open access

https://doi.org/10.14943/doctoral.k13134

https://hdl.handle.net/2115/72343

ワニ類の生態進化史の復元 : 形態計測・系統学的アプローチ

英語

2018-03-22

Hokkaido University

vi, 256p

Crocodylia is a remarkably successful group of archosaurs (crocodylians, birds, and all descendants of their most recent common ancestor) which has dominated the land-water interface since the Late Cretaceous. Because extant species are available for measuring morphology, function, performance, and fitness, Crocodylia is an ideal model group for ecomorphological study. Moreover, ~80 Ma of its clade history allows us to test various evolutionary hypotheses in a geologic time scale. Meanwhile, crocodylian systematics is a classic model case in that morphological and molecular datasets provide contradictory higher-level relationships, which has not reached consensus yet. This dissertation aimed at understanding evolution of ecology in crocodylians thorough morphometrics of trophic and locomotor ecomorphology and reconsideration of the morphological phylogenetic hypothesis, using extant and fossil materials. Comparative growth trajectories of cranial trophic ecomorphology among crocodyloid species revealed the role of postnatal development in shaping trophic diversity in crocodyloid crocodylians. Growth patterns of some traits (e.g. rostrum shape; pterygoid flange depth) are universal among all examined species, suggesting the existence of a shared size-dependent biomechanical constraint in crocodyloids. On the other hand, Gavialis gangeticus, a putative piscivorous taxon, shows unique ontogenies for several other traits (e.g. size of supratemporal fenestra; tooth size disparity), implying that piscivorous adaptation involves shifts in the growth trajectories of trophic ecomorphology. Morphometrics of appendicular and axial skeletons among extant and fossil crocodylians revealed locomotor divergence between two major groups (Alligatoroidea and Crocodyloidea) and uniqueness of G. gangeticus. Comparative limb proportions show that crocodyloids have relatively long stylopodia (humerus and femur) than alligatoroids, indicating that two groups may differ in locomotor functions. G. gangeticus differs in various limb and vertebral metrics, reflecting its distinct locomotor ecology. Some of the postcranial metrics (e.g. limb proportions) are correlated with rostral shape, an indicator of trophic function, suggesting that trophic and locomotor functions might differently delimit the ecological opportunity of alligatoroids and crocodyloids in the evolution of crocodylians. The morphological phylogeny of crocodylians was reconsidered through a new look at East Asian “tomistomines,” and a revised postcranial character sampling. Although a new analysis supported the previous morphological hypothesis of higher level relationships, the alternative tree that was forced to fit the molecular hypothesis was only few steps longer than the unconstrained tree, and two trees were not significantly incongruent with each other. Mapped on the constrained tree that is consistent with the molecular hypothesis, cranial and postcranial characters show a gradual specialization on the lineage of G. gangeticus, preceding the divergence of “gavialoids” in the morphological context.

2018-03-22

甲第13134号

博士(理学)

(主査) 准教授 小林 快次 （総合博物館）, 特任教授 鈴木 德行, 教授 竹下 徹, 准教授 伊庭 靖弘

理学院（自然史科学専攻）

博士論文

Version of Record