1 Keeping a lid on it: muscle scars and the mystery of the

2 Mobergellidae

3

4 TIMOTHY P. TOPPER1,2* and CHRISTIAN B. SKOVSTED1

5

6 1Department of Palaeobiology, Swedish Museum of Natural History, P.O. Box 50007,

7 SE-104 05, Stockholm, Sweden.

8 2Palaeoecosystems Group, Department of Earth Sciences, Durham University, Durham

9 DH1 3LE, UK.

10

11 Mobergellans were one of the first Cambrian skeletal groups to be recognized yet have

12 long remained one of the most problematic in terms of biological function and affinity.

13 Typified by a disc-shaped, phosphatic sclerite the most distinctive character of the

14 group is a prominent set of internal scars, interpreted as representing sites of former

15 muscle attachment. Predominantly based on muscle scar distribution, mobergellans

16 have been compared to brachiopods, bivalves and monoplacophorans, however a

17 recurring theory that the sclerites acted as operculum remains untested. Rather than

18 correlate the number of muscle scars between taxa, here we focus on the percentage of

19 the inner surface shell area that the scars constitute. We investigate two mobergellan

20 species, Mobergella holsti and Discinella micans comparing the Cambrian taxa with the

21 muscle scars of a variety of extant and fossil marine invertebrate taxa to test if the

22 mobergellan muscle attachment area is compatible with an interpretation as operculum.

23 The only skeletal elements in our study with a comparable muscle attachment

24 percentage are gastropod opercula. Complemented with additional morphological 25 information, our analysis supports the theory that mobergellan sclerites acted as an

26 operculum presumably from a tube-living organism. The paucity of tubes co-occurring

27 with mobergellan sclerites could be explained by the transportation and sorting of

28 detached opercula while the corresponding tube remained attached to substrata in

29 shallower water. The opercula perhaps performed a similar role to that seen in serpulid

30 annelids and in neritid gastropods sealing the living chamber of the organism to avoid

31 desiccation or for protection.

32

33 ADDITIONAL KEYWORDS Annelida –Brachiopoda – Cambrian – Discinella –

34 Gastropoda – Mollusca – Operculum – Mobergella

35

36 INTRODUCTION

37

38 Early Cambrian skeletal fossil assemblages are dominated by a sundry of small tubes,

39 shells, plates and spines that most likely represent some of the earliest representatives of

40 animal groups with mineralized hard parts. Some of these fossils represent the shells of

41 complete microscopic individual organisms, but others represent elements (sclerites) of

42 larger composite exoskeletons (scleritomes) that are predominantly disarticulated post-

43 mortem (Bengtson et al., 1990; Skovsted, 2006; Vannier et al., 2007; Topper et al.,

44 2009; Caron et al., 2013; Devaere et al., 2014). The disarticulated nature of Cambrian

45 skeletal fossils (Small Shelly Fossils or SSFs) generally obscures their placement in the

46 metazoan tree, seemingly awaiting the discovery of exceptionally preserved specimens

47 to unveil their scleritome structure and biological affinity (e.g. Chen et al., 1989;

48 Conway Morris & Peel, 1990; Skovsted et al., 2008, 2009, 2011; Larsson et al., 2014).

49 Such discoveries are however rare and the majority of Cambrian skeletal fossils 50 continue to float in the taxonomic ether, hindering our understanding of the earliest

51 animal ecosystems.

52 The problems in assessing such skeletal fossils are exemplified by the

53 mobergellans, a group of small, disc-shaped, phosphatic sclerites. Almost flat in lateral

54 profile, the sclerites can be variously convex or concave and exhibit concentric growth

55 lines on the outer surface (Bengtson, 1968; Skovsted, 2003). Historically, this

56 exclusively Cambrian family (consisting of five genera and at least eight species) was

57 one of the first SSF groups documented (Billings, 1871) and despite attracting

58 considerable interest for over 140 years (Billings, 1871; Hall, 1872; Moberg, 1892;

59 Hedström, 1923, 1930; Bengtson, 1968; Missarzhevsky, 1989; Rozanov & Zhuravlev,

60 1992; Conway Morris & Chapman, 1997; Skovsted, 2003; Streng & Skovsted, 2006;

61 Demidenko et al., 2012), our understanding of their functional morphology and

62 biological affinity remains limited.

63 Indisputably, the most distinctive character of mobergellans is the prominent

64 radiating, roughly bilaterally symmetrical structures on the presumed internal surface

65 (Figs 1A-B, 2A, 4D). These markings, which are the focus of this study, have been

66 interpreted and generally accepted as muscle scars (Moberg, 1892; Bengtson, 1968;

67 Conway Morris & Chapman, 1997; Skovsted, 2003; Streng & Skovsted, 2006) and here

68 we continue to follow this interpretation. The muscle scars in some mobergellans (e.g.

69 Mobergella holsti) are delineated by puncta, visible as fine pores on the inner surface of

70 the sclerite (Bengtson, 1968, fig. 3). This feature however is not ubiquitous across the

71 group (compare Mobergella holsti with Discinella micans Fig. 1A, B; see Bengtson,

72 1968) and appears to be a unique feature without obvious modern analogues. Lacking

73 an abundance of key morphological characteristics, the number of muscle scars has

74 been repeatedly utilised to decipher the biological function and affinity of mobergellan 75 sclerites (Streng & Skovsted, 2006 and references therein). However, the number of

76 muscle scars displayed by mobergellan taxa varies considerably between genera and

77 even intraspecifically (Bengtson, 1968; Conway Morris & Chapman, 1997; Skovsted,

78 2003), exacerbating direct comparisons with fossil and extant taxa. For example,

79 Mobergella holsti (Billings) generally bears 13- 14 muscle scars, Mobergella hexactina

80 Skovsted, 2003 displays 11-12 scars and Discinella micans Moberg, 1892 exhibits 9-10

81 scars. Given this numerical variation, alternate approaches to studies of mobergellans

82 may be more insightful.

83 Studies on shell-bearing taxa, such as brachiopods and molluscs, tend to focus

84 predominantly on the variation in valve morphology (e.g. Stanley, 1970; Haney et al.,

85 2001; Gaspar et al., 2002; Inoue et al., 2013) however a few studies have correlated the

86 size of muscle attachment sites with life position (e.g. endobyssate verse epibyssate

87 taxa, Stanley 1972), environmental parameters (e.g. high energy verse low energy

88 environments, Colmenar et al., 2014), particular biological functions (e.g. the ability to

89 swim in scallops, Gould, 1971) and behavioural adaptations to specific environments

90 (e.g. burrowing in intertidal molluscs, Ansell & Trevallion, 1969 and in the coral boring

91 species of Lithophaga, Morton & Scott, 1980). These studies demonstrate the potential

92 utility of investigating the size of muscle attachment sites in shells, however to help

93 account for intraspecific variation, additional morphological information is regularly

94 used to complement the study (Ansell & Trevallion, 1969; Stanley, 1972; Gould, 1971;

95 Morton & Scott, 1980). Muscle scars are invariably retained in mobergellans and

96 represent the only traces of the animal’s soft body and complemented with the

97 additional morphological characteristics available (such as size, shape and shell

98 convexity/concavity), we believe they provide our best opportunity to understand their

99 function and affinity. 100 In the past, comparisons of mobergellans with modern biological groups have

101 relied heavily on the comparative number and distribution of these muscle scars.

102 Mobergellans have been compared to a variety of groups, including brachiopods

103 (Moberg, 1892), monoplacophorans (Poulsen, 1963; Missarzhevsky, 1989), patellacean

104 gastropods (Hedström, 1923; Poulsen, 1932), bivalves (Valentine & Erwin, 2013) and

105 they have also been suggested as potentially forming a dorsal scleritome (Rozanov &

106 Zhuravlev, 1992). Little consensus has been found, undoubtedly a consequence of a

107 paucity of comparable morphological characters. An early and enduring functional

108 interpretation is that mobergellan sclerites acted as operculum of some hitherto

109 undiscovered tube-dwelling organism (Billings, 1871; Åhman & Martinsson, 1965;

110 Bengtson, 1968). The recurrent nature of this proposal could either partially endorse its

111 legitimacy or merely represents the path of least incongruence. The abundance of

112 phosphatic tubular structures in the Cambrian (e.g. Bengtson et al., 1990; Skovsted,

113 2006; Topper et al., 2009; Devaere et al., 2014; Budd & Jackson, 2016) suggest it is a

114 conceivable theory, yet limited evidence has been presented to support this claim and

115 this is the main hypothesis that we test herein.

116 Here we investigate the percentage of the sclerite inner surface that is occupied

117 by muscle scars of the two most well known mobergellan taxa, Mobergella holsti

118 (Moberg, 1892) and Discinella micans (Hall, 1872). The total surface area that the

119 muscle scars constitute, despite their variation in shape and number, is an aspect of the

120 mobergellans that is yet to be investigated. We take the novel approach of comparing

121 muscle scar/inner surface area of the two Cambrian species with the muscle scar/inner

122 surface area ratios of a variety of extant and fossil invertebrate taxa where the functional

123 significance of the shells and scars are known. The aim of our study is to explore

124 whether the total area of the mobergellan muscle scars, complemented with additional 125 morphological information, is consistent with the interpretation that mobergellan

126 sclerites represent opercula.

127

128 MATERIAL AND METHODS

129

130 Fossil and extant taxa included in this study were selected due to a number of

131 contributing factors, not least of which was the possession of easily distinguished

132 muscle scars. Taxa were specifically chosen as representatives of organisms that

133 mobergellans have been compared to (e.g. bivalved organisms such as bivalves and

134 brachiopods and univalved organisms such as patellacean gastropods and

135 monoplacophorans) or suggested as performing a particular function (e.g. dorsal

136 scleritome and operculum). Taxa included exhibit different modes of life and have been

137 investigated in an attempt to better understand the potential mode of life and function of

138 the mobergellans.

