- Vertebrate skeletal evolution, Developmental evolution of patterning vertebrate dentitions, 3D Modelling (Architecture), I have ranks from 17 - 1, is this for a month?, Dev and evolutionary origins, Acquisition of dental characters on a phylogeny, and Morphological dental diversity from developmental plasticityedit
- I am a retired academic who is research active in the field of evolutionary and developmental anatomy of fish dentitions.edit
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The late Llandovery (early Silurian) of South China has yielded a locally abundant and diverse microvertebrate fauna. This includes scales of the little-known mongolepids, sinacanthid spines and a whole host of as yet unassigned forms.... more
The late Llandovery (early Silurian) of South China has yielded a locally abundant and diverse microvertebrate fauna. This includes scales of the little-known mongolepids, sinacanthid spines and a whole host of as yet unassigned forms. The material recovered provides a considerable amount of new information about the diversity of fish in the South Yangtze biome during the early Silurian, and suggests that ichthyoliths have a future role to play in Lower Palaeozoic stratigraphic correlation across China and into Mongolia and Siberia. A new family of mongolepids, the Shiqianolepidae, is erected, accommodating the new genusShiqianolepiswith the type speciesS. hollandi. The description ofShiqianolepisenables the identification of a differentiated squamation in mongolepid fish, a feature which has not previously been recognised. Two further taxa,Rongolepis cosmeticagen. et sp. nov. andChenolepis asketagen. et sp. nov., of, as yet, uncertain affinities are also erected.
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Although the Harding Sandstone (basal Franklinian, Caradoc, Ordovician) of Canon City, Fremont County, Colorado, USA, has been known for over one hundred years, recent fieldwork in the type area and the preparation of new material has... more
Although the Harding Sandstone (basal Franklinian, Caradoc, Ordovician) of Canon City, Fremont County, Colorado, USA, has been known for over one hundred years, recent fieldwork in the type area and the preparation of new material has yielded a substantial amount of new ...
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... FROM THE DEVONIAN OF WESTERN AUSTRALIA: ... The ultrastructure is described from scan-ning electron micrographs (sem s) of formed surfaces and fracture surfaces, and compared with sem s of recent mammalian and fish material. ...
Research Interests: Morphogenesis, Biological Sciences, Phylogeny, Animals, Fishes, and 5 moreJaw, Dentition, Dentin, Species Specificity, and Tooth
Cartilaginous vertebrate skeletons leave few records as fossils, unless mineralized. Here, we report outstanding preservation of early stages of cartilage differentiation, present in the Devonian vertebrate Palaeospondylus gunni. In large... more
Cartilaginous vertebrate skeletons leave few records as fossils, unless mineralized. Here, we report outstanding preservation of early stages of cartilage differentiation, present in the Devonian vertebrate Palaeospondylus gunni. In large specimens of Palaeospondylus, enlarged, hypertrophic cell spaces (lacunae) are dominant in the cartilage matrix, each defined by thin mineralized matrix, where phosphorus and calcium co-occur. This is comparable to living endochondral cartilage, where cell hypertrophy and matrix mineralization mark the end of an ontogenetic process of cell growth and division before bone formation. New information from small individuals of Palaeospondylus demonstrates that the skeleton comprises mostly unmineralized organic matrix with fewer hypertrophic cell spaces, these occurring only in the central regions of each element. Only here has the surrounding matrix begun to mineralize, differing from the larger specimens in that phosphorus is dominant with little associated calcium at these earlier stages. This reflects cellular control of mineralization in living tissues through phosphate accumulation around hypertrophic cells, with later increase in calcium in the cartilaginous matrix. These features are always associated with endochondral bone development, but in the Palaeospondylus skeleton, this bone never develops. This skeletal state is thus far unique among vertebrates, with two alternative explanations: either later stages of endochondral bone development have been lost in Palaeospondylus, or, in a stepwise acquisition of the mineralized skeleton, these late stages have not yet evolved.
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Research Interests: Biological Sciences, Phylogeny, Fossils, Female, Animals, and 4 moreMale, Fishes, Biological evolution, and Tooth
This introduction to new patterning theories for the vertebrate dentition outlines the historical concepts to explain graded sequences in tooth shape in mammals (incisors, canines, premolars, molars) which change in evolution in a linked... more
This introduction to new patterning theories for the vertebrate dentition outlines the historical concepts to explain graded sequences in tooth shape in mammals (incisors, canines, premolars, molars) which change in evolution in a linked manner, constant for each region. The classic developmental models for shape regulation, known as the ‘regional field’ and ‘dental clone’ models, were inspired by the human dentition, where it is known that the last tooth in each series is the one commonly absent. The mouse, as a valuable experimental model, has provided data to test these models and more recently, based on spatial-temporal gene expression data, the ‘dental homeobox code’ was proposed to specify regions and regulate tooth shape. We have attempted to combine these hypotheses in a new model of the combinatorial homeobox gene expression pattern with the clone and field theories in one of ‘co-operative genetic interaction’. This also explains the genetic absence of teeth in humans ascribed to point mutations in mesenchymally expressed genes, which affect tooth number in each series. J. Exp. Zool. (Mol. Dev. Evol.) 306B, 2006. © 2006 Wiley-Liss, Inc.
Research Interests: Evolutionary Biology, Zoology, Mammals, Humans, Tagging, and 6 moreAnimals, Tag, Biological evolution, Dentition, Odontogenesis, and Tooth
In classical theory, teeth of vertebrate dentitions evolved from co-option of external skin denticles into the oral cavity. This hypothesis predicts that ordered tooth arrangement and regulated replacement in the oral dentition were also... more
In classical theory, teeth of vertebrate dentitions evolved from co-option of external skin denticles into the oral cavity. This hypothesis predicts that ordered tooth arrangement and regulated replacement in the oral dentition were also derived from skin denticles. The fossil batoid ray Schizorhiza stromeri (Chondrichthyes; Cretaceous) provides a test of this theory. Schizorhiza preserves an extended cartilaginous rostrum with closely spaced, alternating saw-teeth, different from sawfish and sawsharks today. Multiple replacement teeth reveal unique newdata frommicro-CT scanning, showing howthe ‘conein-
cone’ series of ordered saw-teeth sets arrange themselves developmentally, to become enclosed by the roots of pre-existing saw-teeth. At the rostrum tip, newly developing saw-teeth are present, as mineralized crown tips within a vascular, cartilaginous furrow; these reorient via two 908 rotations then relocate laterally between previously formed roots. Saw-tooth replacement slows mid-rostrum where fewer saw-teeth are regenerated. These exceptional developmental data reveal regulated order for serial self-renewal, maintaining the saw edge with ever-increasing saw-tooth size. This mimics tooth replacement in hondrichthyans, but differs in the crown reorientation and their enclosure directly between roots of predecessor saw-teeth. Schizorhiza saw-tooth development is decoupled fromthe jawteeth and their replacement, dependent on a dental lamina. This highly specialized rostral saw, derived from diversification of skin denticles, is distinct from the dentition and demonstrates the potential developmental plasticity of skin denticles.
