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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 Panderichthys2, Elpistostege3,4 and Tiktaalik1,5, 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.
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Nature | Vol 579 | 26 March 2020 | 549
Article
Elpistostege and the origin of the vertebrate
hand
Richard Cloutier1,2 ✉, Alice M. Clement2, Michael S. Y. Lee2,3, Roxanne Noël1, Isabelle Béchard1,
Vincent Roy1 & John A. Long2
The evolution of shes 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 sh fossils from the Middle and
Late Devonian period (393–359million years ago)1. These taxa—known as
elpistostegalians—include Panderichthys2, Elpistostege3,4 and Tiktaalik1,5, none of
which has yet revealed the complete skeletal anatomy of the pectoral n. Here we
report a 1.57-metre-long articulated specimen of Elpistostege wat soni 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 n 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 n to date), the n retains lepidotrichia
(n rays) distal to the radials. We suggest that the vertebrate hand arose primarily
from a skeletal pattern buried within the fairly typical aquatic pectoral n of
elpistostegalians. Elpistostege is potentially the sister taxon of all other tetrapods,
and its appendages further blur the line between sh 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 ana-
tomical transformations associated with locomotion
5,9–12
, breathing
13
,
hearing14 and feeding11,15, 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 sapiens17,18.
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 region1 anterior to the pelvic region are preserved.
Elpistostege watsoni was first described from a partial poste-
rior skull roof (accession code: British Museum of Natural History
(BMNH)P.50063) from the Escuminac Formation of Miguasha (Que-
bec) as a ‘stegocephalian’ amphibian4. A second incomplete anterior
half of a skull (accession code: Musée d’Histoire Naturelle de Migua-
sha (MHNM)06-538) was later described as an elpistostegalian fish3.
Until now, the postcranial anatomy of E.watsoni has been known
only from a small patch of articulated scales and vertebral elements
(MHNM06-537)
3
. The specimen of E.watsoni that we describe here
(MHNM06-2067) was discovered in 2010 from laminated bed12 in the
lower part of the Escuminac Formation. It is a complete individual that
is preserved flattened dorsoventrally, although the caudal region is
preserved in lateral view (Fig.1a, b). The dorsal side of the skull, trunk
and the pelvic fins, as well as the ventral side of the pectoral fins, have
been mechanically prepared. Ventral anatomical data are derived from
computed tomography scan images (Fig.2b, c, 3c). Compaction of other
fossil fishes found in similar laminated lithofacies of the Escuminac For-
mation varies between 50 and 83%
24
. As the skull and visceral skeleton
will be described elsewhere (R.C. etal., manuscript in preparation), we
provide only a few images to support the character coding used in our
phylogenetic analysis (Extended Data Figs.1b, 3).
To our knowledge, Elpistostege is now the sole elpistostegalian for
which we have complete knowledge of body shape and proportions.
Elpistostege has a short head, an elongated and slender trunk and rela-
tively short caudal region and small anal fin. The skull (Fig.1, Extended
Data Fig.1a, b) accounts for only 14.4% of the total length; this proportion
https://doi.org/10.1038/s41586-020-2100-8
Received: 30 July 2019
Accepted: 17 January 2020
Published online: 18 March 2020
Check for updates
1Université du Québec à Rimouski, Rimouski, Quebec, Canada. 2College of Science and Engineering, Flinders University, Adelaide, South Australia, Australia. 3Earth Sciences Section, South
Australian Museum, Adelaide, South Australia, Australia. e-mail: richard_cloutier@uqar.ca
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... For correct result, perfect body shaping should stand before biomechanical modeling. The specimens of Elpistostege (Cloutier et al. 2020) and Panderichthys from Tallinn (Boisvert 2005) apparently represent the natural, narrow shape of the body. In the Elpistostege specimen, only the head and shoulder girdle region are somewhat splayed out, while in the Tallinn Panderichthys only, the scaly skin in the mid-trunk region 2 is spread laterally and the tail is lost. ...
... A few not as much squashed specimens of Panderichthys and Elpistostege are available for control. The width and therefore (due to torpedo-like shape) the height of the body at the skull-to-shoulder girdle transition is 15 cm in Panderichthys (Tallinn specimen, studied by Boisvert 2005, preserves natural width of skull and shoulder girdle), and 17 cm in Elpistostege (studied by Cloutier et al. 2020). Both values approach but do not exceed the 20 cm body height theoretical limit deduced above for an abstract walking fish. ...