139 Relationships between members of the mobergellid family are unsettled and

140 some species exhibit features that at first glance are quite different and may not be

141 homologous (e.g. Hippoklosma Conway Morris & Chapman, 1997, see Streng &

142 Skovsted, 2006 for discussion). The focus of the study surrounds Mobergella holsti

143 from the Cambrian of Sweden (Bengtson, 1968) and Discinella micans from the

144 Cambrian of Greenland and Labrador, Canada (Skovsted, 2003, 2006). Mobergella

145 holsti and D. micans were chosen for a number of reasons; both taxa are well known in

146 the scientific literature and are undoubtedly considered to be representatives of the

147 Mobergellidae (Skovsted, 2003). Both species also possess distinguishable muscle scars

148 and lastly well-preserved specimens were readily available in the collections of the

149 Swedish Museum of Natural History. 150 We compare the two Cambrian species to a range of fossil and extant species of

151 molluscs, brachiopods and an annelid. Extant species include the elytra of the polynoid

152 annelid Lepidonotus squamatus (Linnaeus, 1758), the linguliform brachiopod

153 Discinisca lamellosa (Broderip, 1833), the bivalves Ostrea edulis Linnaeus, 1758,

154 Pseudamussium peslutrae (Linnaeus, 1771) and Tellina lineata Turton, 1819, the

155 gastropod limpet Patella sp., the operculum of the gastropod genera Nerita Linnaeus,

156 1758 and Natica Scopoli, 1777 the monoplacophoran Neopilina galatheae Lemche,

157 1957 and plates from the polyplacophoran Chiton tuberculatus Linnaeus, 1758 (Fig. 1).

158 Analyzed fossils species include four taxa generally considered to be

159 monoplacophorans, the Ordovician species Pilina cheyennica Peel, 1977 and Proplina

160 cornutaformis (Walcott, 1879) and the Silurian species Tryblidium reticulata

161 Lindström, 1880 and Kosovina peeli Horný, 2004, the operculum of the Ordovician

162 hyolithid Gompholites striatulus (Barrande, 1847), plates from the Pliocene

163 polyplacophoran Callistochiton spp. (Vendrasco et al., 2012) and the Ordovician

164 bivalved taxon, Angarella jaworowskii Asatkin, 1932 (Dzik 2010). For the sake of

165 simplicity all taxa will be referred to by their generic name for the remainder of the

166 paper. All examined specimens, except for Natica, Gompholites, Pilina, Proplina,

167 Kosovina, Callistochiton and Angarella are housed at the Swedish Museum of Natural

168 History, across the Palaeobiology and Zoology Departments (SMNH).

169 Twenty-five specimens of each taxon housed at the SMNH were included in the

170 analysis. The remaining taxa were measured from published reports that frequently

171 resulted in a reduced number of specimens utilized in the analysis. Due to the rarity of

172 monoplacophoran specimens, especially those exhibiting visible muscle scars, all

173 monoplacophoran measurements were done from published images and despite an

174 extensive search, each monoplacophoran species (fossil and extant) is represented by 175 only a single data point and the single specimen examined of Neopilina is measured

176 from a schematic illustration (Lemche & Wingstrand, 1969). Poor preservation resulted

177 in only a single specimen of Angarella included in the analysis, analyzed from a

178 schematic illustration (Dzik, 2010, fig. 4). Due to the rarity of preserved muscle scars

179 only 5 specimens of Gompholites were included, all from a published report (Martí Mus

180 & Bergström, 2005). The scarcity of published reports illustrating the ventral side of

181 chiton plates that display clear muscle scars resulted in only 8 specimens of

182 Callistochiton examined (Vendrasco et al., 2012), which was further split into

183 intermediate and tail valves to test for variability along the dorsal scleritome. Opercula

184 of Natica species bearing attachment scars are also infrequently illustrated and the 6

185 specimens examined come from a number of reports (Pedriali & Robba, 2004;

186 Hasegawa & Rosenberg, 2011; Costa and Pasorino, 2012; Simone, 2014).

187 A single species was chosen in each case, with the exception of the opercula of

188 Nerita and Natica and the chiton Callistochiton, where a number of species were

189 utilized to compile a comparable dataset. The genera Natica and Nerita were chosen as

190 in terms of morphology and ecology, they are well-studied operculum-bearing taxa

191 (Ansell, 1960; Vermeij, 1973, 1977, 1977) and at least for Nerita specimens, muscle

192 scars are easily visible and specimens were readily available. For the taxa that possess

193 more than a single valve, for consistency, all measurements were taken from the same

194 valve, for example the right valve in Ostrea (see Waller 1981), left valve in

195 Pseudomussium (see Hayami & Okamoto, 1986) right valve in Tellina (see de Freitas

196 Tallarico et al., 2014), the ventral valve in Discinisca (see Williams et al., 2000). For

197 organisms exhibiting an imbricated dorsal scleritome, only intermediate valves in

198 Chiton were examined (see Schwabe 2010), however 4 intermediate valves and 4 tail

199 valves were measured from a published report of Callistochiton (Vendrasco et al., 200 2012) and Lepidonotus elytra came from a variety of positions along the dorsal trunk of

201 the annelid.

202 The authors recognize that not all muscles possessed by an organism will leave

203 discrete scars, however the general distribution and size of muscle attachment areas, is

204 relatively conservative in related organisms and generally share a comparable functional

205 adaptation (Rudwick, 1970; Stanley, 1977). The authors also acknowledge that the

206 outline of specimens may not capture the exact surface area of the interior.

207 Consequently well-preserved specimens exhibiting a limited topographic profile and

208 without a complex sculpture were utilised to minimalise the error and increase the

209 accuracy of surface area measurements.

210 Specimens were photographed in plan view under normal light using a Canon

211 EOS6D digital SLR camera except for Mobergella and Discinella that were imaged

212 using SEM facilites at the Swedish Museum of Natural History and at Uppsala

213 University, Sweden. Measurements and calculations were done with ImageJ (1.49v,

214 available online at http://imagej.nih.gov/ij/download.html, National Institutes of Health,

215 USA). The inner surface of each specimen was outlined and the resulting area

216 calculated. Then each muscle scar was outlined and the total resulting area calculated

217 (for an example see Discinella, Fig. 2) and the percentage of the total area occupied by

218 the muscle scars was tabulated. Box and beanplots (Table S1) were produced with

219 BoxPlotR (available online at http://boxplot.tylerslab.com/, Spitzer et al., 2014;

220 Krzywinski & Altman, 2014). Subsequent analyses, such as normal probability plots, t-

221 tests, one-way ANOVA and Tukey’s Honestly Significant Difference (HSD) test (Table

222 S2) were completed using PAST version 3.1 (Hammer et al., 2001).

223

224 RESULTS 225

226 The percentage of surface area that the muscle scars occupy in relation to the total inner

227 surface area is visually presented as a box (Fig. 3A) and beanplot (Fig. 3B, detailed

228 statistics available in Table S1). The boxplot indicates that Discinella and Mobergella

229 are very similar in terms of muscle scar percentage with a mean of 19.25 and 18.42

230 respectively. The lower and upper quartiles are also similar with Discinella exhibiting a

231 spread of 17.88 to 20.80 with an interquartile range (IQR) of 2.92 and Mobergella from

232 17.04 to 19.99 with an IQR of 2.95 (Fig. 2; Table S1). The 95% confidence interval

233 (CI) of both taxa significantly overlap (Fig. 2). Normal probability plot correlation

234 coefficients were all above the critical value (e.g. >0.9590 for n=26) indicating the data

235 came from a population with a normal distribution. A t-test also indicates that the

236 measurements obtained from both mobergellan taxa do not significantly differ (p-value

237 0.164).

238 The boxplot reveals that muscle scar percentage of both mobergellans when

239 compared to the majority of measured taxa is relatively large and Tukey’s HSD tests

240 showed significant differences between the mobergellans and nearly all other taxa, with

241 the exception of Natica (Table S2). The only overlap of the whiskers of both

242 mobergellan taxa is with the opercula of Natica and Nerita (Fig. 3A). The attachment

243 scar of investigated Natica opercula is very similar to the mobergellans with a mean of

244 18.9. The muscle scar percentage of Nerita opercula is slightly larger in comparison

245 exhibiting a lower quartile of 22.29, an upper quartile of 24.23 (IQR of 1.94) and a

246 mean of 23.16.

247 The muscle scar percentage of the remaining taxa, when compared with the

248 mobergellans is comparatively speaking, considerably lower. The muscle scar area of 249 Discinisca, Ostrea and Pseudomussium considerably overlap (Fig. 3A) and differences

250 between the taxa are not deemed significant (Table S2). There is also some degree of

251 overlap in whisker length between the bivalve Tellina and the limpet Patella,

252 differences of which are not considered significant (p-value 0.99). The differences in

253 the two polyplacophorans, Chiton and Callistochiton are also not significant (Table S2)

254 and of particular note are the similarities in measurements from the intermediate valves

255 of Callistochiton which are basically identical to the tail valves and show no significant

256 difference (p-value 0.99). With the exception of the four fossil monoplacophorans and

257 Angarella (all represented by single data points with recorded measurements of 11.1 to

258 12.5), the mean surface area that the muscle scars constitute of the other eleven taxa is

259 below 10 (Fig. 3). The extant monoplacophoran taxa, Neopilina exhibits a low muscle

260 scar percentage of 5.18, significantly lower than the four fossil monoplacophorans.

261 In a general trend, the three taxa possessing an imbricating dorsal scleritome

262 (Lepidonotus, Chiton and Callistochiton) exhibited the lowest muscle area to sclerite

263 area percentage, followed by the taxa that possess two shells (Ostrea, Pseudomussium,

264 Discinisca and Tellina) and the largest muscle area percentage is displayed by the

265 mobergellans and the opercula of Nerita. This pattern is interrupted by the Ordovician

266 hyolithid Gompholites that possesses a series of small muscle scars that constitute a

267 mean of 4.14 and the limpet Patella that possesses a single horseshoe-shaped scar with

268 a mean of 9.37.

