Mesacanthus mitchelli is an important taxon in elucidating relationships amongst stem chondrichth... more Mesacanthus mitchelli is an important taxon in elucidating relationships amongst stem chondrichthyans, being the best known and most abundant of the oldest known acanthodiform acanthodians. Here we note some newly recognized morphological features and describe the histological structure of the endoskeleton and dermal elements. The jaws are preserved as a single layer of irregularly tessellate bone-like mineralizations. A mandibular bone is not present, contrary to previous descriptions. Fin spines have a wide central cavity that is more than half the width at mid-spine level. Prepelvic spines have a very narrow leading edge ridge and a wide open pulp cavity. Head sensory lines are enclosed by a pair of tesserae with a smooth crown, flat base and concave sides, bordering the sensory line canal. The tiny body scales have a relatively large pulp cavity in the embryonic zone. The large size of the eyes relative to the head and body size in Mesacanthus mitchelli , the structure and small size of the scales relative to those of other acanthodians of a similar size, and the wide central pulp cavity in the spines, all indicate that the species is likely to have undergone paedomorphism.
Serial sectioning of a nodule encapsulating an adult specimen of the arthrodire placoderm Watsono... more Serial sectioning of a nodule encapsulating an adult specimen of the arthrodire placoderm Watsonosteus fletti from the Eday Flagstone Formation (Givetian) in the Orcadian Basin of northern Scotland has revealed the presence of a number of embryos within the adult. This specimen represents the oldest known record of fossilized vertebrate embryos. Thin sections of two of the slices have revealed the detailed histological structure of embryonic plates in placoderms, showing that as previously deduced from visual examination, the outer and inner layers were the first to form. Gut contents preserved near the embryos show that the species had a varied diet, with dermal bone fragments from sarcopterygians and placoderms.
Vertebrate fossils are extremely rare below the Achanarras fish beds and equivalent strata in nor... more Vertebrate fossils are extremely rare below the Achanarras fish beds and equivalent strata in northern Scotland. Here we describe the cheiracanthid acanthodians from the lowest Middle Devonian of this region, comprising partial articulated specimens and squamation patches of two species Cheiracanthus flabellicostatus and C. brevicostatus. Both species were previously only known as isolated scales from the eastern Baltic and Russia. The stratigraphic range of the two species in Scotland extends up into the Achanarras equivalent fish beds in the Moray Firth.
Articulated cheiracanthid acanthodians are relatively rare above the Dickosteus thrieplandi biost... more Articulated cheiracanthid acanthodians are relatively rare above the Dickosteus thrieplandi biostratigraphic zone in the Orcadian Basin, with Cheiracanthus peachi den Blaauwen, Newman & Burrow the only species identified to date. Here we describe two other taxa Fallodentus davidsoni nov. gen. et sp. and Markacanthus costulatus Valiukevičius from the Mey Flagstone Formation. F. davidsoni occurs at the base of the formation, in the Osteolepis panderi biostratigraphic zone, and is readily identified by its robust fin spines which have a wide longitudinal ridge on each side below the groove separating the leading edge from the side of the spine. The taxon is most similar to Homalacanthus concinnus (Whiteaves) from the younger (Frasnian) Escuminac Formation in Quebec, Canada. The unique specimen of Markacanthus costulatus is from the top of the Mey Flagstone Formation. This taxon was previously only known from isolated scales from the upper Narva and Aruküla Regional Stages of the east B...
Spiracles are a general character of gnathostomes (jawed fishes), being present in antiarch placo... more Spiracles are a general character of gnathostomes (jawed fishes), being present in antiarch placoderms, commonly regarded as the most basal gnathostome group. The presence of spiracular tubes in acanthodians has been deduced from grooves on the neurocranium of the derived acanthodiform Acanthodes bronni from the Permian of Germany, but until now these tubes were presumed to lack an external opening, rendering them non‐functional. Here we describe the external spiracular elements in specimens of the Middle Devonian acanthodiforms Cheiracanthus murchisoni, Cheiracanthus latus and Mesacanthus pusillus from northern Scotland, and the internal structure of these elements in C. murchisoni, demonstrating that the spiracle in acanthodiforms differed from all known extant and extinct fishes in having paired cartilage‐pseudobranch structures. This arrangement represents a transitional state between the presumed basal gnathostome condition with an unconstricted first gill slit (as yet not identified in any fossil) and the derived condition with a spiracle and a single pseudobranch derived from the posterior hemibranch of the mandibular arch. We identify the main tissue forming the pseudobranch as elastic cartilage, a tissue previously unrecorded in fossils.