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Comparative study of terrestrial locomotion of four fish genera including Anabas, Channa, Clarias, and Monopterus, was performed in experimental setting with the substrate surface of wet clay. No special adaptations for terrestrial locomotion were found. Every fish uses for propulsion on land what it already has. Eel-shaped Monopterus crawls by body undulations in a serpentine or sidewinding technique, the latter of which was not previously observed beyond snakes. The other three fish genera walk by body oscillations using stiff appendages as propulsors. When they are located anteriorly, as the serrate operculum in Anabas and the preaxial spine of the pectoral fin in Clarias, the propulsion is termed prolocomotor, when posteriorly, as the spiny anal fin in Channa – metalocomotor. Channa is the heaviest fish walking out of water in our days, quite comparable in size with first Devonian tetrapods Acanthostega and Tulerpeton. A theoretical calculation is suggested for the upper size limit of a fish capable of terrestrial walking without special locomotor adaptations. It should be roughly 20 cm in the vertical dimension of the trunk which is just a little above the known size of Devonian tetrapodomorph fishes Panderichthys and Elpistostege. The metalocomotor walking technique of Channa is suggested as the closest extant model for terrestrial locomotion at the fish-tetrapod transition. The major difference is that the metalocomotor propulsor in Channa is represented by the anal fin, while in tetrapodomorphs by the pelvic fins. The sprawled pelvic fins were advantageous in respect of reduced requirement for side-to-side tail swinging. This article is protected by copyright. All rights reserved
... The cranial morphology of Palaeospondylus revealed in this study corresponds to the morphotype of tetrapodomorph crania (see Supplementary Information for detailed comparisons). To infer the phylogenetic position of Palaeospondylus, we performed analyses using a modified version of the character matrix in ref. 17 , with 202 characters coded for 43 taxa (Supplementary Information). Although most characters were necessarily inapplicable to Palaeospondylus, we were able to add 14 codings of character states for this species. ...
... Bayesian analyses were performed using MrBayes 3.2 18 , with the setting coded in the MrBayes script from ref. 17 . For each character matrix, the Bayesian Markov chain Monte Carlo analysis was run four times: each run employed four incrementally heated chains (temperature 0.1) and was 50 million steps long with sampling every 5,000th step. ...
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Palaeospondylus gunni, from the Middle Devonian period, is one of the most enigmatic fossil vertebrates, and its phylogenetic position has remained unclear since its discovery in Scotland in 1890 (ref. 1). The fossil’s strange set of morphological features has made comparisons with known vertebrate morphotype diversity difficult. Here we use synchrotron radiation X-ray micro-computed tomography to show that Palaeospondylus was a sarcopterygian, and most probably a stem-tetrapod. The skeleton of Palaeospondylus consisted solely of endoskeletal elements in which hypertrophied chondrocyte cell lacunae, osteoids and a small fraction of perichondral bones developed. Despite the complete lack of teeth and dermal bones, the neurocranium of Palaeospondylus resembles those of stem-tetrapod Eusthenopteron2 and Panderichthys3, and phylogenetic analyses place Palaeospondylus in between them. Because the unique features of Palaeospondylus, such as the cartilaginous skeleton and the absence of paired appendages, are present in the larva of crown tetrapods, our study highlights an unanticipated heterochronic evolution at the root of tetrapods. Detailed structural analysis of Palaeospondylus gunni from the Middle Devonian period shows strong resemblance to Eusthenopteron and Panderichthys, indicating that it was a sarcopterygian and most probably a stem-tetrapod.
... Essentially the same spiracular morphology as in these tetrapodomorphs, but accompanied by a shorter and wider space for the spiracular canal, is found in the elpistostegids Panderichthys ( Figure 7D) and Tiktaalik (Figure 6F; Brazeau and Ahlberg, 2006;Downs et al., 2008). Elpistostege is probably similar, though the internal part of the spiracular region is currently unknown (Cloutier et al., 2020). Given the shape of the elpistostegid skull, which suggests a surface-skimming lifestyle, it seems likely that the spiracle was used for air-breathing in a manner similar to Polypterus. ...
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... Limbs have a characteristic skeletal pattern of stylopod, zeugopod, and autopod, with a distal region composed of digits and mesopodial bones. While fossil taxa reveal intermediates in these conditions (1)(2)(3), the appendages of extant actinopterygians lack common features that allow comparison among the distal regions. Teleosts, for example, have fins with both endochondral and dermal bones. ...
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... Here, we present an invertebrate fossil from the Late Devonian Escuminac Formation (Miguasha, Quebec, Canada), a UNESCO world heritage site famous for its abundance of well-preserved vertebrate fossils including most major evolutionary groups of Devonian lower vertebrates from jawless fish to stem-tetrapods [29][30][31][32][33][34][35][36][37][38] . Based on morphological similarities of this Canadian fossil with stem-ctenophore fossils from the Cambrian Lagerstätte of the Chinese locality Chengjiang 7,39 , we assess its affinity to stem-group ctenophores ('dinomischids' , Siphusauctum, 'scleroctenophorans' 1,3,7,13 ) and early crown group ctenophores. ...
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