35 results on '"Ahlberg PE"'
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2. The Bothriolepis (Placodermi, Antiarcha) material from the Valentia Slate Formation of the Iveragh Peninsula (middle Givetian, Ireland): Morphology, evolutionary and systematic considerations, phylogenetic and palaeogeographic implications.
- Author
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Dupret V, Byrne HM, Castro N, Hammer Ø, Higgs KT, Long JA, Niedźwiedzki G, Qvarnström M, Stössel I, and Ahlberg PE
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- Animals, Phylogeny, Ireland, China, Biological Evolution, Skull
- Abstract
Material of the antiarch placoderm Bothriolepis from the middle Givetian of the Valentia Slate Formation in Iveragh Peninsula, Ireland, is described and attributed to a new species, B. dairbhrensis sp. nov. A revision of the genus Bothriolepis is proposed, and its taxonomic content and previous phylogenetic analyses are reviewed, as well as the validity of morphologic characteristics considered important for the establishment of the genus, such as the shape of the preorbital recess of the neurocranium. A series of computerised phylogenetic analyses was performed, which reveals that our new species is the sister taxon to the Frasnian Scottish form B. gigantea. New phylogenetic and biogeographic analyses of the genus Bothriolepis together with comparisons between faunal assemblages reveal a first northward dispersal wave from Gondwana to Euramerica at the latest in the mid Givetian. Other Euramerican species of Bothriolepis seem to belong to later dispersal waves from Gondwana, non-excluding southward waves from Euramerica. Questions remain open such as the taxonomic validity and stratigraphic constraints for the most ancient forms of Bothriolepis in China, and around the highly speciose nature of the genus., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Dupret et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)
- Published
- 2023
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3. Exceptional preservation of organs in Devonian placoderms from the Gogo lagerstätte.
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Trinajstic K, Long JA, Sanchez S, Boisvert CA, Snitting D, Tafforeau P, Dupret V, Clement AM, Currie PD, Roelofs B, Bevitt JJ, Lee MSY, and Ahlberg PE
- Subjects
- Animals, Jaw anatomy & histology, Phylogeny, Western Australia, Biological Evolution, Catfishes anatomy & histology, Catfishes classification, Fossils
- Abstract
The origin and early diversification of jawed vertebrates involved major changes to skeletal and soft anatomy. Skeletal transformations can be examined directly by studying fossil stem gnathostomes; however, preservation of soft anatomy is rare. We describe the only known example of a three-dimensionally mineralized heart, thick-walled stomach, and bilobed liver from arthrodire placoderms, stem gnathostomes from the Late Devonian Gogo Formation in Western Australia. The application of synchrotron and neutron microtomography to this material shows evidence of a flat S-shaped heart, which is well separated from the liver and other abdominal organs, and the absence of lungs. Arthrodires thus show the earliest phylogenetic evidence for repositioning of the gnathostome heart associated with the evolution of the complex neck region in jawed vertebrates.
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- 2022
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4. Morphometric analysis of lungfish endocasts elucidates early dipnoan palaeoneurological evolution.
- Author
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Clement AM, Challands TJ, Cloutier R, Houle L, Ahlberg PE, Collin SP, and Long JA
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- Animals, Brain diagnostic imaging, Fishes, Paleontology, Skull anatomy & histology, Skull diagnostic imaging, Biological Evolution, Fossils
- Abstract
The lobe-finned fish, lungfish (Dipnoi, Sarcoptergii), have persisted for ~400 million years from the Devonian Period to present day. The evolution of their dermal skull and dentition is relatively well understood, but this is not the case for the central nervous system. While the brain has poor preservation potential and is not currently known in any fossil lungfish, substantial indirect information about it and associated structures (e.g. labyrinths) can be obtained from the cranial endocast. However, before the recent development of X-ray tomography as a palaeontological tool, these endocasts could not be studied non-destructively, and few detailed studies were undertaken. Here, we describe and illustrate the endocasts of six Palaeozoic lungfish from tomographic scans. We combine these with six previously described digital lungfish endocasts (4 fossil and 2 recent taxa) into a 12-taxon dataset for multivariate morphometric analysis using 17 variables. We find that the olfactory region is more highly plastic than the hindbrain, and undergoes significant elongation in several taxa. Further, while the semicircular canals covary as an integrated module, the utriculus and sacculus vary independently of each other. Functional interpretation suggests that olfaction has remained a dominant sense throughout lungfish evolution, and changes in the labyrinth may potentially reflect a change from nektonic to near-shore environmental niches. Phylogenetic implications show that endocranial form fails to support monophyly of the 'chirodipterids'. Those with elongated crania similarly fail to form a distinct clade, suggesting these two paraphyletic groups have converged towards either head elongation or truncation driven by non-phylogenetic constraints., Competing Interests: AC, TC, RC, LH, PA, SC, JL No competing interests declared, (© 2022, Clement et al.)
- Published
- 2022
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5. Tooth morphology elucidates shark evolution across the end-Cretaceous mass extinction.
