Convergent evolution, the acquisition of morphologically similar traits in unrelated taxa due to similar functional demands or environmental factors, is a common phenomenon in the animal kingdom. Consequently, the occurrence of similar form is used routinely to address fundamental questions in morphofunctional research and to infer function in fossils. However, such qualitative assessments can be misleading and it is essential to test form/function relationships quantitatively. The parallel occurrence of a suite of morphologically convergent craniodental characteristics in three herbivorous, phylogenetically disparate dinosaur clades (Sauropodomorpha, Ornithischia, Theropoda) provides an ideal test case. A combination of computational biomechanical models (Finite Element Analysis, Multibody Dynamics Analysis) demonstrate that despite a high degree of morphological similarity between representative taxa (Plateosaurus engelhardti, Stegosaurus stenops, Erlikosaurus andrewsi) from these clades, their biomechanical behaviours are notably different and difficult to predict on the basis of form alone. These functional differences likely reflect dietary specialisations, demonstrating the value of quantitative biomechanical approaches when evaluating form/function relationships in extinct taxa.
Although Stegosaurus is one of the most iconic dinosaurs, well-preserved fossils are rare and as a consequence there is still much that remains unknown about the taxon. A new, exceptionally complete individual affords the opportunity to describe the anatomy of Stegosaurus in detail for the first time in over a century, and enables additional comparisons with other stegosaurian dinosaurs. The new specimen is from the Red Canyon Ranch Quarry, near Shell Wyoming, and appears to have been so well preserved because it was buried rapidly in a pond or body of standing water immediately after death. The quarry is probably located in the middle part of the Morrison Formation, which is believed to be Tithonian in age in this area. The specimen is referable to Stegosaurus stenops based on the possession of an edentulous anterior portion of the dentary and elevated postzygapophyses on the cervical vertebrae. New information provided by the specimen concerns the morphology of the vertebrae, the iliosacral block and dermal armor. Several aspects of its morphology indicate the individual was not fully skeletally mature at the time of death, corroborating a previous histological study.
The first African dinosaur to be discovered,Paranthodon africanuswas found in 1845 in the Lower Cretaceous of South Africa. Taxonomically assigned to numerous groups since discovery, in 1981 it was described as a stegosaur, a group of armoured ornithischian dinosaurs characterised by bizarre plates and spines extending from the neck to the tail. This assignment has been subsequently accepted. The type material consists of a premaxilla, maxilla, a nasal, and a vertebra, and contains no synapomorphies of Stegosauria. Several features of the maxilla and dentition are reminiscent of Ankylosauria, the sister-taxon to Stegosauria, and the premaxilla appears superficially similar to that of some ornithopods. The vertebral material has never been described, and since the last description of the specimen, there have been numerous discoveries of thyreophoran material potentially pertinent to establishing the taxonomic assignment of the specimen. An investigation of the taxonomic and systematic position ofParanthodonis therefore warranted. This study provides a detailed re-description, including the first description of the vertebra. Numerous phylogenetic analyses demonstrate that the systematic position ofParanthodonis highly labile and subject to change depending on which exemplifier for the clade Stegosauria is used. The results indicate that the use of a basal exemplifier may not result in the correct phylogenetic position of a taxon being recovered if the taxon displays character states more derived than those of the basal exemplifier, and we recommend the use, minimally, of one basal and one derived exemplifier per clade.Paranthodonis most robustly recovered as a stegosaur in our analyses, meaning it is one of the youngest and southernmost stegosaurs.
The exceptionally rare transition to quadrupedalism from bipedal ancestors occurred on three independent occasions in ornithischian dinosaurs. The possible driving forces behind these transitions remain elusive, but several hypotheses-including the development of dermal armour and the expansion of head size and cranial ornamentation-have been proposed to account for this major shift in stance. We modelled the position of the centre of mass (CoM) in several exemplar ornithischian taxa and demonstrate that the anterior shifts in CoM position associated with the development of an enlarged skull ornamented with horns and frills for display/defence may have been one of the drivers promoting ceratopsian quadrupedality. A posterior shift in CoM position coincident with the development of extensive dermal armour in thyreophorans demonstrates this cannot have been a primary causative mechanism for quadrupedality in this clade. Quadrupedalism developed in response to different selective pressures in each ornithischian lineage, indicating different evolutionary pathways to convergent quadrupedal morphology.
Using bone histology, a slow growth rate, uncommon for most dinosaurs, has been interpreted for the highly derived stegosaur Stegosaurus (Ornithischia: Thyreophora) and the basal thyreophoran Scutellosaurus. In this study, we examine whether this slow growth rate also occurs in the more basal stegosaur Kentrosaurus from the Tendaguru beds of Tanzania. The bone histology of six femora of Kentrosaurus representing an ontogenetic series from subadult to adult was studied, as well as one scapula. The primary bone is mainly highly vascularized fibro-lamellar bone with some reticular organization of the vascular canals. In addition to LAGs and annuli, distinctive shifts in the pattern of vascularization occur, which have been interpreted as potential growth marks. The variation in the development of growth marks may reflect annual climatic fluctuations. The overall bone depositional rate, and hence growth rate in Kentrosaurus appears to be higher than in Stegosaurus and Scutellosaurus. Considering that Stegosaurus is the larger-sized of the two stegosaurs, this would be contrary to an earlier supposition that small-bodied dinosaurs have slower growth rates than larger ones. Our finding of rapid rates of bone deposition in Kentrosaurus suggests that slow growth rates previously reported in Scutellosaurus and Stegosaurus are not a phylogenetic characteristic of the Thyreophora. Thus, slow growth rates are not plesiomorphic for the Thyreophora. We propose that the slow growth rates documented in the highly derived Stegosaurus could have been secondarily derived or alternatively that Kentrosaurus is the exception having increased growth rates. Anat Rec, 2013. © 2013 Wiley Periodicals, Inc.