Proc. R. Soc. B (2012) 279, 1613–1620

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*Author

Electron10.1098

doi:10.1098/rspb.2011.1986

Published online 9 November 2011

ReceivedAccepted

Skimming the surface with Burgess Shalearthropod locomotion

Nicholas J. Minter1,*, M. Gabriela Mangano1

and Jean-Bernard Caron2,3

1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon,

Saskatchewan, Canada S7N 5E22Department of Natural History (Palaeobiology section), Royal Ontario Museum, 100 Queen’s Park,

Toronto, Ontario, Canada M5S 2C63Department 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 pre-

vious 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. INTRODUCTIONDiscovered over 100 years ago, the world-famous Burgess

Shale continues to yield important new insights into the evol-

ution and ecology of animals during the Cambrian explosion

[1–5]. The exceptional preservation of soft tissues in the

Burgess Shale permits functional morphological inferences

on the modes of life and palaeoecology of early metazoans

[6,7]. Trace fossils, including trackways and burrows,

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-

ral and palaeoenvironmental constraints on the origin of

functional novelties. Here, we report and discuss the signifi-

cance of the first arthropod trackways discovered from the

Burgess Shale Formation.

Functional morphological studies of the benthic

Burgess Shale arthropods Olenoides [8,9], Naraoia [10],

Sidneyia [11], Molaria [12] and Tegopelte [13] postulated

that they employed slow, low-geared, stable gaits with

many propulsive limbs in contact with the substrate

when walking, and could launch themselves off the sub-

strate to swim or drift. Limb morphology, with spinose

coxae and podomeres, also led to the suggestion that

Olenoides, Naraoia, Sidneyia and Molaria were predators

or scavengers [6,8–12]. The coxae are not known in

for correspondence (nic.minter@usask.ca).

ic supplementary material is available at http://dx.doi.org//rspb.2011.1986 or via http://rspb.royalsocietypublishing.org.

22 September 201119 October 2011 1613

Tegopelte, but it is also inferred to have been a benthic

predator or scavenger [6,13]. The aims of this paper are

to (i) describe the first known arthropod trackways from

the Burgess Shale Formation; (ii) identify their potential

producers; (iii) integrate trackway and body fossil data

to derive the locomotory techniques employed in pro-

ducing the trackways; and (iv) test previous hypotheses

on their locomotory capabilities and mode of life.

2. GEOLOGICAL SETTINGThe Middle Cambrian (Series 3) Burgess Shale Formation

in Yoho National Park (British Columbia, Canada) is a

basinal succession of mudstone and carbonate that has

yielded several fossil assemblages of soft-bodied animals

[14,15]. The trackways were collected from two separate

localities on Mount Stephen and Mount Field (electronic

supplementary material, figure S1) within the Kicking

Horse Shale Member of the Burgess Shale Formation

[14,15]. The Kicking Horse Shale Member comprises

thinly bedded, calcareous muddy siltstone and thin slaty

limestone [14,15]. With the exception of the Sanctacaris

locality on Mount Stephen [16], the Kicking Horse

Shale Member generally contains a low diversity assem-

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].

The exceptional preservation of animals within the

younger (Bathyuriscus Zone) Phyllopod Bed in the Walcott

Quarry Shale Member [14,15] has been attributed to

This journal is q 2011 The Royal Society

(a)

(d)

os

isim

(b) (c)

Figure 1. Burgess Shale arthropod trackways. (a) Detail of the beginning of ROM 61454 (see electronic supplementarymaterial, figures S3–S5 for full trackway) from the Kicking Horse Shale Member on Mount Field. (b) Line drawing of thebeginning of ROM 61454. (c) ROM 61455, from the Kicking Horse Shale Member on Mount Stephen, showing a gradualturn and intermittent paired medial imprints, im. (d) ROM 61456, from the Kicking Horse Shale Member on MountStephen, 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.