269 For a visual comparison of the individual observations and the density of the

270 distributions a beanplot (Kampstra, 2008) has been provided (Fig. 3B). Beanplots

271 display all individual observations (unlike Boxplots) in a one-dimensional plot, but also

272 show the estimated density of distributions and the average (Kampstra, 2008). The

273 variation of the mobergellan muscle area percentage is apparent and both taxa display a 274 similar distribution of individual measurements. The mobergellans also have the highest

275 standard deviation (s.d.) of 2.1 and 2.0 respectively, followed by Patella (1.66) and the

276 opercula of Nerita (1.6) compared with Lepidonotus that recorded the lowest s.d. of

277 0.44 (Table S1). In terms of individual observations, a number of measurements from

278 Discinella and Mobergella overlap with observations from the opercula of Nerita,

279 however no individual observations of either mobergellan species overlap with any

280 individual observations of other taxa included in the analysis.

281

282 DISCUSSION

283

284 The key to unlocking the function and possible affinity of mobergellans rests with their

285 most diagnostic feature; the radiating set of muscle scars on the internal surface of the

286 sclerites. The study presented here represents the first investigation into the total area

287 occupied by the muscle scars relative to the inner surface area of the sclerite, comparing

288 directly the muscle scar area of fossil and extant taxa. Our results show clearly that

289 compared to the majority of taxa analyzed, the percentage of internal surface area that

290 the muscle scars occupy in mobergellan sclerites is larger. In relative size it is only

291 comparable to the muscle scar area observed on the opercula of the gastropod genera

292 Natica and Nerita. The size of the attachment sites indicates that the construction of the

293 muscles would represent a significant investment to the organism and certainly had

294 important functional significance. Based on our data and a review of similar structures

295 in fossil and extant organisms we herein discuss evidence for and against a number of

296 potential functional hypotheses for mobergellan sclerites.

297

298 MOBERGELLANS AS BIVALVED ORGANISMS 299 Mobergellans have been suggested as representing valves from a variety of bivalved

300 organisms. Early comparisons of mobergellans to the crown-group brachiopods were

301 founded on the apparent similarities in shell composition to Cambrian linguliform

302 brachiopods (Hall, 1872). The similarity in musculature arrangement prompted Dzik

303 (2010) to make comparisons with the bivalved Angarella, a possible ancestor to the

304 cephalopods and Erwin & Valentine (2013) recently claimed, without explanation, that

305 Mobergella is a stem bivalve. Bivalved organisms possess a wide range of muscular

306 arrangements, the main impetus of which is the ability to open and close their shell (e.g.

307 Gosling, 2008; Purchon, 2013). Brachiopods for example, typically possess a pair of

308 adductor and diductor muscle (Williams et al., 2000) and bivalves generally possess

309 either a single or pair of adductor muscles (Dillion, 2000; Gosling, 2008; Nielsen, 2012;

310 Purchon, 2013). Brachiopods are predominantly sessile organisms (e.g. Discinisca),

311 their musculature system predominantly required to open and close their valves.

312 (Williams et al., 2000). Bivalves on the other hand display an assortment of life habits;

313 taxa can be sessile (e.g. Ostrea) but can also possess musculature systems that allow the

314 individual to burrow (e.g. Tellina) and to swim (e.g. Pseudomussium). From our results,

315 the burrowing Tellina displays the highest mean, followed by the free-swimming

316 pectinid Pseudomussium and finally the sessile oyster Ostrea (Figs 3).

317 What is immediately apparent though, is that the mean of the muscle scar area for all

318 four bivalved taxa are greatly reduced by approximately one half to a third of the mean

319 of the muscle scar measurements of the mobergellans.

320 The unique distribution of muscle scars in mobergellans does bear a

321 resemblance to the remaining bivalved taxon included in the analysis, Angarella,

322 described from the Ordovician from Siberia (Dzik, 2010). The dorsal valves of

323 Angarella are conical and display 5 paired scars and a narrow posterior scar (Dzik, 324 2010, fig. 4). The ventral valve is flat, bearing a pair of elongate muscle scars and has

325 been interpreted has having been cemented to hard substrates (Dzik, 2010). Despite

326 differences in shell composition (phosphatic verse calcitic), Dzik (2010) noted

327 similarities in the musculature and hypothesized that mobergellans may have also been

328 a bivalved organism, cementing their ‘ventral’ valve to hard substrates in a manner akin

329 to Angarella. The percentages of the inner surface shell area that the scars constitute are

330 markedly different though; Angarella with a much lower mean of 11.09 compared to

331 the mobergellans 19.25 and 18.42 respectively.

332 A possible explanation for the relatively small area of the inner shell occupied

333 by musculature scars, is that generally in bivalved taxa, the entire soft body of the

334 organism is encapsulated within the shell (Williams et al., 2000; Gosling, 2008;

335 Nielsen, 2012; Purchon, 2013). In shell bearing bivalved taxa the ability to fully enclose

336 their soft parts within their protective shell is their primary mode of defense (Vermeij,

337 1973; Leighton, 2003; Topper et al., 2015). Some bivalved taxa do extend their soft

338 tissues considerably beyond the margin of their shell, but also possess the ability to

339 retract their soft parts within their shell in times of stress (Gosling, 2008; Purchon,

340 2013). This complete enclosure or retractability of soft parts may be difficult for an

341 organism to successfully achieve if the musculature occupies nearly one-fifth of the

342 inner surface area of the shell, as demonstrated in the mobergellans. This may be even

343 harder to reconcile if the shells themselves are frequently concave as demonstrated in

344 the mobergellans (Bengtson, 1968; Skovsted, 2003; Streng & Skovsted, 2006; Fig. 4G).

345 This is of course assuming, in a bivalved scenario that both valves are morphologically

346 recognizable as mobergellan sclerites. The inability of the organism to completely

347 enclose their soft tissue within their shell would likely result in an increased

348 susceptibility to predation, with likely lower fitness compared to taxa that could enclose 349 their soft tissue within their shell. Consequently such exposed bivalved taxa are

350 relatively rare in modern marine communities.

351 Bivalved taxa are morphologically diverse, with a vast array of body plans and

352 life habits. Despite the wide range of life habits and variation in musculature

353 arrangement exhibited by the bivalved taxa investigated herein, all display similar

354 muscle scar measurements and individual observations largely overlap (Fig. 3B). The

355 four genera however contrast markedly in valve morphology and musculature

356 distribution (Fig. 1D-F, I) and represent a scenario where the subtle differences in the

357 muscle scar area cannot alone resolve the function and affinity of the skeletal element

358 and any interpretation would benefit from additional morphological information. One

359 characteristic however, that is shared by all the examined bivalved taxa is a dramatically

360 reduced percentage of the inner surface of the shell that the muscle scars constitute.

361 A reduced area occupied by musculature would provide more space for the

362 remaining soft parts and increase the probability that the organism could enclose their

363 entire body within the shell. The ability to close the shell and isolate the internal body of

364 the organism from outside stresses would be pivotal for the survival of the individual.

365 Due to the lack of correlation in muscle size and the life habits of the bivalved taxa

366 investigated, it may be ambitious to interpret the life habit of mobergellan sclerites

367 based only on the area of muscle scars (e.g. burrowing, swimming). However, we

368 consider it unlikely, based on existing morphological evidence, that mobergellans

369 constitute part of a bivalved organism. The number and distribution of muscle scars of

370 mobergellans is not known from extant bivalved taxa and the large area occupied by

371 muscle scars and the almost flat lateral profile of the mobergellan discs (Fig. 4E-G), is

372 not consistent with possessing the capability of sheltering the entire soft body within a

373 bivalved organism. A cemented lifestyle, as proposed for Angarella (Dzik, 2010) may 374 be plausible, however, compared to mobergellans Angarella also exhibits a reduced area

375 occupied by muscle scars, despite possessing a vaguely similar muscle distribution.

376

377 MOBERGELLANS AS UNIVALVED ORGANISMS

378 Hedström (1923, 1930) suggested that mobergellans represent molluscan shells and

379 more recently, Conway Morris & Chapman (1997, p. 977) advocated that of ‘all the

380 metazoan groups, comparisons with the molluscs could be the most fruitful line of

381 further investigation’. The principal reasoning behind this championing of a molluscan

382 affinity is the striking similarities in the pattern of muscle scars of some mobergellans

383 and the single shelled monoplacophorans (Hedström, 1923, 1930). Both groups display

384 a circumferential distribution of muscle scars and at first glance they do bear a putative

385 similarity. However the similarities are rather cursory. Muscle attachment sites in the

386 two groups differ not only in number (Yochelson, 1958; Peel, 1977; Conway Morris &

387 Chapman, 1997; Horný, 2004; Lindberg, 2009; Ruthensteiner et al., 2010), but also

388 significantly in the area that the attachment sites occupy (Fig. 3). The Palaeozoic

389 monoplacophorans, including Tryblidium (holotype reproduced here Fig. 1K with six

390 pairs of muscle scars) all exhibited near identical results, with the muscle scars

391 occupying between 11.3% and 12.5% of the inner surface of the shell. The sites of

392 muscle attachment in the single analyzed extant species, Neopilina is further reduced in

393 terms of area, only occupying 5.1% (Fig. 3A) of the inner surface area, despite the

394 possession of eight pairs of muscle scars (Lemche & Wingstand, 1959). The reason for

395 this reduction in muscle attachment area in the extant monoplacophorans compared with

396 their fossil equivalents is unclear. With each fossil and extant monoplacophoran

397 represented by a single data point, a larger data set would be favourable in solving this

398 question. It is possible that muscle scar area is related to changes in habitat, as extant 399 monoplacophora predominantly live at abyssal depths (Lemche & Wingstrand, 1959;

400 Lindberg, 2009) contrasting with Palaeozoic taxa that are typically recovered from

401 much shallower, limestone settings (e.g. Peel, 1977; Horný, 2004), however further

402 scrutiny is needed.

