The Cambrian Explosion and the Evolution of Protostomes

AbstractThe Cambrian Explosion by nature is a three-phased explosion of animal body plans alongside episodic biomineralization, pulsed change of generic diversity, body size variation, and progressive increase of ecosystem complexity. The Cambrian was a time of crown groups nested by numbers of stem groups with a high-rank taxonomy of Linnaean system (classes and above). Some stem groups temporarily succeeded while others were ephemeral and underrepresented by few taxa. The high number of stem groups in the early history of animals is a major reason for morphological gaps across phyla that we see today. Most phylum-level clades achieved their maximal disparity (or morphological breadth) during the time interval close to their first appearance in the fossil record during the early Cambrian, whereas others, principally arthropods and chordates, exhibit a progressive exploration of morphospace in subsequent Phanerozoic. The overall envelope of metazoan morphospace occupation was already broad in the early Cambrian though it did not reach maximal disparity nor has diminished significantly as a consequence of extinction since the Cambrian. Intrinsic and extrinsic causes were extensively discussed but they are merely prerequisites for the Cambrian Explosion. Without the molecular evolution, there could be no Cambrian Explosion. However, the developmental system is alone insufficient to explain Cambrian Explosion. Time-equivalent environmental changes were often considered as extrinsic causes, but the time coincidence is also insufficient to establish causality. Like any other evolutionary event, it is the ecology that make the Cambrian Explosion possible though ecological processes failed to cause a burst of new body plans in the subsequent evolutionary radiations. The Cambrian Explosion is a polythetic event in natural history and manifested in many aspects. No simple, single cause can explain the entire phenomenon.

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Silicon isotopes reveal a decline in oceanic dissolved silicon driven by biosilicification: A prerequisite for the Cambrian Explosion?

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2021.116959 ◽

2021 ◽

Vol 566 ◽

pp. 116959

Author(s):

Yan Ye ◽

Patrick J. Frings ◽

Friedhelm von Blanckenburg ◽

Qinglai Feng

Keyword(s):

Cambrian Explosion ◽

Silicon Isotopes ◽

Dissolved Silicon

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Documentation by citizen scientists/naturalists of the ‘Cambrian explosion’ in Pennsylvania

Geology Today ◽

10.1111/gto.12343 ◽

2021 ◽

Vol 37 (2) ◽

pp. 57-62

Author(s):

Roger D. K. Thomas

Keyword(s):

Cambrian Explosion

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Basal Cambrian microfossils from the Yangtze Gorges area (South China) and the Aksu area (Tarim block, northwestern China)

Journal of Paleontology ◽

10.1017/s0022336000058108 ◽

2009 ◽

Vol 83 (1) ◽

pp. 30-44 ◽

Cited By ~ 25

Author(s):

Lin Dong ◽

Shuhai Xiao ◽

Bing Shen ◽

Chuanming Zhou ◽

Guoxiang Li ◽

...

Keyword(s):

Geographic Distribution ◽

South China ◽

Northwestern China ◽

Cambrian Explosion ◽

Filamentous Cyanobacteria ◽

Primary Producers ◽

Tarim Block ◽

Eukaryotic Phytoplankton ◽

History Of ◽

Wide Geographic Distribution

The basal Cambrian marks the beginning of an important chapter in the history of life. However, most paleontological work on the basal Cambrian has been focused on skeletal animal fossils, and our knowledge about the primary producers—cyanobacteria and eukaryotic phytoplankton (e.g., acritarchs)—is limited. In this research, we have investigated basal Cambrian acritarchs, coccoidal microfossils, and cyanobacteria preserved in phosphorites and cherts of the Yanjiahe Formation in the Yangtze Gorges area (South China) and the Yurtus Formation in the Aksu area (Tarim Block, northwestern China). Our study confirms the occurrence in these two formations of small acanthomorphic acritarchs characteristic of the basal CambrianAsteridium–Comasphaeridium–Heliosphaeridium(ACH) assemblage. These acritarchs include abundantHeliosphaeridium ampliatimi(Wang, 1985) Yao et al., 2005, commonYurtusia uniformisn. gen. and n. sp., and rareComasphaeridium annulare(Wang, 1985) Yao et al., 2005. In addition, these basal Cambrian successions also contain the clustered coccoidal microfossilArchaeophycus yunnanensis(SonginLuo et al., 1982) n. comb., several filamentous cyanobacteria [Cyanonema majusn. sp.,Oscillatoriopsis longaTimofeev and Hermann, 1979, andSiphonophycus robustum(Schopf, 1968) Knoll et al., 1991], and the tabulate tubular microfossilMegathrix longusL. Yin, 1987a, n. emend. Some of these taxa (e.g.,H. ampliatum, C. annulare, andM. longus) have a wide geographic distribution but occur exclusively in basal Cambrian successions, supporting their biostratigraphic importance. Comparison between the stratigraphic occurrences of microfossils reported here and skeletal animal fossils published by others suggests that animals and phytoplankton radiated in tandem during the Cambrian explosion.

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