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Abstract

Material of tesseraspids (Tesseraspidiformes) is reported from the uppermost Severnaya Zemlya Formation (Lochkovian, Lower Devonian) of the Severnaya Zemlya archipelago, in the Russian Arctic, where it is associated with other vertebrate remains, including corvaspids, acanthodians, and large but rare specimens of osteostracans. The tesseraspid material is not abundant, and most often preserved as a “patchwork” of bony platelets (tesserae), except for a few partly articulated specimens. We redescribe the holotype of Tesseraspis mosaica Karatajūtė-Talimaa, 1983, whose head carapace is preserved as a flattened tube of adjacent tesserae. This material is compared to the already published tesseraspid taxa, i.e., T. tessellata Wills, 1935, T. toombsi Tarlo, 1964, T. mutabilis (Brotzen, 1934), T. oervigi Tarlo, 1964 emend. Dineley and Loeffler, 1976, T. denisoni Tarlo, 1964, and T. talimaae Tarlo, 1965. All species are based upon rare and incomplete material, as no head carapaces associated with trunk and tail are known, and so, the intraspecific variability is also unknown. Distinction between “species” is based on the detail of the superficial sculpture of the tesserae of the head carapaces, which is unsatisfactory. It is concluded that only four of the nominal species can be retained. A review of all other known tessellated pteraspidomorphs indicates that our knowledge of tessellated heterostracans is currently insufficient to support a meaningful classification.
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Abstract

Placoid and polyodontode scales of stem chondrichthyans have been found in the early Lochkovian “Ditton Group” of the Brown Clee Hill district, Shropshire, England and at Talgarth, south Wales. One of the forms is assigned to a new species of Altholepis Karatajūtė-Talimaa, 1997, a genus already recognised from Lochkovian shallow marine deposits in Celtiberia, Spain and the Northwest Territories, Canada as well as the type locality in Podolia, Ukraine. Altholepis salopensis sp. nov. is based on small polyodontode scales with typically three to eight high odontodes; the scale form was previously considered to belong to acanthodian “Nostolepis” robusta (Brotzen, 1934). The structure of other scales formerly assigned to “Nostolepis” robusta has led us to erect a new genus Jolepis for this scale form, which differs from Altholepis in lacking an ordered layout of odontodes. Jolepis robusta (Brotzen, 1934), originally (and possibly still) considered to be an acanthodian, is also known from the Baltic countries, Russia, and northern Germany (ex erratic limestones). Scales of acanthodian Parexus recurvus Agassiz, 1845, and/or possibly from the stem chondrichthyan Seretolepis elegans Karatajūtė-Talimaa, 1968 (scales of these two taxa are barely distinguishable), and of stem chondrichthyan Polymerolepis whitei Karatajūtė-Talimaa, 1968 are also present. Altholepis, Jolepis gen. nov., Seretolepis Karatajūtė-Talimaa, 1968 and Polymerolepis Karatajūtė- Talimaa, 1968 are found in marine deposits elsewhere; the British occurrence of these taxa adds to the debate on the sedimentological origins of the Lower Old Red Sandstone deposits in the Welsh Borderland. The geographic range of several early sharks is now known to extend around the Old Red Sandstone continent and beyond.
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Abstract

Ischnacanthus gracilis (Egerton, 1861), the only ischnacanthiform acanthodian from the Lochkovian Lower Old Red Sandstone of Scotland, is known from hundreds of specimens in institutional collections worldwide. Despite this relative abundance, morphology and histology of its skeletal elements have rarely been investigated. Surface details of spines, dental elements, and scales are often not visible in specimens because they are usually split through the middle. We have examined a broad size range of fish, from 35 mm to 250 mm long. Several intact (not split) specimens have been collected in recent years and acid-prepared to show fine details of the dermal and dental elements. We have also used scanning electron microscopy of scales, jaws and dental elements, denticles and fin spines, and serial thin sectioning of articulated specimens, to document their structure. Some of our notable observations include: identification of ventral lateral lines, double-layered subtessellate calcified cartilage forming the jaws, and the probable occurrence of extraoral tricuspid denticles on the jaws of most fish. Examination of the size range, body proportions and dentition of institutional specimens gives no support for recognising more than one species in the Midland Valley localities.
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