403 Compared to the mobergellans (mean percentage of 19.25 and 18.46

404 respectively) the results from the single shelled monoplacophorans are considerably

405 lower. Furthermore, the shells of mobergellans and monoplacophorans contrast notably

406 in shape and the position of the apex (overhanging the anterior margin in

407 monoplacophorans compared to sub-central in mobergellans). With the exception of

408 possessing a set of radiating muscle scars, similarities between the groups are scant. The

409 other single shelled organism included into the analysis, the gastropod limpet Patella

410 exhibits a single, horseshoe-shaped muscle scar (Fig. 1J). With a mean of 9.37%, the

411 muscle attachment area of Patella is approximately half of the mean of Mobergella and

412 Discinella (Fig. 3).

413 Lacking a second shell to sufficiently enclose their soft parts, taxa that possess

414 only a single shell (e.g. limpet gastropods and monoplacophorans) alternatively utilize

415 the substrata (e.g. rock surfaces) to protectively seal their soft parts within their shell

416 (Vermeij, 1973, 1976, 1977). A similar explanation for the relatively small area of the

417 inner shell occupied by muscle scars can be given for univalved taxa as for bivalved

418 taxa. Except for the muscle scars, nothing is known about the soft parts of mobergellans

419 and an immediate dismissal of monoplacophoran similarities may be unwarranted.

420 However, the large area occupied by the muscle scars and the flat to sometimes concave

421 profile of mobergellan sclerites is not coherent with the ability to encapsulate the entire

422 soft body within a single shelled organism and the lack of reliable congruent

423 morphological characters currently obscures comparisons with univalved taxa. 424

425 THE FORMATION OF A DORSAL SCLERITOME

426 The discovery of Microdictyon sinicum (Chen et al., 1989) and Halkiera evangelista

427 Conway Morris & Peel, 1990 revolutionized the interpretation of Cambrian skeletal

428 assemblages. Both discoveries providing an apparent template for disarticulated

429 sclerites, resulting in a host of skeletal fossils interpreted as constituting cataphract

430 dorsal shields of vermiform bilaterians (Evans & Rowell, 1990; Williams & Holmer,

431 2002; Li & Xiao, 2004). Even, mobergellans have been suggested to represent

432 disarticulated sclerites from a larger dorsal scleritome (Rozanov & Zhuravlev, 1992) but

433 evidence to support this hypothesis is particularly tenuous.

434 Three taxa were investigated in this study that possess a dorsal scleritome, the

435 polyplacophorans Chiton and Callistochiton and the polynoid annelid Lepidonotus.

436 Polynoid scale worms are protected dorsally by a number of large overlapping elytra

437 that are attached to prominent elytrophores via a single attachment point near the centre

438 of each eleytron (Rouse & Pleijel, 2001). Elytra vary noticeably in shape and size,

439 depending on their location on the trunk and elytra representing a variety of positions

440 along the dorsal trunk of Lepidonotus has been included in the analysis. The attachment

441 is not strong and elytra are frequently detached (Rouse & Pleijel, 2001). The apparent

442 ease of elytra removal is seemingly reflected in the size of the attachment scar of

443 Lepidonotus that constitutes an exceptionally small area (mean 3.7%). Lepidonotus

444 scars displayed a low degree of variation (s.d. 0.44) indicating the area that the

445 attachment site is proportional to the size of the sclerite along the length of the dorsal

446 scleritome. Curiously the other investigated dorsal scleritome bearing taxa, Chiton and

447 Callistochiton also exhibits a relatively small muscle attachment area (a pair of oblique

448 muscle scars constituting a mean of 5.4% and 4.6% respectively). To test whether there 449 was variation in muscle attachment area relative to sclerite area of polyplacophorans the

450 intermediate and tail valves of Callistochiton were incorporated into the analysis. The

451 intermediate valves generally possess a pair of muscle scars whilst the tail valves

452 exhibit 4-6 pairs of small muscle scars that flank the midline of the valve (Vendrasco et

453 al., 2012). This type of musculature however is only present on the single tail valve of

454 the polyplacophoran and the individual attachment sites are dramatically reduced in size

455 (Schwabe, 2010; Vendrasco et al., 2012). Measurements were nearly identical for both

456 the intermediate (mean 4.6%) and tail (mean 4.7%) valves of Callistochiton indicating

457 minimal variation in muscle attachment area along the dorsal scleritome of

458 Callistochiton. The results from all three dorsal scleritome bearing taxa are dramatically

459 reduced in comparison to the mobergellans.

460 The principal difficulty with the proposal that mobergellan sclerites formed part

461 of a larger scleritome is the number of muscle scars and the large area on the inner

462 surface of the shell that the musculature occupies. Given their size and prominence, the

463 energy invested by the organism to construct and maintain such musculature

464 attachments seems excessive for the formation of a multi-element scleritome. Every

465 mobergellan sclerite in this hypothetical scleritome would require five to seven pairs of

466 musculature attachments, a rather sophisticated system when compared to the one or

467 two, relatively small attachment sites displayed in Lepidonotus and in the intermediate

468 valves of the polyplacophorans. Hypothetically speaking, if the scleritome of

469 Mobergella consisted of 8 shells (akin to polyplacophorans) and if each site of

470 attachment represented a single muscle, the organism would posses a musculature

471 system consisting of 104-112 muscles. It would likely be more energy efficient for the

472 scleritome-bearing creature, as observed in Lepidonotus, Chiton and Callistochiton, to

473 minimize the investment in the musculature attachment of each sclerite, to take into 474 account the number of sclerites necessary to form a scleritome. Furthermore there is no

475 morphological evidence that indicates that mobergellans shells were juxtaposed to other

476 sclerites or overlapping in an imbricating dorsal scleritome. Mobergellan sclerites are

477 usually almost perfectly circular (Figs 1A,B, 4D) and show no consistent signs of

478 smoothing or grinding on the exterior of the shell that could imply that the shells were

479 in contact during life. Further, conjoined specimens have never been found (unlike

480 among other Cambrian sclerites, e.g. Demidenko, 2004; Skovsted et al., 2015) and

481 mobergellans do not possess any shell extensions that would enhance the suturing of the

482 valves, as seen in extant chitons (Todt et al., 2008; Schwabe, 2010).

483 Morphological parallels with scleritome-bearing organisms in contemporary

484 Cambrian faunal assemblages are depauperate. Sites of muscle attachment have not

485 been documented on the internal surface of most sclerites that are definitely known to

486 have formed part of the dorsal scleritome in Cambrian taxa. This includes a range of

487 isolated lobopodian plates, such as Microdictyon (Bengtson et al., 1990; Zhang &

488 Aldridge, 2007; Topper et al., 2011), Onchyodictyon (Topper et al., 2013), Hallucigenia

489 (Caron et al., 2013) or isolated palaeoscolecid plates (Wrona, 1987; Müller & Hinz

490 Schallreuter, 1993; Barragán et al., 2014). Lobopodians and palaeoscolecids are also

491 generally accepted as members of the ecdyozoans, a group united by the process of

492 moulting their exoskeleton (Topper et al., 2010; Harvey et al., 2010; Daley & Drage,

493 2015). The external concentric growth lines (Bengtson, 1968; Skovsted, 2003) and the

494 cyclic secretion of thin phosphatic lamine in the microstructure of mobergellan shells

495 (Bengtson, 1968; Skovsted, 2003; Streng & Skovsted, 2006) reflect incremental growth

496 by marginal accretion providing no evidence of growth via ecdysis.

497 The only documented account of internal structures that are likened to sites of

498 muscular attachment in Cambrian sclerites are from the tommotiid genus Dailyatia 499 Bischoff, 1976. Dailyatia has recently been interpreted as forming a complex multi-

500 element scleritome of a slug-like bilaterian and individual sclerites can display up to

501 two pairs of attachment sites (Skovsted et al., 2015). Dailyatia sclerites are invariably

502 pyramidal, coiled and torted, impeding accurate measurements of the inner surface area

503 of the sclerite and as such have been omitted from the analysis. The overall pattern of

504 muscle insertion points differs between sclerite type and the shape, size and expression

505 displays a clear variability (Skovsted et al., 2015). Skovsted et al. (2015) suggested this

506 level of variability as potentially representing differences in the muscle insertion

507 requirements of sclerites that occupy different positions within the scleritome (in a

508 similar manner as in the intermediate and tail valves of Callistochiton mentioned

509 above). Mobergellan species although showing variation, do not show such a high level

510 of fluctuation in attachment sites and if the above statement is correct, the lack of

511 comparable variation further discourages support for the hypothesis that mobergellan

512 sclerites constituted part of a larger dorsal scleritome.

513

514 THE OPERCULUM HYPOTHESIS

515 The interpretation that mobergellans represent the operculum of a tube dwelling

516 organism has been the ominous cloud lingering over the enigmatic sclerites since the

517 1870s (Billings, 1871). Only limited evidence has been presented to support this claim

518 (Bengtson, 1968) and the enduring nature of the proposal is seemingly one of least

519 discordance. There are however many lines of evidence to support the claim. Tubular

520 fossils of similar phosphatic composition are frequent constitutes of Cambrian

521 assemblages (Bengtson et al., 1990; Wrona, 2004; Skovsted, 2006; Topper et al., 2009;

522 Skovsted & Peel, 2011). The circular nature of mobergellan sclerites and the tendency

523 for some shells to display flared margins would also appear consistent with life within a 524 tube. Our analysis also supports this theory, with the muscle scars of gastropod

525 opercula, the only taxa analyzed with a somewhat comparable area of the inner shell

526 occupied by muscle scars (Fig 3).

