The role of extrathecal tissue in the construction and functioning of some Ordovician and Silurian retiolitid graptoloids

Spinose and laciniate Ordovician graptoloids are briefly described and an attempt is made to explain the formation and anastomosis of their spines by secretion by cells within the mesenchyme of an extrathecal mantle. It is suggested that in Pipiograptus hesperus and Phormograptus sooneri the ciliated epidermis co­vering the expanded mesenchyme resulted in a smooth ciliary flow of water over the rhabdosome, the food particles being diverted to the zooids along ciliated funnels. In both of these taxa the thecal walls were largely unbandaged, the support and protection of the zooids being taken over by the mantled laci­nia. The umbrella-like ancora of certain non-retiolitid Silurian graptoloids is attributed to secretion beneath an extension of the mantle covering the virgella. Its development early in the growth of the metasicula se­ems only explicable if the ancora functioned as a food gathering organ while the young colony was still at­tached to the sea floor. In the Silurian retiolitids the ancora umbrella has extended back over the thecate part of the rhabdosome, taking over the support of the largely unbandaged thecae. Covered and lined with ciliated epidermis the ancora sleeve would have contributed to the ciliary flow, directing the efferent cur­rents, sometimes from specialized stomata, but more usually from the distal trailing end of the rhab­dosome giving optimum separation from the afferent currents to the thecal apertures, as well as a maxi­mum contribution to the feeding rise.