Article: Intraspecific variation, diversity, revised systematics and type of the Devonian stromatoporoid, Amphipora
Colin W. Stearn
Understanding of the morphological variation and defining characters of the abundant and ubiquitous dendroid stromatoporoid genus Amphipora Schulz has been hindered by the loss of the holotype specimen of the type species, A. ramosa (Phillips). In the absence of a type, the boundaries of the genus Amphipora could only be defined vaguely, yet 12 genera of Amphipora-like fossils have been named to constitute the family Amphiporidae Rukhin, and about 175 species of these genera have been described. Assemblages of amphiporids in a single bed show a wide range of variation but are considered to be the remains of a monospecific stand because a comparable range of variation can be demonstrated along the length of single stems. Some highly variable characters have been used to distinguish genera and species. In particular, the expression of the axial canal, nature and presence of peripheral membranes, thickness of skeletal elements, and density of the skeletal network are shown to be of dubious value as criteria for characterizing genera or species.A neotype for Amphipora ramosa is selected from collections made from the Chercombe Bridge Limestone at the Broadridge Wood Quarry, Newton Abbot, Devon, and revised diagnoses of the species and genus are formulated. The presence of distinct pillars in the neotype and all examined specimens from Devon shows that the four genera whose distinctiveness from Amphipora was based on the presence of this character, should be regarded as junior synonyms. Four other genera are also placed in synonymy with Amphipora. The amphiporid animal is reconstructed as a small, cylindrical, branching, calcified sponge in which the dermal membranes covering the vestibules were locally calcified to form the peripheral membranes and vesicles.