cone’ series of ordered saw-teeth sets arrange themselves developmentally, to become enclosed by the roots of pre-existing saw-teeth. At the rostrum tip, newly developing saw-teeth are present, as mineralized crown tips within a vascular, cartilaginous furrow; these reorient via two 908 rotations then relocate laterally between previously formed roots. Saw-tooth replacement slows mid-rostrum where fewer saw-teeth are regenerated. These exceptional developmental data reveal regulated order for serial self-renewal, maintaining the saw edge with ever-increasing saw-tooth size. This mimics tooth replacement in hondrichthyans, but differs in the crown reorientation and their enclosure directly between roots of predecessor saw-teeth. Schizorhiza saw-tooth development is decoupled fromthe jawteeth and their replacement, dependent on a dental lamina. This highly specialized rostral saw, derived from diversification of skin denticles, is distinct from the dentition and demonstrates the potential developmental plasticity of skin denticles.
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In classical theory, teeth of vertebrate dentitions evolved from co-option of external skin denticles into the oral cavity. This hypothesis predicts that ordered tooth arrangement and regulated replacement in the oral dentition were also... more
In classical theory, teeth of vertebrate dentitions evolved from co-option of
external skin denticles into the oral cavity. This hypothesis predicts that
ordered tooth arrangement and regulated replacement in the oral dentition
were also derived from skin denticles. The fossil batoid ray Schizorhiza stromeri
(Chondrichthyes; Cretaceous) provides a test of this theory. Schizorhiza preserves
an extended cartilaginous rostrum with closely spaced, alternating
saw-teeth, different from sawfish and sawsharks today. Multiple replacement
teeth reveal unique newdata frommicro-CT scanning, showing howthe ‘conein-
cone’ series of ordered saw-teeth sets arrange themselves developmentally,
to become enclosed by the roots of pre-existing saw-teeth. At the rostrum tip,
newly developing saw-teeth are present, as mineralized crown tips within a
vascular, cartilaginous furrow; these reorient via two 908 rotations then relocate
laterally between previously formed roots. Saw-tooth replacement
slows mid-rostrum where fewer saw-teeth are regenerated. These exceptional
developmental data reveal regulated order for serial self-renewal, maintaining
the saw edge with ever-increasing saw-tooth size. This mimics tooth
replacement in chondrichthyans, but differs in the crown reorientation and
their enclosure directly between roots of predecessor saw-teeth. Schizorhiza
saw-tooth development is decoupled fromthe jawteeth and their replacement,
dependent on a dental lamina. This highly specialized rostral saw, derived
from diversification of skin denticles, is distinct from the dentition and
demonstrates the potential developmental plasticity of skin denticles.
external skin denticles into the oral cavity. This hypothesis predicts that
ordered tooth arrangement and regulated replacement in the oral dentition
were also derived from skin denticles. The fossil batoid ray Schizorhiza stromeri
(Chondrichthyes; Cretaceous) provides a test of this theory. Schizorhiza preserves
an extended cartilaginous rostrum with closely spaced, alternating
saw-teeth, different from sawfish and sawsharks today. Multiple replacement
teeth reveal unique newdata frommicro-CT scanning, showing howthe ‘conein-
cone’ series of ordered saw-teeth sets arrange themselves developmentally,
to become enclosed by the roots of pre-existing saw-teeth. At the rostrum tip,
newly developing saw-teeth are present, as mineralized crown tips within a
vascular, cartilaginous furrow; these reorient via two 908 rotations then relocate
laterally between previously formed roots. Saw-tooth replacement
slows mid-rostrum where fewer saw-teeth are regenerated. These exceptional
developmental data reveal regulated order for serial self-renewal, maintaining
the saw edge with ever-increasing saw-tooth size. This mimics tooth
replacement in chondrichthyans, but differs in the crown reorientation and
their enclosure directly between roots of predecessor saw-teeth. Schizorhiza
saw-tooth development is decoupled fromthe jawteeth and their replacement,
dependent on a dental lamina. This highly specialized rostral saw, derived
from diversification of skin denticles, is distinct from the dentition and
demonstrates the potential developmental plasticity of skin denticles.
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Shark and ray (elasmobranch) dentitions are well known for their multiple generations of teeth, with isolated teeth being common in the fossil record. However, how the diverse dentitions characteristic of elasmobranchs form is still... more
Shark and ray (elasmobranch) dentitions are well known for their multiple generations of teeth, with isolated teeth being common in the fossil record. However, how the diverse dentitions characteristic of elasmobranchs form is still poorly understood. Data on the development and maintenance of the dental patterning in this major vertebrate group will allow comparisons to other morphologically diverse taxa, including the bony fishes, in order to identify shared pattern characters for the vertebrate dentition as a whole. Data is especially lacking from the Batoidea (skates and rays), hence our objective is to compile data on embryonic and adult batoid tooth development contributing to ordering of the dentition, from cleared and stained specimens and micro-CT scans, with 3D rendered models. We selected species (adult and embryonic) spanning phylogenetically significant batoid clades, such that our observations may raise questions about relationships within the batoids, particularly with respect to current molecular-based analyses. We include developmental data from embryos of recent model organisms Leucoraja erinacea and Raja clavata to evaluate the earliest establishment of the dentition. Characters of the batoid dentition investigated include alternate addition of teeth as offset successional tooth rows (versus single separate files), presence of a symphyseal initiator region (symphyseal tooth present, or absent, but with two parasymphyseal teeth) and a restriction to tooth addition along each jaw reducing the number of tooth families, relative to addition of successor teeth within each family. Our ultimate aim is to understand the shared characters of the batoids, and whether or not these dental characters are shared more broadly within elasmobranchs, by comparing these to dentitions in shark outgroups. These developmental morphological analyses will provide a solid basis to better understand dental evolution in these important vertebrate groups as well as the general plesiomorphic vertebrate dental condition.
