Page 192 - Introduction to Paleobiology and The Fossil Record
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MASS EXTINCTIONS AND BIODIVERSITY LOSS 179
mammals in Europe and North America. (a) (b) (c)
Later Cenozoic events are less well defi ned.
There was a dramatic extinction among
mammals in North America in the mid-Oli-
gocene, and minor losses of plankton in the
mid-Miocene, but neither event was large.
Planktonic extinctions occurred during the
Pliocene, and these may be linked to disap-
pearances of bivalves and gastropods in tropi-
cal seas. (d) (e)
The latest extinction event, at the end of
the Pleistocene, while dramatic in human
terms, barely qualifies for inclusion. As the
great ice sheets withdrew from Europe and 0 2
North America, large mammals such as mam- cm
moths, mastodons, woolly rhinos and giant Figure 7.12 Disaster taxa after the end-Permian
ground sloths died out. Some of the extinc- mass extinction: the brachiopod Lingula (a), and
tions were related to major climatic changes, the bivalves Claraia (b), Eumorphotis (c),
and others may have been exacerbated by Unionites (d) and Promyalina (e). These were
human hunting activity. The loss of large some of the few species to survive the end-
mammal species was, however, minor in glo- Permian crisis, and they dominated the black
bal terms, amounting to a total loss of less anoxic seabed mudstones for many thousands of
than 1% of species. years after the event.
Recovery after mass extinctions
disaster taxa in the earliest Triassic are the
After mass extinctions, the recovery time is bivalves Claraia, Unionites and Promyalina,
proportional to the magnitude of the event. found in black, anoxic shales everywhere.
Biotic diversity took some 10 myr to recover These animals could presumably cope with
after major extinction events such as the Late poorly oxygenated waters.
Devonian, the end-Triassic and the KT. Recov- Bivalves and brachiopods diversifi ed slowly
ery time after the massive PT event was much in the next 5–10 myr, as did the ammonoids.
longer: it took some 100 myr for total global But other groups had gone forever. The rugose
marine familial diversity to recover to pre- and tabulate corals and other Late Permian
extinction levels. Species-level diversity may reef-builders had been obliterated. The “reef
have recovered sooner, perhaps within 20 or gap” following the PT mass extinction is pro-
30 myr, by the Late Triassic. But the deeper found evidence for a major environmental
diversity of body plans represented by the crisis. The rich tropical reefs of the Late
total number of families took much longer. Permian had all gone, and nothing faintly
It is becoming clear that all the rules resembling a coral reef was seen for 10 myr
change after a profound environmental crisis after the event. When the first tentative reefs
(Jablonski 2005). Disaster taxa prove the reassembled themselves in the Middle Trias-
point (Fig. 7.12). These are species that, for sic, they were composed of a motley selection
whatever reason, are able to thrive in condi- of Permian survivors, a few species of bryo-
tions that make other species quail. Stromato- zoans, stony algae and sponges. It took
lites, for example, in marine environments another 10 myr before corals began to build
and ferns on land make sudden but brief true structural reefs (see p. 289).
appearances. After the PT crisis, the inarticu- The reef gap in the sea is paralleled by the
lated brachiopod Lingula flourished for a “coal gap” on land. Coals are formed from
brief spell, before retiring to the wings. Lingula dead plants, and there were rich coal depo-
is sometimes called a “living fossil” because sits formed through the Carboniferous and
it is a genus that has been known for most of Permian, indicating the presence of lush
the past 500 myr, and it lives today in low- forests. After the acid rain had cleared the
oxygen estuarine muds. Other post-extinction land of plant life, no coal formed during the
