The conodont genus Neognathodus Dunn, 1970, lower to middle Pennsylvanian, Amazonas Basin, Western Gondwana: Biostratigraphic and paleoenvironmental analysis

Abstract

Conodonts are very abundant, present a wide occurrence worldwide and high evolutionary rates during the Paleozoic and the Triassic, providing the recognition of more than 240 biozones over the 300 million years of their geological history. Neognathodus comprises species useful as biostratigraphic markers for the early-to-middle Pennsylvanian succession. In this paper, we analyze the record of this genus within the Amazonas Basin, northern Brazil, based on 18 wells and several outcropping sections, including limestone exploratory mines, related to the marine deposits of the Tapajós Group (Monte Alegre, Itaituba and Nova Olinda formations). Taxa have been revised, updated and some have been reclassified according to current international taxonomy. All records of this genus were morphologically analyzed using stereomicroscope and Scanning Electronic Microscope images, allowing the recognition of six species: N. symmetricus, N. bassleri, N. medadultimus, N. atokaensis, N. bothrops, and N. roundyi. The occurrences of these species as well their areal distribution in the basin were detailed, resulting in several modeling stratigraphic maps. Four biozones are proposed herein, in ascending stratigraphic order: (i) N. symmetricus-N. bassleri interval Zone, (ii) N. bassleri-N. symmetricus interval Zone, (iii) N. atokaensis taxon range Zone, and (iv) N. atokaensis-N. bassleri interval Zone. A middle Bashkirian to middle Moscovian age is assigned to the studied marine deposits of the Tapajós Group. Neognathodus is related worldwide to neritic, warm and oxygenated water environments both in high energy proximal contexts and in lower energy more distal environments, slightly deeper waters and periodically affected by storms.