Early acanthodians from the Lower Silurian of Asia

Abstract

Abundant microvertebrate remains from the Siberian Platform are described as early acanthodians. All are preserved with both excellent morphology and histology. They are assigned to a new order, Tchunacanthida, with two new families, Lenacanthidae and Tchunacanthidae. These comprise two new genera, Lenacanthus and Tchunacanthus with type species L. priscus sp. nov. and T. obruchevisp.nov. The evidence from the morphology and histology is that they are the most ancient acanthodian scales so far found. The total collection of vertebrate material from the Irkutsk amphitheatre is described, together with their geological distribution, geographical range and systematic palaeontology. Head scales, tesserae of three morphotypes, transitional scales and body scales have been found. All comprise morphological sets as determined by comparison of morphology using SEM and of histology using Nomarsky interference optics. Observations of growth were possible from details of concentric lines on the crown and also from incremental layers seen in both horizontal and vertical sections. Starting from the primordial scale, consecutive layers are added, coronally, laterally, and deep in the corium onto the base; these occurred simultaneously in both crown and base. A type of areal-superpositional growth occurred in some body scales but in other scales there was little superpositional growth. It is concluded that Tchunacanthus possessed very solid body armour with tightly joined scales covering a large area of the body, and scale bases deeply set in the corium. Both tissues of the scales, dentine and bone, feature enclosed cells. This character is regarded as primitive within acanthodians, as in derived forms both tissues are acellular. Acanthodian scales are one of many examples of transformation from cellular to acellular tissue in evolution.

The characters of Palaeozoic jawed vertebrates

“…In effect, it is morphologically intermediate between meso-and orthodentine. There are also questions about the distribution of this tissue, as scales from Siberia attributed to the earliest known acanthodians (Karatajute-Talimaa & Smith, 2003) and a pteraspidomorph from the Ordovician of Australia (I. bear semidentine-like tissue with polarized cell spaces. Giles, Rücklin & Donoghue (2013) noted that not all placoderms exhibit a superficial dentinous layer.…”

New data on Silurian vertebrates of southern Siberia

“…Siberian Platform Moskalenko (1968), Karatajute- Talimaa and Predtechenskyi (1995) andŽigaite (2013) Tuva Karatajute-Talimaa (1973), Vladimirskaya et al (1977Vladimirskaya et al ( , 1986, Karatajute-Talimaa (1978), Kulkov et al (1985), Karatajute- Talimaa and Ratanov (2002), Karatajute- Smith (2003, 2004), Zigaite (2004Zigaite ( , 2013, Žigaite and Blieck (2006) anď Zigaite and Karatajute-Talimaa (2008) Severnaya Zemlya Matukhin and Menner (1999), Karatajute-Talimaa and Märss (2002) and Märss and Karatajute-Talimaa (2002) Trans-Baikal Vladimirskaya et al (1986), Rodina et al (2002) and Karatajute-Talimaa and Smith (2003) foldbelts of subduction-accretionary nature, the Middle Paleozoic Anui-Chuya marginal sea trough, the Late Paleozoic-Early Mesozoic Kuznetsk Basin, which is a Hercynian continental intermountain depression, and the Zaisan collisional belt (Dobretsov, 2003;Sennikov et al, 2008;Vladimirov et al, 2008). The complicated structure of the ASFA is formed by gradual northward accretion of Late Neoproterozoic-Paleozoic oceanic, island-arc and active margin terranes and Precambrian microcontinents to the Siberian continental margin (Dobretsov et al, 1995;Dobretsov, 2003;Windley et al, 2007;Safonova et al, 2011).…”

“…were found in varicolored conglomerates, sandstones, and siltstones with rare beds of limestone (Rodina, 2002;Gutak et al, 2005). They were found together with a new acanthodian genus Tchunacanthus in the Lower Silurian Balturino Formation (Staroe Balturino outcrop) in the Irkutsk Amphitheatre (Vladimirskaya et al, 1986;Rodina et al, 2002;Karatajute-Talimaa and Smith, 2003). The Gornaya Shoriya vertebrates occur along with the brachiopods Meristina obtusa (Sowerby), Leptaena depressa (Sowerby), Plectatrypa cf.…”

Precise occlusion and trophic niche differentiation indicate specialized feeding in Early Devonian jawed vertebrates

“…Acanthodians were generally small, spiny fishes that are represented in the fossil record from the Early Silurian (Hanke et al 2001b;Karatajute-Talimaa and Smith 2002;Burrow 2003) to the Permian (Denison 1979) by mostly disarticulated scales, fin spines, and teeth. Several phylogenetic analyses have suggested that acanthodians as a group most likely represent a paraphyletic assemblage of stem chondrichthyans (Brazeau 2009;Davis et al 2012;Zhu et al 2013;Giles et al 2015;Long et al 2015b;Burrow et al 2016).…”