A Neognathodus-based biozonation of the Desmoinesian Series (Pennsylvanian) in the Illinois Basin, USA

The Neognathodus Index (NI) is developed as a primary biostratigraphic indicator for the Desmoinesian Series of the Illinois Basin in Indiana, Illinois and Kentucky. It is based on the gradual evolutionary morphotypic variations of P1 elements of named Neognathodus species. The NI analyses and zonations presented herein are constructed from 472 productive conodont samples from 105 different localities. NIs for the Illinois Basin show morphologic drift in Indiana from the Perth Limestone Member of the Staunton Formation (NI = 2.03) to the Velpen Limestone and Mecca Quarry Shale members of the Linton Formation (NI = 5.04) followed by stability through the Alum Cave (NI = 4.83). Further morphologic drift then occurred through the Providence Limestone Member of the Dugger Formation (NI = 5.43) and continued through the West Franklin Limestone Member of the Shelburn Formation (NI = 6.32) until the extinction of Neognathodus at the end of Desmoinesian. The patterns of drift and stability translate to Neognathodus biozones, and are supported by Kolmogorov–Smirnov tests of standardized Neognathodus morphotype distributions. Localized Neognathodus Index variations are due to morphologic shifts and differential evolution in response to localized environmental conditions. Examination of the NI differences in the states of Illinois, Kentucky, and Indiana reflect similar morphologic simplification of the P1 element from Perth to Velpen and Mecca Quarry in Indiana (NI values 2.03–5.04) and from Seville to Brereton in Illinois and Kentucky (NI values 2.80–5.00). Post-Brereton in Illinois and Kentucky (NI values 5.00–5.65) and post-Providence in Indiana (NI values 5.43–6.32) saw progressive P1 simplification, but it was much more pronounced in Indiana.

新颌牙形石指数（Neognathodus Index，NI）被构建为印第安纳州、伊利诺伊州与肯塔基州境内伊利诺伊盆地德斯莫伊尼亚统的核心生物地层标志。该指数基于已命名新颌牙形石物种P1齿型单元的渐进演化形态型变异。本文所呈现的NI分析与生物地层分带，依托105处不同产地的472件含牙形石（conodont）有效样品搭建而成。

伊利诺伊盆地的NI值呈现印第安纳地区的形态演化漂移序列：从斯汤顿组珀斯灰岩段（NI=2.03），依次过渡至林顿组维尔彭灰岩与梅卡采石场页岩段（NI=5.04），随后在明矾洞段（NI=4.83）进入形态稳定阶段。此后演化漂移再度发生，延续至达格组普罗维登斯灰岩段（NI=5.43），并进一步延伸至谢尔伯恩组西富兰克林灰岩段（NI=6.32），直至新颌牙形石在德斯莫伊尼亚统末期彻底灭绝。

该演化漂移与稳定的分布模式可对应至新颌牙形石生物带，且通过标准化新颌牙形石形态型分布的柯尔莫哥洛夫-斯米尔诺夫检验（Kolmogorov–Smirnov tests）得到验证。局域性新颌牙形石指数的变异，源于针对局域环境条件产生的形态型转变与差异化演化。

对伊利诺伊州、肯塔基州与印第安纳州的NI差异分析显示，三地均呈现相似的P1齿型简化趋势：印第安纳地区从珀斯段至维尔彭段与梅卡采石场段（NI值区间2.03~5.04），伊利诺伊州与肯塔基州则从塞维尔段至布里雷顿段（NI值区间2.80~5.00）。伊利诺伊州与肯塔基州布里雷顿段之后（NI值区间5.00~5.65）、印第安纳州普罗维登斯段之后（NI值区间5.43~6.32）均出现P1齿型的持续简化，但该演化趋势在印第安纳地区表现得更为显著。