Food web reconstruction through isotopic compositions of fossil faeces: insights into the ecology of a late Barremian freshwater ecosystem (Las Hoyas, Cuenca, Spain)

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(1)Repositorio Institucional de la Universidad Autónoma de Madrid https://repositorio.uam.es. Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in:. Cretaceous Research 108 (2020): 104343 DOI: https://doi.org/10.1016/j.cretres.2019.104343. Copyright: © 2019 Elsevier Ltd.. El acceso a la versión del editor puede requerir la suscripción del recurso Access to the published version may require subscription.

(2) 1. The late Barremian ecosystem of Las Hoyas sustained by fishes and. 2. shrimps as inferred from coprofabrics. 3 4. Sandra Barrios-de Pedro1*, Karen Chin2 and Ángela D. Buscalioni1. 5. 1. 6. Autónoma de Madrid. Calle Darwin 2, 28049, Cantoblanco, Madrid (Spain); e-mails:. 7. [email protected], [email protected].. 8. 2. 9. CO, 80309, USA; e-mail: [email protected].. 10. * Corresponding author: [email protected]. Biology Department (Palaeontology) and Centro para la Integración en Paleobiología (CIPb), Universidad. Department of Geological Sciences and Museum of Natural History, University of Colorado Boulder, Boulder,. 11 12. Abstract. 13. Las Hoyas, a locality of the La Huérguina Formation in the southwestern Iberia Basin. 14. (Cuenca province, Spain), is a well-known late Barremian Konservat-Lagerstätte highly rich. 15. in coprolites that have a wide variety of morphologies. Thin sections of twenty specimens. 16. were prepared from seven coprolite morphotypes. All of the examined coprolites exhibit a. 17. microcrystalline calcium phosphate groundmass lacking recrystallization and containing fine. 18. inclusions. Three different coprofabrics have been characterized and reflect the diet and. 19. digestive processes of the animal producers. The fish scale coprofabric is formed by ganoine. 20. scales likely attributed to Semionotiformes; the decapod cuticle coprofabric includes. 21. exoskeletal fragments which might be assigned to Atydae shrimps and Astacidae crayfish. 22. prey. A combination of decapod cuticles and fish scales can occasionally occur in the same. 23. coprolite. The third coprofabric, contains few or no inclusions, and likely denotes the. 24. presence of coprolite producers with very efficient digestive systems. The exceptional. 25. preservation of the Las Hoyas coprolites reveals the trophic importance of small crustaceans. 26. with thin and delicate exoskeletons, a food resource that is not usually evident in coprolites.. 27. The number of coprofabrics relative to the number of possible fecal producers is low, and.

(3) 28. may reflect taphonomic biases. Nevertheless, the coprofabric contents demonstrate that. 29. decapods and fishes were important food resources in this Barremian ecosystem.. 30 31. Keywords. 32. Coprolites, decapod cuticle, exceptional preservation, ganoid scales, trophic interactions.. 33 34 35. 1. Introduction Coprolites represent fossilized residues from the digestive tract of animals, so they. 36. provide information about recently ingested food, the diet of the producer, its feeding. 37. strategy, predator-prey interactions, and prey availability in an ecosystem (Chin et al., 1998,. 38. 2003; Rodríguez-de la Rosa et al., 1998; Ghosh et al., 2003; Northwood, 2005; Richter and. 39. Wedmann, 2005; Schweizer et al., 2006, 2007; Chin, 2007; Khosla et al., 2015; Schwimmer. 40. et al., 2015, Jia et al., 2016). In addition, recent integrative studies, which combine. 41. morphological and analytical procedures, have renewed analyses of geochemical and. 42. molecular evidence within coprolites (Eriksson et al., 2011; Fiorelli et al., 2013; Zatón et al.,. 43. 2015; Super et al., 2018; Qvarnström et al., 2019a).. 44. Importantly, exceptional Mesozoic localities have brought further advantages in the. 45. study of coprolite assemblages, because they (1) preserve a wide variety of morphotypes. 46. within individual coprolite assemblages; (2) provide information on environmental. 47. (sedimentary and taphonomic) biases; and (3) can be related to biodiversity, offering new. 48. information and perspectives about the feeding functionality of an ancient ecosystem.. 49. Integrating taphonomic and palaeoecological evidence from coprolite assemblages sheds light. 50. on relationships between environmental conditions and faecal producers (see for instance,. 51. Rodríguez-de la Rosa et al., 1998; Chin, 2007; Chin et al., 2008, Eriksson et al., 2011;. 52. Niedźwiedzki et al., 2016; Luo et al., 2017, and Segesdi et al., 2017, for several Mesozoic. 53. coprolite assemblages). Thus, the study of coprolite assemblages provides a more.

(4) 54. comprehensive approach to interpreting paleoecosystems than studying isolated coprolite. 55. specimens.. 56 57. The well-known Early Cretaceous Konservat-Lagerstätte of Las Hoyas (La Huérguina. 58. Formation, southwestern Iberian Basin) has yielded almost 2000 coprolites of notable. 59. variation in shapes that have been assigned to twelve morphotypes (Barrios-de Pedro et al.,. 60. 2018; see Fig. 1). Previous analyses based on non-destructive techniques allowed us to. 61. characterize their morphologies and their taphonomic properties (Barrios-de Pedro et al.,. 62. 2018), whereas the study of the features of undigested organic material (abundance of. 63. undigested food, identification of inclusions, size and arrangement, and the effects of the. 64. action of the digestive process) allowed us to explore the digestive strategies of the Las Hoyas. 65. coprolite producers (Barrios-de Pedro and Buscalioni, 2018; see below). Herein, we undertake. 66. the study of thin sections of twenty coprolites, corresponding to seven of the twelve described. 67. morphotypes. Petrographic observations and electron microprobe analyses have been used to. 68. describe the Las Hoyas coprolite coprofabrics. We aim to understand aspects of their. 69. diagenetic processes to evaluate their preservation, and to infer the dominant trophic. 70. interactions of this Barremian freshwater ecosystem. The results are discussed in the context. 71. of the feeding ecology of the Las Hoyas ecosystem to determine whether the coprolite. 72. assemblage can be correlated with the body fossil record, and to formulate hypotheses about. 73. the trophic characteristics of the biota.. 74 75 76. 2. The Las Hoyas geological and palaeoenvironmental settings The upper Barremian Las Hoyas basin is located in the Serranía de Cuenca, which is. 77. part of the Iberian Chain (Fig. 2). The Iberian Chain resulted from the Alpine tectonic. 78. inversion of the Iberian Basin; an Upper Permian–Lower Cretaceous intracratonic extensional. 79. rift basin. The Iberian Basin experienced several rifting events related to the opening of the. 80. central Atlantic and the rotation of the Iberian Plate (Salas and Casas, 1993; Salas et al., 2001;.

