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

An injured Shergoldia laevigata Zhu, Hughes & Peng, 2007 (Trilobita, Asaphida) was collected from the Furongian of Guangxi, South China. The injuries occurred in the left thoracic pleurae possessing two marked V-shaped gaps. It led to substantial transverse shortening of the left pleural segments, with barely perceptible traces of healing. This malformation is interpreted as a sub-lethal attack from an unknown predator. The morphology of injuries and the spatial and temporal distribution of predators indicated that the predatory structure might have been the spines on the ganathobase or ganathobase-like structure of a larger arthropod. There were overlapped segments located in the front of the injuries, and slightly dislocated thoracic segments on the left part of the thorax, suggesting that the trilobite had experienced difficulties during molting. The freshly molted trilobite had dragged forward the old exuvia causing the irregular arrangement of segments. This unusual trilobite specimen indicates that the injuries interfered with molting.

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.
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.
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.

Interference with the molting of trilobite
Previous studies have reported abundant injured trilobites and presented the possible identity of the predators, including information about their behavior (Babcock, 1993;Babcock, 2007;Pates et al., 2017;Bicknell & Paterson, 2018;Bicknell, Paterson & Hopkins, 2019;Pates & Bicknell, 2019). However, there are few direct fossil records showing that injury has disturbed the molting of trilobites (Šnajdr, 1985). The studied specimen has an apparent overlap of segments along with the injuries that are mainly present in the posterior of the cranidium and the front of the thorax, especially in the left part of the exoskeleton. The anterior margin of the injuried third thoracic segment was covered by the unbroken second thoracic segment ( Figs. 2A-2D), indicating that the injury formed before the overlap of the segments. Bottom currents can also cause the overlap and even disruption of trilobite segments, the Sandu Formation formed in a relatively calm environment (Lerosey-Aubril, Zhu & Ortega-Hernández, 2017), although there are overlapping segments on the exuvia and carcass of Shergoldia laevigata in the same horizon, their thoracic segments still maintain imbricated arrangement (Zhu, Hughes & Peng, 2007). There is also overlap of thoracic segments on the exuvia of uninjured trilobites (Daley & Drage, 2016), but it is difficult to determine whether it was caused by molting or other abiotic factors. In contrast to the above two cases, in addition to the overlap of segments in the studied specimen, the left thoracic segments are presented an interlaced arrangement rather than imbricated (Fig. 2), which seems not to be caused by bottom currents and is rather likely caused by the active behavior of trilobite. Moreover, all abnormal arrangements of the segments appeared near the injury, and the overlapped part of the segments was only located before the most serious injury (on the third to fourth thoracic segments). It is speculated that all of the irregular patterns were caused by post-injury molting of Shergoldia laevigata. Namely, the new exoskeleton could not be smoothly separated from the old one due to the unbalanced body with injuries (Drage, 2019;Drage et al., 2019). The trilobite dragged forward the old shell to get rid of the exuvia, which led to the overlap of segments and the dislocated arrangement of thoracic segments, especially near the injuries (Fig. 3). Some previous studies have reported cases of failed molting of a trilobite (McNamara & Rudkin, 1984) and other ecdysozoans García- Bellido & Collins, 2004;Drage & Daley, 2016;Yang et al., 2019), in which the new exoskeletons were preserved under the old exuvia. However, none of the fragments of the new exoskeleton were found under or near the exuvia of Shergoldia laevigata, which implies that the molting might not have failed. Although the injuries complicated the molting process, it was successful and the molted trilobite moved away.

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.