Richard Cloutier | Université du Québec à Rimouski (original) (raw)
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Papers by Richard Cloutier
Geobios, 1995
Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allocht... more Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allochthonous unit of the Palaeozoic orogen encountered between the Strait of Dover and the Rhine river; in northern France and southern Belgium. Heterostracans have been used to ...
Geobios, 1995
Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allocht... more Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allochthonous unit of the Palaeozoic orogen encountered between the Strait of Dover and the Rhine river; in northern France and southern Belgium. Heterostracans have been used to ...
Special publication, 1992
Sciences de la mer, 2020
L’abondance du saumon atlantique (Salmo salar) dans le nord de l’Atlantique a diminué considérabl... more L’abondance du saumon atlantique (Salmo salar) dans le nord de l’Atlantique a diminué considérablement au cours des dernières décennies. Jusqu’à maintenant, peu de stocks ont montré des signes de rétablissement malgré des efforts importants pour renverser cette situation. Les niveaux décroissants d’abondance des saumons adultes de retour en rivière pourraient être associés à une augmentation de la mortalité naturelle en mer découlant d’une réduction de la croissance. La présente étude a pour but de vérifier si 2 indices de croissance scalaire en phase marine (la distance intercirculi maximale atteinte lors de la première saison estivale en mer et la somme des distances intercirculi de la zone de croissance marine des écailles) et l’abondance des retours de saumon dans la rivière Matane (Québec, Canada) appuient cette hypothèse. Bien que ces indices présentent des différences interannuelles significatives, ils affichent une tendance à la hausse de 1964 à 1979 et une tendance à la bai...
PeerJ
The study of development is critical for revealing the evolution of major vertebrate lineages. Co... more The study of development is critical for revealing the evolution of major vertebrate lineages. Coelacanths have one of the longest evolutionary histories among osteichthyans, but despite access to extant representatives, the onset of their weakly ossified endoskeleton is still poorly understood. Here we present the first palaeohistological and skeletochronological study ofMiguashaia bureauifrom the Upper Devonian of Canada, pivotal for exploring the palaeobiology and early evolution of osteogenesis in coelacanths. Cross sections of the caudal fin bones show that the cortex is made of layers of primary bone separated by lines of arrested growth, indicative of a cyclical growth. The medullary cavity displays remnants of calcified cartilage associated with bony trabeculae, characteristic of endochondral ossification. A skeletochronological analysis indicates that rapid growth during a short juvenile period was followed by slower growth in adulthood. Our new analysis highlights the life...
PeerJ, Dec 10, 2021
Background. The megalichthyids are one of several clades of extinct tetrapodomorph fish that live... more Background. The megalichthyids are one of several clades of extinct tetrapodomorph fish that lived throughout the Devonian-Permian periods. They are advanced ''osteolepidid-grade'' fishes that lived in freshwater swamp and lake environments, with some taxa growing to very large sizes. They bear cosmine-covered bones and a large premaxillary tusk that lies lingually to a row of small teeth. Diagnosis of the family remains controversial with various authors revising it several times in recent works. There are fewer than 10 genera known globally, and only one member definitively identified from Gondwana. Cladarosymblema narrienense Fox et al. 1995 was described from the Lower Carboniferous Raymond Formation in Queensland, Australia, on the basis of several well-preserved specimens. Despite this detailed work, several aspects of its anatomy remain undescribed. Methods. Two especially well-preserved 3D fossils of Cladarosymblema narrienense, including the holotype specimen, are scanned using synchrotron or micro-computed tomography (µCT), and 3D modelled using specialist segmentation and visualisation software. New anatomical detail, in particular internal anatomy, is revealed for the first time in this taxon. A novel phylogenetic matrix, adapted from other recent work on tetrapodomorphs, is used to clarify the interrelationships of the megalichthyids and confirm the phylogenetic position of C. narrienense. Results. Never before seen morphological details of the palate, hyoid arch, basibranchial skeleton, pectoral girdle and axial skeleton are revealed and described. Several additional features are confirmed or updated from the original description. Moreover, the first full, virtual cranial endocast of any tetrapodomorph fish is presented and described, giving insight into the early neural adaptations in this group. Phylogenetic analysis confirms the monophyly of the Megalichthyidae with seven genera included (Askerichthys,
Journal of Vertebrate Paleontology, Jun 30, 2022
Nature, Jul 8, 2020
In the 'Data availability' section of this Article, we stated "Morphological data are deposited i... more In the 'Data availability' section of this Article, we stated "Morphological data are deposited in MorphoBank" but at the time of publication the data were not uploaded. All data on our phylogenetic analysis uploaded to MorphoBank Project 3480 are now available at https://morphobank. org/index.php/Projects/ProjectOverview/project_id/3480. The original Article has been corrected online.