(5) 81. Martín-Chivelet et al., 2019). The Upper Jurassic–Lower Cretaceous second rift induced the. 82. development of four palaeogeographic domains in the Iberian Basin (Soria et al., 2000):. 83. Cameros, Central Iberian, Maestrazgo, and Southwestern Iberian. Extensional tectonics in. 84. turn divided each domain into many basins of graben and half-graben type. Las Hoyas was. 85. one of the half-graben type basins into which the Southwestern Iberian Basin was divided. 86. (Fregenal-Martínez, 1998; Fregenal-Martínez and Meléndez, 2000, 2016; Fregenal-Martínez. 87. et al., 2014), and was filled with upper Barremian continental deposits from alluvial,. 88. lacustrine, and palustrine sedimentary settings. The thickest record of these non-marine. 89. deposits (400 m) is found in the Las Hoyas basin. This succession has been recently redefined. 90. as a single vertical sequence made up of two lithostratigraphic units laterally related by a. 91. change of facies: the Tragacete and the La Huérguina Formations (Fregenal-Martínez et al.,. 92. 2017). The Las Hoyas fossil locality forms part of the La Huerguina Formation, which is. 93. primarily composed of a variety of limestone facies, with minor amounts of marl, and sandy. 94. limestone. It has been dated as late Barremian in age (125–127 Ma) based on the charophyte. 95. and ostracod associations (Diéguez et al., 1995; Fregenal-Martínez et al., 2017).. 96 97. The fossiliferous locality of Las Hoyas consists of laminated limestones produced in. 98. the context of a continental (freshwater) subtropical, seasonal, carbonate wetland that overlay. 99. a low-relief karstic terrain. During seasonal flooding and prolonged wet periods lacustrine. 100. sedimentation dominated, including chemical and bioinduced precipitation of calcium. 101. carbonate, and accumulation of thin microbial mats. Dense microbial mats and laminae of. 102. very fine detrital carbonate sediments transported during occasional floods characterized. 103. sedimentation during dry periods when the water column was reduced to probably just a few. 104. centimetres (Fregenal-Martínez and Meléndez, 2000, 2016; Gupta et al., 2008; Buscalioni and. 105. Fregenal-Martínez, 2010; Fregenal-Martínez et al., 2017).. 106.

(6) 107. At a regional scale this Barremian inland wetland was drained by carbonate-rich. 108. freshwater, and comprised a typical environmental mosaic of swampy plains, ponds, lakes,. 109. ephemeral channels, and sloughs, where the Las Hoyas locality would have been a ponding. 110. area. Sediments of wetter periods record the maximum biodiversity of Las Hoyas, although. 111. the sediments of dry periods contain the best preserved and most abundant fossils (Buscalioni. 112. and Fregenal-Martínez, 2010). Previous studies conclude that this difference reflects greater. 113. environmental stress, and the greater prevalence of microbial mats during dry periods that, in. 114. turn, facilitated the rapid burial and the establishment of conditions favourable for. 115. preservation (Martín-Closas, 1999; Gupta et al., 2008; Iniesto et al., 2013, 2015, 2016; Briggs. 116. et al., 2016;). Phosphatization assisted by microbial mats results in extraordinary fossilization. 117. in Las Hoyas with fossils preserving different types of soft tissues such as eyes, muscles, skin,. 118. and several integumentary structures (Martin et al., 2015; Navalón et al., 2015; Poyato-Ariza. 119. and Buscalioni, 2016).. 120 121. The Las Hoyas locality is well-known for the abundance and diversity of plants and. 122. animals. The fossil association at this site is clearly biased towards the abundance and. 123. diversity of obligate aquatic animals, in order of abundance represented by ostracods,. 124. decapods, fishes, aquatic insects, with fewer salamanders and frogs. In comparison,. 125. amphibious organisms (crocodilians are most common), and terrestrial groups. 126. (albanerpetontids, squamates, and dinosaurs) are much less abundant (Buscalioni et al., 2016).. 127. Las Hoyas has yielded a great diversity of plants and animals, estimated to represent 117. 128. families and around 200 species (Buscalioni and Poyato-Ariza, 2016). Animals constitute. 129. approximately 77 % of the total diversity at the species level, with hexapod arthropods. 130. constituting 36 % of the total number of species, and vertebrates 23 %. Among the later, the. 131. actinopterygian fishes (Macrosemiidae, Semionotiformes, Pycnodontiformes, Amiiformes,. 132. and Teleosts) are by far the most diverse (Buscalioni et al., 2016). Fossil burrows in the. 133. deposit are categorized as a particular case of a Mermia ichnoassemblage (Gibert et al., 2016),.

(7) 134. attributed to benthic invertebrate activity, mainly of crustaceans, worms, and insect larvae set. 135. in continental environments (Buatois and Mangano, 2011).. 136 137 138. 3. Coprolites from Las Hoyas: features and queries The coprolite assemblage includes a notable variety of coprolite morphologies, and. 139. twelve different morphotypes have been described: spiral, circular, irregular, elongated,. 140. rosary, ellipsoidal, cylinder, bump-headed lace, fir-tree, cone, straight, and thin lace (Barrios-. 141. de Pedro et al., 2018; see Fig. 1). The specimens are skewed towards rather small sizes, with a. 142. common length between 10–40 mm. In previous studies, non-destructive chemical analysis. 143. with energy dispersive X-ray analysis (EDX) on an environmental scanning electron. 144. microscope was used to characterize their elemental composition and documented a calcium-. 145. phosphate coprolite matrix (Barrios-de Pedro et al., 2018). The taphonomic features of the. 146. coprolites established that their high integrity, absence of desiccation marks, and volume were. 147. congruent with faeces produced and deposited in an aquatic ecosystem. Most of the samples. 148. (96 %) were collected from sedimentary layers related to low water-level conditions and. 149. linked to the presence of microbial mats (Buscalioni and Fregenal-Martínez, 2010; Barrios-de. 150. Pedro et al., 2018). Consequently, the Las Hoyas coprolites are likely autochthonous.. 151 152. SEM and macroscopic analyses of digestive features observed on the coprolite. 153. inclusions (i.e., pitting, corrosion lines, shape of the fractures at the ends, together with the. 154. arrangement, number, and size of undigested remains) were used to order and cluster the. 155. coprolite morphotypes (Barrios-de Pedro and Buscalioni, 2018). Non-metric. 156. multidimensional scaling ordination, an indirect gradient analysis approach which produces. 157. an ordination and attempts to represent the pairwise dissimilarity between objects in a low-. 158. dimensional space, identified three groups. Each group shares a set of digestive alterations. 159. that appear to represent three distinct digestive strategies: ingestion of abundant prey with. 160. limited oral processing, and limited or less effective acid secretions (DS1); ingestion of prey.

