The Origin and Evolution of Mammals - Moodle

the 11 species of marsupial, of which only one,
Alphadon marshi survived. On the other hand, all six
of the placental species survived, Gypsonictops
illuminatus, four species of the palaeoryctid genus
Cimolestes, and another palaeoryctid Batodon tenuis.
Finally, 5 out of the 10 multituberculate species survived.
Archibald (1996) offered a possible explanation
for the much more severe effect on the
marsupials. The re-establishment of the land bridge
across the Bering Straits that resulted from the
regression of the sea allowed the immigration of
several species of mammals, at the very start of
the Palaeocene. These were mainly primitive
‘condylarths’, which are related to the later ungulate
groups of mammals. At this stage, however, the
condylarths such as Purgatorius were small, lacked
extensive herbivorous adaptations, and may have
had a similar diet to the marsupials. Thus he proposed
that competition by the invaders affected
marsupials more than the other two mammal
groups.
To conclude, what is known of the environmental
and ecological setting within which the mammals
underwent their transition from the Mesozoic
to the Tertiary is, to say the least, sketchy and geographically
very limited. In North America there
was a gradual environmental shift during the
Maastrichtian, the last stage of the Cretaceous,
involving an increase in temperature and humidity.
At the K/T boundary this process reversed,
mean temperature falling and conditions becoming
dryer. Simultaneously, there was a dramatic
shift in the vegetation, characterised by a reduction
in angiosperms and an increase in ferns for a short
time. These latter changes may well have been
associated with the bolide impact. One other
important geographical feature at about this time
was the regression of the sea level, causing a large
reduction in coastal areas within North America,
and also opening up a land connection across the
Bering Straits to the Asian land mass (Smith et al.
1994).
THE MESOZOIC MAMMALS 189
For all the detailed effort put into study of the
K/T boundary and the extinction patterns of the
components of the biota in North America, it ought
to be borne in mind that this represents only one
area of the world, and it is impossible to generalise
from it. For one thing, mid-western North America
lies close to the site of the bolide impact, so presumably
would have been disproportionately
affected by that event. Second, it was only one environmental
type, namely a mid-latitude, low-lying
area immediately adjacent to a large epicontinental
sea. Third, once into the Palaeocene it was rapidly
colonised by the new taxa of mammals that are
assumed to have immigrated into the area from
Asia, and which must represent lineages of mammals
that had survived the K/T transition elsewhere
in the world, possibly under very different
circumstances. In fact, several mammalian lineages
are found in the Palaeocene of other parts of the
world that have no initial representation in North
America. In China, Early Palaeocene mammals of
the Shanghuan Formation include both endemic
forms, and primitive members of groups that
appear later in North America, such as the herbivorous
pantodonts and tillodonts (Wang et al. 1998;
Lucas 2001). In South America, Palaeocene faunas
containing diverse marsupials, and indigenous placental
groups leave wide open the question of
where and when they originated, and how they
relate to the North American extinction (Marshall
et al. 1997). As to what happened to mammals at the
K/T boundary of Australia, Africa, and Antarctica,
there is simply no evidence at all.
At any event, the new, exciting Tertiary world
opened with a range of surviving Cretaceous
groups of mammals, and so far as is known no
dinosaurs. Increase in diversity was rapid, with the
known standing diversity in North America rising
from 15 species at the K/T boundary to over 60 a
mere 5 million years later (Wing et al. 1995; Alroy
1999b). This part of the story is pursued in more
detail in the following two chapters.