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Johanson Z.,Natural History Museum in London | Kearsley A.,Natural History Museum in London | den Blaauwen J.,University of Amsterdam | Newman M.,Llanstadwell House | And 2 more authors.
Journal of Experimental Zoology Part B: Molecular and Developmental Evolution | Year: 2012

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. © 2011 Wiley Periodicals, Inc..

Johanson Z.,Natural History Museum in London | Kearsley A.,Natural History Museum in London | den Blaauwen J.,University of Amsterdam | Newman M.,Llanstadwell House | And 2 more authors.
Seminars in Cell and Developmental Biology | Year: 2010

Palaeospondylus gunni (Devonian, Scotland) is an enigmatic vertebrate, assigned to various jawless and jawed groups since its original description. New sections through the whole body allow description of a novel skeletal tissue for Palaeospondylus, comprising the entire skeleton. This tissue is mineralized cartilage and is characterized by large cell spaces embedded in minimal matrix. Bone is completely absent. Calcium phosphate mineralization has a differential topography of deposition within the cartilage that reflects a biogenic origin, despite subsequent diagenetic modification. This combination of hypertrophied cell spaces surrounded by regionalized mineralized matrix differs from all other cartilage in fossil and extant vertebrates. However, it compares most closely to gnathostome endochondral bone in early developmental stages. For example, Palaeospondylus skeletal histology differs from the Devonian agnathan Euphanerops and extant lamprey cartilage. Comparison with mineralized cartilage of armored fossil agnathans and placoderms shows the histology is not comparable to globular calcified cartilage. It also differs from that in extant chondrichthyan mineralized tesserae, which is restricted to a subperichondral zone. Amongst this diversity of calcified cartilage types we discuss various interpretations, including one that implicates tissue either in developmental stasis, before osteoblasts can deposit bone, or at a phylogenetic stage when this step has not evolved. These very different hypotheses highlight difficulties in interpreting fossil ontogenies when phylogenetic relationships are uncertain. Nevertheless, we propose that the composition of the Palaeospondylus skeleton represents a fossilized ontogenetic stage of endochondral bone, a type of bone characteristic of osteichthyan vertebrates. Crown Copyright © 2009.

Burrow C.J.,Geosciences | Newman M.J.,Llanstadwell House | Davidson R.G.,35 Millside Road | Den Blaauwen J.L.,University of Amsterdam
Palaeontology | Year: 2011

Circumorbital dermal bones are found in most groups of early vertebrates that have dermal bony plates on the head. Taxonomic distribution of dermal sclerotic plates on the eye itself is less clear, partly because the eyeball is rarely preserved and sometimes because sclerotic bones have been misinterpreted as circumorbital bones. Based on the examination of climatiid Climatius plus mesacanthid, cheiracanthid and acanthodid acanthodiform acanthodians, we conclude that most, if not all, acanthodiforms and climatiids had sclerotic rings. Presence and number of these elements should be included as a character in phylogenetic analyses of early jawed vertebrates. © The Palaeontological Association.

Newman M.J.,Llanstadwell House | Davidson R.G.,35 Millside Road | Den Blaauwen J.L.,University of Amsterdam | Burrow C.J.,Geosciences
Scottish Journal of Geology | Year: 2011

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.

Newman M.J.,Llanstadwell House | Den Blaauwen J.L.,University of Amsterdam
Scottish Journal of Geology | Year: 2010

A new specimen of the recently described dipnoan fish, Pinnalongus saxoni Newman & den Blaauwen, 2007 from the Scottish Middle Devonian has been collected in Achastle-shore, eastern Caithness. This new specimen whilst consisting of only a head, has many new features not previously described in P. saxoni, such as the lateral line system of the head which is shown to conform to the well known form Dipterus, also from the Middle Devonian of Scotland. The specimen also has features not seen before in any fossil fish such as the ring-like structures associated with pore openings, plus an unusual development of the cosmine cover at the rear of the skull. The presence of both Coccosteus and Thursius in the same deposits confirms that Achastle-shore in situated below the well-known Achanarras fish bed horizon and suggests that P. saxoni could be used as a zone fossil. © The Geological Society of London 2012.

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