The oldest known heterobranch gastropod, Kuskokwimia gen. nov., from the Early Devonian of west-central Alaska, with notes on the early phylogeny of higher gastropods
Bulletin of Geosciences, volume 76, issue 1;
pages: 39 - 54;
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A new gastropod, Kuskokwimia moorei
gen. et sp. nov., with a dextral teleoconch and small, sinistrally coiled protoconch is described from Emsian (late Early Devonian) strata of the Medfra B-4 quadrangle, west-central Alaska. The genus Kuskokwimia
is placed in a new family, the Kuskokwimiidae fam. nov., considered to be the oldest and first undoubted Middle Paleozoic representative of the subclass Heterobranchia. It is suggested that the Heterobranchia have been separated from other gastropod groups at least since the Early Devonian, or perhaps even earlier. Gastropod groups developing a true larval shell (protoconch II) such as the Caenogastropoda, Perunelomorpha, and Mimospirina are favored as possible ancestral groups of the Heterobranchia. Shell heterostrophy (anastrophic as well as inclined heterostrophic coiling) can no longer be considered a unique character of the subclass Heterobranchia, because it is also well documented now in the slit-bearing Archaeogastropoda. The origin of shell heterostrophy seems to be connected with a reorganization of the pallial organs during early shell ontogeny.