(8) 161. with mastication prior to deglutition and/or food fractioning (DS2); and mastication and/or. 162. food fractioning with more effective acid secretions (DS3). The DS1 strategy corresponds to. 163. coprolites with smaller, compressed diameters, including flattened and long shapes, such as. 164. fir-tree, rosary, ellipsoidal, and the lace morphotypes; the DS2 strategy includes circular and. 165. cylinder coprolites; and the DS3 strategy is linked with cone and elongated morphotypes. The. 166. spiral coprolites may represent a form of digestive processing that was intermediate between. 167. the second and the third strategies. Some specimens without visible inclusions (big irregular. 168. and cylinder coprolites) were not classified into any of the proposed digestive strategies by. 169. Barrios-de Pedro and Buscalioni (2018), but are likely linked to mastication of the prey, long. 170. retention time of food, and/or highly effective digestion. This form of digestive processing is. 171. herein characterized as the DS4 strategy.. 172 173. This study seeks to understand the trophic functional dynamics of the Las Hoyas. 174. ecosystem by examining the potential biases of the coprolite association through. 175. characterization of the petrographic fabrics of the coprolites (coprofabrics). We consider the. 176. following questions: (1) is there a clear correspondence between the coprofabrics and. 177. coprolite morphotypes? (2) is there congruence between coprofabrics and feeding strategies?. 178. (3) can the various coprolite morphotypes and coprofabrics be reasonably correlated with the. 179. Las Hoyas body fossil record? To address the last question, we have used the Las Hoyas. 180. database updated in 2019, and the coprolite data from the Las Hoyas research project. 181. (Barrios-de Pedro, 2019).. 182 183 184. 4. Material and methods The coprolites are housed at the Museo de Paleontología de Castilla-La Mancha. 185. (MUPA) in Cuenca, Spain, and belong to the Las Hoyas (LH) collection. Thin sections of. 186. twenty specimens belonging to seven different morphotypes were prepared: circular, cylinder,. 187. spiral, elongated, irregular, straight lace, and thin lace, as described in Barrios-de Pedro et al..

(9) 188. (2018) (Fig. 3A–I). These seven morphotypes were chosen primarily because of their. 189. abundance in the coprolite collection (thin and straight lace) or because of their relatively. 190. large size (cylinder, elongated, and irregular coprolite morphotypes), which makes it possible. 191. to analyse the coprolite material in future studies. A summary of the main features of each. 192. morphotype, including the types and densities (ranging from 1 to 4) of inclusions is provided. 193. in Table 1.. 194 195 196. 4.1. Thin sections A total of fifty-one thin sections were prepared in the Department of Stratigraphy at. 197. the Universidad Complutense de Madrid (UCM) in Spain, and twenty of these samples were. 198. polished. The coprolites were cut transversally with a Buehler IsoMet 1000 precision saw. 199. with a 0.5 mm thick diamond-saw blade. The slabs were mounted on glass slides using the. 200. epoxy resin Araldite 20-20. Lapping was carried out with a Logitech machine. The twenty. 201. thin sections were polished with a Struers RotoPot-35 and diamond paste (DP-Paste P Struers. 202. of ¼ µm). Additionally, three other sections were polished at the University of Colorado. 203. Boulder (USA), with a Buehler Ecomet Grinder and 5 and 0.05 µm aluminum oxide polishing. 204. material.. 205 206 207. 4.2. Microscope and chemical analyses The thin sections were studied at the University of Colorado Boulder (USA), using a. 208. polarized microscope Leica DMR petrographic microscope, and photomicrographs were. 209. taken using a Canon 5D Mark II digital camera. The thin sections were observed with plane. 210. polarized light, cross polarized light, and with the full-wave plate, as the latter made it easier. 211. to observe the inclusions within the coprolite matrices. A summary of the main observation. 212. and measurements obtained from the microscope is provided in Table 2.. 213 214. In order to determine the chemical composition of the elements within the coprolites, qualitative element maps of carbon-coated, polished thin sections were generated with a.

(10) 215. JEOL JXA-8230 electron probe microanalyzer. The microprobe beam was operated at 15.0. 216. kV with a beam diameter of 0.5 to 2 µm. These chemical analyses were accomplished at the. 217. Department of Geological Sciences, University of Colorado Boulder (USA).. 218 219 220. 5. Results All coprolites exhibit a microcrystalline and fairly homogeneous groundmass. 221. composed of calcium phosphate. Three different coprofabrics have been characterized. 222. considering the presence or absence of inclusions in the coprolite matrix and the types of. 223. inclusions present: the nondescript coprofabric (Fig. 4A–C); the fish scale coprofabric (Fig.. 224. 4D–F); and the decapod cuticle coprofabric (Fig. 4G–I). Microprobe maps showing the. 225. distribution of elements are also useful for coprofabric characterization. The chemical. 226. composition of host-rock is predominantly calcium carbonate and other trace elements, such. 227. as potassium, silicon, and iron.. 228 229. 5.1. Amorphous ground mass (coprolite matrix). 230. Although the ground masses of all of the Las Hoyas coprolites are largely. 231. structureless, they share some unusual and enigmatic features. When observed with plane. 232. polarized or cross polarized light, dark (black to brown; Fig. 5A–F) linear structures are. 233. evident which sometimes partially obscure undigested inclusions. High magnification reveals. 234. that the dark structures are formed of 1.5–6.5 micron-sized spheres, which are arranged in. 235. linear configurations (Fig. 5D). These dark linear features were not observed with. 236. backscattered imaging.. 237. In some coprolites, the ground mass also contains patchy areas with scattered to. 238. aggregated granular material, visible in plane polarized and cross polarized light (Fig. 5G–L).. 239. The individual granular particles range from 3.2 to 20.5 microns in diameter and are. 240. birefringent with the full-wave plate (lambda plate). They are sometimes grouped in masses. 241. which are white-to-brown in plane polarized light (Fig. 5G–I). The organization, appearance,.