A number of partial articulated specimens of Cheiracanthus peachi nov. sp. have been collected fr... more A number of partial articulated specimens of Cheiracanthus peachi nov. sp. have been collected from the Mey Flagstone Formation and Rousay Flagstone Formation within the Orcadian Basin of northern Scotland. The new, robust-bodied species is mainly distinguished by the scale ornament of radiating grooves rather than ridges. Compared to other Cheiracanthus species in the Orcadian Basin, C. peachi nov. sp. has quite a short range making it a useful zone fossil. As well as describing the general morphology of the specimens, we have also described and figured SEM images of scales and histological sections of all elements, enabling identification of other, isolated remains. Of particular biological interest is the identification of relatively robust, tooth-like gill rakers. Finally, the species has also been identified from isolated scales in Belarus, where it appears earlier and has a longer stratigraphical range, implying the species evolved in the marine deposits of the east and migrated west into the Orcadian Basin via the river systems.
Abstract The Lower Old Red Sandstone terranes of the Midland Valley of Scotland and the Anglo-Wel... more Abstract The Lower Old Red Sandstone terranes of the Midland Valley of Scotland and the Anglo-Welsh Basin have been considered as separate realms due to the rarity of fish species common to both areas. Although in the first half of the 19th century the osteostracan Cephalaspis lyelli was thought to occur in both terranes this was shown in be incorrect in the latter part of that century. It was not until 1968 that it was demonstrated that the thelodont agnathan Turinia pagei occurred in both terranes. This species has a much wider distribution across the whole of the Old Red Sandstone continent, but its presence in both realms indicates they were connected either directly or indirectly. In 2012 it was suggested that the osteostracan Janaspis watsoni might be present in both basins and in 2013 the acanthodian Parexus recurvus was shown to definitely occur in both. Here we show that other acanthodian genera and species were present in both regions during the Lochkovian (earliest Devonian). Co-specific plants also occur in both terranes during the Lochkovian. As there is no evidence of a marine connection to the Midland Valley in the Lochkovian, the only logical conclusion is that the connection between the two terranes was fluvial.
ABSTRACT Disarticulated elements in a large, uncompressed regurgitate from Tillywhandland Quarry ... more ABSTRACT Disarticulated elements in a large, uncompressed regurgitate from Tillywhandland Quarry (Lochkovian), as well as serial sections of an articulated specimen, reveal the three-dimensional shape and structure of fin spines, scapulocoracoid and dermal plates, and the histological structure of dermal and endoskeletal hard tissues of the climatiid acanthodian Climatius reticulatus. Globular calcified cartilage is the only form of mineralization of the head endoskeleton, with the jaws preserved as double-layered globular calcified cartilage. Tooth whorls are borne on both the upper and lower jaws and comprise a vascularized bone base and tooth cusps composed of a vascular network and mesodentine, without a central pulp cavity. The short admedian spine is the only strongly laterally compressed spine; the anterior dorsal fin spine has a wide, splayed base. No evidence was found of large dermal plates between the pectoral fin spine and the median lorical plates, with the prepectoral pinnal plates with spines being the only large paired dermal components of the shoulder girdle. The anterior lorical plate, pinnal plates, and tesserae on the scapulocoracoid bear ornament comparable to the postbranchial apronic ornament in acanthothoracid placoderms. Branchiostegal plates have a thin, dense inner bone layer and an outer dentinous ornament layer but lack a middle osteodentine layer; other postcranial plates and fin spines have a smoothsurfaced inner layer of bone and a thick middle osteodentine layer. Body scales have a crown with areal growth zones formed of Stranggewebe and syncitial mesodentine on a cellular bone base.
Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2004
The large Middle Devonian osteostracan Cephalaspis magnifica Traquair, 1893a, from the Late Eifel... more The large Middle Devonian osteostracan Cephalaspis magnifica Traquair, 1893a, from the Late Eifelian Upper Caithness Flagstone Group of Caithness, Scotland, is redescribed on the basis of the holotype and a second, hitherto undescribed specimen. This species is assigned to a new genus, Trewinia gen. nov. and, on account of its probable lack of lateral cephalic fields, is regarded as a possible member of the Escuminaspididae, which are hitherto only known from the Late Devonian of Quebec, Canada. Other characters of the head shield of T. magnifica also accord with the structure of the largest known escuminaspidid Escuminaspis. The morphology, relationships and biogeography of the few other Middle and Late Devonian osteostracans are discussed. North American osteostracans are generally quite distinctive from European ones throughout the Devonian, and only few taxa seem common to the two areas. The Escuminaspididae could be one of these, and this would agree with similar distributions ...
ABSTRACT The first full description of the Lochkovian acanthodian Euthacanthus gracilis, presente... more ABSTRACT The first full description of the Lochkovian acanthodian Euthacanthus gracilis, presented here, shows that the species shares many characters with the type species Euthacanthus macnicoli supporting its retention in this genus. Euthacanthus elegans is deemed to be a junior synonym of E. gracilis. E. gracilis differs from E. macnicoli in having large 'umbellate' scales edging the main lateral line canal, and small body scales. The crown morphology of the enlarged lateral line scales are similar to isolated 'umbellate' scales described world-wide in the Siluro-Devonian. They have the same general morphology as the sensory line scales on some other acanthodians including the sensory line scales on the head of Ischnacanthus gracilis and the lateral lines in Brochoadmones milesi. Neither of these articulated forms is closely related to Euthacanthus. The crown morphology of the normal flank scales is similar to that of the type species, and also the acritolepid acanthodian Acritolepis urvantsevi. Acritolepids are ischnacanthiform acanthodians, and are not closely related to Euthacanthus. This similarity indicates that scale crown morphology without histology is not always a useful tool in identifying species or genera from isolated remains.
Mesacanthus mitchelli is an important taxon in elucidating relationships amongst stem chondrichth... more Mesacanthus mitchelli is an important taxon in elucidating relationships amongst stem chondrichthyans, being the best known and most abundant of the oldest known acanthodiform acanthodians. Here we note some newly recognized morphological features and describe the histological structure of the endoskeleton and dermal elements. The jaws are preserved as a single layer of irregularly tessellate bone-like mineralizations. A mandibular bone is not present, contrary to previous descriptions. Fin spines have a wide central cavity that is more than half the width at mid-spine level. Prepelvic spines have a very narrow leading edge ridge and a wide open pulp cavity. Head sensory lines are enclosed by a pair of tesserae with a smooth crown, flat base and concave sides, bordering the sensory line canal. The tiny body scales have a relatively large pulp cavity in the embryonic zone. The large size of the eyes relative to the head and body size in Mesacanthus mitchelli , the structure and small size of the scales relative to those of other acanthodians of a similar size, and the wide central pulp cavity in the spines, all indicate that the species is likely to have undergone paedomorphism.
Serial sectioning of a nodule encapsulating an adult specimen of the arthrodire placoderm Watsono... more Serial sectioning of a nodule encapsulating an adult specimen of the arthrodire placoderm Watsonosteus fletti from the Eday Flagstone Formation (Givetian) in the Orcadian Basin of northern Scotland has revealed the presence of a number of embryos within the adult. This specimen represents the oldest known record of fossilized vertebrate embryos. Thin sections of two of the slices have revealed the detailed histological structure of embryonic plates in placoderms, showing that as previously deduced from visual examination, the outer and inner layers were the first to form. Gut contents preserved near the embryos show that the species had a varied diet, with dermal bone fragments from sarcopterygians and placoderms.
Vertebrate fossils are extremely rare below the Achanarras fish beds and equivalent strata in nor... more Vertebrate fossils are extremely rare below the Achanarras fish beds and equivalent strata in northern Scotland. Here we describe the cheiracanthid acanthodians from the lowest Middle Devonian of this region, comprising partial articulated specimens and squamation patches of two species Cheiracanthus flabellicostatus and C. brevicostatus. Both species were previously only known as isolated scales from the eastern Baltic and Russia. The stratigraphic range of the two species in Scotland extends up into the Achanarras equivalent fish beds in the Moray Firth.
