The Origin and Evolution of Mammals - Moodle

diversity on Earth (Janis 1993). Fully tropical rain
forests extended as far as latitude 30 o , and subtropical,
angiosperm dominated forests to 60 o . At even
higher latitudes there were forests containing
broadleaf deciduous trees, such as that revealed by
the rich fossil localities of Ellesmere Island at 75 o
north, and the cool-temperate forests preserved on
Seymour Island on the Antarctic Peninsula (Wing
and Sues 1992; Prothero 1994). In the context of
such favourable environmental conditions of warm
temperatures, very high primary productivity, and
a large habitable area, the existing mammalian
groups radiated extensively, and a number of new
taxa appeared.
The fauna of North America, Europe, and Asia
was quite similar, although with distinct endemism
at lower taxonomic levels, indicating that interdispersal
between these northern areas during the
Early and mid-Eocene was relatively free (Ting
1998; Agustí and Antón 2002). Land connections
existed between northeastern America and Europe,
one via Greenland and Norway, and a second further
south, via Greenland, Iceland, the Faroes, and
Scotland. The dry Bering Strait connected North
America with Asia (Prothero 1994). In this single,
extensive landmass the archaic Palaeocene groups
continued to radiate. Tillodonts and large pantodonts,
and various of the ‘condylarth’ lineages flourished.
The creodonts, particularly the hyaenodontids,
were the prominent carnivorous mammals, for the
Carnivora were still relatively small, though abundant
animals.
The newly introduced modern orders were also
rapidly diversifying. Perissodactyls in particular
soon became an important group of medium to
large-sized herbivores, perhaps because their
hindgut fermentation system was better adapted for
the high fibre content of many Eocene forest plants
(Janis 1989). They diversified into the early representatives
of the modern families of horses, rhinos, and
tapirs, and also included bizarre, large-bodied,
browsing groups, the chalicotheres and especially at
this time the brontotheres. Forest adapted lemuriform
primates were another very diverse and abundant
group. On the other hand, the rodents and
artiodactyls, two orders destined eventually to
become amongst the most diverse of all placental
groups, remained relatively modest during the
LIVING AND FOSSIL PLACENTALS 281
Eocene. This was also the time of appearance of two
radically new kinds of placental body plan. Bats are
recorded from the Early Eocene and rapidly gained
a worldwide distribution including, uniquely for
placentals, dispersal to Australia. Amphibious members
of both cetaceans and sirenians appeared in the
Early Eocene, and by the Middle to Late Eocene permanently
aquatic versions had evolved.
Far less is known about Eocene mammals in
the southern continents, although from what the
fossil record does reveal, it was quite as rich as in
Laurasia. At present, Early and Middle Eocene
mammal fossils of Africa include isolated teeth of
creodonts, ‘condylarths’, and basal primates, which
presumably entered the continent from Eurasia by
an unknown route at an unknown time, and
the undoubtedly indigenous afrotherian order
Proboscidea. It is not until the Late Eocene and the
fossil fauna of the Jebel Qatrini Formation of
Fayum in Egypt that a clear African picture
emerges. The herbivore fauna was dominated by
the afrotherian orders Proboscidea and Hyracoidea,
and the embrithopod Arsinoitherium, a fauna no
doubt culminating from the radiation in isolation of
these orders throughout the Eocene and possibly
earlier. The few non-afrotherian placentals included
the continued presence of creodonts and primates,
amongst the latter of which were early anthropoids
inexplicably contemporaneous with those of Asia.
Marine mammals found here, Cetacea, and Sirenia,
are palaeobiogeographically easier to account for.
In South America, the xenungulates and
meridiungulate groups radiated throughout the
Eocene. One of the more unexpected features of
meridiungulate evolution is the presence of high
crowned, hypsodont teeth in Mid-Eocene notoungulates
(MacFadden 1997). These kinds of teeth are
associated with grazing, suggesting that grasslands
had started to spread in South America 10–15 Ma
earlier than in the northern continents. The carnivorous
mammals of South America were represented
not by placentals, but by the borhyaenid
marsupials.
Antarctica was still attached to South America at
this time, and to judge from the presence of
meridiungulates and xenarthrans from the mid-
Eocene of Seymour Island, off the Antarctic
Peninsula, the two landmasses had a contiguous