Sinodiversograptus—Its occurrence in Australia and northern Canada

Sinodiversograptus Mu and Chen, 1962, from the Monograptus turriculatus Zone of the lower Telychian (=Upper Llandovery, Silurian), is the earliest known Silurian graptolite genus to bear thecal cladia. Only one species of sinodiversograptid is known, this being S. lientanensis (Mu, 1948). A mature specimen of this species is illustrated in Figure 1. The genus Sinodiversograptus has been portrayed as such in both the Treatise (Bulman, 1970) and in Rickards et al.'s (1977) analysis of the evolution of the Silurian and Devonian graptoloids. Mu and Chen (1962), however, in their detailed study of the astogeny of the conspecific S. multibrachiatus, noted that the development of Sinodiversograptus may be divided into three stages.1) Streptograptid stage (Figure 2.1)—development is as in a normal monograptid, with the first theca arising from the sicula. Subsequent thecae are added sympodially. Rickards et al. (1977) noted that the thecae of Sinodiversograptus are not streptograptid in form (i.e., retroverted, with an aperture slit-like laterally, but possessing a median upturned lip—see Chen, 1986, for illustrations of chemically isolated streptograptid thecae), but are hooked (Figure 2.8), an observation also made by the present author of the thecae of Mu and Chen's (1962) specimens (housed in the Nanjing Institute of Geology and Palaeontology). They most closely resemble the thecae of Monograptus runcinatus Lapworth, 1876, as illustrated, for example, by Bjerreskov (1975). This first stage of development might, therefore, more correctly be termed the monograptid stage.