Maletz Jörg, Lerosey-Aubril Rudy
Institute für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany.
Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, USA.
Evol Dev. 2025 Jun;27(2):e70010. doi: 10.1111/ede.70010.
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.
在群体羽鳃纲动物（笔石纲）中，有两种生长模式：单轴生长和合轴生长。最早的群体笔石纲动物可能是通过单轴生长发育而来的，这种群体形成模式在现存的笔石动物诺曼拟管笔石中有详细记录。这种生长方式涉及一个永久性的末端个虫，以及在其后面依次出芽的额外个虫，随着这个末端个虫的收缩柄（裸茎）伸长。这个过程反映在分泌的外壳结构——线管的特定特征中。最近，基于线管特征，首次在一个化石分类单元——寒武纪的双胞树形笔石中确定了单轴生长，因为没有保存个虫。在其他类似于树形笔石的寒武纪分类单元中，单轴生长似乎也有可能，尽管由于材料零碎，证据仍然有限。合轴生长的特征是随着新芽的形成，短暂的末端个虫依次被取代，这在笔石纲的化石记录中有广泛记载。这种生长模式是寒武纪到泥盆纪绝大多数树笔石目和正笔石目的特征。系统发育证据表明，合轴生长是从笔石类的单轴生长进化而来的，但这种进化转变的潜在机制仍不清楚。
