Hands-on sea urchin
digital model, mounted 3D print
This model of the Late Jurassic sea urchin Rhabdocidaris lacks several details, not least the smaller spines between the primary ones or along the ampullary fields. However, it is suitable as a hands-on object, if handled with due care.
Among sea urchins, Cidaridae (pencil urchins) are relatively primitive, with few and very large spines. The family has existed from the Paleozoic era to the present day. In Rhabdocidaris, one can see the division of labour among the primary spines: the lower ones are short and, with their partially thickened ends, touch roughly the same plane, thus functioning as legs. The higher ones are heavily spined themselves and show pycnodont fish or sea turtles where it is best not to bite.