Biology Letters, Feb 1, 2023
Actinopterygians are the most diversified clade of extant vertebrates. Their impressive morpholog... more Actinopterygians are the most diversified clade of extant vertebrates. Their impressive morphological disparity bears witness to tremendous ecological diversity. Modularity, the organization of biological systems into quasi-independent anatomical/morphological units, is thought to increase evolvability of organisms and facilitate morphological diversification. Our study aims to quantify patterns of variational modularity in a model actinopterygian, the zebrafish ( Danio rerio ), using three-dimensional geometric morphometrics on osteological structures isolated from micro-CT scans. A total of 72 landmarks were digitized along cranial and postcranial ossified regions of 30 adult zebrafishes. Two methods were used to test modularity hypotheses, the covariance ratio and the distance matrix approach. We find strong support for two modules, one comprised paired fins and the other comprised median fins, that are best explained by functional properties of subcarangiform swimming. While the skull is tightly integrated with the rest of the body, its intrinsic integration is relatively weak supporting previous findings that the fish skull is a modular structure. Our results provide additional support for the recognition of similar hypotheses of modularity identified based on external morphology in various teleosts, and at least two variational modules are proposed. Thus, our results hint at the possibility that internal and external modularity patterns may be congruent.
Canadian Journal of Earth Sciences, Mar 1, 2023
Ontogeny is of paramount importance to understand evolutionary relationships of organisms. Howeve... more Ontogeny is of paramount importance to understand evolutionary relationships of organisms. However, in the fossil record, early developmental stages are rarely preserved because of their unmineralized skeleton. Here, we describe the ontogeny of the Devonian jawless fish, Euphanerops longaevus, based on observations from more than 3500 specimens. The ontogeny of Euphanerops is compared with the development of the median fins of the extant jawless fish, the sea lamprey ( Petromyzon marinus). From a subsample of 216 specimens of Euphanerops, we define three ontogenetic stages: larvae (2–38 mm total length (TL), mostly composed of “ Scaumenella mesacanthi”), juveniles (28–98 mm TL), and adults (≥90 mm TL) based on the degree of skeletal development, chemical and microscopic compositions. Larvae display three cranial plates, a simple branchial apparatus, notochordal elements, and caudal fin supports. In juveniles, we document the development of paired anteroventral, anal, and median dorsal fins. Given how little information is generally available on ostracoderm ontogeny, the growth series of Euphanerops provides an unparalleled opportunity to understand the development of early vertebrate characters.
Evolution & Development, Nov 1, 2009
Morphological integration can respond to environmental conditions, a response that may be dynamic... more Morphological integration can respond to environmental conditions, a response that may be dynamic through ontogeny. Among fishes, brook charrs (Salvelinus fontinalis) display a trophic polymorphism that makes it a good species for analyzing the ontogeny of morphological integration. To better understand the processes regulating variation and integration, we assess the ontogenetic dynamics of covariances and developmental progress for populations of S. fontinalis from two habitats that differ in water velocity; lake and stream. Geometric morphometrics and developmental progress were evaluated on 751 and 198 specimens, respectively. In both habitats, most ossification events occur before the transition from alevin to juvenile. This threshold defines two distinct periods. During the first period representing free-embryos and alevins, there are important shape changes and rapid ossification, integration tends to be relatively low and decreasing and the variance of shape drastically decreases. During the juvenile period, the rate of shape change decreases and the onset of ossification is nearly complete, plus integration increases and shape variance stabilizes. While we find two distinct developmental periods, we nonetheless find a notable stability underlying the ontogenetic dynamics of variability as well as gradual change in the structure of covariation within each habitat. Our results imply that the variability of juvenile body shape does not seem to retain signals of variability determined early in ontogeny and warrants caution in using juvenile as guides to the earlier causes of variability. Overall, this study highlights the difficulty of inferring causes of integration from studies of static covariance.
