1
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Widrig KE, Navalón G, Field DJ. Paleoneurology of stem palaeognaths clarifies the plesiomorphic condition of the crown bird central nervous system. J Morphol 2024; 285:e21710. [PMID: 38760949 DOI: 10.1002/jmor.21710] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2024] [Revised: 04/29/2024] [Accepted: 05/03/2024] [Indexed: 05/20/2024]
Abstract
Lithornithidae, an assemblage of volant Palaeogene fossil birds, provide our clearest insights into the early evolutionary history of Palaeognathae, the clade that today includes the flightless ratites and volant tinamous. The neotype specimen of Lithornis vulturinus, from the early Eocene (approximately 53 million years ago) of Europe, includes a partial neurocranium that has never been thoroughly investigated. Here, we describe these cranial remains including the nearly complete digital endocasts of the brain and bony labyrinth. The telencephalon of Lithornis is expanded and its optic lobes are ventrally shifted, as is typical for crown birds. The foramen magnum is positioned caudally, rather than flexed ventrally as in some crown birds, with the optic lobes, cerebellum, and foramen magnum shifted further ventrally. The overall brain shape is similar to that of tinamous, the only extant clade of flying palaeognaths, suggesting that several aspects of tinamou neuroanatomy may have been evolutionarily conserved since at least the early Cenozoic. The estimated ratio of the optic lobe's surface area relative to the total brain suggests a diurnal ecology. Lithornis may provide the clearest insights to date into the neuroanatomy of the ancestral crown bird, combining an ancestrally unflexed brain with a caudally oriented connection with the spinal cord, a moderately enlarged telencephalon, and ventrally shifted, enlarged optic lobes.
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Affiliation(s)
- Klara E Widrig
- Department of Earth Sciences, University of Cambridge, Cambridge, UK
| | - Guillermo Navalón
- Department of Earth Sciences, University of Cambridge, Cambridge, UK
| | - Daniel J Field
- Department of Earth Sciences, University of Cambridge, Cambridge, UK
- Museum of Zoology, University of Cambridge, Cambridge, UK
- Fossil Reptiles, Amphibians and Birds Section, The Natural History Museum, London, UK
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2
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Navalón G, Chiappe LM, Martinelli AG, Nava W, Field DJ. Fossil basicranium clarifies the origin of the avian central nervous system and inner ear. Proc Biol Sci 2022; 289:20221398. [PMID: 36168759 PMCID: PMC9515635 DOI: 10.1098/rspb.2022.1398] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Among terrestrial vertebrates, only crown birds (Neornithes) rival mammals in terms of relative brain size and behavioural complexity. Relatedly, the anatomy of the avian central nervous system and associated sensory structures, such as the vestibular system of the inner ear, are highly modified with respect to those of other extant reptile lineages. However, a dearth of three-dimensional Mesozoic fossils has limited our knowledge of the origins of the distinctive endocranial structures of crown birds. Traits such as an expanded, flexed brain, a ventral connection between the brain and spinal column, and a modified vestibular system have been regarded as exclusive to Neornithes. Here, we demonstrate all of these ‘advanced’ traits in an undistorted braincase from an Upper Cretaceous enantiornithine bonebed in southeastern Brazil. Our discovery suggests that these crown bird-like endocranial traits may have originated prior to the split between Enantiornithes and the more crownward portion of avian phylogeny over 140 Ma, while coexisting with a remarkably plesiomorphic cranial base and posterior palate region. Altogether, our results support the interpretation that the distinctive endocranial morphologies of crown birds and their Mesozoic relatives are affected by complex trade-offs between spatial constraints during development.
