July 2024
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16 Reads
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July 2024
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16 Reads
June 2022
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34 Reads
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1 Citation
Scottish Journal of Geology
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 recognised 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 mineralisations. 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.
November 2021
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330 Reads
June 2021
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308 Reads
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5 Citations
Papers in Palaeontology
The ‘acanthodian’ fishes provide key anatomical insights into the deepest branches of the chondrichthyan stem group. We review the anatomy of the acanthodian Vernicomacanthus uncinatus from the Lochkovian (Lower Devonian, 419.2–410.8 Ma) of Scotland based on eight articulated fossils, one of which is newly described. Broadly, the anatomy of V. uncinatus fits with that of contemporaneous acanthodians such as Climatius and Parexus, with a head covered by robust tesserae, an enlarged postorbital scale, an armoured shoulder girdle, and many pairs of ventrolateral spines. However, it departs from this anatomy in key respects. Its pectoral fin spines are obliquely ridged and posteriorly denticulated, similarly to Carboniferous gyracanth stem‐group chondrichthyans. Its scales consist of multiple anteroposteriorly aligned odontodes, similarly to many Palaeozoic ‘sharks’. And its endoskeletal shoulder girdle may have a posterolateral angle, previously observed only in shark‐like chondrichthyans. We propose that the differences between V. uncinatus and its congeneric, V. waynensis, which include potentially phylogenetically significant characters of the shoulder girdle and spines, are sufficient to erect a new genus for V. waynensis:Dobunnacanthus gen. nov. The scales of Vernicomacanthus are identical to those of the ‘shark’ scale genus Altholepis, suggesting that some such scales may instead belong to taxa with acanthodian‐like gross anatomies. Based on these scales we highlight potential patterns in chondrichthyan scale evolution, in particular the axial addition of odontodes. Anatomical similarities between Vernicomacanthus and gyracanths, highlighted by previous authors, may indicate the existence of a grade including these and similar acanthodian‐grade taxa placed relatively crownwards in the chondrichthyan stem‐group.
April 2021
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288 Reads
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9 Citations
Norwegian Journal of Geology
January 2021
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12 Reads
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4 Citations
September 2020
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61 Reads
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2 Citations
Palaeontology
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.
September 2020
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48 Reads
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3 Citations
Scottish Journal of Geology
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 Baltic region. The dorsoventral preservation of the head region in the F. davidsoni specimens reveals clearly the position of the ceratohyal cartilages in a cheiracanthid, as well as showing for the first time that there is a basihyal cartilage anterior to the ceratohyals.
July 2020
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132 Reads
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3 Citations
Skeletal bone enlargement (hyperossification) was thought to only occur in endochondral and dermal bones (e.g., endoskeletons of marine tetrapods and dermal bones of certain jawless vertebrates, placoderms, and teleost fishes). However, in some arthrodiran placoderms (basal jawed vertebrates), i.e., Millerosteus minor, Compagopiscis croucheri, Eastmanosteus calliaspis, several types of bone enlargement also occur within the endoskeleton, affecting the vertebral column. Significantly, placoderm endoskeletal bone is thought to be thin-walled, ossifying only in the fibrous layer surrounding a cartilage precursor (historically called perichondral bone), rather than endochondrally/dermally. Hyperossification differs among these three species, revealing a range of internal and external bone morphologies undescribed for the placoderm endoskeleton. Thus, neural arches of M. minor are swollen in external appearance, in cross-section showing considerable deposition of layered, compact bone. In contrast, the arches of E. calliaspis are unswollen externally, with the neural spine cortex composed of the thin perichondral bone expected for placoderms. Histologically, though, the arch comprises numerous layers of perichondral bone. In C. croucheri, as in E. calliaspis, the spine and arch are unswollen externally, but the perichondral bone is thickened and compact, more similar to the condition in M. minor. Vertebral hyperossification in these taxa differs considerably in the degree and mode of cortical tissue thickening, via addition of tissue to the bone’s external (periosteal) and/or internal (endosteal) surfaces. Hyperossifcation in these arthrodires demonstrates that increases in endoskeletal bone mass are not restricted to crown-group gnathostomes (Chondrichthyes + Actinopterygii), representing a first step in the evolution of this process, involving modifications to cortical bone layers.
July 2020
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61 Reads
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3 Citations
Scottish Journal of Geology
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.
... Of the four families recognised within the Acanthodiformes, the Mesacanthidae are mostly distinguished by features that are considered 'primitive' because they are shared with 'Climatiiformes' sensu lato -having one pair of prepelvic spines, a shallowly inserted dorsal spine, an unmineralised endocranium, and a simple mandibular joint (Burrow, 2021). As noted by Burrow et al. (2022), recent cladistic analyses (e.g., Zhu et al. 2022, extended data figure 9b) retrieve mesacanthids as the sister group to most other acanthodiforms. ...
June 2022
Scottish Journal of Geology
... varied cartilage tissues that have been described in other stemchondrichthyans (Burrow et al. 2018, Maisey et al. 2020, Burrow and Blaauwen 2021. The thickness of this tissue is variable through the skeleton, for example being thicker in the mandibular arches than in the branchial arches, particularly around the mandibular articulation (Fig. 5C). ...