(11) 242. and composition of the granules are very similar to that of the host-rock (Fig. 5G–I), so it is. 243. likely that these granules within the fossilized faecal masses correspond to sediment (from the. 244. bottom of the lake or from land). The granules occasionally occur in cylinder coprolites (MUPA-LH17075, MUPA-. 245 246. LH21055, MUPA-LH21067, MUPA-LH22141 and MUPA-LH28719a), irregular specimen. 247. (MUPA-LH28775), elongated coprolite (MUPA-LH16005), and straight lace specimen. 248. (MUPA-LH22485; see Table 2).. 249 250. 5.2. Nondescript coprofabric: few or no inclusions Coprolites containing few or no inclusions exhibit a relatively homogeneous. 251 252. microcrystalline groundmass, in which very small fossil inclusions (ranging from 0.07 to 0.9. 253. mm) are scarce or absent. In fact, the presence of undigested inclusions was not detected. 254. during previous macroscale examinations of the broken surfaces of these coprolites.. 255. Nevertheless, some thin sections reveal remains of tiny, unidentifiable inclusions that might. 256. be altered bony fragments (cylinder coprolites MUPA-LH21055, MUPA-LH22141 (Fig. 6A–. 257. C), and MUPA-LH28719a, and the spiral coprolite MUPA-LH30816; see Fig. 6D, and Table. 258. 2).. 259 260. 5.3. Fish scale coprofabric: bony and fish scale contents. 261. The fish scale coprofabric contains both fish scales and fragments of undetermined. 262. bony inclusions. These inclusions range from 0.25 to 5.33 mm in length, and from 0.06 to. 263. 2.35 mm in width (see Table 2). Bony fragments can be recognized by microstructural. 264. features such as the presence of osteocyte lacunae and lamellar compact bone (Fig. 6). The. 265. fish scales have rounded ends and contours, and some preserve an acute process. Some of the. 266. preserved scales show two layers: a pluristratified thin ganoine layer and a rather thick bony. 267. basal plate (Fig. 6E–F). In all observed scales, the dentine layer is absent. Histological details. 268. of the bony plate, such as growth lines, tubules, and osteocyte lacunae can be histologically.

(12) 269. differentiated in some of the sections (Fig. 6G–L). The scales appear isolated within the. 270. coprolite matrix, with occasional exceptions where several specimens (up to five) are. 271. observed in proximity (Fig. 4E).. 272. Electron microprobe elemental maps of thin sections show the distribution of elements. 273. within the coprolite (i.e. coprolite matrices and inclusions; Fig. 7 A–O). Both the scales and. 274. bony inclusions are richer in calcium than the coprolite matrix, with concentrations. 275. comparable to the host-rock. Phosphorus (virtually absent in the host-rock) is slightly more. 276. abundant in these inclusions than in the coprolite matrix, and is particularly elevated in the. 277. ganoine layers (Fig. 7A–G). Sodium is present in low concentrations, although the ganoine. 278. layer is richer than the rest of the scale. The magnesium map reveals that the host-rock is. 279. somewhat enriched relative to the fish scale and the coprolite matrix, though the magnesium. 280. signal in the enamel layer is a bit higher than that of the rest of the scale (Fig. 7G).. 281 282 283. 5.4. Decapod cuticle coprofabric: thin arthropod cuticle contents Distinctive, thin laminar inclusions are identified as crustacean cuticle in the decapod. 284. cuticle coprofabric. The cuticle fragments appear as thread-like structures both on coprolite. 285. surfaces (Barrios-de Pedro et al., 2018), and in thin sections (see Fig. 4I, 8A–F). These. 286. laminar inclusions are mostly evident as flat strips, but a few have ring-like, oval or frayed. 287. configurations. They range from 0.03 to 0.39 mm in length and from 0.01 to 0.08 mm in. 288. width (see Table 2 and Fig. 8A). The circular shapes are likely transverse sections of. 289. appendages which appear as isolated or as apparently associated segments (Fig. 8A–B).. 290. Clusters of up to 15 possible appendages are densely packed in one small area of irregular. 291. coprolite MUPA-LH28253 (Fig. 4I). These inclusions have pointed or torn ends.. 292. Cross sections of the thin, laminar inclusions resemble the cuticles of small decapod. 293. exoskeletons. In modern crustaceans, the exoskeleton is divided into distinct layers:. 294. epicuticle, exocuticle, endocuticle, and membranous layer (Taylor et al., 2015), and are. 295. enriched (except for the membranous layer) with hardening minerals (usually calcite and.

(13) 296. amorphous calcium carbonate) (Roer et al., 2015; Chin et al., 2017; Mergelsberg et al., 2019).. 297. In extant decapods the thin epicuticle contains lipoproteins impregnated with calcium salts,. 298. and both the exocuticle and endocuticle layers are composed of chitin-protein fibres with. 299. calcium, magnesium and phosphorus (Taylor et al., 2015; Mergelsberg et al., 2019). In. 300. contrast, insect cuticles are rarely mineralized (Roer et al., 2015). The different layers within. 301. the decapod cuticle in the coprolites were not discerned at the levels of magnification utilized. 302. in this study, however, a clear lamellar structure is evident (Fig. 8D). Furthermore, the. 303. microprobe maps show that the laminar inclusions are clearly richer in calcium, and slightly. 304. richer in phosphorus and magnesium than the coprolite matrix, which is consistent with. 305. decapod cuticle (Fig. 7H–O).. 306 307 308. 5.5. Correspondence between coprofabrics and morphotypes Of the twenty coprolite specimens studied, 40 % have the nondescript coprofabric, 35. 309. % exhibit the fish scale coprofabric, and the remaining 25 % show the decapod cuticle. 310. coprofabric (Fig. 9). The coprolites with nondescript coprofabrics correspond to circular. 311. (MUPA-LH13638), cylinder (MUPA-LH17075, MUPA-LH21055, MUPA-LH28719a, and. 312. MUPA-LH22141), irregular (MUPA-LH28775), and the spiral (MUPA-LH30816). 313. morphotypes. Scales are identified in cylinders (MUPA-LH9534, MUPA-LH15900, and. 314. MUPA-LH21067), elongated (MUPA-LH16005), and irregulars (MUPA-LH27015, MUPA-. 315. LH30254, and MUPA-LH33099; see Table 2). Decapod inclusions are common in the matrix. 316. of straight lace (MUPA-LH22485), thin lace (MUPA-LH9591, MUPA-LH22505, and. 317. MUPA-LH35426) and in the irregular (MUPA-LH28253) morphotypes (see Table 2).. 318. The coprolites exhibiting the nondescript coprofabric have a broader range of. 319. diameters (3–32 mm) with a mean of 12.2 mm; the fish scale coprofabric is associated with. 320. cylinder and elongated coprolites, ranging from 4–15 mm in width; and the decapod cuticle. 321. coprofabric is most commonly found in lace morphotypes (thin lace and straight lace) with. 322. small diameters, ranging from 2–5 mm (Fig. 9). Although most coprofabrics seemed to be.

