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- DISCOVERING THYLACOLEO -
(page 2)
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    Some seven years later Owen (1866) elaborated on his description after receiving additional skull fragments from Darling Downs which were sent to him by Edward Hill of Sydney.  When pieced back together, these fragments made a much more complete specimen than the skull from lake Colongulac.  The alveoli of the maxillary's pre-carnassial teeth were all present and the canine itself was still in position.  Owen then determined that Thylacoleo must have had an enormous tusk-like I1/ and two smaller incisors on the premaxilla while the maxilla must have carried the small, blunt canine followed by one or two small premolars positioned medially to the anterior root of the carnassial.  He also made mention of the double wear facet on the medially-directed occlusal surface of P3/ which shows a slicing action was had occurred against both P/3 and M/1.  Owen also pointed out the fact that the glenoid cavity was not semicylindrical in design as in typical placental carnivores.  Though it is transversely elongate and bears processes limiting lateral excursion of the condyle, its articulating surface is slightly convex ventrally and extends forward for about 2 cm from the postglenoid process.

    Information on the mandible was provided through a photo and a drawing of Wellington Caves material sent by Mr. Gerard Krefft, Curator at the Australian Museum, Sydney.  From these, Owen demonstrated that there was only one large tooth at the anterior perimeter of the dentary, though its morphology could not be determined as the crown was no longer present.  Only two small alveoli lay between this tooth and the mandibular carnassial.  Based upon this pictorial information, Owen presumed that the thylacoleonid taxonomic relationship lay with the diprotodontid marsupials as opposed to the carnivorous dasyurids.  Owen gave Thylacoleo's dental formula as I 3-3 / ?, C 1 / ?, P 2-2 / 2-2, M 1-1 / 2-2 = 24 denoting his incertitude as to whether a second incisor or a canine lay behind I/1 and considering that the small anterior premolar was double rooted.  Owen restated his belief in the carnivorous nature of Thylacoleo, and recorded that it "exemplifies the simplest and most effective dental machinery for predatory life and carnivorous diet known in the mammalian class, It is the extreme modification, to this end, of the Diprotodont type of Marsupialia" (Owen 1866:  81).

    More fossil material from Darling Downs had reached Owen by 1871 and from this he described I1/, C1/, the two small premolars anterior to the maxillary carnassial and a mandibular incisor in which the base of the crown was still present.  However, the premaxillary-maxillary suture was not well defined and appeared to run through the alveolus of I3/ which Owen had therefore labeled as C1/, while the actual canine became P1/.  The photos provided by Krefft had served as a basis for illustrations of isolated teeth in this paper and, in the next year, Krefft (1872b) cited that his images of I2/, I3/, and C1/ had been labeled P1/, C1/, and P3/ respectively by Owen.  This mistake was soon made clear.   In 1868, Flower had published a drawing of the skull in which the canine was correctly labeled and in 1876 McCoy provided a comprehensive description of the path of the premaxillary-maxillary suture in new fossil specimens from Lake Colongulac.  The suture was demonstrated to approach the alveolus of I3/ but then, at the periphery of the jaw, it was redirected back to run through the alveolus of the canine.

    The serrated enamel on the lateral margin of each I/1 was noted by both Owen (1871) and Krefft (1872b).  Krefft also pointed out that the enamel was restricted to the tips of the crowns on the interdental surfaces of these particular teeth.  He also noted wear facets in this region which indicate that there was movement between the mandibular incisors and that, thus, the mandibular symphysis could not have been fused.  Each of the large maxillary first incisors were shown to bear a wear facet on its lingual surface illustrating that the mandibular incisors passed behind the maxillary ones upon jaw closure.  The crowns of I2/ and I3/ also exhibited evidence of abrasion at their tips as a result of contact with I/1.

    The next notable breakthrough in the study of Thylacoleo carnifex came about in 1883 when Owen (1883a) ardently acknowledged receipt of a maxilla and a mandible from the Wellington Valley, both of which still retained all of their teeth.  The mandible possessed an intact, transversely-elongated condyle and a deeply inflected mandibular angle but the coronoid process, unfortunately, was broken.  The broad, anteriorly-directed profile of the latter was finally established in 1887 from a cast of a complete dentary forwarded to Owen to G. P. Ramsay, successor to Krefft.  In this, Owen's final compendiary publication regarding Thylacoleo, he reprised his conjecture that the animal was a diprotodontid which had become adapted for a life of carnivory.
 