527 Gastropod opercula are primarily utilized in closing the aperture of the shell

528 when the organism has withdrawn (Vermeij, 1971; Hunt, 1976; Checa & Jiménez-

529 Jiménez, 1998; Vermeij & Williams, 2007). The significance of the opercula in species

530 of Nerita for example, are well documented (e.g. Vermeij, 1971, 1973). The tight

531 closure of the shell offering protection and to assist in the avoidance of water loss and

532 subsequent desiccation (Vermeij, 1971, 1973). Typically congregating in large numbers

533 in high-intertidal habitats, the neritid gastropods are subjected to tidal influence and are

534 periodically exposed (Vermeij, 1973; Chelazzi, 1982). Possessing an ability to maintain

535 a reservoir of water within the shell, individuals can retract their foot, enclosing their

536 soft parts in the shell behind their calcareous operculum where the individual can

537 remain (Vermeij, 1973, 1987). The majority of neritids can remain in this dormant state

538 for weeks or even months at a time, until optimal conditions reappear (Vermeij, 1973).

539 The thick operculum also provides a defensive barrier by restricting predatory access to

540 the retracted soft parts of the organism while the individual is exposed (Bertness et al.,

541 1981). Naticids as predatory gastropods, display a very different lifestyle to the grazing

542 neritids (Ansell, 1960; Berry, 1982; Hughes, 1985), yet they employ their opercula in a

543 similar manner, sealing their aperture in times of environmental or predatory stress.

544 That said, not all opercula exhibit large muscle scars on their internal surface, as

545 shown by the operculum of the Ordovician hyolithid Gompholite (Martí Mus &

546 Bergström, 2005). This disparity can be explained by the contrasting functions that

547 hyolithid opercula and gastropod opercula perform. Hyolithids possessed a complex

548 musculature system that was used to open and close their opercula but primarily utilized 549 for providing a wide range of movement to their helens (Martí Mus & Bergström, 2005,

550 2007; Martí Mus et al., 2014). Helens are long, curved skeletal elements that hyolithids

551 presumably used to orient and stabilize themselves to potentially facing water currents

552 for optimal filter- (or suspension) feeding (Martí Mus & Bergström, 2005; Martí Mus et

553 al., 2014). Further, hyolithid opercula exhibit prominent internal projections, clavicles

554 and cardinal processes (Marti Mus & Bergström, 2005) that presumably functioned as

555 levers for muscular action, effectively reducing the force (and hence muscular size)

556 needed to perform functions such as closing the aperture of the conch.

557 Based on musculature comparisons, there is no evidence to suggest that the

558 mobergellan shells functioned in a similar manner to hyolithid opercula, providing

559 leverage and articulating other skeletal elements for movement. Concerning the size of

560 the muscle attachment sites of mobergellans and gastropod opercula (Table S2) the

561 overlap of individual observations (Fig. 3) suggests that mobergellan sclerites may have

562 functioned as an operculum of an organism, closing the aperture during times of

563 predatory or desiccation stress. There is insufficient evidence however, to suggest that

564 mobergellan sclerites are opercula of a gastropod. Gastropod opercula are primarily

565 calcareous and also tend to vary dramatically in shape and size (e.g. Checa & Jiménez-

566 Jiménez, 1998; Vermeij & Williams, 2007) much unlike the phosphatic mobergellan

567 sclerites that are consistently circular in outline (Bengtson, 1968; Skovsted, 2003; Fig.

568 1A,B). However, representing attachment to a portion of the organism rather than

569 housing the entire body of the animal, would resolve the mobergellan conundrum of

570 concave shells and would also explain the relative high percentage of the internal

571 surface area that is utilized by musculature. The large area of the opercula inner surface

572 occupied by muscle scars (Figs 1H, 3) is a reflection of the strength in which the

573 individual seals the aperture in times of stress (Vermeij 1973; Kaim and Sztajner 2004) 574 and the attachment area of some gastropod species can constitute nearly 50% of the

575 total area (Pimenta et al., 2008). Therefore we suggest that mobergellans were opercula

576 employed to firmly seal the aperture and safely enclose and protect the presumably

577 tube-dwelling animal.

578 The major stumbling block of the operculum hypothesis has been the apparent

579 lack of a corresponding co-occurring tube. Why would an organism housed in a tube of

580 negligible preservation potential possess a relatively thick, densely laminated

581 phosphatic operculum? While not catastrophic to the hypothesis, the absentee tube has

582 presented a hurdle that is challenging to negotiate. Phosphatic tubes, such as

583 hyolithellids are extremely common constitutes in Cambrian faunal assemblages,

584 (Bengtson et al., 1990; Skovsted, 2006: Topper et al., 2009; Skovsted & Peel, 2011;

585 Devaere et al., 2014) yet are rarely associated with mobergellans and vice versa. There

586 have been a few occurrences where phosphatic tubes and mobergellan sclerites co-

587 occur, such as New York (Landing & Bartowski, 1996), Siberia (Rozanov & Zhuravlev,

588 1992) and Greenland (Skovsted, 2006). Re-examination of material from Venenäs,

589 Sweden (Åhman & Martinsson, 1965; Bengtson, 1968, Fig. 4A-D) has also revealed a

590 few phosphatic tubes that are comparable in size to mobergellan sclerites (Fig. 4A-C).

591 Despite these glimpses of association between tube and possible operculum, it is not a

592 persistent occurrence and the disassociation requires attention.

593 The dearth of tubes co-occurring with mobergellan sclerites could be explained

594 by speculating that the organism lived attached to the substrate and that after death and

595 soft part decomposition, the opercula were whisked away by currents and transported to

596 their place of deposition. Bengtson (1968) speculated that the organism lived attached to

597 the substrate however; he doubted such a hypothesis, stating that the discrepancy in

598 sizes between tube and opercula would be too large to be explained by means of sorting 599 or taphonomic factors. There is some truth in that statement, however this is under the

600 assumption that upon the death of the animal, the tube, like the opercula, would be

601 isolated and carried away in the water currents. Here we suggest that the tube may have

602 been cemented or at least firmly attached to the substrate and remaining attached after

603 the death and decay of the animal.

604 The opercula would eventually dissociate from the decaying organism and be

605 transported away, leaving the tube behind. Significantly, mobergellan sclerites have a

606 tendency to be recovered from transported sediments, such as turbidite deposits, debris

607 surrounding archaeocyathan bioherms or in relatively coarse reworked siliciclastic

608 settings (see Table 1). Retrieval from transported and reworked sediments could explain

609 the dissociation between tube and sclerite, if the tube remained firmly attached to the

610 substrate in its original environment. In many faunal assemblages where mobergellan

611 sclerites are present, they are present in an extreme abundance (Table 1). It is worth

612 considering that the high accumulation of mobergellan specimens in many samples

613 (Table 1) could be a result of the transportation and sorting of detached opercula from a

614 gregarious shallow-water tube-dwelling animal.

615 Organisms such as serpulid polychaetes inhabit calcareous tubes that are firmly

616 attached or even encrusted to the substrata (ten Hove & Kupriyanova, 2009). Serpulids

617 can form dense aggregations and tubes also typical remain attached to the substrata long

618 after the death of the animal (Dill & Fraser, 1997; ten Hove & Kupriyanova, 2009;

619 Ippolitov et al., 2014). Interestingly serpulids possess an operculum that serves as a

620 plug when the worm responds to shadows, touch or water movement and withdraws

621 into the safety of its tube (Dill & Fraser, 1997; ten Hove & Kupriyanova, 2009;

622 Ippolitov et al., 2014). The two structures are clearly not homologous (the serpulid

623 operculum is a modified ciliated tentacle or radiole, sometimes calcareous in 624 composition) and there is no evidence to suggest a serpulid affinity of mobergellans.

625 Serpulid opercula also do not bear any muscle scars and consequently were not included

626 in this analysis. Serpulids however, do present a similar situation to that interpreted for

627 mobergellans as in fossil assemblages serpulid opercula and tubes are seldom found in

628 association (Ippolitov et al., 2014).

629 A wide variety of phopshatic tubes frequently occur in Cambrian deposits, the

630 large majority of which have an unknown biological affinity. A definitive relationship

631 between mobergellans and phosphatic tubes is difficult to establish without finding an

632 articulated specimen. Although, the operculum hypothesis remains somewhat

633 speculative, it is possible to envisage a situation where an operculum-bearing organism

634 inhabited a tube that was firmly attached to substrata in a shallow water environment. In

635 this scenario, the operculum, would function as a defensive structure, firmly sealing the

636 aperture of the tube if the organism felt threatened by a predator or if the tubes became

637 exposed from falling tide, in an analogous manner to how neritid gastropods and

638 serpulid polychaete annelids utilize their opercula (Vermeij, 1973; ten Hove &

639 Kupriyanova, 2009; Ippolitov et al., 2014). The death of the organism resulted in the

640 disassociation of the operculum that was subsequently transported and deposited

641 elsewhere. The major drawback of this hypothesis is that the biological affinity of the

642 mobergellans remains unanswered. Whilst there is convincing evidence to support the

643 mobergellan shell having functioned as an operculum there is currently insufficient

644 evidence to place the Cambrian fossil into any known biological group.

645

646 CONCLUSIONS

647 648 Documenting biotas from the early Cambrian sheds light on biological diversity, body

649 plan complexity and evolutionary history of many animal groups and is pivotal in

650 understanding the events surrounding one of the greatest biological diversification

651 events in the history of life. It is frustrating that the function of many of the minute

652 skeletal fossils from the early stages of this diversification is unknown and many cannot

653 be assigned with confidence to particular biological groups. Mobergellens lack a broad

654 suite of distinguishing morphological characters, however the sheer size of the muscle

655 scars expressed on the internal surface of the shell stresses that the muscle system must

656 have held an important functional significance.

