Mode of life of planktonic graptolites: flotation structure in Ordovician Dicellograptus sp.

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Abstract
An unusual isolated specimen representing an immature rhabdosome of Dicellograptus sp. from the Middle Ordovician Athens Shale of Alabama has a flattened sphere attached to the distal end of the nema. The flattened sphere can be interpreted as a float that probably conferred buoyancy on the rhabdosome. Such floats, which became detached in mature rhabdosomes, may be common in the fossil record. The presence of nematophorous siculae in all planktonic graptolites suggests that for immature rhabdosomes a passive, buoyant mode of life, attained by means of a float, might have been widespread taxonomically. This mode of life is compatible with the theory of passive response for the mode of life of mature rhabdosomes, but it is difficult to reconcile with the theory of automobility.