Microanatomy of Early Devonian book lungs - PubMed

Fine structure of the book lungs in Devonian and Recent arachnids. (a) Longitudinal section through an almost complete Palaeocharinus sp. (Arachnida: Trigonotarbida), PBM 3530, from the Early Devonian Rhynie cherts of Scotland. (b) Same specimen, close-up of the book lungs—see box in (a). (c) Detail of the distal parts of medial lamellae revealing bridging trabeculae traversing the air space and spines from the lamellar edge—see box in (b). (d) Comparative transverse histological section through the book-lung lamellae of a Recent tetrapulmonate arachnid, Damon sp. (Amblypygi; 1.5 μm) stained with methylene blue–azure II revealing the same pattern of trabeculae and spines. (e) Comparative longitudinal histological section through the book-lung lamellae of a Recent scorpion, Heterometrus longimanus (1.5 μm) stained with methylene blue–azure II revealing the same pattern of trabeculae and spines. (f) Three-dimensional reconstruction (using

Bitplane Imaris

) of the fossil lung in PBM 3530 showing a distal lamellar region. Images generated from a digitally combined stack of photographs taken at successive planes of focus: reconstruction of the surface of the lamellae and their trabeculae; trabeculae are shown here only between two lamellae on the polished plane of section (see also electronic supplementary material 1 and 2). Lamellae are fused towards the midline of the body. Note that these spines resolved on the lamellar edges would have been more pointed in life, but the reconstruction programme tends to blur them artificially. A, air sac; At, atrium; Bl, book lung; Bt, bridging trabeculae; Cc, chelicera; E, epidermis cell; F, region of lamellar fusion; H, haemolymph cell; L, lamella (haemolymph space); Lc, leg coxae; N, nucleus; O, opisthosoma; P, prosoma; Pc, pillar cells; Pp, pedipalp; S, spine; Sp, spiracle; St, sternite; T, trabecula; ant, anterior; post, posterior.