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Affiliation(s)
- Guillermo Navalón
- Unidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, Madrid, Spain.,Department of Earth Sciences, University of Cambridge, Cambridge, UK
| | - Luis M Chiappe
- Dinosaur Institute, Natural History Museum of Los Angeles, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
| | - Agustín G Martinelli
- Sección Paleontología de Vertebrados, CONICET-Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', Buenos Aires, Argentina
| | - William Nava
- Museu de Paleontologia de Marília, Marília, São Paulo, Brazil
| | - Daniel J Field
- Department of Earth Sciences, University of Cambridge, Cambridge, UK.,Museum of Zoology, University of Cambridge, Cambridge, UK
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3
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New Remains of Scandiavis mikkelseni Inform Avian Phylogenetic Relationships and Brain Evolution. DIVERSITY 2021. [DOI: 10.3390/d13120651] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Although an increasing number of studies are combining skeletal and neural morphology data in a phylogenetic context, most studies do not include extinct taxa due to the rarity of preserved endocasts. The early Eocene avifauna of the Fur Formation of Denmark presents an excellent opportunity for further study of extinct osteological and endocranial morphology as fossils are often exceptionally preserved in three dimensions. Here, we use X-ray computed tomography to present additional material of the previously described taxon Scandiavis mikkelseni and reassess its phylogenetic placement using a previously published dataset. The new specimen provides novel insights into the osteological morphology and brain anatomy of Scandiavis. The virtual endocast exhibits a morphology comparable to that of modern avian species. Endocranial evaluation shows that it was remarkably similar to that of certain extant Charadriiformes, yet also possessed a novel combination of traits. This may mean that traits previously proposed to be the result of shifts in ecology later in the evolutionary history of Charadriiformes may instead show a more complex distribution in stem Charadriiformes and/or Gruiformes depending on the interrelationships of these important clades. Evaluation of skeletal and endocranial character state changes within a previously published phylogeny confirms both S. mikkelseni and a putative extinct charadriiform, Nahmavis grandei, as charadriiform. Results bolster the likelihood that both taxa are critical fossils for divergence dating and highlight a biogeographic pattern similar to that of Gruiformes.
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4
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Watanabe A, Balanoff AM, Gignac PM, Gold MEL, Norell MA. Novel neuroanatomical integration and scaling define avian brain shape evolution and development. eLife 2021; 10:68809. [PMID: 34227464 PMCID: PMC8260227 DOI: 10.7554/elife.68809] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2021] [Accepted: 06/15/2021] [Indexed: 12/17/2022] Open
Abstract
How do large and unique brains evolve? Historically, comparative neuroanatomical studies have attributed the evolutionary genesis of highly encephalized brains to deviations along, as well as from, conserved scaling relationships among brain regions. However, the relative contributions of these concerted (integrated) and mosaic (modular) processes as drivers of brain evolution remain unclear, especially in non-mammalian groups. While proportional brain sizes have been the predominant metric used to characterize brain morphology to date, we perform a high-density geometric morphometric analysis on the encephalized brains of crown birds (Neornithes or Aves) compared to their stem taxa—the non-avialan coelurosaurian dinosaurs and Archaeopteryx. When analyzed together with developmental neuroanatomical data of model archosaurs (Gallus, Alligator), crown birds exhibit a distinct allometric relationship that dictates their brain evolution and development. Furthermore, analyses by neuroanatomical regions reveal that the acquisition of this derived shape-to-size scaling relationship occurred in a mosaic pattern, where the avian-grade optic lobe and cerebellum evolved first among non-avialan dinosaurs, followed by major changes to the evolutionary and developmental dynamics of cerebrum shape after the origin of Avialae. Notably, the brain of crown birds is a more integrated structure than non-avialan archosaurs, implying that diversification of brain morphologies within Neornithes proceeded in a more coordinated manner, perhaps due to spatial constraints and abbreviated growth period. Collectively, these patterns demonstrate a plurality in evolutionary processes that generate encephalized brains in archosaurs and across vertebrates.
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Affiliation(s)
- Akinobu Watanabe
- Department of Anatomy, New York Institute of Technology College of Osteopathic Medicine, Old Westbury, United States.,Division of Paleontology, American Museum of Natural History, New York, United States.,Department of Life Sciences Vertebrates Division, Natural History Museum, London, United Kingdom
| | - Amy M Balanoff
- Division of Paleontology, American Museum of Natural History, New York, United States.,Department of Psychological and Brain Sciences, Johns Hopkins University, Baltimore, United States
| | - Paul M Gignac
- Division of Paleontology, American Museum of Natural History, New York, United States.,Department of Anatomy and Cell Biology, Oklahoma State University Center for Health Sciences, Tulsa, United States
| | - M Eugenia L Gold
- Division of Paleontology, American Museum of Natural History, New York, United States.,Biology Department, Suffolk University, Boston, United States
| | - Mark A Norell
- Division of Paleontology, American Museum of Natural History, New York, United States
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5
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Torres CR, Norell MA, Clarke JA. Bird neurocranial and body mass evolution across the end-Cretaceous mass extinction: The avian brain shape left other dinosaurs behind. SCIENCE ADVANCES 2021; 7:eabg7099. [PMID: 34330706 PMCID: PMC8324052 DOI: 10.1126/sciadv.abg7099] [Citation(s) in RCA: 22] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/22/2021] [Accepted: 06/15/2021] [Indexed: 05/30/2023]
Abstract
Birds today are the most diverse clade of terrestrial vertebrates, and understanding why extant birds (Aves) alone among dinosaurs survived the Cretaceous-Paleogene mass extinction is crucial to reconstructing the history of life. Hypotheses proposed to explain this pattern demand identification of traits unique to Aves. However, this identification is complicated by a lack of data from non-avian birds. Here, we interrogate survivorship hypotheses using data from a new, nearly complete skull of Late Cretaceous (~70 million years) bird Ichthyornis and reassess shifts in bird body size across the Cretaceous-Paleogene boundary. Ichthyornis exhibited a wulst and segmented palate, previously proposed to have arisen within extant birds. The origin of Aves is marked by larger, reshaped brains indicating selection for relatively large telencephala and eyes but not by uniquely small body size. Sensory system differences, potentially linked to these shifts, may help explain avian survivorship relative to other dinosaurs.
