Past climate change - Climate Change: Biological and Human Aspects - page 106

Present ( (

Past

Fig. 3.5 Thewidediversiicationoffernsdidnotsolelytakeplacewhentheydominatedmanyterrestrialbiomes(as

representedinpanel(a))butextantfernsdiversiiedlater(moreasrepresentedinpanel(b)),illingthenew
environmentalnichescreatedbytheangiosperms.

the golden age of a high number of fern species was prior to the Cretaceous (135
mya). It was previously thought that this time before the Cretaceous, when fern-
dominated biomes were far more common, was when the great diversity of ferns
arose. It was similarly thought that there were fewer species of fern evolving actually
in the Cretaceous due to the rise and increase in angiosperms (flowering plants).
Supposedly, the rise and diversification of one offset the diversification of the other,
as they were competitors. Of course, some fern species - indeed many - have died
out over the past couple of hundred million years, but there is evidence that the
richness of fern species we see today does not appear to have stemmed from before
the Cretaceous. Conversely, genetic and systematic analysis (of species' evolutionary
relatedness) suggests later development of the range of modern ferns that coincided
with - not preceded - the rise of angiosperms in the Cretaceous.
This may at first seem counterintuitive. It would seem likely that there were more
of today's species early on when the ferns dominated many terrestrial ecosystems,
not later, with only a few surviving extinction episodes and competition from the
rise of more advanced plants. However, what actually appears to have happened is
that the new angiosperm communities established in the Cretaceous also created new
environmental niches, and of course the ferns were already there, poised to exploit
them (Schneider et al., 2004). The rise of angiosperms was therefore coincident with,
and not subsequent to, the diversification of the fern species we see today and the
fern diversification was in fact a response to the rise in angiosperms (Figure 3.5).
The relevance of this to the late-Miocene expansion of C 4 grasses (14-9 mya) is that
the expansion of grasses also provided new ecological niches, hence the evolutionary
opportunity for speciation (the formation of new species). One speciation event that
began at this time, and which arguably has had the greatest impact on the planet, was
that of the Hominoidea, the apes. This branch of speciation continued ultimately to
include modern humans, Homo sapiens , but began back in the late Miocene. One
of the early Hominoidea speciations was that of the sivapithecines, whose remains