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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
Elpistostege and the origin of the vertebrate
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)
, while trackway fossils
showing digitate impressions of limbs suggest an earlier origin for this
. 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
, breathing
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
: 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
, the vertebrae
, the scale patterning
and very little on the pelvic fin and girdle morphology
. Although more
than 60specimens
of Tiktaalik have been found, most of the anatomy
of this species has been described from a fairly complete individual for
which the skull
, pectoral and pelvic fins and girdles
, scales
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
. 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%
. 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
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:
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... 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. ...
Full-text available
The spiracular region, comprising the hyomandibular pouch together with the mandibular and hyoid arches, has a complex evolutionary history. In living vertebrates, the embryonic hyomandibular pouch may disappear in the adult, develop into a small opening between the palatoquadrate and hyomandibula containing a single gill-like pseudobranch, or create a middle ear cavity, but it never develops into a fully formed gill with two hemibranchs. The belief that a complete spiracular gill must be the ancestral condition led some 20th century researchers to search for such a gill between the mandibular and hyoid arches in early jawed vertebrates. This hypothesized ancestral state was named the aphetohyoidean condition, but so far it has not been verified in any fossil; supposed examples, such as in the acanthodian Acanthodes and symmoriid chondrichthyans, have been reinterpreted and discounted. Here we present the first confirmed example of a complete spiracular gill in any vertebrate, in the galeaspid (jawless stem gnathostome) Shuyu. Comparisons with two other groups of jawless stem gnathostomes, osteostracans and heterostracans, indicate that they also probably possessed full-sized spiracular gills and that this condition may thus be primitive for the gnathostome stem group. This contrasts with the living jawless cyclostomes, in which the mandibular and hyoid arches are strongly modified and the hyomandibular pouch is lost in the adult. While no truly aphetohyoidean spiracular gill has been found in any jawed vertebrate, the recently reported presence in acanthodians of two pseudobranchs suggests a two-step evolutionary process whereby initial miniaturization of the spiracular gill was followed, independently in chondrichthyans and osteichthyans, by the loss of the anterior pseudobranch. On the basis of these findings we present an overview of spiracular evolution among vertebrates.
... 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. ...
Full-text available
One of the central problems of vertebrate evolution is understanding the relationship among the distal portions of fins and limbs. Lacking comparable morphological markers of these regions in fish and tetrapods, these relationships have remained uncertain for the past century and a half. Here we show that Gli3 functions in controlling the proliferative expansion of distal progenitors are shared among dorsal and paired fins as well as tetrapod limbs. Mutant knockout gli3 fins in medaka ( Oryzias latipes ) form multiple radials and rays, in a pattern reminiscent of the polydactyly observed in Gli3 -null mutant mice. In limbs, Gli3 controls both anterior–posterior patterning and cell proliferation, two processes that can be genetically uncoupled. In situ hybridization, quantification of proliferation markers, and analysis of regulatory regions reveal that in paired and dorsal fins, gli3 plays a main role in controlling proliferation but not in patterning. Moreover, gli3 down-regulation in shh mutant fins rescues fin loss in a manner similar to how Gli3 deficiency restores digits in the limbs of Shh mutant mouse embryos. We hypothesize that the Gli3/Shh gene pathway preceded the origin of paired appendages and was originally involved in modulating cell proliferation. Accordingly, the distal regions of dorsal fins, paired fins, and limbs retain a deep regulatory and functional homology that predates the origin of paired appendages.
... 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. ...
Full-text available
Like other soft-bodied organisms, ctenophores (comb jellies) produce fossils only under exceptional taphonomic conditions. Here, we present the first record of a Late Devonian ctenophore from the Escuminac Formation from Miguasha in eastern Canada. Based on the 18-fold symmetry of this disc-shaped fossil, we assign it to the total-group Ctenophora. Our phylogenetic analyses suggest that the new taxon Daihuoides jakobvintheri gen. et sp. nov. falls near Cambrian stem ctenophores such as 'dinomischids' and 'scleroctenophorans'. Accordingly, Daihuoides is a Lazarus-taxon, which postdates its older relatives by over 140 million years, and overlaps temporally with modern ctenophores, whose oldest representatives are known from the Early Devonian. Our analyses also indicate that the fossil record of ctenophores does not provide strong evidence for or against the phylogenomic hypothesis that ctenophores are sister to all other metazoans.
