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Patterns of Bilateral Asymmetry and Allometry in Late Devonian Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts Sabrina Renaud, Beverley Ecalle, Pénélope Claisse, Anne-lise Charruault, Ronan Ledevin, Catherine Girard To cite this version: Sabrina Renaud, Beverley Ecalle, Pénélope Claisse, Anne-lise Charruault, Ronan Ledevin, et al.. Pat- terns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts. Palaeontology, Wiley, 2020, 64 (1), pp.137 - 159. 10.1111/pala.12513. hal-03317732 HAL Id: hal-03317732 https://hal.archives-ouvertes.fr/hal-03317732 Submitted on 9 Aug 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. [Palaeontology, 2020, pp. 1–23] PATTERNS OF BILATERAL ASYMMETRY AND ALLOMETRY IN LATE DEVONIAN POLYGNATHUS CONODONTS by SABRINA RENAUD1 , BEVERLEY ECALLE2 ,PEN ELOPE CLAISSE2 , ANNE-LISE CHARRUAULT2,RONANLEDEVIN1,3 and CATHERINE GIRARD2 1Laboratoire de Biometrie et Biologie Evolutive, UMR 5558 CNRS, Universite Claude Bernard Lyon 1, Universite de Lyon, 69100 Villeurbanne, France 2Institut des Sciences de l’Evolution de Montpellier (ISEM), Universite Montpellier, CNRS, EPHE, IRD, Montpellier, France; [email protected] 3Current address: UMR5199 PACEA, Universite de Bordeaux, Allee Geoffroy Saint Hilaire (Bat. B8), 33615 Pessac, France Typescript received 5 June 2020; accepted in revised form 17 September 2020 Abstract: Conodont animals were early jawless vertebrates element geometry. The 3D analysis delivered further insights equipped with a feeding apparatus composed of several into the relationship between the geometry of the elements tooth-like elements. The P1 elements, at the rear of the appa- and the constraints related to occlusion. The 2D analysis ratus, were characterized by a robust shape and rapid mor- allowed a quantitative assessment of the variation among spe- phological evolution. Occlusion occurred between paired cies and through time. Allometry and bilateral asymmetry right and left P1 elements, occasioning some bilateral asym- were differently expressed depending on the species consid- metry, which, together with allometric growth, may partially ered, suggesting that constraints imposed on pairing by the obliterate the temporal differences. The present study aims to morphology of the elements varied even among related spe- disentangle these different components of morphological vari- cies. The within-species variation was so important that it lar- ation in Late Devonian Polygnathus P1 conodont elements. gely obliterated temporal trends; a relationship of Polygnathus An extensive 2D geometric morphometric analysis of the plat- shape and conodont biofacies variations through the Famen- form shape was performed through the Famennian record of nian nevertheless suggested an evolution driven by ecological two outcrops. This analysis was completed by a 3D study on a interactions between conodont genera. subset of conodont elements. The 2D and 3D morphometric quantifications provided highly congruent results, showing Key words: conodont, Famennian, bilateral asymmetry, that the 2D shape constitutes a good approximation of the allometry, Polygnathus, occlusion. A FUNCTIONAL feeding strategy is an evident prerequisite (Purnell et al. 2007). A drawback is that the functional for any growth and survival. Through life history, animals understanding required as a fundament for a valid eco- therefore repeatedly evolved buccal tooth-like structures morphological approach may be more approximate for of various shapes and complexity to ensure food com- extinct animals without modern relatives. minution. Because of this function in food handling, Conodonts were small eel-like animals that were these elements are mineralized to resist abrasion. They important part of the marine fauna from the late Cam- have therefore a good preservation potential in the fossil brian to the Late Triassic (Briggs et al. 1983; Aldridge record. Variations in the geometry of such buccal ele- et al. 1986, 1993). Isotopic data have recently shown that ments can trace variations in resource exploitation, they were first level consumers (Balter et al. 2019). Con- because different food resources require different func- odont animals displayed a complex feeding apparatus, in tional properties to be efficiently processed (Linde et al. which the anterior elements formed a structure allowing 2004; Price et al. 2011). An ecomorphological approach them to trap food that was further processed by the pos- can thus shed light on the diet of extinct species (Boyer terior elements (Aldridge