- Author
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Bazzi M, Campione NE, Ahlberg PE, Blom H, and Kear BP
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- Animals, Ecosystem, Biological Evolution, Extinction, Biological, Fossils anatomy & histology, Sharks anatomy & histology, Tooth anatomy & histology
- Abstract
Sharks (Selachimorpha) are iconic marine predators that have survived multiple mass extinctions over geologic time. Their prolific fossil record is represented mainly by isolated shed teeth, which provide the basis for reconstructing deep time diversity changes affecting different selachimorph clades. By contrast, corresponding shifts in shark ecology, as measured through morphological disparity, have received comparatively limited analytical attention. Here, we use a geometric morphometric approach to comprehensively examine tooth morphologies in multiple shark lineages traversing the catastrophic end-Cretaceous mass extinction-this event terminated the Mesozoic Era 66 million years ago. Our results show that selachimorphs maintained virtually static levels of dental disparity in most of their constituent clades across the Cretaceous-Paleogene interval. Nevertheless, selective extinctions did impact apex predator species characterized by triangular blade-like teeth. This is particularly evident among lamniforms, which included the dominant Cretaceous anacoracids. Conversely, other groups, such as carcharhiniforms and orectolobiforms, experienced disparity modifications, while heterodontiforms, hexanchiforms, squaliforms, squatiniforms, and †synechodontiforms were not overtly affected. Finally, while some lamniform lineages disappeared, others underwent postextinction disparity increases, especially odontaspidids, which are typified by narrow-cusped teeth adapted for feeding on fishes. Notably, this increase coincides with the early Paleogene radiation of teleosts as a possible prey source, and the geographic relocation of disparity sampling "hotspots," perhaps indicating a regionally disjunct extinction recovery. Ultimately, our study reveals a complex morphological response to the end-Cretaceous mass extinction and highlights an event that influenced the evolution of modern sharks., Competing Interests: The authors have declared that no competing interests exist.
- Published
- 2021
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6. The developmental relationship between teeth and dermal odontodes in the most primitive bony fish Lophosteus .
- Author
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Chen D, Blom H, Sanchez S, Tafforeau P, Märss T, and Ahlberg PE
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- Animals, Phylogeny, Skin diagnostic imaging, Synchrotrons, Tooth diagnostic imaging, X-Ray Microtomography, Biological Evolution, Dentition, Fishes anatomy & histology, Fossils, Odontogenesis, Skin anatomy & histology, Tooth anatomy & histology
- Abstract
The ontogenetic trajectory of a marginal jawbone of Lophosteus superbus (Late Silurian, 422 Million years old), the phylogenetically most basal stem osteichthyan, visualized by synchrotron microtomography, reveals a developmental relationship between teeth and dermal odontodes that is not evident from the adult morphology. The earliest odontodes are two longitudinal founder ridges formed at the ossification center. Subsequent odontodes that are added lingually to the ridges turn into conical teeth and undergo cyclic replacement, while those added labially achieve a stellate appearance. Stellate odontodes deposited directly on the bony plate are aligned with the alternate files of teeth, whereas new tooth positions are inserted into the files of sequential addition when a gap appears. Successive teeth and overgrowing odontodes show hybrid morphologies around the oral-dermal boundary, suggesting signal cross-communication. We propose that teeth and dermal odontodes are modifications of a single system, regulated and differentiated by the oral and dermal epithelia., Competing Interests: DC, HB, SS, PT, TM, PA No competing interests declared, (© 2020, Chen et al.)
- Published
- 2020
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7. Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates.
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Vaškaninová V, Chen D, Tafforeau P, Johanson Z, Ekrt B, Blom H, and Ahlberg PE
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- Animals, Czech Republic, Electron Microscope Tomography, Fossils, Phylogeny, Synchrotrons, Tooth anatomy & histology, Biological Evolution, Dentition, Jaw anatomy & histology, Vertebrates anatomy & histology, Vertebrates classification
- Abstract
The dentitions of extant fishes and land vertebrates vary in both pattern and type of tooth replacement. It has been argued that the common ancestral condition likely resembles the nonmarginal, radially arranged tooth files of arthrodires, an early group of armoured fishes. We used synchrotron microtomography to describe the fossil dentitions of so-called acanthothoracids, the most phylogenetically basal jawed vertebrates with teeth, belonging to the genera Radotina , Kosoraspis , and Tlamaspis (from the Early Devonian of the Czech Republic). Their dentitions differ fundamentally from those of arthrodires; they are marginal, carried by a cheekbone or a series of short dermal bones along the jaw edges, and teeth are added lingually as is the case in many chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). We propose these characteristics as ancestral for all jawed vertebrates., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)
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- 2020
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8. Tyrannosaurid-like osteophagy by a Triassic archosaur.
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Qvarnström M, Ahlberg PE, and Niedźwiedzki G
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- Animals, Poland, Biological Evolution, Dinosaurs anatomy & histology, Dinosaurs classification, Dinosaurs physiology, Feeding Behavior physiology, Fossils
- Abstract
Here we present evidence for osteophagy in the Late Triassic archosaur Smok wawelski Niedźwiedzki, Sulej and Dzik, 2012, a large theropod-like predator from Poland. Ten medium to large-sized coprolites are matched, by their dimensions and by association with body fossils and footprints, to S. wawelski. The coprolites contain fragments of large serrated teeth as well as up to 50 percent by volume of bone fragments, with distinct fragmentation and angularity, from several prey taxa. This suggests pronounced osteophagy. Further evidence for bone-crushing behaviour is provided by isolated worn teeth, bone-rich regurgitalites (fossil regurgitates) and numerous examples of crushed or bite-marked dicynodont bones, all collected from the same bone-bearing beds in the Lipie Śląskie clay-pit. Several of the anatomical characters related to osteophagy, such as a massive head and robust body, seem to be shared by S. wawelski and the tyrannosaurids, despite their wide phylogenetic separation. These large predators thus provide evidence of convergence driven by similar feeding ecology at the beginning and end of the age of dinosaurs.
