Among the three pairs of maxillary incisors, the first pair is enormous,
curved and caniniform, meeting at the tips and exhibiting wear facets on
their neighboring surfaces. An inclined groove on the posterior surface
of each has been created by the lower incisors. I2
is small but I3 possesses a large crown
which is oblique anteriorly and lingually.
perfectly preserved incisor (cast) from the Wellington
The small blunt maxillary canine is located in the palate posterior to
I3 and has no resemblance at all to the
homologous tooth in most carnivores. The first two maxillary premolars
are small as well and are positioned at the anterior margin of P3.
The third maxillary premolar is a massive structure adapted for cutting,
which may in some specimens measure up to 57 mm in length and has a lingually
oriented occlusal surface. As this tooth becomes worn down, the buccal
side of the root becomes further elongated faster than the lingual surface
and therefore the crown 'rotates' lingually in order to maintain a close
contact with P3. The margin of the maxilla appears
to grow downwards at the same time, thereby increasing the vaulting of
the palate. Such a rotation has been recorded by Mellett (1969) in
the maxillary carnassial teeth (M1 and
Hyaenodon but this action
does not appear to occur in fissipede carnivores.
The single maxillary molar is positioned obliquely on the lingual side
of the posterior root of P3. It displays
an anteromedial wear surface symmetrical with its function as a stop for
the mandibular first molar.
buccally to converge
with the occlusal surface of P3 and grew
in this direction during life as appears to have been the situation with
(Mellett, 1969). The lower sectorial surface bears a facet on the
trigonid of M1 so that both P3
and M1 shear past P3. M2
is quite small and is rarely present in fossil material.
In the lower jaw, the single pair of recumbent incisors are positioned
with their tips posterior to I1 when at
rest. Wear occurs at the tip and upon the outer margin of the crown
and an interdental facet is present on the contiguous incisor surfaces.
As is the case with the maxillary dentition, in the mandible there are
two small premolars at the anterolingual margin of the lower carnassial.
The latter tooth is inclined
crown of a left maxillary 3rd premolar. Wellington Caves, NSW.
The image provided by
Thylacoleo's skull morphology is therefore
that of a marsupial that displays adaptations to carnivory which are similar
to those seen in placentals except in the form of the condyle and the glenoid
fossa, the reduction of the canine teeth, the procumbent I1,
and the enlarged I1.
Reconstruction of the masticatory musculature of Thylacoleo by using
a dissection of the structurally similar phalangerid, Phalanger maculatus,
as a guide (Finch, 1981) demonstrated that the temporalis muscle constituted
approximately 56% of the total adductor musculature, while the masseter
complex (superficial and deep masseters and zygomaticomandibularis) contributed
34% and the pterygoids a mere 10%. Figures were given by Turnbull
(1970) for a wide variety of mammals which he arranges into four basic
categories: A, generalized or unspecialised type; B, carnivore-shear or
"scissors" type; C, ungulate-grinding or "mill" type; and D, rodent-gnawing
or "forward-shift" type. There is only a difference of degree between
categories A and B as, in each, the temporalis is dominant and the smallest
of the adductor muscle groups is the pterygoid. The forward-shift
category (D) does not correspond with the grinding category (C) in that
the predominating masseter complex is notably larger and the temporalis
and pterygoid ae inclined to be more reduced than in category C.
It would seem that Phalanger is intermediate between categories
C and D. Examination of its skull demonstrates a glenoid fossa which
is not closed anteriorly and possesses an articulating surface which is
ventrally convex. This suggests a forward shift mechanism to allow
occlusion of the incisors as, at rest, the tip of I1
lies anteriorly to I1. To attaining
this position, the condyle is moved anteriorly on to the convexity of the
glenoid fossa which permits contact of the incisor tips. Also, this
prevents occlusion of the cheek teeth at times when the incisors are in
In difference to Phalanger, however, Thylacoleo exhibits
a dominant temporalis and a small pterygoid which conspicuously places
the animal in the carnivore-shear category (B). Van Valen (1969:
p. 114) makes note that the masseter complex exceeds the temporalis in
Thylacoleo but provides no supporting information.
