Vertebrates

14,000,000 Leading Edge Experts on the ideXlab platform

Scan Science and Technology

Contact Leading Edge Experts & Companies

Scan Science and Technology

Contact Leading Edge Experts & Companies

The Experts below are selected from a list of 173145 Experts worldwide ranked by ideXlab platform

David Jandzik - One of the best experts on this subject based on the ideXlab platform.

  • lamprey lecticans link new vertebrate genes to the origin and elaboration of vertebrate tissues
    Developmental Biology, 2021
    Co-Authors: Zachary D Root, David Jandzik, Cara Allen, Margaux Brewer, Marek Romasek, Tyler A Square, Daniel Meulemans Medeiros
    Abstract:

    Abstract The evolution of Vertebrates from an invertebrate chordate ancestor involved the evolution of new organs, tissues, and cell types. It was also marked by the origin and duplication of new gene families. If, and how, these morphological and genetic innovations are related is an unresolved question in vertebrate evolution. Hyaluronan is an extracellular matrix (ECM) polysaccharide important for water homeostasis and tissue structure. Vertebrates possess a novel family of hyaluronan binding proteins called Lecticans, and studies in jawed Vertebrates (gnathostomes) have shown they function in many of the cells and tissues that are unique to Vertebrates. This raises the possibility that the origin and/or expansion of this gene family helped drive the evolution of these vertebrate novelties. In order to better understand the evolution of the lectican gene family, and its role in the evolution of vertebrate morphological novelties, we investigated the phylogeny, genomic arrangement, and expression patterns of all lecticans in the sea lamprey (Petromyzon marinus), a jawless vertebrate. Though both P. marinus and gnathostomes each have four lecticans, our phylogenetic and syntenic analyses are most consistent with the independent duplication of one of more lecticans in the lamprey lineage. Despite the likely independent expansion of the lamprey and gnathostome lectican families, we find highly conserved expression of lecticans in vertebrate-specific and mesenchyme-derived tissues. We also find that, unlike gnathostomes, lamprey expresses its lectican paralogs in distinct subpopulations of head skeleton precursors, potentially reflecting an ancestral diversity of skeletal tissue types. Together, these observations suggest that the ancestral pre-duplication lectican had a complex expression pattern, functioned to support mesenchymal histology, and likely played a role in the evolution of vertebrate-specific cell and tissue types.

  • Lamprey Lecticans Link New Vertebrate Genes to the Origin and Elaboration of Vertebrate Tissues
    2020
    Co-Authors: Zachary D Root, David Jandzik, Cara Allen, Margaux Brewer, Marek Romasek, Tyler A Square, Daniel Meulemans Medeiros
    Abstract:

    The evolution of Vertebrates from an invertebrate chordate ancestor involved the evolution of new organs, tissues, and cell types. It was also marked by the origin and duplication of new gene families. If, and how, these morphological and genetic innovations are related is an unresolved question in vertebrate evolution. Hyaluronan is an extracellular matrix (ECM) polysaccharide important for water homeostasis and tissue structure. Vertebrates possess a novel family of hyaluronan binding proteins called Lecticans, and studies in jawed Vertebrates (gnathostomes) have shown they function in many of the cells and tissues that are unique to Vertebrates. This raises the possibility that the origin and/or expansion of this gene family helped drive the evolution of these vertebrate novelties. In order to better understand the evolution of the lectican gene family, and its role in the evolution of vertebrate morphological novelties, we investigated the phylogeny, genomic arrangement, and expression patterns of all lecticans in the sea lamprey (Petromyzon marinus), a jawless vertebrate. Though both P. marinus and gnathostomes have four lecticans, our phylogenetic and syntenic analyses suggest lamprey lecticans are the result of one or more cyclostome-specific duplications. Despite the independent expansion of the lamprey and gnathostome lectican families, we find highly conserved expression of lecticans in vertebrate-specific and mesenchyme-derived tissues. We also find that, unlike gnathostomes, lamprey expresses its lectican paralogs in distinct subpopulations of head skeleton precursors, potentially reflecting an ancestral diversity of skeletal tissue types. Together, these observations suggest that the ancestral pre-duplication lectican had a complex expression pattern, functioned to support mesenchymal histology, and likely played a role in the evolution of vertebrate-specific cell and tissue types.