Background Mutations in the genes that encode the human γ-secretase subunits Presenilin-1, Presenilin Enhancer Protein 2, and Nicastrin (NCSTN) are associated with familial hidradenitis suppurativa (HS); and, regarding Presenilin Enhancer Protein 2, also with comorbidity for the hereditary pigmentation disorder Dowling-Degos disease. Objective Here, the consequences of targeted inactivation of ncstn, the zebrafish homologue of human NCSTN, were studied. Methods After morpholino (MO)-mediated ncstn-knockdown, the possibilities of phenotype rescue through co-injection of ncstn-MO with wildtype zebrafish ncstn or human NCSTN mRNA were investigated. Further, the effects of the co-injection of a human missense, nonsense, splice-site, and frameshift mutation were studied. Results MO-mediated ncstn-knockdown resulted in a significant reduction in melanophore morphology, size and number; and alterations in their patterns of migration and distribution. This phenotype was rescued by co-injection of zebrafish ncstn RNA, human NCSTN RNA, or a construct encoding the human NCSTN missense mutation p.P211R. Conclusion Human NCSTN mutations encoding null alleles confer loss-of-function regarding pigmentation homeostasis in zebrafisch. In contrast, the human missense mutation p.P211R was less harmful, asserting sufficient residual ncstn activity to maintain pigmentation in zebrafish. Since fish lack the anatomical structures affected by HS, our data suggest that the zebrafish ncstn gene and the human NCSTN gene have probably acquired different functions during evolution. In fish, one major role of ncstn is the maintenance of pigmentation homeostasis. In contrast, one of the roles of NCSTN in humans is the prevention of inflammatory processes in the adnexal structures of the skin, as seen in familial HS.
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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, Cladarosymblema, Ectosteorhachis, Mahalalepis, Megalichthys, Palatinichthys, and Sengoerichthys ). The position of the megalichthyids as sister group to canowindrids, crownward of “osteolepidids” (e.g., Osteolepis and Gogonasus ), but below “tristichopterids” such as Eusthenopteron is confirmed, but our findings suggest further work is required to resolve megalichthyid interrelationships.
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The evolution of tetrapod limbs from paired fins has long been of interest to both evolutionary and developmental biologists. Several recent investigative tracks have converged to restructure hypotheses in this area. First, there is now general agreement that the limb skeleton is patterned by one or more Turing-type reaction–diffusion, or reaction–diffusion–adhesion, mechanism that involves the dynamical breaking of spatial symmetry. Second, experimental studies in finned vertebrates, such as catshark and zebrafish, have disclosed unexpected correspondence between the development of digits and the development of both the endoskeleton and the dermal skeleton of fins. Finally, detailed mathematical models in conjunction with analyses of the evolution of putative Turing system components have permitted formulation of scenarios for the stepwise evolutionary origin of patterning networks in the tetrapod limb. The confluence of experimental and biological physics approaches in conjunction with deepening understanding of the developmental genetics of paired fins and limbs has moved the field closer to understanding the fin-to-limb transition. We indicate challenges posed by still unresolved issues of novelty, homology, and the relation between cell differentiation and pattern formation.
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The fossils assigned to the tetrapod stem group document the evolution of terrestrial vertebrates from lobe-finned fishes. During the past 18 years the phylogenetic structure of this stem group has remained remarkably stable, even when accommodating new discoveries such as the earliest known stem tetrapod Tungsenia and the elpistostegid (fish-tetrapod intermediate) Tiktaalik. Here we present a large lobe-finned fish from the Late Devonian period of China that disrupts this stability. It combines characteristics of rhizodont fishes (supposedly a basal branch in the stem group, distant from tetrapods) with derived elpistostegid-like and tetrapod-like characters. This mélange of characters may reflect either detailed convergence between rhizodonts and elpistostegids plus tetrapods, under a phylogenetic scenario deduced from Bayesian inference analysis, or a previously unrecognized close relationship between these groups, as supported by maximum parsimony analysis. In either case, the overall result reveals a substantial increase in homoplasy in the tetrapod stem group. It also suggests that ecological diversity and biogeographical provinciality in the tetrapod stem group have been underestimated.
