Injuries and molting interference in a trilobite from the Cambrian (Furongian) of South China

Introduction

Numerous trilobite exoskeleton deformities have been documented, including abnormal healing, hyperplasia, deformation, and missing or fractured segments. The causes of these deformities are usually thought to be injuries, developmental disorders, and diseases (Owen, 1985; Babcock, 1993; Pates et al., 2017; Bicknell & Pates, 2020). The evaluation of injuries caused by predator attack is useful for presenting the interactions between predators and trilobites, and for reconstructing the food web and ecological structure in deep time (Klompmaker et al., 2019). Furthermore, such predatorial injuries are used to uncover behavioral information (Babcock & Robison, 1989; Babcock, 1993; Pates et al., 2017; Bicknell, Paterson & Hopkins, 2019). The injuries caused by predators have mainly been detected on the edges of trilobites, especially in the thoraces and pygidia, and are generally considered to have been non-lethal (Babcock, 2003; Babcock, 2007), while cephalic attacks are more often fatal (Pates & Bicknell, 2019). Although numerous studies have evaluated injured trilobites (e.g., Owen, 1985; Rudkin, 1985; Babcock, 1993; Babcock, 2003; Babcock, 2007; Zhu et al., 2007; Schoenemann, Clarkson & Høyberget, 2017; Bicknell & Paterson, 2018; Bicknell & Pates, 2020; Bicknell & Holland, 2020; Zong, 2020), most predators remain unidentified, except some carnivores with trilobite fragments in their guts or coprolites (Vannier & Chen, 2005; Vannier, 2012; Zacaï, Vannier & Lerosey-Aubril, 2016; Bicknell & Paterson, 2018; Kimmig & Pratt, 2018). The shapes of the Cambrian trilobite injuries suggest that some predators may have been radiodonts (Babcock & Robison, 1989; Babcock, 1993; Nedin, 1999). Other predator candidates include cephalopods, echinoderms, fish, and other larger arthropods (Bruton, 1981; Briggs & Collins, 1988; Babcock, 1993; Fatka, Budil & Grigar, 2015; Jago, García-Bellido & Gehling, 2016; Bicknell & Paterson, 2018; Bicknell et al., 2018a; Zhai et al., 2019).

Moreover, although it has been inferred that injuries did interfere with daily activities of trilobites, there are rare direct fossil records (Šnajdr, 1985). Herein, I discuss an injured Shergoldia laevigata Zhu, Hughes & Peng, 2007 from the Cambrian (Furongian) of Jingxi, Guangxi, South China. The exoskeletal injuries suggest that the predatory structure might have been the spines on the gnathobase or gnathobase-like structure of a larger arthropod. In addition, the findings indicate that these injuries would have caused difficulties for trilobite during molting, but did not cause molting failure.

Materials & Methods

The described Shergoldia laevigata specimen, housed in the State Key Laboratory of Biogeology and Environmental Geology, China University of Geoscience (Wuhan), was discovered from the Cambrian (Furongian)-aged Sandu Formation of Guole Town, Jingxi County, Guangxi Zhuang Autonomous Region, South China (Fig. 1) (Zhu, Hughes & Peng, 2007). The Sandu Formation is represented by calcareous mudstones, siltstones, and argillaceous banded limestones, which formed most probably in the uppermost part of the continental slope (Lerosey-Aubril, Zhu & Ortega-Hernández, 2017). The Sandu Formation is richly fossiliferous, containing abundant, well-preserved articulated trilobites (Han et al., 2000; Zhu, 2005; Zhu, Hughes & Peng, 2007; Zhu, Hughes & Peng, 2010), non-trilobite arthropods (Lerosey-Aubril, Ortega-Hernández & Zhu, 2013; Lerosey-Aubril, Zhu & Ortega-Hernández, 2017), echinoderms (Han & Chen, 2008; Chen & Han, 2013; Zamora et al., 2017; Zamora, Zhu & Lefebvre, 2013; Zhu, Zamora & Lefebvre, 2014), brachiopods, graptolites (Zhan et al., 2010), hyolithids, cnidarians, algae, and some exceptionally preserved soft-bodied fossils (Zhu et al., 2016).

Trilobites from the Sandu Formation have been classified into at least 25 genera (Zhu, 2005; Zhu, Hughes & Peng, 2007; Zhu, Hughes & Peng, 2010); however, only six abnormal specimens have been documented from this formation (five Tamdaspis jingxiensis Zhu et al., 2007 and one Guangxiaspis guangxiensis Zhou in Zhou et al., 1977; Zhu, 2005; Zhu et al., 2007; Zong, 2020). The injured Shergoldia laevigata was collected from the grey-yellow calcareous mudstones. The specimen is from the Probinacunaspis nasalis–Peichiashania hunanensis Zone of the Furongian, Jiangshanian (Peng, 2009; Zhu et al., 2016).

The fossil in Fig. 2C was whitened with magnesium oxide powder, and all photographs were captured using a Nikon D5100 camera with a Micro-Nikkor 55 mm F3.5 lens.