Articulated cheiracanthid acanthodians are relatively rare above the Dickosteus thrieplandi biost... more Articulated cheiracanthid acanthodians are relatively rare above the Dickosteus thrieplandi biostratigraphic zone in the Orcadian Basin, with Cheiracanthus peachi den Blaauwen, Newman & Burrow the only species identified to date. Here we describe two other taxa Fallodentus davidsoni nov. gen. et sp. and Markacanthus costulatus Valiukevičius from the Mey Flagstone Formation. F. davidsoni occurs at the base of the formation, in the Osteolepis panderi biostratigraphic zone, and is readily identified by its robust fin spines which have a wide longitudinal ridge on each side below the groove separating the leading edge from the side of the spine. The taxon is most similar to Homalacanthus concinnus (Whiteaves) from the younger (Frasnian) Escuminac Formation in Quebec, Canada. The unique specimen of Markacanthus costulatus is from the top of the Mey Flagstone Formation. This taxon was previously only known from isolated scales from the upper Narva and Aruküla Regional Stages of the east B...
Spiracles are a general character of gnathostomes (jawed fishes), being present in antiarch placo... more Spiracles are a general character of gnathostomes (jawed fishes), being present in antiarch placoderms, commonly regarded as the most basal gnathostome group. The presence of spiracular tubes in acanthodians has been deduced from grooves on the neurocranium of the derived acanthodiform Acanthodes bronni from the Permian of Germany, but until now these tubes were presumed to lack an external opening, rendering them non‐functional. Here we describe the external spiracular elements in specimens of the Middle Devonian acanthodiforms Cheiracanthus murchisoni, Cheiracanthus latus and Mesacanthus pusillus from northern Scotland, and the internal structure of these elements in C. murchisoni, demonstrating that the spiracle in acanthodiforms differed from all known extant and extinct fishes in having paired cartilage‐pseudobranch structures. This arrangement represents a transitional state between the presumed basal gnathostome condition with an unconstricted first gill slit (as yet not identified in any fossil) and the derived condition with a spiracle and a single pseudobranch derived from the posterior hemibranch of the mandibular arch. We identify the main tissue forming the pseudobranch as elastic cartilage, a tissue previously unrecorded in fossils.
A number of partial articulated specimens of Cheiracanthus peachi nov. sp. have been collected fr... more A number of partial articulated specimens of Cheiracanthus peachi nov. sp. have been collected from the Mey Flagstone Formation and Rousay Flagstone Formation within the Orcadian Basin of northern Scotland. The new, robust-bodied species is mainly distinguished by the scale ornament of radiating grooves rather than ridges. Compared to other Cheiracanthus species in the Orcadian Basin, C. peachi nov. sp. has quite a short range making it a useful zone fossil. As well as describing the general morphology of the specimens, we have also described and figured SEM images of scales and histological sections of all elements, enabling identification of other, isolated remains. Of particular biological interest is the identification of relatively robust, tooth-like gill rakers. Finally, the species has also been identified from isolated scales in Belarus, where it appears earlier and has a longer stratigraphical range, implying the species evolved in the marine deposits of the east and migrated west into the Orcadian Basin via the river systems.
Abstract The Lower Old Red Sandstone terranes of the Midland Valley of Scotland and the Anglo-Wel... more Abstract The Lower Old Red Sandstone terranes of the Midland Valley of Scotland and the Anglo-Welsh Basin have been considered as separate realms due to the rarity of fish species common to both areas. Although in the first half of the 19th century the osteostracan Cephalaspis lyelli was thought to occur in both terranes this was shown in be incorrect in the latter part of that century. It was not until 1968 that it was demonstrated that the thelodont agnathan Turinia pagei occurred in both terranes. This species has a much wider distribution across the whole of the Old Red Sandstone continent, but its presence in both realms indicates they were connected either directly or indirectly. In 2012 it was suggested that the osteostracan Janaspis watsoni might be present in both basins and in 2013 the acanthodian Parexus recurvus was shown to definitely occur in both. Here we show that other acanthodian genera and species were present in both regions during the Lochkovian (earliest Devonian). Co-specific plants also occur in both terranes during the Lochkovian. As there is no evidence of a marine connection to the Midland Valley in the Lochkovian, the only logical conclusion is that the connection between the two terranes was fluvial.