  • evolution of the endothelin pathway drove neural crest cell diversification
    Nature, 2020
    Co-Authors: David Jandzik, Marek Romasek, Tyler A Square, James L Massey, Haley Paula Stein, Andrew Wesley Hansen, Amrita Purkayastha
    Abstract:

    Neural crest cells (NCCs) are migratory, multipotent embryonic cells that are unique to Vertebrates and form an array of clade-defining adult features. The evolution of NCCs has been linked to various genomic events, including the evolution of new gene-regulatory networks1,2, the de novo evolution of genes3 and the proliferation of paralogous genes during genome-wide duplication events4. However, conclusive functional evidence linking new and/or duplicated genes to NCC evolution is lacking. Endothelin ligands (Edns) and endothelin receptors (Ednrs) are unique to Vertebrates3,5,6, and regulate multiple aspects of NCC development in jawed Vertebrates7-10. Here, to test whether the evolution of Edn signalling was a driver of NCC evolution, we used CRISPR-Cas9 mutagenesis11 to disrupt edn, ednr and dlx genes in the sea lamprey, Petromyzon marinus. Lampreys are jawless fishes that last shared a common ancestor with modern jawed Vertebrates around 500 million years ago12. Thus, comparisons between lampreys and gnathostomes can identify deeply conserved and evolutionarily flexible features of vertebrate development. Using the frog Xenopus laevis to expand gnathostome phylogenetic representation and facilitate side-by-side analyses, we identify ancient and lineage-specific roles for Edn signalling. These findings suggest that Edn signalling was activated in NCCs before duplication of the vertebrate genome. Then, after one or more genome-wide duplications in the vertebrate stem, paralogous Edn pathways functionally diverged, resulting in NCC subpopulations with different Edn signalling requirements. We posit that this new developmental modularity facilitated the independent evolution of NCC derivatives in stem Vertebrates. Consistent with this, differences in Edn pathway targets are associated with differences in the oropharyngeal skeleton and autonomic nervous system of lampreys and modern gnathostomes. In summary, our work provides functional genetic evidence linking the origin and duplication of new vertebrate genes with the stepwise evolution of a defining vertebrate novelty.

  • Evolution of the new vertebrate head by co-option of an ancient chordate skeletal tissue
    Nature, 2015
    Co-Authors: David Jandzik, Maria V. Cattell, Aaron T. Garnett, Tyler A Square, Daniel M. Medeiros
    Abstract:

    A defining feature of Vertebrates (craniates) is a pronounced head that is supported and protected by a robust cellular endoskeleton. In the first Vertebrates, this skeleton probably consisted of collagenous cellular cartilage, which forms the embryonic skeleton of all Vertebrates and the adult skeleton of modern jawless and cartilaginous fish. In the head, most cellular cartilage is derived from a migratory cell population called the neural crest, which arises from the edges of the central nervous system. Because collagenous cellular cartilage and neural crest cells have not been described in inVertebrates^ 1 , the appearance of cellular cartilage derived from neural crest cells is considered a turning point in vertebrate evolution^ 2 . Here we show that a tissue with many of the defining features of vertebrate cellular cartilage transiently forms in the larvae of the invertebrate chordate Branchiostoma floridae (Florida amphioxus). We also present evidence that during evolution, a key regulator of vertebrate cartilage development, SoxE , gained new cis- regulatory sequences that subsequently directed its novel expression in neural crest cells. Together, these results suggest that the origin of the vertebrate head skeleton did not depend on the evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head. We further propose that the evolution of cis -regulatory elements near an ancient regulator of cartilage differentiation was a major factor in the evolution of the vertebrate head skeleton. The origins of the vertebrate head have perplexed scientists for centuries. It is a distinctive structure with no obvious counterpart in the invertebrate world. It is thought that the skeleton of the head of the first Vertebrates would have been built of collagenous cellular cartilage derived from the neural crest, a migratory cell population that arises from the edges of the central nervous system. But non-vertebrate chordates have no neural crest, making the origin of this cartilage hard to fathom. Using pharmacological perturbations, histology and gene expression, Daniel Medeiros and colleagues identify a tissue virtually indistinguishable from vertebrate cellular cartilage that forms transiently in metamorphosing larvae of the invertebrate chordate, amphioxus. This suggests that the appearance of the vertebrate head skeleton did not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head. A tissue with many of the defining features of vertebrate cellular cartilage is shown to form transiently in larvae of the invertebrate chordate amphioxus, indicating that the origin of the vertebrate head skeleton depended not on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head.

Tyler A Square - One of the best experts on this subject based on the ideXlab platform.

