Figure 1

Hipparionin metapodial histology observed under polarised light using a 1/4λ filter. Scale: 0.5 mm. (a) Primary bone tissue formed by fibrolamellar complex (FLC) in the innermost cortex of the Hipparion truyolsi metatarsal IPS-28842. The arrow points out the transition between a FLC with higher parallel fibered bone component (PFB) to a FLC with higher fibrous component and larger osteons. (b) Laminar bone tissue in the inner cortex of the Hipparion cf. sebastopolitanum (primigenium morphotype) PNT-22. (c) Radial vascular canals present in the first growth cycle of the Hipparion philippus (dietrichi morphotype) metatarsal PER-342. (d) Secondary bone tissue in the lateral posterior area of the Hipparion gromovae metacarpal IPS-101807. Dense Haversian systems (HS) are concentrated in the contact zone with the lateral metapodials, and lamellar tissue is filling the resorption cavities near the medullary area (arrow indicating resorption line).