A home on the sea floor: Benthic graptolites from North German glacial erratic boulders and beyond

Benthic graptolites were the prime inhabitants of the shallow water sea floor of the early Palaeozoic oceans. Here, starting in the early Cambrian, benthic encrusting and erect growing graptolites evolved into numerous species and genera that are difficult to identify in the usual flattened preservation. Especially, the erect growing dendroids were commonly ripped off from their attachment sites and transported by currents into deeper water regions. Three-dimensionally preserved dendroid graptolites from glacial erratic boulders have proved to be essential for the taxonomic and phylogenetic interpretation of these taxa due to their often superior preservation. Two species from the Callograptidae and Dendrograptidae, found in a single glacial erratic boulder of Sandbian (Late Ordovician) age show the tubarium construction and allow a revised interpretation of autothecae and bithecae for the Callograptidae. The Callograptidae bear parallel-sided, slender thecae of a uniform type in most species. Robust taxa form very complex stipes with strongly overlapping thecae. Slender taxa show often a zigzag main stipe with long thecal tubes extending to alternate sides and a ventral row of thecae exhibiting a zigzag succession. These are here regarded as the autothecae, while the laterally projecting thecae are identified as bithecae. Dendrograptus shows simple, serially aligned, aperturally widening autothecae with rutellate apertures and alternating bithecae on the sides of the stipes.