657 The muscle scars of the mobergellans Mobergella holsti and Discinella micans

658 are very similar occupying nearly 20% of the total inner surface of the sclerite. The

659 large area occupied by muscle scars, the distribution of the muscle scars, the lateral

660 profile and the composition of mobergellan sclerites is not compatible with representing

661 a univalved mollusc or a single shell of a brachiopod or another bivalved organism. The

662 large muscle scar area and the lack of comparable structures in Cambrian taxa also do

663 not provide any evidence to suggest that the sclerites amassed to form part of a dorsal

664 scleritome. The area that the muscle scars constitute in mobergellen sclerites is

665 comparable to the muscle scars on the opercula of gastropods. Combined with their

666 circular shape and the tendency of valves to be variously concave with flared margins,

667 the sclerite morphology appears consistent with life in a tube. Gastropod opercula are

668 utilized to seal the aperture and enclose the entire soft body of the organism inside the

669 shell in times of predatory or desiccation stress. The similarities in muscle scar area

670 suggest that mobergellans could have performed a similar function. The dissociation in

671 fossil assemblages of the tube and mobergellan sclerties a result of transportation as

672 mobergellan shells are invariably documented from reworked and transported 673 sediments. The organism would have lived in a tube firmly attached to the substrate in

674 shallow water environments. The opercula becoming detached after death and carried

675 away in the currents, leaving the tube still firmly attached to substrate.

676 Despite these advances in regards to the function of the sclerites, there still remains

677 insufficient evidence to place the mobergellans in the metazoan tree and their biological

678 affinities continue to be shrouded in mystery.

679

680 ACKNOWLEDGEMENTS

681

682 We are grateful to Stefan Bengtson (Swedish Museum of Natural History_SMNH) for

683 access to mobergellan specimens that were collected from the Cambrian of Sweden

684 over 50 years ago. We also must thank Lena Gustavsson and Anna Persson for

685 permission to wander through the annelid and mollusc collections at the Department of

686 Zoological, SMNH. We also thank Luke Strotz (Yale University) for statistical

687 assistance. The SMNH is thanked for funding and the manuscript benefited from

688 constructive reviews by xxxxx.

689

690 REFERENCES

691

692 Åhman E, Martinsson A. 1965. Fossiliferous Lower Cambrian at Äspelund on the

693 Skäggenäs Peninsula. GFF 87:139-151

694

695 Ansell, A.D. 1960. Observations on predation of Venus striatula (Da Costa) by Natica

696 alderi (Forbes). Journal of Molluscan Studies 34: 157-164.

697 698

699 Ansell AD, Trevallion A. 1969. Behavioural adaptations of intertidal molluscs from a

700 tropical sandy beach. Journal of Experimental Marine Biology and Ecology 4: 9-35

701

702 Barragán T, Esteve J, García-Bellido DC, Zamora S, Álvaro JJ. 2014. New Middle

703 Cambrian palaeoscolecid sclerites of Hadimopanella oezgueli from the Cantabrian

704 Mountains, northern Spain. GFF 136: 22-25.

705

706 Barrande J. 1847. Pugiunculus, ein fossils Pteropoden Geschlecht. Neus Jahg für Min

707 Geog Geol und Petref 1847: 554-558.

708

709 Bengtson S. 1968. The problematic genus Mobergella from the Lower Cambrian of the

710 Baltic area. Lethaia 1: 325-351.

711

712 Bengtson S, Conway Morris S, Cooper BJ, Jell PA, Runnegar BN. 1990. Early

713 Cambrian fossils from South Australia. Memoirs of the Association of Australasian

714 Palaeontologists 9, pp. 364.

715

716 Berry AJ. 1982. Predation by Natica maculosa Lamarck (Naticidae: Gastropoda) upon

717 the trochacean gastropod Umbonium vestiarium (L.) on a Malaysian shore. Journal of

718 Experimental Marine Biology and Ecology 64: 71-89.

719

720 Bertness MD, Garrity SD, Levings SC. 1981. Predation pressure and gastropod

721 foraging: a tropical-temperate comparison. Evolution 35: 995-1007.

722 723 Bischoff GCO. 1976. Dailyatia, a new genus of the Tommotiidae from Cambrian strata

724 of SE Australia (Crustacea, Cirripedia). Senckenberg lethaia 57: 1-33.

725

726 Broderip WJ. 1833. New species accompanied by characters. Proceedings of the

727 Zoological Society of London 1: 4-8.

728

729 Billings E. 1871. Proposed new genus of Pteropoda [Hyolithellus]. Canadian Nature

730 and Geology 6, pp. 240.

731

732 Budd GE, Jackson IS. 2016. Ecological innovations in the Cambrian and the origins of

733 the crown group phyla. Philosophical Transactions of the Royal Society B, 371(1685).

734 Doi:10.1098/rstb.2015.0287.

735

736 Caron JB, Smith MR, Harvey TH. 2013. Beyond the Burgess Shale: Cambrian

737 microfossils track the rise and fall of hallucigeniid lobopodians. Proceedings of the

738 Royal Society B: Biological Sciences 280: 20131613.

739

740 Checa, AG, Jiménez-Jiménez, AP. 1998. Constructional morphology, origin, and

741 evolution of the gastropod operculum. Paleobiology 24: 109-132.

742

743 Chen J-Y, Hou X-G, Lu H-Z. 1989. Early Cambrian netted scale-bearing wormlike

744 sea animal. Acta Paleontologica Sinica 28: 1-16.

745 746 Chelazzi G. 1982. Behavioural adaptation of the gastropod Nerita polita L. on different

747 shores at Aldabra Atoll. Proceedings of the Royal Society B: Biological Sciences 215:

748 451-467

749

750 Colmenar J, Harper DA, Villas E. 2014. Morphofunctional analysis of Svobodaina

751 species (Brachiopoda, Heterorthidae) from south‐ western Europe. Palaeontology 57:

752 193-214.

753

754 Conway Morris S, Peel JS. 1990. Articulated halkieriids from the Lower Cambrian of

755 north Greenland. Nature 345: 802-805.

756

757 Conway Morris S, Chapman AJ. 1997. Mobergellans from the Lower Cambrian of

758 Mongolia, Sweden, and the United States: molluscs or opercula of incertae sedis?

759 Journal of Paleontology 71: 968-984.

760

761 Costa PMS, Pastorino G. 2012. New Naticidae (Gastropoda) from Brazil. The

762 Nautilus 126: 25-32.

763

764 Daley AC, Drage HB. 2015. The fossil record of ecdysis, and trends in the moulting

765 behaviour of trilobites. Arthropod structure and development

766 doi:10.1016/j.asd.2015.09.004

767

768 de Freitas Tallarico L, Passos FD, Machado FM, Campos A, Recco-Pimentel SM,

769 Introíni GO. 2014. Bivalves of the São Sebastião Channel, north coast of the São Paulo

770 State, Brazil. Check List 10: 97-105. 771

772 Demidenko YE. 2004. New data on the sclerite morphology of the tommotiid species

773 Lapworthella fasciculata. Paleontological Zhurnal 38: 134-140

774

775 Demidenko YE, Parkhaev PY, Rozanov AY. 2012. Morphological variability and

776 types of preservation of Mobergella radiolata – a potential index species for the GSSP

777 of Cambrian Stage 3. In: Zhao YL, Zhu MY, Peng J, Gaines RR, Parsley RL eds.

778 Cryogenian-Ediacaran to Cambrian Stratigraphy and Paleontology of Guizhou, China.

779 Journal of Guizhou University, Natural Science 29, pp. 157-158.

780

781 Devaere L, Clausen S, Monceret E, Tormo N, Cohen H, Vachard D. 2014.

782 Lapworthellids and other skeletonised microfossils from the Cambrian Stage 3 of the

783 northern Montagne Noire, southern France. Annales de Paléontology 100: 175-191.

784

785 Dill LM, Fraser AH. 1997. The worm re-turns: hiding behavior of a tube-dwelling

786 marine polychaete, Serpula vermicularis. Behavioural Ecology 8: 186-193.

787

788 Dillon RT. 2000. The ecology of freshwater molluscs. Cambridge University Press.

789

790 Dzik J. 2010. Brachiopod identity of the alleged monoplacophoran ancestors of

791 cephalopods. Malacologia 52: 97-113.

792

793 Erwin DH, Valentine JW. 2013. The Cambrian Explosion. Roberts and Company

794 Publishers, Colorado.

795 796 Evans KR, Rowell AJ. 1990. Small shelly fossils from Antarctica: an Early Cambrian

797 faunal connection with Australia. Journal of Paleontology 64: 692-700.

798

799 Gaspar MB, Santos MN, Vasconcelos P, Monteiro CC. 2002. Shell morphometric

800 relationships of the most common bivalve species (Mollusca: Bivalvia) of the Algarve

801 coast (southern Portugal). Hydrobiologia 477: 73-80

802

803 Gosling E. 2008. Bivalve molluscs: biology, ecology and culture. John Wiley & Sons.

804

805 Gould SJ. 1971. Muscular mechanics and the ontogeny of swimming in scallops.

806 Palaeontology 14: 61-94.

807

808 Hall J. 1872. Notes on some new or imperfectly known forms among the Brachiopoda.

809 etc. - Annual Report to the Regents of the University of the State of New York, Report

810 23, Appendix G: 244-247.

811

812 Haney RA, Mitchell CE, Kim K. 2001. Geometric morphometric analysis of patterns

813 of shape change in the Ordovician brachiopod Sowerbyella. Palaios 16: 115-125.

814

815 Hammer Ø, Harper DAT, Ryan PD. 2001. PAST-Palaeontological statistics. www.

816 uv. es/~ pardomv/pe/2001_1/past/pastprog/past. pdf, acessado em, 25(07), 2009.

817

818 Harvey TH, Dong X, Donoghue PC. 2010. Are palaeoscolecids ancestral

819 ecdysozoans? Evolution and Development 12: 177-200.