  • lamprey lecticans link new vertebrate genes to the origin and elaboration of vertebrate tissues
    Developmental Biology, 2021
    Co-Authors: Zachary D Root, David Jandzik, Cara Allen, Margaux Brewer, Marek Romasek, Tyler A Square, Daniel Meulemans Medeiros
    Abstract:

    Abstract The evolution of Vertebrates from an invertebrate chordate ancestor involved the evolution of new organs, tissues, and cell types. It was also marked by the origin and duplication of new gene families. If, and how, these morphological and genetic innovations are related is an unresolved question in vertebrate evolution. Hyaluronan is an extracellular matrix (ECM) polysaccharide important for water homeostasis and tissue structure. Vertebrates possess a novel family of hyaluronan binding proteins called Lecticans, and studies in jawed Vertebrates (gnathostomes) have shown they function in many of the cells and tissues that are unique to Vertebrates. This raises the possibility that the origin and/or expansion of this gene family helped drive the evolution of these vertebrate novelties. In order to better understand the evolution of the lectican gene family, and its role in the evolution of vertebrate morphological novelties, we investigated the phylogeny, genomic arrangement, and expression patterns of all lecticans in the sea lamprey (Petromyzon marinus), a jawless vertebrate. Though both P. marinus and gnathostomes each have four lecticans, our phylogenetic and syntenic analyses are most consistent with the independent duplication of one of more lecticans in the lamprey lineage. Despite the likely independent expansion of the lamprey and gnathostome lectican families, we find highly conserved expression of lecticans in vertebrate-specific and mesenchyme-derived tissues. We also find that, unlike gnathostomes, lamprey expresses its lectican paralogs in distinct subpopulations of head skeleton precursors, potentially reflecting an ancestral diversity of skeletal tissue types. Together, these observations suggest that the ancestral pre-duplication lectican had a complex expression pattern, functioned to support mesenchymal histology, and likely played a role in the evolution of vertebrate-specific cell and tissue types.

  • Lamprey Lecticans Link New Vertebrate Genes to the Origin and Elaboration of Vertebrate Tissues
    2020
    Co-Authors: Zachary D Root, David Jandzik, Cara Allen, Margaux Brewer, Marek Romasek, Tyler A Square, Daniel Meulemans Medeiros
    Abstract:

    The evolution of Vertebrates from an invertebrate chordate ancestor involved the evolution of new organs, tissues, and cell types. It was also marked by the origin and duplication of new gene families. If, and how, these morphological and genetic innovations are related is an unresolved question in vertebrate evolution. Hyaluronan is an extracellular matrix (ECM) polysaccharide important for water homeostasis and tissue structure. Vertebrates possess a novel family of hyaluronan binding proteins called Lecticans, and studies in jawed Vertebrates (gnathostomes) have shown they function in many of the cells and tissues that are unique to Vertebrates. This raises the possibility that the origin and/or expansion of this gene family helped drive the evolution of these vertebrate novelties. In order to better understand the evolution of the lectican gene family, and its role in the evolution of vertebrate morphological novelties, we investigated the phylogeny, genomic arrangement, and expression patterns of all lecticans in the sea lamprey (Petromyzon marinus), a jawless vertebrate. Though both P. marinus and gnathostomes have four lecticans, our phylogenetic and syntenic analyses suggest lamprey lecticans are the result of one or more cyclostome-specific duplications. Despite the independent expansion of the lamprey and gnathostome lectican families, we find highly conserved expression of lecticans in vertebrate-specific and mesenchyme-derived tissues. We also find that, unlike gnathostomes, lamprey expresses its lectican paralogs in distinct subpopulations of head skeleton precursors, potentially reflecting an ancestral diversity of skeletal tissue types. Together, these observations suggest that the ancestral pre-duplication lectican had a complex expression pattern, functioned to support mesenchymal histology, and likely played a role in the evolution of vertebrate-specific cell and tissue types.