Environmental Biology of Fishes, Sep 1, 1991
Affiche présentée dans le cadre du Colloque de l'ARC, «La culture de la recherche au collégia... more Affiche présentée dans le cadre du Colloque de l'ARC, «La culture de la recherche au collégial», dans le cadre du 82e Congrès de l'Acfas, Université Concordia, Montréal, le 14 mai 2014.La plupart des études en paléontologie utilisant les technologies de tomographie sont effectuées sur un nombre limité de fossiles en raison des coûts et du temps requis. L’état de préservation, la taille ainsi que le type de roche entourant les fossiles peuvent également représenter des obstacles. Dans les cas où la reconstitution en trois dimensions (3D) ne nécessite pas l’investigation de l’anatomie interne, les numériseurs 3D offrent une solution non invasive pour obtenir des modèles 3D sans les limites imposées par les autres technologies. Les résultats sont rapides et facilement exploitables pour la recherche et la gestion de collections. Dix-neuf fossiles bien conservés d’un poisson du Dévonien, Bothiolepis canadensis (Miguasha, Québec), ont été numérisés à une résolution maximale afin de développer un nouveau protocole de reconstitution 3D. Le traitement et le nettoyage des données brutes ont été effectués dans le logiciel d'acquisition et de traitement de données du numériseur. Un logiciel de sculpture numérique a été utilisé pour corriger les fossiles numérisés et pour assembler le modèle 3D. Ce dernier est la reconstitution la plus objective et la plus précise de ce poisson fossile effectuée à ce jour. Le protocole vous est présenté en réalité augmentée via l’application mobile gratuite PaléoRA
Morphological and developmental similarities, and interactions among developing structures are in... more Morphological and developmental similarities, and interactions among developing structures are interpreted as evidences of modularity. Such similarities exist between the dorsal and anal fins of living actinopterygians: (1) both fins differentiate in the same direction [dorsal and anal fin patterning module (DAFPM)], and (2) radials and lepidotrichia differentiate in the same direction [endoskeleton and exoskeleton module (EEM)]. To infer the evolution of these common developmental patternings among osteichthyans, we address (1) the complete description and quantification of the DAFPM and EEM in a living actinopterygian (the rainbow trout Oncorhynchus mykiss) and (2) the presence of these modules in fossil osteichthyans (coelacanths, lungfishes, porolepiforms and ‘osteolepiforms’). In Oncorhynchus, sequences of skeletal elements are determined based on (1) apparition (radials and lepidotrichia), (2) chondrification (radials), (3) ossification (radials and lepidotrichia), and (4) seg...
Entomologia Experimentalis et Applicata, 2018
Under the hypothesis of environmentally cued hatching (ECH), eggs are stimulated to hatch by cues... more Under the hypothesis of environmentally cued hatching (ECH), eggs are stimulated to hatch by cues favouring larval survival. Here we investigated whether the bird-parasitic nest fly Philornis downsi Dodge & Aitken (Diptera: Muscidae) exhibits ECH in response to the presence of suitable hosts and environmental conditions. Philornis downsi is an invasive in the Galapagos Islands and ECH could help to explain its success in a novel habitat. We found that the presence of hosts (nesting birds) per se does not accelerate hatching time in P. downsi, but that nesting birds do produce a microclimate, in terms of temperature and/or humidity that is conducive to faster hatching and higher hatching success. Eggs that are laid under extremely dry conditions die rather than delay their hatching time. We also found that P. downsi eggs hatch more rapidly when the substrate is very wet. Furthermore, larvae that hatch rapidly survive longer under starvation conditions. We suggest that eggs hatching more rapidly can either take advantage of optimal conditions for larval growth (host presence) or escape unfavourable microclimatic conditions (as larvae are mobile).
Geobios, 1995
Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allocht... more Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allochthonous unit of the Palaeozoic orogen encountered between the Strait of Dover and the Rhine river; in northern France and southern Belgium. Heterostracans have been used to ...