et al. 1987; Purnell & Donoghue 2008; Gomez Cano et al. 2013). In remote times, such an 1997). These buccal elements provide the almost exclusive approach can improve the understanding of past food fossil remains of these former soft-bodied animals to get chains and their variation through time and space insights into their palaeobiology, including phylogenetic doi: 10.1111/pala.12513 1 relationships (Donoghue et al. 2000, 2008; Sweet 1988), In the present study, a landmark-based geometric mor- functional constraints (Donoghue 2001a) and response to phometric approach was used to quantify the shape of environmental changes (Renaud & Girard 1999; Balter Polygnathus P1 elements in two outcrops through which et al. 2008; Girard & Renaud 2012; Guenser et al. 2019). Famennian environmental variations were recently inves- The buccal elements are most often found isolated, tigated (Girard et al. 2020a), allowing a comparison because of the dislocation of the apparatus during fos- between morphological variations and the environmental silization, and because of the procedure of chemical dis- context. Extensive sampling through the Famennian sec- solutions most often used to find them in the sediments. tion provided abundant material. For six species, the high Among the different elements composing an apparatus, number of specimens (>100) allowed a 2D morphometric the platform elements (P1), located at the rear of the con- analysis of temporal variation. The geometry of Poly- odont mouth, were characterized by a robust shape and gnathus P1 elements is however characterized by ridges rapid morphological evolution that made them useful and grooves that are typically three-dimensional struc- stratigraphic tools since decades. Their abundance has tures, suggesting that they could properly be described allowed extensive morphometric studies (Klapper & Fos- only using a 3D approach (Martınez-Perez et al. 2016). A ter 1986; Roopnarine et al. 2004; Jones 2009; Jones et al. 3D geometric morphometric analysis was therefore 2009; Chen et al. 2016; Hogancamp et al. 2016; Zimmer- performed on a subset encompassing elements of two man et al. 2018; Guenser et al. 2019) shedding light on species. the morphological variation of many genera. The plat- The questions investigated were: form elements of Palmatolepis, Icriodus and Ancyrodella 1. Could the 2D description of the platform margin have been shown to vary synchronously according to constitute a good approximation of the 3D geometry short-term environmental fluctuations (Renaud & Girard of Polygnathus elements? This was assessed by com- 1999; Girard & Renaud 2008). In the latter genus, how- paring 2D and 3D geometric morphometric quantifi- ever, temporal changes are partly masked by allometric cation of a subset of Po. glaber and Po. communis. variations, constituting an important source of within- 2. Is the asymmetry of pairing during occlusion associ- species morphological variation in several conodont gen- ated with a bilateral asymmetry of the platform shape, era (Roopnarine et al. 2004; Chen et al. 2016). suggestive of functional constraints on the element Further, the study of Palmatolepis P1 elements demon- geometry, and conversely, can patterns of bilateral strated a morphological trend from broad to narrow ele- asymmetry in element geometry raise questions about ments through the Famennian (Girard & Renaud 2012). the constraints related to occlusion? Right–left differ- This period, previously thought to be rather stable, was ences in element shape were characterized to address recently shown to be marked by complex environmental this question. changes interrelating temperature, eustatic and tectonic 3. Is the pattern of shape variation conserved across spe- variations (Girard et al. 2020a). The relative role of these cies? This was addressed by assessing the direction of different factors on Palmatolepis evolution is still unclear; allometric shape changes as well as the balance furthermore, ecological interactions between the actors of between allometry and bilateral asymmetry in the dif- the nektonic fauna, including not only conodonts but also ferent species. bony fishes and chondrichthyans, rather than abiotic 4. Despite the extensive within-group variation, can changes per se, could have driven the morphological evo- consistent temporal variations in Polygnathus P1 plat- lution of buccal elements (Gauchey et al. 2014). The form shape be evidenced, and if yes, can a biotic or comparison with another conodont record throughout
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