Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

Living vertebrates are divided into those that possess a fully formed and fully mineralised skeleton (gnathostomes) versus those that possess only unmineralised cartilaginous rudiments (cyclostomes). As such, extinct phylogenetic intermediates of these living lineages afford unique insights into the evolutionary assembly of the vertebrate mineralised skeleton and its canonical tissue types. Extinct jawless and jawed fishes assigned to the gnathostome stem evidence the piecemeal assembly of skeletal systems, revealing that the dermal skeleton is the earliest manifestation of a homologous mineralised skeleton. Yet the nature of the primitive dermal skeleton, itself, is poorly understood. This is principally because previous histological studies of early vertebrates lacked a phylogenetic framework required to derive evolutionary hypotheses. Nowhere is this more apparent than within Heterostraci, a diverse clade of primitive jawless vertebrates. To this end, we surveyed the dermal skeletal histology of heterostracans, inferred the plesiomorphic heterostracan skeleton and, through histological comparison to other skeletonising vertebrate clades, deduced the ancestral nature of the vertebrate dermal skeleton. Heterostracans primitively possess a four‐layered skeleton, comprising a superficial layer of odontodes composed of dentine and enameloid; a compact layer of acellular parallel‐fibred bone containing a network of vascular canals that supply the pulp canals (L1); a trabecular layer consisting of intersecting radial walls composed of acellular parallel‐fibred bone, showing osteon‐like development (L2); and a basal layer of isopedin (L3). A three layered skeleton, equivalent to the superficial layer L2 and L3 and composed of enameloid, dentine and acellular bone, is possessed by the ancestor of heterostracans + jawed vertebrates. We conclude that an osteogenic component is plesiomorphic with respect to the vertebrate dermal skeleton. Consequently, we interpret the dermal skeleton of denticles in chondrichthyans and jawless thelodonts as independently and secondarily simplified. J. Morphol. 276:657–680, 2015. © 2015 The Authors Journal of Morphology Published by Wiley Periodicals, Inc.     

Histology of “placoderm” dermal skeletons: Implications for the nature of the ancestral gnathostome

The vertebrate dermal skeleton has long been interpreted to have evolved from a primitive condition exemplified by chondrichthyans. However, chondrichthyans and osteichthyans evolved from an ancestral gnathostome stem‐lineage in which the dermal skeleton was more extensively developed. To elucidate the histology and skeletal structure of the gnathostome crown‐ancestor we conducted a histological survey of the diversity of the dermal skeleton among the placoderms, a diverse clade or grade of early jawed vertebrates. The dermal skeleton of all placoderms is composed largely of a cancellar architecture of cellular dermal bone, surmounted by dermal tubercles in the most ancestral clades, including antiarchs. Acanthothoracids retain an ancestral condition for the dermal skeleton, and we record its secondary reduction in antiarchs. We also find that mechanisms for remodeling bone and facilitating different growth rates between adjoining plates are widespread throughout the placoderms. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.     

New birkeniid anaspid from the Lower Devonian of Scotland and its phylogenetic implications

Abstract: A new possible stem gnathostome, Kerreralepis carinata gen. et sp. nov., is described on the basis of a single specimen from the Lower Devonian of the island of Kerrera in the Inner Hebrides, Scotland. It is recognized as an anaspid by the chevron‐like arranged rod‐shaped scales on the trunk, gill openings extending behind the orbits in a slanting row and a series of median dorsal ridge scales. This specimen also has a series of median ventral plates, indicating the presence of a preanal fin‐fold, which in turn has consequences for interpretations of other problematic stem gnathostomes and their phylogenetic context. A cladistic analysis supports a monophyletic Anaspida including the scale‐covered birkeniids but excluding Lasanius as well as anaspid‐like forms such as Euphanerops and Jamoytius. The establishment of a new genus and species increases the diversity of anaspids and allows for a more detailed study of anaspid interrelationships. An ingroup analysis using Lasanius as an outgroup resolves Birkenia as a rather basal anaspid, sister to all other anaspids, alternatively sister to a clade represented by the taxa from Ringerike, Norway, and the closely associated taxon from Saaremaa Island, Estonia. These topologies agree rather well with the present fossil record of anaspids.     