Geobios, 1995
Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allocht... more Heterostraci; Osteostraci and Thelodonti are known from the Lower Devonian of the Ardenne Allochthonous unit of the Palaeozoic orogen encountered between the Strait of Dover and the Rhine river; in northern France and southern Belgium. Heterostracans have been used to ...
Special publication, 1992
Sciences de la mer, 2020
L’abondance du saumon atlantique (Salmo salar) dans le nord de l’Atlantique a diminué considérabl... more L’abondance du saumon atlantique (Salmo salar) dans le nord de l’Atlantique a diminué considérablement au cours des dernières décennies. Jusqu’à maintenant, peu de stocks ont montré des signes de rétablissement malgré des efforts importants pour renverser cette situation. Les niveaux décroissants d’abondance des saumons adultes de retour en rivière pourraient être associés à une augmentation de la mortalité naturelle en mer découlant d’une réduction de la croissance. La présente étude a pour but de vérifier si 2 indices de croissance scalaire en phase marine (la distance intercirculi maximale atteinte lors de la première saison estivale en mer et la somme des distances intercirculi de la zone de croissance marine des écailles) et l’abondance des retours de saumon dans la rivière Matane (Québec, Canada) appuient cette hypothèse. Bien que ces indices présentent des différences interannuelles significatives, ils affichent une tendance à la hausse de 1964 à 1979 et une tendance à la bai...
PeerJ
The study of development is critical for revealing the evolution of major vertebrate lineages. Co... more The study of development is critical for revealing the evolution of major vertebrate lineages. Coelacanths have one of the longest evolutionary histories among osteichthyans, but despite access to extant representatives, the onset of their weakly ossified endoskeleton is still poorly understood. Here we present the first palaeohistological and skeletochronological study ofMiguashaia bureauifrom the Upper Devonian of Canada, pivotal for exploring the palaeobiology and early evolution of osteogenesis in coelacanths. Cross sections of the caudal fin bones show that the cortex is made of layers of primary bone separated by lines of arrested growth, indicative of a cyclical growth. The medullary cavity displays remnants of calcified cartilage associated with bony trabeculae, characteristic of endochondral ossification. A skeletochronological analysis indicates that rapid growth during a short juvenile period was followed by slower growth in adulthood. Our new analysis highlights the life...
PeerJ, Dec 10, 2021
Background. The megalichthyids are one of several clades of extinct tetrapodomorph fish that live... more Background. The megalichthyids are one of several clades of extinct tetrapodomorph fish that lived throughout the Devonian-Permian periods. They are advanced ''osteolepidid-grade'' fishes that lived in freshwater swamp and lake environments, with some taxa growing to very large sizes. They bear cosmine-covered bones and a large premaxillary tusk that lies lingually to a row of small teeth. Diagnosis of the family remains controversial with various authors revising it several times in recent works. There are fewer than 10 genera known globally, and only one member definitively identified from Gondwana. Cladarosymblema narrienense Fox et al. 1995 was described from the Lower Carboniferous Raymond Formation in Queensland, Australia, on the basis of several well-preserved specimens. Despite this detailed work, several aspects of its anatomy remain undescribed. Methods. Two especially well-preserved 3D fossils of Cladarosymblema narrienense, including the holotype specimen, are scanned using synchrotron or micro-computed tomography (µCT), and 3D modelled using specialist segmentation and visualisation software. New anatomical detail, in particular internal anatomy, is revealed for the first time in this taxon. A novel phylogenetic matrix, adapted from other recent work on tetrapodomorphs, is used to clarify the interrelationships of the megalichthyids and confirm the phylogenetic position of C. narrienense. Results. Never before seen morphological details of the palate, hyoid arch, basibranchial skeleton, pectoral girdle and axial skeleton are revealed and described. Several additional features are confirmed or updated from the original description. Moreover, the first full, virtual cranial endocast of any tetrapodomorph fish is presented and described, giving insight into the early neural adaptations in this group. Phylogenetic analysis confirms the monophyly of the Megalichthyidae with seven genera included (Askerichthys,
Journal of Vertebrate Paleontology, Jun 30, 2022
Nature, Jul 8, 2020
In the 'Data availability' section of this Article, we stated "Morphological data are deposited i... more In the 'Data availability' section of this Article, we stated "Morphological data are deposited in MorphoBank" but at the time of publication the data were not uploaded. All data on our phylogenetic analysis uploaded to MorphoBank Project 3480 are now available at https://morphobank. org/index.php/Projects/ProjectOverview/project_id/3480. The original Article has been corrected online.
