Development of the graptoloid rhabdosome

Proximal development of the dichograptoid rhabdosome is re-examined in detail. Two major types are recognised: the artus type (= dichograptid type of Bulman) in which th 1¹ is dicalycal, and the isograptid type (= isograptid + leptograptid types of Bulman) in which theca 1² is dicalycal. In branching dichograptids, distal stipe division takes place by replication of the thecal budding sequence employed in the initial dichotomy of the rhabdosome and is always of isograptid type. Dichotomies beyond the first are unknown in species with initial development of other than isograptid type. The thecal budding pattern and stipe composition of dendroids can be interpreted in terms of the structure of, and thecal notation used for, graptoloids and it is thus possible to compare directly the mode of stipe division in the two groups. It is found that both distal and proximal dichotomy in dendroids are achieved by a pattern of thecal budding closely comparable with that of the isograptid type of division in graptoloids. The isograptid development type is therefore thought to have been directly derived from dendroids, and is the primitive type for Graptoloidea. The artus type was derived via three or more independent lineages, at least one of which (that leading to the artus group of pendent didymograptids) involved an ancestor with isograptid development. Proximal end characters of Phyllograptus suggest that this genus and biserial graptolites of the family Diplograptidae shared a common ancestor.