Quantifying shell outline variability in extant and fossilLaqueus(Brachiopoda: Terebratulida): are outlines good proxies for long-looped brachidial morphology and can they help us characterize species?

Natalia López Carranza,Sandra J Carlson

doi:10.1017/pab.2020.56

Abstract

AbstractExtant and extinct terebratulide brachiopod species have been defined primarily on the basis of morphology. What is the fidelity of morphological species to biological species? And how can we test this fidelity with fossils? Taxonomically and phylogenetically, the most informative internal feature in the brachiopod suborder Terebratellidina is the geometrically complex long-looped brachidium, which is highly fragile and only rarely preserved in the fossil record. Given this, it is essential to test other sources of morphological data, such as valve outline shape, when trying to recognize and identify species. We analyzed valve outlines and brachidia in the genusLaqueusto explore the utility of shell shape in discriminating extant and fossil species. Using geometric morphometric methods, we quantified valve outline variability using elliptical Fourier methods and tested whether long-looped brachidial morphology correlates with shell outline shape. We then built classification models based on machine learning algorithms using outlines as shape variables to predict fossil species’ identities. Our results demonstrate that valve outline shape is significantly correlated with long-looped brachidial shape and that even relatively simple outlines are sufficiently morphologically distinct to enable extantLaqueusspecies to be identified, validating current taxonomic assignments. These are encouraging results for the study and delimitation of fossil terebratulide species, and their recognition as biological species. In addition, machine learning algorithms can be successfully applied to help solve species recognition and delimitation problems in paleontology, especially when morphology can be characterized quantitatively and analyzed statistically.

Highlights

Can biological species be recognized and delineated in the fossil record from morphology alone? This question is fundamental to most studies in evolutionary paleobiology, and only recently has the relationship between morphological and biological species been tested empirically (e.g., Bichain et al 2007; González-Wevar et al 2011; Reisser et al 2012; Pieri et al 2018; Smith et al 2018; Vaux et al 2018)
The questions we focus on in this study ask whether valve outline alone can clearly distinguish morphological clusters that proxy for biological species in Laqueus, a genus of terebratulide brachiopod
The resulting elliptical Fourier coefficients were used as input variables for a principal component analysis (PCA) to examine the overall pattern of variation and a canonical variate analysis (CVA) to determine if species could be discriminated based on outline shape

Summary

Introduction

Can biological species be recognized and delineated in the fossil record from morphology alone? This question is fundamental to most studies in evolutionary paleobiology, and only recently has the relationship between morphological and biological species been tested empirically (e.g., Bichain et al 2007; González-Wevar et al 2011; Reisser et al 2012; Pieri et al 2018; Smith et al 2018; Vaux et al 2018). The classic concept of a biological species (Mayr 1942) based on reproductive isolation is assumed to be manifested as well by a demonstrated lack of gene flow and in the clustering of organisms by size and shape For this reason, some have claimed that morphological species, more often than not, proxy for biological species and can be identified and delimited with confidence (e.g., Gingerich 1979; Bose 2012)—and paleontologists can breathe a sigh of relief. Others claim that biological species can only be recognized reliably on the basis of molecular data (e.g., Hebert et al 2003; Robinson et al 2016)—and paleontologists have no truly robust basis upon which to recognize, and study, species This possibility is obviously concerning to morphological paleontologists but is often assumed to be so uncommon that it can be disregarded with some confidence.

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