Formation and structure of scales in the Australian lungfish, neoceratodus forsteri (Osteichthyes: Dipnoi)

Abstract

The large elasmoid scales of the Australian lungfish, Neoceratodus forsteri, are formed within the dermis by unpigmented scleroblasts, growing within a collagenous dermal pocket below a thick glandular epidermis. The first row of scales, on the trunk of the juvenile lungfish, appears below the lateral line of the trunk, single in this species, at around stage 53. The scales, initially circular in outline, develop anteriorly and posteriorly from the point of initiation in the mid-trunk region, and rows are added alternately below the line, and above the line, until they reach the dorsal or ventral midline, or the margins of the fins. Scales develop later on the ventral surface of the head, from a separate centre of initiation. Scales consist of three layers, all produced by scleroblasts of dermal origin. The outermost layer of interlocking plates, or squamulae, consists of a mineralised matrix of fine collagen fibrils, covered by unmineralised collagen and a single layer of cells. Squamulae of the anterior and lateral surfaces are ornamented with short spines, and the mineralised tissue of the posterior surface is linked to the pouch by collagen fibrils. The innermost layer, known as elasmodin, consists of bundles of thick collagen fibrils and cells arranged in layers. An intermediate layer, made up of collagen fibrils, links the outer and inner layers. The elasmoid scales of N. forsteri can be compared with scale types among other osteichthyan groups, although the cellsand canaliculi in the mineralised squamulae bear little resemblance to typical bone.

Skin structure in the snout of the Australian lungfish, Neoceratodus forsteri (Osteichthyes: Dipnoi)

“…Cosmine nodules may also be found buried by subsequent growth of additional elements (Denison, 1968). It is not present in the scales and epidermis of living lungfish (Kemp, 2012b;Kerr, 1955;Zylberberg, 1988). Cosmine is located between the pore canals and flask organs where the tubules terminate in fossils.…”

“…Presence of lymphatic vessels in tissues of the snout may be protective, as this part of the lungfish is frequently abraded or cut during feeding activities, and the epithelium of the snout has no scales. The trunk and other parts of the head of the lungfish have a heavy cover of imbricating scales (Kemp, 2012b;Pridmore and Barwick, 1993), and the thick epithelium secretes large amounts of mucus. The snout has some cells to secrete mucus, but no scale cover.…”

“…The skin of the snout and lips in the living Australian lungfish is soft, and free of scales at all stages of the life cycle (Kemp, 2012b). In external view, the skin of the snout is pitted, with perforations of various sizes, for goblet cells and sense organs, and apart from this, individual epithelial cells are visible in surface view (Fig.…”

“…On the snout they are single, and on the dorsal, lateral and ventral surfaces of the head they are arranged in pit lines (Kemp, 1999). Rows of electroreceptors run above and below the lateral line of the trunk (Kemp, 2012b). Despite a close topographic association with the dermal papillae and the sensory lines of the head, electroreceptors are separate in structure and in function.…”

Ornamentation of dermal bones of Placodermi from the Lower Devonian of Morocco as a measure of biodiversity

“…In modern fishes it is possible to differentiate species on the basis of microscopic analysis of single scales that appear identical on a macroscale (Kaur & Dua, 2004;Esmaeili & Gholami, 2011;Kemp, 2012). The aim of the present note is to determine whether or not it is possible to describe structural differences in fragments of single plates of placoderm armour, and whether or not these differences can be considered distinctive between species.…”