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Affiliation(s)
- Christopher R Torres
- Department of Integrative Biology, University of Texas at Austin, Austin, TX, USA.
- Jackson School of Geoscience, University of Texas at Austin, Austin, TX, USA
- Heritage College of Osteopathic Medicine, Ohio University, Athens, OH, USA
| | - Mark A Norell
- Richard Gilder Graduate School, American Museum of Natural History, New York, NY, USA
- Division of Paleontology, American Museum of Natural History, New York, NY, USA
| | - Julia A Clarke
- Department of Integrative Biology, University of Texas at Austin, Austin, TX, USA.
- Jackson School of Geoscience, University of Texas at Austin, Austin, TX, USA
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6
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Early CM, Iwaniuk AN, Ridgely RC, Witmer LM. Endocast structures are reliable proxies for the sizes of corresponding regions of the brain in extant birds. J Anat 2020; 237:1162-1176. [PMID: 32892372 DOI: 10.1111/joa.13285] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2020] [Revised: 07/01/2020] [Accepted: 07/03/2020] [Indexed: 12/15/2022] Open
Abstract
Endocasts are increasingly relied upon to examine avian brain evolution because they can be used across extant and extinct species. The endocasts of birds appear to be relatively faithful representatives of the external morphology of their brains, but it is unclear how well the size of a surface feature visible on endocasts reflects the volume of the underlying brain region. The optic lobe and the Wulst are two endocast structures that are clearly visible on the external surface of avian endocasts. As they overlie two major visual regions of the brain, the optic tectum and hyperpallium, the surface areas of the optic lobe and Wulst, respectively, are often used to infer visual abilities. To determine whether the surface area of these features reflects the volume of the underlying brain regions, we compared the surface areas of the optic lobes and Wulsts from digital endocasts with the volumes of the optic tecta and hyperpallia from the literature or measured from histological series of brains of the same species. Regression analyses revealed strong, statistically significant correlations between the volumes of the brain regions and the surface areas of the overlying endocast structures. In other words, the size of the hyperpallium and optic tectum can be reliably inferred from the surface areas of the Wulst and optic lobe, respectively. This validation opens the possibility of estimating brain-region volumes for extinct species in order to gain better insights in their visual ecology. It also emphasizes the importance of adopting a quantitative approach to the analysis of endocasts in the study of brain evolution.
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Affiliation(s)
- Catherine M Early
- Biology Department, Science Museum of Minnesota, Saint Paul, MN, USA.,Department of Biological Sciences, Ohio University, Athens, OH, USA.,Florida Museum of Natural History, University of Florida, Gainesville, FL, USA
| | - Andrew N Iwaniuk
- Department of Neuroscience, University of Lethbridge, Lethbridge, AB, Canada
| | - Ryan C Ridgely
- Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH, USA
| | - Lawrence M Witmer
- Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH, USA
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7
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Knoll F, Kawabe S. Avian palaeoneurology: Reflections on the eve of its 200th anniversary. J Anat 2020; 236:965-979. [PMID: 31999834 DOI: 10.1111/joa.13160] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2017] [Revised: 12/28/2019] [Accepted: 01/07/2020] [Indexed: 12/13/2022] Open
Abstract
In birds, the brain (especially the telencephalon) is remarkably developed, both in relative volume and complexity. Unlike in most early-branching sauropsids, the adults of birds and other archosaurs have a well-ossified neurocranium. In contrast to the situation in most of their reptilian relatives but similar to what can be seen in mammals, the brains of birds fit closely to the endocranial cavity so that their major external features are reflected in the endocasts. This makes birds a highly suitable group for palaeoneurological investigations. The first observation about the brain in a long-extinct bird was made in the first quarter of the 19th century. However, it was not until the 2000s and the application of modern imaging technologies that avian palaeoneurology really took off. Understanding how the mode of life is reflected in the external morphology of the brains of birds is but one of several future directions in which avian palaeoneurological research may extend. Although the number of fossil specimens suitable for palaeoneurological explorations is considerably smaller in birds than in mammals and will very likely remain so, the coming years will certainly witness a momentous strengthening of this rapidly growing field of research at the overlap between ornithology, palaeontology, evolutionary biology and neurosciences.