Ray-finned fishes (Actinopterygii) are the dominant vertebrate group today (þ30 000 species, predominantly teleosts), with great morphological diversity, including their dentitions. How dental morphological variation evolved is best... more
Ray-finned fishes (Actinopterygii) are the dominant vertebrate group today (þ30 000 species, predominantly teleosts), with great morphological diversity, including their dentitions. How dental morphological variation evolved is best addressed by considering a range of taxa across actinopterygian phylogeny;
here we examine the dentition of Polyodon spathula (American paddlefish), assigned to the basal group Acipenseriformes. Although teeth are present and functional in young individuals of Polyodon, they are completely absent in adults. Our current understanding of developmental genes operating in the dentition is primarily restricted to teleosts; we show that shh and bmp4, as highly conserved epithelial and mesenchymal genes for gnathostome tooth development, are similarly expressed at Polyodon tooth loci, thus extending this conserved developmental pattern within the Actinopterygii. These genes map spatio-temporal tooth initiation in Polyodon larvae and provide newdata in both oral and pharyngeal tooth sites.Variation in cellular intensity of shh maps timing of tooth morphogenesis, revealing a second odontogenic wave as alternate sites within tooth rows, a dental pattern also present in more derived actinopterygians. Developmental timing for each tooth field in Polyodon follows a gradient, from rostral to caudal and ventral to dorsal, repeated during subsequent loss of teeth. The transitory Polyodon dentition is modified by cessation of tooth addition and loss. As such, Polyodon represents a basal actinopterygian model for the evolution of developmental novelty: initial conservation, followed by tooth loss, accommodating the adult trophic modification to filter-feeding.
here we examine the dentition of Polyodon spathula (American paddlefish), assigned to the basal group Acipenseriformes. Although teeth are present and functional in young individuals of Polyodon, they are completely absent in adults. Our current understanding of developmental genes operating in the dentition is primarily restricted to teleosts; we show that shh and bmp4, as highly conserved epithelial and mesenchymal genes for gnathostome tooth development, are similarly expressed at Polyodon tooth loci, thus extending this conserved developmental pattern within the Actinopterygii. These genes map spatio-temporal tooth initiation in Polyodon larvae and provide newdata in both oral and pharyngeal tooth sites.Variation in cellular intensity of shh maps timing of tooth morphogenesis, revealing a second odontogenic wave as alternate sites within tooth rows, a dental pattern also present in more derived actinopterygians. Developmental timing for each tooth field in Polyodon follows a gradient, from rostral to caudal and ventral to dorsal, repeated during subsequent loss of teeth. The transitory Polyodon dentition is modified by cessation of tooth addition and loss. As such, Polyodon represents a basal actinopterygian model for the evolution of developmental novelty: initial conservation, followed by tooth loss, accommodating the adult trophic modification to filter-feeding.
The outer armour of fossil jawless fishes (Heterostraci) is, predominantly, a bone with a superficial ornament of dentine tubercles surrounded by pores leading to flask-shaped crypts (ampullae). However, despite the extensive bone... more
The outer armour of fossil jawless fishes (Heterostraci) is, predominantly, a
bone with a superficial ornament of dentine tubercles surrounded by pores
leading to flask-shaped crypts (ampullae). However, despite the extensive
bone present in these early dermal skeletons, damage was repaired almost
exclusively with dentine. Consolidation of bone, by dentine invading and filling
the vascular spaces, was previously recognized in Psammolepis and other
heterostracans but associated with ageing and dermal shieldwear (reparative).
Here, we describe wound repair by deposition of dentine directly onto a bony
scaffold of fragmented bone. An extensive wound response occurred from
massive deposition of dentine(reactionary), traced from tubercle pulp cavities
and surrounding ampullae. These structures may provide the cells to make
reparative and reactionary dentine, as in mammalian teeth today in response
to stimuli (functional wear or damage). We suggest in Psammolepis, repair
involved mobilization of these cells, in response to a local stimulatory mechanism,
for example, predator damage. By comparison, almost no new bone is
detected in repair of the Psammolepis shield. Dentine infilling bone vascular
tissue spaces of both abraded dentine and wounded bone suggests recruitment
of this process has been evolutionarily conserved over 380 Myr and
precedes osteogenic skeletal repair.
bone with a superficial ornament of dentine tubercles surrounded by pores
leading to flask-shaped crypts (ampullae). However, despite the extensive
bone present in these early dermal skeletons, damage was repaired almost
exclusively with dentine. Consolidation of bone, by dentine invading and filling
the vascular spaces, was previously recognized in Psammolepis and other
heterostracans but associated with ageing and dermal shieldwear (reparative).
Here, we describe wound repair by deposition of dentine directly onto a bony
scaffold of fragmented bone. An extensive wound response occurred from
massive deposition of dentine(reactionary), traced from tubercle pulp cavities
and surrounding ampullae. These structures may provide the cells to make
reparative and reactionary dentine, as in mammalian teeth today in response
to stimuli (functional wear or damage). We suggest in Psammolepis, repair
involved mobilization of these cells, in response to a local stimulatory mechanism,
for example, predator damage. By comparison, almost no new bone is
detected in repair of the Psammolepis shield. Dentine infilling bone vascular
tissue spaces of both abraded dentine and wounded bone suggests recruitment
of this process has been evolutionarily conserved over 380 Myr and
precedes osteogenic skeletal repair.
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Classically the oral dentition with teeth regulated into a successional iterative order was thought to have evolved from the superficial skin denticles migrating into the mouth at the stage when jaws evolved. The canonical view is that... more
Classically the oral dentition with teeth regulated into a successional iterative order was thought
to have evolved from the superficial skin denticles migrating into the mouth at the stage when
jaws evolved. The canonical view is that the initiation of a pattern order for teeth at the mouth
margin required development of a sub-epithelial, permanent dental lamina. This provided regulated
tooth production in advance of functional need, as exemplified by the Chondrichthyes. It had been
assumed that teeth in the Osteichthyes form in this way as in tetrapods. However, this has been
shown not to be true for many osteichthyan fish where a dental lamina of this kind does not form,
but teeth are regularly patterned and replaced. We question the evolutionary origin of pattern
information for the dentition driven by new morphological data on spatial initiation of skin
denticles in the catshark. We review recent gene expression data for spatio-temporal order of
tooth initiation for Scyliorhinus canicula, selected teleosts in both oral and pharyngeal dentitions,
and Neoceratodus forsteri. Although denticles in the chondrichthyan skin appear not to follow a
strict pattern order in space and time, tooth replacement in a functional system occurs with
precise timing and spatial order. We suggest that the patterning mechanism observed for the oral
and pharyngeal dentition is unique to the vertebrate oro-pharynx and independent of the skin
system. Therefore, co-option of a successional iterative pattern occurred in evolution not from the
skin but from mechanisms existing in the oro-pharynx of now extinct agnathans.
to have evolved from the superficial skin denticles migrating into the mouth at the stage when
jaws evolved. The canonical view is that the initiation of a pattern order for teeth at the mouth
margin required development of a sub-epithelial, permanent dental lamina. This provided regulated
tooth production in advance of functional need, as exemplified by the Chondrichthyes. It had been
assumed that teeth in the Osteichthyes form in this way as in tetrapods. However, this has been
shown not to be true for many osteichthyan fish where a dental lamina of this kind does not form,
but teeth are regularly patterned and replaced. We question the evolutionary origin of pattern
information for the dentition driven by new morphological data on spatial initiation of skin
denticles in the catshark. We review recent gene expression data for spatio-temporal order of
tooth initiation for Scyliorhinus canicula, selected teleosts in both oral and pharyngeal dentitions,
and Neoceratodus forsteri. Although denticles in the chondrichthyan skin appear not to follow a
strict pattern order in space and time, tooth replacement in a functional system occurs with
precise timing and spatial order. We suggest that the patterning mechanism observed for the oral
and pharyngeal dentition is unique to the vertebrate oro-pharynx and independent of the skin
system. Therefore, co-option of a successional iterative pattern occurred in evolution not from the
skin but from mechanisms existing in the oro-pharynx of now extinct agnathans.
