Complex axial growth patterns in an early Cambrian trilobite from South Australia

Abstract

The exceptional fossil record of trilobites provides our best window on developmental processes in early euarthropods, but data on growth dynamics are limited. Here, we analyse post-embryonic axial growth in the Cambrian trilobite Estaingia bilobata from the Emu Bay Shale, South Australia. Using threshold models, we show that abrupt changes in growth trajectories of different body sections occurred in two phases, closely associated with the anamorphic/epimorphic and meraspid/holaspid transitions. These changes are similar to the progression to sexual maturity seen in certain extant euarthropods and suggest that the onset of maturity coincided with the commencement of the holaspid period. We also conduct hypothesis testing to reveal the likely controls of observed axial growth gradients and suggest that size may better explain growth patterns than moult stage. The two phases of allometric change in E. bilobata , as well as probable differing growth regulation in the earliest post-embryonic stages, suggest that observed body segmentation patterns in this trilobite were the result of a complex series of changing growth controls that characterized different ontogenetic intervals. This indicates that trilobite development is more complex than previously thought, even in early members of the clade.

Reassessing a cryptic history of early trilobite evolution

“…They are one of the largest and most successful Paleozoic groups, persisting for some 270 million years, and represented by over 22,000 described species 14 , 15 . This excellent fossil record—a result of their easily-preserved, biomineralized exoskeleton that was molted many times during life—can be used to address important questions concerning early animal evolution 16 , 17 . Trilobites have been viewed as exemplary for the argument of deep bilaterian (and therefore metazoan) divergence dates, and formed an important part of the argument for early proponents of this view 4 , 18 , 19 .…”

Malformed individuals of the trilobiteEstaingia bilobatafrom the Cambrian Emu Bay Shale and their palaeobiological implications

“…Emuellids in particular may have been more prone to teratologies as they added new segments through euanamorphosis (Paterson & Edgecombe, 2006). These trilobites certainly had unusual patterns of trunk segment development compared with other forms (Holmes et al 2021a). This may have increased the likelihood of developmental malformations, indicating possible evidence for increased developmental plasticity within Cambrian trilobites compared to younger forms (Webster, 2007).…”

Life cycle evolution in the trilobitesBalangiaandDuyunaspisfrom the Cambrian Series 2 (Stage 4) of South China

“…duyunensis , but there was no sufficient evidence to confirm the correlation between sexual maturation of trilobite and its holaspid period ( Hughes, Minelli & Fusco, 2006 ). Based on the changes in growth trajectories, the onset of sexual maturity in the trilobite Estaingia bilobata is thought to coincide with the onset of the holaspid period ( Holmes, Paterson & García-Bellido, 2021b ). However, the changes in the growth trajectory of E .…”