- Published
- 2019
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9. Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur.
- Author
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Lindgren J, Sjövall P, Thiel V, Zheng W, Ito S, Wakamatsu K, Hauff R, Kear BP, Engdahl A, Alwmark C, Eriksson ME, Jarenmark M, Sachs S, Ahlberg PE, Marone F, Kuriyama T, Gustafsson O, Malmberg P, Thomen A, Rodríguez-Meizoso I, Uvdal P, Ojika M, and Schweitzer MH
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- Adaptation, Physiological, Adipose Tissue anatomy & histology, Adipose Tissue chemistry, Animals, Dermis anatomy & histology, Dermis chemistry, Dolphins, Epidermis anatomy & histology, Epidermis chemistry, Female, Keratinocytes chemistry, Lipids analysis, Male, Melanins analysis, Melanophores chemistry, Porpoises, Proteins analysis, Biological Evolution, Body Temperature Regulation, Dinosaurs anatomy & histology, Dinosaurs physiology, Fossils, Homeostasis
- Abstract
Ichthyosaurs are extinct marine reptiles that display a notable external similarity to modern toothed whales. Here we show that this resemblance is more than skin deep. We apply a multidisciplinary experimental approach to characterize the cellular and molecular composition of integumental tissues in an exceptionally preserved specimen of the Early Jurassic ichthyosaur Stenopterygius. Our analyses recovered still-flexible remnants of the original scaleless skin, which comprises morphologically distinct epidermal and dermal layers. These are underlain by insulating blubber that would have augmented streamlining, buoyancy and homeothermy. Additionally, we identify endogenous proteinaceous and lipid constituents, together with keratinocytes and branched melanophores that contain eumelanin pigment. Distributional variation of melanophores across the body suggests countershading, possibly enhanced by physiological adjustments of colour to enable photoprotection, concealment and/or thermoregulation. Convergence of ichthyosaurs with extant marine amniotes thus extends to the ultrastructural and molecular levels, reflecting the omnipresent constraints of their shared adaptation to pelagic life.
- Published
- 2018
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10. A tetrapod fauna from within the Devonian Antarctic Circle.
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Gess R and Ahlberg PE
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- Animals, Antarctic Regions, Fossils, Jaw anatomy & histology, South Africa, Biological Evolution, Vertebrates anatomy & histology
- Abstract
Until now, all known fossils of tetrapods (limbed vertebrates with digits) and near-tetrapods (such as Elpistostege , Tiktaalik , and Panderichthys ) from the Devonian period have come from localities in tropical to subtropical paleolatitudes. Most are from Laurussia, a continent incorporating Europe, Greenland, and North America, with only one body fossil and one footprint locality from Australia representing the southern supercontinent Gondwana. Here we describe two previously unknown tetrapods from the Late Devonian (late Famennian) Gondwana locality of Waterloo Farm in South Africa, then located within the Antarctic Circle, which demonstrate that Devonian tetrapods were not restricted to warm environments and suggest that they may have been global in distribution., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)
- Published
- 2018
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11. Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution.
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Clement AM, King B, Giles S, Choo B, Ahlberg PE, Young GC, and Long JA
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- Animals, Australia, Bayes Theorem, Ear, Inner anatomy & histology, Ear, Inner diagnostic imaging, Ear, Inner physiology, Extinction, Biological, Fishes classification, Fishes physiology, Fossils diagnostic imaging, Fossils history, History, Ancient, Skull diagnostic imaging, Skull physiology, X-Ray Microtomography, Biological Evolution, Fishes anatomy & histology, Fossils anatomy & histology, Phylogeny, Skull anatomy & histology
- Abstract
The skull of ' Ligulalepis ' from the Early Devonian of Australia (AM-F101607) has significantly expanded our knowledge of early osteichthyan anatomy, but its phylogenetic position has remained uncertain. We herein describe a second skull of ' Ligulalepis ' and present micro-CT data on both specimens to reveal novel anatomical features, including cranial endocasts. Several features previously considered to link ' Ligulalepis' with actinopterygians are now considered generalized osteichthyan characters or of uncertain polarity. The presence of a lateral cranial canal is shown to be variable in its development between specimens. Other notable new features include the presence of a pineal foramen, the some detail of skull roof sutures, the shape of the nasal capsules, a placoderm-like hypophysial vein, and a chondrichthyan-like labyrinth system. New phylogenetic analyses place ' Ligulalepis ' as a stem osteichthyan, specifically as the sister taxon to 'psarolepids' plus crown osteichthyans. The precise position of 'psarolepids' differs between parsimony and Bayesian analyses., Competing Interests: AC No competing interests declared, (© 2018, Clement et al.)
