Graptoloids were colonial hemichordates which lived in a branching rhabdosome that was created by distal accretion and is seen to have a wide variety of shapes and features that evolved relatively quickly.
Each soft bodied zooid (often not preserved) lived within a tubular structure called a 'theca' and the following younger zooid lived within the theca above, often taking years for the colony to be fully grown. The most prominent feature is the number of stipes or branches hanging from the attaching 'virgella' (spine at the proximal end). Attached to each theca were a variety of lobes, spines, cladia (secondary branches) and appendages made from a type of peridermal fabric known as fuselli. By taking into account the shapes of the thecal tubes and apertures (openings), the appendages, the shape of the whole rhabdosome, the position of the 'virgella' and overall lengths and widths of the graptoloid it is possible to identify these fossils to a high level of accuracy which makes them a very useful tool for high resolution relative aging of rocks. (Cooper et al. 2012)
Monograptids - graptoloids with only a single stipe
Reclined - where the stipes bend back on the virgella
Didymograptids - the earliest graptolites with only 2 stipes
Tetragraptids - graptolites with 4 stipes
Dendroids - the earliest graptolites with many interconnected stipes, which give a net like appearance
Labelled graptolite morphology
(Wilkinson J. 2015)
Each zooid is believed to have had ciliated lophophores for filter feeding, similar to many planktonic animals today unselectively eating what ever was available in the water. Living with the marine water column, graptolites were mainly planktonic but it is believed they were able to control the depth within the water column. Melchin and DeMont (1995) summised that the most likely method for graptolites to control their placement within the water column was by soft bodied swimming appendages rather than the floatation devices previously hypothesised. Lateral movement however was believed to be due mainly to seasonal and larger ocean currents resulting in graptolite fossils being found in a wide range of lithofacies and species being widely distributed around the world.
Mainly due to anoxic sea floors (which were much more common in these times than today), where graptolite would sink to the sea floor and their rhabdosomes would not be eaten, as the lack of oxygen minimised the number of animals living in these environments. However due to their fragile structures you only tend to find graptolites in very low energy zones such as laminated beds and mainly as just graphite films on bedding planes. However graptolites can be exceptionally preserved in 3D within nodules and when pyritised. (Zalasiewicz et al. 2009)
Graptolites make excellent index and zonal fossils as they meet the criteria of being widely distributed around the world (relatively evenly) within anoxic oceanic sediments. They were also rapidly evolving which means that assemblage turnover is fairly regular and so it is possible to identify a number of biozones within each of the epochs and thus resolve geological time into smaller sections when relatively dating a facies.
Previous view that graptolites were attached to a floatation device
(adapted from Maletz J. 2013)
www.earthsciences.hku.hk (2015) Writing on the rocks
Swimming appendages from the feeding tentacles (A) or with wing like extensions (B)
(Melchin and DeMont, 1995)