Xingliang Zhang Current understanding on the Cambrian Explosion: questions and answers 14 July 2021
Was the Darwin’s dilemma resolved?
The contrast between Precambrian rocks almost barren of animal fossils and Cambrian rocks in which they may abound had puzzled scientists for over 100 years and was referred to Darwin’s dilemma (e.g., Schopf 2000; Conway Morris 2006). However, Darwin himself inferred “… if the theory be true, it is indisputable that before the lowest Silurian (revised to Cambrian in later editions) stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian (Cambrian) age to the present day; and that during these vast periods the world swarmed with living creatures (Darwin 1859: 287).” The discovery of Eozoon canadense, once identified as fossilized shells of giant foraminiferans and later reinterpreted as a pseudofossil (see discussion in Schopf 2000), in the Precambrian Laurentian Formation of Canada was cited by Darwin to support his inference about the existence of living beings long before the Cambrian Period (Darwin 1872). On the one hand, Darwin thought the Cambrian Explosion was inexplicable using his theory and on the other hand he seemed to firmly believe his explanations. This paradoxical combination may lead him draw a conclusion in the end of the Chapter 9 of On the imperfection of geological record, which stated “On this view the difficulties (to his theory of evolution) above discussed are greatly diminished or even disappear (Darwin 1859: 311)”. Taking this case into consideration, it was difficult to know if Darwin really had a dilemma on the Cambrian Explosion.
The hunt for Precambrian fossils has never been ceasing since 1850s. Specimens of Eozoon were first discovered in 1858, a year before publication of Darwin’s theory of evolution, although their biological interpretation finally collapsed. Since then, of course, much has been learnt. It is, however, a very difficult journey! The existence of life before the Cambrian became widely accepted until 1950s (Schopf 2000). Multidisciplinary investigations did extend the life history deep into Archean (e.g., Shen et al. 2001; Duda et al. 2016; Tashiro et al. 2017; Lepot 2020; Mißbach et al. 2021) but only heightened the sharpness of the beginnings of animal fossils across the Ediacaran–Cambrian transition. All fossil records still point to an explosion of animal life near the beginning of the Cambrian.
What does the fossil record tell us?
Precambrian palaeontologists are at loss to search for animal remains in ancient rocks, but results are somewhat awkward. The findings do not extend the dawn of animal life much further back. Body, trace, and chemical remains of animals are not as ancient as Darwin predicted but have been found in the time slice just before the Cambrian, i.e., late Ediacaran (~ 580.0–538.8 Ma). Moreover, most Ediacaran candidates of animals are a bit weird and thus difficult to be placed in any known animal group. More familiar animals arrived in the early Cambrian.
Most living animal phyla made their first appearance in the fossil record during the first 20 million years of the Cambrian Period (Erwin et al. 2011; Zhang and Shu 2014; Shu et al. 2014).
What is the nature of the Cambrian Explosion?
Explosion of fossils resulted from explosion of animals! Fossil record does capture the explosive evolution of animal body plans along with biomineralization, body size variation and increase of ecosystem complexity.
However, a major burst of bilateral body plans took place in the first 20 Ma of the Cambrian . Taking the Euarthropoda, the most diversified phylum, for example, molecular clock studies constrained the origin between 561 and 530 Ma , spanning into the Cambrian. The earliest arthropod trace dates to the Fortunian (~ 538.8–529.0 Ma) (Mángano and Buatois 2020). In addition, divergence time analyses using morphological data also recover a Cambrian origin for the Euarthropoda (Wolfe 2017). The estimated timing of origin is remarkably congruent with the fossil record.
Simply there are morphological gaps between phyla. By analyzing the timing of appearance of Linnaean rank taxa in the fossil record, paleontologists realized that the limits on animal disparity were early in animal evolutionary history. By the reinterpretation of the Burgess Shale fauna, Gould suggested a pattern of rapid, maximal disparity in the early history followed by later removal of most groups (stem groups) by extinction that leaves large morphological gaps among high-rank clades. This pattern is applicable to high-rank clades and metazoans as a whole. Particularly, he argued that that the morphological disparity of arthropods at a single locality (Burgess Shale) surpassed all extant arthropods, which inspired considerable efforts to understand disparity. Subsequent quantitative studies have shown that most clades achieved their maximal disparity (or morphological breadth) during a short time interval close to their first appearance in the fossil record in the early Cambrian. A more recent study by mapping of fossil and living metazoan morphospace demonstrated that the majority of phylum-level clades achieved maximal initial disparity in the Cambrian and that the overall disparity was already very broad in the early history of animal evolution, although the envelope of disparity explored by the Metazoa has increased through geological time. It is worth mentioning that new discoveries of weird forms in Cambrian deposits would increase the morphological breadth of Cambrian animals.
XingLiang Zhang Causes and consequences of the Cambrian explosion 22 November 2013
Newly Discovered Fossil Species Named After Star Wars Starship
The 500-million-year-old species is a distant relative of today’s crabs, spiders, and insects.
David F. Coppedge Soft Tissue in Biggest Ever Cambrian Fossil Bonanza March 23, 2019 2
This is in shale rock, but could hardly be better preserved by amber. It looks as though the creatures was buried instantly. If not, this amount of detail would not have survived a day. Nature summarizes the context of this fossil bed, and notes the delicacy of soft-bodied creatures preserved in this fashion. What does it take to preserve details of a jellyfish, or a comb jelly?
‘A stroke of luck’
Finding fossilized nerve tissue from the Cambrian period, which took place between about 543 million and 490 million years ago, is a “rarity,” Ortega-Hernández said. “It’s really a stroke of luck.” Scientists uncovered the first evidence of a fossilized arthropod brain from the Cambrian period about a decade ago, according to a 2012 report in the journal NatureCommunications; 4
Feiyang Chen First report on Guanshan Biota (Cambrian Stage 4) at the stratotype area of Wulongqing Formation in Malong County, Eastern Yunnan, China 5
The Guanshan Biota (Cambrian Series 2, Stage 4) in the Yunnan Province of South China, has produced abundant and diverse, exquisitely preserved fossils that often retain soft tissues and organs. The fossils are preserved by carbonaceous compressions, pyritization, phosphatization and clay mineral replacement, and display high fidelity preservation of soft tissues, similar to that seen in the Chengjiang and other Guanshan Biota localities west of the Xiaojiang Fault. New discoveries of soft-bodied assemblage from the Kanfuqing section on the eastern side of the Xiaojiang Fault has increased the occurrences of the Guanshan Biota, and suggest that similar faunal assemblages were deposited across the basin after the transgression of the eastern Yunnan Sea.
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