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The rainbow trout (Oncorhynchus mykiss) as a developmental model surpasses both zebrafish and mouse for a more widespread distribution of teeth in the oro-pharynx as the basis for general vertebrate odontogenesis, one in which replacement... more
The rainbow trout (Oncorhynchus mykiss) as a developmental model surpasses
both zebrafish and mouse for a more widespread distribution of teeth in the oro-pharynx as the basis
for general vertebrate odontogenesis, one in which replacement is an essential requirement. Studies
on the rainbow trout have led to the identification of the initial sequential appearance of teeth,
through differential gene expression as a changing spatio-temporal pattern, to set in place the
primary teeth of the first generation, and also to regulate the continuous production of replacement
tooth families. Here we reveal gene expression data that address both the field and clone theories for
patterning a polyphyodont osteichthyan dentition. These data inform how the initial pattern may be
established through up-regulation at tooth loci from a broad odontogenic band. It appears that
control and regulation of replacement pattern resides in the already primed dental epithelium at the
sides of the predecessor tooth. A case is presented for the developmental changes that might have
occurred during vertebrate evolution, for the origin of a separate successional dental lamina, by
comparison with an osteichthyan tetrapod dentition (Ambystoma mexicanum). The evolutionary
origins of such a permanent dental lamina are proposed to have occurred from the transient one
demonstrated here in the trout. This has implications for phylogenies based on the homology of teeth
as only those developed from a dental lamina. Utilising the data generated from the rainbow trout
model, we propose this as a standard for comparative development and evolutionary theories of the
vertebrate dentition. J. Exp. Zool. (Mol. Dev. Evol.) 306B, 2006.
both zebrafish and mouse for a more widespread distribution of teeth in the oro-pharynx as the basis
for general vertebrate odontogenesis, one in which replacement is an essential requirement. Studies
on the rainbow trout have led to the identification of the initial sequential appearance of teeth,
through differential gene expression as a changing spatio-temporal pattern, to set in place the
primary teeth of the first generation, and also to regulate the continuous production of replacement
tooth families. Here we reveal gene expression data that address both the field and clone theories for
patterning a polyphyodont osteichthyan dentition. These data inform how the initial pattern may be
established through up-regulation at tooth loci from a broad odontogenic band. It appears that
control and regulation of replacement pattern resides in the already primed dental epithelium at the
sides of the predecessor tooth. A case is presented for the developmental changes that might have
occurred during vertebrate evolution, for the origin of a separate successional dental lamina, by
comparison with an osteichthyan tetrapod dentition (Ambystoma mexicanum). The evolutionary
origins of such a permanent dental lamina are proposed to have occurred from the transient one
demonstrated here in the trout. This has implications for phylogenies based on the homology of teeth
as only those developed from a dental lamina. Utilising the data generated from the rainbow trout
model, we propose this as a standard for comparative development and evolutionary theories of the
vertebrate dentition. J. Exp. Zool. (Mol. Dev. Evol.) 306B, 2006.
Research Interests:
This introduction to new patterning theories for the vertebrate dentition outlines the historical concepts to explain graded sequences in tooth shape in mammals (incisors, canines, premolars, molars) which change in evolution in a linked... more
This introduction to new patterning theories for the vertebrate dentition outlines
the historical concepts to explain graded sequences in tooth shape in mammals (incisors, canines,
premolars, molars) which change in evolution in a linked manner, constant for each region. The classic
developmental models for shape regulation, known as the ‘regional field’ and ‘dental clone’ models,
were inspired by the human dentition, where it is known that the last tooth in each series is the one
commonly absent. The mouse, as a valuable experimental model, has provided data to test these
models and more recently, based on spatial-temporal gene expression data, the ‘dental homeobox code’
was proposed to specify regions and regulate tooth shape. We have attempted to combine these
hypotheses in a new model of the combinatorial homeobox gene expression pattern with the clone and
field theories in one of ‘co-operative genetic interaction’. This also explains the genetic absence of teeth
in humans ascribed to point mutations in mesenchymally expressed genes, which affect tooth number
in each series. J. Exp. Zool. (Mol. Dev. Evol.) 306B, 2006.
the historical concepts to explain graded sequences in tooth shape in mammals (incisors, canines,
premolars, molars) which change in evolution in a linked manner, constant for each region. The classic
developmental models for shape regulation, known as the ‘regional field’ and ‘dental clone’ models,
were inspired by the human dentition, where it is known that the last tooth in each series is the one
commonly absent. The mouse, as a valuable experimental model, has provided data to test these
models and more recently, based on spatial-temporal gene expression data, the ‘dental homeobox code’
was proposed to specify regions and regulate tooth shape. We have attempted to combine these
hypotheses in a new model of the combinatorial homeobox gene expression pattern with the clone and
field theories in one of ‘co-operative genetic interaction’. This also explains the genetic absence of teeth
in humans ascribed to point mutations in mesenchymally expressed genes, which affect tooth number
in each series. J. Exp. Zool. (Mol. Dev. Evol.) 306B, 2006.
