First Trace and Body Fossil Evidence of a Burrowing, Denning Dinosaur David J
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Proc. R. Soc. B doi:10.1098/rspb.2006.0443 Published online First trace and body fossil evidence of a burrowing, denning dinosaur David J. Varricchio1,*, Anthony J. Martin2 and Yoshihiro Katsura3 1Earth Sciences, Montana State University, Bozeman, MT 59717, USA 2Department of Environmental Studies, Emory University, Atlanta, GA 30322, USA 3Gifu Prefectural Museum, 1989 Oyana, Seki-shi, Gifu 501-3941, Japan A fossil discovery in the mid-Cretaceous Blackleaf Formation of southwest Montana, USA, has yielded the first trace and body fossil evidence of burrowing behaviour in a dinosaur. Skeletal remains of an adult and two juveniles of Oryctodromeus cubicularis gen. et sp. nov., a new species of hypsilophodont-grade dinosaur, were found in the expanded distal chamber of a sediment-filled burrow. Correspondence between burrow and adult dimensions supports Oryctodromeus as the burrow maker. Additionally, Oryctodromeus exhibits features of the snout, shoulder girdle and pelvis consistent with digging habits while retaining cursorial hindlimb proportions. Association of adult and young within a terminal chamber provides definitive evidence of extensive parental care in the Dinosauria. As with modern vertebrate cursors that dig, burrowing in Oryctodromeus may have been an important adaptation for the rearing of young. Burrowing also represents a mechanism by which small dinosaurs may have exploited the extreme environments of polar latitudes, deserts and high mountain areas. The ability among dinosaurs to find or make shelter may contradict some scenarios of the Cretaceous–Paleogene impact event. Burrowing habits expand the known range of nonavian dinosaur behaviours and suggest that the cursorial ancestry of dinosaurs did not fully preclude the evolution of different functional regimes, such as fossoriality. Keywords: Dinosauria; Ornithopoda; Ichnology; burrow; parental care; Blackleaf Formation 1. INTRODUCTION 2. SYSTEMATIC PALAEONTOLOGY Despite the longevity and predominance of dinosaur Ornithischia Seeley, 1887 lineages throughout the Mesozoic, the radiation of Ornithopoda Marsh 1881 nonavian dinosaurs remained morphologically restricted Euornithopoda Sereno 1986 in comparison with the diversity found among Cenozoic Oryctodromeus cubicularis gen. et sp. nov. mammals (Sereno 1999). Originating from small bipedal cursors, nonavian dinosaurs maintained primarily (a) Etymology terrestrial habits throughout their history (Sereno The generic name comes from the Greek words orycto and 1999). Even the climbing capabilities hypothesized for dromeus, meaning the ‘digging runner’. The species name, the dinosaurian ancestors of birds remain quite con- cubicularis, translates to ‘of the lair’, referring to the troversial (Zhang et al. 2002; Padian 2004). Although inferred denning habit of the taxon. skeletal features suggest a digging and tearing habit in the dinosaurs Heterodontosaurus and Mononykus (Norman (b) Holotype et al. 2004; Senter 2005) and earthen nest structures MOR 1636a consists of the following elements: fused represent the products of digging behaviour in the premaxillae; posterior/occipital region of the brain- theropod Troodon (Varricchio et al. 1997) and titanosaur case; three cervical, six dorsal, sacrum and 23 caudal sauropods (Chiappe et al. 2004), the nonavian dinosaur vertebrae; three dorsal ribs; scapulocoracoid; scapula; radiation has been regarded as devoid of true burrowing coracoid; humerus; ulna; radius; tibiae; distal fibula; and forms (Sereno 1999; Robertson et al. 2004). A novel metatarsal IV (figures 3 and 4). discovery (Museum of the Rockies, MOR, 1636) from the mid-Cretaceous of Montana consists of the skeletal (c) Paratype remains of an adult and two juvenile dinosaurs preserved MOR 1636b includes cranial and postcranial materials within an infilled burrow (figures 1 and 2). Here, we found in association with the holotype and represents two describe this dinosaur, Oryctodromeus cubicularis, as a new juveniles with most linear dimensions measuring 55% to genus and species of hypsilophodont (here considered a 65% those of the adult (figure 4). grade of basal ornithopods), and present the first trace and body fossil evidence for fossorial behaviour in a (d) Locality and horizon nonavian dinosaur. Recognition of such behaviour has The new specimens come from the upper portion of the implications for dinosaur ecology, reproduction, distri- mid-Cretaceous Blackleaf Formation near the Lima Peaks, bution, evolution and extinction. Beaverhead Co., Montana, USA (figure 1). Dyman & Nichols (1988) designate this sequence as the ‘volcaniclastic * Author for correspondence ([email protected]). lithofacies’, a 270 m thick lithologically heterogeneous unit