Abstract

In order to maintain a phyletic classification within the Blastoidea, the polyphyletic order Spiraculata will require revision or replacement. Previously created familial and subfamilial names suffice for most of the independent pentremitid lineages, but the new family Ambolostomatidae and the new subfamily Belocrininae are created for the genera Amobolostoma and Belocrinus, respectively.

The genera placed in the spiraculate blastoid family Pentremitidae apparently had at least six independent origins from fissiculate and spiraculate incestors. The late Siegenian pentremitid Belocrinus appears before the well-documented lineage of Pentremitidea-Conuloblastus-Hyperoblastus, which contains the transition from a fissiculate to a spiraculate blastoid and is not clearly related to other pentremitids. The Middle Devonian genera Devonoblastus and Eleutherocrinus are related hyperoblastids and probably developed from an unknown early hyperoblastid. The Famennian Petaloblastus could be derived from Hyperoblastus (Emsian-Givetian) by exposing the lancet plates and lengthening the deltoid crest so that it became visible in lateral view. The Early Mississippian (Kinderhookian, Tournaisian) fissiculate genus Pentremoblastus could have become a new spiraculate genus morphologically similar to Pentremites by bridging its open fissiculate slit to create spiracular pores and reducing the anal deltoids to two; the new spiraculate genus retains deltoid crests. The presence of a conspicuous spiracular septum (a presumed primitive character) in the earliest species heretofore assigned to Pentremites and undescribed intermediate forms from the Kinderhookian (Tournaisian) Lodgepole Formation of Montana suggest that Pentremites could have arisen by merging the 10 spiracles found in Strongyloblastus. Ambolostoma has morphologic features which suggest that it is unrelated to other genera presently placed in the Pentremitidae. The Permian genera Calycoblastus and Rhopaloblastus, although placed in the family Pentremitidae, were regarded by Fay (1968) as derived from the spiraculate troosticrinid genus Metablastus.

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