820 821 Hasegawa K, Rosenberg GA. 2011. Taxonomic Note on Natica saitoi Kuroda and

822 Habe in Kuroda, Oyama and Habe, 1971 (Gastropoda, Naticidae). Bulletin of the

823 National Museum of Natural Sciences, Series A 37: 113-119.

824

825 Hayami I, Okamoto T. 1986. Geometric regularity of some oblique sculptures in

826 pectinid and other bivalves: recognition by computer simulations. Paleobiology 433-

827 449.

828

829 Hedström H. 1923. On "Discinella holsti Mbg." and Scapha antiquissima

830 (Markl.) of the division Patellacea. Sveriges Geological Underskrift (C) 314: 1-13

831

832 Hedström H. 1930. Mobergella versus Discinella; Paterella versus Scapha &

833 Archaeophiala (some questions on nomenclature). Sveriges Geological Underskrift (C

834 362: 1-8.

835

836 Horný RJ. 2004. Kosovina, a new Silurian tryblidiid genus (Mollusca, Tergomya) from

837 Bohemia (Czech Republic). Acta Musei Nationalis Pragae, Series B, Natural History

838 60: 143-148.

839

840 Hove ten, HA, Kupriyanova EK. 2009. Taxonomy of Serpulidae (Annelida,

841 Polychaeta): the state of affairs. Zootaxa 2036: 1-126.

842

843 Hughes RN. 1985. Predatory behaviour of Natica unifasciata feeding intertidally on

844 gastropods. Journal of Molluscan Studies 51: 331-335.

845 846 Hunt S. 1976. The gastropod operculum: a comparative study of the composition of

847 gastropod opercular proteins. Journal of Mollusca Studies 42: 251-260.

848

849 Inoue K, Hayes DM, Harris JL, Christian AD. 2013. Phylogenetic and morphometric

850 analyses reveal ecophenotypic plasticity in freshwater mussels Obovaria jacksoniana

851 and Villosa arkansasensis (Bivalvia: Unionidae). Ecology and Evolution 3: 2670-2683

852

853 Ippolitov AP, Vinn O, Kupriyanova EK, Jäger M. 2014. Written in stone: history of

854 serpulid polychaetes through time. Memoirs of the Museum of Victoria 71: 123-159.

855

856 Kaim A, Sztajner P. 2005. The opercula of neritopsid gastropods and their

857 phylogenetic importance. Journal of Molluscan Studies 71: 211-219.

858

859 Koneva SP. 1983. Mobergella iz nizhnego kembriya seletinskogo sinklinoriya. In:

860 Apollonov MK, Bandaletv SM, Ivshin NK eds. Stratigrafiya i paleontologiya niznego

861 paleozoya Kazakhstana, Alma-Ata (Akademiya Kazakhskoj SSR), pp. 110-112

862

863 Krzywinski M, Altman N. 2014. Points of significance: Visualizing samples with box

864 plots. Nature methods 11: 119-120.

865

866 Landing E, Bartowski KE. 1996. Oldest shelly fossils from the Taconic allochthon

867 and late Early Cambrian sea-levels in eastern Laurentia. Journal of Paleontology 70:

868 741-761.

869 870 Larsson CM, Skovsted CB, Brock GA, Balthasar U, Topper TP, Holmer LE. 2014.

871 Paterimitra pyramidalis from South Australia: scleritome, shell structure and evolution

872 of a lower Cambrian stem group brachiopod. Palaeontology 57: 417-446.

873

874 Leighton LR. 2003. Predation on brachiopods. In: Kelly PH, Kowalewski, N, Hansen

875 TA eds. Predator—Prey Interactions in the Fossil Record, Springer US, pp. 215-237.

876

877 Lemche H. 1957. A new living deep-sea mollusc of the Cambro-Devonian class

878 Monoplacophora. Nature 179: 413-416.

879

880 Lemche H, Wingstand KG. 1959. The anatomy of Neopilina galatheae Lemche, 1957

881 (Mollusca, Tryblidiacea). Galathea Report, Copenhagen 3: 9-71.

882

883 Li G, Xiao S. 2004. Tannuolina and Micrina (Tannuolinidae) from the Lower

884 Cambrian of eastern Yunnan, South China, and their scleritome reconstruction. Journal

885 of Paleontology 78: 900-913

886

887 Lindberg DR. 2009. Monoplacophorans and the origin and relationships of mollusks.

888 Evolution: Education and Outreach 2: 191-203.

889

890 Lindström G. 1880. Fragmenta silurica. Samson & Wallin, Stockholm.

891

892 Linnaeus CV. 1758. Systema naturae per regna tria naturae, secundum classes, ordines,

893 genera, species, cum characteribus, differentiis, synonymis, locis. Ed. 10, Tomus 1. L.

894 Salvii, Stockholm, Sweden, pp. 823. 895

896 Linnaeus CV. 1771. Regni animalis, Appendix. Insecta, pp. 529-543.

897

898 Martí Mus M, Bergström J. 2005. The morphology of hyolithids and its functional

899 implications. Palaeontology 48:1139-1167

900

901 Martí Mus M, Bergström J. 2007. Skeletal microstructure of helens, lateral spines of

902 hyolithids. Palaeontology 50: 1231-1243.

903

904 Martí Mus M, Jeppsson L, Malinky JM. 2014. A complete reconstruction of the

905 hyolithid skeleton. Journal of Paleontology 88: 160-170.

906

907 Missarzhevsky VV 1989. Drevnejshie skeletnye okamenelosti i stratigrafiya

908 pogranichnykh tolshch dokembriya i kembriya. Trudy Geolog Inst AN SSSR 443: 1-237.

909

910 Moberg JC. 1892. Omen nyupptickt fauna i block af kambrisk sandsten, insamlade av

911 Dr. N.O. Holst. Geol Frren i Stockholm Forhan 14: 103-120.

912

913 Morton B, Scott PJB. 1980. Morphological and functional specializations of the shell,

914 musculature and pallial glands in the Lithophaginae (Mollusca: Bivalvia). Journal of

915 Zoology 192: 179-203.

916

917 Müller K, Hinz-Schallreuter I. 1993. Palaeoscolecid worms from the Middle

918 Cambrian of Australia. Palaeontology 36: 549-591.

919 920 Nielsen C. 2012. Animal evolution: interrelationships of the living phyla. Oxford

921 University Press

922

923 Peel JS. 1977. Relationship and internal structure of a new Pilina (Monoplacophora)

924 from the late Ordovician of Oklahoma. Journal of Paleontology 51: 116-122.

925

926 Pimenta AD, do Couto DR, Santos Costa PM. 2008. A new species and a new record

927 of Muricidae (Gastropoda) from Brazil: genera Pterynotus and

928 Leptotrophon. Nautilus 122: 244-251.

929

930 Poulsen V. 1963. Notes on Hyolithellus Billings, 1871, Class Pogonophora Johannson,

931 1937. i kommission hos Ejnar Munksgaard.

932

933 Purchon RD. 2013. The biology of the Mollusca. Elsevier.

934

935 Rouse G, Pleijel F. 2001. Polychaetes. Oxford University Press. London.

936

937 Rozanov AY, Zhuravlev AY. 1992. The Lower Cambrian fossil record of the Soviet

938 Union. In: Lipps JH, Signor PW eds. Origin and early evolution of the Metazoa, New

939 York (Plenum Press), pp. 205-282.

940

941 Rudwick MJS. 1970. Living and Fossil Brachiopods. London: Hutchinson and Co.

942

943 Ruthensteiner B, Schröpel V, Haszprunar G. 2010. Anatomy and affinities of

944 Micropilina minuta Warén, 1989 (Monoplacophora: Micropilinidae). Journal of 945 Molluscan Studies 76: 323-332.

946

947 Schwabe E. 2010. Illustrated summary of chiton terminology. Spixiana 33: 171-194.

948

949 Simone LR. 2014. Taxonomic study on the molluscs collected during the Marion-

950 Dufresne expedition (MD55) off SE Brazil: the Naticidae (Mollusca: Caenogastropoda).

951 Zoosystema 36: 563-93.

952

953 Skovsted CB. 2003. Mobergellans (problematica) from the Cambrian of Greenland,

954 Siberia and Kazakhstan. Paläontogische Zeitschrift 77: 429-443.

955

956 Skovsted CB. 2006. Small Shelly fauna from the upper Lower Cambrian Bastion and

957 Ella Island formations, north-east Greenland. Journal of Paleontology 80: 1087-1112.

958

959 Skovsted CB, Brock GA, Paterson JR, Holmer LE, Budd GE 2008. The scleritome

960 of Eccentrotheca from the Lower Cambrian of South Australia: lophophorate affinities

961 and implications for tommotiid phylogeny. Geology 36: 171-174.

962

963 Skovsted CB, Holmer LE, Larsson CM, Högström AE, Brock GA, Topper TP,

964 Balthasar U, Petterson Stolk, S, Paterson, JR. 2009. The scleritome of Paterimitra:

965 an Early Cambrian stem group brachiopod from South Australia. Proceedings of the

966 Royal Society of London B: Biological Sciences, rspb-2008

967

968 Skovsted CB, Brock GA, Topper TP, Paterson JR, Holmer LE. 2011. Scleritome

969 construction, biofacies, biostratigraphy and systematics of the tommotiid Eccentrotheca 970 helenia sp. nov. from the early Cambrian of South Australia. Palaeontology 54: 253-

971 286.

972

973 Skovsted CB, Peel JS. 2011. Hyolithellus in life position from the Lower Cambrian of

974 North Greenland. Journal of Paleontology 85: 37-47.

975

976 Skovsted C, Betts M, Topper T, Brock GAB 2015. The early Cambrian tommotiid

977 genus Dailyatia from South Australia. Memoirs of the Association of Australasian

978 Palaeontologists 48: 1-117.

979

980 Spencer LM. 1980. Paleoecology of a Lower Cambrian archaeocyathid inter-reef fauna

981 from southern Labrador, pp.153, pls. 7.