Biology Letters, Feb 1, 2023
Actinopterygians are the most diversified clade of extant vertebrates. Their impressive morpholog... more Actinopterygians are the most diversified clade of extant vertebrates. Their impressive morphological disparity bears witness to tremendous ecological diversity. Modularity, the organization of biological systems into quasi-independent anatomical/morphological units, is thought to increase evolvability of organisms and facilitate morphological diversification. Our study aims to quantify patterns of variational modularity in a model actinopterygian, the zebrafish ( Danio rerio ), using three-dimensional geometric morphometrics on osteological structures isolated from micro-CT scans. A total of 72 landmarks were digitized along cranial and postcranial ossified regions of 30 adult zebrafishes. Two methods were used to test modularity hypotheses, the covariance ratio and the distance matrix approach. We find strong support for two modules, one comprised paired fins and the other comprised median fins, that are best explained by functional properties of subcarangiform swimming. While the skull is tightly integrated with the rest of the body, its intrinsic integration is relatively weak supporting previous findings that the fish skull is a modular structure. Our results provide additional support for the recognition of similar hypotheses of modularity identified based on external morphology in various teleosts, and at least two variational modules are proposed. Thus, our results hint at the possibility that internal and external modularity patterns may be congruent.
Canadian Journal of Earth Sciences, Mar 1, 2023
Ontogeny is of paramount importance to understand evolutionary relationships of organisms. Howeve... more Ontogeny is of paramount importance to understand evolutionary relationships of organisms. However, in the fossil record, early developmental stages are rarely preserved because of their unmineralized skeleton. Here, we describe the ontogeny of the Devonian jawless fish, Euphanerops longaevus, based on observations from more than 3500 specimens. The ontogeny of Euphanerops is compared with the development of the median fins of the extant jawless fish, the sea lamprey ( Petromyzon marinus). From a subsample of 216 specimens of Euphanerops, we define three ontogenetic stages: larvae (2–38 mm total length (TL), mostly composed of “ Scaumenella mesacanthi”), juveniles (28–98 mm TL), and adults (≥90 mm TL) based on the degree of skeletal development, chemical and microscopic compositions. Larvae display three cranial plates, a simple branchial apparatus, notochordal elements, and caudal fin supports. In juveniles, we document the development of paired anteroventral, anal, and median dorsal fins. Given how little information is generally available on ostracoderm ontogeny, the growth series of Euphanerops provides an unparalleled opportunity to understand the development of early vertebrate characters.
Evolution & Development, Nov 1, 2009
Morphological integration can respond to environmental conditions, a response that may be dynamic... more Morphological integration can respond to environmental conditions, a response that may be dynamic through ontogeny. Among fishes, brook charrs (Salvelinus fontinalis) display a trophic polymorphism that makes it a good species for analyzing the ontogeny of morphological integration. To better understand the processes regulating variation and integration, we assess the ontogenetic dynamics of covariances and developmental progress for populations of S. fontinalis from two habitats that differ in water velocity; lake and stream. Geometric morphometrics and developmental progress were evaluated on 751 and 198 specimens, respectively. In both habitats, most ossification events occur before the transition from alevin to juvenile. This threshold defines two distinct periods. During the first period representing free-embryos and alevins, there are important shape changes and rapid ossification, integration tends to be relatively low and decreasing and the variance of shape drastically decreases. During the juvenile period, the rate of shape change decreases and the onset of ossification is nearly complete, plus integration increases and shape variance stabilizes. While we find two distinct developmental periods, we nonetheless find a notable stability underlying the ontogenetic dynamics of variability as well as gradual change in the structure of covariation within each habitat. Our results imply that the variability of juvenile body shape does not seem to retain signals of variability determined early in ontogeny and warrants caution in using juvenile as guides to the earlier causes of variability. Overall, this study highlights the difficulty of inferring causes of integration from studies of static covariance.