(14) 323. dominated by one type of inclusion, there are exceptions: the cylinder coprolite MUPA-. 324. LH21067 (fish scale coprofabric) has a putative cuticle inclusion in its matrix, and the thin. 325. lace coprolite MUPA-LH35426 (decapod cuticle coprofabric) also contains some bones,. 326. although cuticle inclusions are more abundant (Fig. 8E).. 327 328 329. 5.6. Congruence between coprofabrics and digestive strategies As noted above, the Las Hoyas coprofabrics types appear to be related to diameter. 330. which also bears on feeding strategies (Fig. 10A–B, as defined by Barrios de Pedro and. 331. Buscalioni, 2018; see section 3: Coprolites from Las Hoyas). There is a clear correspondence. 332. between the coprofabric dominated by decapod cuticle inclusions, and the DS1 feeding. 333. strategy (no mastication of prey with a short gut retention time) that corresponds to lace. 334. morphotypes (straight and thin lace) with smaller coprolite diameters (Fig. 9, 10B). The. 335. coprofabric dominated by scales and bones occurs in coprolites (spirals, cylinders, and. 336. elongated) classified in the DS2 and DS3 feeding strategies (mastication of the prey, longer. 337. gut retention time, and effective gut processing, respectively). However, the nondescript. 338. coprofabric does not show a clear correspondence with digestive strategies because it includes. 339. a variety of morphotypes (circular, cylinders, and irregulars) that occupy different feeding. 340. strategies.. 341 342 343. 5.7. Relative abundances of coprolite morphotypes and vertebrate skeletal fossils When the relative abundance of the Las Hoyas coprolite morphotypes and vertebrate. 344. body fossil records are compared, it is evident that fish account for most of the skeletal fossils. 345. (approximately 94.6 % of the vertebrate fossils) as well as the coprolites, since more than 47. 346. % of the coprolites are attributed to fish [spiral, bump-headed lace, straight lace, and thin. 347. lace]; Fig. 11A–B). However, the estimated 26.2 % of coprolite morphotypes potentially. 348. attributable to tetrapods (cylinder, elongated, cone, and circular) is much greater than the. 349. relative abundance of tetrapod body fossils (~1.4 % of body fossils collected). Therefore, the.

(15) 350. relative number of tetrapod coprolites appears to be larger than the number of skeletal fossils. 351. from their potential producers.. 352 353 354. 6. Discussion The Las Hoyas coprolite association is characterized by the high relative abundance of. 355. specimens and by exceptional preservation. The elemental maps generated by electron. 356. microprobe analyses are consistent with the predominantly phosphatic composition already. 357. determined in previous EDX analyses (Barrios de Pedro et al., 2018). Phosphorus enrichment. 358. in faeces has been related to the elimination of this micronutrient through the faeces, in which. 359. other micronutrients such as silica, copper, iron, manganese, zinc, nickel, and chromium form. 360. part of the faecal composition (data corresponding to fish faeces and to omnivorous. 361. crustaceans in Geesey et al., 1984). High phosphorus levels in faeces have been also linked to. 362. the accumulation of bacteria throughout the digestive tract, and even to the presence of. 363. microbial communities that may favour phosphatogenesis during early diagenesis (Eriksson et. 364. al., 2011; Luo et al., 2018). The size and distribution of the black linear structures (Fig. 5A–F). 365. suggest microbial activity since they resemble similar micron-scale spheres that appear to. 366. represent a diagenetic signature indicative of ancient bacterial activity (Wilby and Martill,. 367. 1992; Bajdek et al., 2015).. 368 369. Mineralization with calcium phosphate is the most common type of coprolite. 370. preservation, and is thought to be representative of carnivorous scats, in which the calcium. 371. and phosphorus elements came from dietary contents (i.e., bones, teeth, exoskeletal parts of. 372. decapods, and components of animal soft tissues) (e.g. Häntzschel et al., 1968; Chin et al.,. 373. 1998). In all the coprolites analysed from Las Hoyas, the calcium phosphate groundmass is. 374. relatively homogeneous in texture, suggesting similar taphonomic processes. The lack or. 375. minimal amount of recrystallization of the ground mass and permineralization by. 376. allocthonous minerals are indicative of good preservation. In addition, the coprolite inclusions.

(16) 377. exhibit well-preserved internal microstructure. Therefore, differences among the coprofabrics. 378. appear to primarily reflect the diets and digestive processes of the animals that dwelt in the. 379. Las Hoyas ecosystem.. 380 381 382. 6.1. Coprofabric content: prey animals Two of the three coprofabrics contain aquatic prey that reflect the most abundant. 383. dwellers of the Las Hoyas aquatic milieu: fishes (a total of 4923 specimens collected) and. 384. decapods (a total of 2342 specimens collected). The high percentage of aquatic prey is. 385. noteworthy even though 96 % of the coprolites were recovered from the drier facies. This is. 386. consistent, however, with the large number of aquatic body fossils that were also recovered. 387. from the drier facies.. 388 389. The taxonomic identity of the crustacean prey can be correlated with the body fossil. 390. record. The shapes and sizes of the laminated cuticle, with apparently articulated pleopods,. 391. are congruent with the body fossils of the Las Hoyas decapods which have appendage widths. 392. from 0.01 to 0.02 mm (SBdP pers. obs. [2019]). The Atyidae shrimp Delclosia martinelli and. 393. the Astacidae Austropotamobius llopisi crayfish are abundant at the locality, and include a. 394. variety of ontogenetic stages, starting as small as 5 mm in body length (Garassino, 2016).. 395. Crustacean cuticles have been rarely recognized in coprolites (Chin et al., 2008, 2017; Zatoń. 396. and Rakociński, 2014; Silva et al., 2017).. 397 398. Fish scales are commonly found in coprolites with cylindrical and irregular shapes. 399. (Zangerl and Richardson, 1963; Coy, 1995; Owocki et al., 2012; Bajdek et al., 2015;. 400. Qvarnström et al., 2017; Segesdi et al., 2017). Some thin sections of the Las Hoyas coprolites. 401. show fish scales with a similar pattern to that of the isolated ganoine scales described for. 402. Adrianaichthys, a Lepisosteiform from the Early Cretaceous Kem-Kem beds in Morocco. 403. (Meunier et al., 2016, text-fig. 3). Because no histological analyses have yet been performed.

