Reconstructing the diet of a 505-million-year-old arthropod: Sidneyia inexpectans from the Burgess Shale fauna

Abstract

The feeding ecology of the 505-million-year-old arthropod Sidneyia inexpectans from the middle Cambrian (Series 3, Stage 5) Burgess Shale fauna (British Columbia, Canada) is revealed by three lines of evidence: the structure of its digestive system, the fossilized contents of its gut and the functional anatomy of its appendages. The digestive tract of Sidneyia is straight, tubular and relatively narrow in the trunk region. It is enlarged into a pear-shaped area in the cephalic region and stretches notably to form a large pocket in the abdomen. The mouth is ventral, posteriorly directed and leads to the midgut via a short tubular structure interpreted as the oesophagus. Anteriorly, three pairs of glands with internal, branching tubular structures open into the digestive tract. These glands have equivalents in various Cambrian arthropod taxa (e.g. naraoiids) and modern arthropods. Their primary function was most likely to digest and assimilate food. The abdominal pocket of Sidneyia concentrates undigested skeletal elements and various residues. It is interpreted here as the functional analogue of the stercoral pocket of some extant terrestrial arachnids (e.g. Araneae, Solifugae), whose primary function is to store food residuals and excretory material until defecation. Analysis of the gut contents indicates that Sidneyia fed largely on small ptychopariid trilobites, brachiopods, possibly agnostids, worms and other undetermined animals. Sidneyia was primarily a durophagous carnivore with predatory and/or scavenging habits, feeding on small invertebrates that lived at the water-sediment interface. There is no evidence for selective feeding. Its food items (e.g. living prey or dead material) were grasped and manipulated ventrally by its anterior appendages, then macerated into ingestible fragments and conveyed to the mouth via the converging action of strong molar-like gnathobases. Digestion probably took place within the anterior midgut via enzymes secreted in the glands. Residues were transported through the digestive tract into the abdominal pocket. The storage of faeces suggests infrequent feeding. The early diagenetic three-dimensional preservation of the digestive glands and abdominal pocket may be due to the capacity of Sidneyia to store Phosphorus and Calcium (e.g. spherites) in its digestive tissues during life as do, for example, modern horseshoe crabs.

An early Cambrian chelicerate from the Emu Bay Shale, South Australia

“…), and is similar to the phosphatized gut of Sidneyia (Zacaï et al . ), but contrasts with that described by Bergstöm et al . () for Naraoia , where the composition of the gut was much the same as that of the surrounding sediment.…”

Coprolites in mid-Cambrian (Series 2-3) Burgess Shale-type deposits of Nevada and Utah and their ecological implications

“…The dense accumulation of agnostoid and other skeletal fragments indicates that the producer was able to digest most soft tissue associated with the prey and that it was likely their main food source, as omnivores usually produce a coprolite composed of a mix of sediment, organic matter, and exoskeletal fragments (Vannier & Chen 2005, Zacaï et al 2016, Peel 2017. The coprolite shows similarities to the ellipsoid aggregates described from the Maotianshan Shale, which have been attributed to large arthropods (Vannier & Chen 2005), consistent with our predator/scavenger interpretation.…”

Abnormal xiphosurids, with possible application to Cambrian trilobites

“…Although the broad mechanics of predation (e.g., crushing, breaking, striking) were likely similar, the predator groups are very different. Cambrian trilobite predators-Euarthropoda Sidneyia inexpectans, Wisangocaris barbarahardyae Jago et al, 2016, a Fuxianhuia-like arthropod, and perhaps other trilobites (Bruton 1981;Zhu et al, 2004;Daley et al, 2013;Jago et al, 2016;Zacaï et al, 2016;Bicknell and Paterson, 2018)-were very different to the predators of extant xiphosurids (see Walls et al, 2002). Nonetheless, these preliminary results highlight that such comparative research could be explored further.…”

“…Furthermore, paleontologists have been intrigued by the morphological similarities of L. polyphemus and fossil xiphosurids like Yunnanolimulus luopingensis Zhang et al, 2009 (Guanling Formation, China, Triassic;Hu et al, 2017), Mesolimulus walchi (Desmarest, 1822) (Solnhofen Limestone, Germany, Jurassic; Sekiguchi and Sugita, 1980;Smith and Berkson, 2005) and Limulus darwini Kin and Błaże-jowski, 2014 (Sławno Limestone, Kcynia Formation, Poland, Late Jurassic;Błażejowski, 2015). Finally, L. polyphemus is a useful modern analogue for exploring how extinct gnathobase-bearing arthropods consumed food, including large eurypterids (Selden, 1981;Poschmann et al, 2016), Sidneyia inexpectans Walcott, 1911(Zacaï et al, 2016Bicknell et al, 2018b;Bicknell and Paterson, 2018), and Alacaris mirabilis Yang et al 2018.…”