Genome biology of the cyclostomes and insights into the evolutionary biology of vertebrate genomes

The jawless vertebrates (lamprey and hagfish) are the closest extant outgroups to all jawed vertebrates (gnathostomes) and can therefore provide critical insight into the evolution and basic biology of vertebrate genomes. As such, it is notable that the genomes of lamprey and hagfish possess a capacity for rearrangement that is beyond anything known from the gnathostomes. Like the jawed vertebrates, lamprey and hagfish undergo rearrangement of adaptive immune receptors. However, the receptors and the mechanisms for rearrangement that are utilized by jawless vertebrates clearly evolved independently of the gnathostome system. Unlike the jawed vertebrates, lamprey and hagfish also undergo extensive programmed rearrangements of the genome during embryonic development. By considering these fascinating genome biologies in the context of proposed (albeit contentious) phylogenetic relationships among lamprey, hagfish, and gnathostomes, we can begin to understand the evolutionary history of the vertebrate genome. Specifically, the deep shared ancestry and rapid divergence of lampreys, hagfish and gnathostomes is considered evidence that the two versions of programmed rearrangement present in lamprey and hagfish (embryonic and immune receptor) were present in an ancestral lineage that existed more than 400 million years ago and perhaps included the ancestor of the jawed vertebrates. Validating this premise will require better characterization of the genome sequence and mechanisms of rearrangement in lamprey and hagfish.     

Lamprey as an evo-devo model: lessons from comparative embryology and molecular phylogenetics

Lamprey, the living jawless vertebrate, has been regarded as one of the most primitive groups of vertebrates. The evolutionary phylogenetic position of the lamprey promises to provide hints about the origin of the vertebrate genome as well as the origin of the body plan, a part of which may be written in the genome. Since the lamprey split from the gnathostome lineage early in the history of vertebrates, the shared developmental mechanisms in lampreys and gnathostomes can be regarded as possessed by the hypothetical common ancestor of these animals, whereas the gnathostome-specific developmental mechanisms that are absent from lampreys indicate that they are relatively new, added to the developmental program only after the split of gnathostomes. Thus, the sequential establishment of the gnathostome body plan is inherently related to the history of genomic duplication events. In this review, recent molecular developmental and evolutionary molecular research on the living lampreys are summarized and discussed, taking vertebrate comparative morphology and embryology into consideration.     

Early vertebrate evolution

Debate over the origin and evolution of vertebrates has occupied biologists and palaeontologists alike for centuries. This debate has been refined by molecular phylogenetics, which has resolved the place of vertebrates among their invertebrate chordate relatives, and that of chordates among their deuterostome relatives. The origin of vertebrates is characterized by wide‐ranging genomic, embryologic and phenotypic evolutionary change. Analyses based on living lineages suggest dramatic shifts in the tempo of evolutionary change at the origin of vertebrates and gnathostomes, coincident with whole‐genome duplication events. However, the enriched perspective provided by the fossil record demonstrates that these apparent bursts of anatomical evolution and taxic richness are an artefact of the extinction of phylogenetic intermediates whose fossil remains evidence the gradual assembly of crown gnathostome characters in particular. A more refined understanding of the timing, tempo and mode of early vertebrate evolution rests with: (1) better genome assemblies for living cyclostomes; (2) a better understanding of the anatomical characteristics of key fossil groups, especially the anaspids, thelodonts, galeaspids and pituriaspids; (3) tests of the monophyly of traditional groups; and (4) the application of divergence time methods that integrate not just molecular data from living species, but also morphological data and extinct species. The resulting framework will provide for rigorous tests of rates of character evolution and diversification, and of hypotheses of long‐term trends in ecological evolution that themselves suffer for lack of quantitative functional tests. The fossil record has been silent on the nature of the transition from jawless vertebrates to the jawed vertebrates that have dominated communities since the middle Palaeozoic. Elucidation of this most formative of episodes likely rests with the overhaul of early vertebrate systematics that we propose, but perhaps more fundamentally with fossil grades that await discovery.     

Otx expression during lamprey embryogenesis provides insights into the evolution of the vertebrate head and jaw

Agnathan or jawless vertebrates, such as lampreys, occupy a critical phylogenetic position between the gnathostome or jawed vertebrates and the cephalochordates, represented by amphioxus. In order to gain insight into the evolution of the vertebrate head, we have cloned and characterized a homolog of the head-specific gene Otx from the lamprey Petromyzon marinus. This lamprey Otx gene is a clear phylogenetic outgroup to both the gnathostome Otx1 and Otx2 genes. Like its gnathostome counterparts, lamprey Otx is expressed throughout the presumptive forebrain and midbrain. Together, these results indicate that the divergence of Otx1 and Otx2 took place after the gnathostome/agnathan divergence and does not correlate with the origin of the vertebrate brain. Intriguingly, Otx is also expressed in the cephalic neural crest cells as well as mesenchymal and endodermal components of the first pharyngeal arch in lampreys, providing molecular evidence of homology with the gnathostome mandibular arch and insights into the evolution of the gnathostome jaw.     

