Contributions to Zoology, 84 (1) – 2015Nicole Klein; Alexandra Houssaye; James M. Neenan; Torsten M. Scheyer: Long bone histology and microanatomy of Placodontia (Diapsida: Sauropterygia)

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Microanatomical description

The humerus and femur of Psephoderma alpinum both have a central free medullary cavity that is surrounded by scattered large erosion cavities (in a perimedullary region), which reach far into the cortex (Table 1; Figs 2C, D; 3C, D). The erosion cavities are mainly roundish to oval shaped. In the humerus, they are randomly organized whereas those in the femur are arranged in circumferential rows. Vascular density is low (Table 2, 3).

The photograph of the midshaft cross section of the Henodus humerus indicates a relatively large medullary region consisting of large cavities and randomly organized trabecles (Table 1; Fig. 2E) surrounding a rather small medullary cavity. The vascular system is dominated by a radial organization and contains longitudinal and radial canals as far as observable. In general, microstructure of Henodus seems to be comparable to that of Placodontia indet. aff. Cyamodus humeri (see below).

Midshaft samples of humeri of Placodontia indet. aff. Cyamodus usually show a small free medullary cavity (Figs 2F-M; 4E-L) that is lined by a thin layer of endosteal bone, those that were not taken exactly at midshaft have a medullary region consisting of small cavities, endosteal bone deposits, and scattered pockets of calcified cartilage (Figs 4F, J). The medullary region is then surrounded by a sharp line, which consists of a very thin layer of endosteal bone. Vascular density in the humeri is poor in the innermost cortex but always high in the middle and outer cortex (Table 2; Figs 2F-M; 4E-L). Contrary to the condition in other vertebrates vascular density increases towards the outer cortex in Placodontia indet. aff. Cyamodus humeri. Long and thick, mainly radial vascular canals forming a primary trabecle-like architecture dominate locally the inner to middle cortex of the pre- and postaxial bone sides. The entire cortex of humeri MHI 1096, SMNS 15937, and SMNS 59831 is spongeous-like in organization: although the surface occupied by bony tissues represents much more than 50% of the cortex surface, the high amount of primary osteons with large vascular spaces confers to the cortex a spongeous aspect (Table 2, 3; Figs 2, 7). In the other samples, the inner cortex can be locally spongeous-like and/or trabecular-like. All samples share a spongeous-like organization of the outer cortex (Fig. 7). The spongeous-like organization is the result of locally primary spongeous and trabecular bone and secondarily widened primary osteons.

The midshaft of a humerus (SMNS 59827), which is thought to represent aff. Placodus (Vogt 1983; Rieppel 1995) was micro-CT-scanned. The resolution of the resulting pictures is poor, which is however often the case for Muschelkalk samples (NK, pers. obsv.). A medullary cavity in the center of the cross section cannot be identified and the organization seems to be rather loose, most likely indicating a medullary region (Fig. 2O). The vascular system resembles that of humeri of Placodontia indet. aff. Cyamodus. The outer cortex clearly shows large vascular canals (Fig. 2O).


Fig. 4. Details of medulla and inner cortex of Placodontia long bones. A, Humerus in polarized light and B, Femur in normal light of Paraplacodus broilii (PIMUZ T5845). The medullary region consists of endosteal deposits with pockets of calcified cartilage in between. In the femur large erosion cavities occur in the centre of the section. C, Humerus (PIMUZ A/III 1476) and D, Femur (PIMUZ A/III 0735) of Psephoderma alpinum both in polarized light. Note that the medullary cavities are surrounded by a distinct perimedullary region. E-L, Placodontia indet. aff. Cyamodus humeri all in polarized light. Except for SMNS 54569 (Fig. 3), which represents a more proximal sample and MHI 697 all midshaft samples exhibit a small free cavity. SMNS 54569 (Fig. 3) and MHI 697 (Fig. 3F) show a medullary region infilled by secondary trabecels, sediment filled cavities, and pockets of calcified cartilage. E, MHI 1096. F, MHI 697. G, SMNS 59831. H, SMNS 15891. I, SMNS 54582. J, SMNS 54569. K, MHI 2112/6. L, SMNS 15937. M-O, Humeri of the marine reptile Horaffia kugleri. Horaffia kugleri has a small free cavity surrounded by a medullary region at midshaft. M, MHI 2112/4. N, SMNS 84816. O, MHI 2112-1.

The humerus and femur of Paraplacodus broilii both have a medullary region that is densely packed with endosteal deposits and pockets of calcified cartilage (Figs 2A, B; 4A, B). The bone tissue is nearly avascular (Table 2; Figs 6C, D).

The medullary region of humerus MB.R. 454 (Placodontia indet.) is very large (Figs 2P, 5A). The area consists of thin secondary trabecles surrounding intertrabecular spaces. Humerus IGWH 9 (Placodontia indet.) also shows a large medullary region and a trabecle-like architecture at the postaxial bone side (Figs 2Q, 5B). The medullary region is filled by endosteal deposits. Vascular density is moderate (MB.R. 454) or high (IGWH 9) (Table 2). Bone compactness increases towards the outer cortex in both samples.

Femora IGWH 23 and SMNS 84545 (both Placodontia indet.) both have a large medullary region that consists of several generations of secondary trabecles (Fig. 5C, D). Please note that the cortex is incomplete in IGWH 23 at the dorsal and preaxial bone sides due to preservation (Fig. 25). Ventrally, large irregularly shaped cavities are aligned in circumferential rows. The cortex is in both scattered with randomly shaped and oriented large cavities, resulting in a spongeous-like tissue (Fig. 2R, S).


Fig. 5. Details of medulla and inner cortex of Placodontia indet. humeri and femora. All bones have a large medulla consisting of secondary trabecles and large cavities. A, Medulla of humerus MB.R. 454 in normal light. B, Medulla of humerus IGWH 9 in polarized light. C, Medulla of femur SMNS 84545 in normal light. D, Medulla of femur IGWH 23 in polarized light.

Midshaft sections of placodont femora MB.R. 814.2, MB.R. 961, and MB.R. 812 (Figs 2U, V, X) have an off-centered free medullary cavity, which corresponds to a medullary region in the more distal section SMNS 54578 (Fig. 2W). The medullary region in non-midshaft sample SMNS 54578 consists of secondary trabecles. Vascular density in all specimens is high (Table 2). The vascular canals have undergone resorption resulting in the formation of large cavities. All samples show locally (mainly at the dorsopostaxial bone sides) a radial or irregularly formed trabecle-like architecture and locally spongeous-like tissue. Vascular density decreases towards the outer cortex. One placodont femur (MB.R. 965) was micro-CT-scanned. The quality of the resulting pictures documents a similar microstructure as was described for the femora above: most of the cross section shows a large medullary region followed by spongeous-like tissue in the inner cortex (Fig. 2T). Locally a trabecle-like architecture is visible. Only the outer cortex region is compact.