Past climate change - Climate Change: Biological and Human Aspects - page 89

It is well established that in the Cretaceous the large supercontinent of Pangea
continued to break up and that there was considerable volcanic activity (such as
the Deccan Traps in north-west India, discussed above). New oceanic currents were
established, and old ones diminished or ceased. The late Cretaceous was a time of
major sea-level transgressions and regressions and in particular no fewer than six
such events have been identified. Globally, oceanic isotopic data (such as forams;
see Chapter 2) from the tropics and also belemnites (a small, extinct cephalopod or
octopus-type mollusc) from higher latitudes suggest a consistent picture of at least two
periods of warming and cooling (Frakes et al., 1992). Add to this the regional climatic
changes due to the aforementioned circulation, and times were hard, and especially
so for those creatures more wedded to a specific biome. Considered this way, it is
not surprising that the dinosaurs were in decline. Whether or not an asteroidal coup
de gr ace was required to terminate the Mesozoic age of the dinosaurs we may never
know for certain. That such an event was necessary for so complete a termination
is, however, likely. For some the debate will continue, as it did with an exchange of
lucid articles and letters over many issues of Geoscientist in 2003-4.
The end-Cretaceous clearing of many species meant that when ecosystems
recovered there were vacant ecological niches. This, and the recovery period of
environmental change, facilitated speciation of surviving fauna. Of relevance to this
topic's readers, among the new species to arise were the placental mammals from
Eutheria, the forebears of the mammals. Such an early Placentalian was Maelestes
gobiensis (Wible et al., 2007). Environmental change is an important factor in biolo-
gical evolution.

3.3.9 TheEocene(55-34mya)andtheInitialEoceneThermalMaximum
(

∼ 55mya)

Following the Cretaceous-Tertiary dinosaur extinction 65 mya, the archaic mammals
began to diversify, but further change was to come. Prior to 55 mya, although the
temperature globally was warmer than today, there is a case that the terrestrial climate
was more seasonal than it previously had been. Many of the major landmasses had
already tectonically moved towards the northern high latitudes and so globally there
may have been a more deciduous type of vegetation. Certainly, folivorous (leaf-eating)
mammals were not apparent until the Eocene.
So, the late Palaeocene and the Eocene was already a warm time. The Eocene
itself began 55.8 mya and ended 33.9 mya. A few million years into the Eocene was
warmest sustained part of this time but at the very beginning of the Eocene there was
a short (geologically) and abrupt warm event at 55.8 mya that was, for a brief while
(a hundred thousand years), even warmer than the later, more sustained, warmest part
of the Eocene (see Figure 3.2): the Eocene (Climatic) Maximum 4 , which occurred
50-52 mya. This longer period of warmth in the already warm Eocene was towards
the end of the first quarter of the Eocene. During this warm time there was even a
period of some 800 000 years when it appears that Arctic waters were much fresher

4

Note that Eocene (Climatic) Maximum is not the same as Initial Eocene Thermal Maximum
(IETM)/Palaeocene-Eocene Thermal Maximum (PETM).