- Published
- 2018
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12. Non-marine palaeoenvironment associated to the earliest tetrapod tracks.
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Qvarnström M, Szrek P, Ahlberg PE, and Niedźwiedzki G
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- Animals, Paleontology, Biological Evolution, Fossils, Vertebrates
- Abstract
Opinions differ on whether the evolution of tetrapods (limbed vertebrates) from lobe-finned fishes was directly linked to terrestrialization. The earliest known tetrapod fossils, from the Middle Devonian (approximately 390 million years old) of Zachełmie Quarry in Poland, are trackways made by limbs with digits; they document a direct environmental association and thus have the potential to help answer this question. However, the tetrapod identity of the tracks has recently been challenged, despite their well-preserved morphology, on account of their great age and supposedly shallow marine (intertidal or lagoonal) depositional environment. Here we present a new palaeoenvironmental interpretation of the track-bearing interval from Zachełmie, showing that it represents a succession of ephemeral lakes with a restricted and non-marine biota, rather than a marginal marine environment as originally thought. This context suggests that the trackmaker was capable of terrestrial locomotion, consistent with the appendage morphology recorded by the footprints, and thus provides additional support for a tetrapod identification.
- Published
- 2018
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13. A Devonian tetrapod-like fish reveals substantial parallelism in stem tetrapod evolution.
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Zhu M, Ahlberg PE, Zhao WJ, and Jia LT
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- Animals, Bayes Theorem, Phylogeny, Biological Evolution, Fishes anatomy & histology, Fishes classification
- Abstract
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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- 2017
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14. The origin of novel features by changes in developmental mechanisms: ontogeny and three-dimensional microanatomy of polyodontode scales of two early osteichthyans.
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Qu Q, Sanchez S, Zhu M, Blom H, and Ahlberg PE
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- Animal Scales anatomy & histology, Animals, China, Fishes classification, Fishes growth & development, Phylogeny, Biological Evolution, Fishes anatomy & histology, Fossils
- Abstract
Recent advances in synchrotron imaging allow us to study the three-dimensional (3D) histology of vertebrate fossils, including microfossils (e.g. teeth and scales) of early jawed vertebrates. These microfossils can often be scanned at submicron resolution (<1 µm) because of their small size. The resulting voxel (3D pixel) stacks can be processed into virtual thin sections revealing almost every internal detail of the samples, comparable to traditional thin sections. In addition, 3D models of the internal microanatomical structures, such as embedded odontodes and vasculature, can be assembled and examined in situ. Scales of two early osteichthyans, Psarolepis romeri from the Early Devonian of China and Andreolepis hedei from the Late Silurian of Sweden, were scanned using propagation phase-contrast synchrotron X-ray microtomography (PPC-SRµCT), and 3D models of internal canal systems and buried odontodes were created from the scans. Based on these new data, we review the evolutionary origin of cosmine and its associated pore-canal system, which has been long recognized as a synapomorphy of sarcopterygians. The first odontode that appeared during growth shows almost identical morphology in the two scales, but the second odontode of the Psarolepis scale shows a distinctive morphology with several pores on the surface. It is suggested that a shift from ridge-like odontode to pore-bearing odontode was the key step in the origin of cosmine, which was then elaborated further in more-derived sarcopterygians. We perform a detailed comparison between the two scales and propose a primary homology framework to generate microanatomical characters, which can be used in the phylogenetic analysis of early osteichthyans when more 3D data become available. Our results highlight the importance of 3D data for the study of histology and ontogeny of the dermal skeleton of early jawed vertebrates, especially scales of the polyodontode type. The traditional microvertebrate collection is not only useful for biostratigraphic studies, but also preserves invaluable biological information about the growth of vertebrate hard tissues. Today, we are only beginning to understand the biological meaning of the new 3D data. The increasing availability of such data will enable, and indeed require, a complete revision of traditional palaeohistological studies on early vertebrates., (© 2016 Cambridge Philosophical Society.)
- Published
- 2017
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15. A Silurian maxillate placoderm illuminates jaw evolution.
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Zhu M, Ahlberg PE, Pan Z, Zhu Y, Qiao T, Zhao W, Jia L, and Lu J
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- Animals, Dental Arch anatomy & histology, Fossils, Paleontology, Phylogeny, Biological Evolution, Fishes anatomy & histology, Fishes classification, Mandible anatomy & histology, Maxilla anatomy & histology
- Abstract
The discovery of Entelognathus revealed the presence of maxilla, premaxilla, and dentary, supposedly diagnostic osteichthyan bones, in a Silurian placoderm. However, the relationship between these marginal jaw bones and the gnathal plates of conventional placoderms, thought to represent the inner dental arcade, remains uncertain. Here we report a second Silurian maxillate placoderm, which bridges the gnathal and maxillate conditions. We propose that the maxilla, premaxilla, and dentary are homologous to the gnathal plates of placoderms and that all belong to the same dental arcade. The gnathal-maxillate transformation occurred concurrently in upper and lower jaws, predating the addition of infradentary bones to the lower jaw., (Copyright © 2016, American Association for the Advancement of Science.)
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- 2016
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16. A Devonian predatory fish provides insights into the early evolution of modern sarcopterygians.
