似哺乳类爬行动物和哺乳类动物脑颅侧壁构造类型的演替

本文对从原始似哺乳爬行动物进化到高级兽类哺乳动物过程中脑颅侧壁所发生的形态演化进行了分析,根据构造上的不同,将这些动物的脑颅侧壁的构造方式划分为四个构造类型,代表四个进化阶段。

SUCCESSION OF THE STRUCTURAL TYPES OF THE SIDEWALL OF BRAINCASE IN MAMMAL-LIKE REPTILES AND MAMMALS

The structure of the sidewall of the braincase in mammal-like reptiles and mammals is reviewed;there are four recognized patterns in them;each represents a successive evolutionary level of the struction of the sidewall of braincase in the evolutionary course from the primitive pelycosaurs to the extant eutherians: 1.In the first stage of the lower mammal-like reptiles,the type of the periotic of the si- dewall of braincase may be taken as the sign of this evolutionary level of the mammal-like rep- tiles.On the sidewall of braincase,there are only the bones of the ear capsule,the epiptery- goid stands outside the cavum epiptercum,not expanding as a plate and not taking part in the sidewall of braincase,the anterior lamina of the petrosal has not yet developed.The trige- minal nerves go out of the braincase through tire prootic incisure.The squamosals locate be- hind the periotic and take no part in the structure of the sidewall of braincase.The patterns of the structure of the sidewall of braincase of the Pelycosauria and the Therapsida`(except for the therocephalians and the cynodontids)may be attributed to this type of the sidewall of braincase. 2.In the stage of the advanced mammal-like reptiles,the epipterygoid type of the sidewall of braincase may be taken as the sign of the evolutionary level of the mammal-like reptiles.On the sidewall of braincase,besides the periotic,the expanded ascending process of the epiptery- goid and the anterior lamina of the petrosal(including the antero-dorsal process of the prootic and the lamina obturans——Presley 1976)also take part in the structure of the sidewall of braincase.There remains a larger gap between the ascending process of the epipterygoid and obitosphenoid.The cavum epiptericum has been enclosed by the osseous wall and begins to re- duce.The trigeminal nerves(V_2+V_3)come out of the braincase through the prootic incisure or the seam between the ascending process of the epipterygoid and the anterior lamina of pe- trosal,or the fenestra ovalis(fenestra pseudoovalis)and the fenestra rotunda(fenestra pseudo- rotunda)in the lamina obturans.The squamosal locates in the back of the periotie and rakes no part in the structure of the sidewall of braincase.The patterns of the sidewall of brain- case of the advanced therapsids(therocephalians and cynodontids)and the Atheria(morganu- codontids,triconodontids,docodontids,amphilestids?,but except for multituberculats and mo- notremes)may be attributed to this type of the sidewall of braincase. 3.In the stage of the lower therian mammals,the type of the lamina obturans of the sidewall of braincase may be taken as the sign of the evolutionary level of the mammals.The entire sidewall of the braincase is nearly to be complete and leaves a little gap between the lamina obturans(anterior lamina of the petrosal)and the orbitosphenoid.The lamina obtu- tans(anterior lamina of the petrosal)has very well developed and occupied most of the area of the orbitemporal,and the epipterygoid has lost its ascending process and became a small one under the lamina obturans.The cavum epiptericum has degenerated.The trigeminal nerves (V_2+V_3)come out of the braincase through the fenestra ovalis(fenestra pseudovalis)and the fenestra rotunda(fenestra pseudorotunda),which locate in the lamina obturans or near by the sutural line between the epipterygoid and the lamina obturans.The squamosal still does not partake in the structure of the sidewall of braincase,but its lower part begins to degerate, so a part of the periotic has occured of the surface of the sidewall of braincase.The post- temporal fenestra remains.To this type of the sidewall of braincase the patterns of the struc- ture of the sidewall of braincase of the Monotremes and the Multituberculata belong,and that of the Symmetrodonta and Eupantotheria may be attributed. 4.In the stage of the advanced theria mammals,the squamosal type of the sidewall of braincase may be taken as the sign of the evolutionary level of the mammals.The sidewall of braincase is already complete and consists mainly of the alisphenoid and the squamosal,the petrosal contributes only a little part of it.The squamosal has taken in the structure of the si- dewall of braincase and moved to the front of the petrosal The post-temporalfenestra lost. The epipterygoid and the lamina obturans fused into the alisphenoid.The cavum epiptericum vanished.The maxillary branch and mandibular branch of the trigeminal nerve come out of the braincase through the fenestra ovalis and the fenestra rotunda located within the alisphenoid generally.The patterns of the structure of the sidewall of braincase of the placentals and the marsupials and their direct common ancester——the Metatherian-Eutherian grade may to this type of the sidewall of braincase.