中期更新統の大阪層群から発見されたマチカネワニ(Toyotamaphimeia machikanensis)の模式標本について,本論文で詳細な再記載を行った.系統解析は,48分類群(2外群)の165形質を使って行った.その結果,420ステップで323個の最節約樹が得られた.厳密合意樹は,マチカネワニが明らかにマレーガビアル亜科(Tomistominae)に含まれることと,同亜科の唯一の現生種(マレーガビアルTomistoma schlegelii)の姉妹群であることを示している.さらに,系統樹の樹形は,マレーガビアル亜科がヨーロッパで起源したあとマチカネワニとマレーガビアルの分岐群が40万年前までに東アジアに拡散してきたことを支持し,これまでの研究と整合性を持つ.マチカネワニの定義についても改定した.従来は上顎の最大の歯は7番目と言われていたが,実際には12,13番目の方が7番目より大きい.上顎の7番より後方の歯はそれより前方の歯に比べてより密に並ぶ.上顎の8番から12番の歯は,かみ合ったときに下顎の歯の外側に位置するが,13番から16番の歯は対応する下顎歯と噛みあうことから,後方の歯は破砕の用途に使われたと考えられる.岸和田市から発見された類似のマレーガビアル亜科のワニ化石は,以前にはマチカネワニと考えられていたが,マチカネワニとは異なる分類群で,より原始的な種の可能性がある.
The holotype of Toyotamaphimeia machikanensis, discovered from the Middle Pleistocene sediments of the Osaka Group in Osaka Prefecture of Japan, is redescribed in details in this study. Phylogenetic analysis is conducted with 165 characters for 48 taxa (two outgroups). The analysis produces 323 most parsimonious trees of 420 steps. The strict consensus tree suggests that Toyotamaphimeia is deeply nested within the clade Tomistominae and is a sister taxon to the only extant tomistomine crocodylian, Tomistoma schlegelii. The topology of the phylogenetic trees implies that tomistomines originated in Europe as previously suggested and the clade of Toyotamaphimeia machikanensis plus Tomistoma schlegelii dispersed into eastern Asia by or prior to 400 thoudsand years ago. Diagnoses of Toyotamaphimeia machikanensis are also revised. Although previous studies stated that maxillary tooth 7 was the largest in Toyotamaphimeia, maxillary teeth twelfth and thirteenth maxillary teeth are actually larger than the seventh. Maxillary teeth posterior to the seventh are more closely placed than anterior ones. Maxillary teeth eighth to twelfth are placed lateral to the dentary teeth in occlusion, whereas maxillary teeth thirteenth to sixteenth meet with corresponding dentary teeth, suggesting that posterior teeth of the maxilla and dentary may have been used for crushing. A tomistomine crocodylian from the Kishiwada Town, previously referred to Toyotamaphimeia machikanensis, is not comparable with the holotype of Toyotamaphimeia and considered here as a different taxon, possibly more primitive than Toyotamaphimeia machikanensis.