982

983 Spitzer M, Wildenhain J, Rappsilber J, Tyers M. 2014. BoxPlotR: a web tool for

984 generation of box plots. Nature methods 11: 121-122.

985

986 Stanley SM. 1970. Relation of shell form to life habits of the Bivalvia (Mollusca) (Vol.

987 125). Geological Society of America.

988

989 Stanley SM. 1972. Functional morphology and evolution of byssally attached bivalve

990 mollusks. Journal of Paleontology 46: 165-212.

991

992 Streng M, Skovsted, CB. 2006. A new mobergellan (small shelly fossils) from the

993 early Middle Cambrian of Morocco and its significance. Paläontologische Zeitschrift,

994 80: 209-220. 995

996 Todt C, Okusu A, Schander C, Schwabe E. 2008. Solenogastres, Caudofoveata and

997 Polyplacophora. In: Ponder WF, Lindberg DR eds. Phylogeny and Evolution of the

998 Mollusca, pp. 71-96.

999

1000 Topper TP, Brock GA, Skovsted CB, Paterson JR 2009. Shelly Fossils from the

1001 Lower Cambrian Pararaia bunyerooensis Zone, Flinders Ranges, South Australia.

1002 Memoirs of the Association of Australasian Palaeontologists 37: 199-246.

1003

1004 Topper TP, Brock GA, Skovsted CB, Paterson JR. 2010. Palaeoscolecid scleritome

1005 fragments with Hadimopanella plates from the early Cambrian of South Australia.

1006 Geological Magazine 147: 86-97.

1007

1008 Topper TP, Brock GA, Skovsted CB, Paterson JR. 2011. Microdictyon plates from

1009 the lower Cambrian Ajax Limestone of South Australia: Implications for species

1010 taxonomy and diversity. Alcheringa 35: 427-443.

1011

1012 Topper TP, Skovsted CB, Peel JS, Harper DA. 2013. Moulting in the lobopodian

1013 Onychodictyon from the lower Cambrian of Greenland. Lethaia 46: 490-495.

1014

1015 Topper TP, Strotz LC, Holmer LE, Zhang Z, Tait NN, Caron JB. 2015.

1016 Competition and mimicry: the curious case of chaetae in brachiopods from the middle

1017 Cambrian Burgess Shale. BMC Evolutionary Biology 15(1). Doi:10.1186S12862-015-

1018 0314-4.

1019 1020 Vannier J, Steiner M, Renvoisé E, Hu SX, Casanova JP. 2007. Early Cambrian

1021 origin of modern food webs: evidence from predator arrow worms. Proceedings of the

1022 Royal Society of London B: Biological Sciences 274: 627-633.

1023

1024 Vendrasco MJ, Eernisse DJ, Powell II, Fernandez CZ. 2012. Polyplacophora

1025 (Mollusca) from the San Diego Formation: a remarkable assemblage of fossil chitons

1026 from the Pliocene of Southern California. Natural History Museum of Los Angeles

1027 County Contributions to Science 520: 15-72

1028

1029 Vermeij GJ. 1971. Temperature relationships of some tropical Pacific intertidal

1030 gastropods. Marine Biology 10: 308-314.

1031

1032 Vermeij GJ. 1973. Morphological patterns in high-intertidal gastropods: adaptive

1033 strategies and their limitations. Marine Biology 20: 319-346.

1034

1035 Vermeij GJ. 1976. Interoceanic differences in vulnerability of shelled prey to crab

1036 predation. Nature 260: 135-136.

1037

1038 Vermeij GJ. 1977. The Mesozoic marine revolution: evidence from snails, predators

1039 and grazers. Paleobiology 3: 245-258.

1040

1041 Vermeij GJ. 1987. The dispersal barrier in the tropical Pacific: implications for

1042 molluscan speciation and extinction. Evolution 41: 1046-1058.

1043 1044 Vermeij GJ, Williams ST. 2007. Predation and the geography of opercular thickness

1045 in turbinid gastropods. Journal of Molluscan Studies 73: 67-73.

1046

1047

1048 Waller TR. 1981. Functional morphology and development of veliger larvae of the

1049 European oyster, Ostrea edulis. Smithsonian Contributions to Zoology 328: 1-70.

1050

1051 Williams A, Carlson SJ, Brunton CHC. 2000. Classification. In: Kaesler RL ed.

1052 Treatise on invertebrate paleontology, vol. H. Kansas: The Geological Society

1053 of America; 2000. pp. 1–29.

1054

1055 Williams, A. Holmer, LE. 2002. Ornamentation and inferred growth, functions and

1056 affinities of the sclerites of the problematic Micrina. Palaeontology 45: 845-873.

1057

1058 Wrona R 1987. Cambrian microfossil Hadimopanella Gedik from glacial erratics in

1059 West Antarctica. Palaeotologica Polonica 49: 37-48.

1060

1061 Wrona R. 2004. Cambrian microfossils from glacial erratics of King George Island,

1062 Antarctica. Acta Palaeonlogica Polonica 49: 13-56.

1063

1064 Yochelson EL. 1958. Some lower Ordovician monoplacophoran molluscs from

1065 Missouri. Journal of the Washington Academy of Sciences 48: 8–14.

1066

1067 Zhang XG, & Aldridge RJ. 2007. Development and diversification of trunk plates of

1068 the Lower Cambrian lobopodians. Palaeontology 50: 401-415. 1069

1070 Figure 1. Selected taxa investigated in this study. A, Mobergella holsti (Moberg, 1892),

1071 SMNH X5780, Cambrian of Sweden, scale bar 1 mm. B, Discinella micans (Hall,

1072 1872), SMNH X5781, Cambrian of North-East Greenland, scale bar 1 mm. C, Chiton

1073 tuberculatus Linnaeus, 1758, SMNH Mo181927, Santa Marta, Columbia. D, Ostrea

1074 edulis Linnaeus, 1758, SMNH 149696, the Netherlands, Yerseke, scale bar 3 cm. E,

1075 Pseudomussium peslutrae (Linnaeus, 1771), SMNH 57951, Skagerrak Sweden, off

1076 Idefjord, Säcken, depth 60-180 m, scale bar 1.5 cm. F, Tellina lineata Turton, 1819,

1077 SMNH. G, Lepidonotus squamatus (Linnaeus, 1758), SMNH 113623, west coast of

1078 Sweden, scale bar 1 mm. H, Opercula of Nerita peloronta Linnaeus, 1758, SMNH

1079 149950, Caribbean Sea, Saint Barthelemy. I, Discinisca lamellosa (Broderip, 1833),

1080 SMNH Br150543, Namibia. J, Patella sp., SMNH Mo181928, Western Australia. K,

1081 Tryblidium reticulatum Lindström 1880, SMNH Mo150432, Silurian of Sweden. All

1082 scale bars 1 cm unless otherwise stated. Taxa incorporated in the analysis from

1083 published reports are not illustrated here.

1084

1085 Figure 2. Discinella micans (Hall, 1872). A, SEM image of Discinella, SMNH X5781.

1086 B, Schematic example of outlining the total inner surface and the respective muscle

1087 scars. Scale bar 1 mm.

1088

1089 Figure 3. Box and beanplot results of the percentage of the inner surface occupied by

1090 muscle scars A Boxplot. Center lines show the medians; box limits indicate the 25th

1091 and 75th percentiles as determined by R software; whiskers extend 1.5 times the

1092 interquartile range from the 25th and 75th percentiles, outliers are represented by dots;

1093 crosses represent sample means; bars indicate 95% confidence intervals of the means. n 1094 = 25 for all taxa, except Angarella, Neopilina, Pilina, Proplina, Tryblidium and

1095 Kosovina (n=1), Gompholites (n=5), Natica (n=6) and Callistochiton (tail) and

1096 Callistochiton (int.), both (n=4). Tail and int. refer to the tail valves and intermediate

1097 valves of Callistochiton. B Beanplot. Black lines show the means; white lines represent

1098 individual data points; polygons represent the estimated density of the data.

1099

1100 Figure 4. Co-occuring tubular fragments with Mobergella holsti from the Cambrian of

1101 Sweden and flat to concave Discinella micans specimens from the Cambrian of

1102 Labrador, Canada. A, Tubular fragment, SMNH X5784. B-C, Tubular fragment,

1103 SMNH X5785. D Mobergella, SMNH X5782. E, lateral view of Discinella, SMNH

1104 X5783, F, lateral view of Discinella, SMNH X5786. G, lateral view of concave

1105 Discinella, SMNH X5787. All scale bars 1 mm.

1106

1107 Table 1. Table showing distribution, abundance and lithological details of the strata that

1108 mobergellan species have been documented from.

1109

1110 SUPPORTING INFORMATION

1111

1112 Table S1. Detailed statistics of the dataset, including mean, median and whisker length

1113 and quartile length of investigated taxa.

1114

1115 Table S2. A Results from a one-way ANOVA test. B Pairwise comparison results from

1116 a Tukey’s Honestly Significant Difference (HSD) test. Tukey’s Q below the diagonal,

1117 p-value above the diagonal. Significant comparisons are in light blue. a

b A Neopilina + Angarella + Pilina + Tryblidium + Kosovina + Proplina + Ostrea + Pseudomussium + Tellina + Patella + Discinisca + Callistochiton (int.) + Callistochiton (tail) + Chiton + Lepidonotus + Gompholites + Nerita + Nacita + Mobergella + Discinella +

5 10 15 20 25

B Neopilina Angarella Pilina Tryblidium Kosovina Proplina Ostrea Pseudomussium Tellina Patella Discinisca Callistochiton (int.) Callistochiton (tail) Chiton Lepidonotus Gompholites Nerita Nacita Mobergella Discinella

5 10 15 20 25