  • evolution of the endothelin pathway drove neural crest cell diversification
    Nature, 2020
    Co-Authors: David Jandzik, Marek Romasek, Tyler A Square, James L Massey, Haley Paula Stein, Andrew Wesley Hansen, Amrita Purkayastha
    Abstract:

    Neural crest cells (NCCs) are migratory, multipotent embryonic cells that are unique to Vertebrates and form an array of clade-defining adult features. The evolution of NCCs has been linked to various genomic events, including the evolution of new gene-regulatory networks1,2, the de novo evolution of genes3 and the proliferation of paralogous genes during genome-wide duplication events4. However, conclusive functional evidence linking new and/or duplicated genes to NCC evolution is lacking. Endothelin ligands (Edns) and endothelin receptors (Ednrs) are unique to Vertebrates3,5,6, and regulate multiple aspects of NCC development in jawed Vertebrates7-10. Here, to test whether the evolution of Edn signalling was a driver of NCC evolution, we used CRISPR-Cas9 mutagenesis11 to disrupt edn, ednr and dlx genes in the sea lamprey, Petromyzon marinus. Lampreys are jawless fishes that last shared a common ancestor with modern jawed Vertebrates around 500 million years ago12. Thus, comparisons between lampreys and gnathostomes can identify deeply conserved and evolutionarily flexible features of vertebrate development. Using the frog Xenopus laevis to expand gnathostome phylogenetic representation and facilitate side-by-side analyses, we identify ancient and lineage-specific roles for Edn signalling. These findings suggest that Edn signalling was activated in NCCs before duplication of the vertebrate genome. Then, after one or more genome-wide duplications in the vertebrate stem, paralogous Edn pathways functionally diverged, resulting in NCC subpopulations with different Edn signalling requirements. We posit that this new developmental modularity facilitated the independent evolution of NCC derivatives in stem Vertebrates. Consistent with this, differences in Edn pathway targets are associated with differences in the oropharyngeal skeleton and autonomic nervous system of lampreys and modern gnathostomes. In summary, our work provides functional genetic evidence linking the origin and duplication of new vertebrate genes with the stepwise evolution of a defining vertebrate novelty.

  • Evolution of the new vertebrate head by co-option of an ancient chordate skeletal tissue
    Nature, 2015
    Co-Authors: David Jandzik, Maria V. Cattell, Aaron T. Garnett, Tyler A Square, Daniel M. Medeiros
    Abstract:

    A defining feature of Vertebrates (craniates) is a pronounced head that is supported and protected by a robust cellular endoskeleton. In the first Vertebrates, this skeleton probably consisted of collagenous cellular cartilage, which forms the embryonic skeleton of all Vertebrates and the adult skeleton of modern jawless and cartilaginous fish. In the head, most cellular cartilage is derived from a migratory cell population called the neural crest, which arises from the edges of the central nervous system. Because collagenous cellular cartilage and neural crest cells have not been described in inVertebrates^ 1 , the appearance of cellular cartilage derived from neural crest cells is considered a turning point in vertebrate evolution^ 2 . Here we show that a tissue with many of the defining features of vertebrate cellular cartilage transiently forms in the larvae of the invertebrate chordate Branchiostoma floridae (Florida amphioxus). We also present evidence that during evolution, a key regulator of vertebrate cartilage development, SoxE , gained new cis- regulatory sequences that subsequently directed its novel expression in neural crest cells. Together, these results suggest that the origin of the vertebrate head skeleton did not depend on the evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head. We further propose that the evolution of cis -regulatory elements near an ancient regulator of cartilage differentiation was a major factor in the evolution of the vertebrate head skeleton. The origins of the vertebrate head have perplexed scientists for centuries. It is a distinctive structure with no obvious counterpart in the invertebrate world. It is thought that the skeleton of the head of the first Vertebrates would have been built of collagenous cellular cartilage derived from the neural crest, a migratory cell population that arises from the edges of the central nervous system. But non-vertebrate chordates have no neural crest, making the origin of this cartilage hard to fathom. Using pharmacological perturbations, histology and gene expression, Daniel Medeiros and colleagues identify a tissue virtually indistinguishable from vertebrate cellular cartilage that forms transiently in metamorphosing larvae of the invertebrate chordate, amphioxus. This suggests that the appearance of the vertebrate head skeleton did not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head. A tissue with many of the defining features of vertebrate cellular cartilage is shown to form transiently in larvae of the invertebrate chordate amphioxus, indicating that the origin of the vertebrate head skeleton depended not on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head.

Sebastian M. Shimeld - One of the best experts on this subject based on the ideXlab platform.

