the ‘Quaternary’ (or Recent) that charted the appearance
of recognizably modern plants and animals and the influence of the
great ice ages.

This general pattern has stood the test of time remarkably well.
All modern geological timescales continue to recognize these
relatively crude, but fundamental, subdivisions: Paleozoic,
Mesozoic, Cenozoic, Recent. However, refinements in the way
the fossil record can be examined for example, through the use
of high-resolution microscopy, the identification of chemical
signatures associated with life, and the more accurate dating of
rocks enabled by radioactive isotope techniques have led to a more
precise timescale of Earth history.

The part of the timescale that we are most concerned with in this
book is the Mesozoic Era, comprising three geological periods:
the Triassic (245–200 Ma), the Jurassic (200–144 Ma), and the
Cretaceous (144–65 Ma). Note that these periods of time are not by
any means equal in duration. Geologists were not able to identify a
metronome-like tick of the clock measuring the passing of Earth
time. The boundaries between the periods were mapped out in the
last two centuries by geologists who were able to define particular
rock types and, very often, their constituent fossils, and this is
usually reflected in the names chosen for the periods. The term
‘Triassic’ originates from a triplet of distinctive rock types
(known as the Lias, Malm, and Dogger); the ‘Jurassic’ hails from
a sequence of rocks identified in the Jura Mountains of France;
while the name ‘Cretaceous’ was chosen to reflect the great
thickness of chalk (known as Kreta in Greek) such as that which forms the White Cliffs of Dover and is found widely across Eurasia
and North America.

The earliest dinosaurs known have been identified in rocks dated
at 225 Ma, from the close of the Triassic (a period known as the
Carnian), in Argentina and Madagascar. Rather disconcertingly,
these earliest remains are not rare, solitary examples of one type of
creature: the common ancestor of all later dinosaurs. To date at
least four, possibly five, different creatures have been identified:
three meat-eaters (Eoraptor, Herrerasaurus, and Staurikosaurus),
a tantalizingly incomplete plant-eater named Pisanosaurus, and an
as-yet-unnamed omnivore. One conclusion is obvious: these are not
the earliest dinosaurs. In the Carnian there was clearly a diversity of
early dinosaurs. This indicates that there must have been dinosaurs
living in the Middle Triassic (Ladinian-Anisian) that had ‘fathered’
the Carnian diversity. So we know for a fact that the story of
dinosaur origins, both the time and the place, is incomplete.