Research Interests:
Repeated tooth initiation occurs often in nonmammalian vertebrates (polyphyodontism), recurrently linked with tooth shedding and in a definite order of succession. Regulation of this process has not been genetically defined and it is... more
Repeated tooth initiation occurs often in
nonmammalian vertebrates (polyphyodontism), recurrently
linked with tooth shedding and in a definite order of
succession. Regulation of this process has not been
genetically defined and it is unclear if the mechanisms for
constant generation of replacement teeth (secondary
dentition) are similar to those used to generate the primary
dentition.We have therefore examined the expression pattern
of a sub-set of genes, implicated in tooth initiation in mouse, in
relation to replacement tooth production in an osteichthyan
fish (Oncorhynchus mykiss). Two epithelial genes pitx2, shh
and one mesenchymal bmp4 were analyzed at selected
stages of development for O. mykiss. pitx2 expression is
upregulated in the basal outer dental epithelium (ODE) of the
predecessor tooth and before cell enlargement, on the
postero-lingual side only. This coincides with the site for
replacement tooth production identifying a region responsible
for further tooth generation. This corresponds with the
expression of pitx2 at focal spots in the basal oral epithelium
during initial (first generation) tooth formation but is now subepithelial
in position and associated with the dental epithelium
of each predecessor tooth. Co-incidental expression of bmp4
and aggregation of the mesenchymal cells identifies the
epithelial–mesenchymal interactions and marks initiation of
the dental papilla. These together suggest a role in tooth site
regulation by pitx2 together with bmp4. Conversely, the
expression of shh is confined to the inner dental epithelium
during the initiation of the first teeth and is lacking from the
ODE in the predecessor teeth, at sites identified as those for
replacement tooth initiation. Importantly, these genes
expressed during replacement tooth initiation can be used
as markers for the sites of ‘‘set-aside cells,’’ the committed
odontogenic cells both epithelial and mesenchymal, which
together can give rise to further generations of teeth. This
information may show how initial pattern formation is
translated into secondary tooth replacement patterns, as a
general mechanism for patterning the vertebrate dentition.
Replacement of the marginal sets of teeth serves as a basis
for discussion of the evolutionary significance, as these
dentate bones (dentary, premaxilla, maxilla) form the
restricted arcades of oral teeth in many crown-group
gnathostomes, including members of the tetrapod stem group.
nonmammalian vertebrates (polyphyodontism), recurrently
linked with tooth shedding and in a definite order of
succession. Regulation of this process has not been
genetically defined and it is unclear if the mechanisms for
constant generation of replacement teeth (secondary
dentition) are similar to those used to generate the primary
dentition.We have therefore examined the expression pattern
of a sub-set of genes, implicated in tooth initiation in mouse, in
relation to replacement tooth production in an osteichthyan
fish (Oncorhynchus mykiss). Two epithelial genes pitx2, shh
and one mesenchymal bmp4 were analyzed at selected
stages of development for O. mykiss. pitx2 expression is
upregulated in the basal outer dental epithelium (ODE) of the
predecessor tooth and before cell enlargement, on the
postero-lingual side only. This coincides with the site for
replacement tooth production identifying a region responsible
for further tooth generation. This corresponds with the
expression of pitx2 at focal spots in the basal oral epithelium
during initial (first generation) tooth formation but is now subepithelial
in position and associated with the dental epithelium
of each predecessor tooth. Co-incidental expression of bmp4
and aggregation of the mesenchymal cells identifies the
epithelial–mesenchymal interactions and marks initiation of
the dental papilla. These together suggest a role in tooth site
regulation by pitx2 together with bmp4. Conversely, the
expression of shh is confined to the inner dental epithelium
during the initiation of the first teeth and is lacking from the
ODE in the predecessor teeth, at sites identified as those for
replacement tooth initiation. Importantly, these genes
expressed during replacement tooth initiation can be used
as markers for the sites of ‘‘set-aside cells,’’ the committed
odontogenic cells both epithelial and mesenchymal, which
together can give rise to further generations of teeth. This
information may show how initial pattern formation is
translated into secondary tooth replacement patterns, as a
general mechanism for patterning the vertebrate dentition.
Replacement of the marginal sets of teeth serves as a basis
for discussion of the evolutionary significance, as these
dentate bones (dentary, premaxilla, maxilla) form the
restricted arcades of oral teeth in many crown-group
gnathostomes, including members of the tetrapod stem group.
Research Interests:
In two species of Heterodontus, H. portusjacksoni and H. galeatus, the first scales to develop form two opposing rows along the caudal fin axis on both the left and right sides of the fin. The opposing rows originate from an initial scale... more
In two species of Heterodontus, H. portusjacksoni and H. galeatus, the first
scales to develop form two opposing rows along the caudal fin axis on both the
left and right sides of the fin. The opposing rows originate from an initial scale
located on either side of the posterior tip of the caudal fin, with subsequent
scales erupting in a posterior to anterior direction along the tail axis. These
scale rows may strengthen tail movements, providing aeration in the egg case,
but are lost later in ontogeny. Development of subsequent body scales shows
a more irregular origin and arrangement, from anterior to posterior, to cover
the dorsal and ventral lobes of the caudal fin. Although the early developmental
pattern of the scale associated with the Heterodontus caudal fin has not been
previously described, several chondrichthyan taxa, including chimeroids, likewise
possess ordered rows of flank scales early in ontogeny that are subsequently lost.
These ordered scales contrast with previous suggestions that chondrichthyan
scale development is entirely random. Instead, regulated and sequential
development of scales may be a plesiomorphic character for both chondrichthyans
and osteichthyans, with the less organized arrangement in later ontogenetic
stages being a derived condition within Chondrichthyes.
scales to develop form two opposing rows along the caudal fin axis on both the
left and right sides of the fin. The opposing rows originate from an initial scale
located on either side of the posterior tip of the caudal fin, with subsequent
scales erupting in a posterior to anterior direction along the tail axis. These
scale rows may strengthen tail movements, providing aeration in the egg case,
but are lost later in ontogeny. Development of subsequent body scales shows
a more irregular origin and arrangement, from anterior to posterior, to cover
the dorsal and ventral lobes of the caudal fin. Although the early developmental
pattern of the scale associated with the Heterodontus caudal fin has not been
previously described, several chondrichthyan taxa, including chimeroids, likewise
possess ordered rows of flank scales early in ontogeny that are subsequently lost.
These ordered scales contrast with previous suggestions that chondrichthyan
scale development is entirely random. Instead, regulated and sequential
development of scales may be a plesiomorphic character for both chondrichthyans
and osteichthyans, with the less organized arrangement in later ontogenetic
stages being a derived condition within Chondrichthyes.
Research Interests:
Regular scale patterning, restricted to the caudalmost tail and organized into two opposing rows on each side of the tail, is observed in few chondrichthyans. These evenly spaced scales, in dorsal and ventral rows, develop in an iterative... more
Regular scale patterning, restricted to the
caudalmost tail and organized into two opposing
rows on each side of the tail, is observed in few
chondrichthyans. These evenly spaced scales, in
dorsal and ventral rows, develop in an iterative
sequence from the caudal tip, either side of the
notochord. They are subsequently lost as a
scattered pattern of placoid scales develops
on the body and fins. An identical organized
pattern is observed in tail scales of Scyliorhinus
canicula (catshark), where the expression of
sonic hedgehog signal is restricted to the
epithelium of developing scales and remains
localized to the scale pocket. Regulation of
iterative scale position by sonic hedgehog is
deeply conserved in vertebrate phylogeny.
