Larval shells and shell microstructures of exceptionally well-preserved Late Carboniferous Gastropods from the Buckhorn Asphalt deposit (Oklahoma, USA) [Larven-Gehäuse und Gehäuse-Mikrostrukturen von außergewöhnlich gut erhaltenen unterkarbonischen Gastropoden vom Buckhorn-Asphaltlager (Oklahoma, USA).]

Gastropods from the mid-Pennsylvanian Buckhorn Asphalt Quarry in Oklahoma (U.S.A.) are very well preserved. Commonly, protoconchs and original shell microstructure are present and unaltered. Two pleurotomarioid gastropod species of the genera Salterospira and Paragoniozona are preserved with the early ontogenetic shell and with a nacreous shell structure. Two other nacreous archaeogastropods with a rotelliform shell lack a selenizone and are closely related to the Anomphalidae. They are tentatively assigned to the genus Anomphalus. A fifth tiny archaeogastropod lacks nacre and has exclusively crossed lamellar shell microstructure. It represents the oldest known archaeogastropod that lacks nacre. Microdoma conicum represents the trochomorph archaeogastropods; it has an inner nacreous layer, a crossed acicular layer, and an outermost thin calcitic layer. The protoconch of Microdoma is typical for archaeogastropods. The construction of the shell indicates that most archaeogastropods from Buckhorn Asphalt deposit have a nacreous inner layer, but one type has a crossed lamellar microstructure, like the modern Phasianellinae and Fissurelloidea, indicating the antiquity of this structural type within the Vetigastropoda (Archaeogastropoda). The Neritimorpha are present with the two neritopsid genera Naticopsis and Trachydomia. Naticopsis has a thick inner crossed lamellar layer and a thin prismatic outer calcitic layer. The families Goniasmidae, Orthonemidae, Pseudozygopleuridae, and Meekospiridae represent the Caenogastropoda. All reported species of these families have a crossed lamellar shell structure and a caenogastropod-type protoconch. There is now clear evidence that the Goniasmidae are Caenogastropoda, though they have a selenizone, a feature that is almost unknown in modern caenogastropods. Only a few modern caenogastropods have slit-like structures in their shell, e.g. the heteropod Atlanta and the worm snail Siliquaria. Both are distantly related to the slit-bearing Goniasmidae and it is unlikely that the slit is homologous in these groups. The fact that these ancient caenogastropods had a slit in the outer lip is most important for the reconstruction of the phylogeny of the Gastropoda. The Goniasmidae are present with the genera Goniasma (one species), Stegocoelia (three species), and Cerithioides (one species). The Orthonemidae are represented by the genera Orthonema and Palaeostylus. The genus Orthonema is present with a dextral species and the sinistral species O. sinistrorsa. Species of Orthonema have the same type of a heliciform larval shell that is present in the Goniasmidae. Therefore, the two families are probably closely related. Palaeostylus is present with a newly described species, Palaeostylus batteni n. sp. The Pseudozygopleuridae are present with a single species. Pseudozygopleura peoriense. The common species Girtyspira minuta belongs to the family Meekospiridae. The telcoconch has a faint spiral striation and a polished surface The protoconch/teleoconch transition indicates that its protoconch is paucispiral and ends with a faint, straight prosocline thread. The Heterostropha are present with the order Allogastropoda represented by two species of the genera Donaldina and Pseudaclisina, of which Pseudoaclisina is reported from North America for the first time.