Sarcopterygian Research Articles

A somitic contribution to the apical ectodermal ridge is essential for fin formation.

The transition from fins to limbs was an important terrestrial adaptation, but how this crucial evolutionary shift arose developmentally is unknown. Current models focus on the distinct roles of the apical ectodermal ridge (AER) and the signaling molecules that it secretes during limb and fin outgrowth. In contrast to the limb AER, the AER of the fin rapidly transitions into the apical fold and in the process shuts off AER-derived signals that stimulate proliferation of the precursors of the appendicular skeleton. The differing fates of the AER during fish and tetrapod development have led to the speculation that fin-fold formation was one of the evolutionary hurdles to the AER-dependent expansion of the fin mesenchyme required to generate the increased appendicular structure evident within limbs. Consequently, a heterochronic shift in the AER-to-apical-fold transition has been postulated to be crucial for limb evolution. The ability to test this model has been hampered by a lack of understanding of the mechanisms controlling apical fold induction. Here we show that invasion by cells of a newly identified somite-derived lineage into the AER in zebrafish regulates apical fold induction. Ablation of these cells inhibits apical fold formation, prolongs AER activity and increases the amount of fin bud mesenchyme, suggesting that these cells could provide the timing mechanism proposed in Thorogood's clock model of the fin-to-limb transition. We further demonstrate that apical-fold inducing cells are progressively lost during gnathostome evolution;the absence of such cells within the tetrapod limb suggests that their loss may have been a necessary prelude to the attainment of limb-like structures in Devonian sarcopterygian fish.

Morphology, Characterization and Distribution of Retinal Photoreceptors in the South American (Lepidosiren paradoxa) and Spotted African (Protopterus dolloi) Lungfishes

Lungfishes are the closest living relatives of the ancestors to all terrestrial vertebrates and have remained relatively unchanged since the early Lochkovin period (410 mya). Lungfishes, therefore, represent a critical stage in vertebrate evolution and their sensory neurobiology is of considerable interest. This study examines the ultrastructure of the retina of two species of lungfishes: the South American lungfish, Lepidosiren paradoxa and the spotted African lungfish, Protopterus dolloi in an attempt to assess variations in photoreception in these two ancient groups of sarcopterygian (lobe-finned) fishes. In juvenile P. dolloi, the retina contains one rod and two cone photoreceptor types (one containing a red oil droplet), while only one rod and one cone photoreceptor type is present in adult L. paradoxa. Both species lack double cones. The large size and inclusion of oil droplets in both species apart from one of the cone photoreceptor types in P. dolloi suggests that L. paradoxa and P. dolloi are adapted for increasing sensitivity. However, the complement of photoreceptor types suggests that there may be a major difference in the capacity to discriminate colour (dichromatic and monochromatic photoreception in P. dolloi and L. paradoxa, respectively). This study suggests that the visual needs of these two species may differ.

A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians.

Crown or modern sarcopterygians (coelacanths, lungfishes, and tetrapods) differ substantially from stem sarcopterygians, such as Guiyu and Psarolepis, and a lack of transitional fossil taxa limits our understanding of the origin of the crown group. The Onychodontiformes, an enigmatic Devonian predatory fish group, seems to have characteristics of both stem and crown sarcopterygians but is difficult to place because of insufficient anatomical information. We describe the new skull material of Qingmenodus, a Pragian (~409-million-year-old) onychodont from China, using high-resolution computed tomography to image internal structures of the braincase. In addition to its remarkable similarities with stem sarcopterygians in the ethmosphenoid portion, Qingmenodus exhibits coelacanth-like neurocranial features in the otic region. A phylogenetic analysis based on a revised data set unambiguously assigns onychodonts to crown sarcopterygians as stem coelacanths. Qingmenodus thus bridges the morphological gap between stem sarcopterygians and coelacanths and helps to illuminate the early evolution and diversification of crown sarcopterygians.