Lasanius,an exceptionally preserved Silurian jawless fish from Scotland

The fossil record of non-biomineralizing, soft-bodied taxa is our only direct evidence of the early history of vertebrates. A robust reconstruction of the affinities of these taxa is critical to unlocking vertebrate origins and understanding the evolution of skeletal tissues, but these taxa invariably have unstable and poorly supported phylogenetic positions. At the cusp between mineralized bony vertebrates and entirely soft-bodied vertebrates is the enigmatic Lasanius, a purported anaspid from the Silurian of Scotland. Interpretations of its affinity and significance are conflicted, principally because of its poorly understood anatomy due to taphonomic distortion and loss of soft-tissues. Here we use an array of modern techniques to reassess the anatomy of Lasanius via a comprehensive study of 229 complete and partial specimens. A new reconstruction clarifies the identity and position of preserved features, including paired sensory organs, a notochord, and digestive tract, supporting the vertebrate affinities of this genus. SEM-EDS trace element mapping suggests a bone-like composition of mineralized parts, but finds no evidence for mineralized dermal armour. Phylogenetic analysis recovers Lasanius as an early stem-cyclostome, and subsequent analysis supports the rejection of alternative placements (such as stem-gnathostome). We highlight that while distinguishing between the early cyclostome and gnathostome condition is problematic, increasing confidence in the anatomy of key taxa, such as Lasanius, is vital for increased stability throughout the early vertebrate tree.     

A fate-map for cranial sensory ganglia in the sea lamprey

Cranial neurogenic placodes and the neural crest make essential contributions to key adult characteristics of all vertebrates, including the paired peripheral sense organs and craniofacial skeleton. Neurogenic placode development has been extensively characterized in representative jawed vertebrates (gnathostomes) but not in jawless fishes (agnathans). Here, we use in vivo lineage tracing with DiI, together with neuronal differentiation markers, to establish the first detailed fate-map for placode-derived sensory neurons in a jawless fish, the sea lamprey Petromyzon marinus, and to confirm that neural crest cells in the lamprey contribute to the cranial sensory ganglia. We also show that a pan-Pax3/7 antibody labels ophthalmic trigeminal (opV, profundal) placode-derived but not maxillomandibular trigeminal (mmV) placode-derived neurons, mirroring the expression of gnathostome Pax3 and suggesting that Pax3 (and its single Pax3/7 lamprey ortholog) is a pan-vertebrate marker for opV placode-derived neurons. Unexpectedly, however, our data reveal that mmV neuron precursors are located in two separate domains at neurula stages, with opV neuron precursors sandwiched between them. The different branches of the mmV nerve are not comparable between lampreys and gnatho-stomes, and spatial segregation of mmV neuron precursor territories may be a derived feature of lampreys. Nevertheless, maxillary and mandibular neurons are spatially segregated within gnathostome mmV ganglia, suggesting that a more detailed investigation of gnathostome mmV placode development would be worthwhile. Overall, however, our results highlight the conservation of cranial peripheral sensory nervous system development across vertebrates, yielding insight into ancestral vertebrate traits.     

A three‐dimensional placoderm (stem‐group gnathostome) pharyngeal skeleton and its implications for primitive gnathostome pharyngeal architecture

The pharyngeal skeleton is a key vertebrate anatomical system in debates on the origin of jaws and gnathostome (jawed vertebrate) feeding. Furthermore, it offers considerable potential as a source of phylogenetic data. Well‐preserved examples of pharyngeal skeletons from stem‐group gnathostomes remain poorly known. Here, we describe an articulated, nearly complete pharyngeal skeleton in an Early Devonian placoderm fish, Paraplesiobatis heinrichsi Broili, from Hunsrück Slate of Germany. Using synchrotron light tomography, we resolve and reconstruct the three‐dimensional gill arch architecture of Paraplesiobatis and compare it with other gnathostomes. The preserved pharyngeal skeleton comprises elements of the hyoid arch (probable ceratohyal) and a series of branchial arches. Limited resolution in the tomography scan causes some uncertainty in interpreting the exact number of arches preserved. However, at least four branchial arches are present. The final and penultimate arches are connected as in osteichthyans. A single median basihyal is present as in chondrichthyans. No dorsal (epibranchial or pharyngobranchial) elements are observed. The structure of the pharyngeal skeleton of Paraplesiobatis agrees well with Pseudopetalichthys from the same deposit, allowing an alternative interpretation of the latter taxon. The phylogenetic significance of Paraplesiobatis is considered. A median basihyal is likely an ancestral gnathostome character, probably with some connection to both the hyoid and the first branchial arch pair. Unpaired basibranchial bones may be independently derived in chondrichthyans and osteichthyans.     