The orbit (eye socket) of Thylacoleo's skull.  The arrow points to the postorbital bar.  Among marsupials, this feature known to exist only in Thylacoleo and Thylacosmilus.  In this particular specimen, the centre of the postorbital bar is missing.  In an intact skull, the orbit would be completely surrounded by a ring of bone.
    Of the skull anatomy discussed thus far, no mention has yet been made of two nearly unique features of Thylacoleo.  The orbit of this animal is delineated posteriorly by a slender bar that is missing from most fossil specimens.  This postorbital section of bone is also found in the extinct Thylacosmilus of South America, but is not present in other marsupials.  It was illustrated without comment in a reconstruction of the skull in Lydekker's catalouge (1887, fig. 28) but notice was drawn to this peculiar characteristic by Anderson (1929) and by Gill (1954).  The second feature of note, one which is totally unique to Thylacoleo, was also remarked upon by Anderson.  Thylacoleo's sphenopalatine foramen, through which the orbit and nasal cavity are in contact, is significantly larger, relatively, than that seen in any other land mammal.  No explanation for why this should be has been put forward.  In 1956, Woods produced an in depth description of the skull of Thylacoleo based upon the Darling Downs specimens held at the Queensland Museum.  From one skull he was able to create an endocranial cast which he compared with the one described by Gervais as far back as 1869.

    Among the marsupials, the brain of Thylacoleo would appear to most closely resemble that of the phascolomids.  The ear region (including the stapes) as well as the temporomandibular joint, however, show affinities to those of the phascolarctids.  Woods also pointed out the fact that no deciduous teeth had been found in any specimen.

    Presently, skull and dental material of Thylacoleo carnifex have been found in every state in Australia.  The Western Australian and Tasmanian specimens are few in number as compared to that discovered in the other states, but it would seem that they belonged to animals that were of smaller build than those hailing from the eastern states.  This discrepancy in size has been examined by Finch and Freedman (1982).
 

POSTCRANIAL MATERIAL:

    A metacarpal was described by Owen (1859), a radius, ulna and terminal phalanx (1883a) and a pelvic girdle (1883b) while De Vis made a description of a femur (1887) and a radius (1900) and Longman (1925) classified a calacaneum to Thylacoleo.  Ungual phalanges were discovered by Krefft, one of which he described (1870a) as Mylodon australis although Owen (1871, 1877) was inclined to consider them as being Thylacoleonine.  Although many of these specimens had been recovered from the same localities as Thylacoleo skulls, however, there was no solid evidence that they were genuinely parts of a Thylacoleo.

    The sole published record of postcranial material which was undoubtedly referable to Thylacoleo was that of Wells and Nichol (1977).  They described the Thylacoleo manus in both articulated and disassembled manner and were of the belief that the forelimb stance was digitigrade.  According to these authors considerable mobility would have been exhibited by the pollex and appeared to be pseudo-opposable to the pisiform.  The pes was not complete but its structure inferred a plantigrade posture and a hallux that was divergent from the other digits.

THE MOREE THYLACOLEO:
 

    A nearly complete skeleton of Thylacoleo was discovered in a loam pit in 1966 near Moree in northern New South Wales.  The right scapula and right ilium were still in association with the vertebrae and the vertebral groups articulated together to form a complete column apart from the terminal caudal units.  The limb bones were broken but could be pieced back together to make a complete forelimb and hindlimb which were missing only the pes.  The adult specimen was in association with a juvenile, represented only by a large skull fragment and a section of the lower jaw.  Found with these was the crushed skull of a Thylacoleo pouch-young.  This material has been used in examinations of mastication, feeding habits and locomotion (Finch and Freedman 1982, Finch 1981).
The Moree Thylacoleo, one of the most complete skeletons ever found of T. carnifex.  Unearthed at a loam pit in New South Wales in 1966, it offered the first solid glimpse of Thylacoleo's postcranial anatomy.
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Information on this page is based primarily upon the publication The Discovery and Interpretation of Thylacoleo Carnifex (Thylacoleonidae, Marsupialia) authored by M. E. Finch, presented in Carnivorous Marsupials, 45:537-551, Royal Zoological Society of N.S.W., Mosman, 1982.  All rights reserved and acknowledged.

Reference listing from the publication

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