Environmental Biology of Fishes, Sep 1, 1991
Affiche présentée dans le cadre du Colloque de l'ARC, «La culture de la recherche au collégia... more Affiche présentée dans le cadre du Colloque de l'ARC, «La culture de la recherche au collégial», dans le cadre du 82e Congrès de l'Acfas, Université Concordia, Montréal, le 14 mai 2014.La plupart des études en paléontologie utilisant les technologies de tomographie sont effectuées sur un nombre limité de fossiles en raison des coûts et du temps requis. L’état de préservation, la taille ainsi que le type de roche entourant les fossiles peuvent également représenter des obstacles. Dans les cas où la reconstitution en trois dimensions (3D) ne nécessite pas l’investigation de l’anatomie interne, les numériseurs 3D offrent une solution non invasive pour obtenir des modèles 3D sans les limites imposées par les autres technologies. Les résultats sont rapides et facilement exploitables pour la recherche et la gestion de collections. Dix-neuf fossiles bien conservés d’un poisson du Dévonien, Bothiolepis canadensis (Miguasha, Québec), ont été numérisés à une résolution maximale afin de développer un nouveau protocole de reconstitution 3D. Le traitement et le nettoyage des données brutes ont été effectués dans le logiciel d'acquisition et de traitement de données du numériseur. Un logiciel de sculpture numérique a été utilisé pour corriger les fossiles numérisés et pour assembler le modèle 3D. Ce dernier est la reconstitution la plus objective et la plus précise de ce poisson fossile effectuée à ce jour. Le protocole vous est présenté en réalité augmentée via l’application mobile gratuite PaléoRA
Morphological and developmental similarities, and interactions among developing structures are in... more Morphological and developmental similarities, and interactions among developing structures are interpreted as evidences of modularity. Such similarities exist between the dorsal and anal fins of living actinopterygians: (1) both fins differentiate in the same direction [dorsal and anal fin patterning module (DAFPM)], and (2) radials and lepidotrichia differentiate in the same direction [endoskeleton and exoskeleton module (EEM)]. To infer the evolution of these common developmental patternings among osteichthyans, we address (1) the complete description and quantification of the DAFPM and EEM in a living actinopterygian (the rainbow trout Oncorhynchus mykiss) and (2) the presence of these modules in fossil osteichthyans (coelacanths, lungfishes, porolepiforms and ‘osteolepiforms’). In Oncorhynchus, sequences of skeletal elements are determined based on (1) apparition (radials and lepidotrichia), (2) chondrification (radials), (3) ossification (radials and lepidotrichia), and (4) seg...
Entomologia Experimentalis et Applicata, 2018
Under the hypothesis of environmentally cued hatching (ECH), eggs are stimulated to hatch by cues... more Under the hypothesis of environmentally cued hatching (ECH), eggs are stimulated to hatch by cues favouring larval survival. Here we investigated whether the bird-parasitic nest fly Philornis downsi Dodge & Aitken (Diptera: Muscidae) exhibits ECH in response to the presence of suitable hosts and environmental conditions. Philornis downsi is an invasive in the Galapagos Islands and ECH could help to explain its success in a novel habitat. We found that the presence of hosts (nesting birds) per se does not accelerate hatching time in P. downsi, but that nesting birds do produce a microclimate, in terms of temperature and/or humidity that is conducive to faster hatching and higher hatching success. Eggs that are laid under extremely dry conditions die rather than delay their hatching time. We also found that P. downsi eggs hatch more rapidly when the substrate is very wet. Furthermore, larvae that hatch rapidly survive longer under starvation conditions. We suggest that eggs hatching more rapidly can either take advantage of optimal conditions for larval growth (host presence) or escape unfavourable microclimatic conditions (as larvae are mobile).
Les fossiles comme outils dans l’interprétation de l’évolution des nageoires chez les poissons A... more Les fossiles comme outils dans l’interprétation de l’évolution des nageoires chez les poissons
Auteurs : Olivier Larouche(1), Miriam L. Zelditch(2) et Richard Cloutier(1)
(1) Laboratoire de Paléontologie et de Biologie évolutive, Université du Québec à Rimouski; Membres du CSBQ.
(2) Museum of Paleontology, University of Michigan.