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Lu J, Zhu M, Ahlberg PE, Qiao T, Zhu Y, Zhao W, and Jia L
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- Animals, Paleontology, Biological Evolution, Fishes, Fossils, Predatory Behavior
- Abstract
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.
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- 2016
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17. New genomic and fossil data illuminate the origin of enamel.
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Qu Q, Haitina T, Zhu M, and Ahlberg PE
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- Amelogenin genetics, Animals, China, Dental Enamel Proteins genetics, Dentin, Evolution, Molecular, Fish Proteins genetics, Multigene Family genetics, Skin anatomy & histology, Skin chemistry, Skull chemistry, Tooth anatomy & histology, Tooth chemistry, Alkaloids, Biological Evolution, Dental Enamel, Fishes genetics, Fossils, Genome genetics, Genomics, Pyrroles
- Abstract
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.
- Published
- 2015
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18. Copulation in antiarch placoderms and the origin of gnathostome internal fertilization.
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Long JA, Mark-Kurik E, Johanson Z, Lee MS, Young GC, Min Z, Ahlberg PE, Newman M, Jones R, den Blaauwen J, Choo B, and Trinajstic K
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- Animals, Female, Fossils, Male, Models, Biological, Phylogeny, Sex Characteristics, Vertebrates anatomy & histology, Biological Evolution, Copulation physiology, Fertilization physiology, Fishes anatomy & histology, Fishes physiology, Jaw, Vertebrates physiology
- Abstract
Reproduction in jawed vertebrates (gnathostomes) involves either external or internal fertilization. It is commonly argued that internal fertilization can evolve from external, but not the reverse. Male copulatory claspers are present in certain placoderms, fossil jawed vertebrates retrieved as a paraphyletic segment of the gnathostome stem group in recent studies. This suggests that internal fertilization could be primitive for gnathostomes, but such a conclusion depends on demonstrating that copulation was not just a specialized feature of certain placoderm subgroups. The reproductive biology of antiarchs, consistently identified as the least crownward placoderms and thus of great interest in this context, has until now remained unknown. Here we show that certain antiarchs possessed dermal claspers in the males, while females bore paired dermal plates inferred to have facilitated copulation. These structures are not associated with pelvic fins. The clasper morphology resembles that of ptyctodonts, a more crownward placoderm group, suggesting that all placoderm claspers are homologous and that internal fertilization characterized all placoderms. This implies that external fertilization and spawning, which characterize most extant aquatic gnathostomes, must be derived from internal fertilization, even though this transformation has been thought implausible. Alternatively, the substantial morphological evidence for placoderm paraphyly must be rejected.
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- 2015
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19. A primitive placoderm sheds light on the origin of the jawed vertebrate face.
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Dupret V, Sanchez S, Goujet D, Tafforeau P, and Ahlberg PE
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- Animals, Brain anatomy & histology, Face anatomy & histology, Fishes classification, Lampreys anatomy & histology, Neural Crest anatomy & histology, Phylogeny, Biological Evolution, Fishes anatomy & histology, Fossils, Jaw anatomy & histology
- Abstract
Extant vertebrates form two clades, the jawless Cyclostomata (lampreys and hagfishes) and the jawed Gnathostomata (all other vertebrates), with contrasting facial architectures. These arise during development from just a few key differences in the growth patterns of the cranial primordia: notably, the nasal sacs and hypophysis originate from a single placode in cyclostomes but from separate placodes in gnathostomes, and infraoptic ectomesenchyme migrates forward either side of the single placode in cyclostomes but between the placodes in gnathostomes. Fossil stem gnathostomes preserve cranial anatomies rich in landmarks that provide proxies for developmental processes and allow the transition from jawless to jawed vertebrates to be broken down into evolutionary steps. Here we use propagation phase contrast synchrotron microtomography to image the cranial anatomy of the primitive placoderm (jawed stem gnathostome) Romundina, and show that it combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of cyclostomes and the galeaspid (jawless stem gnathostome) Shuyu. This combination seems to be primitive for jawed vertebrates, and suggests a decoupling between ectomesenchymal growth trajectory, ectomesenchymal proliferation, and cerebral shape change during the origin of gnathostomes.
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- 2014
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20. Did terrestrial diversification of amoebas (amoebozoa) occur in synchrony with land plants?
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Fiz-Palacios O, Romeralo M, Ahmadzadeh A, Weststrand S, Ahlberg PE, and Baldauf S
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- Amoebozoa genetics, Biodiversity, Ecosystem, Islands, Oceans and Seas, Plant Dispersal, Plants classification, Plants genetics, Uncertainty, Amoebozoa classification, Biological Evolution, Models, Genetic, Phylogeny
- Abstract
Evolution of lineage diversification through time is an active area of research where much progress has been made in the last decade. Contrary to the situation in animals and plants little is known about how diversification rates have evolved in most major groups of protist. This is mainly due to uncertainty about phylogenetic relationships, scarcity of the protist fossil record and the unknown diversity within these lineages. We have analyzed the evolutionary history of the supergroup Amoebozoa over the last 1000 million years using molecular dating and species number estimates. After an origin in the marine environment we have dated the colonization of terrestrial habitats by three distinct lineages of Amoebozoa: Dictyostelia, Myxogastria and Arcellinida. The common ancestor of the two sister taxa, Dictyostelia and Myxogastria, appears to have existed before the colonization of land by plants. In contrast Arcellinida seems to have diversify in synchrony with land plant radiation, and more specifically with that of mosses. Detection of acceleration of diversification rates in Myxogastria and Arcellinida points to a co-evolution within the terrestrial habitats, where land plants and the amoebozoans may have interacted during the evolution of these new ecosystems.
