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.