Ontogeny of the Furongian (late Cambrian) trilobite Proceratopyge cf. P. Lata Whitehouse from northern Victoria Land, Antarctica, and the evolution of metamorphosis in trilobites

Abstract

There were multiple origins of metamorphosis‐undergoing protaspides in trilobite evolution: within the superfamilies Remopleuridioidea, Trinucleoidea, and within the Order Asaphida. Recent studies have revealed that the protaspides of the Cambrian representatives of the Remopleuridioidea and the Trinucleoidea did not undergo metamorphosis. However, ontogeny of the Cambrian members of the Order Asaphida has remained unknown. This study documents the ontogeny of the Furongian asaphoidean ceratopygid trilobite, Proceratopyge cf. P. lata Whitehouse, from northern Victoria Land, Antarctica. Two stages for the protaspid phase, five developmental stages for the post‐protaspid cranidia, and ten stages for the post‐protaspid pygidia have been identified. Interestingly, the protaspis directly developed into a meraspis without metamorphosis. A new cladistic analysis resulted in a single most parsimonious tree, according to which the presence of the bulbous commutavi protaspis turns out to be a synapomorphy for Asaphidae + Cyclopygoidea, not a synapomorphy for the Order Asaphida as previously suggested. In addition, it is inferred that there was convergent evolution of indirectly‐developing commutavi protaspides during the Furongian and Early Ordovician. Metamorphosis‐entailing planktonic larvae evolved in many different metazoan lineages near the Cambrian–Ordovician transition, due to the escalating ecological pressure of the Great Ordovician Biodiversification Event. Since the bulbous commutavi protaspid morphology is thought to be an adaptation for a planktonic life mode, the convergent evolution of the indirect development in the three trilobite lineages at this period might have been a result of adaptation to the early phase of the Great Ordovician Biodiversification Event.

Ontogeny of the two co-occurring middle Furongian (late Cambrian) shumardiid trilobites and the protaspid morphology of shumardiids

“…Harper, Zhan, & Jin, 2015). Park et al (2016) ascribed the convergent evolution of commutavi protaspides during the Furongian – Early Ordovician to adaptation to ecological pressure in the early phase of GOBE. The appearance of metamorphosis-undergoing commutavi protaspis in shumardiids, the small and blind trilobite group, might also have been due to the onset of ecological pressure in the early phase of GOBE.…”

Middle Cambrian slope deposits in northern Victoria Land, Antarctica: Fingerprinting small carbonate platforms dominated by grainy carbonates and microbial reefs

“…The Spurs successions consist largely of shale, with thinly bedded sandstone, carbonate-block-bearing conglomerates and limestone lenses, and are interpreted as downslope and deeper shelf deposits (Laird and Bradshaw, 1983;Kim et al, 2018). Lower and upper boundaries of the formation are somewhat diachronous across the terrane, but they are approximately assigned as late Miaolingian to early Furongian based on trilobite biozones yielded from shale, limestone and carbonate intraclast conglomerate (Cooper et al, 1996;Geyer and Shergold, 2000;Park et al, 2016).…”

The trouble with trilobites: classification, phylogeny and the cryptogenesis problem

“…The two key synapomorphies that Fortey & Chatterton (1988) considered to unite members of the Asaphida have been subsequently criticized, with both the ventral median cephalic suture (Figs 2b, 3b; Whittington, 2003;Zhu et al 2007Zhu et al , 2010Park & Choi, 2009Park et al 2014) and the bulbous 'commutavi' (or 'asaphoid-type') protaspis ( Fig. 2c, d; Park & Kihm, 2015;Park et al 2016;Park, 2018) thought to have evolved independently across disparate groups. Even so, this does not render these characters as inoperative for diagnosing Asaphida.…”