See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/307421475 Ichnological evidence for large predatory dinosaurs in the Wessex Formation (Wealden Group, Early Cretaceous) of the Isle of Wight. Article · August 2016 CITATIONS 0 1 author: Jeremy Lockwood University of Portsmouth 7 PUBLICATIONS 50 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Invertebrate bio-erosion of bone from the Early Cretaceous. View project New spinosaurid dinosaur finds from the Wessex Formation (Wealden Group, Early Cretaceous) of the Isle of Wight, UK. View project All content following this page was uploaded by Jeremy Lockwood on 30 August 2016. The user has requested enhancement of the downloaded file. Proc. Isle of Wight Nat. Hist. Archaeol. Soc. 30 (pub 2016) ICHNOLOGICAL EVIDENCE FOR LARGE PREDATORY DINOSAURS IN THE WESSEX FORMATION (WEALDEN GROUP, EARLY CRETACEOUS) OF THE ISLE OF WIGHT Jeremy Lockwood Abstract Theropod footprints from the Wessex Formation are briefly reviewed and a particularly large example discovered on the south-west coast of the Isle of Wight is described. Footprint lengths ranged from 10-50cms (modal group 30-40cms) suggesting the most frequent hip height for theropods was 1.5m and the largest 2.5m. The distribution is compared with data on ornithopod prints and the implications for our understanding of the growth and size of the Island’s predatory dinosaurs (Baryonyx sp., Neovenator salerii and Eoyrannus lengii) are discussed. Introduction The Wessex Formation (Wealden Group) on the Isle of Wight can be described ichnologically as a category 3a deposit (Lockley 1991, 33) i.e. footprints and bones occur in about equal proportions and the footprints are generally consistent with known skeletal remains. The bulk of exposed footprints are preserved as natural casts of footprints (convex hyporeliefs) which are exhumed by marine erosion and cliff falls and found as blocks of sandstone on the foreshore. The majority of large prints at Hanover Point (Grid Reference SZ 370849-385834) show characteristics of the ichnogenus Caririichnium (Lockwood et al. 2014, 718), suggesting an ornithopod track maker. On the Island they have, with some justification, been assigned to ‘Iguanodon’, and historically were some of the first to be described (Beckles 1851, 117; Mantell 1854, 238). Theropod tracks are less common in the Wessex Formation, presumably due to predator-prey ratios limiting their population numbers. This is mirrored in the body fossil record of larger theropods, where articulated material is rare, but includes partial specimens of the spinosaurid Baryonyx sp., the tyrannosauroid Eotyrannus lengii and a rather more complete skeleton of the carcharodontosaurid Neovenator salerii. This report reviews theropod tracks in the Wessex Formation and describes an unusually large and well preserved specimen found in the Chilton Chine Sandstone Member in January 2016. Geological setting The Early Cretaceous rocks of the Wessex Formation underlie the Vectis Formation as part of the Wealden Group with the oldest units situated just above the Hauterivian– Barremian boundary (Allen & Wimbledon 1991, 511). Lack of volcanic ash and fossils that provide reliable indicators for biostratigraphic correlation means that the dating of the deposits is imprecise. However, work on fossil pollens and spores suggests that the exposed Wessex Formation of the Wealden Group on the Isle of Wight is entirely Barremian (Hughes & McDougall 1990), while carbon-isotope stratigraphy places local plant remains, known as ‘the Pine Raft’ (Fig. 1), at approximately 125 Ma (Robinson & Hesselbo 2004, 142). The Wessex is principally made up of varicoloured mudstones and interbedded sandstones originating in floodplain, river channel and point bar deposits (Sweetman 2011, 56). The sandstone beds, which occur throughout the succession, were laid down as point bar, levee and channel infills in meandering river systems or in times of flood as crevasse splay events (Insole et al. 1998, 4). These latter events have resulted in an important source of footcasts on the Island. The palaeoenvironment is interpreted as a 103 sequence of alluvial meander plains which overran the Wessex Sub-basin (Allen 1998, 201), together with seasonally ephemeral lakes and ponds (Martill & Naish 2001, 40). The climate was variable ‘Mediterranean’ and the differentiated soils show swelling and shrinkage features typical of modern, warm, semi-arid areas (Allen 1998, 208). The uneven ring structure in the locally found fossil conifer Pseudofrenelopsis paraceramosa also indicates a probable annual change from hot-drier to cool-wetter weather of a Mediterranean type rather than monsoonal (Francis 1987). Higher ground to the north had forested areas of fir trees, with cycads and tree ferns also present. Forest fires and floods were common, washing plant debris into the basin. High-sinuosity rivers in the basin provided for a rich riparian ecosystem (Sweetman 2011, 59) and an excellent environment for the preservation of dinoturbation as evidenced by today’s footprint