The identification of ‘feather-like’ fossils in the Palaeozoic: Algae, hydroids, or graptolites?

‘Feather-like’ fossils, showing the construction from a main stem and bearing lateral unbranched stipes, thus reminding of the construction of a bird’s feather, are common in the Palaeozoic fossil record. A review of these ‘feather-like’ fossils, including new specimens from the late Cambrian (Furongian) Guole Biota of Guangxi, South China, the Ordovician Louredo Formation of Portugal, and the Ordovician Castillejo and Barriga Shale formations of Spain, shows that this particular construction can be found in at least three different groups of organisms: Graptolithina (Hemichordata, Pterobranchia), Hydrozoa (Cnidaria), and macroalgae (Rhodophyta, Phaedophyta, Chlorophyta). The Graptolithina can be differentiated from the other groups by the development of the colonial housing construction. Their tubarium is formed through a glandular secretion of fusellar full or half rings, a highly characteristic feature, forming the interconnected thecal tubes for the zooids. The colonial constructions of the Hydrozoa are based on a dermal development, the periderm, and not formed as a regular glandular secretion from a zooid as in the Graptolithina, thus showing an undifferentiated organic wall. Algae do not have any colonial development in the form of separate modules and show smooth outlines. They also lack the more durable recalcitrant outer coating or housing constructions of Hydrozoa and Graptolithina and are more difficult to preserve in the fossil record.