Mid-Carboniferous conodonts and their evolution: new evidence from Guizhou, South China

摘要

We studied details of the conodont assemblages in two deep-water sections spanning the mid-Carboniferous boundary at Guizhou, South China. Forty-six species/subspecies were identified belonging to 13 genera, including Lochriea, Gnathodus, Declinognathodus, Idiognathoides, Idiognathodus, Neognathodus, Neolochriea, Swadelina, 'Streptognathodusii', Cavusgnathus, Adetognathus, Rhachistognathus and Hindeodus. The evolutionary relationships among the species of Gnathodus, Neognathodus, Declinognathodus, Idiognathoides, Idiognathodus and Swadelina are discussed based on their P-1 and P-2 elements, as well as those of the species of Lochriea and Neolochriea. The early Pennsylvanian species of Declinognathodus are specifically studied with a focus on their taxonomy and evolution because of their biostratigraphical significance. These species can be distinguished as being part of the ridged or nodose groups by the parapet features of their P-1 elements. The ridged group comprises D. praenoduliferus, D. bernesgae, D. lateralis, the newly recognized taxa D. tuberculosus sp. nov. and D. intermedius sp. nov., and D. cf. pseudolateralis. The nodose group includes D. inaequalis, D. noduliferus and D. japonicus. The number of rostral-ventral nodes of the Declinognathodus P-1 elements are gradually reduced stratigraphically. Declinognathodus tuberculosus sp. nov. is considered to be the ancestor of the other ridged Declinognathodus species following a reduction in the number of rostral-ventral nodes; D. intermedius sp. nov. is a species that acts as a link within the lineage of the genera Declinognathodus and Swadelina via the development of an accessory lobe; D. cf. pseudolateralis is considered to be a morphotype that gave rise to the oldest Idiognathodus species, I. primulus, via D. lateralis with the shortening of its groove; and D. bernesgae is interpreted as the ancestor of the genus Idiognathoides following a transition wherein it loses its single node. The ancestry of the ridged group was probably derived from the ridged species Gnathodus postbilineatus. The nodose group may either be derived from D. tuberculosus sp. nov., or from G. girtyi simplex. The mid-Carboniferous boundary in the studied sections can be recognized by the first appearance datum of Declinognathodus tuberculosus sp. nov. Neognathodus is probably derived from Gnathodus girtyi simplex, while species of Neolochriea share a common ancestor with Lochriea commutata.