Graptolite zooids are thought to have been similar in appearance to modern pterobranch zooids, and in particular to those of Rhabdopleura. A cephalic shield would have been needed to secrete the collagenous bandages seen on the surface of grap-tolite rhabdosomes, and the fact that this surface was accessible to the zooids suggests the absence of external soft tissue.
In planktic graptoloids one of the most noticeable adaptations is to an extremely regular colony shape and it seems likely that this was an adaptation to living freely in water. In a planktic mode of life the dominant control is often hydrodynamic -that is, the need to stay afloat and in the correct orientation with respect to food-bearing currents. Graptoloids achieved this in a variety of ways. Many independently evolved morphologies of graptoloids were designed to rotate as they moved, improving stability and increasing the amount of water sampled by each zooid. For very simple shapes (such as biserial forms and monograptids), stability and the correct orientation, with the sicula facing into a current, was achieved by having a nema. This acted like a tail on a kite, keeping its position downstream of the main colony.
The distribution of graptoloids seems to have been correlated with nutrient levels in the Lower Palaeozoic oceans. In many sections across the ocean shelves, graptolite diversity is highest near the shelf edge where upwelling currents would have supplied nitrates and phosphates to the primary producers on which graptoloids fed. In some areas, graptoloid blooms - similar to the blooms seen in modern plankton - can be identified, associated with thin volcanic ash bands. These bands represent eruptions that would have supplied vital micronutrients, especially iron and zinc, to the oceans.
A benthic dendroid graptolite up to 20 cm in height. Well over 1000 zooids could have been accommodated in a fully grown colony. The rhabdosome is cone shaped, and held rigid by a series of dissepiments and attached to the sea floor by a holdfast.
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