Abnormal xiphosurids, with possible application to Cambrian trilobites

Abstract

Xiphosurida comprise an archetypal arthropod group of considerable interest to both biological and palaeontological researchers. This appeal is generated by a combination of unique anatomical features, utility as modern analogues for extinct arthropod groups, and an impressive fossil record. Although xiphosurids have been extensively studied, there are few published examples of abnormal specimens. Abnormalities in xiphosurids have mostly been attributed to injuries (either self-inflicted, from mating, or predation) or teratologies (developmental and genetic malfunctions). Here we summarise all previously recorded extant xiphosurid abnormalities and describe new examples of injuries and teratologies to Limulus polyphemus and Tachypleus tridentatus. Furthermore, we present the first evidence of injured fossil xiphosurids: Euproops danae and Mesolimulus walchi. We identify two main groups of telson teratologies and document new 'U' shaped cephalothoracic injuries to the anterior cephalothoracic margins of L. polyphemus and T. tridentatus. We show 'V' and 'W' shaped injuries to E. danae and M. walchi cephalothoracic sections. A further specimen of E. danae is described, which likely represents plastic deformation of a recently moulted exoskeleton, rather than an abnormality sensu stricto. We compare injuries on extant xiphosurids to extinct Cambrian trilobite injuries to suggest that rare cephalic injuries to trilobites were incurred during soft-shelled exoskeletal stages. Reviewing xiphosurid injuries through time is a pivotal step towards understanding how Recent and extinct arthropods responded to injuries.

An enigmatic Arthropoda from the Upper Triassic (Carnian) southwestern Gondwana (Argentina)

“…3, 5.1) has an irregularity on the left margin of the shield (Fig. 5.1, c), like the cephalothoracic injuries found in extant xiphosurans (see Bicknell et al, 2018, fig. 2.5).…”

“…The asymmetrical split occurring at different lengths of this structure in the two fossil specimens suggests that it could be taphonomic breakage rather than a taxonomic feature. Similarly, divided telsons in extant xiphosurans have been reported as teratological abnormalities (see Bicknell et al, 2018, fig. 1), so this possibility should not be ruled out.…”

“…5.3) (Dunlop and Selden, 1997; Lamsdell, 2013). Nevertheless, the absence of ornamentation in our specimens could be due to a taphonomic issue, because spines and other structures are most likely to be lost during the process of fossilization (Bicknell et al, 2018).…”

Injured trilobites within a collection of dinosaurs: Using the Royal Tyrrell Museum of Palaeontology to document Cambrian predation

“…Specimens from Cambrian-aged deposits within the TMP collection were reviewed for trilo-bites showing some form of abnormality; features identified by their aberrant morphology (see discussions in Owen, 1985;Bicknell et al, 2018c;Bicknell and Pates, 2019; and references therein). Within the ~5500 examined specimens, six abnormal individuals were identified.…”

“…Despite the primary focus on dinosaur and other vertebrate groups, the TMP collection also houses an array of invertebrate taxa (Motz et al, 2001). To expand the use of this diverse collection, and to align with recent research documenting Cambrian predation (Fatka et al, 2015;Pates et al, 2017;Bicknell et al, 2018c;Bicknell and Pates, 2019;Klompmaker et al, 2019;Pates and Bicknell, 2019), Cambrian-aged fossils were reviewed for abnormal trilobite specimens. We present six new examples of abnormal trilobites, representing the taxa Gabriellus kierorum, Ogygopsis klotzi, Modocia typicalis, and the first record of an abnormal Hemirhodon amplipyge.…”