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In a recent paper by Reilly and White (2003) published in the journal "Science",
the motor patterns in the abdominal hypaxial muscles of the Grey
short-tailed opossum (Monodelphis domestica) and the Virginia
opossum (Didelphis virginiana) were studied during treadmill
locomotion using video
fluoroscopy. Reilly and White note:
"The
animals (opossums) used only symmetrical gaits (i.e., each step
is dominated by diagonal couplets of support). As in generalized
amniotes,
these species do not exhibit asymmetrical gaits (such as bounds) during
sustained locomotion".
They argued that if the function of the epipubic bones was to support the
pouch, they would be observed to move symmetrically during gait.
Their results showed this not to be the case, with the epipubic bones moving
asymmetrically. Reilly and White state: |
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Thylacine
epipubic bones (circled).
Specimen:
Ab2127.
Courtesy:
Bristol Museum & Art Gallery (UK).
Photo: International Thylacine
Specimen Database 5th Revision 2013.
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"Since the first description of epipubic bones in 1698, their functions
and those of the associated abdominal muscles of monotremes and marsupial
mammals have remained unresolved. We show that each epipubic bone
is part of a kinetic linkage extending from the femur, by way of the pectineus
muscle, to the epipubic bone, through the pyramidalis and rectus abdominis
muscles on one side of the abdomen, and through the contralateral external
and internal oblique muscles to the vertebrae and ribs of the opposite
side. This muscle series is activated synchronously as the femur
and contralateral forelimb are retracted during the stance phase in locomotion.
The epipubic bone acts as a lever that is retracted (depressed) to stiffen
the trunk between the diagonal limbs that support the body during each
step. This cross-couplet kinetic linkage and the stiffening function
of the epipubic bone appear to be the primitive conditions for mammals".
In relation to marsupials that have reduced epipubic bones, such as the
thylacine, they note:
"Evidence of release from this gait constraint comes from mammals that
have reduced epipubic bones (for example, the extinct
Tasmanian tiger) or conjoined epipubic bones (for example, bandicoots),
or that have lost the connection of the pectineus with the epipubic bones
(such as wombats and large kangaroos and wallabies). These animals
use more complex asymmetrical gaits (half bounds, bounds, and gallops),
have more erect limb postures, or have shifted to bipedal locomotion".
Professor
Heinz Moeller, in several key papers, contributed much to our understanding
of the skeletal relationships of the thylacine. Moeller (1980) investigated
the growth dependent changes to the skeleton of the thylacine. His
study examined 22 skeletons, of which 12 were deemed adult, 7 sub-adult,
and 3 juvenile based on their full complement of molar teeth. |
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Fig.
2, p. 68, "Growth dependent changings in the skeleton proportions of
Thylacinus cynocephalus (Harris, 1808)". (Moeller 1980).
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Moeller noted that the thylacine is poorly adapted for fast running, as
its legs are comparatively short. The length of the extremities correspond
to those of an isometrically enlarged Tiger
quoll (Dasyurus maculatus), an arboreal type of dasyurid.
Moeller found that the trunk length of the youngest thylacine was more
than 50% shorter than the mean of the corresponding length of the adults.
He observed that the body proportions change during growth; the skull becomes
smaller (skull length by nearly 6%), the anterior and posterior extremities
become relatively shorter (6%), there is a relative prolongation of the
radius (3%) and a shortening of the third metacarpus (2%), the femur and
tibia become 1% and 2% longer respectively, and the relative length of
the scapula and pelvis become longer by 4% and 2%.
Moeller (1968) took biometric measurements from a series of skeletons of
various predatory mammal species, comparing the length of the limbs to
that of the trunk (i.e. the spinal column minus the tail). He found
that the thylacine's legs proved to be proportionately much shorter than
those of a wolf, and are consistent in form with those of dasyurid marsupials.
He notes that the limb length ratio infers that the thylacine relies more
on stalking rather than lengthy pursuit of its prey, and consequently would
be more suited to living in forest habitats than open fields. Of
the placental carnivores studied by Moeller, the most comparable in limb
proportions to the thylacine is the Clouded leopard (Neofelis nebulosa),
a native of the Asian rainforests. Moeller also demonstrated that
in order to preserve the muscles' mechanical efficiency, the greatest lengthening
in canid limbs is to be found at the wrist and ankle. However, the
legs of the thylacine are not as proportionately long as those of dogs
and other canids, and it likely stalks its prey in a cat-like manner in
preference to actively chasing it. |