  • Hmx gene conservation identifies the evolutionary origin of vertebrate cranial ganglia
    2020
    Co-Authors: Vasileios Papdogiannis, Hugo J. Parker, Marianne E. Bronner, Alessandro Pennati, Cedric Patthey, Sebastian M. Shimeld
    Abstract:

    The evolutionary origin of Vertebrates included innovations in sensory processing associated with the acquisition of a predatory lifestyle. Vertebrates perceive external stimuli through sensory systems serviced by cranial sensory ganglia (CSG) which develop from cranial placodes; however understanding the evolutionary origin of placodes and CSGs is hampered by the gulf between living lineages and difficulty in assigning homology between cell types and structures. Here we use the Hmx gene family to address this question. We show Hmx is a constitutive component of vertebrate CSG development and that Hmx in the tunicate Ciona is able to drive the differentiation program of Bipolar Tail Neurons (BTNs), cells previously thought neural crest homologs. Using Ciona and lamprey transgenesis we demonstrate that a unique, tandemly duplicated enhancer pair regulated Hmx in the stem-vertebrate lineage. Strikingly, we also show robust vertebrate Hmx enhancer function in Ciona, demonstrating that deep conservation of the upstream regulatory network spans the evolutionary origin of Vertebrates. These experiments demonstrate regulatory and functional conservation between Ciona and vertebrate Hmx, and confirm BTNs as CSG homologs. Our analysis also identifies derived evolutionary changes, including a genetic basis for secondary simplicity in Ciona and unique regulatory complexity in Vertebrates.

  • evolutionary crossroads in developmental biology cyclostomes lamprey and hagfish
    Development, 2012
    Co-Authors: Sebastian M. Shimeld, Phillip C J Donoghue
    Abstract:

    Lampreys and hagfish, which together are known as the cyclostomes or ‘agnathans’, are the only surviving lineages of jawless fish. They diverged early in vertebrate evolution, before the origin of the hinged jaws that are characteristic of gnathostome (jawed) Vertebrates and before the evolution of paired appendages. However, they do share numerous characteristics with jawed Vertebrates. Studies of cyclostome development can thus help us to understand when, and how, key aspects of the vertebrate body evolved. Here, we summarise the development of cyclostomes, highlighting the key species studied and experimental methods available. We then discuss how studies of cyclostomes have provided important insight into the evolution of fins, jaws, skeleton and neural crest.

  • chordate βγ crystallins and the evolutionary developmental biology of the vertebrate lens
    Comparative Biochemistry and Physiology B, 2007
    Co-Authors: Kumars Riyahi, Sebastian M. Shimeld
    Abstract:

    Several animal lineages, including the Vertebrates, have evolved sophisticated eyes with lenses that refract light to generate an image. The nearest invertebrate relatives of the Vertebrates, such as the ascidians (sea squirts) and amphioxus, have only basic light detecting organs, leading to the widely-held view that the vertebrate lens is an innovation that evolved in early Vertebrates. From an embryological perspective the lens is different from the rest of the eye, in that the eye is primarily of neural origin while the lens derives from a non-neural ectodermal placode which invaginates into the developing eye. How such an organ could have evolved has attracted much speculation. Recently, however, molecular developmental studies of sea squirts have started to suggest a possible evolutionary origin for the lens. First, studies of the Pax, Six, Eya and other gene families have indicated that sea squirts have areas of non-neural ectoderm homologous to placodes, suggesting an origin for the embryological characteristics of the lens. Second, the evolution and regulation of the βγ-crystallins has been studied. These form one of the key crystallin gene families responsible for the transparency of the lens, and regulatory conservation between the βγ-crystallin gene in the sea squirt Ciona intestinalis and the vertebrate visual system has been experimentally demonstrated. These data, together with knowledge of the morphological, physiological and gene expression similarities between the C. intestinalis ocellus and vertebrate retina, have led us to propose a hypothesis for the evolution of the vertebrate lens and integrated vertebrate eye via the co-option and combination of ancient gene regulatory networks; one controlling morphogenetic aspects of lens development and one controlling the expression of a gene family responsible for the biophysical properties of the lens, with the components of the retina having evolved from an ancestral photoreceptive organ derived from the anterior central nervous system.

