Investigation of growth pattern and life mode of Desmostylus using X ray CT and serial tissue sections

通过对北太平洋地区中新世苔藓的头骨、下颌骨的X射线CT图像和磨牙的系列组织切片，对其生长方式和生活型进行了研究。对14种颅骨和20种下颌材料进行了X-射线CT检查，发现其牙科公式为0.1.3.3/1.1.3.3，应分别修订为早中新世的0.1.2.3/2.1.2.3和中中新世的0.0.2-1.3/2.1.2-1.3。分析还揭示了它们一生中牙齿替换和移位的模式。桥柄类物种的本质特征是几乎所有第二代牙齿的顺序移位：即，下第二、第三门牙和尖牙像磨牙一样顺序萌出，上前磨牙和下前磨牙的每一颗牙齿都有…更多的S在他们的一生中也相继爆发。在14个样本中的10个头骨中，发现了明显的性别二态。它表现为磨牙和尖牙相对于颅骨大小的大小差异。对于从组织学角度观察硬组织发育与回答生活方式问题的关系，在一些离体牙和颅骨上的原位观察到周期性的Retzius线。这些品系的浓度被认为是出生、断奶、繁殖(或分娩)、病理经历等生理反应的结果。在桥刺的标本中，在女性头骨上长出的第二和第三磨牙上也观察到了这种周期性的Retzius线集中。因为这样的牙齿是在生命的后期发育起来的，所以在桥刺属雌性个体的第二和第三磨牙上出现的这种周期性的Retzius线可能是繁殖历史的信号，对于像剑齿目动物这样的灭绝的神秘哺乳动物来说，这仍然是未知的。较少

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 … More 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