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, 274 (1616), 1361-8

First Trace and Body Fossil Evidence of a Burrowing, Denning Dinosaur

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First Trace and Body Fossil Evidence of a Burrowing, Denning Dinosaur

David J Varricchio et al. Proc Biol Sci.

Abstract

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.

Figures

Figure 1
Figure 1
Map of southwestern Montana, USA, showing the distribution of mid-Cretaceous rocks of the Blackleaf and Frontier Formations. Star denotes the location of MOR 1636. Modified from Dyman & Nichols (1988).
Figure 2
Figure 2
Burrow for O. cubicularis. (a) Upper portion of tunnel, which from its upper erosional end, U, slopes down making first a right turn followed by a left turn. The dotted oval represents the cross-section of the tunnel, as it turns left towards the terminal chamber. Smaller tunnels, t1 and t2, of possible commensal burrowers project off either side. Scale bar, 15 cm. (b) Lower portion of burrow showing the last section of tunnel (32 cm wide) expanding into the chamber, CH. View is similar to that of (a), with Z–Z′ representing equivalent portions in both the photographs. This edge of the tunnel was damaged during excavation. Bone occurred in the last portion of the tunnel and chamber; plaster now covers the original weathered face. Note the burrow cuts across the contact between the host mudstones. (c) Close-up view from above of the upper set of small accessory burrows (t1), showing them diverging from the upper aspect of the main burrow (mb). Scale bar, 10 cm. (d) Schematic showing the preserved portion of the MOR 1636 burrow. The sinuous burrow crosscuts a series of mudrocks (units 1–3) before terminating in the exposed chamber. Two episodes of infilling by coarser material (units (i) and (ii)) preserved the main burrow and the smaller subsidiary burrows (t1 and t2 in figure 2a) as natural casts. The first episode of infilling buried the bones as a dense concentration near the base of the lower unit (i) with a few small elements occurring higher up. The full extant of the terminal chamber remains unknown, as recent weathering had exposed it and its bony contents prior to discovery.
Figure 3
Figure 3
Adult cranial elements for the O. cubicularis holotype (MOR 1636a). (a) Fused premaxillae in right lateral view. Premaxillae bears five teeth posterior to a small beak and a transversely expanded and roughened anterior face, f. (b) Ventral view of posterior braincase showing elongate paroccipital processes, poc, and steeply sided ventral ‘box’, b, just anterior to the occipital condyle, oc. Scale bar, 1 cm.
Figure 4
Figure 4
Adult and juvenile postcranial elements for O. cubicularis holotype and paratype (MOR 1636a and b, respectively). (a) Anterior dorsal vertebrae in posterior view with steeply inclined transverse processes. (b) Sacrum with seven vertebrae, two pairs of fused dorsal ribs, d, and an articulation for the pubis, p. Note long attachment for ilium. (c) Fused left scapulocoracoid with large acromion process with scapular spine, s, and strong posterior bend of the distal blade. (d,e) Left juvenile and right adult humeri in anterior view. (f) Left juvenile ilium in lateral view with long post- and short pre-acetabular portions. Scale bar, 2 cm.
Figure 5
Figure 5
Phylogenetic position of O. cubicularis gen. et sp. nov. among ornithopods and some potential basal marginocephalians (Xu et al. 2006). This single most parsimonious tree has 377 steps, retention index (RI) of 0.66 and consistency index (CI) of 0.47. Bootstrap analysis with 1000 replicates found greater than 50% support for all nodes, except those among Agilisaurus, Yandusaurus and Othnelia and for Thescelosaurus and Parksosaurus. The Montana hypsilophodont clade occurred in 60% and ZephyrosaurusOrodromeus in 58% of the replicates. Analysis used PAUP v. 4.0b10 (Swofford 1998) and the matrix of Scheetz (1999) with three modifications: (i) adding one character in the absence/presence of fused premaxillae, (ii) altering character 81 (number of sacral vertebrae) to accommodate the presence of seven sacrals, and (iii) correcting two typographic errors for Orodromeus in characters 90 and 98. The final matrix consisted of 21 taxa and 125 characters. Oryctodromeus was scored as follows: 001?? ?1000/????? ?????//????? ?111?/01000 0???0//0??00 1??1?/200?? ?0?00//1?0?? ??001/110?? 11131//41022 ?1112/0?100 ??21?//???10 10110/?1200 100??//????1. Characters and character states as in Scheetz (1999) with fused premaxillae as character 125.

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