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- 2013
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21. Fossil musculature of the most primitive jawed vertebrates.
- Author
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Trinajstic K, Sanchez S, Dupret V, Tafforeau P, Long J, Young G, Senden T, Boisvert C, Power N, and Ahlberg PE
- Subjects
- Animals, Fishes classification, Phylogeny, Biological Evolution, Fishes anatomy & histology, Fishes genetics, Fossils, Maxillofacial Development genetics, Neck Muscles anatomy & histology
- Abstract
The transition from jawless to jawed vertebrates (gnathostomes) resulted in the reconfiguration of the muscles and skeleton of the head, including the creation of a separate shoulder girdle with distinct neck muscles. We describe here the only known examples of preserved musculature from placoderms (extinct armored fishes), the phylogenetically most basal jawed vertebrates. Placoderms possess a regionalized muscular anatomy that differs radically from the musculature of extant sharks, which is often viewed as primitive for gnathostomes. The placoderm data suggest that neck musculature evolved together with a dermal joint between skull and shoulder girdle, not as part of a broadly flexible neck as in sharks, and that transverse abdominal muscles are an innovation of gnathostomes rather than of tetrapods.
- Published
- 2013
- Full Text
- View/download PDF
22. Vertebral architecture in the earliest stem tetrapods.
- Author
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Pierce SE, Ahlberg PE, Hutchinson JR, Molnar JL, Sanchez S, Tafforeau P, and Clack JA
- Subjects
- Animals, Phylogeny, Synchrotrons, X-Ray Microtomography, Biological Evolution, Extremities anatomy & histology, Fossils, Spine anatomy & histology, Vertebrates anatomy & histology
- Abstract
The construction of the vertebral column has been used as a key anatomical character in defining and diagnosing early tetrapod groups. Rhachitomous vertebrae--in which there is a dorsally placed neural arch and spine, an anteroventrally placed intercentrum and paired, posterodorsally placed pleurocentra--have long been considered the ancestral morphology for tetrapods. Nonetheless, very little is known about vertebral anatomy in the earliest stem tetrapods, because most specimens remain trapped in surrounding matrix, obscuring important anatomical features. Here we describe the three-dimensional vertebral architecture of the Late Devonian stem tetrapod Ichthyostega using propagation phase-contrast X-ray synchrotron microtomography. Our scans reveal a diverse array of new morphological, and associated developmental and functional, characteristics, including a possible posterior-to-anterior vertebral ossification sequence and the first evolutionary appearance of ossified sternal elements. One of the most intriguing features relates to the positional relationships between the vertebral elements, with the pleurocentra being unexpectedly sutured or fused to the intercentra that directly succeed them, indicating a 'reverse' rhachitomous design. Comparison of Ichthyostega with two other stem tetrapods, Acanthostega and Pederpes, shows that reverse rhachitomous vertebrae may be the ancestral condition for limbed vertebrates. This study fundamentally revises our current understanding of vertebral column evolution in the earliest tetrapods and raises questions about the presumed vertebral architecture of tetrapodomorph fish and later, more crownward, tetrapods.
- Published
- 2013
- Full Text
- View/download PDF
23. Palaeontology: Birth of the jawed vertebrates.
- Author
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Ahlberg PE
- Subjects
- Animals, Fishes anatomy & histology, Fishes classification, Western Australia, Biological Evolution, Fertilization physiology, Fishes embryology, Fishes physiology, Fossils, Viviparity, Nonmammalian physiology
- Published
- 2009
- Full Text
- View/download PDF
24. The pectoral fin of Panderichthys and the origin of digits.
- Author
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Boisvert CA, Mark-Kurik E, and Ahlberg PE
- Subjects
- Animals, Fishes classification, Biological Evolution, Extremities anatomy & histology, Fishes anatomy & histology, Fossils
- Abstract
One of the identifying characteristics of tetrapods (limbed vertebrates) is the presence of fingers and toes. Whereas the proximal part of the tetrapod limb skeleton can easily be homologized with the paired fin skeletons of sarcopterygian (lobe-finned) fish, there has been much debate about the origin of digits. Early hypotheses interpreted digits as derivatives of fin radials, but during the 1990s the idea gained acceptance that digits are evolutionary novelties without direct equivalents in fish fin skeletons. This was partly based on developmental genetic data, but also substantially on the pectoral fin skeleton of the elpistostegid (transitional fish/tetrapod) Panderichthys, which appeared to lack distal digit-like radials. Here we present a CT scan study of an undisturbed pectoral fin of Panderichthys demonstrating that the plate-like 'ulnare' of previous reconstructions is an artefact and that distal radials are in fact present. This distal portion is more tetrapod-like than that found in Tiktaalik and, in combination with new data about fin development in basal actinopterygians, sharks and lungfish, makes a strong case for fingers not being a novelty of tetrapods but derived from pre-existing distal radials present in all sarcopterygian fish.