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rapid burial by mud-rich slurry flows [17]. This indicates

that the animals were transported, although recent tapho-

nomic analysis suggests transport and decay were

negligible, and most benthic animals were probably buried

within their habitats [18]. Geochemical analysis of Burgess

Shale-type deposits indicates exaerobic conditions, with the

anoxic–oxic boundary occurring at the sediment–water

interface, rather than anoxic bottom water [19]. Trace fos-

sils are preserved in situ and therefore represent reliable

indicators of local environmental conditions. Simple bur-

rows and trails within the Burgess Shale Formation

[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

fossil assemblages are also found in association with cara-

paces of soft-bodied animals in the Stephen Formation of

Canada [22,23] and Chengjiang [24] and Kaili biotas of

China [25,26]. Such occurrences in Burgess Shale-type

deposits have been attributed to increased benthic oxygen

Proc. R. Soc. B (2012)

levels following soft-tissue stabilization under anoxic con-

ditions [21]. High rates of sedimentation have also been

invoked to explain the near absence of burrows in parts of

the Burgess Shale Formation [17], although the emplace-

ment of simple trails only requires short colonization

windows [27]. The diminutive nature and shallow

penetration depth of the trace fossils in the Stephen For-

mation are indicative of oxygen-deficient conditions, and

suggest that oxygen rather then sedimentation rate is the

primary control on their occurrence in Burgess Shale-type

deposits [23].

3. TRACKWAY DESCRIPTIONThe five known trackway specimens are reposited at

the Royal Ontario Museum, Toronto, Canada (ROM).

The longest trackway (ROM 61454) is preserved for

over 3 m (figure 1a,b; electronic supplementary material,

figures S2–S5). The trackway comprises overlapping

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series of up to 25 tracks. It is slightly dimorphic in form.

The tracks on one side are elliptical to curvilinear in shape

and orientated obliquely to the midline of the trackway,

while those on the other are more elongate and orientated

perpendicularly to the midline. The sets of tracks are also

all offset and orientated obliquely relative to the long-axis

of the trackway and the direction of travel. The external

width is 100–120 mm. Tracks are 2–27 mm long and

spaced 7–28 mm apart. The stride is 200–230 mm.

ROM 61455 preserves a trackway making a gradual

turn (figure 1c). The external width is 120–135 mm.

The trackway splays on the outside of the turn, making

the series more apparent. Highly overlapping series of

up to 25 tracks are observed and the stride is 90–95 mm

on the outside of the turn, decreasing to 60–85 mm on

straighter sections. Tracks are circular to comma-shaped,

2–15 mm long and spaced 5–21 mm apart. Intermit-

tent paired medial imprints are present. ROM 61456

preserves a trackway making a tight U-turn (figure 1d).

The series splay strongly on the outside of the turn,

although a high degree of overprinting obscures the precise

number of tracks per series. Short spurs of tracks are pre-

sent on the inside of the turn. Tracks range from elliptical

to curvilinear in shape and at least 20 are present within

each series. The external width is approximately 70 mm,

and tracks are 5–18 mm long and spaced 8–22 mm apart.

Several smaller trackways with differing orientations are

preserved in ROM 61457 (electronic supplementary

material, figure S6) with external widths of 58–70 mm

and overlapping series of up to 22 elliptical to curvilinear

tracks. Tracks are 3–10 mm long and spaced 7–22 mm

apart. The stride is 145–150 mm. ROM 61458 is a small

slab that preserves a partial trackway similar to those of

ROM 61457. All trackways can be ascribed to the ichno-

genus Diplichnites (electronic supplementary material).

Helminthoidichnites-like trails are also associated with the

trackways (electronic supplementary material, figure S7).

4. PRODUCER IDENTIFICATIONTrace fossil morphology is influenced by the anatomy of

the producer, its behaviour and the substrate conditions.

Compared with other types of trace fossils, trackways are

more strongly influenced by, and therefore more reflective

of, the anatomy of the producer [28]. Fortey & Seilacher

[29] established a set of criteria for assigning a particular

trace fossil taxon to a specific producer: (i) close associ-

ation in the field; (ii) concurrent stratigraphic range;

(iii) minimal choice of available candidates; (iv) consistent

size range; and (v) consistent biogeographic range. These

ideas can be built upon to create criteria for assigning indi-

vidual trackways or assemblages of trackways to producers

with varying levels of confidence and corresponding

approximate biotaxonomic rank. In increasing order,

these assignment levels are: (i) general correlation of track-

way features with the body plan of an animal group (class

or higher equivalent taxonomic rank at the stem or crown-

group levels); (ii) correlation of features with coeval body

fossils (class–order); (iii) correlation of features with

coeval body fossils from the same palaeobiogeographic

region and environment (order–family); (iv) correlation

of features with body fossils from the same formation

(order–genus); and (v) producer at the end or in close

association with the trackway (species).