Corticotropin-releasing hormone family evolution: five ancestral genes remain in some lineages.

The evolution of the peptide family consisting of corticotropin-releasing hormone (CRH) and the three urocortins (UCN1-3) has been puzzling due to uneven evolutionary rates. Distinct gene duplication scenarios have been proposed in relation to the two basal rounds of vertebrate genome doubling (2R) and the teleost fish-specific genome doubling (3R). By analyses of sequences and chromosomal regions, including many neighboring gene families, we show here that the vertebrate progenitor had two peptide genes that served as the founders of separate subfamilies. Then, 2R resulted in a total of five members: one subfamily consists of CRH1, CRH2, and UCN1. The other subfamily contains UCN2 and UCN3. All five peptide genes are present in the slowly evolving genomes of the coelacanth Latimeria chalumnae (a lobe-finned fish), the spotted gar Lepisosteus oculatus (a basal ray-finned fish), and the elephant shark Callorhinchus milii (a cartilaginous fish). The CRH2 gene has been lost independently in placental mammals and in teleost fish, but is present in birds (except chicken), anole lizard, and the nonplacental mammals platypus and opossum. Teleost 3R resulted in an additional surviving duplicate only for crh1 in some teleosts including zebrafish (crh1a and crh1b). We have previously reported that the two vertebrate CRH/UCN receptors arose in 2R and that CRHR1 was duplicated in 3R. Thus, we can now conclude that this peptide-receptor system was quite complex in the ancestor of the jawed vertebrates with five CRH/UCN peptides and two receptors, and that crh and crhr1 were duplicated in the teleost fish tetraploidization.

The Oldest Actinopterygian Highlights the Cryptic Early History of the Hyperdiverse Ray-Finned Fishes

Osteichthyans comprise two divisions, each containing over 32,000 living species [1]: Sarcopterygii (lobe-finned fishes and tetrapods) and Actinopterygii (ray-finned fishes). Recent discoveries from China highlight the morphological disparity of early sarcopterygians and extend their origin into the late Silurian [2-4]. By contrast, the oldest unambiguous actinopterygians are roughly 30 million years younger, leaving a long temporal gap populated by fragments and rare body fossils of controversial phylogenetic placement [5-10]. Here we reinvestigate the enigmatic osteichthyan Meemannia from the Early Devonian (∼415 million years ago) of China, previously identified as an exceptionally primitive lobe-finned fish [3, 7, 11, 12]. Meemannia combines "cosmine"-like tissues taken as evidence of sarcopterygian affinity with actinopterygian-like skull roof and braincase geometry, including endoskeletal enclosure of the spiracle and a lateral cranial canal. We report comparable histological structures in undoubted ray-finned fishes and conclude that they are general osteichthyan features. Phylogenetic analysis places Meemannia as an early-diverging ray-finned fish, resolving it as the sister lineage of Cheirolepis [13] plus all younger actinopterygians. This brings the first appearance of ray-fins more in line with that of lobe-fins and fills a conspicuous faunal gap in the otherwise diverse late Silurian-earliest Devonian vertebrate faunas of the South China Block [4].

Immunohistochemical and Western Blotting Analyses of Ganoine in the Ganoid Scales of Lepisosteus oculatus: an Actinopterygian Fish.

