Larven-Gehäuse und Gehäuse-Mikrostrukturen von außergewöhnlich gut erhaltenen unterkarbonischen Gastropoden vom Buckhorn-Asphaltlager (Oklahoma, USA)

Abstract

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 generaSalterospira andParagoniozona 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 genusAnomphalus. 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 ofMicrodoma 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 generaNaticopsis andTrachydomia. 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 heteropodAtlanta and the worm snailSiliquaria. 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 generaGoniasma (one species),Stegocoelia (three species), andCerithioides (one species). The Orthonemidae are represented by the generaOrthonema andPalaeostylus. The genusOrthonema is present with a dextral species and the sinistral speciesO. sinistrorsa. Species ofOrthonema 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 speciesGirtyspira minuta belongs to the family Meekospiridae. The teleoconch 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 generaDonaldina andPseudaclisina, of whichPseudoaclisina is reported from North America for the first time. The exceptional preservation increases the number of shell characters considerably (protoconch, shell microstructure, micro-ornaments), even for seemingly well established taxa. The knowledge of these features greatly improves the systematics of several major groups of Late Palaeozoic gastropods and represents an important source of information for future phylogenetic analyses.