Considerable morphological variation is found in the mammalian skull. In this thesis, ecological mechanism underlying mammalian cranial diversity and developmental mechanism that produce mammalian cranial diversity are discussed.

Craniodental morphology of 41 primate species with differing dietary patterns was comparatively analyzed. The subfamily Colobinae was selected for comparison since this taxon encapsulates a wide array of dietary variation and serves as an excellent context for identifying covariance patterns between morphology and diet among primates. Colobine species that feed on resistant seeds are found to exhibit increased leverages of the masticatory muscles for biting, more medially positioned tooth rows, and significantly reduced molar rows compared to the primates which rarely exploit resistant food. In particular, the reduction of distal portion of the molar row is suggested to be related to be a trade off with increased mechanical leverage, preventing the distal molars from being pushed into the posterior region of the mandible and to keep the midline muscle resultant force enclosed within the triangle of support. In addition, while seed predators show overall increase of masticatory leverages, morphological divergence between mature seed eaters and immature seed predators is also identified. Mature seed eaters show leverage emphasis for postcanine biting, while immature seed predators exhibit higher leverage at the anterior dentition. High leverage at the anterior dentition is suggested to be adaptation for removing the puncture-resistant husks of unripe fruits and gaining access to the immature seeds (sclerocarpic harvesting). Furthermore, I demonstrate that evolution of bite force increase occurred several times in the colobine lineage and that such bite force increase is clearly related to the increased emphasis on seed eating. Thus, the craniodental variation found in colobine primates is suggested to reflect adaptation for differing diet.

Using three closely related squirrel species, I comparatively analyzed the craniofacial divergence of sympatric species and tested whether such variation can be linked to ecological difference of these species. Results show that species which extensively feed on hard seeds and tough tree bark possesses a greater mechanical advantage of the masseter and temporalis muscles for chewing, and a more robust zygomatic arch, compared to species which feed mainly on softer food items. Thus, species that feed on resistant food is capable of generating greater bite force at a given muscle force at the cheek teeth, and its zygomatic arch is more resistant against the stresses generated by the masseter muscle. In addition, I tested the hypothesis that orbital orientation is related to arboreality and terrestriality. While arboreal species were indicated to have more convergent orbits, terrestrial ones exhibited more divergent and more dorsally oriented orbits. More convergent orbits will facilitate binocular vision, and thus a better stereoscopic depth perception. On the other hand, rather than more convergent orbits, more divergent orbits will be advantageous for terrestrial animals since it will provide wider visual field and increase the chance of predator detection. There have been various debates on whether orbital orientation is associated with the degree of arboreal or terrestrial lifestyle, but my results give support to the hypothesis that more arboreal species should have more convergent orbits and that more terrestrial species should exhibit more laterally oriented orbits. Although some authors have interpreted convergent orbits as a tradeoff with having larger eyes in nocturnal species, results provided here does not support this model since it cannot explain the orbital differences among the three squirrel species which are all diurnal species. I also suggest that not only divergent but also dorsally positioned orbit may be the key feature of terrestrial small mammals, since the ground dwelling rodents can detect predators approaching from dorsal direction more easily with their dorsally positioned obits.

To investigate the role of ontogenesis in producing adult morphological diversity, the most comprehensive analysis to date on osteogenesis of 22 cranial elements in 34 species of mammals was conducted, spanning over nine orders across mammals with various life history. Results demonstrate that mammals are characterized by the delayed onset of pterygoid relative to other bones compared to the sauropsid outgroups. Given the evolutionary fact that the pterygoid bone has experienced a relative size reduction in mammals from the condition of early synapsids, it is suggested that the late shift of the pterygoid is linked to the evolutionary reduction of this bone in mammals. In addition, I find that the moles (family Talpidae) are characterized by the extremely early development of the bones that constitute the vomeronasal complex (nasal, vomer, palatine, frontal, and pterygoid). Since the vomeronasal complex is relatively enlarged compared to those of the moles' close relatives, shrews and hedgehogs, and since vomeronasal complex plays an important role in the moles, I suggest that the early developmental shift of vomeronasal bones in moles may reflect their ecological emphasis on this morphological complex. In addition, I detect the pattern of step-by-step early shifts of the occipital bones (supraoccipital, basioccipital, and exoccipital) in human evolution. Basioccipital moves earlier in Euarchontoglires, supraoccipital shifts earlier in Primates, exoccipital moves earlier in Catarrhini, and exoccipital shifts further earlier in Homo sapiens. Other major heterochronies that have occurred in the mammalian evolution are: the late shifts of the vomer and supraoccipital in Placentalia when splitting with Marsupialia, the late shift of the pterygoid in Marsupialia when splitting with Placentalia, and the late shifts of the presphenoid and alisphenoid in Chiroptera when splitting from Pegasoferae. Although previous study on much more limited species sampling proposed that the sequence of cranial ossification is quite conservative and that few heterochronic shifts have occurred along the mammalian evolution, my comprehensive study on cranial ossification sequence reveals that numerous sequence heterochronies have occurred along the evolution of mammals and suggests that sequence heterochrony may have played a significant role in shaping the evolution of the mammalian skull.

