The Origin and Evolution of Mammals - Moodle

(d)
Figure 6.6 (continued).
Thylacosmilus
Muizon et al. (1997) compared the primitive
Tiupampan sparassodont Mayulestes with the
didelphimorphs Pucadelphys and Andinodelphys
from the same locality. They found a number of similarities
suggesting a relationship, such as a medial
process of the squamosal, and an enlarged, slightly
procumbent first upper incisor and staggered
second lower incisor. Limited though this evidence
is, it is rather more convincing because it does
take cranial anatomy into account as well as dental.
In fact, this is a reversion to another long-held
view, that sparassodonts are simply specialised
carnivores derived from a basal didelphimorph
stock (Marshall 1978).
Mayulestes (Fig. 6.6(a)) is the oldest known sparassodont
and is represented by a skull and partial
skeleton (Muizon 1998). It is placed in a separate
family Mayulestidae, along with the contemporary
Allqokirus that is only known from isolated teeth.
Mayulestes has retained the primitive marsupial
dental formula of I5/4 : C1/1 : P3/3 : M4/4, and
the structure of the molars is also of the basic
marsupial form. However, the skull does show the
diagnostic sparassodont characters of a contribution
(e)
LIVING AND FOSSIL MARSUPIALS 203
Prothylacinus
of the squamosal to the roof of the alisphenoid
sinus, and the loss of the prootic canal. The postcranial
skeleton is indicative of a mammal with an
agile, scansorial habit; the fore and hindlimbs had
considerable three-dimensional mobility and the tail
was probably prehensile. Muizon (1998) believed
that the mode of life would have been comparable
to that of a small pine marten, and suggested that
this was the primitive habit, not only of sparassodonts,
but of marsupials generally.
Members of the more progressive, and far more
abundant and diverse family Borhyaenidae appear
in the slightly younger Itaboraí fauna, for example
Patene. The Early Eocene Callistoe (Fig. 6.6(b)) is
related to the sabre-toothed Thylacosmilus, as indicated
by its already very enlarged canines. The
borhyaenids subsequently went on to radiate as
middle-sized and larger carnivores of the South
American Tertiary (Marshall 1978), a role shared
amongst the mammals only with a few of the largest
didelphids, and modest-sized procyonid placentals
that entered the continent in the Late Miocene. In fact,
the largest predators were amongst the flightless
phorusrhacid birds, with some species approaching