record which extends through numerous horizons. Distinguishing theropod from ornithopod footprints The usual features differentiating theropod from ornithopod footprints may be summarised as: The presence of claw marks Segmentation of the digital impressions due to phalangeal pads. A more acute angle of divarication of digits II and IV (Lockley 1991, 47). A long digit III compared to the lateral digits, i.e. greater mesaxony (Lockley 2009, 419). Slender digits which are distally acuminate. A curved digit III and sometimes curved lateral digits (Thulborn 1990, 221). Length greater than the width, usually with a ratio above 1.25 (Moratalla et al. 1988, 404). However many of these defining characteristics are based on morphologies which can usually only be seen distinctly in archetypal small Jurassic ichnogenera such as Grallator Hitchcock 1858 (see Fig.2A) and Anchisauripus Lull 1904. Unfortunately some of these factors do not scale isometrically and can be problematical when trying to separate larger ornithopod and theropod footprints. Digital divarication can vary widely in a single trackway, claw marks are often absent and as size increases segmentation becomes less pronounced (this is especially notable in the Cretaceous, where marked segmentation is unusual) and the degree of mesaxony is reduced. Substrate consistency and gait are also important e.g. collapse of substrate walls or very shallow impressions can simulate narrow digits. Morphological convergence in large tridactyl prints and preconceived ideas based on Jurassic ichnogenera have both been cited as a cause of diagnostic difficulties in Cretaceous assemblages (Lockley et al. 2011, 172; Castanera et al. 2013, 1; Moratalla et al. 1988, 396). Thulborn & Wade (1984, 434) also felt that theropod prints could be separated into those with longer gracile digits and those with shorter more robust ones. They attributed the former to dinosaurs such as Allosaurus and Megalosaurus, and the latter to tyrannosaurids. Furthermore in the Wessex Formation Valdosaurus canaliculatus Galton 1977, a dryosaurid with long narrow digits, may have produced theropod-like prints adding to the confusion (osteometric data does however make it unlikely that these prints would have exceeded 25cms). Moratalla et al. (1988) used multivariate analysis to identify measurements possessing high discriminatory value in determining the theropod or ornithopod origins of footprints. For example if foot length (FL):width ratio >1.25 the chance of the print being theropod is 80%. A series of nine measurements collectively provide a useful tool although there are still diagnostic problems with large prints and Moratalla et al. (1988, 404) suggested that measurements were complemented with non-metrical observations such as claw marks. This report accepts the current diagnostic difficulties, but finding footprints that meet all of the above criteria would have excluded most specimens. The ‘Iguanodon’ tracks on the Isle of Wight have distinctive padded, rounded and blunt ended toes which are of similar sizes but weakly mesaxonic and the pragmatic approach was taken of considering tridactyl footprints with narrow tapering digits and pronounced mesaxony to be theropodal in origin. Specimens All Wessex Formation footprints in the collections at Dinosaur Isle Museum exhibiting theropod morphology were examined and photographed. Also included were photographs of foot casts and prints from the Isle of Wight collected by the author over a ten year period. There were some borderline examples especially with larger prints but only typical theropod prints were selected. Eight theropod tracks recorded at Chilton Chine (Grid Reference SZ 408822) on the south coast of the Island by William Blows (Pond et al. 2014, 748) were included in the data calculations, giving a sample size of 46. One specimen (Fig. 2. LL, Fig. 3. C) has a FL of nearly 70cms so is exceptionally large (by comparison Tyrannosaurus rex foot lengths = 86cms (Lockley & Hunt 1994, 213) and 72cms (Manning et al. 2008, 645)) and has narrow pads on the surviving lateral digit and digit III. Despite some characteristic large ornithopod features it possesses 104 considerable mesaxony and a fairly narrow digit III making it atypical; however distortion due to substrate collapse or dragging of digit III may account for the appearance and it has not been included in the analysis. Other specimens exhibited FLs ranging from 10 – 50cms. Figure 2. Outlines of probable theropod foot casts from the Wessex Formation. Placed in order of increasing FL, scale bars 10cms. A. IWCMS:2011.31 B. MIWG:5768 C.T. Photographed at Yaverland. D.F.G.I.J.L.O.Q.U.W.Y.Z. AA.EE.FF.GG.HH.II.JJ.LL Photographed at Hanover Point. E.V.BB.CC. Photographed at Chilton Chine. H. IWCMS:2014.22 K. Fig. 6. (Radley 1994, 206). M. MIWG:5348 N. MIWG:unaccessioned. P. IWCMS:1995.60 R. IWCMS:1996.160 S. MIWG:2010.9 X. IWCMS:1995.61. DD. IWCMS:2000.398. KK. IWCMS:2016.273. Figure 3. A. Typical example of ichnogenus Grallator from the Jurassic – not represented on the Isle of Wight.