(17) 404. on the Las Hoyas actinopterygian scales, the taxonomic attribution of the prey cannot be. 405. precisely determined. Nonetheless, among the different neopterygian groups represented in. 406. Las Hoyas, the semionotiform species Lepidhoyas and Hoyasotes, and the basal teleostean. 407. Pleuropholis show thick shiny ganoine scales (Poyato-Ariza and Martín-Abad, 2016).. 408 409. 6.2. Feeding strategies and possible source animals The coprofabrics appear to be correlated with the feeding strategies classification, with. 410 411. each one represented by a subset of coprolite morphotypes (Fig. 10). Based upon relative. 412. coprolite sizes, coprofabrics, and skeletal fossils from the site, it appears that small to. 413. medium-sized-fishes and some archosaurs, were the likely faecal producers (see Fig. 11 for. 414. taxa).. 415 416. The delicate and abundant decapod cuticle inclusions are components of a coprofabric. 417. that matches with the feeding strategy DS1, that implies no mastication of the prey and a short. 418. retention time of food in the digestive system. The lace coprolite morphotypes (straight and. 419. thin lace) with abundant prey inclusions can be tentatively attributed to medium-sized. 420. teleosteans, and/or to small amiiforms and semionotiforms. The diets of both ancient and. 421. extant fishes support this interpretation. Fossilized fish gut contents containing crustaceans. 422. and fry fish skeletons have been reported in the Pachyrhizodontidei teleosteans, Rhacolepis. 423. and Notelops (Wilby and Martill, 1992) from the Aptian locality of Araripe (Santana. 424. Formation, Brazil). The gut contents of a medium-sized amiiform, Calamopleurus cylindricus. 425. from the same Lower Cretaceous formation reveals an almost complete fish skeleton (Mulder,. 426. 2013), and the living amiiform Amia calva is catalogued as a non-specific predator that eat. 427. insects, fish, crustaceans, and amphibians (Eschmeyer, 2004). These observations reinforce. 428. the view that extinct and living Amiiformes have utilized their sharp, conical teeth to hunt a. 429. variety of prey (Poyato-Ariza and Martín-Abad, 2013, 2016). On the other hand, the pointed. 430. to blunt teeth of the Semionotiformes (Lepidhoyas and Hoyasotes) was probably optimal for.

(18) 431. feeding on animals with moderately hard exoskeletons, such as macruran decapods (Poyato-. 432. Ariza and Martín-Abad, 2016). Nevertheless, although we infer that the decapod crustacean. 433. coprofabric is characteristic of small fish, we note that the stomach contents of the. 434. enantiornithe bird Eualulavis found at Las Hoyas also contains decapod exoskeletal cuticle. 435. (Sanz et al., 1996), and could be classified as having a DS1 feeding strategy.. 436 437. Although the lace coprolite morphotypes do not exhibit a spiral morphology, the spiral. 438. valve of the extant amiiform Amia calva is not formed by a coiling twisting of the whole. 439. thickness of the intestine (Hilton, 1900) as occurs in acipenseriforms, lungfish, bichirs,. 440. sharks, rays, and skates (Jain, 1983; Hassanpour and Joss, 2009). Instead, it is made by a set. 441. of powerful muscular bands transversely set with several longitudinal folds, and is classified. 442. as a ‘rudimentary spiral valve’ (Burton and Burton 2017); as such their faecal masses would. 443. not be internally coiled and produce typical spiral coprolites.. 444 445. The fish scale coprofabric is congruent with both prey mastication and longer retention. 446. of food (DS2) as well as more effective gut processing (DS3), so several types of animals. 447. could have produced faeces that were preserved with this coprofabric. Zatoń et al. (2017). 448. reported a coprolite with a cylindrical-spiral shape that they tentatively attributed to a. 449. coelacanth, and fish scales and other skeletal elements have been recovered from the guts of. 450. extant coelacanths (Uyeno and Tsutsumi, 1991). The dwarf hybodontiform chondrichthyan. 451. (Soler-Gijón et al., 2016) probably produced the tiny spiral coprolite (MUPA-LH30816),. 452. which is consistent with the DS3 feeding strategy (Fig. 10). The larger-diameter cylinder and. 453. elongated coprolites may have been generated by fishes or by medium-sized archosaurs;. 454. fragments of undigested bone (and occasionally muscle) have been found within theropod and. 455. other archosaur coprolites (Chin et al., 1998, 2003; Stone et al., 2000; Rinehart et al., 2005;. 456. Chin and Bishop, 2007; Bajdek, 2018; Qvarnström et al., 2019b), and fish remains were. 457. identified in the stomach of Enantiornithes from other Lower Cretaceous localities (Zhou et.

