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

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ABSTRACT

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.

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Histology of Loricopteraspis serrata. NRM‐PAL C.5939, SEM BSE section through the cephalothoracic shield (A); NRM‐PAL C.5940, SRXTM section through a scale unit (B); NRM‐PAL C.5941, SEM BSE section through a tubercle, showing the enemeloid capping layer (C); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of a tubercle, showing the arrangement of dentine canaliculi radiating from a pulp canal (D); NRM‐PAL C.5942, etched SEM section through a wall of L2, showing homogenous core and lamellar margins pervaded by an orthogonal fabric of thread‐like spaces (E); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of L3, showing Sharpey's fibres trending in two principal orientations (F). Sup., superficial layer; L1, layer 1; L2, layer 2; L3, layer 3; C.W., incomplete cross wall; E/D jun., Enameloid/dentine junction. Scale bar equals 606 μm in (A), 227 μm in (B), 179 μm in (C), 55 μm in (D), 30 μm in (E) and 56 μm in (F).
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jmor20370-fig-0002: Histology of Loricopteraspis serrata. NRM‐PAL C.5939, SEM BSE section through the cephalothoracic shield (A); NRM‐PAL C.5940, SRXTM section through a scale unit (B); NRM‐PAL C.5941, SEM BSE section through a tubercle, showing the enemeloid capping layer (C); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of a tubercle, showing the arrangement of dentine canaliculi radiating from a pulp canal (D); NRM‐PAL C.5942, etched SEM section through a wall of L2, showing homogenous core and lamellar margins pervaded by an orthogonal fabric of thread‐like spaces (E); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of L3, showing Sharpey's fibres trending in two principal orientations (F). Sup., superficial layer; L1, layer 1; L2, layer 2; L3, layer 3; C.W., incomplete cross wall; E/D jun., Enameloid/dentine junction. Scale bar equals 606 μm in (A), 227 μm in (B), 179 μm in (C), 55 μm in (D), 30 μm in (E) and 56 μm in (F).

Mentions: The dermal skeleton of Lepidaspis is composed of a broad cephalothoracic shield with body armour of separate scale units. The cephalothoracic shield is constructed from four distinct layers (Fig. 2A); a superficial layer of oak leaf‐shaped tubercles, a discontinuous compact layer of canals (L1), a cancellous layer (L2) and a thin basal layer (L3). The scale units equate to the superficial, compact and basal layer of the shield, but lack the middle cancellous division (Fig. 2B). The Lepidaspis dermal skeleton shows no evidence of cell lacunae within the mineral matrix.


Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton
Histology of Loricopteraspis serrata. NRM‐PAL C.5939, SEM BSE section through the cephalothoracic shield (A); NRM‐PAL C.5940, SRXTM section through a scale unit (B); NRM‐PAL C.5941, SEM BSE section through a tubercle, showing the enemeloid capping layer (C); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of a tubercle, showing the arrangement of dentine canaliculi radiating from a pulp canal (D); NRM‐PAL C.5942, etched SEM section through a wall of L2, showing homogenous core and lamellar margins pervaded by an orthogonal fabric of thread‐like spaces (E); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of L3, showing Sharpey's fibres trending in two principal orientations (F). Sup., superficial layer; L1, layer 1; L2, layer 2; L3, layer 3; C.W., incomplete cross wall; E/D jun., Enameloid/dentine junction. Scale bar equals 606 μm in (A), 227 μm in (B), 179 μm in (C), 55 μm in (D), 30 μm in (E) and 56 μm in (F).
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jmor20370-fig-0002: Histology of Loricopteraspis serrata. NRM‐PAL C.5939, SEM BSE section through the cephalothoracic shield (A); NRM‐PAL C.5940, SRXTM section through a scale unit (B); NRM‐PAL C.5941, SEM BSE section through a tubercle, showing the enemeloid capping layer (C); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of a tubercle, showing the arrangement of dentine canaliculi radiating from a pulp canal (D); NRM‐PAL C.5942, etched SEM section through a wall of L2, showing homogenous core and lamellar margins pervaded by an orthogonal fabric of thread‐like spaces (E); NRM‐PAL C.5940, SRXTM volume rendered virtual thin section of L3, showing Sharpey's fibres trending in two principal orientations (F). Sup., superficial layer; L1, layer 1; L2, layer 2; L3, layer 3; C.W., incomplete cross wall; E/D jun., Enameloid/dentine junction. Scale bar equals 606 μm in (A), 227 μm in (B), 179 μm in (C), 55 μm in (D), 30 μm in (E) and 56 μm in (F).
Mentions: The dermal skeleton of Lepidaspis is composed of a broad cephalothoracic shield with body armour of separate scale units. The cephalothoracic shield is constructed from four distinct layers (Fig. 2A); a superficial layer of oak leaf‐shaped tubercles, a discontinuous compact layer of canals (L1), a cancellous layer (L2) and a thin basal layer (L3). The scale units equate to the superficial, compact and basal layer of the shield, but lack the middle cancellous division (Fig. 2B). The Lepidaspis dermal skeleton shows no evidence of cell lacunae within the mineral matrix.

View Article: PubMed Central - PubMed

ABSTRACT

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.

No MeSH data available.


Related in: MedlinePlus