Monopodial and Sympodial Growth Modes in the Colonial Graptolithina (Hemichordata, Pterobranchia)

Two growth modes are recognized in colonial pterobranchs (Graptolithina): monopodial growth and sympodial growth. The earliest colonial Graptolithina likely developed through monopodial growth, a mode of colony formation well-documented in the extant graptolite Rhabdopleura normani. This growth involves a permanent terminal zooid and the sequential budding of additional zooids behind it, as the contractile stalk (gymnocaulus) of this terminal zooid elongates. This process is reflected in specific features of the secreted housing structure, the tubarium. Recently, monopodial growth was identified for the first time in a fossil taxon—the Cambrian dithecodendrid Tarnagraptus—based on tubarium characteristics, as no zooids were preserved. Monopodial growth also appears probable in other Cambrian taxa resembling Tarnagraptus, although evidence remains limited due to fragmentary materials. Sympodial growth, characterized by transient terminal zooids that are sequentially replaced as new buds form, is extensively documented in the fossil record of the Graptolithina. This growth mode characterizes the vast majority of Cambrian to Devonian Dendroidea and Graptoloidea. Phylogenetic evidence suggests sympodial growth evolved from monopodial growth in graptolithines, but the mechanisms underlying this evolutionary transition remain unclear.