Pneumaticity, the early years: Wealden Supergroup dinosaurs and the hypothesis of saurischian pneumaticity

Saurischian dinosaurs were pneumatic animals. The presence of invasive skeletal foramina leading to large internal chambers within the skeleton strongly indicate the presence of avian-style skeletal pneumaticity of the skeleton in sauropodomorphs and non-avian theropods. While the hypothesis of skeletal pneumaticity has undergone a renaissance in recent years, it was initially promoted during the late 1800s after dinosaur fossils from the English Lower Cretaceous Wealden Supergroup led Richard Owen and Harry Seeley to note the pneumatic, bird-like features of the vertebrae they described (Hermann von Meyer had also briefly alluded to skeletal pneumaticity in dinosaurs during the 1830s). In describing the theropod Becklespinax altispinax from the Hastings Beds Group (at the time referred to Megalosaurus), Richard Owen proposed that the laminae on the neural arch served to house ‘parts of the lungs’. He evidently imagined Becklespinax to exhibit avian-style post-cranial skeletal pneumaticity. In 1870 Harry Seeley described two sauropod vertebrae from the Wealden Supergroup, naming them Ornithopsis hulkei. Contrary to what is often stated, Seeley did not identify Ornithopsis as a pterosaur, but as an animal that might ‘bridge over’ the gap between birds and pterosaurs, while at the same time having some affinity with dinosaurs. The lateral foramina and internal bony cavities of one of these specimens were regarded by Seeley as allowing ‘the prolongation of the peculiarly avian respiratory system into the bones’, and he emphasized ‘the lightest and airiest plan’ of the specimen. In 1876 Owen described the Wessex Formation sauropod Chondrosteosaurus gigas. While regarding the lateral fossae as probably having ‘lodged a saccular process of the lung’, Owen now took the opportunity to attack Seeley’s claims of pneumaticity in Ornithopsis, arguing that the internal cavities in Chondrosteosaurus ‘were occupied in the living reptile by unossified cartilage, or chondrine’. The name Chondrosteosaurus gigas (‘giant cartilage and bone lizard’) also looks like a direct assault on Seeley’s proposal of a pneumatic vertebral interior. Owen’s actions seem odd given that he was familiar with the internal morphology of avian vertebrae (which are often strikingly similar to those of sauropods). However, both authors have proved insighful in correctly identifying skeletal pneumaticity during this early phase of dinosaur research. A thorough historical review of early ideas on dinosaurian pneumaticity is still required. In terms of its significance for early dinosaur discoveries, the Lower Cretaceous Wealden Supergroup of southern England must rank as one of the most important geological units. It yielded Mantell’s original Iguanodon material during the 1820s, the armoured dinosaur Hylaeosaurus armatus during the 1830s, the earliest sauropod discoveries during the 1840s, and what proved to be a pivotal form in early ideas on the evolutionary relationship between dinosaurs and birds, Hypsilophodon, in 1869. Despite the fact that Wealden exposures have been well explored and extensively studied since the early 1800s, they continue to yield new dinosaurs, with recently described taxa including the spinosauroid Baryonyx walkeri (Charig & Milner 1986), the ankylosaur Polacanthus rudgwickensis (Blows 1996), the allosauroid Neovenator salerii (Hutt et al. 1996), the basal tyrannosauroid Eotyrannus lengi (Hutt et al. 2001), the extremely unusual neosauropod Xenoposeidon proneneukos (Taylor & Naish 2007) and a large (as yet unnamed) tetanuran theropod of uncertain affinities (Benson et al. 2009). The term ‘Wealden’ refers to a series of nonmarine mudstones, sandstones and other strata that were deposited in two sub-basins located in what is now SE England: the Weald sub-basin of the English mainland; and the Wessex sub-basin of the Isle of Wight and Dorset (Martill & Naish 2001; Radley 2004, 2006a, b). While the strata of both the Weald and Wessex sub-basins were previously referred to as ‘the Wealden Group’, they are now known as the Wealden Supergroup (Fig. 1). Within the Weald sub-basin, the oldest unit is the Berriasian–Valanginian Hastings Beds Group. Younger than the Hastings Beds Group, but also occurring within the Weald sub-basin, is the Weald Clay Group: this unit is mostly Hauterivian and Barremian, but might extend into the Aptian as well (Allen & Wimbledon 1991). Finally, within the Wessex sub-basin, the Wealden Group (sensu stricto) is mostly Barremian and From: Moody, R. T. J., Buffetaut, E., Naish, D. & Martill, D. M. (eds) Dinosaurs and Other Extinct Saurians: A Historical Perspective. Geological Society, London, Special Publications, 343, 229–236. DOI: 10.1144/SP343.13 0305-8719/10/$15.00 # The Geological Society of London 2010. extends into the Aptian (Kerth & Hailwood 1988). It includes the Wessex and Vectis formations, both of which crop out on the Isle of Wight. While the Wealden Supergroup is often noted as an important unit for discoveries that have shed new light on dinosaur diversity, less well appreciated is that dinosaurs from the Wealden have also proved important in terms of shaping our views on dinosaur palaeobiology. Among the most interesting and vexing, and arguably most important, aspect of dinosaur palaeobiology is the fact that saurischians (and not ornithischians so far as we know) exhibited skeletal pneumaticity: a system of air sacs and pneumatic diverticula were present in at least some of the vertebrae, with basal forms exhibiting shallow pneumatic fossae on their vertebral centra and derived forms possessing internalized pneumatic cavities connected to foramina located within the fossae (Britt 1993; Wedel 2003a, b, 2004, 2007; O’Connor 2006). Pterosaurs also exhibited skeletal pneumaticity, raising the possibility that it was ancestral for ornithodirans and secondarily lost in ornithischians (Bonde & Christiansen 2003; Butler et al. 2009). Skeletal pneumaticity has also been inferred for some basal archosauriforms (Erythrosuchus africanus) and crocodile-group archosaurs, which could suggest that it was primitive for crown-group archosaurs or even for a more inclusive clade (Gower 2001). However, the bony vertebral fossae present in these taxa do not communicate with internal chambers, and were argued by O’Connor (2006) to resemble structures that, in crocodilians and other extant reptiles, contain adipose tissue and are not pneumatic. While it can be argued that we are still in the early stages in our understanding of the distribution, phylogeny and function of pneumaticity, there are indications that its presence may correlate not only with pulmonary structure and function but also with metabolism and growth rates (Bonde & Christiansen 2003; Wedel 2003b; O’Connor 2006). A thorough review on the history of thoughts about saurischian pneumaticity has yet to appear. Here, I examine the role that Wealden Supergroup dinosaurs had in early ideas on skeletal pneumaticity. Institutional abbreviations: HASMG, Hastings Museum and Art Gallery, Hastings, UK; NHM UK, Natural History Museum, London, UK (formerly the British Musuem (Natural History)). Becklespinax, the ‘first’ pneumatic