The likely areas of origin and insertion of the adductor muscles were outlined
on a precise drawing of the skull and the midpoints of the the two attachments
of each muscle were joined in order to obtain further insight into the
masticatory function in Thylacoleo. This displayed the direction
of pull of the muscle's "central" fibre. Taking this into consideration,
along with the distance of the carnassial tooth from the condyle (resistance
arm) and the length of the effort lever arm (fulcrum to centre of the muscle
insertion area), it is possible to evaluate the useful power (E) of a muscle.
This is done from Turnbull's formula (1970):
E = M x FL
x F× x r
Where M represents the percentage of weight of the muscle and r is the
ratio of the effort lever arm to the resistance lever arm. FL
and F× are correction factors which counterbalance the
deviation, in the parasagittal and traverse planes, of fiber direction
from the plane of jaw closure. A muscle that is oriented at 90 degrees
to the lever arm in the parasagittal plane is regarded as capable of exerting
its full force to close the jaw and thus FL
= 1. Any departure from 90 degrees, however, will lessen its performance
until a muscle lying parallel to the lever arm has FL
= 0. Likewise, F× = 1 if the muscle, observed from
the posterior aspect of the skull, is vertically oriented (lying parallel
to the coronoid process). F× tends toward zero as
the muscle tends towards a horizontal orientation.
Of the total useful power (E) produced by the adductor muscles of Thylacoleo,
the temporalis exerts 62%, the superficial masseter 10%, the deep masseter
11%, the zygomaticomandibularis 14% and the pterygoid 3% (Finch, 1981).
Upon comparing these figures to the percentage weights, only the zygomaticomandibularis
and ptrygoid exert less force than would be expected from their relative
mass. Examination of the probable fibre direction reveals the reason.
These two muscles approach a horizontal rather than a vertical direction.
Obviously, their functions are not solely adductor. Scapino (1965)
proposes that in a canid, where the mandibular symphysis is not fused,
these two muscles act to regulate the angel at which the mandibular carnassial
approaches the maxillary one by slanting the dentary to allow the most
effective shear. This would also appear to be the case in Thylacoleo
in which the mandibular symphysis is unfused. Also, only limited
lateral excursion of the condyle is possible at the glenoid fossa in Thylacoleo
and, even though the carnassial teeth continue to grow as they become worn
down, correct positioning of the cutting surfaces demands some adjustment
to the stance of the lower blade.
Upon examination of
Thylacoleo's skull structure and reconstructed
musculature, it can be proposed that this animal was capable of exerting
a powerful slicing action with the carnassial teeth and a forceful puncturing
bite with the four convergent first incisors. Wear facets exhibit
that there was occlusion between I1 and the three
maxillary incisors which would allow for the biting and tearing of smaller
pieces of food. Wear present on the posterolateral surface of the
canine could only have been produced by occlusion with the anterior buttress
of P3 when the jaw was in the forward position.
The position of M1 and a facet on the posterolateral
wall of M1 upholds Woods' (1956) belief that the maxillary
molar was a stop for M1 as the jaw moved back into
the resting position.
The fact that Thylacoleo does not possess a battery of grinding
molars illustrates that it could not have subsisted on plant material.
The massive dimensions of the carnassial teeth and the concomitant carnivore
adaptations indicate a carnivorous habit. However, Thylacoleo
has retained the ancestral forward-shift mechanism along with the reduction
of the canines, features which are not carnivore characteristics.
A similar case can be seen in the giant panda, Ailuropoda melanoleuca
which has been shown by Davis (1964) to have evolved in the opposite direction.
Descended from an omnivore-carnivore stock, it has become a specialized
herbivore while still retaining a number of carnivore characteristics.
Wells et al. (1982) is in general concurrence with the theory that Thylacoleo
was a carnivore, but sees the animal as a bone-crusher capable of pivoting
its mandible laterally on one condyle, thereby enabling it to occlude the
I1 with the three ipsilateral maxillary incisors.