The Origin and Evolution of Mammals - Moodle

(a)
(b)
Djarthia Thylacotinga
(c) (d)
Tingamarra Bobbschaefferia
Figure 6.9 Tingamarra mammals. (a) Upper molar of the
didelphimorph-like Djarthia murgonensis in occlusal view (redrawn
after Godthelp et al. 1999). (b) Isolated, incomplete upper molar of
Thylacotinga bartholomaii (redrawn after Long et al. 2002). (c) Lower
molar of Tingamarra porterorum in lingual, occlusal, and labial views,
with (d) lower molar of Bobbshaefferia flumensis also in lingual,
occlusal, and labial views for comparison (Woodburne and
Case 1996).
reduced in area and the stylar cusps are equal in
size to the main cusps, creating a broad, crushing
tooth. Yet other Tingamarran teeth have been
attributed to the Microbiotheria (Archer et al. 1999)
but this is also very tentative indeed.
The most enigmatic Tingamarran fossil of all
is a single lower molar tooth named Tingamarra
(Fig. 6.9(c)). Godthelp et al. (1992) claimed that it is
not a marsupial because it lacks the twinning of the
entoconid and hypoconulid and other features that
are so characteristic of marsupials. Instead, they
attributed it to the primitive placental group
LIVING AND FOSSIL MARSUPIALS 209
Condylarthra which, if true, would make it the only
early Cenozoic placental known from Australia.
Woodburne and Case (1996) challenged their interpenetration
and proposed instead that Tingamarra
is actually a marsupial, possibly a member of the
ameridelphian family Protodidelphidae. Members
of this family, of which the most familiar is
Bobbschaefferia (Fig. 6.9(d)), have only been found
otherwise in the Palaeocene Itaboraí deposits
of South America, and therefore, on this view,
Tingamarra would be an ameridelphian group common
to both continents. An alternative possibility is
that Tingamarra is a specialised australidelphian
convergent upon such forms. Only better specimens
are likely to resolve the question of its true
identity. Thus the tantalising nature of these fragmentary
Tingamarra mammals can be appreciated.
At one extreme, they could all be basal members of
the radiation of Australidelphia, and to varying
degrees exhibiting a dental morphology convergent
on other marsupial groups. At the other extreme,
there could be representatives of as many as three
ameridelphian taxa, didelphids, polydolopoids,
and protodidelphids, and of microbiotheres that
are also common to both continents. This conclusion
would indicate that at the time of the
Palaeocene–Eocene boundary there was virtually
a single marsupial fauna throughout Gondwana.
Only more diagnostic material will help to resolve
the issue, which will be returned to later.
Microbiotheria
Once Szalay’s (1982) comparative study of ankle
structure, and subsequent molecular and morphological
analyses confirmed that the surviving South
American microbiothere Dromiciops (Fig. 6.1(e) and
6.10(a)) is related to the Australian rather than the
South American marsupials, the biogeographic and
stratigraphic history of the group became very
important. Characterised by the elongation of the
talonid of the lower molars, and reduction of the
stylar shelf of the upper molars, the earliest record
is Khasia (Fig. 6.10(b)), molar teeth of which have
been found in the Palaeocene Tiu Pampa locality of
Argentina (Muizon 1991). Various other genera of
microbiotheres occur through the South American
fossil record, for instance Microbiotherium itself
(Fig. 6.10(c)) in the Eocene and Miocene. They are