In order to compare its characteristics with those of jaw tooth collar enamel, normally developing and experimentally regenerating ganoine from ganoid scales of Lepisosteus oculatus (spotted gar), an actinopterygian fish species, was examined by Western blotting and immunohistochemistry. Amelogenin, a major enamel matrix protein (EMP), is widely found from sarcopterygian fish to mammals. Therefore, we used antimammalian amelogenin antibodies and antisera: an antibody against bovine amelogenin; antiserum against porcine amelogenin; and region-specific antibodies or antiserum against the C-terminus, middle region, or N-terminus of porcine amelogenin in this study. Positive immunoreactivity with the antibody against bovine amelogenin, antiserum against porcine amelogenin, and the middle and C-terminal region-specific antibodies was detected in both normally developing and regenerating ganoine matrix, as well as in granules found within inner ganoine epithelial cells. These immunohistochemical analyses indicated that the Lepisosteus ganoine matrix contains EMP-like proteins with epitopes similar to mammalian amelogenins. In Western blotting analyses of regenerating ganoid scales with the antibovine amelogenin antibody, two protein bands with molecular weights of approximately 78 and 65 kDa were detected, which were similar to those found in Lepisosteus tooth enamel. Our study suggests that in Lepisosteus, EMP-like proteins in the ganoine matrix corresponded to those in tooth enamel. However, it was revealed that the 78 and 65 kDa EMP-like proteins were different from 27 kDa bovine amelogenin.

Resolving the Phylogenetic Position of Coelacanth: The Closest Relative Is Not Always the Most Appropriate Outgroup.

Determining the phylogenetic relationship of two extant lineages of lobe-finned fish, coelacanths and lungfishes, and tetrapods is important for understanding the origin of tetrapods. We analyzed data sets from two previous studies along with a newly collected data set, each of which had varying numbers of species and genes and varying extent of missing sites. We found that in all the data sets the sister relationship of lungfish and tetrapods was constructed with the use of cartilaginous fish as the outgroup with a high degree of statistical support. In contrast, when ray-finned fish were used as the outgroup, which is taxonomically an immediate outgroup of lobe-finned fish and tetrapods, the sister relationship of coelacanth and tetrapods was supported most strongly, although the statistical support was weaker. Even though it is generally accepted that the closest relative is an appropriate outgroup, our analysis suggested that the large divergence of the ray-finned fish as indicated by their long branch lengths and different amino acid frequencies made them less suitable as an outgroup than cartilaginous fish.

The Lungfish Transcriptome: A Glimpse into Molecular Evolution Events at the Transition from Water to Land.

Lungfish and coelacanths are the only living sarcopterygian fish. The phylogenetic relationship of lungfish to the last common ancestor of tetrapods and their close morphological similarity to their fossil ancestors make this species uniquely interesting. However their genome size, the largest among vertebrates, is hampering the generation of a whole genome sequence. To provide a partial solution to the problem, a high-coverage lungfish reference transcriptome was generated and assembled. The present findings indicate that lungfish, not coelacanths, are the closest relatives to land-adapted vertebrates. Whereas protein-coding genes evolve at a very slow rate, possibly reflecting a “living fossil” status, transposable elements appear to be active and show high diversity, suggesting a role for them in the remarkable expansion of the lungfish genome. Analyses of single genes and gene families documented changes connected to the water to land transition and demonstrated the value of the lungfish reference transcriptome for comparative studies of vertebrate evolution.

Tetrapod V1R-like ora genes in an early-diverging ray-finned fish species: the canonical six ora gene repertoire of teleost fish resulted from gene loss in a larger ancestral repertoire.

