Morphology, systematic position, and distribution of Bothriocidaroid echinoids

This is a brief review of conceptions of morphology, distribution and systematic position of the most ancient echinoid group—order Bothriocidaroida. It comprises 3 genera: Bothriocidaris, Neobothriocidaris, and Unibothriocidaris. More than 20 species of these genera are distributed from Upper Ordovician (Caradocian) to Lower Silurian (Llandoveri) in North Western Europe and North America. The question of whether this group belongs to the class Echinoidea or not has been the subject to discussions several times, particularly based on cladistic methodology. In my opinion the peculiarities of bothriocidaroids are quite typical of archetype definition of this class, however, so-called unique modalities of its merons are actually prone to variation both inside the group and among other echinoid groups. For example, Bothriocidaris maquoketensis has a structure transitional from uniserial to biserial interambulacra, however uniserial interambulacra are found in Paleozoic family Cravenechinidae and in some Cretaceous holasteroids and Cenozoic clypeasteroids. Unfortunately, such an important feature as the apical system is found only in one genus Bothriocidaris.