Les poissons constituent un groupe à la fois très riche en espèces mais aussi en diversité de formes corporelles, notamment en raison de différences dans la configuration des nageoires. Afin de mieux comprendre l’origine de ces différences, il est nécessaire de clarifier le scénario évolutif traduisant l’apparition séquentielle des nageoires. Celui-ci n’est pas bien compris pour deux raisons principales : (1) les relations phylogénétiques entre les agnathes (poissons sans mâchoires) et les gnathostomes (poissons avec mâchoires) ne font pas l’objet d’un consensus, et (2) les relations d’homologie entre les nageoires des agnathes et celles des gnathostomes sont incertaines. Or, la majorité des agnathes ne sont connus que par des taxons fossiles. De ce fait, une bonne connaissance des états de caractères de ces taxons fossiles est essentielle pour interpréter la séquence évolutive d’apparition des nageoires. À ces fins, un super-arbre phylogénétique a été produit, synthétisant les résultats de 118 analyses phylogénétiques récentes, puis les données de présence/absence et de nombre des nageoires ont été superposées sur ce super-arbre. La répartition des caractères dans le super-arbre suggère entre autres que les nageoires médianes et paires seraient toutes deux apparues d’abord sous la forme de structures allongées qui auraient donc été les précurseurs des nageoires aux insertions plus exigües que l’on retrouve chez les poissons plus avancés.
Nature, 2020
The evolution of fishes to tetrapods (four-limbed vertebrates) was one of the most important tran... more The evolution of fishes to tetrapods (four-limbed vertebrates) was one of the most important transformations in vertebrate evolution. Hypotheses of tetrapod origins rely heavily on the anatomy of a few tetrapod-like fish fossils from the Middle and Late Devonian period (393-359 million years ago) 1. These taxa-known as elpistostegalians-include Panderichthys 2 , Elpistostege 3,4 and Tiktaalik 1,5 , none of which has yet revealed the complete skeletal anatomy of the pectoral fin. Here we report a 1.57-metre-long articulated specimen of Elpistostege watsoni from the Upper Devonian period of Canada, which represents-to our knowledge-the most complete elpistostegalian yet found. High-energy computed tomography reveals that the skeleton of the pectoral fin has four proximodistal rows of radials (two of which include branched carpals) as well as two distal rows that are organized as digits and putative digits. Despite this skeletal pattern (which represents the most tetrapod-like arrangement of bones found in a pectoral fin to date), the fin retains lepidotrichia (fin rays) distal to the radials. We suggest that the vertebrate hand arose primarily from a skeletal pattern buried within the fairly typical aquatic pectoral fin of elpistostegalians. Elpistostege is potentially the sister taxon of all other tetrapods, and its appendages further blur the line between fish and land vertebrates. The first tetrapods known from skeletal remains date back to the Late Devonian period (about 374 million years ago) 6,7 , while trackway fossils showing digitate impressions of limbs suggest an earlier origin for this clade 8. Over the past decade, fossils that provide information on the fish-to-tetrapod transition have been used to better understand anatomical transformations associated with locomotion 5,9-12 , breathing 13 , hearing 14 and feeding 11,15 , with regard to the change in habitat from water to land. Until now, the terrestrialization of vertebrates has primarily been a matter of comparing six relatively well-known Devonian taxa among stem-group tetrapods 16 : a true piscine sarcopterygian, Eusthe-nopteron foordi; a piscine elpistostegalian, Panderichthys rhombolepis; a near-tetrapod elpistostegalian, Tiktaalik roseae; and three true basal tetrapods, Acanthostega gunnari, Ventastega curonica and Ichthyostega sp. Here we adopt an apomorphy-based definition of tetrapods as 'all organisms derived from the first sarcopterygian to have possessed digits homologous with those in Homo sapiens' 17,18. However, these inferences regarding terrestrialization rely critically on the handful of specimens that have been referred to elpistostega-lians, none of which has been completely described. The postcranial anatomy of Panderichthys is primarily restricted to the morphology of the pectoral fins and girdle 2,19,20 , the vertebrae 2,21 , the scale patterning 22 and very little on the pelvic fin and girdle morphology 9. Although more than 60 specimens 1,10 of Tiktaalik have been found, most of the anatomy of this species has been described from a fairly complete individual for which the skull 1,15 , pectoral and pelvic fins and girdles 5,10,23 , scales 22 and the trunk region 1 anterior to the pelvic region are preserved.