BackgroundChemical senses serve a multitude of essential functions across the animal kingdom. Vertebrates employ four GPCR families to detect odors, among them the v1r/ora gene family. The V1R family is known to evolve rapidly in the lobe-finned lineage giving rise to tetrapods, but the homologous ORA family consists of just six highly conserved genes in teleost fish, with direct orthologs in the lobe-finned fish coelacanth. Thus, the teleost repertoire of six canonical ora genes was assumed to be the ancestral feature before the divergence of ray-finned and lobe-finned fish. So far, this hypothesis has not been tested with earlier diverging ray-finned fish.ResultsWe have newly identified the complete ora gene repertoires of five teleost species, and of spotted gar, a basal ray-finned fish, using thorough data mining and extensive phylogenetic analysis. The genomes of eight further teleost species were re-analyzed for their ORA repertoires. We report that direct orthologs of the six canonical ora genes (ora1-6) were present in all newly analyzed species, with faithfully preserved exon/intron structure and mostly preserved genomic arrangement in symmetric pairs for ora1-4. In four teleost species including medaka and cave fish we observe species-specific gene duplication events. Thus, the ora gene repertoire in teleost fish is not quite as strictly conserved as previously assumed. In fact, the examination of non-synonymous vs. synonymous substitution rates (dN/dS) shows pronounced negative selection in five of the six ora genes, but also rare occurrence of positive selection in ora3 and ora6. Surprisingly, spotted gar possesses beyond the six canonical genes three additional genes, ora7-8b, orthologous to coelacanth genes v1r07-10. No orthologs for these genes were found in teleosts and cartilaginous fish.ConclusionsEarly diverging ray-finned fish such as the spotted gar possess several v1r-like genes previously assumed to be restricted to the lobe-finned lineage, but now found to be already present in the most recent common ancestor of lobe- and ray-finned fish. Thus, the presence of just six canonical ora genes in many teleost species is not the ancestral feature of the ray-finned lineage, but caused by loss of two ancestral genes in teleosts.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2399-6) contains supplementary material, which is available to authorized users.

Vertebrate paleontological exploration of the Upper Cretaceous succession in the Dakhla and Kharga Oases, Western Desert, Egypt

The Campanian and Maastrichtian stages are very poorly documented time intervals in Africa's record of terrestrial vertebrate evolution. Upper Cretaceous deposits exposed in southern Egypt, near the Dakhla and Kharga Oases in the Western Desert, preserve abundant vertebrate fossils in nearshore marine environments, but have not yet been the focus of intensive collection and description. Our recent paleontological work in these areas has resulted in the discovery of numerous new vertebrate fossil-bearing localities within the middle Campanian Qusier Formation and the upper Campanian-lower Maastrichtian Duwi Formation. Fossil remains recovered from the Campanian-aged Quseir Formation include sharks, rays, actinopterygian and sarcopterygian fishes, turtles, and rare terrestrial archosaurians, including some of the only dinosaurs known from this interval on continental Africa. The upper Campanian/lower Maastrichtian Duwi Formation preserves sharks, sawfish, actinopterygians, and marine reptiles (mosasaurs and plesiosaurs). Notably absent from these collections are representatives of Mammalia and Avialae, both of which remain effectively undocumented in the Upper Cretaceous rocks of Africa and Arabia. New age constraints on the examined rock units is provided by 23 nannofossil taxa, some of which are reported from the Duwi Formation for the first time. Fossil discoveries from rock units of this age are essential for characterizing the degree of endemism that may have developed as the continent became increasingly tectonically isolated from the rest of Gondwana, not to mention for fully evaluating origin and diversification hypotheses of major modern groups of vertebrates (e.g., crown birds, placental mammals).

Bioelectric fields of the African marbled lungfish Protopterus aethiopicus (Sarcopterygii: Protopteridae), African (Heterotis niloticus) and South American silver (Osteoglossum bicirrhosum) arowanas (Actinopterygii: Osteoglossidae): Primitive electrogenesis?

Sporadic weak electric discharges were recorded from the African marbled lungfish (Protopterus aethiopicus), African arowana (Heterotis niloticus) and South American silver arowana (Osteoglossum bicirrhosum). In all species, the discharges were registered from a pair of freely swimming individuals left for several hours in an experimental tank, or evoked by tactile stimulation in the lungfish and the African arowana. Maximum amplitudes of recoded pulses ranging from 1.5 mV in Osteoglossum to 10 mV in Protopterus and Heterotis, as well as the coordinated discharge patterns, allow us to suggest the existence of anatomically and physiologically specialized electrogenic tissue in these species. The phylogenetic and evolutionary significance of primitive electrogenesis in the lobe-finned and bony-tongue fishes is discussed.

