The Origin and Evolution of Mammals - Moodle

188 THE ORIGIN AND EVOLUTION OF MAMMALS
indicate a shift from an area of poorly drained soils
and meandering river channels to conditions of
standing ponds and swamps. The conclusion of
Johnson and Hickey (1990) is that the abruptness
and extensiveness of extinction of the plants were
too great to be accounted for solely by climatic
change. Rather, they strongly support the idea of a
sudden, catastrophic cause. Palynological studies
of fossil pollen grains and spores in the western
interior are more equivocal. That of Nichols and
Fleming (1990) supports the impact hypothesis of a
sudden extinction event co-incidental with the iridium
anomaly. Furthermore, the discovery of the
celebrated K/T ‘fern-spike’, a large rise in the percentage
of fern spores in palynological samples of
the time, points to a suddenly imposed environmental
shift. However, another palynological study,
by Sweet et al. (1993), indicates that the floral extinctions
actually began significantly earlier than the
K/T boundary in far northern latitudes, which is
consistent not with an impact but with a gradual
climatic deterioration that affected higher latitudes
tens of thousands of years earlier than lower ones.
Taking these studies together, there is a good case to
be made from the evidence of the fossil flora for an
impact induced catastrophic extinction in North
America that was superimposed on an environment
that was altering more gradually anyway, as
indeed most environments are most of the time.
What very limited evidence is currently available
suggests that the flora of other parts of the world
did not suffer the abrupt mass-extinction evident
in North America (Archibald 1996). Studies across
the K/T boundary in New Zealand (Johnson
1993), Northern Russia (Golovneva 1994), and
Antarctica (Askin 1990; Elliot et al. 1994) all fail to
produce evidence for a single large extinction, and
are more compatible with gradual environmental
change.
The described pattern of taxonomic change suffered
by the various elements of the fauna, including
the dinosaurs and the mammals, is no less
ambiguous than that of the flora. Even more so than
in the case of the plants, knowledge of the extinction
of tetrapods across the K/T boundary is at
present virtually completely restricted to the North
American western interior sequence, and notably
the fossil vertebrates of the Late Cretaceous
Hell Creek Formation and the overlying earliest
Palaeocene Tullock formation in Montana. Regarding
the dinosaurs, there is still no agreement on
whether there was a gradual decline and that the
K/T boundary only marks the end of that process,
or whether there was a catastrophic extinction at
that time. Sheehan et al. (1991) made a very detailed
collection and found no statistically significant
decline in number of dinosaur families through
the Hell Creek Formation, which remained at eight
throughout. They inferred from this that the
dinosaur extinction was a single, large, and rapid
event. However, Hurlbert and Archibald (1995)
pointed out that the taxonomic level of family is too
insensitive to detect a decline, since a large proportion
of their contained species could have disappeared
without whole families becoming extinct. It
seems that the fossil record even here is simply
inadequate to distinguish between a gradual
decline and a catastrophic mass extinction of the
group. Indeed, it cannot even indicate whether any
dinosaurs survived until the actual K/T boundary.
The highest stratigraphic level of any specimen
recorded by Sheehan et al. (1991) was 60 cm below
the boundary, a result equally explicable as the true
absence of later dinosaurs, or as an artefact due to
the increased rarity of fossil preservation in the
later part of the Hell Creek Formation.
Archibald and Bryant (1990; Archibald 1991,
1996) made a detailed study of the fossil collections
of vertebrates across the K/T boundary in northeastern
Montana. After allowing for occurrences in
places other than this area, and for pseudoextinction,
where a species is believed to have evolved
into a new species rather than become extinct, they
documented species level survival percentages for
all the major vertebrate taxa. Even after making
these adjustments, there is still the possibility of
large errors because of the relatively low resolution
of this fossil record. Certain groups disappeared
completely, notably the dinosaurs but also the
freshwater sharks. Only 30% of the squamate
species survived, but amphibians (100%), turtles
(88%), and crocodiles (80%) were barely affected.
As far as the mammals are concerned, about 44% of
the latest Cretaceous species also occur in the earliest
Palaeocene, but different taxonomic groups
were very differently affected. The hardest hit were