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:

thylacine epipubic bones
Thylacine epipubic bones (circled).
Specimen: Ab2127.
Courtesy: Bristol Museum & Art Gallery (UK).
Photo: International Thylacine Specimen Database 5th Revision 2013.
    "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.

body and limb proportions in Thylacinus
Fig. 2, p. 68, "Growth dependent changings in the skeleton proportions of Thylacinus cynocephalus (Harris, 1808)".  (Moeller 1980).

    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.

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