These scales also reveal an archaic histological
structure of a dentine type found in the oldest
known shark scales from the Ordovician and
Silurian. This combination of regulated pattern
and ancient dentine occurs only in the tail,
representing the primary scalation. Scattered
body scales in elasmobranchs such as S. canicula
originate secondarily from differently regulated
development, one with typical orthodentine
around a central pulp cavity. These observations
emphasize the modular nature of chondrichthyan
scale development and illustrate previously
undetected variation as an atavism in extant
chondrichthyan dentine.
Keywords: Scyliorhinus; chondrichthyan evolution;
scale development; dentine structure
caudalmost tail and organized into two opposing
rows on each side of the tail, is observed in few
chondrichthyans. These evenly spaced scales, in
dorsal and ventral rows, develop in an iterative
sequence from the caudal tip, either side of the
notochord. They are subsequently lost as a
scattered pattern of placoid scales develops
on the body and fins. An identical organized
pattern is observed in tail scales of Scyliorhinus
canicula (catshark), where the expression of
sonic hedgehog signal is restricted to the
epithelium of developing scales and remains
localized to the scale pocket. Regulation of
iterative scale position by sonic hedgehog is
deeply conserved in vertebrate phylogeny.
These scales also reveal an archaic histological
structure of a dentine type found in the oldest
known shark scales from the Ordovician and
Silurian. This combination of regulated pattern
and ancient dentine occurs only in the tail,
representing the primary scalation. Scattered
body scales in elasmobranchs such as S. canicula
originate secondarily from differently regulated
development, one with typical orthodentine
around a central pulp cavity. These observations
emphasize the modular nature of chondrichthyan
scale development and illustrate previously
undetected variation as an atavism in extant
chondrichthyan dentine.
Keywords: Scyliorhinus; chondrichthyan evolution;
scale development; dentine structure
Research Interests:
Experimental evidence that the neural crest participates in tooth development in any osteichthyan fish has so far been lacking. Using vital dye cell-lineage tracking, we demonstrate that trigeminal stream neural crest cells contribute to... more
Experimental evidence that the neural crest
participates in tooth development in any osteichthyan fish
has so far been lacking. Using vital dye cell-lineage tracking,
we demonstrate that trigeminal stream neural crest cells
contribute to the dental papilla of developing teeth in the
Australian lungfish. Trigeminal neural crest cells labeled
before migration have been traced during the earliest stages
of tooth development. Neural crest cells froma single midbrain
locus were relocated as ectomesenchyme in all developing
teeth of the lungfish regardless of their topographical position
in the dentition. These cells remain at the dental papilla
interface and become cells committed to dentine production.
Our findings provide the first cell-lineage evidence that cranial
neural crest is fated to ectomesenchyme for tooth development
and dentine production in the living sister-group
to tetrapods. This shows that cranial neural crest contribution
to teeth is conserved from this node on the tetrapod
phylogeny.
participates in tooth development in any osteichthyan fish
has so far been lacking. Using vital dye cell-lineage tracking,
we demonstrate that trigeminal stream neural crest cells
contribute to the dental papilla of developing teeth in the
Australian lungfish. Trigeminal neural crest cells labeled
before migration have been traced during the earliest stages
of tooth development. Neural crest cells froma single midbrain
locus were relocated as ectomesenchyme in all developing
teeth of the lungfish regardless of their topographical position
in the dentition. These cells remain at the dental papilla
interface and become cells committed to dentine production.
Our findings provide the first cell-lineage evidence that cranial
neural crest is fated to ectomesenchyme for tooth development
and dentine production in the living sister-group
to tetrapods. This shows that cranial neural crest contribution
to teeth is conserved from this node on the tetrapod
phylogeny.
Research Interests:
We report a temporal order of tooth addition in the Australian lungfish where timing of tooth induction is sequential in the same pattern as osteichthyans along the lower jaw. The order of tooth initiation in Neoceratodus starts from the... more
We report a temporal order of tooth addition in the Australian lungfish where timing of tooth induction is sequential in the same pattern as osteichthyans along the lower jaw. The order of tooth initiation in Neoceratodus starts from the midline tooth, together with left and right ones at jaw position 2, followed by 3
and then 1. This is the pattern order for dentary teeth of several teleosts and what we propose represents a stereotypic initiation pattern shared with all osteichthyans, including the living sister group to all tetrapods, the Australian lungfish. This is contrary to previous opinions that the lungfish dentition is otherwise
derived and uniquely different. Sonic hedgehog (shh) expression is intensely focused on tooth positions at different times corresponding with their initiation order. This deployment of shh is required for lungfish tooth induction, as cyclopamine treatment results in complete loss of these teeth when applied before they develop. The temporal sequence of tooth initiation is possibly regulated by shh and is know to be required for dentition pattern in other osteichthyans, including cichlid fish and snakes. This reflects a shared developmental process with jawed vertebrates at the level of the tooth module but differs with the lack of replacement teeth.
and then 1. This is the pattern order for dentary teeth of several teleosts and what we propose represents a stereotypic initiation pattern shared with all osteichthyans, including the living sister group to all tetrapods, the Australian lungfish. This is contrary to previous opinions that the lungfish dentition is otherwise
derived and uniquely different. Sonic hedgehog (shh) expression is intensely focused on tooth positions at different times corresponding with their initiation order. This deployment of shh is required for lungfish tooth induction, as cyclopamine treatment results in complete loss of these teeth when applied before they develop. The temporal sequence of tooth initiation is possibly regulated by shh and is know to be required for dentition pattern in other osteichthyans, including cichlid fish and snakes. This reflects a shared developmental process with jawed vertebrates at the level of the tooth module but differs with the lack of replacement teeth.
Research Interests:
For a dentition representing the most basal extant gnathostomes, that of the shark can provide us with key insights into the evolution of vertebrate dentitions. To detail the pattern of odontogenesis, we have profiled the expression of... more
For a dentition representing the most basal extant gnathostomes, that of the shark can provide us with key
insights into the evolution of vertebrate dentitions. To detail the pattern of odontogenesis, we have profiled
the expression of sonic hedgehog, a key regulator of tooth induction. We find in the catshark (Scyliorhinus
canicula) that intense shh expression first occurs in a bilaterally symmetrical pattern restricted to broad
regions in each half of the dentition in the embryo jaw. As in the mouse, there follows a changing temporal
pattern of shh spatial restriction corresponding to epithelial bands of left and right dental fields, but also a
subfield for symphyseal teeth. Then, intense shh expression is restricted to loci coincident with a temporal
series of teeth in iterative jaw positions. The developmental expression of shh reveals previously undetected
timing within epithelial stages of tooth formation. Each locus at alternate, even then odd, jaw positions
establishes precise sequential timing for successive replacement within each tooth family. Shh appears first
in the central cusp, iteratively along the jaw, then reiteratively within each tooth for secondary cusps. This
progressive, sequential restriction of shh is shared by toothed gnathostomes and conserved through 500
million years of evolution.
