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The Hypothesis of Saurischian Pneumaticity

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The Hypothesis of Saurischian Pneumaticity Geological Society, London, Special Publications Pneumaticity, the early years: Wealden Supergroup dinosaurs and the hypothesis of saurischian pneumaticity Darren Naish Geological Society, London, Special Publications 2010; v. 343; p. 229-236 doi:10.1144/SP343.13 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by University of Portsmouth on 6 October 2010 © 2010 Geological Society of London Pneumaticity, the early years: Wealden Supergroup dinosaurs and the hypothesis of saurischian pneumaticity DARREN NAISH School of Earth & Environmental Sciences, Burnaby Building, Burnaby Road, University of Portsmouth, Portsmouth PO1 3QL, UK (e-mail: [email protected]) Abstract: Saurischian dinosaurs were pneumatic animals. The presence of invasive skeletal for- amina 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 Cretac- eous Wealden Supergroup led Richard Owen and Harry Seeley to note the pneumatic, bird-like fea- tures 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 ver- tebrae (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 discov- unnamed) tetanuran theropod of uncertain affinities eries, the Lower Cretaceous Wealden Supergroup of (Benson et al. 2009). southern England must rank as one of the most The term ‘Wealden’ refers to a series of non- important geological units. It yielded Mantell’s marine mudstones, sandstones and other strata that original Iguanodon material during the 1820s, the were deposited in two sub-basins located in what armoured dinosaur Hylaeosaurus armatus during is now SE England: the Weald sub-basin of the the 1830s, the earliest sauropod discoveries during English mainland; and the Wessex sub-basin of the 1840s, and what proved to be a pivotal form in the Isle of Wight and Dorset (Martill & Naish early ideas on the evolutionary relationship 2001; Radley 2004, 2006a, b). While the strata of between dinosaurs and birds, Hypsilophodon,in both the Weald and Wessex sub-basins were pre- 1869. Despite the fact that Wealden exposures viously referred to as ‘the Wealden Group’, they have been well explored and extensively studied are now known as the Wealden Supergroup since the early 1800s, they continue to yield new (Fig. 1). Within the Weald sub-basin, the oldest dinosaurs, with recently described taxa including unit is the Berriasian–Valanginian Hastings Beds the spinosauroid Baryonyx walkeri (Charig & Group. Younger than the Hastings Beds Group, Milner 1986), the ankylosaur Polacanthus rudg- but also occurring within the Weald sub-basin, is wickensis (Blows 1996), the allosauroid Neovenator the Weald Clay Group: this unit is mostly Hauteri- salerii (Hutt et al. 1996), the basal tyrannosauroid vian and Barremian, but might extend into the Eotyrannus lengi (Hutt et al. 2001), the extremely Aptian as well (Allen & Wimbledon 1991). unusual neosauropod Xenoposeidon proneneukos Finally, within the Wessex sub-basin, the Wealden (Taylor & Naish 2007) and a large (as yet 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. 230 D. NAISH 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 & Chris- tiansen 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)). Fig. 1. Stratigraphic diagram showing the approximate correlations between the Weald sub-basin units of the English mainland and the Wessex sub-basin units of the Isle of Wight and Dorset. Based on Kerth and Hailwood Becklespinax, the ‘first’ pneumatic (1988), Allen and Wimbledon (1991), Martill & Naish theropod (2001) and Radley (2004, 2006a, b). Among the few theropods that are known from the Hastings Beds Group is NHMUK R1828, currently extends into the Aptian (Kerth & Hailwood 1988). It known as Becklespinax altispinax (Paul 1988). Rep- includes the Wessex and Vectis formations, both of resented only by three articulated dorsal vertebrae which crop out on the Isle of Wight. discovered by Samuel Beckles some time prior to While the Wealden Supergroup is often noted as 1855 (Owen 1855), these three vertebrae are all an important unit for discoveries that have shed new that we have of this dinosaur. However, in describ- light on dinosaur diversity, less well appreciated is ing the specimen, Owen (1856) referred to some that dinosaurs from the Wealden have also proved other material (another two vertebrae and two important in terms of shaping our views on dinosaur ribs) that apparently belonged to it but have since palaeobiology. Among the most interesting and been lost. The proximal end of the robust right vexing, and arguably most important, aspect of tibia HASM G.378 from the Hastings Beds Group, dinosaur palaeobiology is the fact that saurischians identified as Allosauroidea indet., was suggested (and not ornithischians so far as we know) exhibited by Naish (2003) to perhaps be referable to B. altis- skeletal pneumaticity: a system of air sacs and pneu- pinax. It remains impossible to evaluate this in the matic diverticula were present in at least some of the absence of better B. altispinax remains. vertebrae, with basal forms exhibiting shallow Like many Wealden Supergroup dinosaurs, B. pneumatic fossae on their vertebral centra and altispinax has had a tumultuous nomenclatural derived forms possessing internalized pneumatic history, and I have largely avoided this area here cavities connected to foramina located within the (a full revision of B. altispinax is in preparation). fossae (Britt 1993; Wedel 2003a, b, 2004, 2007; However, Rauhut (2000) argued that Huene (1923) O’Connor 2006). Pterosaurs also exhibited skeletal proposed the generic name Altispinax for NHMUK pneumaticity, raising the possibility that it was R1828, and not for the indeterminate tooth regarded ancestral for ornithodirans and secondarily lost in as the type of Megalosaurus dunkeri by Kuhn ornithischians (Bonde & Christiansen 2003; Butler (1939) and Olshevsky (1991). Rauhut therefore con- et al. 2009). Skeletal pneumaticity has also been cluded that B. altispinax should be referred to as inferred for some basal archosauriforms (Erythrosu- Altispinax altispinax. This can be interpreted as chus africanus) and crocodile-group archosaurs, contradicting key statements in the literature. On which could suggest that it was primitive for naming Altispinax, Huene (1923) included Megalo- crown-group archosaurs or even for a more inclus- saurus dunkeri and M. oweni within the genus. He ive clade
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