| Budget Amount *help |
¥3,100,000 (Direct Cost: ¥3,100,000)
Fiscal Year 2004: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2003: ¥1,500,000 (Direct Cost: ¥1,500,000)
Fiscal Year 2002: ¥1,100,000 (Direct Cost: ¥1,100,000)
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| Research Abstract |
The growth pattern and life mode of the Miocene North Pacific tethythere, Desmostylus, was investigated based on X-ray CT images of skulls and mandibles and serial tissue sections of molar teeth.
Examinations of more than 14 cranial and 20 mandibular material, represented by the individuals of juveniles, sub-adults, adults, and sexually both males and females using X-ray CT revealed that the dental formula of Desmostylus, previously considered as 0.1.3.3/1.1.3.3, should be revised as 0.1.2.3/2.1.2.3 for the Early Miocene species D.japonicus and 0.0.2-1.3/2.1.2-1.3 for the Middle Miocene species D.hesperus, respectively. The analyses also revealed the patterns of tooth replacements and displacements during their life. The substantial peculiarity in the species of Desmostylus is the sequential displacements of almost all second-generation teeth : i.e., the lower second, third incisors and canine have erupted sequentially just like the molars, and each tooth of upper and lower premolars ha
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s also erupted sequentially throughout their life.
Among the 10 skulls out of 14 samples, distinct sexual dimorphism was recognized. It was expressed by the size differences of molar teeth as well as canine teeth relative to the skull size. It suggests that the sexual dimorphism in Desmostylus may be continually emphasized by male individuals during their later life.
As for the examination of hard tissue development from histological perspective in relation to answering questions for their life mode, periodic concentrations of Retzius lines were observed in some isolated teeth and those in situ on a skull. The concentrations of such lines are thought to be results of physiological responses from birth, weaning, breeding (or give birth), pathological experiences and so on. In the case of samples of Desmostylus, such periodic concentrations of Retzius lines were also observed in the second and third molars erupted on a female skull. Because such teeth are developed during the later stage of their life, these periodic concentrations of Retzius lines seen in the second and third molars in a female individual of Desmostylus maybe signals of the reproduction histories, which are still unknown for such extinct, enigmatic mammals as desmostylians at all. Less
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