Citation: Coelho, M.V.; Cupello, C.; Brito, P.M.; Otero, O. Living Taxa and Their Importance in Understanding the Extinct Diversity: A Look at Polypterid Pinnules. Diversity 2023, 15, 517. https://doi.org/10.3390/ d15040517 Academic Editors: Jürgen Kriwet and Michael Wink Received: 31 January 2023 Revised: 28 March 2023 Accepted: 30 March 2023 Published: 3 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diversity Article Living Taxa and Their Importance in Understanding the Extinct Diversity: A Look at Polypterid Pinnules Marcos Vinícius Coelho 1, * , Camila Cupello 1 , Paulo M. Brito 1,2 and Olga Otero 3 1 Departamento de Zoologia, Instituto de Biologia-IBRAG, Universidade do Estado do Rio de Janeiro, Rio de Janeiro 20550-900, RJ, Brazil 2 UMR 8067, Département Adaptations du Vivant, Muséum National d’Histoire Naturelle, 75005 Paris, France 3 PALEVOPRIM, UMR CNRS-UP 7262, Université de Poitiers, 86073 Poitiers, France * Correspondence: delimacoelhomv@gmail.com Abstract: Pinnules are the peculiar, impaired spines that carry each of the numerous finlets that constitute the dorsal fins of polypterid fishes (Cladistia and Polypteriformes). Previous studies, including a recent detailed paper on the comparative analysis of the morphology of pinnules in most of the extant species (genera Polypterus and Erpetoichthys), suggest that they display unique characteristics that allow for species identification. Since most of the polypterid fossil records are composed of scales that lack specific characteristics and isolated pinnules, this work aims to test their taxonomic relevance before comparing the fossil pinnule morphologies across the fossil records in order to evaluate polypterid paleodiversity. Therefore, we describe the intra-individual and intra- specific morphological variations of the pinnules in the extant species Polypterus bichir. Furthermore, we compared it with the various morphologies described in the Polypteridae family. We report intra-individual variability related to the anteroposterior axis of the dorsal fin. We also report morphological differences in the pinnules among specimens that overlap those anticipated among different species, concluding that the pinnule morphology cannot support taxonomic purposes in polypterid fishes in their current state. Keywords: Polypteriformes; pinnules; morphological variation; taxonomy 1. Introduction The dorsal fin of all Polypteriformes is divided into several finlets [1–3]. Each finlet is composed of a central spine (called a pinnule), a lepidotrichia partially fused to the pinnule and divided into secondary rami, and a membrane connecting a finlet to the anterior part of the next finlet (Figure 1)[4,5]. Pinnules are only found in Polypteriformes. They are the most unique and distinguished characteristic of the group, and they represent a considerable part of the known fossils of the group, usually found isolated and disarticulated (e.g., [2,4,6–8]). The first isolated fossil pinnules were reported in 1988 for Polypterus sp. [6], and a wealth of material is described as such in Cenozoic outcrops from Africa (e.g., [9–20]). In 1996, isolated material was reported from the Coniacian in Niger, and in 1997, more pinnules were described from the Cenomanian in Sudan, allowing the description of new extinct taxa. All these fossils equate to a total of six genera that comprise 17 species, of which two are Polypterus species [2,4,6–8]. Since pinnules are an exclusive characteristic of Polypteriformes [1,21,22], fossil pinnules are confidently assigned to polypteriform fish. Conversely, diagnostic values for distinguishing species appear fragile, and only two critical works regarding the validity of pinnules as taxonomic units in extant fish have been published so far. In 2018, Coelho et al. [23] described the intraindividual morphological variation relative to the position on the dorsal fin in five specimens from four different species. They pointed out that the descriptions of fossil species based on pinnule morphology do not consider individual variations, and thus, the isolated material described as different species might belong to the same taxon. In 2020, Meunier and Gayet [5] Diversity 2023, 15, 517. https://doi.org/10.3390/d15040517 https://www.mdpi.com/journal/diversity