The evolution of paired fins

Summary The Archipterygium is Gegenbaur’s most lasting contribution to the study of vertebrate limb evolution. This transformational hypothesis of gill arches to limb girdles, rays to fins, and proposal of a vertebrate fin-limb groundplan, is generally treated as a flawed alternative to the more widely accepted lateral fin-fold hypothesis of vertebrate limb evolution. When compared to the phylogenetic distribution and diversity of fins and limbs, both hypotheses fail. Dermal skeletal lateral folds, spines and keels originate repeatedly in vertebrate evolution, but paired fins with girdles originate at pectoral level and are anteroposteriorly restricted. Pelvic fins emerge later in phylogeny; pectoral and pelvic appendages primitively differ. Endoskeletal girdles never exhibit characteristics of gill arches. The emergent sequence of paired fin evolution depends upon phylogenetic hypotheses within which extant agnathan interrelationships are uncertain; positions of jawless fossil fish along the gnathostome stem are insecure; the fossil data set is patchy. However, certain features of the data set are robust. This has prompted a reconsideration of Gegenbaur’s hypothesized arch-girdle relationship, and an iterative homology between scapulocoracoid and extrabranchial cartilages is suggested. No transformation of arch to girdle is necessarily implied, but some signal of developmental relatedness is predicted.     

Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history

The fossil record of early vertebrates has been influential in elucidating the evolutionary assembly of the gnathostome bodyplan. Understanding of the timing and tempo of vertebrate innovations remains, however, mired in a literal reading of the fossil record. Early jawless vertebrates (ostracoderms) exhibit restriction to shallow-water environments. The distribution of their stratigraphic occurrences therefore reflects not only flux in diversity, but also secular variation in facies representation of the rock record. Using stratigraphic, phylogenetic and palaeoenvironmental data, we assessed the veracity of the fossil records of the jawless relatives of jawed vertebrates (Osteostraci, Galeaspida, Thelodonti, Heterostraci). Non-random models of fossil recovery potential using Palaeozoic sea-level changes were used to calculate confidence intervals of clade origins. These intervals extend the timescale for possible origins into the Upper Ordovician; these estimates ameliorate the long ghost lineages inferred for Osteostraci, Galeaspida and Heterostraci, given their known stratigraphic occurrences and stem–gnathostome phylogeny. Diversity changes through the Silurian and Devonian were found to lie within the expected limits predicted from estimates of fossil record quality indicating that it is geological, rather than biological factors, that are responsible for shifts in diversity. Environmental restriction also appears to belie ostracoderm extinction and demise rather than competition with jawed vertebrates.     

A new evolutionary scenario for the vertebrate jaw

The jaw is one of the earliest innovations in vertebrate history. Several recent findings suggest a scenario for jaw evolution as a progression of changes in pharyngeal developmental mechanisms. The lamprey, an extant jawless vertebrate, constitutes a model for the pre-gnathostome ancestry. Comparing expression patterns of regulatory genes between the gnathostome and lamprey embryos may enable us to get a glimpse of the essential changes that were responsible for the evolution of the jaw. We hypothesize that a specific topographical change of inductive tissue interactions to be described here brought about the jaw as an evolutionary novelty.     

The origin of developmental mechanisms underlying vertebral elements: implications from hagfish evo-devo

The origins of the vertebral elements and the underlying developmental mechanisms have so far remained unclear, largely due to the unusual axial skeletal morphology of hagfish, one of two extant jawless vertebrate clades. Hagfish axial supporting tissue is generally believed to consist of the notochord and cartilaginous fin rays only. However, careful investigations of whether vertebral elements are truly absent in hagfish are scarce, and it is also unclear whether the axial skeletal morphology of the hagfish is an ancestral or a derived condition. To address these questions, we re-examined the axial skeletal morphology of the Japanese inshore hagfish (Eptatretus burgeri). Based on a report published a century ago which implied the existence of vertebral elements in hagfish, we conducted anatomical and histological analyses of the hagfish axial skeletal systems and their development. Through this analysis, we demonstrate that hagfish possesses sclerotome-derived cartilaginous vertebral elements at the ventral aspect of the notochord. Based on (i) molecular phylogenetic evidence in support of the monophyly of cyclostomes (hagfish and lampreys) and jawed vertebrates (gnathostomes), and (ii) the morphology of the vertebral elements in extant gnathostomes and cyclostomes, we propose that the embryos of the common ancestor of all vertebrates would have possessed sclerotomal cells that formed the segmentally arranged vertebral elements attached to the notochord. We also conclude that the underlying developmental mechanisms are likely to have been conserved among extinct jawless vertebrates and modern gnathostomes.