Figure 3 in On cosmine: its origins, biology and implications for sarcopterygian interrelationships

Figure 3. – Morphology and histology of porolepiform holoptychiid scales. A: Holoptychius nobilissimus (MB.f. 1994.1) and B: Laccognathus panderi (MB.f. 1833,2) from the Middle – Late Devonian of Latvia. 1: External view of the scales (arrow points anteriorly); 2: Internal view of the scales; 3: General view of the cross section; 4: Inset from the cross section showing the arrangement of the histological layers. Note the ornamental and histological differences between both scales: in Holoptychius the exposed area is ornamented with thick longitudinal ridges solely made of dense bone resting upon the spongiosa (vascular bone); in Laccognathus the exposed area shows numerous dentine tubercles, capped with a thin enamel layer (i.e. odontodes), older odontodes from previous generations are embedded in the underlying spongiosa (black arrowheads). In both scales, the internal surface is flat and smooth and is solely formed by the isopedine (lamellar bone) layer, which is relatively thicker in Holoptychius than in Laccognathus. Abbreviations: d, dentine; e, enamel; lb, lamellar bone; vb, vascular bone. Scale bars: A/B2 = 1 cm; A/B3 = 5 mm; A/B4 = 500 μm.