On wing disparity and morphological variation of the Santana Group pterosaurs

Pterosaurs were widely spread throughout the Mesozoic Era, populating the whole world. Among this great diversity, two groups are commonly found in Brazil: the Anhangueridae and Tapejaridae. These can be mainly identified by cranial synapomorphies. However, because of the fragility of the pterosaur skeleton and rarity of the fossilisation process, the fossils found are usually incomplete, which hampers a proper taxonomic identification of the specimens. The specific proportions of these two groups of pterosaurs were obtained from bibliographic data and measurements of specimens. Eight Anhangueridae-like and seven Tapejaridae were used: Anhanguera piscator, Anhanguera santanae, Anhanguera spielbergi, Araripesaurus castilhoi, Barbosania gracilisrostris and three Anhangueridae sp. indet.; Sinopterus dongi, Tapejara wellnhoferi and five Tapejaridae sp. indet. We find that proportions of the humerus, wing metacarpal, first phalanx of the wing digit, femur and tibia are sufficient to identify partial remains of Araripe pterosaurs. A principal component analysis shows that each clade has different, non-overlapping scores in the studied ratios and these can be used with precision. Specific bone ratios for fast identification of anhanguerids and tapejarids are given, opening a broader way to diagnostic fragmentary bones.

Jaw mechanics of the pterosaur skull construction and the evolution of toothlessness

In the present study, pterosaur skull constructions were analysed using a combined approach of finite element analysis (FEA), static investigations as well as applying classical beam theory and lever mechanics. The study concentrates on the operating regime „bite“, where loads are distributed via the dentition or a keratinous rhamphotheca into the skull during jaw occlusion. As a first step, pterosaur tooth constructions were analysed. The different morphologies of the tooth construction determine specific operational ranges, in which the teeth perform best (= greatest resistance against failure). The incomplete enamel-covering of the pterosaur tooth constructions thereby leads to a reduction of strain and stress and to a greater lateral elasticity than for a complete enamel cover. This permits the development of high and lateral compressed tooth constructions. Further stress-absorption occurs in the periodontal membrane, although its mechanical properties can not be clarified unambiguously. A three-dimensionally preserved skull of Anhanguera was chosen as a case-study for the investigation of the skull constructions. CT-scans were made to get information about the internal architecture, supplemented by thin-sections of a rostrum of a second Anhanguera specimen. These showed that the rostrum can be approximated as a double-walled triangular tube with a large central vacuity and an average wall-thickness of the bony layers of about 1 mm. On base of the CT-scans, a stereolithography of the skull of Anhanguera was made on which the jaw adductor and abductor muscles were modelled, permitting to determine muscular forces. The values were used for the lever mechanics, cantilever and space frame analysis. These studies and the FEA show, that the jaw reaction forces are critical for the stability of the skull construction. The large jugal area ventral to the orbita and the inclined occipital region act as buttresses against these loads. In contrast to the orbitotemporal region which is subject to varying loading conditions, the pattern in the rostrum is less complex. Here, mainly bending in dorsal direction and torsion occur. The hollow rostrum leads to a reduction of weight of the skull and to a high bending and torsional resistance. Similar to the Anhanguera skull construction, the skulls of those pterosaur taxa were analysed, from which enough skull material is know to permit a reliable reconstruction. Furthermore, FEA were made from five selected taxa. The comparison of the biomechanical behaviour of the different skull constructions results in major transformational processes: elongation of rostra, inclination of the occipital region, variation of tooth morphology, reduction of the dentition and replacement of teeth by a keratinous hook or rhamphotheca, fusion of naris and antorbital fenestra, and the development of bony and soft-tissue crests. These processes are discussed for their biomechanical effects during bite. Certain optional operational ranges for feeding are assigned to the different skull constructions and previous hypotheses (e.g. skimming) are verified. Using the principle of economisation, these processes help to establish irreversible transformations and to define possible evolutionary pathways. The resulting constructional levels and the structural variations within these levels are interpreted in light of a greater feeding efficiency and reduction of bony mass combined with an increased stability against the various loads. The biomechanical conclusive pathways are used for comparison and verification of recent hypothesis of the phylogenetic systematics of pterosaurs.