Proc. R. Soc. B (2012)

The majority of Cambrian trackways are broadly

attributed to trilobites [30,31] on the basis of a corre-

lation between trackway features and the general

trilobite body plan (assignment level i), or more specific

groups [32] on the basis of correlation with coeval body

fossils from the same palaeobiogeographic region and

environment (assignment level iii). Trackways from the

shallow marine Bright Angel Shale of Arizona have been

correlated with a producer similar to the coeval (but

deeper-water) Burgess Shale arthropod Habelia [33]

(assignment level ii). A second trace fossil from the

same formation was identified as a trackway, potentially

related to a lobopodian animal resembling Aysheaia

[34], although this particular trace fossil may instead be

an unusual expression of a serially and alternating

branched burrow system [34,35].

The Burgess Shale trackways described herein are

unique in their large size and occurrence in Cambrian Bur-

gess Shale-type deposits. The trackways are of different

sizes (58–135 mm wide) and probably come from differ-

ent stratigraphic levels within the Kicking Horse Shale

Member, and were thus produced by different individuals.

However, they all share the same characteristics of relative

dimensions and numbers of tracks, and were therefore all

made by the same type of animal. Comparison of key

trackway characters (including size and number of tracks)

with potential producers from the Burgess Shale is facili-

tated by the exceptional preservation of soft tissues in

census and time-averaged communities [18]. This provides

the potential opportunity to correlate the trackways with

body fossils from the same formation (assignment level iv).

The producers of the Burgess Shale trackways were large

(up to 300 mm long and 135 mm wide) and possessed at

least 25 pairs of walking limbs. Among known Burgess

Shale and other Lower–Middle Cambrian arthropods

with walking limbs (electronic supplementary material,

table S1), only Tegopelte [13] (figure 2) fits these criteria,

reaching 280 mm in length, 140 mm in width and posses-

sing 33 pairs of walking limbs. The posteriormost limbs are

reduced and are unlikely to have been used in locomotion

[13]. Fragmentary remains of Sidneyia from the Middle

Cambrian of Utah are estimated to approach the appropri-

ate size [36], although Sidneyia only has nine pairs of

walking limbs [11]. All other known Burgess Shale arthro-

pods with walking limbs are too small to have made all but

the smallest trackways, but still possess insufficient num-

bers of appendages (electronic supplementary material,

table S1). Saperion, from the Lower Cambrian Chengjiang

fauna of China, reaches 151 mm in length and possesses

approximately 25 pairs of walking limbs [37].

Tegopelte is the largest benthic arthropod from the

Burgess Shale (figure 2; electronic supplementary material,

figure S8) and is at least twice the size of any other known

benthic Burgess Shale bilaterian. It is also one of the rarest,

with the only two known specimens occurring from the

Walcott Quarry Shale Member [13]. Tegopelte was orig-

inally described as a ‘soft-bodied trilobite’ with a thin

unmineralized divided exoskeleton [13]. Subsequent

analysis has determined that the divisions are folds, arising

from compaction during preservation, in an undivided

dorsal shield [37]. Tegopelte is often regarded as belonging

to the lamellipedians [38], a group that includes trilobites;

however, the position of this group is largely unconstrained

[39]. Phylogenetic analysis has resolved a sister relationship

LaLa?

Ra

ReHLe

gd

(a) (d)

(c)

n

(b)Ran

H

al

Figure 2. Tegopelte gigas from the Walcott Quarry Shale Member at the Walcott Quarry. (a) Holotype (USNM 189201) showing

the hypostome, H, left antennae, La, right antennae, Ra, notches, n, at the anterior and posterior end and the alimentarycanal, al. (b) Counterpart of the holotype showing the gut diverticulae, gd. (c) Detail of gut diverticulae. (d) Paratype(USNM 189200) showing the additional presence of a left eye, Le, and right eye, Re. Eyes are tear-shaped and probably ventralin position. Scale bars: (a,b,d ) 30 mm; (c) 10 mm.

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of Tegopelte with Saperion and position in the Order

Helmetiida/Conciliterga [40,41]. Body fossils of Tegopelte

are not known from the Kicking Horse Shale Member,

but the trackways can be attributed to Tegopelte (or, at the

very least, a tegopeltid) on the basis of correlation with

body fossils from the same formation (assignment level iv).

5. LOCOMOTIONAnimal locomotion can be defined by three parameters:

the gait ratio (p : r), which is the proportion of the step

cycle that a limb spends in the forward, recovery, protrac-

tion period (p) to that spent in the backward, propulsive,

retraction period (r); the successive phase difference (suc),

which is the proportion of the step cycle that a limb moves

before the limb in front; and the opposite phase difference

(opp), which is the proportion of the step cycle that a limb

on one side of the body moves after the paired limb on the

opposing side of the body [42,43]. These parameters can

be derived from trackways and a reconstruction of the

producer (electronic supplementary material), and have

been used to reconstruct the locomotion of numerous

extinct animals [44–46].

The Burgess Shale trackways reveal that their producers

were capable of performing fast, high-geared walking gaits

and also able to ‘skim’ across the substrate with few limbs

in contact with the substrate at any one point in time. Track-

way analysis (electronic supplementary material) yields gait

ratios of 9.3 : 0.7 and 9.4 : 0.6, indicating that limbs only

spent 7 and 6 per cent of the step cycle in the propulsive

backstroke phase, for ROM 61454 (figure 1a,b and elec-

tronic supplementary material, figures S3–S5) and ROM

61457 (electronic supplementary material, figure S6),

respectively. Successive limbs on one side of the body

Proc. R. Soc. B (2012)

would have moved in advance of the limb in front by

88 per cent of the step cycle. The trackways have opposite

symmetry and so paired limbs on either side of the body

moved synchronously. These parameters indicate that

only one to two pairs of limbs were in contact with the sub-

strate at any one point in time and that metachronal waves

of eight limbs appeared to travel backwards along the

body with short gaps between the placement of proceed-

ing limbs (figure 3; electronic supplementary material,

figures S9a and S10, and animation S1).

ROM 61455 demonstrates a slower but still high-geared

gait. Analysis of the straighter section yields a gait ratio of

7.6 : 2.4, where limbs spent 24 per cent of the step cycle

in the propulsive backstroke phase and successive limbs

moved in advance of the limb in front by 69 per cent of

the step cycle. The trackway has opposite symmetry and

so paired limbs on either side of the body moved synchro-

nously. These parameters indicate that six pairs of limbs

were in contact with the substrate at any one point in

time, and metachronal waves of three limbs appear to

have travelled backwards along the body with short gaps

between the placement of proceeding limbs (electronic

supplementary material, figure S9b).

ROM 61456 indicates the producer was capable of

performing tight turns (figure 1d); however, unlike trilo-

bites, Tegopelte has an unarticulated dorsal shield rather

than an articulated dorsal exoskeleton. Detailed examin-

ation of Tegopelte and the trackway demonstrates how

the execution of a tight turn can be ameliorated with an

arthropod with an unarticulated dorsal exoskeleton. In

Tegopelte, the compression of gill filaments on the surface

of the cuticle suggests that its dorsal shield was thin and

would have had some flexibility. In the trackway, the

track series on the outside of the turn are splayed, but

time(a) (b)

limb

1

2

3

4

5

6

7

8

9

suc

pr

t0

t1

t2

t3

t4

t5

t6

t7

t8

t9

t0

33

33

33

33

33

33

33

33

33

33

25

25

25

25

20 19

21 20

22 21

23 22

19

20

21

22

23

11

12

13 5

6

7

8

9 1

1

1

2

16

1

1

1

17

18 10

1525 24 23

25 24

25

25

25

25

24

14

4

3

25

25 10 2

1

1

1

1

1

1

18 17

19 18

18

9 1

9

10

11

12

13

14

15

16

17

18

t1

t2

t3

t4

t5

t6

t7

t8

t9

1

2

3

4

5

6

7

8

9

10

Figure 3. Locomotion and trackway production of a tegopeltid animal, based on ROM 61454 using a gait ratio of 9.3 : 0.7 andsuccessive phase difference (suc) of 0.88. (a) Manton gait diagram. Horizontal axis represents progression through time. The

phase relationships of the anterior 10 limbs on one side of the body are depicted. Each complete step cycle of a limb is rep-resented by a couplet of a thick downward sloping line, corresponding to the relative duration of the propulsive, backward,retraction phase (r) when the limb is in contact with the substrate; and a thin upward sloping line corresponding to the relativeduration of the recovery, forward, protraction phase (p) when the limb is lifted above the substrate. The successive phase differ-

ence is the proportion of the step cycle that a limb n þ 1 moves in advance of the limb n in front. Vertical time lines (t0 2 t9)capture snapshots of the positions of the limbs that are depicted in (b). (b) Time-lapse trackway production. ‘Gait still’ snap-shots represent the positions of the limbs and development of the trackway between the placement of limbs n and n þ 1 in theproceeding step cycle. Light-coloured limbs indicate those in contact with the substrate. The en-echelon arrangement of theseries of tracks has been exaggerated to show the individual tracks.

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only with a low degree of curvature, and short spurs of

tracks are present on the inside of the turn (figure 1d).

Such a tight turn could therefore be achieved by a

tegopeltid through a combination of limited lateral flexure

and side-stepping.

The reconstructed method of trackway production

shows that the anterior tracks of each series are produced

by the posterior limbs (figures 3 and 4). In ROM 61454,

the anterior tracks of each series are situated more

medially to the midline of the trackway (figure 1a,b; elec-

tronic supplementary material, figures S3–S5), which is

consistent with the method of production and the poster-

iormost limbs being shorter in Tegopelte. The en-echelon

arrangement of tracks in this trackway and ROM 61457

results from the body of the producer being offset obli-

quely relative to the long axis of the trackway and

direction of travel. This is caused by the animal being

partly buoyed up while producing the trackway and

could be related to a current or the producer being less

able to control the yaw of its body while employing a

high-geared gait ratio. There are no tool marks or

scours preserved with the trackways to indicate the

action of a current. These may not be observed if

the trackways are undertracks [47]; however, undertracks

are unlikely to be produced by an animal that was buoyed

up. Also, in ROM 61457, several trackways have dif-

fering orientations (electronic supplementary material,

figure S6), but were all produced by high-geared gaits,

and this is inconsistent with the producers all being

Proc. R. Soc. B (2012)

subject to a unidirectional current. The alternative is

that the producers were less able to control the yaw of

their body at these higher-geared gaits, owing to fewer

limbs being in contact with the substrate at any one

point in time, compared with more normal trackways pro-

duced by lower-geared walking gaits (figure 1c).

6. DISCUSSIONThe discovery of trackways in the Burgess Shale permits

direct insights into animal behaviour and environmental

conditions during the Cambrian. The trackways can be

attributed to the Burgess Shale arthropod Tegopelte,

or at least a tegopeltid (figure 4). No other types of

trackways have been found so far in the Burgess Shale

Formation. On the basis of functional morphology,

Tegopelte was hypothesized as employing a walking gait

that was slow and low-geared (p : r ¼ 3.75 : 6.25,

suc ¼ 0.125), with many propulsive limbs in contact

with the substrate at any one point in time, but could

also perform higher-geared gaits to launch from the sub-

strate and drift or swim [13]. The trackways provide

direct evidence to largely support this hypothesis,

showing that such animals could indeed employ fast,

high-geared gaits akin to ‘skimming’ across the substrate.

The trackways also demonstrate that they were capable of

a slower and lower-geared walking gait that is similar to

those calculated for subaqueously walking myriapods

[45]. This walking gait is higher-geared than that

Figure 4. Reconstruction of Tegopelte gigas producing a trackway. Copyright q 2011 Marianne Collins.

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previously proposed for Tegopelte, although it falls within

the hypothesized locomotory end-members [13].

On the basis of functional morphology, Tegopelte is

considered to have been much like the large Sidneyia

and smaller Olenoides, Naraoia and Molaria in being a

benthic predator or scavenger [6]. The trackway evidence

supports the hypothesis of a predatory or scavenging

mode of life, demonstrating that they could employ fast,

high-geared gaits and were also capable of performing

tight turns. Fast, high-geared gaits could potentially be

associated with predator avoidance, although the produ-

cers of these trackways would have been twice the size

of any other benthic Burgess Shale arthropod and over

half the estimated size of the largest Anomalocaris [48].

In addition, our re-examination of Tegopelte identified the

presence of gut diverticulae (figure 2b,c). This provides

strong evidence of a predatory or scavenging mode of life

[49,50]. The presence of eyes in Tegopelte [13] was con-

sidered to be uncertain [40], but they are confirmed

herein and are probably ventral in position (figure 2d and

electronic supplementary material, figure S8a).

The presence of trackways, along with interface trails,

in the Kicking Horse Shale Member of the Burgess

Shale Formation also permits inferences on environ-

mental conditions. Burrows have been observed in other

members of the Burgess Shale Formation and indicate

at least intermittent oxygenation of the sediment [20],

while the diminutive and shallow nature of burrows and

trails in the adjacent Stephen Formation suggests that

oxygen was a major control on their occurrence [23].

The trackways described herein are the first to be discov-

ered from the Burgess Shale Formation and they are

unique for Burgess Shale-type deposits in their large

size. They provide evidence of arthropods living within

the environment, extending the stratigraphic range of

tegopeltids, and indicate that the bottom waters must

have been sufficiently oxygenated, at least intermittently,

to sustain large benthic animals during deposition of the

oldest member of the Burgess Shale Formation. The pres-

ervation of such trackways would also have required that

the substrate was dewatered and stiff owing to breaks in,

or slow rates of, sedimentation. Contrary to younger

Proc. R. Soc. B (2012)

Burgess Shale localities [14], the fossil-bearing layers of

the Kicking Horse Shale Member do not lie directly at

the base of the Cathedral Escarpment, and are of low

diversity and abundance. This might suggest that the

Kicking Horse Shale Member represented an environ-

ment generally unfavourable for a rich community to

thrive. The presence of large trackways also suggests

that the general scarcity of soft-tissue preservation

may be taphonomic, with a greater prevalence of oxygen

and/or low sedimentation rates throughout deposition

of the Kicking Horse Shale Member.

The Burgess Shale biota provides key insights into the

evolution and ecology of body plans during the Cambrian

explosion. The discovery of arthropod trackways from the

Kicking Horse Shale Member highlights their application

as indicators of environmental conditions. The results

also provide insights into the locomotory capabilities of

extinct animals that can be used to test functional mor-

phological hypotheses on modes of life. Integrated trace

and body fossil evidence provides an innovative and

dynamic picture of arthropods such as Tegopelte as active

carnivores. Early marine communities were therefore

occupied not just by large nektonic predators such as

Anomalocaris, but also by large benthic predators, empha-

sizing the importance of such lifestyles in shaping early

complex marine communities and evolution during the

Cambrian explosion.

We thank Parks Canada for collecting permits (to ROM—Desmond Collins and J.-B.C.), and Doug Erwin, MarkFlorence and John Pojeta (Smithsonian Institution) foraccess to the Tegopelte specimens and technical assistance.N.J.M.’s research is supported through a Governmentof Canada Post-doctoral Research Fellowship under theCanadian Commonwealth Scholarship Programme.M.G.M.’s research is supported by Natural Sciences andEngineering Research Council (NSERC) Discovery Grant311727 and University of Saskatchewan StartUp fund411435. J.-B.C.’s research is supported by NSERCDiscovery Grant 341944 and ROM research and fieldworkgrants. Marianne Collins produced figures 3b and 4,and electronic supplementary material, figure S10 andanimation S1. This is Royal Ontario Museum BurgessShale project number 34. The manuscript benefitted from

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the comments of Greg Edgecombe as the Associate Editor,and two anonymous reviewers.

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