副牙形石的演化趋向  

EVOLUTIONARY TRENDS OF PARACONODONTS

作　　者：董熙平[1] 

机构地区：[1]北京大学地质学系

出　　处：《地质论评》1997年第5期498-502,共5页Geological Review

基　　金：国家自然科学基金(编号NSFC49272077)

摘　　要：原牙形石Gapparodus属外部形态的演化趋向可能是沟的增加,从“二沟型”至“三沟型”。这与早奥陶世Scolopodus属的演化有某些类似之处。单锥副牙形石的演化趋向尚不清楚。Westergaardodina的演化趋向表现在:①齿体轮廓:主要表现在长/宽比的变化,二齿型的种长/宽比的变化趋向是逐渐增大;三齿型的齿体长/宽比的变化趋向是由小变大,再由大变小。②基部开口和侧部开口的大小:二齿型的种侧部开口的总的趋势是逐渐变小;三齿型的齿体基部开口和侧部开口总的趋势是逐渐变小,但也有例外。③侧齿突的方向和形态:三齿型的种侧齿突的演化趋向与正笔石目笔石枝的演化趋向或多或少有些类似之处。原始类型两侧齿突水平延伸,随后出现两侧齿突上斜或上曲类型。至晚寒武世出现两侧齿突近于平行,垂直向上延伸的类型,多数三齿型的种属于此种类型。二齿型的种侧齿突方向的演化不明显。原始类型两侧齿突近于平行,垂直向上延伸。至晚寒武世开始出现两侧齿实向外分散,长短不一的类型。中寒武世晚期至晚寒武世早期还出现侧齿突下部发育隆脊和凹坑的类型。但是,一直残存至奥陶纪的种仍为两侧齿突近于平行,垂直向上延伸的类型。The protoconodont genus Gapparodus seems to have an evolutionary trend of the increase of furrows, i. e. the trend from 'two furrows type' to 'three furrows type'. This evolutionary trend is more or less similar to that of the Tremadocian euconodont Scolopodus. Little is known about the evolutionary trends of the coniform paraconodonts. The evolutionary trends of Wester-gaadodina seem to be manifested in three aspects as follows: CD the length/width ratio, (2) the size of basal and lateral openings, and (3) the direction of the lateral projections. The length/ width ratio increases gradually within the bicuspidate species, whereas the length/width ratio increases early and decreases late within the tricuspidate sclerits. The size of lateral openings decreases gradually within the bicuspidate species, while the size of basal and lateral openings decreases gradually within the tricuspidate sclerites, but there are few exceptions. The evolutionary development of the directions of the lateral projections within the tricuspidate species is more or less similar to that of the stipes of Graptoloidea. The lateral projections extended almost horizontally in the original form, and later they became reclined or reflexed in the intermediate form. During the Late Cambrian, the lateral projections extended vertically, with each other nearly parallel, and most species of tricuspidate Westergaadodina belong to this type. The evolutionary development of the directions of the lateral projections within the bicuspidate species seems to be less regular. The lateral projections of the original form extended vertically, with each other nearly parallel. The diverging lateral projections of different lengths appeared in the Late Cambrian. However, the lateral projections of the form that survived into the Ordovician still extended vertically, with each other nearly parallel.

关 键 词：副牙形石 演化 早奥陶世 晚寒武世 地层 古动物 

分 类 号：Q915.642[天文地球—古生物学与地层学] Q915.641[生物学—古生物学]