(19) 458. al., 2002; Wang et al., 2016; O'Connor, 2019). The coprolite morphotypes with fish scale. 459. coprofabrics, with a smaller number of inclusions (density of 2–3, see Table 1), and with. 460. notable digestive modification (DS2 and DS3 in Barrios de Pedro and Buscalioni, 2018; see. 461. Fig. 10) suggest a higher digestive efficiency in comparison to most fishes. Turtles cannot be. 462. discarded as putative producers of cylinder coprolites with inclusions, because cylinder-. 463. shaped morphotypes have been ascribed to turtles (Hunt and Lucas, 2012; Bajdek et al.,. 464. 2019). Furthermore, it has been found that bony inclusions within coprolites attributed to. 465. turtles appear acid-etched, and digestion within extant turtles is characterized by long gut. 466. retention (Bajdek et al., 2019, and references herein).. 467. The nondescript coprofabric, with few or no inclusions, presents an interesting case. It. 468 469. may be regarded as the result of the extraordinary digestive efficacy, characterizing the DS4. 470. feeding strategy. For example, extant crocodilian digestion normally involves the elimination. 471. of indigestible food due to the emetic digestive mechanism (i.e. hair, chitin and decapod. 472. exoskeletal parts; Andrews et al., 2000; Wallace, 2006), the corrosion of enamel and ganoine,. 473. the capacity to decalcify bones of ingested prey in the gut (Fisher, 1981; Milan, 2012; Milan. 474. et al., 2012) (particularly when the prey are fish or amphibians; Wallace, 2006), and the. 475. capacity to retain food until everything is almost digested (Fernández-Jalvo and Andrews,. 476. 2016). However, it is also possible that coprolites with nondescript coprofabrics could be. 477. linked to organisms that fed on soft-bodied prey (Thulborn, 1991). Finally, diagenetic. 478. alteration of coprolite contents may have also contributed to the absence of conspicuous. 479. inclusions in this coprofabric, though this seems unlikely in view of the un-recrystallized. 480. coprolite ground mass.. 481 482. 6.3. Reconciling coprofabric data with numbers of coprolite morphotypes and skeletal. 483. fossils.

(20) 484. One of the most notable results from this study is that only three distinct coprofabrics. 485. were identified. This is low in comparison with the vertebrate diversity and the number of. 486. coprolite morphotypes recovered at Las Hoyas. Based upon coprofabric data alone, the. 487. prevalence of recognizable decapod and fish scale inclusions suggest that small fishes and. 488. shrimps were the most common and abundant food sources in the aquatic paleoenvironments. 489. of Las Hoyas. While this is probably true, the rich fossil record suggests that other food. 490. resources were also important. Juvenile non-chanid teleosts (lesser than 15 mm in body. 491. length) were likely planktivorous (Poyato-Ariza, 2005; Poyato-Ariza and Martín-Abad,. 492. 2016), and aquatic lissamphibia probably fed upon zooplankton and small insects (Buscalioni. 493. et al., 2016; Evans, 2016). The absence of evidence for other food resources in the Las Hoyas. 494. coprofabrics may be due to taphonomic complexities. Ingested plankton, insects, and other. 495. labile foods may not be recognizable after digestion and mineralization of the faeces. It is also. 496. possible that droppings containing such foods were eaten by other consumers or were not. 497. deposited in settings conducive to fossilization. Finally, insufficient sampling (due to. 498. restrictions on destructive analyses) may have made it unlikely to find less common. 499. coprofabrics.. 500 501. Most of the coprolite morphotypes are inferred to correspond to vertebrate predators at. 502. Las Hoyas (Barrios-de Pedro et al., 2018). Although coprolite morphotypes ascribed to fish. 503. (the lace morphotypes) are most common at the locality, several morphotypes have been. 504. attributed to tetrapods (cylinder, elongated, cone, and circular coprolites; see Fig. 11B).. 505. Nevertheless, we did not find a clear and exclusive correspondence between morphotypes and. 506. coprofabrics, as occurred in other exceptional assemblages such as the Upper Cretaceous of. 507. Sweden (Eriksson et al., 2011). Thus, there is general but non-exclusive correlation between. 508. the coprofabrics and the coprolite morphotypes and their contents, and there appears to be. 509. better correlation between coprofabrics, type of feeding strategy, and coprolite diameter.. 510. Taphonomic issues may have played a role in the discrepancy between the number of.

(21) 511. coprofabrics (three) and coprolite morphotypes (twelve). However, dietary differences during. 512. different ontogenetic stages may also account for some of the differences among morphotypes. 513. (Barrios de Pedro, 2019). This interpretation is supported by the correspondence between. 514. coprofabric type and coprolite diameter (Fig. 9).. 515 516. 7. Conclusions. 517. The petrographic and microprobe analyses of the Las Hoyas coprolites facilitate recognition. 518. of three distinct coprofabrics. The coprofabric contents suggest that: (1) both aquatic and. 519. terrestrial vertebrate predators fed upon fishes and decapods; (2) numerous omnivorous. 520. animals consumed different species of fishes and decapods; and (3) the presence of selective. 521. feeders in the ecosystem cannot be discarded, based on the histological similarity of the fish. 522. scales. There is general but non-exclusive correlation between the coprofabrics, the coprolite. 523. morphotypes, and their contents, and there appears to be better correlation between the. 524. coprofabrics, coprolite diameter, and the feeding strategies proposed for the Las Hoyas faecal. 525. producers. The differences among coprofabrics reflect the general diets and digestive. 526. processes of the animals from Las Hoyas but are likely influenced by taphonomic factors.. 527. Nevertheless, these coprofabrics suggest that decapods and fishes were the most common. 528. prey chosen by the Las Hoyas predators (medium-sized-fishes and archosaurs, respectively),. 529. and played an important role in sustaining this Barremian ecosystem. The coprolites from Las. 530. Hoyas do not offer a reasonably proxy to the diversity of organisms of the ecosystem, but. 531. reflect a good approximation of trophic interactions of Las Hoyas animals.. 532 533. Acknowledgements. 534. Thanks to Mercedes Llandres and Santiago Langreo at the Museo de Paleontología de. 535. Castilla-La Mancha for allowing us access to the coprolite collection. Special thanks go to. 536. Juan Carlos Salamanca at the Department of Stratigraphy in the Universidad Complutense de. 537. Madrid for the thin sections, and Aaron Bell at the Department of Geological Sciences at the.

