DISCOVERY OF MONOGRAPTIDS IN BASAL PART OF LOWER SILURIAN FROM S. ANHUI WITH SPECIAL REFERENCE TO THEIR ORIGIN
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Abstract:
For a long time, the evolutionary series of a idoraptids-dimorphograptids-monograptids has been generally recognized by graptolite researchers. In the past years, the akidograptids were found to appear in the Lower Silurian Parakidograptus acuminatus Zone, while the dimorphograptids in the P. acuminatus and Orthograpius vesiculosus Zones, but the monograptids appeared as late as in the O. vesiculosus zone; this evolutionary series can be easily accepted by other people. Chen Xu and Lin Yao-kun (1978) put forward that the graptolites with uniserial portion composed of only one theca should represent the primitive species of the various genera of the Dimorphograptidae; these are just the species directly evolving from the Akidograptus after the loss of its first theca (th12) in the second row. Later, since the second row thecae in the rhabdosome began to be lost one after another, the uniserial portion became gradually longer and finally evolved itself to the Monograptus (Chen Xu and Lin Yao-kun, 1978. p. 15). Recently, monograptid graptolites have been successively found from the Glyptograptus persculptus Zone and its equivalent horizons at the base of the Silurian in Britain, Denmark and such areas as Qingyang of Anhui, Qijiang of Sichuan in China. Stratigraphically, the dimorphograptid species appeared much later than the earliest monograptids. Therefore, it is rather questionable whether the evolutionary series which was generally recognized in the past really exists. Bulman (1970) pointed out: "Regarded from the viewpoint of adult rhabdosome, the dimorphogrptids occupy morphologically an intermediate portion between Diplograptidae and Monograptidae (and between Diplograptina and Monograptina), but as with the dicranograptids they were probably not phyletically intermediate." Then, after all, what kind of graptolites did the monograptids evolve from? The present paper will make a further probation into this problem. The Pristiograptus antiquatus sp. nov. appearing in the Parakidograptus primarius Zone at the basal part of the Kaochiapien Formation at Zhangcunxu of Qingyang, Anhui and the Atavograptus primitivus (Li)in the Parakidograptus acuminatus Zone of the same formation at Helixi of Ningguo, Anhui are the earliest monograptids in China, providing significante evidence for researches on the origin of monograptids. In these two species, the rhabdosome is extremely slender and slightly curving towards the dorsum, the sicula can be clearly observed, and the first theca (th11) grows directly upwards after being produced from the upper or middle part of the sicula; some of the thecae are slightly depressed along the basal part of the ventral margin to the shallow apertural excavation, appearing more or less like those of the Leptograptus. In the developmental type of the proximal end and the thecal characters of the rhabdosome, the Pristiograptus antiquatus sp. nov. yielded from the upper part of the Parakidograptus primarius Zone in the Kaochiapien Formation at Zhangcunxu of Qingyang is very much similar to the Parakidograptus primarius sp. nov. produced from the lower and upper parts of the same formation at the same locality, while Atavograptus primitivus (Li) yielded from the Parakidograptus acuminatus Zone in the Kaochiapien Formation at Helixi of Ningguo is very close to the Parakidograptus angustitubus Li produced from the lower part of the Parakidograptus primarius Zone in the same formation at Zhangcunxu of Qingyang. The first row branch of the rhabdosome in the above-mentioned Parakidogrptus primarius can hardly be distinguished from the rhabdosome of Pristiograptus antiquatus. Similarly, the first row branch of the rhabdosome in the Parakidograptus a ngustitubus is almost entirely the same as the rhabdosome of Atavograptus primitivus (text-fig. 1). parakidograptus primarius and P. angutitubus appeared some that earlier, while Pristiograptus antiquatus and Atavograptus primitivus somewhat later. Therefore, the writer believes that the latter probably evolved right from the former respectively which had lost all of their thecae in the second row, without any intermediate forms existing in between, their relation to each other can be shown in the Chinese diagram.