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Understanding the evolutionary transformation of fish fins into tetrapod limbs is a fundamental problem in biology. The search for antecedents of tetrapod digits in fish has remained controversial because the distal skeletons of limbs and fins differ structurally, developmentally, and histologically. Moreover, comparisons of fins with limbs have been limited by a relative paucity of data on the cellular and molecular processes underlying the development of the fin skeleton. Here, we provide a functional analysis, using CRISPR/Cas9 and fate mapping, of 5' hox genes and enhancers in zebrafish that are indispensable for the development of the wrists and digits of tetrapods. We show that cells marked by the activity of an autopodial hoxa13 enhancer exclusively form elements of the fin fold, including the osteoblasts of the dermal rays. In hox13 knockout fish, we find that a marked reduction and loss of fin rays is associated with an increased number of endochondral distal radials. These discoveries reveal a cellular and genetic connection between the fin rays of fish and the digits of tetrapods and suggest that digits originated via the transition of distal cellular fates.
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Species composition of the genus Obruchevichthys Vorobyeva, 1977, previously based on two specimens from the Upper Frasnian (Upper Devonian) of Latvia and Leningrad Region of Russia is revised. The precise locality of the latter specimen was considered by the author of this taxon as unknown. Archives recently found in collection and field research in the presumable locality allowed the rediscovery of this important locality along the Sondala River (east of the Leningrad Region). This provenance is furthermore supported by spectrometric testing. Despite the presence of several subadult features hindering its attribution by earlier authors to a separate taxon, new observations on its morphology revealed that several characters of the material from Russia significantly differ from those of the type specimen that resulted in its attribution to a new genus and species Weberepeton sondalensis gen. et sp. nov. These differences mainly consist in a number of lower jaw characters, such as relative size of marginal teeth, development rate of the adsymphysial plate, orientation of the precoronoid fossa, and dermal ornamentation.
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There is no obvious morphological counterpart of the autopod (wrist/ankle and digits) in living fishes. Comparative molecular data may provide insight into understanding both the homology of elements and the evolutionary developmental mechanisms behind the fin to limb transition. In mouse limbs the autopod is built by a "late" phase of Hoxd and Hoxa gene expression, orchestrated by a set of enhancers located at the 5' end of each cluster. Despite a detailed mechanistic understanding of mouse limb development, interpretation of Hox expression patterns and their regulation in fish has spawned multiple hypotheses as to the origin and function of "autopod" enhancers throughout evolution. Using phylogenetic footprinting, epigenetic profiling, and transgenic reporters, we have identified and functionally characterized hoxD and hoxA enhancers in the genomes of zebrafish and the spotted gar, Lepisosteus oculatus, a fish lacking the whole genome duplication of teleosts. Gar and zebrafish "autopod" enhancers drive expression in the distal portion of developing zebrafish pectoral fins, and respond to the same functional cues as their murine orthologs. Moreover, gar enhancers drive reporter gene expression in both the wrist and digits of mouse embryos in patterns that are nearly indistinguishable from their murine counterparts. These functional genomic data support the hypothesis that the distal radials of bony fish are homologous to the wrist and/or digits of tetrapods.
Around 370 million years ago, a distant relative of a modern lungfish began a most extraordinary adventure-emerging from the water and laying claim to the land. Over the next 70 million years, this tentative beachhead had developed into a worldwide colonization by ever-increasing varieties of four-limbed creatures known as tetrapods, the ancestors of all vertebrate life on land. This new edition of Jennifer A. Clack's groundbreaking book tells the complex story of their emergence and evolution. Beginning with their closest relatives, the lobe-fin fishes such as lungfishes and coelacanths, Clack defines what a tetrapod is, describes their anatomy, and explains how they are related to other vertebrates. She looks at the Devonian environment in which they evolved, describes the known and newly discovered species, and explores the order and timing of anatomical changes that occurred during the fish-to-tetrapod transition.
The origin of tetrapods from their fish antecedents, approximately 400 million years ago, was coupled with the origin of terrestrial locomotion and the evolution of supporting limbs. Polypterus is a member of the basal-most group of ray-finned fish (actinopterygians) and has many plesiomorphic morphologies that are comparable to elpistostegid fishes, which are stem tetrapods. Polypterus therefore serves as an extant analogue of stem tetrapods, allowing us to examine how developmental plasticity affects the 'terrestrialization' of fish. We measured the developmental plasticity of anatomical and biomechanical responses in Polypterus reared on land. Here we show the remarkable correspondence between the environmentally induced phenotypes of terrestrialized Polypterus and the ancient anatomical changes in stem tetrapods, and we provide insight into stem tetrapod behavioural evolution. Our results raise the possibility that environmentally induced developmental plasticity facilitated the origin of the terrestrial traits that led to tetrapods.