On the features of embryonic cleavage in diverse fish species

Literature on the earliest steps of fish embryogenesis (including a number of "non-model" species) has been considered. The main attention has been paid to the loss of cleavage division synchrony and the first latitudinal cleavage furrow. In teleostean embryos, the features of their meroblastic cleavage are not rigidly associated with egg size. The midblastula transition (in a form clearly enough) occurs in some chondrostean and teleostean fishes, but it has not been detected in the representatives of sarcopterygian and chondrichthyan fishes.

New genomic and fossil data illuminate the origin of enamel.

Enamel, the hardest vertebrate tissue, covers the teeth of almost all sarcopterygians (lobe-finned bony fishes and tetrapods) as well as the scales and dermal bones of many fossil lobe-fins. Enamel deposition requires an organic matrix containing the unique enamel matrix proteins (EMPs) amelogenin (AMEL), enamelin (ENAM) and ameloblastin (AMBN). Chondrichthyans (cartilaginous fishes) lack both enamel and EMP genes. Many fossil and a few living non-teleost actinopterygians (ray-finned bony fishes) such as the gar, Lepisosteus, have scales and dermal bones covered with a proposed enamel homologue called ganoine. However, no gene or transcript data for EMPs have been described from actinopterygians. Here we show that Psarolepis romeri, a bony fish from the the Early Devonian period, combines enamel-covered dermal odontodes on scales and skull bones with teeth of naked dentine, and that Lepisosteus oculatus (the spotted gar) has enam and ambn genes that are expressed in the skin, probably associated with ganoine formation. The genetic evidence strengthens the hypothesis that ganoine is homologous with enamel. The fossil evidence, further supported by the Silurian bony fish Andreolepis, which has enamel-covered scales but teeth and odontodes on its dermal bones made of naked dentine, indicates that this tissue originated on the dermal skeleton, probably on the scales. It subsequently underwent heterotopic expansion across two highly conserved patterning boundaries (scales/head-shoulder and dermal/oral) within the odontode skeleton.

Allometric growth in the extant coelacanth lung during ontogenetic development.

Coelacanths are lobe-finned fishes known from the Devonian to Recent that were long considered extinct, until the discovery of two living species in deep marine waters of the Mozambique Channel and Sulawesi. Despite extensive studies, the pulmonary system of extant coelacanths has not been fully investigated. Here we confirm the presence of a lung and discuss its allometric growth in Latimeria chalumnae, based on a unique ontogenetic series. Our results demonstrate the presence of a potentially functional, well-developed lung in the earliest known coelacanth embryo, and its arrested growth at later ontogenetic stages, when the lung is clearly vestigial. The parallel development of a fatty organ for buoyancy control suggests a unique adaptation to deep-water environments. Furthermore, we provide the first evidence for the presence of small, hard, flexible plates around the lung in L. chalumnae, and consider them homologous to the plates of the ‘calcified lung' of fossil coelacanths.

African Lungfish Reveal the Evolutionary Origins of Organized Mucosal Lymphoid Tissue in Vertebrates

One of the most remarkable innovations of the vertebrate adaptive immune system is the progressive organization of the lymphoid tissues that leads to increased efficiency of immune surveillance and cell interactions. The mucosal immune system of endotherms has evolved organized secondary mucosal lymphoid tissues (O-MALT) such as Peyer's patches, tonsils, and adenoids. Primitive semi-organized lymphoid nodules or aggregates (LAs) were found in the mucosa of anuran amphibians, suggesting that O-MALT evolved from amphibian LAs ∼250 million years ago. This study shows for the first time the presence of O-MALT in the mucosa of the African lungfish, an extant representative of the closest ancestral lineage to all tetrapods. Lungfish LAs are lymphocyte-rich structures associated with a modified covering epithelium and express all IGH genes except for IGHW2L. In response to infection, nasal LAs doubled their size and increased the expression of CD3 and IGH transcripts. Additionally, de novo organogenesis of inducible LAs resembling mammalian tertiary lymphoid structures was observed. Using deep-sequencing transcriptomes, we identified several members of the tumor necrosis factor (TNF) superfamily, and subsequent phylogenetic analyses revealed its extraordinary diversification within sarcopterygian fish. Attempts to find AICDA in lungfish transcriptomes or by RT-PCR failed, indicating the possible absence of somatic hypermutation in lungfish LAs. These findings collectively suggest that the origin of O-MALT predates the emergence of tetrapods and that TNF family members play a conserved role in the organization of vertebrate mucosal lymphoid organs.