- Published
- 2008
- Full Text
- View/download PDF
25. Ventastega curonica and the origin of tetrapod morphology.
- Author
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Ahlberg PE, Clack JA, Luksevics E, Blom H, and Zupiņs I
- Subjects
- Animals, Fossils, Pelvic Bones anatomy & histology, Phylogeny, Shoulder anatomy & histology, Skull anatomy & histology, Biological Evolution, Fishes anatomy & histology
- Abstract
The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaalik roseae. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and partial pelvis of Ventastega curonica from the Late Devonian of Latvia, a transitional intermediate form between the 'elpistostegids' Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega. Ventastega is the most primitive Devonian tetrapod represented by extensive remains, and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms.
- Published
- 2008
- Full Text
- View/download PDF
26. Palaeontology: a firm step from water to land.
- Author
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Ahlberg PE and Clack JA
- Subjects
- Animals, Extremities anatomy & histology, Extremities physiology, Fishes classification, History, Ancient, Paleontology, Phylogeny, Time Factors, Water, Biological Evolution, Environment, Fishes anatomy & histology, Fishes physiology, Fossils
- Published
- 2006
- Full Text
- View/download PDF
27. Homologies and cell populations: a response to Sánchez-Villagra and Maier.
- Author
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Ahlberg PE and Koentges G
- Subjects
- Animals, Biomarkers, Muscle, Skeletal anatomy & histology, Scapula embryology, Biological Evolution, Mammals anatomy & histology, Scapula anatomy & histology
- Published
- 2006
- Full Text
- View/download PDF
28. Tetrapod-like middle ear architecture in a Devonian fish.
- Author
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Brazeau MD and Ahlberg PE
- Subjects
- Animals, Ear, Middle physiology, Fishes physiology, Gait physiology, History, Ancient, Respiration, Biological Evolution, Ear, Middle anatomy & histology, Extremities physiology, Fishes anatomy & histology, Fossils
- Abstract
Few fossils show the incipient stages of complex morphological transformations. For example, the earliest stages in the remodelling of the spiracular tract and suspensorium (jaw suspension) of osteolepiform fishes into the middle ear of tetrapods have remained elusive. The most primitive known tetrapods show a middle ear architecture that is very different from osteolepiforms such as Eusthenopteron, with little indication of how this transformation took place. Here we present an analysis of tetrapod middle ear origins that is based on a detailed study of Panderichthys, the immediate sister taxon of tetrapods. We show that the spiracular region is radically transformed from osteolepiforms and represents the earliest stages in the origin of the tetrapod middle ear architecture. The posterior palatoquadrate of Panderichthys is completely tetrapod-like and defines a similarly tetrapod-like spiracular tract. The hyomandibula has lost its distal portion, representing a previously unrecognized advance towards a stapes-like morphology. This spiracular specialization suggests that the middle ear of early tetrapods evolved initially as part of a spiracular breathing apparatus.
- Published
- 2006
- Full Text
- View/download PDF
29. Neural crest origins of the neck and shoulder.
- Author
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Matsuoka T, Ahlberg PE, Kessaris N, Iannarelli P, Dennehy U, Richardson WD, McMahon AP, and Koentges G
- Subjects
- Amphibians embryology, Animals, Bone Development, Cell Movement, Fossils, Humans, Mammals embryology, Mesoderm cytology, Mesoderm metabolism, Mice, Models, Biological, Neck pathology, Neural Crest cytology, Stem Cells cytology, Stem Cells metabolism, Biological Evolution, Cell Lineage, Neck embryology, Neural Crest metabolism, Shoulder embryology
- Abstract
The neck and shoulder region of vertebrates has undergone a complex evolutionary history. To identify its underlying mechanisms we map the destinations of embryonic neural crest and mesodermal stem cells using Cre-recombinase-mediated transgenesis. The single-cell resolution of this genetic labelling reveals cryptic cell boundaries traversing the seemingly homogeneous skeleton of the neck and shoulders. Within this assembly of bones and muscles we discern a precise code of connectivity that mesenchymal stem cells of both neural crest and mesodermal origin obey as they form muscle scaffolds. The neural crest anchors the head onto the anterior lining of the shoulder girdle, while a Hox-gene-controlled mesoderm links trunk muscles to the posterior neck and shoulder skeleton. The skeleton that we identify as neural crest-derived is specifically affected in human Klippel-Feil syndrome, Sprengel's deformity and Arnold-Chiari I/II malformation, providing insights into their likely aetiology. We identify genes involved in the cellular modularity of the neck and shoulder skeleton and propose a new method for determining skeletal homologies that is based on muscle attachments. This has allowed us to trace the whereabouts of the cleithrum, the major shoulder bone of extinct land vertebrate ancestors, which seems to survive as the scapular spine in living mammals.
