The Origin and Evolution of Mammals - Moodle

208 THE ORIGIN AND EVOLUTION OF MAMMALS
probably nuts and other hard food, and that in
addition to the gnawing and breaking function of
the incisors, a particularly well-developed tongue
may have been involved, working in association
with the symphyseal floor. Nothing is known of the
rest of the skeleton.
A second groeberiid, Klohnia (Fig. 6.8(d)), has been
described from the late Eocene/Early Oligocene by
Flynn and Wyss (1999), and it too is poorly known,
with little more than partial dentitions preserved.
A third possible member, Patagonia (Pascual and
Carlini 1987), was originally described as a member
of yet another family of specialised, rodent-like
caenolestoids. It is based on a single partial lower
jaw with a gliriform incisor. Flynn and Wyss (1999)
made a preliminary analysis of the interrelationships
and concluded that Groeberia, Klohnia, and
Patagonia do indeed constitute a valid family, and
that Groeberiidae is indeed the sister group of
Argyrolagidae.
Australidelphia: the Australian radiation
As discussed earlier, the Australasian marsupials
form a monophyletic group, provided that the living
microbiotherian Dromiciops and its fossil relatives are
admitted. Unlike the South American radiation, in
Australia the marsupials included middle- to largesized
herbivores as well as the carnivores. Indeed,
apart from the minute number of monotremes,
numerous bats, and a few dozen species of murid
rodents that entered the continent in the later part
of the Cenozoic from Asia, the entire pre-human
mammalian fauna was marsupial. Also in contrast
to South America, the fossil record of Tertiary
Australian marsupials during the critical early stages
is extremely poor (Vickers-Rich et al. 1991; Long et al.
2002). Nothing at all is known of the group during
the Palaeocene, and the earliest record is the very
modest Tingamarra fauna of Queensland, which is
dated approximately 55 Ma, and so is latest
Palaeocene or earliest Eocene. This is followed by
another relatively huge gap; the next window on to
the radiation is not until Late Oligocene times,
around 25 Ma, with several fossil-yielding localities,
including the start of the great tropical rain forest
Riversleigh system, with its abundance of beautifully
preserved fossils continuing through the Miocene
(Archer et al. 1991). Other important, though less rich
fossil-bearing areas from the Late Oligocene include
the Etadunna Formation in the Lake Eyre Basin of
South Australia, and the Namba Formation which is
also in South Australia. With one exception, all the
Oligocene and Miocene marsupials are accommodated
in one or another of the four extant Australian
orders. At a lower taxonomic level, 19 of the total of
23 families of australidelphians listed by McKenna
and Bell (1997) occur in the Oligocene–Miocene, and
of the 10 extant families only 4 are as yet unknown as
fossils from that time. Thus, by Riversleigh times a
taxonomically richer version of what is essentially
the modern marsupial fauna existed; nevertheless
there are a few surprises and mysteries.
The Tingamarra marsupials
The Early Eocene or possibly Late Palaeocene fossils
from the Tingamarra Local Fauna, near Murgon in
southeastern Queensland, are the oldest marsupial
mammals from Australasia (Godthelp et al. 1992).
Unfortunately, only a few very fragmentary jaws and
isolated teeth are preserved, and none of them are
clearly attributable to particular modern groups.
Djarthia (Fig. 6.9(a)) was described by Godthelp et al.
(1999) on the basis of several jaws and teeth. Its upper
molars are dilambdodont, having a V-shaped centrocrista,
and there are four well-developed stylar cusps.
The proportions of the talonid and trigonid of the
lower molars are of standard marsupial pattern.
From these dental characters, it is impossible to tell
whether Djarthia is a didelphid or a basal australidelphian,
yet this is a fundamentally important issue
for understanding Australian marsupial history.
If Djarthia is a didelphid, then it means that both
ameridelphians and australidelphians occurred
together there. If it is a basal australidelphian, then
the Australian radiation can be accounted for as a
consequence of a single didelphid-like immigrant
into the continent prior to the Eocene.
A second Tingamarra form is equally tantalising.
Archer et al. (1993) described isolated teeth of
Thylacotinga (Fig. 6.9(b)) as a possible peramelemorph.
However, this view has been revised, and
Long et al. (2002) regard it as a possible member of
the South American and Antarctic group Polydolopoidea.
The molar teeth have swollen, bunodont
cusps. The stylar shelf of the upper molar is