New genera and species having the Fissurisepta shell form,
with a generic level phylogenetic analysis (Gastropoda: Fissurellidae).
McLean, J. H. & D. L. Geiger. 1998. Los Angeles County Museum of Natural History
Science Series 475: 1-32.
Six genera having an interior septum ad an apical or subapical foramen are
defined on characters of shell sculpture, shell profile, radula, epipodium,
and ctenidial structure. Four genera obliterate the protoconch by expansion
of the foramen at maturity Altrix Palmer, 1942, Fissurisepta Seguenza,
1862, and the new genera Clathrosepta and Cornisepta. Two new
genera retain the protoconch at maturity: Manganesepta and Profundisepta.
All described species previously assigned to Fissurisepta are tentatively
assigned among these genera.
New species described are Manganesepta hessleri on manganese nodules
from the north equatorial Pacific near Clipperton Island, 4500 m; Clathrosepta
depressa from Volcano 5, Eastern Pacific Rise at 13oN, 1160 m;Clathrosepta
becki from hot vents at Manus Basin, east of Papua New Guinea, 2494 m; Cornisepta
levinae from Volcano 6, Eastern Pacific Rise at 13oN, 1775 m; and Cornisepta
verenae from Axial Seamount, Juan de Fuca Ridge, 1530 m.
A hypothesis for the evolution of these genera is offered, based on a cladistic
analysis of morphological characters. Outgroup genera are the scissurellid genus
Anatoma Woodward, 1859, and the fissurellid genus Emarginula Lamarck,
1801, which is first recorded from the Middle Triassic. Additional genera included
in the analysis are Cranopsis A. Adams, 1860, and Puncturella
Lowe, 1827, in which the apical whorl is retained, and Diodora Gray,
1821 in which the septum is reduced to a truncate callus.
Analysis of 22 characters for the 10 genera produced a single most parsimonious
tree. The traditional sequence of Emarginula, Cranopsis, Puncturella,
and Diodora is confirmed. The genera Clathrosepta, Fissurisepta,
and Cornisepta showed the highest number of derived character states.
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