Past climate change - Climate Change: Biological and Human Aspects - page 102

Brittany, France, today. (Although again it needs to be remembered that seasonality
was probably higher, with a greater difference between Eocene summer and winter
temperatures compared to lower latitudes.)
Even though we do not know much of the detail of ecological change that took
place during the IETM/PETM, what we do know is that many ecosystems saw
great disruption. For example, a study of plant fossils in present-day USA showed
that initial Eocene maximum floras were a mixture of native and migrant lineages
and that plant shifts were large and rapid (occurring within 10 000 years). Leaf
margin analysis (LMA) of 23 dicotyledon species suggested that temperatures in the
present-day USA area rose by approximately 5 ◦ C during the initial Eocene maximum
compared to LMA of fossils 250 000 years beforehand and is consistent with isotope-
derived estimates. Leaf area analysis (LAA) suggests increased precipitation but
some local reductions and a reduction (approximately 40%) early on in the event.
This overall increase from LAA is again in line with isotopic evidence elsewhere
on the planet suggesting an increase in precipitation during the IETM/PETM (Wing
et al., 2005).
It is likely that a similar event today, with a global human population of several
billion fragmenting the natural landscape, would not see such successful species
migration as happened in the Eocene. Furthermore, if the event were faster (due
to our 21st-century pulse of a similar amount of carbon being more rapid) there
would be considerable dislocation of largely human-managed ecosystems (such as
agricultural land). Consequently, it is likely that there would be much human suffering
(and economic dislocation). Whether it would have significant long-term evolutionary
implications is simply not known; even without climate change, humans are affecting
many ecosystems globally and causing biodiversity loss, which itself is significant.
In terms of the IETM's long-term biological impact, as a result of the early Eocene
climatic-maximum ecosystem disruption the older groups of mammals began to
decline and were replaced by a diversification of the newer, modern orders of mam-
mals. This diversification favoured smaller creatures. Smaller creatures have a higher
surface-area-to-volume ratio and so tend to lose heat faster than larger animals, which
would have been an advantage in a hot climate. In 2012 Ross Secord, Jonathan Bloch,
Stephen Chester and colleagues reported a high-resolution analysis of the fossil record
across the IETM/PETM. It showed a decrease in the size of equids (horse family)
of around 30% across this CIE followed by around a 76% increase in size after it.
Carnivorous mammals also diversified across the IETM/PETM but at first were more
generalist and they did not assume the role of larger predators until the late Eocene
(Janis, 2001). The Eocene also saw the order Primates begin to diversify, having first
appeared in basal (early) Eocene deposits of North America and Europe and slightly
later in Asia.
A question that has arisen is which was more important to mammal evolution:
the IETM climate change or the end-Cretaceous extinction? Clearly both affected
mammalian evolution but different researchers emphasise the importance of different
events. An international research team led by Robert Meredith of the University of
California in 2011 constructed a molecular supermatrix for mammalian families and
examined genome data (using relaxed clock analyses). Their results suggest that it