Helically arranged cross struts in azhdarchid pterosaur cervical vertebrae and their biomechanical implications

Abstract

Azhdarchid pterosaurs, the largest flying vertebrates, remain poorly understood, with fundamental aspects of their palaeobiology unknown. X-ray computed tomography reveals a complex internal micro-architecture for three-dimensionally preserved, hyper-elongate cervical vertebrae of the Cretaceous azhdarchid pterosaur, Alanqa sp. Incorporation of the neural canal within the body of the vertebra and elongation of the centrum result in a "tube within a tube" supported by helically distributed trabeculae. Linear elastic static analysis and linearized buckling analysis, accompanied with a finite element model, reveal that as few as 50 trabeculae increase the buckling load by up to 90%, implying that a vertebra without the trabeculae is more prone to elastic instability due to axial loads. Subsuming the neural tube into the centrum tube adds considerable stiffness to the cervical series, permitting the uptake of heavy prey items without risking damage to the cervical series, while at the same time allowing considerable skeletal mass reduction.

Keywords: Animal Morphology; Imaging Anatomy; Paleobiology; Paleontology.

Graphical abstract

Figure 1

Cervical vertebra of Alanqa sp. from the Kem Kem Group of Morocco Cervical vertebra of Alanqa sp. FSAC-KK 5077 in (A), dorsal view, (B), ventral view, (C), right lateral view, (D), anterior view, (E), posterior view. Scale bars represent 10 mm. Abbreviations: pz, prezygapophysis; ct, cotyle; ns, neural spine; su, sulci; fo, foramina; lpf, lateral pneumatic foramina (See Table S1 for measurements).

Figure 2

Images of XCT scan of cervical vertebra of Alanqa sp Images of digital model generated from XCT scans of cervical vertebra FSAC-KK 5077 showing the internal architecture (A and B) and simplification diagrams of the internal structure (C and D), dorsal (A (above neural canal), (C and E) (cut through neural canal)) and anterior (B and D) views cut transversely through the cervical (position indicated by arrows). All views to same scale; scale bar represents 10 mm.

Figure 3

Graphs displaying relative buckling and relative safety factor for a simplified azhdarchid cervical vertebra modeled with varying numbers of trabeculae (A) Relative increase of buckling load proportional factor (LPF) versus number of trabeculae (average values and standard deviation). (B) Safety factor (SF) in tension and in compression determined via a static analysis (average values reported) averaged values rated to the no trabeculae condition. The safety factor refers to the strain level. When the number of the trabeculae increases, it increases the mechanical bonding between the external shell of the vertebra and the internal channel; this introduces an increased stress concentration (and therefore strain level) onto the wall of the internal neural tube. In fact, the internal neural tube is the area where stresses and strains are highest, as shown in Figure 4.

Figure 4

Distribution of the maximum principal strain on the finite element (FE) model; a cross-section showing the effect of the load transfer via the trabeculae (A–C) (A) 50 trabeculae; (B) 150 trabeculae (optimum number), and (C) 450 trabeculae.