- Published
- 2005
- Full Text
- View/download PDF
30. The origin of the internal nostril of tetrapods.
- Author
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Zhu M and Ahlberg PE
- Subjects
- Animals, China, Fishes anatomy & histology, Fishes physiology, History, Ancient, Models, Biological, Nose physiology, Phylogeny, Respiration, Time Factors, Biological Evolution, Fossils, Nose anatomy & histology, Vertebrates anatomy & histology, Vertebrates physiology
- Abstract
The choana, a unique 'internal nostril' opening from the nasal sac into the roof of the mouth, is a key part of the tetrapod (land vertebrate) respiratory system. It was the first component of the tetrapod body plan to evolve, well before the origin of limbs, and is therefore crucial to our understanding of the beginning of the fish-tetrapod transition. However, there is no consensus on the origin of the choana despite decades of heated debate; some have claimed that it represents a palatally displaced external nostril, but others have argued that this is implausible because it implies breaking and rejoining the maxillary-premaxillary dental arcade and the maxillary branch of nerve V. The fossil record has not resolved the dispute, because the choana is fully developed in known tetrapod stem-group members. Here we present new material of Kenichthys, a 395-million-year-old fossil fish from China, that provides direct evidence for the origin of the choana and establishes its homology: it is indeed a displaced posterior external nostril that, during a brief transitional stage illustrated by Kenichthys, separated the maxilla from the premaxilla.
- Published
- 2004
- Full Text
- View/download PDF
31. Comment on "The early evolution of the tetrapod humerus".
- Author
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Ahlberg PE
- Subjects
- Animals, Scotland, Biological Evolution, Fossils, Humerus anatomy & histology, Vertebrates anatomy & histology
- Published
- 2004
- Full Text
- View/download PDF
32. A uniquely specialized ear in a very early tetrapod.
- Author
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Clack JA, Ahlberg PE, Finney SM, Dominguez Alonso P, Robinson J, and Ketcham RA
- Subjects
- Adaptation, Physiological, Animals, Fishes anatomy & histology, Fishes physiology, Phylogeny, Skull anatomy & histology, Water, Biological Evolution, Ear anatomy & histology, Ear physiology, Fossils, Hearing physiology, Vertebrates anatomy & histology, Vertebrates physiology
- Abstract
The Late Devonian genus Ichthyostega was for many decades the earliest known tetrapod, and the sole representative of a transitional form between a fish and a land vertebrate. However, despite being known since 1932 (ref. 1) from a large collection of specimens, its morphology remained enigmatic and not what was expected of a very primitive tetrapod. Its apparent specializations led it to be considered as a "blind offshoot" or "sidebranch" off the tetrapod family tree, and recent cladistic analyses have disagreed about its exact phylogenetic position within the tetrapod stem group. In particular, its braincase and ear region defied interpretation, such that conventional anatomical terms seemed inapplicable. Using new material collected in 1998 (ref. 9), preparation of earlier-collected material, and high-resolution computed tomography scanning, here we identify and interpret these problematic anatomical structures. They can now be seen to form part of a highly specialized ear, probably a hearing device for use in water. This represents a structurally and functionally unique modification of the tetrapod otic region, unlike anything seen in subsequent tetrapod evolution. The presence of deeply grooved gill bars as in its contemporary Acanthostega suggest that Ichthyostega may have been more aquatically adapted than previously believed.
- Published
- 2003
- Full Text
- View/download PDF
33. Palaeontology: first Devonian tetrapod from Asia.
- Author
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Zhu M, Ahlberg PE, Zhao W, and Jia L
- Subjects
- Animals, China, Dentition, Mandible anatomy & histology, Biological Evolution, Fossils, Vertebrates anatomy & histology, Vertebrates classification
- Published
- 2002
- Full Text
- View/download PDF
34. Zebrafish in context: uses of a laboratory model in comparative studies.
- Author
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Metscher BD and Ahlberg PE
- Subjects
- Animals, Classification, Phylogeny, Biological Evolution, Vertebrates anatomy & histology, Zebrafish anatomy & histology
- Abstract
With the recent interest in the reintegration of evolutionary and developmental biology has come a growing need for understanding the phylogenetic relations and degree of generality of the model organisms upon which we rely so heavily. In vertebrate biology the zebrafish Danio rerio has become a paradigmatic system for studies at levels of organization from molecular to interspecific. Studies of model systems in development are often techniques-driven rather than questions-based; however, informative hypotheses for developmental research can be derived from phylogenetic distributions of characters. With some understanding of how general the characters of interest are, a thoughtful comparison of the requirements of the questions with the lists of available embryos, reagents, and protocols can guide choices of new vertebrate models. We describe here the phylogenetic placement of zebrafish within the vertebrate world and discuss how generally observations on zebrafish can be taken to apply. We outline a practical protocol for investigating development in a comparative context, illustrated with an example from an ongoing study of teleost tail fin evolution. The principles and procedures presented here apply equally well to any comparative study with an interest in evolution, at any level of phylogeny from intraspecific studies to comparisons across phyla., (Copyright 1999 Academic Press.)
- Published
- 1999
- Full Text
- View/download PDF
35. How to keep a head in order.
- Author
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Ahlberg PE
- Subjects
- Animals, Branchial Region embryology, Branchial Region innervation, Chick Embryo, Head innervation, Humans, Jaw embryology, Morphogenesis, Muscles embryology, Neural Crest embryology, Skull embryology, Biological Evolution, Head embryology
- Published
- 1997
- Full Text
- View/download PDF
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