Stratigraphy and depositional environments of the Late Famennian (Late Devonian) of Southern Belgium and characterization of the Strud locality

AbstractThe Famennian (Upper Devonian,c. 372 to 359 Ma) strata of Belgium have recently received much attention after the discoveries of early tetrapod remains and outstandingly preserved continental arthropods. The Strud locality has yielded a diverse flora and fauna including seed-plants, tetrapods, various placoderm, actinopterygian, acanthodian and sarcopterygian fishes, crustaceans (anostracans, notostracans, conchostracans and decapods) and a putative complete insect. This fossil assemblage is one of the oldest continental – probably fresh-water – ecosystems with a considerable vertebrate and invertebrate diversity. The study of the palaeoenvironment of the Strud locality is crucial because it records one of the earliest and most important phases of tetrapod evolution that took place after their emergence but before their terrestrialization. It raises the question of environmental and ecological conditions for the Devonian aquatic ecosystem and the selection pressures occurring at the onset of tetrapod terrestrialization. The present study characterized the fluvial facies of the Upper Famennian sedimentary rocks of Strud and the surrounding areas. The exceptional preservation of arthropods and plants in the main fossiliferous layers is explained by rapid burial in the fine-grained sediment of the quiet and confined flood plain environment. Newly investigated fossiliferous sections in the Meuse–Samson area led to the description and correlation of key sections (Strud, Wierde and Jausse sections, complemented by the less continuous Haltinne, Huy and Coutisse sections). Moreover, the investigated sections allowed a review of the age of the fossiliferous horizon, which is now definitely considered to be Late Famennian in age.

Evolution of the Vertebrate Resistin Gene Family.

Resistin (encoded by Retn) was previously identified in rodents as a hormone associated with diabetes; however human resistin is instead linked to inflammation. Resistin is a member of a small gene family that includes the resistin-like peptides (encoded by Retnl genes) in mammals. Genomic searches of available genome sequences of diverse vertebrates and phylogenetic analyses were conducted to determine the size and origin of the resistin-like gene family. Genes encoding peptides similar to resistin were found in Mammalia, Sauria, Amphibia, and Actinistia (coelacanth, a lobe-finned fish), but not in Aves or fish from Actinopterygii, Chondrichthyes, or Agnatha. Retnl originated by duplication and transposition from Retn on the early mammalian lineage after divergence of the platypus, but before the placental and marsupial mammal divergence. The resistin-like gene family illustrates an instance where the locus of origin of duplicated genes can be identified, with Retn continuing to reside at this location. Mammalian species typically have a single copy Retn gene, but are much more variable in their numbers of Retnl genes, ranging from 0 to 9. Since Retn is located at the locus of origin, thus likely retained the ancestral expression pattern, largely maintained its copy number, and did not display accelerated evolution, we suggest that it is more likely to have maintained an ancestral function, while Retnl, which transposed to a new location, displays accelerated evolution, and shows greater variability in gene number, including gene loss, likely evolved new, but potentially lineage-specific, functions.

Evolution of urea transporters in vertebrates: adaptation to urea's multiple roles and metabolic sources.