January 2021
... Our understanding of the relationships of early jawed vertebrates has changed markedly over the past decades (Brazeau, 2009;Brazeau & Friedman, 2014;Burrow & Rudkin, 2014;Li et al., 2021;Lu & Zhu, 2008;Miller et al., 2003;Zhu et al., 2001Zhu et al., , 2021Zhu et al., , 2022. Along with the discovery of new taxa has come restudy of long-known species, with acanthodians of particular interest (e.g., Brazeau, 2012;Burrow et al., 2008Burrow et al., , 2013Burrow et al., , 2015Dearden et al., 2019Dearden et al., , 2021Snyder et al., 2017;Turner et al., 2005;Warren et al., 2000). Although traditionally assigned to the Acanthodii (e.g., Denison, 1979), recent cladistic analyses (Andreev et al., 2022;Burrow et al., 2016;Coates et al., 2018;Giles et al., 2015) show the Gyracanthidae Woodward, 1906 amongst a cluster of basally branching stem chondrichthyan taxa, crownward of the Acanthodii sensu lato Coates et al., 2018). ...
June 2021
Papers in Palaeontology
... In fact, similar functions fulfills other nuclear matrix protein SAF-B (Renz and Fackelmayer, 1996;Kipp et al., 2000;Cherney et al., 2023). 31,50,51 It was shown that the fibrils formed by the abundant nuclear matrix proteins lamins also bind DNA (Ludérus et al., 1992(Ludérus et al., , 1994Pennarun et al., 2023) 52,53,54 and chromatin histones (Taniura et al., 1995). 55 Nuclear matrix proteins can be active participants in genetic processes. ...
September 1994
... Type species and holotype: Cheirolepis trailli Agassiz, 1835 [31]. Included species: C. aleshkai, Plax, 2020 [4]; C. canadensis, Whiteaves, 1881 [33]; C. gaugeri, Gross, 1973 [32]; C. gracilis, Gross, 1973 [32]; C. jonesi, Newman et al., 2021 [34]; C. schultzei, Arratia and Cloutier, 2004 [35]; and C. bychovensis, Plax, 2022 [17]. Material: 21 scales (13 scales from Belarus and 8 scales from Lithuania). ...
April 2021
Norwegian Journal of Geology
... The best known of these is Coccosteus cuspidatus, a freshwater/brackish [13,14] coccosteomorph represented by several complete specimens [15]. Extensive postcranial material and/or body outlines are also known for the non-eubrachythoracid arthrodires Holonema westolii [16], Africanaspis doryssa, and A. edmountaini [17]; the coccosteomorphs Millerosteus minor, Dickosteus threiplandi, Watsonosteus fletti [18], Plourdosteus canadensis [19], and Incisoscutum ritchei [20]; the aspinothoracidan Amazichthys trinajsticae [21]; and Rhachiosteus pterygiatus, a eubrachythoracid of uncertain phylogenetic position (see [22][23][24]). Limited post-thoracic material has been reported for several other arthrodires, including Paramylostoma arcualis [25], Heintzichthys gouldii [26,27], and Dunkleosteus terrelli itself [28,29]. ...
September 2020
Palaeontology
... Into the 21 st century, more detailed work based on acanthodians had identified conspecific forms not only in the Baltic Region and the Orcadian Basin, but also Belarus, Spitsbergen, and Severnaya Zemlya (Burrow et al. 2016(Burrow et al. , 2020(Burrow et al. , 2021den Blaauwen et al. 2019;Newman et al. 2019Newman et al. , 2021b. The reason for acanthodian synonymy is discussed later in this article. ...
September 2020
Scottish Journal of Geology
... This osteological disorder has already been referred as a response to adverse environmental factors (Chang et al., 2008;Greenwood, 1992;Schlüter & Kohring, 2002), hydrostatic maintenance, floating aid (Paig-tran et al., 2016), fungal infestation (Grabda, 1982), a natural consequence of age (Aguilera et al., 2017;Gauldie & Czochanska, 1990;Smith-Vaniz & Carpenter, 2007). Although consensus has not been reached on the causes and possible consequences of hyperostosis, several studies reported the occurrence of the condition in fossil records (Béarez, 1997;Chang et al., 2008;Gaudant & Meunier, 1996;Guzmán & Polaco, 2002;Lidour et al., 2020;Schlüter & Kohring, 2002;Schlüter et al., 1992;Tyler et al., 1992;van Mesdag et al., 2020). ...
July 2020
... Into the 21 st century, more detailed work based on acanthodians had identified conspecific forms not only in the Baltic Region and the Orcadian Basin, but also Belarus, Spitsbergen, and Severnaya Zemlya (Burrow et al. 2016(Burrow et al. , 2020(Burrow et al. , 2021den Blaauwen et al. 2019;Newman et al. 2019Newman et al. , 2021b. The reason for acanthodian synonymy is discussed later in this article. ...
July 2020
Scottish Journal of Geology
... It has been demonstrated in many previous publications (e.g., Burrow et al., 2020b) that there are five fish-bearing faunal zones in the Middle Devonian Orcadian Basin. These zones equate well to the biostratigraphy of the Baltic region (Newman et al., 2017), Belarus and elsewhere in Laurussia (Plax and Newman, 2021). ...
January 2020
Palaeontologia Electronica