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Affiliation(s)
- Fabien Knoll
- ARAID-Fundación Conjunto Paleontológico de Teruel-Dinópolis, Teruel, Spain.,Departamento de Paleobiología, Museo Nacional de Ciencias Naturales-CSIC, Madrid, Spain
| | - Soichiro Kawabe
- Institute of Dinosaur Research, Fukui Prefectural University, Fukui, Japan.,Fukui Prefectural Dinosaur Museum, Fukui, Japan
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8
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Beyond Endocasts: Using Predicted Brain-Structure Volumes of Extinct Birds to Assess Neuroanatomical and Behavioral Inferences. DIVERSITY-BASEL 2020. [DOI: 10.3390/d12010034] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The shape of the brain influences skull morphology in birds, and both traits are driven by phylogenetic and functional constraints. Studies on avian cranial and neuroanatomical evolution are strengthened by data on extinct birds, but complete, 3D-preserved vertebrate brains are not known from the fossil record, so brain endocasts often serve as proxies. Recent work on extant birds shows that the Wulst and optic lobe faithfully represent the size of their underlying brain structures, both of which are involved in avian visual pathways. The endocasts of seven extinct birds were generated from microCT scans of their skulls to add to an existing sample of endocasts of extant birds, and the surface areas of their Wulsts and optic lobes were measured. A phylogenetic prediction method based on Bayesian inference was used to calculate the volumes of the brain structures of these extinct birds based on the surface areas of their overlying endocast structures. This analysis resulted in hyperpallium volumes of five of these extinct birds and optic tectum volumes of all seven extinct birds. Phylogenetic ANCOVA (phyANCOVA) were performed on regressions of the brain-structure volumes and endocast structure surface areas on various brain size metrics to determine if the relative sizes of these structures in any extinct birds were significantly different from those of the extant birds in the sample. Phylogenetic ANCOVA indicated that no extinct birds studied had relative hyperpallial volumes that were significantly different from the extant sample, nor were any of their optic tecta relatively hypertrophied. The optic tectum of Dinornis robustus was significantly smaller relative to brain size than any of the extant birds in our sample. This study provides an analytical framework for testing the hypotheses of potential functional behavioral capabilities of other extinct birds based on their endocasts.
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9
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Multiphase progenetic development shaped the brain of flying archosaurs. Sci Rep 2019; 9:10807. [PMID: 31346192 PMCID: PMC6658547 DOI: 10.1038/s41598-019-46959-2] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2018] [Accepted: 07/05/2019] [Indexed: 11/10/2022] Open
Abstract
The growing availability of virtual cranial endocasts of extinct and extant vertebrates has fueled the quest for endocranial characters that discriminate between phylogenetic groups and resolve their neural significances. We used geometric morphometrics to compare a phylogenetically and ecologically comprehensive data set of archosaurian endocasts along the deep evolutionary history of modern birds and found that this lineage experienced progressive elevation of encephalisation through several chapters of increased endocranial doming that we demonstrate to result from progenetic developments. Elevated encephalisation associated with progressive size reduction within Maniraptoriformes was secondarily exapted for flight by stem avialans. Within Mesozoic Avialae, endocranial doming increased in at least some Ornithurae, yet remained relatively modest in early Neornithes. During the Paleogene, volant non-neoavian birds retained ancestral levels of endocast doming where a broad neoavian niche diversification experienced heterochronic brain shape radiation, as did non-volant Palaeognathae. We infer comparable developments underlying the establishment of pterosaurian brain shapes.
