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BIOLOGY:
- ANATOMY -
SKULL AND SKELETON: DENTITION (page 1)
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    The thylacine is the largest marsupial carnivore to exist into modern times.  Although resembling a canid in its external appearance, its dentition is unique.  Below is a comparison between the dental formula of the thylacine, fox and wolf.  Dental formulae are used to indicate the number of each type of tooth (Incisor, Canine, Premolar and Molar) for a given species.  The dental formula refers to one side of the lower and upper jaw and is doubled to obtain the total complement of teeth for the animal.
 
. I C P M Total
Thylacine (Thylacinus cynocephalus) 4/3 1/1 3/3 4/4 46
Fox (Vulpes vulpes) 3/3 1/1 3/3 3/3 40
Wolf (Canis lupus) 3/3 1/1 3/4 3/3 42
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dentition of the adult thylacine skull
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Dentition of the adult thylacine skull.  View diagrams of complete dentition.
Left: the premaxillary bone (PB) bearing the incisor teeth (1-4) and maxillary bone (MB) bearing the canine tooth (1), premolars (1-3) and molar teeth (1-4).  Right: The dentary (D) or mandible bears all four types of tooth - incisors (1-3) canine (1), premolars (1-3) and molars (1-4).
Photos: International Thylacine Specimen Database Fourth revision 2011.
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    The jaws of the thylacine are long, and the teeth of moderate size.  In contrast to some of the more specialised dasyurids, in which the tooth row has undergone a shortening, that of Thylacinus is elongate.  The most remarkable feature pertaining to marsupial dentition is that these mammals only possess one set of teeth that are homologous to the milk teeth (dentes decidui) in placentals.  In marsupials, the only tooth that is ever replaced is the third premolar in both the upper and lower jaws (Flower 1867).  This normally occurs during the pouch period, and is therefore rarely seen.  Consequently, these functional dentes permanentes will be in use throughout the animal's life.
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adult thylacine skull with exposed tooth roots
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Historical preparation of an adult thylacine skull with bone removed to expose the tooth roots.
Specimen: NMS Z1886.16.10.  Courtesy: National Museum of Scotland (Edinburgh).
Photo: Nicholas Ayliffe - International Thylacine Specimen Database 4th Revision 2011.
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    A thylacine skull is classified as "adult" with the eruption of the fourth upper molar teeth.  Historically, George P. Harris (1808), in his type description of the dental formulae of the thylacine wrote: "Twelve molars in the upper jaw and 14 in the lower".  As such, Harris was describing a specimen in which the upper fourth molars had not yet erupted, and therefore, on the basis of the complete dental formula, not deemed fully adult.  This omission was remarked upon by Temminck (1824b): "He (Harris) or those 
charged to publish his manuscript made a serious fault, since it enumerates the number of molars to the upper jaw as 6 on each row or 12 in total contrary to the fact that one finds 7 molars everywhere in the 2 jaws and that the total number of death teeth is 4 more than the true Dasyurids.  It is more than probable that Harris gave the indication of the dental system of a juvenile animal but in this case should have made mention of it".

    Thylacine dentition is characterised by enamel rills or folds along the ridge of the crown of the lower incisors.  J. Grant (1846) provided the first description of the enamel rills.  Because of their depth, the crown rills produce typical grinding patterns that change with age.  This loss of tooth structure is

dentition of the juvenile (left) and adult (right) thylacine skull
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Juvenile (specimen NMS Z1993.70).
Courtesy: National Museum of Scotland (Edinburgh).
Adult (specimen OUM 7935).
Courtesy: Oxford University Museum of Natural History.
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Comparison of the dentitions of the juvenile and adult thylacine skull.
Photos: International Thylacine Specimen Database 4th Revision 2011.
termed attrition.  Dental attrition is the result of the mechanical forces encountered during feeding on solid food, and from direct contact with the opposing teeth.  Initially, the first visible signs of attrition are seen within the surface enamel of the tooth, but over time this proceeds to the underlying dentine.  With specific reference to the incisors, these teeth become shorter with age.  Moeller (1997) showed how changes observed in the grinding pattern of the crown of the lower incisors could be used as a relatively accurate method of determining the age of a thylacine from its skull.
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References
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back to: Skull (page 3) return to the subsection's introduction forward to: Dentition (page 2)


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