  • molecular evidence from ciona intestinalis for the evolutionary origin of vertebrate sensory placodes
    Developmental Biology, 2005
    Co-Authors: Francoise Mazet, James A Hutt, Josselin Milloz, John Millard, Anthony Graham, Sebastian M. Shimeld
    Abstract:

    Cranial sensory placodes are focused areas of the head ectoderm of Vertebrates that contribute to the development of the cranial sense organs and their associated ganglia. Placodes have long been considered a key character of Vertebrates, and their evolution is proposed to have been essential for the evolution of an active predatory lifestyle by early Vertebrates. Despite their importance for understanding vertebrate origins, the evolutionary origin of placodes has remained obscure. Here, we use a panel of molecular markers from the Six, Eya, Pax, Dach, FoxI, COE and POUIV gene families to examine the tunicate Ciona intestinalis for evidence of structures homologous to vertebrate placodes. Our results identify two domains of Ciona ectoderm that are marked by the genetic cascade that regulates vertebrate placode formation. The first is just anterior to the brain, and we suggest this territory is equivalent to the olfactory/adenohypophyseal placodes of Vertebrates. The second is a bilateral domain adjacent to the posterior brain and includes cells fated to form the atrium and atrial siphon of adult Ciona. We show this bares most similarity to placodes fated to form the vertebrate acoustico-lateralis system. We interpret these data as support for the hypothesis that sensory placodes did not arise de novo in Vertebrates, but evolved from pre-existing specialised areas of ectoderm that contributed to sensory organs in the common ancestor of Vertebrates and tunicates.

David E K Ferrier - One of the best experts on this subject based on the ideXlab platform.

  • more than one to four via 2r evidence of an independent amphioxus expansion and two gene ancestral vertebrate state for myod related myogenic regulatory factors mrfs
    Molecular Biology and Evolution, 2020
    Co-Authors: Madeleine Emma Aaseremedios, Clara Collllado, David E K Ferrier
    Abstract:

    The evolutionary transition from inVertebrates to Vertebrates involved extensive gene duplication, but understanding precisely how such duplications contributed to this transition requires more detailed knowledge of specific cases of genes and gene families. Myogenic differentiation (MyoD) has long been recognized as a master developmental control gene and member of the MyoD family of bHLH transcription factors (myogenic regulatory factors [MRFs]) that drive myogenesis across the bilaterians. Phylogenetic reconstructions within this gene family are complicated by multiple instances of gene duplication and loss in several lineages. Following two rounds of whole-genome duplication (2R WGD) at the origin of the Vertebrates, the ancestral function of MRFs is thought to have become partitioned among the daughter genes, so that MyoD and Myf5 act early in myogenic determination, whereas Myog and Myf6 are expressed later, in differentiating myoblasts. Comparing chordate MRFs, we find an independent expansion of MRFs in the invertebrate chordate amphioxus, with evidence for a parallel instance of subfunctionalization relative to that of Vertebrates. Conserved synteny between chordate MRF loci supports the 2R WGD events as a major force in shaping the evolution of vertebrate MRFs. We also resolve vertebrate MRF complements and organization, finding a new type of vertebrate MRF gene in the process, which allowed us to infer an ancestral two-gene state in the Vertebrates corresponding to the early- and late-acting types of MRFs. This necessitates a revision of previous conclusions about the simple one-to-four origin of vertebrate MRFs.

Marco Stampanoni - One of the best experts on this subject based on the ideXlab platform.

  • fossil jawless fish from china foreshadows early jawed vertebrate anatomy
    Nature, 2011
    Co-Authors: Philippe Janvier, Zhikun 盖志琨 Gai, Philip C J Donoghue, Min 朱敏 Zhu, Marco Stampanoni
    Abstract:

    Most living Vertebrates are jawed Vertebrates (gnathostomes), and the living jawless Vertebrates (cyclostomes), hagfishes and lampreys, provide scarce information about the profound reorganization of the vertebrate skull during the evolutionary origin of jaws. The extinct bony jawless Vertebrates, or 'ostracoderms', are regarded as precursors of jawed Vertebrates and provide insight into this formative episode in vertebrate evolution. Here, using synchrotron radiation X-ray tomography, we describe the cranial anatomy of galeaspids, a 435-370-million-year-old 'ostracoderm' group from China and Vietnam. The paired nasal sacs of galeaspids are located anterolaterally in the braincase, and the hypophyseal duct opens anteriorly towards the oral cavity. These three structures (the paired nasal sacs and the hypophyseal duct) were thus already independent of each other, like in gnathostomes and unlike in cyclostomes and osteostracans (another 'ostracoderm' group), and therefore have the condition that current developmental models regard as prerequisites for the development of jaws. This indicates that the reorganization of vertebrate cranial anatomy was not driven deterministically by the evolutionary origin of jaws but occurred stepwise, ultimately allowing the rostral growth of ectomesenchyme that now characterizes gnathostome head development.