Keywords: catshark; sonic hedgehog; dentition development; tooth patterning; evolution dentition
insights into the evolution of vertebrate dentitions. To detail the pattern of odontogenesis, we have profiled
the expression of sonic hedgehog, a key regulator of tooth induction. We find in the catshark (Scyliorhinus
canicula) that intense shh expression first occurs in a bilaterally symmetrical pattern restricted to broad
regions in each half of the dentition in the embryo jaw. As in the mouse, there follows a changing temporal
pattern of shh spatial restriction corresponding to epithelial bands of left and right dental fields, but also a
subfield for symphyseal teeth. Then, intense shh expression is restricted to loci coincident with a temporal
series of teeth in iterative jaw positions. The developmental expression of shh reveals previously undetected
timing within epithelial stages of tooth formation. Each locus at alternate, even then odd, jaw positions
establishes precise sequential timing for successive replacement within each tooth family. Shh appears first
in the central cusp, iteratively along the jaw, then reiteratively within each tooth for secondary cusps. This
progressive, sequential restriction of shh is shared by toothed gnathostomes and conserved through 500
million years of evolution.
Keywords: catshark; sonic hedgehog; dentition development; tooth patterning; evolution dentition
Although the lungfish (Dipnoi) belong within the Osteichthyes, their dentitions are radically different from other osteichthyans. Lungfish dentitions also show a uniquely high structural disparity during the early evolution of the group,... more
Although the lungfish (Dipnoi) belong within the
Osteichthyes, their dentitions are radically different from other
osteichthyans. Lungfish dentitions also show a uniquely high
structural disparity during the early evolution of the group,
partly owing to the independent variation of odontogenic and
odontoclastic processes that are tightly and stereotypically
coordinated in other osteichthyans. We present a phylogenetic
analysis of early lungfishes incorporating a novel
approach to coding these process characters in preference to
the resultant adult dental morphology. The results only
partially resolve the interrelationships of Devonian dipnoans,
but show that the widely discussed hypothesis of separate
tooth-plated, dentine-plated, and denticulated lineages is
unlikely to be true. The dipnoan status of Diabolepis is
corroborated. Lungfish dentitions seem to have undergone
extensive and nonparsimonious evolution during the early
history of the group, but much of the resulting disparity can be
explained by a modest number of evolutionary steps in the
underlying developmental processes, those for dental
formation (odontogenic) and those for the remodeling of
dentine tissue (odontoclastic). Later in lungfish evolution, this
disparity was lost as the group settled to a pattern of dental
development that is just as stereotypic as, but completely different from, that of other osteichthyans.
Osteichthyes, their dentitions are radically different from other
osteichthyans. Lungfish dentitions also show a uniquely high
structural disparity during the early evolution of the group,
partly owing to the independent variation of odontogenic and
odontoclastic processes that are tightly and stereotypically
coordinated in other osteichthyans. We present a phylogenetic
analysis of early lungfishes incorporating a novel
approach to coding these process characters in preference to
the resultant adult dental morphology. The results only
partially resolve the interrelationships of Devonian dipnoans,
but show that the widely discussed hypothesis of separate
tooth-plated, dentine-plated, and denticulated lineages is
unlikely to be true. The dipnoan status of Diabolepis is
corroborated. Lungfish dentitions seem to have undergone
extensive and nonparsimonious evolution during the early
history of the group, but much of the resulting disparity can be
explained by a modest number of evolutionary steps in the
underlying developmental processes, those for dental
formation (odontogenic) and those for the remodeling of
dentine tissue (odontoclastic). Later in lungfish evolution, this
disparity was lost as the group settled to a pattern of dental
development that is just as stereotypic as, but completely different from, that of other osteichthyans.
The histological composition of the galeaspid cephalothoracic skeleton has been much debated: here we attempt to resolve this through the analysis of well-preserved remains of galeaspids from Yunnan Province, and Tarim Basin, Xinjiang... more
The histological composition of the galeaspid cephalothoracic skeleton has been much debated: here we
attempt to resolve this through the analysis of well-preserved remains of galeaspids from Yunnan Province, and Tarim
Basin, Xinjiang Uygur Autonomous Region, China. Our results indicate that the galeaspid dermoskeleton is dominantly
composed from an acellular laminar bone in which the mineral is organised into cylindrical crystal bundles that are
arranged into three orthogonal sets with associated extrinsic fiber spaces, a unique histology for which the term galeaspedin
is coined. This is permeated by a coarse vascular plexus that divides the dermoskeleton into upper and lower zones,
and the upper zone into distinct tesserae which, like the bounding vascular network, are polygonal in outline.
The outer surface of the dermoskeleton is ornamented by a series of tubercles centered on tesserae, the latter composed
partly from galeaspedin, and partly from a capping layer of microspherulitic, acellular bone, similar to the limiting layer
of bone of elasmoid scales. Neither dentine nor enameloid is present, nor do the tissue compositions or their arrangement
indicate an odontogenic origin.
The endoskeleton is composed of an outer zone of globular calcified cartilage in contact with the dermoskeleton
through a poorly mineralized intermediate zone. The inner zone is finely laminated, resulting from progressive zones of
calcification embracing the calcospherites in a direction away from the dermoskeleton. There is no persuasive histological
evidence for the presence of appositional perichondral bone. As in osteostracans, the galeaspid endoskeleton is interpreted
as an expanded neurocranium. However, the presence of a calcified cartilaginous neurocranium in galeaspids in
the absence of a perichondral bone layer indicates that these two histogenic components have distinct evolutionary
origins. The presence of perichondral bone is a synapomorphy of osteostracans and jawed vertebrates, while the presence
of a mineralized neurocranium unites galeaspids to this clade (possibly also including pituriaspids).
INTRODUCTION
Galeaspids constitute an extinct clade (Silurian-Devonian) of
jawless vertebrates endemic to China (P’an and Dineley, 1988),
Tarim (Wang, Wang, and Zhu, 1996) and northern Vietnam
(Thanh and Janvier, 1987). Phylogenetic analyses of early vertebrates
consistently resolve the group, with the osteostracans and
pituriaspids, as immediate sister taxa to the placoderms and
crown-group gnathostomes (Janvier, 1981a, 1996a; Forey and
Janvier, 1993; Forey, 1995; Donoghue, Forey, and Aldridge,
2000; Donoghue and Smith, 2001), mainly on the basis of hard
tissue histological characteristics. As such, the galeaspids occupy
a critical position in our understanding of the assembly of the
body plan of jawed vertebrates, and of gnathostomes in general.