ABSTRACT Disarticulated elements in a large, uncompressed regurgitate from Tillywhandland Quarry ... more ABSTRACT Disarticulated elements in a large, uncompressed regurgitate from Tillywhandland Quarry (Lochkovian), as well as serial sections of an articulated specimen, reveal the three-dimensional shape and structure of fin spines, scapulocoracoid and dermal plates, and the histological structure of dermal and endoskeletal hard tissues of the climatiid acanthodian Climatius reticulatus. Globular calcified cartilage is the only form of mineralization of the head endoskeleton, with the jaws preserved as double-layered globular calcified cartilage. Tooth whorls are borne on both the upper and lower jaws and comprise a vascularized bone base and tooth cusps composed of a vascular network and mesodentine, without a central pulp cavity. The short admedian spine is the only strongly laterally compressed spine; the anterior dorsal fin spine has a wide, splayed base. No evidence was found of large dermal plates between the pectoral fin spine and the median lorical plates, with the prepectoral pinnal plates with spines being the only large paired dermal components of the shoulder girdle. The anterior lorical plate, pinnal plates, and tesserae on the scapulocoracoid bear ornament comparable to the postbranchial apronic ornament in acanthothoracid placoderms. Branchiostegal plates have a thin, dense inner bone layer and an outer dentinous ornament layer but lack a middle osteodentine layer; other postcranial plates and fin spines have a smoothsurfaced inner layer of bone and a thick middle osteodentine layer. Body scales have a crown with areal growth zones formed of Stranggewebe and syncitial mesodentine on a cellular bone base.
Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 2004
The large Middle Devonian osteostracan Cephalaspis magnifica Traquair, 1893a, from the Late Eifel... more The large Middle Devonian osteostracan Cephalaspis magnifica Traquair, 1893a, from the Late Eifelian Upper Caithness Flagstone Group of Caithness, Scotland, is redescribed on the basis of the holotype and a second, hitherto undescribed specimen. This species is assigned to a new genus, Trewinia gen. nov. and, on account of its probable lack of lateral cephalic fields, is regarded as a possible member of the Escuminaspididae, which are hitherto only known from the Late Devonian of Quebec, Canada. Other characters of the head shield of T. magnifica also accord with the structure of the largest known escuminaspidid Escuminaspis. The morphology, relationships and biogeography of the few other Middle and Late Devonian osteostracans are discussed. North American osteostracans are generally quite distinctive from European ones throughout the Devonian, and only few taxa seem common to the two areas. The Escuminaspididae could be one of these, and this would agree with similar distributions ...
ABSTRACT The first full description of the Lochkovian acanthodian Euthacanthus gracilis, presente... more ABSTRACT The first full description of the Lochkovian acanthodian Euthacanthus gracilis, presented here, shows that the species shares many characters with the type species Euthacanthus macnicoli supporting its retention in this genus. Euthacanthus elegans is deemed to be a junior synonym of E. gracilis. E. gracilis differs from E. macnicoli in having large 'umbellate' scales edging the main lateral line canal, and small body scales. The crown morphology of the enlarged lateral line scales are similar to isolated 'umbellate' scales described world-wide in the Siluro-Devonian. They have the same general morphology as the sensory line scales on some other acanthodians including the sensory line scales on the head of Ischnacanthus gracilis and the lateral lines in Brochoadmones milesi. Neither of these articulated forms is closely related to Euthacanthus. The crown morphology of the normal flank scales is similar to that of the type species, and also the acritolepid acanthodian Acritolepis urvantsevi. Acritolepids are ischnacanthiform acanthodians, and are not closely related to Euthacanthus. This similarity indicates that scale crown morphology without histology is not always a useful tool in identifying species or genera from isolated remains.
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