Five of the six genera described in the systematics have biostratigraphic usefulness, only Cavusgnathus has shown no potential for zonation, of rocks of this age. For the remaining five, the precision, utility, and reliability vary considerably. Neognathodus has piroved to be the choice for zonation in all particulars. It occurs in nearly all units (all units within its range), it shows a constant distribution of kinds between samples of the same age (no local environmental controls manifest wiitftln the genus), it is common enough to usually provide adequate material from samples jof normal size, and most importantly, it underwent rapid evolution. Work with this enus reinforces the earlier idea (Merrill, 1972b) that its potential is still much greater than its present applications. For example, there is little difficulty in distinguishing neognathodids from Brereton and Oak Gtove localities (provided sufficiently large samples), although these units are presently placed in a single zone/subzone (Table 3).
Taken collectively, the Idiognathodus-Streptognathodus plexus provides Some useful biostratigraphic tools. Ultimately it shoudd provide the very best tools for Pennsylvanian biostratigraphy, nevertheless, the overwhelming abundance of idiognathodids that are presently taxondmically indistinguishable reduces their usefulness in most units (Merrill, 1974, p. 21-22). The influx of streptognathodids (S. excelsus in form-taxonomy) in the Spar land and Lonsdale tends to set these units apart. Different streptognathodids (S. oanoellosus) intlte Cramer make that unit readily distinguishable from all others in the study area,. In spite of thkese capabilities, this generic plexus is in a nearly hopeless taxonomic/nomenclatorial muddle. Wors,e
Figures & Tables
“Pennsylvanian rocks exposed in Knox, Peoria, Fulton, and Schuyler Counties in northwestern Illinois include 11 major marine units. The oldest marine unit is late Atokan and the youngest is early Missourian; the remainders are Desmoinesian in age. Each unit has produced at least some conodonts, and the more than 200 samples produced an aggregate total in excess of 160,000 conodont specimens. These can be grouped into not fewer than 78 kinds, considered species in disjunct element taxonomy. At least 10 multielement genera and 40 multielement species are represented. Six new species: Diplognathodus illinoisensis, Neognathodus metanodosus, N. polynodosus, N. oligonodsus, N. anodus, and Gondolella pulchra are described. Although all marine beds were formed in shallow water, generally near shore, they represent the products of highly diverse environments. Conodont biofacies are sharply differentiated and mirror this diversity. Ecologic controls that effected conodont distribution are believed to have been salinity, energy, pH, and possibly biologic antagonism. Neognathodus is the most useful conodont genus for biostratigraphy in these rocks. Four zones and subzones are based on species of this genus and it has permitted relatively precise interregional correlations. Secondary zonations can be based on other genera that supplement the Neognathodus zonation and assist in identifying units. In decreasing importance these are Gondolella, the Idiognathodus-Streptognathodus plexus, and Diplognathodus.”