Skimming the Surface with Burgess Shale Arthropod Locomotion Nicholas J

Downloaded from http://rspb.royalsocietypublishing.org/ on January 24, 2017 Proc. R. Soc. B (2012) 279, 1613–1620 doi:10.1098/rspb.2011.1986 Published online 9 November 2011 Skimming the surface with Burgess Shale arthropod locomotion Nicholas J. Minter1,*, M. Gabriela Mańgano1 and Jean-Bernard Caron2,3 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2 2Department of Natural History (Palaeobiology section), Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario, Canada M5S 2C6 3Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, Canada M5S 3B2 The first arthropod trackways are described from the Middle Cambrian Burgess Shale Formation of Canada. Trace fossils, including trackways, provide a rich source of biological and ecological information, including direct evidence of behaviour not commonly available from body fossils alone. The discovery of large arthropod trackways is unique for Burgess Shale-type deposits. Trackway dimensions and the requi- site number of limbs are matched with the body plan of a tegopeltid arthropod. Tegopelte, one of the rarest Burgess Shale animals, is over twice the size of all other benthic arthropods known from this locality, and only its sister taxon, Saperion, from the Lower Cambrian Chengjiang biota of China, approaches a similar size. Biomechanical trackway analysis demonstrates that tegopeltids were capable of rapidly skimming across the seafloor and, in conjunction with the identification of gut diverticulae in Tegopelte, supports previous hypotheses on the locomotory capabilities and carnivorous mode of life of such arthropods. The trackways occur in the oldest part (Kicking Horse Shale Member) of the Burgess Shale Formation, which is also known for its scarce assemblage of soft-bodied organisms, and indicate at least intermittent oxygenated bottom waters and low sedimentation rates. Keywords: Burgess Shale; Cambrian; ichnology; locomotion; trackway; palaeoecology 1. INTRODUCTION Tegopelte, but it is also inferred to have been a benthic Discovered over 100 years ago, the world-famous Burgess predator or scavenger [6,13]. The aims of this paper are Shale continues to yield important new insights into the evol- to (i) describe the first known arthropod trackways from ution and ecology of animals during the Cambrian explosion the Burgess Shale Formation; (ii) identify their potential [1–5]. The exceptional preservation of soft tissues in the producers; (iii) integrate trackway and body fossil data Burgess Shale permits functional morphological inferences to derive the locomotory techniques employed in pro- on the modes of life and palaeoecology of early metazoans ducing the trackways; and (iv) test previous hypotheses [6,7]. Trace fossils, including trackways and burrows, on their locomotory capabilities and mode of life. provide direct evidence for animal behaviour in the fossil record, and can add further crucial insights for unravelling the palaeoecology of their producers and placing tempo- 2. GEOLOGICAL SETTING ral and palaeoenvironmental constraints on the origin of The Middle Cambrian (Series 3) Burgess Shale Formation functional novelties. Here, we report and discuss the signifi- in Yoho National Park (British Columbia, Canada) is a cance of the first arthropod trackways discovered from the basinal succession of mudstone and carbonate that has Burgess Shale Formation. yielded several fossil assemblages of soft-bodied animals Functional morphological studies of the benthic [14,15]. The trackways were collected from two separate Burgess Shale arthropods Olenoides [8,9], Naraoia [10], localities on Mount Stephen and Mount Field (electronic Sidneyia [11], Molaria [12] and Tegopelte [13] postulated supplementary material, figure S1) within the Kicking that they employed slow, low-geared, stable gaits with Horse Shale Member of the Burgess Shale Formation many propulsive limbs in contact with the substrate [14,15]. The Kicking Horse Shale Member comprises when walking, and could launch themselves off the sub- thinly bedded, calcareous muddy siltstone and thin slaty strate to swim or drift. Limb morphology, with spinose limestone [14,15]. With the exception of the Sanctacaris coxae and podomeres, also led to the suggestion that locality on Mount Stephen [16], the Kicking Horse Olenoides, Naraoia, Sidneyia and Molaria were predators Shale Member generally contains a low diversity assem- or scavengers [6,8–12]. The coxae are not known in blage of soft-bodied animals [14,15]. The Kicking Horse Shale Member is the oldest unit (Glossopleura Zone) of the Burgess Shale Formation [14,15]. *Author for correspondence ([email protected]). The exceptional preservation of animals within the Electronic supplementary