Received 21 December 2006 1 This journal is q 2007 The Royal Society Accepted 9 February 2007 2 D. J. Varricchio et al. Burrowing, denning dinosaur light greenish grey and weakly calcareous. This unit MT exhibits extensive dark reddish grey mottling over its 113° 112° 111° uppermost portion. Additionally, carbonate nodules as much as 4.5 cm in diameter form a discontinuous layer of Butte ID WY 20–25 cm below the sharp upper contact of the unit. ° 46 A dark greenish-grey claystone succeeds vertically. This 25 cm thick unit is calcareous with a platy structure and Bozeman grades upwards into a greenish grey blocky mudstone with some organics. This third unit encloses the upper portion of the sedimentary structure (figure 2a,d ) and bears a Dillon sharp, irregular contact with an overlying burrowed dark red claystone. A thin (approx. 30 cm thick) sandstone and additional mudrocks represent the remaining strata visible 45° Montana Wyoming at this outcrop. The lithology of the structure contrasts sharply with the Lima surrounding host mudrock and facilitates recognition of the structure in the field (figure 2). The lower, bone- bearing portion consists of 25–30 cm of medium to fine, Idaho 50 km calcareous greenish-grey sandstone, moderately sorted with abundant plagioclase and other volcanogenic grains Figure 1. Map of southwestern Montana, USA, showing the distribution of mid-Cretaceous rocks of the Blackleaf and (figure 2d ). Small (3–6 mm), rounded mud clasts are Frontier Formations. Star denotes the location of MOR common in the coarser basal portion. This structureless 1636. Modified from Dyman & Nichols (1988). unit fines upwards with an increasing clay content. A thin, 1–2 cm thick, grey claystone separates this lower sand- consisting of abundant mudstone, siltstone, lithic-rich stone from an upper fine sandstone. The bedding parallels sandstone and subordinate conglomerate (Zartman et al. that of the host mudrocks. Reduced clay content, grey 1995). Low in this lithofacies, mudstone is olive green to colour and larger (approx. 10 mm), platy mud clasts grey, variably calcareous and structureless. Mudstone of the distinguish this unit from the otherwise similar, lower middle and upper portions may be porcelanitic of various sandstone. A thin (less than 1 cm) rim of tan claystone colours (red, grey or green), bentonitic with ‘popcorn- with abundant calcite veins separates the lower portion of texture’ weathering, or strongly calcareous, non-porcelanitic the structure from the host mudrocks. and grading laterally into micritic limestone (Dyman & The preserved portion of the structure measures more Nichols 1988). Siltstone may be tan to grey, rippled, than 210 cm long and is sinuous and semi-helical. bioturbated and variably calcareous. It occurs as thin Throughout most of its length, the structure maintains a discontinuous beds within mudstone or above sandstone fairly constant oval cross-section measuring 30–38 cm in lenticular bodies (Dyman & Nichols 1988). Sandstone is high!30–32 cm wide before ending abruptly as a partially typically fine-grained, variably calcareous and thin to preserved expansion. From its upper eroded limit, the medium bedded. In the Lima Peaks area, sandstone structure slopes downwards at 68 for 60 cm, turns sharply contains a high proportion (as much as 50%) of volcanic right and continues down at 188 for another 70 cm and plagioclase grains (Dyman et al. 1988). (figure 2a,d ). It then turns sharply left and proceeds a These sedimentary rocks represent deposition on a short distance (40 cm) before expanding (figure 2b,d ). broad flood plain with relatively small fluvial channels at the Over its course, the structure drops nearly 50 cm from the western, proximal edge of the foreland basin (Dyman & horizontal. Erosion had exposed the terminal expansion Nichols 1988). The high volcaniclastic content reflects the and its bone content by the time of discovery. Excavation proximity of nearby volcanism associated with the develop- of fossils further destroyed aspects of the structure, ment of the Idaho batholith (Dyman & Nichols 1988; particularly the upper portions of the expansion, prior to Dyman et al. 1988). Bivalves, unidentified reptilian bone its recognition and photo documentation (figure 2). fragments, leaves and wood, including the genus Tempskya, Hence, the expansion’s preserved dimensions, 40–50 cm represent the only macrofossils previously described for the long!45 cm wide, represents only a portion of its original Blackleaf Formation (Cobban et al. 1976; Dyman & size. The bone assemblage consisted of a densely packed Nichols 1988). The upper, volcaniclastic lithofacies may jumble of adult and juvenile bones concentrated in the correlate with the Vaughn Member described from more basal 7 cm of the lower sandstone just prior to and northern outcrops near Great Falls,