Dynamic and synchronous changes in metazoan body size during the Cambrian Explosion

Abstract

Many aspects of the drivers for, and evolutionary dynamics of, the cambrian explosion are poorly understood. Here we quantify high-resolution changes in species body size in major metazoan groups on the Siberian Platform during the early Cambrian (ca. 540-510 Million years ago (Ma)). Archaeocyath sponges, hyolith lophophorates, and helcionelloid mollusc species show dynamic and synchronous trends over million-year timescales, with peaks in body size during the latest tommotian/early Atbadanian and late Atdabanian/early Botoman, and notably small body sizes in the middle Atdabanian and after the Sinsk anoxic extinction event, starting ca. 513 Ma. These intervals of body size changes are also mirrored in individual species and correlate positively with increased rates of origination and broadly with total species diversity. Calcitic brachiopods (rhynchonelliformeans), however, show a general increase in body size following the increase in species diversity through this interval: phosphatic brachiopods (linguliformeans) show a body size decrease that negatively correlates with diversity. Both brachiopod groups show a rapid recovery at the Sinsk event. the synchronous changes in these metrics in archaeocyath, hyoliths and helcionelloids suggest the operation of external drivers through the early cambrian, such as episodic changes in oxygenation or productivity. But the trends shown by brachiopods suggests a differing physiological response. Together, these dynamics created both the distinct evolutionary record of metazoan groups during the cambrian explosion and determined the nature of its termination.The Cambrian Explosion, ca. 540-510 Ma records the dramatic diversification of metazoans and metazoan-dominated ecosystems, where the drivers may well be complex, with multiple feedbacks between dynamic environmental change and evolutionary response. Many typical early Cambrian groups such as archaeocyath sponges and hyoliths either went extinct or declined significantly during the Botoman-Toyonian extinction (ca. 513-508 Ma), starting with extensive shallow marine anoxia ca. 513 Ma, known as the 'Sinsk Event' 1 . By contrast brachiopods and trilobites show modest changes in diversity across the Sinsk Event and continued to diversify thereafter 2 . The Sinsk Event hence effectively marks the end of the canonical Cambrian Explosion, and resets the trajectory for subsequent metazoan evolution.Potential measures of the evolutionary dynamics across radiation events include temporal changes in biodiversity, varying origination and extinction rates, and changes in individual body size (soft tissue mass). Body size scales with many factors including individual metabolic rate, trophic status, temperature, and systematic affinity, and Cope's rule (where lineages evolve larger body sizes over time), has been shown to be common over many taxa and long timescales 3 . This is not surprising as body size correlates with many measures of ecological success in benthic invertebrates, such as enhanced fecundity, competitive superiority,...

Current understanding on the Cambrian Explosion: questions and answers

“…However, the variation of the linear body size was in interesting paces: the size of archaeocyath, hyolith, and helcionelloid mollusc species varied in a synchronous rhythm over million-year timescales, with peaks positively correlated with origination rate and total species diversity, which appear to be a consequence of external drivers. The response of brachiopods is quite different, calcitic shells showing a general increase in body size while phosphatic shells decreasing in size through time (Zhuravlev and Wood 2020).…”

“…The large size was reached before the Cambrian Explosion. A quantitative study of skeletal species from Siberia demonstrated marked changes in maximum linear body size dimensions but no unidirectional trend throughout the early Cambrian (Zhuravlev and Wood 2020). However, the variation of the linear body size was in interesting paces: the size of archaeocyath, hyolith, and helcionelloid mollusc species varied in a synchronous rhythm over million-year timescales, with peaks positively correlated with origination rate and total species diversity, which appear to be a consequence of external drivers.…”

Phylogenetic response of naraoiid arthropods to early–middle Cambrian environmental change

“…It has been observed that hypercalcified forms such as archaeocyaths were especially adversely affected in the Stage 4 extinctions, possibly because anoxia impedes biomineralization as a result of reduced surface‐water carbonate supersaturation (Zhuravlev & Wood 2020). Calcite biomineralization may likewise have been detrimental to trilobites during anoxic pulses, and is possibly a factor in accounting for differences in survivorship compared to non‐biomineralized arthropods such as naraoiids, although it need be noted that other arthropods exhibit a high hypoxic sensitivity (John & Walker 2016; Zhang & Cui 2016).…”

“…2017). Extinctions in Cambrian Series 2, for example, include the disappearance of the sponge group Archaeocyatha in the Cambrian Stage 4 Sinsk Event, and the loss of many lineages of trilobites, most notably of the Order Redlichiida, including the Olenellina towards the end of Series 2 and the Redlichiidae at the Series 2 – Miaolingian boundary (Zhuravlev & Wood 1996, 2018, 2020; Palmer 1998; Fagetter et al . 2017; Wood et al .…”

Enduring evolutionary embellishment of cloudinids in the Cambrian

“…Following the putative extinction of the Ediacaran biota, animals showed significant increase in diversity and disparity over a relatively short interval of geological time during which origination rates greatly exceeded extinction rates [ 3 ]. The Cambrian Explosion led to defensive strategies against predation, including significant size increase [ 4 ]. However, the framework on which this novelty was built remains shrouded because Ediacaran antecedents are unknown, evidence of lineages spanning the Ediacaran–Cambrian transition and the Cambrian Explosion being scant.…”