(22) 538. University of Colorado Boulder for the microprobe analyses. Many thanks to Hugo Martín-. 539. Abad and Oscar Cambra-Moo for their advice. The authors thank to Michal Zatoń and Diego. 540. Kietzmann for their constructive comments on the original manuscript.. 541 542. Funding for the analyses was provided by the Spanish Ministerio de Economía y. 543. Competitividad (MINECO) through the project CGL-2013-42643-P. The fellowship reference. 544. BES-2014-070985 of the Program for the Training of Researchers of the MINECO helped. 545. fund this project, and the PhD travel grant reference EEBB-I-2018-12926 “Ayudas a la. 546. movilidad predoctoral para la realización de estancias breves en centros de I+D” of the. 547. MINECO made it possible to carry out this work at the University of Colorado Boulder. We. 548. also acknowledge Dr. Hayes W. Caldwell and Mrs. Margaret H. Caldwell for support of the. 549. Chin laboratory at the University of Colorado Boulder.. 550 551. References. 552. Andrews, P.L.R., Axelsson, M., Franklin, C., Holmgren, S., 2000. The emetic response in a. 553 554. reptile. Journal of Experimental Biology, 203: 1625–1632. Bajdek, P., Qvarnström, M., Owocki, K., Sulej, T., Sennikov, A.G., Golubev, V.K.,. 555. Niedzwiedzki, G., 2015. Microbiota and food residues including possible evidence of. 556. pre-mammalian hair in Upper Permian coprolites from Russia. Lethaia. DOI:. 557. 10.1111/let.12156.. 558 559 560. Bajdek, P., 2018. Comparative digestive physiology of archosaurs with notes on bird origins. PeerJ PrePrints, 6: e26902v2. Bajdek, P., Szczygielski, T., Kapuścińska, A., Sulej, T., 2019. Bromalites from a turtle-. 561. dominated fossil assemblage from the Triassic of Poland. Palaeogeography. 562. Palaeoclimatology Palaeoecology, 520: 214–228..

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(34) 854 855. Figure 3. Macroscale photos of representative coprolites studied. (A) Circular coprolite,. 856. MUPA-LH13638b. (B) Cylinder coprolite, MUPA-LH15900a. (C) Cylinder coprolite,. 857. MUPA-LH17075. (D) Spiral coprolite, MUPA-LH30816. (E) Elongated coprolite, MUPA-. 858. LH16005b. (F) Irregular coprolite, MUPA-LH28775. (G) Irregular coprolite, MUPA-. 859. LH30254a. (H) Straight lace coprolite, MUPA-LH22485b. (I) Thin lace coprolite, MUPA-. 860. LH22505b. Scale bar in (A) = 2 mm. Scale bars in (B–I) = 5 mm.. 861 862. Figure 4. Coprolites with coprofabrics identified in the Las Hoyas coprolites. (A). 863. Nondescript coprofabric; cylinder coprolite MUPA-LH28719a: macroscale photo, (B) thin. 864. section in plane polarized light, and (C) with full-wave plate. (D) Fish scale coprofabric;. 865. cylinder coprolite MUPA-LH9534: macroscale photo, (E) thin section with plane polarized. 866. light, and (F) full-wave plate. (G) Decapod cuticle coprofabric; irregular coprolite MUPA-. 867. LH28253; macroscale photo, (H) thin section in plane polarized light, and (I) with full-wave. 868. plate. Scale bars in (A), (D), and (G) = 5 mm. All other scale bars = 200 µm. Abbreviations:. 869. CM = Coprolite matrix; DL = Dark lines; EL = Enamel layer; Gl = Glass; GM = Granular. 870. material; Sc = Scale; Sd = Sediment.. 871 872. Figure 5. Coprolite ground mass features. (A) Thin section of coprolite matrix showing. 873. dark linear features; cylinder coprolite MUPA-LH22141 in plane polarized light, (B) cross-. 874. polarized light view, and (C) with full-wave plate. (D) Thin section showing close-up view of. 875. dark linear structures; cylinder coprolite MUPA-LH15900 in plane polarized light. (E) Thin. 876. section of coprolite matrix showing dark linear structures; cylinder coprolite MUPA-. 877. LH17075 in plane polarized light, and (F) with full-wave plate. (G) Thin section of coprolite. 878. matrix showing granular material; cylinder coprolite MUPA-LH21055 in plane polarized. 879. light, (H) cross-polarized light view, and (I) with full-wave plate. (J) Thin section of coprolite. 880. matrix showing small granular material; cylinder coprolite MUPA-LH17075 in plane.

(35) 881. polarized light, (K) cross-polarized light view, and, (L) with full-wave plate. Yellow arrows. 882. point to birefringent masses. Scale bars in (A–C) = 500 µm; scale bars in (D) and (J–L) = 50. 883. µm; scale bars in (E–I) = 200 µm. Abbreviations: BL = Brown lines; CM = Coprolite matrix;. 884. DL = Dark lines; GM = Granular material; Sd = Sediment.. 885 886. Figure 6. Photomicrographs of skeletal inclusions in coprolites with nondescript and fish. 887. scale coprofabric. (A) Thin section showing fish scale; cylinder coprolite MUPA-LH22141. 888. in plane polarized light, (B) cross-polarized light view, and (C) with full-wave plate. Arrows. 889. point to granular material. (D) The only undetermined bony inclusion found in spiral coprolite. 890. MUPA-LH30816, with full-wave plate. (E) Bones and one scale exhibiting an enamel layer;. 891. elongated coprolite MUPA-LH16005, with full-wave plate. (F) detail of pluristratified. 892. ganoine in a scale in cylinder coprolite MUPA-LH15900, with full-wave plate. (G) Sections. 893. of fish bones that could be fin elements; irregular coprolite MUPA-LH27015 in cross-. 894. polarized light. Note that interior of bones is apparently filled with coprolite matrix. (H). 895. Osteocyte lacunae in irregular coprolite MUPA-LH30254. (I) Growth marks and osteocyte. 896. lacunae in irregular coprolite MUPA-LH27015, with full-wave plate. (J) Fish scale in. 897. elongated coprolite MUPA-LH16005; full wave plate. Arrows point to growth marks and. 898. putative osteocyte lacunae. (K) Putative scales in cylinder coprolite MUPA-LH15900, with. 899. full-wave plate. Blue rectangle indicates the area shown in (L). (L) Osteocyte lacunae in a. 900. putative scale in MUPA-LH15900. Scale bars in (A–C), (E), and (G) = 200 µm; scale bars in. 901. (D), (F), and (L) = 50 µm; scale bars in (H–K) = 100 µm. Abbreviations: BO = Bone; CM =. 902. Coprolite matrix; DL = Dark lines; EL = Enamel layer; Gl = Glass; GM = Granular material;. 903. Sc = Scale; Sd = Sediment; UB = Undetermined bone.. 904 905. Figure 7. Photomicrographs and microprobe elemental maps of fish scale (A–G) and. 906. decapod cuticle (H–O). (A) Thin section of ganoid scale in cylinder coprolite MUPA-. 907. LH9534, with full-wave plate, and (B) back-scattered electron (BSE) image. Arrow points to.