Taxonomic and evolutionary pattern revisions resulting from geometric morphometric analysis of Pennsylvanian Neognathodus conodonts, Illinois Basin

Abstract

Conodont fossils are highly valuable for Paleozoic biostratigraphy and for interpreting evolutionary change, but identifying and describing conodont morphologies, and characterizing gradual shape variation remain challenging. We used geometric morphometric (GM) analysis to conduct the first landmark-based morphometric analysis of the biostratigraphically useful conodont genus Neognathodus. Our objective is to assess whether previously defined morphotype groups are reliably distinct from one another. As such, we reevaluate patterns of morphologic change in Neognathodus P1elements, perform maximum-likelihood tests of evolutionary modes, and construct novel, GM-based biozonations through a Desmoinesian (Middle Pennsylvanian) section in the Illinois Basin. Our GM results record the entire spectrum of shape variability among Neognathodus morphotypes, thus alleviating the problem of documenting and classifying gradual morphologic transitions between morphotypes. Statistically distinct GM groups support previously established classifications of N. bassleri, N. bothrops, and N. roundyi. Statistically indistinct pairs of GM groups do not support literature designations of N. medadultimus and N. medexultimus, and N. dilatus and N. metanodosus, and we synonymize each pair. Maximum-likelihood tests of evolutionary modes provide the first statistical assessment of Neognathodus evolutionary models in the Desmoinesian. The most likely evolutionary models are an unbiased random walk or a general random walk. We name four distinct biozones through the Desmoinesian using GM results, and these align with previous biozonation structure based on the Neognathodus Index (NI), illustrating that Neognathodus-based biostratigraphic correlations would not change between GM or NI methods. The structural similarity between both biozonations showcases that determining GM-based biozones is not redundant, as this comparison validates using landmark-based GM work to construct viable biozonations for subsequent stratigraphic correlations. Although this study is limited to the Illinois Basin, our quantitative methodology can be applied broadly to test taxonomic designations of additional genera, interpret statistically robust evolutionary patterns, and construct valid biozones for this significant chordate group.

Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts

“…2016; Zimmerman et al . 2018; Guenser et al . 2019) shedding light on the morphological variation of many genera.…”

Morphological trends across the Norian/Rhaetian boundary within Late Triassic conodonts in western Canada: implications for protracted paleoenvironmental disturbance preceding the end-Triassic mass extinction

“…The platform landmark analysis methodology employed by the present study follows that utilized by prior morphometric analyses of Late Pennsylvanian conodont pectiniform elements (Hogancamp et al 2016; Hogancamp and Barrick 2018; Hogancamp 2020). This landmarking methodology emphasizes the positions of distinct anatomical features, in contrast to morphometric studies of conodont pectiniform elements that focus more on the traced outline of elements (Klapper and Foster 1986, 1993; Renaud and Girard 1999; Girard et al 2007; Zimmerman et al 2018; Guenser et al 2019). The outline-focused methodology has the advantage of not requiring identification of many common point features, resulting in specimen pools for analysis that accommodate a greater degree of morphological variability, potentially even including multiple genera.…”

Potential of closed contour analysis for species differentiation and holotype designation: a case study on lower Norian (Upper Triassic) conodonts

“…Zimmermann et al . (2018) performed landmark analysis on P 1 elements of seven Late Carboniferous Neognathodus species but recognized only five different morphotypes within their sample population. Based on their findings, they synonymized two pairs of statistically indistinguishable species.…”