The results presented in this thesis suggest that ecological factors, such as dietary and locomotive patterns, have driven the diversification of cranial morphology in mammals. It is clear that the developmental sequence of the cranium have changed dramatically along the mammalian evolution. It is possible that the timing of initiation of an organ in the embryo may affect its final size at maturation and that such heterochrony in the relative developmental schedule plays a significant role in producing the morphological diversity found in organisms.

本論文は八章からなる。第一章は本研究の着眼点について語った導入部である。最初に論文の全体像を見渡しながら、形態と機能に関する今日的な論点を浮かび上がらせた。第二章はアフリカコロブス類の顔面頭蓋の形態学的変異と食性適応の関係を頭蓋形態データから論じている。比較的近縁の系統内で多様な食性を見せるアフリカコロブス類の各種について、食性に適応した頭蓋の機能形態学的特質を明らかにした。第三章は実際の食餌の物理学的性状を考慮し、頭蓋の適応様式を骨計測学的データから議論している。とくに種子食種が進化するアジアコロブス類において、咀嚼筋構築と関連してどのような顎や歯の形状の特徴が機能性を具備するかを探る研究になっている。第四章はコロブス類における咀嚼力の形態学的分析となっている。三次元骨計測学より形態学的に咀嚼筋の能力を推定し、歯列において生起する咀嚼力を定量化することを試みた。これにより骨構造をもとに、形態学的特徴と実際の咀嚼力および適応食性を論理的に関係づけることに成功している。第五章は、インドシナ半島に同所的に分布するタイワンリス類とカオナガリス類を例に、食性の違いが微妙な頭蓋の形態に現れていることを示唆した。第六章は、同所的に分布する三種のリスの頭骨形態を比較し、互いに近縁ながら、眼窩の位置や頭蓋のプロポーションなどにおいて樹上生と地上性に適応的分化を遂げていることを示した。第七章は、有胎盤類の頭蓋の骨化パターンの異時性を検討し、分子遺伝学的な系統分岐の各ポイントにおいて、骨化パターンにどのような進化学的変化を生じ、それが系統の適応戦略にいかに影響を与えているかを解析した。第八章は総括である。

以下に評価を述べる。本論文は、観点の異なる各章から、「哺乳類の頭蓋とはいかなるものか?」という壮大な問題点に切り込んだ、きわめて斬新で創造性に富む研究である。データの量、議論の深さ、論理性の厳格さ、そして考察部分に凝縮された進化形態学上の面白みなど、すべての点について世界最高水準の論文であると高く評価できる。構成する各章の内容それぞれも十分に吟味されたものであり、落ち着いた表現や慎重な言い回しなどを見ても、学界を説得する力強い作品に仕上がっていると判断することができる。形態と生態を統計学的に相互交流させる、三次元入力テクノロジーを解析と結び付ける、そして、発生学的データを進化学的分岐と適応の関係になぞらえて議論するなど、手法や論旨の新しさに関しても、既存の知見をはるかに上回る高度な論議力を具備している。

なお、本論文第2章、3章、5章の一部は、遠藤秀紀、押田龍夫、Nguyen Xuan Dang、Dang Ngoc Can、木村順平、佐々木基樹、本川雅治、Nguyen Troung Son、林田明子、新宅勇太との共同研究であるが、論文提出者が主体となって分析及び検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。