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10
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Field DJ, Hanson M, Burnham D, Wilson LE, Super K, Ehret D, Ebersole JA, Bhullar BAS. Complete Ichthyornis skull illuminates mosaic assembly of the avian head. Nature 2018; 557:96-100. [DOI: 10.1038/s41586-018-0053-y] [Citation(s) in RCA: 62] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2017] [Accepted: 03/21/2018] [Indexed: 11/09/2022]
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11
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Knoll F, Chiappe LM, Sanchez S, Garwood RJ, Edwards NP, Wogelius RA, Sellers WI, Manning PL, Ortega F, Serrano FJ, Marugán-Lobón J, Cuesta E, Escaso F, Sanz JL. A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds. Nat Commun 2018; 9:937. [PMID: 29507288 PMCID: PMC5838198 DOI: 10.1038/s41467-018-03295-9] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2017] [Accepted: 02/01/2018] [Indexed: 11/15/2022] Open
Abstract
Fossils of juvenile Mesozoic birds provide insight into the early evolution of avian development, however such fossils are rare. The analysis of the ossification sequence in these early-branching birds has the potential to address important questions about their comparative developmental biology and to help understand their morphological evolution and ecological differentiation. Here we report on an early juvenile enantiornithine specimen from the Early Cretaceous of Europe, which sheds new light on the osteogenesis in this most species-rich clade of Mesozoic birds. Consisting of a nearly complete skeleton, it is amongst the smallest known Mesozoic avian fossils representing post-hatching stages of development. Comparisons between this new specimen and other known early juvenile enantiornithines support a clade-wide asynchronous pattern of osteogenesis in the sternum and the vertebral column, and strongly indicate that the hatchlings of these phylogenetically basal birds varied greatly in size and tempo of skeletal maturation. Fossil juvenile Mesozoic birds are exceedingly rare and can provide important insight into the early evolution of avian development. Here, Knoll et al. describe one of the smallest known Mesozoic avians, which indicates a clade-wide asynchronous pattern of osteogenesis and great variation in basal bird hatchling size and skeletal maturation tempo.
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Affiliation(s)
- Fabien Knoll
- ARAID-Fundación Conjunto Paleontológico de Teruel-Dinopolis, 44002, Teruel, Spain. .,School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK.
| | - Luis M Chiappe
- The Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, CA, 90007, USA
| | - Sophie Sanchez
- Department of Organismal Biology, Uppsala University, 752 36, Uppsala, Sweden.,European Synchrotron Radiation Facility, 38000, Grenoble, France
| | - Russell J Garwood
- School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK.,Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
| | - Nicholas P Edwards
- School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK.,Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA
| | - Roy A Wogelius
- School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
| | - William I Sellers
- School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
| | - Phillip L Manning
- School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK.,Department of Geology and Environmental Geosciences, College of Charleston, SC, 29424, Charleston, USA
| | - Francisco Ortega
- Facultad de Ciencias, Universidad Nacional de Educación a Distancia, 28040, Madrid, Spain
| | - Francisco J Serrano
- The Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, CA, 90007, USA.,Facultad de Ciencias, Universidad de Málaga, 29010, Málaga, Spain
| | - Jesús Marugán-Lobón
- The Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, CA, 90007, USA.,Facultad de Ciencias, Universidad Autónoma de Madrid, 28049, Madrid, Spain
| | - Elena Cuesta
- Facultad de Ciencias, Universidad Autónoma de Madrid, 28049, Madrid, Spain
| | - Fernando Escaso
- Facultad de Ciencias, Universidad Nacional de Educación a Distancia, 28040, Madrid, Spain
| | - Jose Luis Sanz
- Facultad de Ciencias, Universidad Autónoma de Madrid, 28049, Madrid, Spain
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12
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Pritchard AC, Nesbitt SJ. A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida. ROYAL SOCIETY OPEN SCIENCE 2017; 4:170499. [PMID: 29134065 PMCID: PMC5666248 DOI: 10.1098/rsos.170499] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/17/2017] [Accepted: 09/05/2017] [Indexed: 06/07/2023]
Abstract
The Triassic Period saw the first appearance of numerous amniote lineages (e.g. Lepidosauria, Archosauria, Mammalia) that defined Mesozoic ecosystems following the end Permian Mass Extinction, as well as the first major morphological diversification of crown-group reptiles. Unfortunately, much of our understanding of this event comes from the record of large-bodied reptiles (total body length > 1 m). Here we present a new species of drepanosaurid (small-bodied, chameleon-like diapsids) from the Upper Triassic Chinle Formation of New Mexico. Using reconstructions of micro-computed tomography data, we reveal the three-dimensional skull osteology of this clade for the first time. The skull presents many archaic anatomical traits unknown in Triassic crown-group reptiles (e.g. absence of bony support for the external ear), whereas other traits (e.g. toothless rostrum, anteriorly directed orbits, inflated endocranium) resemble derived avian theropods. A phylogenetic analysis of Permo-Triassic diapsids supports the hypothesis that drepanosaurs are an archaic lineage that originated in the Permian, far removed from crown-group Reptilia. The phylogenetic position of drepanosaurids indicates the presence of archaic Permian clades among Triassic small reptile assemblages and that morphological convergence produced a remarkably bird-like skull nearly 100 Myr before one is known to have emerged in Theropoda.
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Affiliation(s)
- Adam C. Pritchard
- Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, USA
- Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06520-8109, USA
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