However, this understanding rests in large part upon a correct
interpretation of the homologies of the tissues constituting the
galeaspid skeleton and, unfortunately, there is no clear consensus
on this issue.
The structure and histological composition of the galeaspid
skeleton has been interpreted in many different ways, largely
because early studies were based on inferences of the presence,
composition, and organisation of various tissue layers from macroscopic
observations of unprepared fractured surfaces. Thus,
Janvier (1981a) and P’an (1984) interpreted small polygonal impressions
as the basal layer of the dermoskeleton, derived from
an original honeycomb sub-structure, comparable with the condition
in heterostracans
attempt to resolve this through the analysis of well-preserved remains of galeaspids from Yunnan Province, and Tarim
Basin, Xinjiang Uygur Autonomous Region, China. Our results indicate that the galeaspid dermoskeleton is dominantly
composed from an acellular laminar bone in which the mineral is organised into cylindrical crystal bundles that are
arranged into three orthogonal sets with associated extrinsic fiber spaces, a unique histology for which the term galeaspedin
is coined. This is permeated by a coarse vascular plexus that divides the dermoskeleton into upper and lower zones,
and the upper zone into distinct tesserae which, like the bounding vascular network, are polygonal in outline.
The outer surface of the dermoskeleton is ornamented by a series of tubercles centered on tesserae, the latter composed
partly from galeaspedin, and partly from a capping layer of microspherulitic, acellular bone, similar to the limiting layer
of bone of elasmoid scales. Neither dentine nor enameloid is present, nor do the tissue compositions or their arrangement
indicate an odontogenic origin.
The endoskeleton is composed of an outer zone of globular calcified cartilage in contact with the dermoskeleton
through a poorly mineralized intermediate zone. The inner zone is finely laminated, resulting from progressive zones of
calcification embracing the calcospherites in a direction away from the dermoskeleton. There is no persuasive histological
evidence for the presence of appositional perichondral bone. As in osteostracans, the galeaspid endoskeleton is interpreted
as an expanded neurocranium. However, the presence of a calcified cartilaginous neurocranium in galeaspids in
the absence of a perichondral bone layer indicates that these two histogenic components have distinct evolutionary
origins. The presence of perichondral bone is a synapomorphy of osteostracans and jawed vertebrates, while the presence
of a mineralized neurocranium unites galeaspids to this clade (possibly also including pituriaspids).
INTRODUCTION
Galeaspids constitute an extinct clade (Silurian-Devonian) of
jawless vertebrates endemic to China (P’an and Dineley, 1988),
Tarim (Wang, Wang, and Zhu, 1996) and northern Vietnam
(Thanh and Janvier, 1987). Phylogenetic analyses of early vertebrates
consistently resolve the group, with the osteostracans and
pituriaspids, as immediate sister taxa to the placoderms and
crown-group gnathostomes (Janvier, 1981a, 1996a; Forey and
Janvier, 1993; Forey, 1995; Donoghue, Forey, and Aldridge,
2000; Donoghue and Smith, 2001), mainly on the basis of hard
tissue histological characteristics. As such, the galeaspids occupy
a critical position in our understanding of the assembly of the
body plan of jawed vertebrates, and of gnathostomes in general.
However, this understanding rests in large part upon a correct
interpretation of the homologies of the tissues constituting the
galeaspid skeleton and, unfortunately, there is no clear consensus
on this issue.
The structure and histological composition of the galeaspid
skeleton has been interpreted in many different ways, largely
because early studies were based on inferences of the presence,
composition, and organisation of various tissue layers from macroscopic
observations of unprepared fractured surfaces. Thus,
Janvier (1981a) and P’an (1984) interpreted small polygonal impressions
as the basal layer of the dermoskeleton, derived from
an original honeycomb sub-structure, comparable with the condition
in heterostracans
Research Interests:
Lungfish have a long evolutionary history, first appearing in the Early Devonian, with three genera extant. Lungfish dentitions were particularly diverse and have been afocus of study for many years. Although diverse, all dentitions can... more
Lungfish have a long evolutionary history, first appearing in the Early Devonian, with three genera extant. Lungfish dentitions were particularly diverse and have been afocus of study for many years. Although diverse, all dentitions can be derived from a toothplated dentition, where components of this dentition, in terms of tooth structures and the processes controlling development, have become dissociated and free to vary.
Despite previous suggestions that lungfish dentitions are not homologous to dentitions in other sarcopterygian and actinopterygian taxa (osteichthyans), new research on
Neoceratodus forsteri indicates several shared similarities in terms of genes involved in dental patterning, tooth origin and positioning on the jaw, and contribution of neural crest cells to tooth development. Future research should expand on these early results,and continue particularly to study genes from the ‘core dental gene network’, found in other fishes
Despite previous suggestions that lungfish dentitions are not homologous to dentitions in other sarcopterygian and actinopterygian taxa (osteichthyans), new research on
Neoceratodus forsteri indicates several shared similarities in terms of genes involved in dental patterning, tooth origin and positioning on the jaw, and contribution of neural crest cells to tooth development. Future research should expand on these early results,and continue particularly to study genes from the ‘core dental gene network’, found in other fishes
Research Interests:
The bearing that agnathans have on the origin of jawed vertebrates is one of the great unsolved problems in vertebrate phylogeny. Here we propose a mechanism for the evolution of jaws in vertebrates based on a combination of evidence from... more
The bearing that agnathans have on the origin of jawed vertebrates is one of the great unsolved
problems in vertebrate phylogeny. Here we propose a mechanism for the evolution of jaws in
vertebrates based on a combination of evidence from the fossil record and from experimental
developmental biology. In chick embryos, osteogenesis can be evoked experimentally from scleral
mesenchyme by the same mandibular epithelium that evokes osteogenesis in the jaws. Sclerotic
bones appeared before jaws in vertebrate phylogeny and scleral ossicles and jaw skeletons are
linked by shared developmental processes. As only one group of fossil agnathans—the
Osteostraci—has ossified sclerotic bones, they alone possessed the inherent developmental
potential to develop bone in the lower jaws and are also considered the most probable sister taxon to gnathostomes
problems in vertebrate phylogeny. Here we propose a mechanism for the evolution of jaws in
vertebrates based on a combination of evidence from the fossil record and from experimental
developmental biology. In chick embryos, osteogenesis can be evoked experimentally from scleral
mesenchyme by the same mandibular epithelium that evokes osteogenesis in the jaws. Sclerotic
bones appeared before jaws in vertebrate phylogeny and scleral ossicles and jaw skeletons are
linked by shared developmental processes. As only one group of fossil agnathans—the
Osteostraci—has ossified sclerotic bones, they alone possessed the inherent developmental
potential to develop bone in the lower jaws and are also considered the most probable sister taxon to gnathostomes