Protanystropheus antiquus, (Huene, 1905) (Huene, 1905)

Rytel, Adam, Surmik, Dawid, Szczygielski, Tomasz, Spiekman, Stephan N. F., Kamp, Thomas van de, Zuber, Marcus & Scheyer, Torsten M., 2024, Unique internal anatomy of vertebrae as a key factor for neck elongation in Triassic archosauromorphs, Zoological Journal of the Linnean Society 202 (3), pp. 1-21 : 6

Bone histology and internal structure of vertebrae of ‘ P ’. antiquus

The CT examination of vertebrae was performed to nondestructively investigate the internal structure of each specimen and to assess the thickness of compact and cancellous bone in the different regions of the vertebrae. Within the anteroposteriorly terminal portions of all scanned ‘ P ’. antiquus centra, the core was filled with dense cancellous bone (identified in CT images so of uncertain tissue composition) and the cortex was relatively thin ( Figs 2–4 ), whereas in the middle portion, where the internal cavity was revealed, the cortical bone was distinctly thicker. The transverse sections revealed the changes of the cortical bone thickness and the shape of internal cavities, with the mid-centrum being roughly cylindrical. The cortex was thinner near the anteroposteriorly terminal portions of the vertebrae and its thickness gradually increased up to approximately four times the original thickness (about 10% of bone diameter), near the midpoint ( Fig. 3 ). The density of the cancellous bone of the anteroposteriorly terminal portions of the centrum decreased medially, with larger cavities being present closer to the midpoint. The transition to the empty internal cavity was relatively sharp. The anterior and posterior ends of the internal cavity were roughly hemispherical and surrounded circumferentially by marginalized trabeculae ( Fig. 3 ). Long intermittent trabeculae composed of parallel-fibred bone moderately remodelled with endosteal secondary lamellar bone ( Fig. 4D ) crossed the internal cavity in varied directions, mainly diagonally. In some vertebrae they divided the internal cavity into several smaller terminus-like pockets ( Figs 2B , 3 ). The neural canal was separated ventrally from the internal cavity by a horizontal, plate-like lamella consisting of endosteal lamellar bone ( Figs 2B , 3 , 4A–C ), which was generally thinner than the internal trabeculae composed of the same tissue. This separation was not continuous, the neural canal floor was perforated by large openings in some specimens ( Figs 2B and 3C–G ) and seemed to disappear completely in others. For example, in MGUWr 3889s at least two large openings were present in the lamella separating the neural canal from the internal cavity. In MGUWr 3902 and GPIH 5194c there appeared to be no bony separation between the neural canal and the internal cavity for most of the length of the centrum. Because no fragments of broken bone were present anywhere inside of the neural canal or the internal cavity of the centrum, and this condition was present in several specimens approximately in the same area, it appears not to be an artifact of preservation. The cortex was predominately composed of parallel-fibred matrix ( Fig. 4 ), with locally (especially in the dorsolateral part) highly organized arrays of mineralized collagen fibres. No rest lines were present. The vascularization was moderate, and its pattern was radial, which was especially evident in the ventral part of the vertebrae ( Fig. 4A–C, H–I ). Inside the neural arch, mostly dorsally to the neural canal (inside of the base of the neural spine) and in some specimens laterally to the neural canal there was a region of occurrence of secondary trabecular bone ( Fig. 4J ). Both the cavity and the neural canal were clear cut and lined with a thin layer of endosteal lamellar bone ( Fig. 4E–J ). Sharpey’s fibres could be seen extending throughout the cortex, especially in the dorsolateral and ventrolateral regions of the centrum ( Fig. 4C, G–I ).

Figures

Figure 2. GIUS-7-3674, cervical of ‘P’.antiquus, surface model in left lateral view (A) and a longitudinal CT cross section (B). Notice the thin, but generally continuous neural canal floor.Anatomical abbreviations: ic—internal cavity; la—lamella that separates the neural canal from the internal cavity; nc—neural canal; ns—neural spine; poz—postzygapophysis; prz—prezygapophysis.

Figure 3. CT transverse cross sections (from anterior to posterior) of ‘P’.antiquus cervical MGUWr 3889s (surface model in left lateral view) showing internal structure of the vertebra. Sectioning planes are marked with lines. Note the presence of a large internal cavity (ic), thicker cortex layer (tc), and slanted trabeculae (st) spanning across the internal chamber. Trabecular bone (tb) is present in the anteroposteriorly terminal portions of the centrum.

Figure 4. Thin sections of ‘P’.antiquus cervical vertebrae SUT-MG/F/Tvert/2 (A, E, F, H, I) and GIUS-7-3674 (B–D, G, J). A–C, thin sections in normal transmitted light (A, B) and in polarized light (C). The slight lateral asymmetry results from minor antero-posterior skewing of the plane of sectioning. D, close-up of the internal supporting trabeculae in polarized light, presenting heavy remodelling. E–F, the lateral cortex of the centrum in polarized light (E) and polarized light with lambda compensator (F) showing lack of distinctive zonation and loosely ordered bone structure. G, cortex of the dorsolateral region of the centrum displaying more pronounced zonation and dorsal internal slanted trabeculae in polarized light. H–I, cortex of the ventrolateral region of centrum showing radial vascularization and a well-developed bundle of Sharpey’s fibres in polarized light (H) and polarized light with lambda compensator (I). J, the trabecular bone of the neural arch in polarized light.Arrows indicate a perforation of the neural canal floor. Anatomical abbreviations: elb—endosteal lamellar bone; ic—internal cavity; nc—neural canal; ns—neural spine; pfb—parallel-fibred bone; pvc—primary vascular canals; sf—Sharpey’s fibres; st—slanted trabeculae; tb—trabecular bone.Scale bars for panels A–C equal 5 mm and 0.5 mm for D–J.