Interpretation of Locomotory Adaptations:

    The Moree Thylacoleo specimen was the first virtually complete specimen of Thylacoleo to be found and provides the basis for limb comparisons with those of mammal species whose locomotory habits are known.

    Howell (1944), Hildebrand (1954, 1961), Smith and Savage (1956) and Savage (1977) have discussed various indices of intra- and inter-limb lengths which are distinctive of different mammalian locomotory methods.  The morphological trends which become evident include the retention of the primitive attribute of a somewhat shorter fore- than hind limb in quadrupedal walking animals, the elongation of the distal limb structures in cursorial forms (e.g. ungulates) and for the evolution of short yet strong legs in fossorial and swimming species.  However, one should bear in mind that the forelimb may serve functions apart from locomotion, which may modify the adaptation of the limb.

life reconstruction of Thylacoleo carnifex - Mauricio Antón
Life reconstruction of Thylacoleo carnifex.  Courtesy: Mauricio Antón.

    In this preliminary examination (Finch, 1982), measurements were made of the length of the vertebral column (taken as atlas - last lumbar vertebra), humerus, radius, metacarpal 3, femur and tibia.  Attention was only given to the Marsupialia, although the placental lion (Panthera leo) was included for comparison.  Skeletal elements from the collections of the Western Australian Museum were made available by Dr. W. D. L. Ride.  Figures in parentheses give the number of specimens from which measurements were made, and where possible, the measurements of both right and left limbs of each animal were procured.

   Order Marsupicarnivora:

    Family Dasyuridae: [Antechinus flavipes (1), Dasyurus geoffroii ( I ), Sarcophilus harrisii ( I )]

   Order Peramelina:

    Families Thylacomyidae and Peramelidae: [Macrotis lagotis (1), lsoodon obesulus (2)]

   Order Diprotodonta:

    Families Petauridae and Phalangeridae: [Pseudocheirus  occidentalis  (2), Trichosurus 
        vulpecula (4)]
    Family  Vombatidae: [Lasiorhinus lalifrons (2), Vombatus hirsutus (1)]
    Family Phascolarctidae: [Phascolarctos cinereus (2)]
    Families Macropodidae and Potoroidae: [Macropus sp. (1), Bettongia lesuer (1)]

    The most useful comparisons which can be made in the instance of the Moree Thylacoleo are: (a) humerus + radius + metacarpal 3 / length of vertebral column (atlas to last lumbar) x 100; (b) length of forelimb / length of hind limb x 100; (c) length of tibia / length of femur x 100; (d) length of radius / length of humerus x 100.  The frequently used ratio of hind limb to vertebral column length could not be evaluated since the pes was missing from the Moree specimen.

    Ratio (a), forelimb/vertebral column, distinguishes cursorial and saltatorial forms.  Animals belonging to the first two groups have a tendency to exhibit comparatively long forelimbs in relation to the length of the vertebral column.  The forelimbs of Thylacoleo are notably longer than those of such species as the arboreal Pseudocheirus and Trichosurus and the smaller, cursorial Dasyurus.  The ratios for the fossorial wombats, Vombatus and Lasiorhinus, and the essentially saltatorial Bettongia, Isoodon and Antechinus are low, ranging from 43 to 53.

    Ratio (b), the intermembral index, correspondingly allows the division of cursorial and arboreal from saltatorial animals.  It is notable, however, that wombats tend to have limb ratios which are similar to those of climbing animals.  Again, Thylacoleo is grouped among the larger, carnivorous forms.

    Within the hind limb, the tibia/femur index (c) has been observed by Howell (1944) to be high in saltatorial animals and in high-speed runners, but low in walking forms such as cattle, lions, and hyenas.  In this index, Thylacoleo can be placed amongst the placental lion and wombats.  Obviously, Thylacoleo could not have been a speedy, cursorial animal and its femoro-tibial index of 82 does not approach that of the arboreal possums, Trichosurus and Pseudocheirus (102-103).  The difference between Thylacoleo and Sarcophilus is generally about 100 in unspecialised animals, ris-approximately equal length.

    The final ratio, (d), that of radius/humerus is usually 100 in unspecialised animals, moving up to 140 in saltatorial types, to 120 in swift cursorial forms and dropping down below 100 in fossorial and walking varieties.  The placental lion has a humero-radial index of merely 90 (Howell 1944) regardless of its ability to make short bursts of speed.  Among the marsupials, Thylacoleo displays a much higher index (115) than its placental counterpart and it is therefore grouped with animals which are adapted to climbing rather than running or walking.

    How the koala (Phascolarctos) is categorized among the four ratios presents something of a puzzle.  It appears close to Thylacoleo in each instance, and there is reasonable dissimilarity in its indices from those of the other arboreal marsupials, the possums.  Archer (1976) however, proposes that the Phascolarctidae and their close relations the Vombatidae evolved directly from diprotodont ancestors bearing selenodont teeth and that the Phalangerids are a completely independent evolutionary lineage from the basic stock.  This theory has gained support from comparative serology studies performed by Kirsch (1977).  If this is the case, the divergence between the adaptations of koalas and possums is not surprising.

    Judging from the limb proportions studied, Thylacoleo appears to have been a cursorial marsupial which was not capable of rapid locomotion.  It exhibited no climbing adaptations, although the relatively long radius places it among the arboreal and saltatorial forms which have forelimbs that are commonly used for manipulative and food gathering purposes as well as for locomotion.

    Based upon present studies, the image that has been formed of Thylacoleo is that of a robustly built animal which normally has a deliberate walk, but that may have also been capable of short bursts of speed.  The unusually long forelimb with its enormous claw on the first digit was probably used in manipulating food toward the mouth, and would indicate a predatory rather than scavenging nature.  The pseudo-opposability of Thylacoleo's powerful first digit suggests a scansorial habit to Wells and Nichol (1977) and, considering the phalangerid characteristics of the Thylacoleonidae, this may have been the case. Thylacoleo, the "pouched lion" may have been more aptly named Thylacopardus or "pouched leopard" if the complete skeleton had been available for examination in the nineteenth century.

Acknowledgement: This subsection of Thylacoleo Revealed has been referenced (in part) from: FINCH, M. E., 1982. The Discovery and Interpretation of Thylacoleo carnifex (Thylacoleonidae, Marsupialia). In "Carnivorous Marsupials - Vol. 2" (Ed. M. Archer). Roy. Zool. Soc. N.S.W.: Sydney. pp. 537-51.
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