ABSTRACT
The articulate spire-bearers are presently divided into three (or four) orders, but the monophyletic (from Rhynchonellida), versus polyphyletic origin (from Rhynchonellida and Orthida) of these orders is still in dispute. In terms of their spiral lophophore orientation, they can be divided into two groups, (1) those with medially to dorsomedially-directed spiralia, the Atrypida (Llandeilo-Frasne), and (2), those with laterally to lateroventrally directed spiralia, the Athyridida (Caradoc-Jurassic) and Spiriferida-Spiriferinida (Ashgill-Juras- sic), excluding the ‘spire-bearing’ Thecideidina (Triassic-Recent). In the atrypoids, the oldest taxa have a complete or partial jugum, but by the Silurian nearly all show the jugum split into two separate jugal processes. In the athyroids simple jugate stocks are the ancestral character, but in the Silurian and Devonian highly complex variations of the jugum evolved, including extensions which produced a parallel set of spiral whorls, and those with a jugal web (Mesozoic). In the spiri- feroids, the jugum remained essentially very simple throughout the group’s history, and was commonly lost in some. The Spiriferida form a large group, usually divided into three suborders [or sometimes split into separate orders, the impunctate Spiriferida, and punctate Spiriferinida]. Atrypoids had an impunctate shell wall, but one family, the Punctatry- pidae had a fenestrate (‘mega-punctate’) shell: the athyroids and spiriferoids (like the Rhynchonellida) comprise both impunctate and punctate stocks, with the impunctate condition always preceding in the fossil record, and punctation repetitively evolved. Deltiodont/cyrtomatodont teeth, and strophic/astrophic hinges, were apparently also repetitive features in all stocks of spire-bearers. This is emphasized by the fact that within atrypids alone, the orthid-like hinge and teeth of Silurian Tuvaella and orthid-strophomenoid hinge and teeth of Devonian Davidsonia and Prodavidsonia evolved independently from different lineages at different times.