material is available at http://dx.doi.org/ younger (Bathyuriscus Zone) Phyllopod Bed in the Walcott 10.1098/rspb.2011.1986 or via http://rspb.royalsocietypublishing.org. Quarry Shale Member [14,15] has been attributed to Received 22 September 2011 Accepted 19 October 2011 1613 This journal is q 2011 The Royal Society Downloaded from http://rspb.royalsocietypublishing.org/ on January 24, 2017 1614 N. J. Minter et al. Burgess Shale trackways (a) (b) (c) (d) os im is Figure 1. Burgess Shale arthropod trackways. (a) Detail of the beginning of ROM 61454 (see electronic supplementary material, figures S3–S5 for full trackway) from the Kicking Horse Shale Member on Mount Field. (b) Line drawing of the beginning of ROM 61454. (c) ROM 61455, from the Kicking Horse Shale Member on Mount Stephen, showing a gradual turn and intermittent paired medial imprints, im. (d) ROM 61456, from the Kicking Horse Shale Member on Mount Stephen, showing a sharp turn with outer splaying of tracks, os, and short inner spurs of tracks, is. All specimens preserved in negative epirelief. Scale bars, 100 mm. rapid burial by mud-rich slurry flows [17]. This indicates levels following soft-tissue stabilization under anoxic con- that the animals were transported, although recent tapho- ditions [21]. High rates of sedimentation have also been nomic analysis suggests transport and decay were invoked to explain the near absence of burrows in parts of negligible, and most benthic animals were probably buried theBurgessShaleFormation[17], although the emplace- within their habitats [18]. Geochemical analysis of Burgess ment of simple trails only requires short colonization Shale-type deposits indicates exaerobic conditions, with the windows [27]. The diminutive nature and shallow anoxic–oxic boundary occurring at the sediment–water penetration depth of the trace fossils in the Stephen For- interface, rather than anoxic bottom water [19]. Trace fos- mation are indicative of oxygen-deficient conditions, and sils are preserved in situ and therefore represent reliable suggest that oxygen rather then sedimentation rate is the indicators of local environmental conditions. Simple bur- primary control on their occurrence in Burgess Shale-type rows and trails within the Burgess Shale Formation deposits [23]. [14,18,20] and other Burgess Shale-type deposits of the USA [21] demonstrate at least intermittent oxygenation and in situ colonization of the sediment. Diminutive trace 3. TRACKWAY DESCRIPTION fossil assemblages are also found in association with cara- The five known trackway specimens are reposited at paces of soft-bodied animals in the Stephen Formation of the Royal Ontario Museum, Toronto, Canada (ROM). Canada [22,23] and Chengjiang [24] and Kaili biotas of The longest trackway (ROM 61454) is preserved for China [25,26]. Such occurrences in Burgess Shale-type over 3 m (figure 1a,b; electronic supplementary material, deposits have been attributed to increased benthic oxygen figures S2–S5). The trackway comprises overlapping Proc. R. Soc. B (2012) Downloaded from http://rspb.royalsocietypublishing.org/ on January 24, 2017 Burgess Shale trackways N. J. Minter et al. 1615 series of up to 25 tracks. It is slightly dimorphic in form. The majority of Cambrian trackways are broadly The tracks on one side are elliptical to curvilinear in shape attributed to trilobites [30,31] on the basis of a corre- and orientated obliquely to the midline of the trackway, lation between trackway features and the general while those on the other are more elongate and orientated trilobite body plan (assignment level i), or more specific perpendicularly to the midline. The sets of tracks are also groups [32] on the basis of correlation with coeval body all offset and orientated obliquely relative to the long-axis fossils from the same palaeobiogeographic region and of the trackway and the direction of travel. The external environment (assignment level iii). Trackways from the width is 100–120 mm. Tracks are 2–27 mm long and shallow marine Bright Angel Shale of Arizona have been spaced 7–28 mm apart. The stride is 200–230 mm. correlated with a producer similar to the coeval (but ROM 61455 preserves a trackway making a gradual deeper-water) Burgess Shale arthropod Habelia [33] turn (figure 1c). The external width is 120–135 mm. (assignment level ii). A second trace fossil from the The trackway splays on the outside of the turn, making same formation was identified as a trackway, potentially the series more apparent. Highly overlapping series of related to a lobopodian animal resembling Aysheaia up to 25 tracks are observed and the stride is 90–95 mm [34], although this particular trace fossil may instead be on the outside of the turn,

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