Results

The injured Shergoldia laevigata is preserved as a nearly complete dorsal exoskeleton (30.5 mm long) without librigena, suggesting an exuvia (Daley & Drage, 2016; Drage, 2019). The posterior of the cranidium overlies the first two thoracic segments, this is most pronounced on the left side (Fig. 2). In addition, the first thoracic segment covered most of the left pleural segment of the second thoracic segment, as well as the anterior margin of the right pleural segment. Similarly, most of the first thoracic segment was covered by the posterior area of the fixigena, particularly on its left side. Moreover, the left pleural segments of the fourth to eighth thoracic segments presented an interlaced arrangement, i.e., the anterior margin of the fourth thoracic segment extended upon the third thoracic segment, and the seventh extended upon the sixth (Fig. 2), while there was a typical imbricated arrangement in the right pleural region.

The malformation is on the left part of the exoskeleton, while the medial (axial) and right sections are undamaged. The left thoracic segments are shorter than those on the right side and show limited healing. There are two injuries: one on the third to fourth thoracic segments, and one on the seventh thoracic segment. Two pleural segments are truncated by 3.3 mm because of the first asymmetric V-shaped injury; the most seriously damaged part is the contact site of the two thoracic segments, where there is a V-shaped injury. The second injury truncates the left pleural section of the seventh thoracic segment by 3.5 mm.

Discussion

Possible origin of the injuries and potential predatory structure

Trilobites that are malformed due to predatory attacks have typically V-, U-, or W-shaped injuries (Owen, 1985; Babcock, 1993; Pratt, 1998; Jago & Haines, 2002; Zamora et al., 2011; Pates et al., 2017; Bicknell & Paterson, 2018; Bicknell & Pates, 2020), with a few showing in bay-shaped injuries (Fatka, Budil & Grigar, 2015). Furthermore, there is occasionally signs of healing or regeneration (Rudkin, 1979; McNamara & Tuura, 2011; Pates et al., 2017). In the present specimen, the injuries have traces of healing and are therefore considered evidence of a predatory attack. The two injuries have a similar degree of healing without any regeneration, suggesting that these injuries may have been incurred in the same inter-molt stage.

In the past, the most commonly suggested Cambrian predators are considered to have been the radiodonts, especially anomalocaridids and amplectobeluids, as their frontal appendages and oral cone were extremely effective predatory structures (Whittington & Briggs, 1985; Babcock, 1993; Zamora et al., 2011). Cambrian arthropods or arthropod-like organisms with gnathobases are also considered possible predators, similar to the modern horseshoe crab (Bicknell et al., 2018a; Bicknell et al., 2021). Some amplectobeluid genera have been documented with gnathobase-like structures (Cong et al., 2017; Cong et al., 2018), suggesting that amplectobeluid radiodonts may have been predators of Cambrian trilobites (Bicknell & Pates, 2020). In addition, some trilobites and predatory arthropods with reinforced gnathobasic spines on the protopodal sections of their walking legs are also considered as potential predators (Bruton, 1981, Conway Morris & Jenkins, 1985; Zacaï, Vannier & Lerosey-Aubril, 2016; Bicknell et al., 2018a; Bicknell et al., 2018b; Bicknell, Paterson & Hopkins, 2019; Bicknell & Holland, 2020).

However, so far, the youngest amplectobeluid and anomalocaridid are from the Drumian and Guzhangian, respectively (Lerosey-Aubril et al., 2014; Lerosey-Aubril et al., 2020). Most Furongian and Ordovician radiodonts belong to the family Hurdiidae, which do not have endites of alternating size, and all members of this family are considered to be sediment sifters or suspension feeders (Daley, Budd & Caron, 2013; Daley et al., 2013; Lerosey-Aubril & Pates, 2018; Van Roy, Daley & Briggs, 2015; Pates et al., 2020). Moreover, no radiodonts were discovered in the Sandu Formation. So, the radiodonts are unlikely to have caused the injuries in the Shergoldia laevigata specimen. Gnathobases have a slight size gradation of spines along the gnathal edge (Stein, 2013), or a saw-toothed pattern with spines of alternating sizes (Bicknell et al., 2018a). These spines may caused smaller injuries, that is missing one or two separate thoracic segments, on the edges of trilobites. The arthropods Aglaspella sanduensis (Lerosey-Aubril, Ortega-Hernández & Zhu, 2013) and Glypharthrus trispinicaudatus, Mollisonia-like arthropods, unnamed aglaspidid-like arthropods, Perspicaris-like bivalve arthropods (Zhu et al., 2016; Lerosey-Aubril, Zhu & Ortega-Hernández, 2017), and some larger trilobites (Zhu, 2005) were discovered in the Sandu Formation at the same site. Therefore, the predator who attacked the studied Shergoldia laevigata specimen may be one of these arthropods.

Conclusions

The Shergoldia laevigata specimen has substantially shorter pleural segments of the third to fourth and seventh thoracic segments, with signs of lightly healing in the injuries incurred during a sub-lethal predator attack. The degree of healing in both injuries and the distribution of the injuries show that they may have been caused in the same inter-molt stage. Based on the morphology of the injuries and the spatial and temporal distribution of predators, the predatory structure may have been the spines on the gnathobase or gnathobase-like structure of a larger arthropod. The conspicuous overlapping of the segments and dislocated arrangement of the thoracic segments, especially in the left pleural region and near the injuries, shows that the injured S. laevigata encountered certain obstacles during molting. The trilobite dragged the old exuvia forward, which led to the irregular arrangement of the segments. Such configuration can demonstrate that even provisionally healed injury can cause certain complication of the molting process in trilobites.