In the two decades since the first cloning of the mammalian kidney urea transporter (UT-A), UT genes have been identified in a plethora of organisms, ranging from single-celled bacteria to metazoans. In this review, focusing mainly on vertebrates, we first reiterate the multiple catabolic and anabolic pathways that produce urea, then we reconstruct the phylogenetic history of UTs, and finally we examine the tissue distribution of UTs in selected vertebrate species. Our analysis reveals that from an ancestral UT, three homologues evolved in piscine lineages (UT-A, UT-C and UT-D), followed by a subsequent reduction to a single UT-A in lobe-finned fish and amphibians. A later internal tandem duplication of UT-A occurred in the amniote lineage (UT-A1), followed by a second tandem duplication in mammals to give rise to UT-B. While the expected UT expression is evident in excretory and osmoregulatory tissues in ureotelic taxa, UTs are also expressed ubiquitously in non-ureotelic taxa, and in tissues without a complete ornithine-urea cycle (OUC). We posit that non-OUC production of urea from arginine by arginase, an important pathway to generate ornithine for synthesis of molecules such as polyamines for highly proliferative tissues (e.g. testis, embryos), and neurotransmitters such as glutamate for neural tissues, is an important evolutionary driving force for the expression of UTs in these taxa and tissues.

Polypterus and the evolution of fish pectoral musculature.

Polypterus, a member of the most primitive living group of ray-finned fishes, has demonstrated the ability to perform fin-assisted terrestrial locomotion, a behavior that indicates a complex pectoral musculoskeletal system. Review of the literature reveals that many aspects of the pectoral muscular anatomy of Polypterus are still unclear, with a number of conflicting descriptions. We provide a new interpretation of the pectoral musculature using soft tissue-enhanced microCT scanning and gross anatomical dissection. The results demonstrate a complex musculature, with six independent muscles crossing the glenoid-fin joint. Comparisons with other bony-fish (Osteichthyes), including both ray-finned (Actinopterygii) and lobed-fin (Sarcopterygii) fish, indicate the presence of novel muscles within Polypterus: coracometapterygialis I+II and the zonopropterygialis medialis. Examination of these muscular additions in the context of osteichthyan phylogeny indicates that this represents a previously unrecognized event in the evolution of pectoral musculature in Osteichthyes. Despite its phylogenetic position as a basal actinopterygian, the musculature of Polypterus has more similarities both anatomically and functionally with that of sarcopterygians. This anatomy, along with other features of Polypterus anatomy such as lobed fins, ventral paired lungs, and a large spiracle, may make it a good model for inferences of stem tetrapod locomotion.

First occurrence of a mawsoniid coelacanth in the Early Jurassic of Europe

ABSTRACT—Coelacanths form a clade of sarcopterygian fishes (lobe-finned vertebrates) that today is represented by a single genus, Latimeria. This genus belongs to a lineage of marine coelacanths, the latimeriids, whose fossils are common in the Jurassic and the Cretaceous deposits of Europe and North America. During the same periods, another lineage of fresh/brackish water coelacanths, the mawsoniids, occurred in South America, Africa, and Madagascar. Mawsoniids are supposed to have originated during the Triassic in North America and were assumed to have subsequently dispersed to South America during the Jurassic, before reaching western Africa during the Early Cretaceous. Previous hypotheses advocated that mawsoniid coelacanths reached Europe during the Late Cretaceous, suggesting the dispersal of freshwater organisms from Africa to Europe during this period. We here reevaluate this scenario based on the reexamination of the coelacanth Trachymetopon from the Early Jurassic of Germany. Although this genus is known from remarkably well-preserved material, its relationships to other Mesozoic coelacanths remained unsolved. An anatomical investigation shows that Trachymetopon shares many common features with the Late Jurassic–Late Cretaceous mawsoniids Mawsonia and Axelrodichthys from western Gondwana, such as the absence of a descending process of the supratemporal, the presence of ossified ribs, and the skull roof and cheek bones ornamented by conspicuous coarse rugosities and ridges. A phylogenetic analysis of morphological characters places Trachymetopon within Mawsoniidae. We suggest that mawsoniid coelacanths were already present in Europe from the Early Jurassic onwards, challenging previous paleobiogeographic scenarios.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP