SKIMMING THE SURFACE WITH BURGESS SHALE ARTHROPOD LOCOMOTION

Transcription

1 ELECTRONIC SUPPLEMENTARY MATERIAL SKIMMING THE SURFACE WITH BURGESS SHALE ARTHROPOD LOCOMOTION Nicholas J. Minter 1, M. Gabriela Mángano 1, Jean-Bernard Caron 2,3 1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada 2 Department of Natural History (Palaeobiology section), Royal Ontario Museum, 100 Queen s Park, Toronto, Ontario, M5S 2C6, Canada 3 Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada CONTENTS 1. Ichnotaxonomy 2. Trackway analysis (a) ROM (b) ROM (c) ROM Supplementary references 1. ICHNOTAXONOMY The trackways consist of two parallel track rows, each comprising overlapping series of up to 25 tracks (see figure S2 for trackway terminology). Individual tracks are elliptical to curvilinear in shape and orientated obliquely to perpendicularly to the mid-line of the trackway. The trackways have opposite symmetry. It is most parsimonious to ascribe the trackways to the ichnogenus Diplichnites Dawson Diplichnites consists of simple trackways with two parallel rows of tracks with individual tracks being blunt to elongate, closely spaced, and orientated approximately perpendicularly to the mid-line of the trackway (Briggs et al. 1979;

2 Fillion and Pickerill 1990; Buatois et al. 1998). Diplichnites and particularly D. gouldi (Gevers et al. 1971) are also often used to refer to trackways with overlapping enechelon series of tracks (Seilacher 1955; Osgood and Drennen 1975; Fillion and Pickerill 1990; Trewin and McNamara 1995; Smith et al. 2003; Minter et al. 2007). ROM 61455, 61456, and can therefore be referred to Diplichnites; although such common usage of D. gouldi does not strictly adhere to the diagnosis of Diplichnites. ROM is slightly dimorphic in nature, with the tracks on one side being more deeply impressed, ranging from elliptical to curvilinear in form, and orientated obliquely to the mid-line of the trackway; whilst the tracks on the other side are more elongate, faintly preserved, curvilinear, and all orientated perpendicularly to the midline. This is similar to the condition in the ichnogenus Dimorphichnus Seilacher Dimorphichnus encompasses asymmetrical trackways with two different types of tracks, those on one side being straight or sigmoidal scratch-like tracks and those on the other side being blunt, and all tracks are orientated obliquely to the mid-line of the trackway (Seilacher 1955; Fillion and Pickerill 1990). ROM lacks such a row of blunt tracks and the curvilinear, more scratch-like, tracks are orientated perpendicularly to the midline of the trackway. This particular trackway is therefore most parsimoniously referred to Diplichnites in line with the other material. 2. TRACKWAY ANALYSIS Animal locomotion can be defined by three parameters: the gait ratio (p:r), the proportion of the step cycle that a limb spends in the forward, recovery, protraction period (p) to that spent in the backward, propulsive, retraction period (r); the successive phase difference (suc), the proportion of the step cycle that a limb moves before the limb in front; and the opposite phase difference (opp), 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 (Manton 1950, 1952a, b, 1954, 1977; Briggs et al. 1979; Braddy 2001). The proportion of the step cycle that a limb spends in the backward, propulsive, retraction period (r) is also often referred to as the duty factor (Ting et al. 1994; Martinez et al. 1998; Ashley-Ross et al. 2009). Manton (1950, 1952a, b, 1954, 1977) pioneered a method for deriving these parameters using simple equations and data from trackways and a reconstruction of the producer. These parameters are also

3 those for which values have been postulated for various Burgess Shale arthropods on the basis of functional morphology and therefore allow comparison between results. Tegopelte was originally reconstructed as employing a slow, low-geared walking gait, similar to that of the trilobite Olenoides, with a gait ratio of 3.75:6.25 and successive phase difference of (Whittington 1975, 1980, 1985). Naraoia (Whittington 1977), Sidneyia (Bruton 1981) and Molaria (Whittington 1981) have also been reconstructed as employing slow, low-geared gaits. Naraoia was postulated as using a gait ratio of 3.75:6.25 and successive phase difference of 0.17 (Whittington 1977). Sidneyia was postulated as using a gait ratio of 2:8 and successive phase difference of 0.2 (Bruton 1981). Molaria was postulated as using a gait ratio of 3.5:6.5 and successive phase difference of 0.17 (Whittington 1981). It was also suggested that these arthropods could employ higher-geared gaits to launch from the substrate to drift or swim. The gait ratio (p:r) is usually expressed as a proportion of ten. The backward, propulsive, retraction period (r) of the step cycle is calculated as: r = backstroke 10 (1) stride where the stride is measured from the trackway and the backstroke is obtained from a reconstruction of the angle of swing of the limbs of the producer and scaling it to the trackway. The total angle of swing of the limbs of Olenoides is considered to be 16º, incorporating 4º from in front of the transverse plane and 12º behind the transverse plane (Whittington 1975, 1980). Whittington (1985) assumed the same degree of movement for the limbs of Tegopelte. Following this assumption, the backstroke distance that a limb moves through during the propulsive retraction period is: backstroke = lsin4 + lsin12 (2) where l is the limb length and is assumed to be half the external width of the trackway. The forward, recovery, protraction period (p) of the step cycle is calculated as:

4 p =10 r (3) The equation for calculating the successive phase difference (suc) is derived from Manton (1950) and Briggs et al. (1979): The distance (x) separating the track of limb n from the subsequent track of limb n+1 (i.e. the limb behind limb n) is: x = d s (4) where d is the distance travelled by the animal during the time between the footfalls of two successive limbs; and s is the exsagittal distance, obtained from a reconstruction, separating the middle of the bases of the two limbs. d = yt (5) where y is speed; and t is the time separating the footfalls of the two successive limbs: t = stride ( suc stride ) y (6) substituting (6) into (5) gives: d = stride ( suc stride) (7) and substituting (7) into (4) gives: x = stride ( suc stride) s (8) The above equations of Manton (1950) and Briggs et al. (1979) were intended to allow construction of a hypothetical trackway based on a presumed successive phase difference. If x, the distance separating the track of limb n from the subsequent track

5 of limb n+1, can be measured from a trackway then suc can be calculated by rearranging (8) to: suc = stride s x stride (9) If suc is > 0.5, the metachronal wave appears to pass backwards along the body of the animal (Manton 1950, 1977) and the number of limbs, N, employed in a metachronal wave is calculated as: 1 N = 1 suc (10) (a) ROM This trackway has a mean external width of mm and stride of mm (figures 1a, b, S3-5). To simplify analysis, the lengths of all limbs are assumed to be uniform once the size of the producer has been scaled to the mean external width of the trackway. Using this data gives a backstroke of 15.0 mm from (2) and gait ratio (p:r) of 9.3:0.7 from (1 and 3). The presence of distinct series means that it is possible to confidently measure the distance (x) separating the track of limb n from the subsequent track of limb n+1. The mean value of x is 16.8 mm. To simplify analysis, the exsagittal distance (s) is assumed to be uniform between all limbs in the producer after it has been scaled to the external width of the trackway. This gives a value of s of 9.5 mm. Using these values gives a suc of 0.88 from (9) and the number of limbs, N, in a metachronal wave is 8 from (10). The trackway has opposite symmetry and so opp is 0. The gait ratio of 9.3:0.7 is very high-geared and implies that the limbs have a very short propulsive, backstroke, retraction phase (r) with a duration of just 7% of the step cycle compared to 93% of the step cycle spent in the forward, recovery, protraction phase (p). It also indicates that only 7%, or 1.75 pairs, of the 25 pairs of limbs are in contact with the substrate at any one point in time. The suc of 0.88 indicates that a limb n+1 begins its step cycle in advance of the limb n in front by 88% of its step cycle. A suc of greater than 0.5 results in the propulsive limbs

6 converging and metachronal waves appearing to pass backwards along the body rather than forwards (Manton 1950, 1977). These gait parameters are represented diagrammatically in figure S9a. The suc of 0.88 is in agreement Manton (1977) who noted that as the proportion of the step cycle spent in the propulsive, backstroke, retraction phase decreases, the successive phase difference increases. In addition, Smith et al. (2003) noted that as gait ratios become higher-geared then suc tends towards: suc r 10 (11) (i.e. limb n is placed on the substrate at the same time as limb n+1 is lifted from the substrate), although in this particular case it is: suc 1 r 10 (12) because suc > 0.5 and the metachronal wave appears to pass backwards along the body with limb n+1 being placed on the substrate after limb n is lifted from the substrate and limb n+1 is in the proceeding step cycle compared to that of limb n. From (1) it is evident that the calculation of r is dependent on the backstroke and hence the assumed 16º angle of swing for the limbs. figure S9a shows that the particular gait parameters calculated result in limb n being lifted from the substrate at the end of its propulsive, backward, retraction phase just prior to limb n+1 being placed on the substrate and beginning the proceeding propulsive, backward, retraction phase. Greater angles of swing would result in lower-geared gait ratios with longer duration propulsive, backward, retraction phases. Hence, if suc remains the same, there would be periods when successive limbs would both be on the substrate and they would overcross. This would be mechanically unsuitable and so the assumed 16º angle of swing is reasonable. Speed is governed by the relative duration of the propulsive, backstroke, retraction phase (r), the stride duration and the angle of swing of the limbs. Absolute speed cannot be determined using the above equations, but a decrease in the relative duration of the backstroke, assuming that the angle of swing remains constant, results

7 in an increase in speed at the same or shorter stride durations (Manton 1952a, b, 1977). In the majority of arthropods, the relative duration of the backstroke and the stride duration decrease in unison with increasing speed (Manton 1952a, b). Animals can adopt higher-geared gaits when moving subaqueously at the same (Martinez et al. 1998) or higher (Ashley-Ross et al. 2009) speeds as on land. Burgess Shale arthropods were all moving within the same subaqueous medium and whilst the duration of the stride is unknown for a tegopeltid executing gaits of 9.3:0.7 and 3.75:6.25, the shorter relative duration of the backstroke in the former gait would have resulted in a greater speed. Time line and gait still t 0 (figures S9a, S10) shows that as limb 1 is starting its propulsive, backward, retraction phase, limb 9 in the other metachronal wave is half way through its propulsive phase. When limb 9 completes its propulsive phase and is lifted off the substrate, limb 18 begins its propulsive phase and limb 1 finishes its propulsive phase half way through that of limb 18. Therefore, on average, only 1.75 pairs of limbs are in contact with the substrate at any one point in time. Gait stills of steps between the placement of limbs n and n+1 (t 0-9 ) and the animation of trackway production show that the anterior tracks of a series are produced by the posterior limbs of the producer (figures S9a, S10; electronic supplementary animation). The direction of travel can be deduced from push-back mounds in the trackway and the anterior tracks of the series are positioned more medially than the posterior tracks. The posterior limbs are shorter in Tegopelte; as such, the derived gait parameters and implications for the mode of trackway production are in agreement with additional evidence from the trackway and anatomy of the producer. (b) ROM One of the trackways in ROM has a mean external width of 62.0 mm and stride of mm (figure S6). This gives a backstroke of 8.6 mm from (2) and gait ratio (p:r) of 9.4:0.6 from (1 and 3). The mean value of x is 12.7 mm. Scaling the proportions of the reconstruction of Tegopelte (Whittington 1985) to the size of the trackway gives a value of s of 5.4 mm. This gives a suc of 0.88 from (9) and the number of limbs, N, in a metachronal wave is 8 from (10). The trackway has opposite symmetry and so opp is 0.

8 These gait parameters are essentially identical to those for ROM The gait ratio is very high-geared with limbs spending just 6% of the step cycle in the propulsive, backward, retraction phase, indicating that 6%, or 1.3 pairs, of the 22 pairs of limbs are in contact with the substrate at any one point in time. Limbs moved in advance of the limb in front by 88% of the step cycle, resulting in propulsive limbs converging and metachronal waves of eight limbs appearing to pass backwards along the body. (c) ROM This trackway preserves a gradual turn (figure 1c). Stride length is exaggerated on the outside of turns and so analysis is restricted to the straighter section of the trackway. The straight section of ROM has a mean external width of mm and stride of 71.7 mm. This gives a backstroke of 17.3 mm from (2) and gait ratio of (p:r) of 7.6:2.4 from (1 and 3). The mean value of x is 11.0 mm. Scaling the proportions of the reconstruction of Tegopelte (Whittington 1985) to the size of the trackway gives a value of s of 10.9 mm. This gives a suc of 0.69 from (9) and the number of limbs, N, in a metachronal wave is 3 from (10). The trackway has opposite symmetry and so opp is 0. This is a slower and lower-geared gait ratio than the other trackways and results in a higher degree of overlap among series. It is more akin to a walking gait and is similar to those calculated for subaqueous walking myriapods (Smith et al. 2003). The gait ratio implies that limbs spent 24% of the step cycle in the propulsive, backward, retraction phase and that 24%, or 6 pairs, of the 25 pairs of limbs were in contact with the substrate at any one point in time. The suc of 0.69 indicates that limbs moved in advance of the limb in front by 69% of the step cycle, resulting in propulsive limbs converging and metachronal waves of 3 limbs appearing to pass backwards along the body. These gait parameters are summarized diagrammatically in figure S9b. As for ROM 61454, figure S9b shows that the particular gait parameters calculated for ROM result in a limb n being lifted from the substrate at the end of its propulsive, backward, retraction phase just prior to limb n+1 being placed on the substrate and beginning the proceeding propulsive, backward, retraction phase. Hence, the assumed 16º angle of swing used to calculate the backstroke is reasonable.

9 Time line and gait still t 0 (figure S9b) shows that as limb 1 is starting its propulsive, backward, retraction phase, limbs 4 and 7 are part way through their propulsive phases, limb 10 is just completing its propulsive phase and is being lifted off the substrate, and limbs 17, 20 and 23 are part way through their propulsive phases. Therefore, on average, 6 pairs of limbs are in contact with the substrate at any one point in time. 3. SUPPLEMENTARY REFERENCES Ashley-Ross, M. A., Lundin, R. & Johnson, K. L Kinematics of level terrestrial and underwater walking in the California newt, Taricha torosa. J. Exp. Biol. A 311, Braddy, S. J Trackways arthropod locomotion. In Palaeobiology II (eds D. E. G. Briggs & P. R. Crowther), pp Oxford, Blackwell Science. Briggs, D. E. G The morphology, mode of life, and affinities of Canadaspis perfecta (Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale, British Columbia. Phil. Trans. R. Soc. B 281, Briggs, D. E. G. & Collins, D The arthropod Alalcomenaeus cambricus Simonetta, from the Middle Cambrian Burgess Shale of British Columbia. Palaeontology 42, Briggs, D. E. G. & Robison, R. A Exceptionally preserved nontrilobite arthropods and Anomalocaris from the Middle Cambrian of Utah. Univ. Kansas Paleontol. Contrib. 111, Briggs, D. E. G., Rolfe, W. D. I. & Brannan, J A giant myriapod trail from the Namurian of Arran, Scotland. Palaeontology 22, Bruton, D. L The arthropod Sidneyia inexpectans, Middle Cambrian, Burgess Shale, British Columbia. Phil. Trans. R. Soc. B 295, Bruton, D. L. & Whittington, H. B Emeraldella and Leanchoilia, two arthropods from the Burgess Shale, Middle Cambrian, British Columbia. Phil. Trans. R. Soc. B 300, Buatois, L. A., Mángano, M. G., Maples, C. G. & Lanier, W. P Taxonomic reassessment of the ichnogenus Beaconichnus and additional examples from the Carboniferous of Kansas, U.S.A. Ichnos 5,

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