The reason for as well as the consequence of the cambrian explosion in animal evolution

Susumu Ohno

doi:10.1007/pl00000055

Survival and selection biases in early animal evolution and a source of systematic overestimation in molecular clocks

Interface Focus ◽

10.1098/rsfs.2019.0110 ◽

2020 ◽

Vol 10 (4) ◽

pp. 20190110 ◽

Cited By ~ 2

Author(s):

Graham E. Budd ◽

Richard P. Mann

Keyword(s):

Fossil Record ◽

Null Hypothesis ◽

Cambrian Explosion ◽

Molecular Clocks ◽

Statistical Features ◽

Statistical Structure ◽

Animal Evolution ◽

Evolutionary Radiation

Important evolutionary events such as the Cambrian Explosion have inspired many attempts at explanation: why do they happen when they do? What shapes them, and why do they eventually come to an end? However, much less attention has been paid to the idea of a ‘null hypothesis’—that certain features of such diversifications arise simply through their statistical structure. Such statistical features also appear to influence our perception of the timing of these events. Here, we show in particular that study of unusually large clades leads to systematic overestimates of clade ages from some types of molecular clocks, and that the size of this effect may be enough to account for the puzzling mismatches seen between these molecular clocks and the fossil record. Our analysis of the fossil record of the late Ediacaran to Cambrian suggests that it is likely to be recording a true evolutionary radiation of the bilaterians at this time, and that explanations involving various sorts of cryptic origins for the bilaterians do not seem to be necessary.

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Ediacaran Doushantuo-type biota discovered in Laurentia

Communications Biology ◽

10.1038/s42003-020-01381-7 ◽

2020 ◽

Vol 3 (1) ◽

Author(s):

Sebastian Willman ◽

John S. Peel ◽

Jon R. Ineson ◽

Niels H. Schovsbo ◽

Elias J. Rugen ◽

...

Keyword(s):

Environmental Change ◽

East Asia ◽

The Other ◽

Cambrian Explosion ◽

Doushantuo Formation ◽

Microbial World ◽

Environmental Distribution ◽

Animal Evolution ◽

Siliciclastic Rocks ◽

North Greenland

Abstract The Ediacaran period (635–541 Ma) was a time of major environmental change, accompanied by a transition from a microbial world to the animal world we know today. Multicellular, macroscopic organisms preserved as casts and molds in Ediacaran siliciclastic rocks are preserved worldwide and provide snapshots of early organismal, including animal, evolution. Remarkable evolutionary advances are also witnessed by diverse cellular and subcellular phosphatized microfossils described from the Doushantuo Formation in China, the only source showing a diversified assemblage of microfossils. Here, we greatly extend the known distribution of this Doushantuo-type biota in reporting an Ediacaran Lagerstätte from Laurentia (Portfjeld Formation, North Greenland), with phosphatized animal-like eggs, embryos, acritarchs, and cyanobacteria, the age of which is constrained by the Shuram–Wonoka anomaly (c. 570–560 Ma). The discovery of these Ediacaran phosphatized microfossils from outside East Asia extends the distribution of the remarkable biota to a second palaeocontinent in the other hemisphere of the Ediacaran world, considerably expanding our understanding of the temporal and environmental distribution of organisms immediately prior to the Cambrian explosion.

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Survival and selection biases in early animal evolution and a source of systematic overestimation in molecular clocks

10.31233/osf.io/j45yf ◽

2019 ◽

Cited By ~ 1

Author(s):

Graham Budd ◽

Richard P. Mann

Keyword(s):

Fossil Record ◽

Null Hypothesis ◽

Cambrian Explosion ◽

Molecular Clocks ◽

Statistical Structure ◽

Animal Evolution ◽

Evolutionary Radiation ◽

Large Groups ◽

Time Of Origin ◽

Looking Back

Important evolutionary events such as the Cambrian Explosion have inspired many attempts at explanation: why do they happen when they do? What shapes them, and why do they eventually come to an end? However, much less attention has been paid to the idea of a “null hypothesis” – that certain features of such diversifications arise simply through their statistical structure. Looking back from our own perspective to the origins of large groups such as the arthropods, or even the animals themselves, will expose features that look causal but are in fact inevitable. Here we review these features with particular regard to the Cambrian explosion. We conclude that the fossil record of the late Ediacaran to Cambrian is very likely to be recording a true evolutionary radiation at this time; and show how the unusually rapid nature of this event leads to characteristic over-estimation of its time of origin by molecular clock methods - an artefact that is likely to apply to other unusually fast radiations too.

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Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians

Proceedings of The Royal Society B Biological Sciences ◽

10.1098/rspb.2013.1613 ◽

2013 ◽

Vol 280 (1767) ◽

pp. 20131613 ◽

Cited By ~ 32

Author(s):

Jean-Bernard Caron ◽

Martin R. Smith ◽

Thomas H. P. Harvey

Keyword(s):

Whole Body ◽

Cambrian Explosion ◽

Burgess Shale ◽

Animal Evolution ◽

Middle Cambrian ◽

Animal Body ◽

Cuticular Structures

Burgess Shale-type deposits are renowned for their exquisite preservation of soft-bodied organisms, representing a range of animal body plans that evolved during the Cambrian ‘explosion’. However, the rarity of these fossil deposits makes it difficult to reconstruct the broader-scale distributions of their constituent organisms. By contrast, microscopic skeletal elements represent an extensive chronicle of early animal evolution—but are difficult to interpret in the absence of corresponding whole-body fossils. Here, we provide new observations on the dorsal spines of the Cambrian lobopodian (panarthropod) worm Hallucigenia sparsa from the Burgess Shale (Cambrian Series 3, Stage 5). These exhibit a distinctive scaly microstructure and layered (cone-in-cone) construction that together identify a hitherto enigmatic suite of carbonaceous and phosphatic Cambrian microfossils—including material attributed to Mongolitubulus , Rushtonites and Rhombocorniculum —as spines of Hallucigenia -type lobopodians. Hallucigeniids are thus revealed as an important and widespread component of disparate Cambrian communities from late in the Terreneuvian (Cambrian Stage 2) through the ‘middle’ Cambrian (Series 3); their apparent decline in the latest Cambrian may be partly taphonomic. The cone-in-cone construction of hallucigeniid sclerites is shared with the sclerotized cuticular structures (jaws and claws) in modern onychophorans. More generally, our results emphasize the reciprocal importance and complementary roles of Burgess Shale-type fossils and isolated microfossils in documenting early animal evolution.

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When Life Got Smart: The Evolution of Behavioral Complexity Through the Ediacaran and Early Cambrian of NW Canada

Journal of Paleontology ◽

10.1666/13-066 ◽

2014 ◽

Vol 88 (2) ◽

pp. 309-330 ◽

Cited By ~ 58

Author(s):

Calla Carbone ◽

Guy M. Narbonne

Keyword(s):

Three Dimensional ◽

Sensory Information ◽

Behavioral Responses ◽

Subsequent Development ◽

Cambrian Explosion ◽

Feeding Strategies ◽

Early Cambrian ◽

Filter Feeding ◽

Animal Evolution ◽

Deposit Feeding

Ediacaran and early Cambrian strata in NW Canada contain abundant trace fossils that record the progressive development of complex behavior in early animal evolution. Five feeding groups can be recognized: microbial grazing, deposit-feeding, deposit-feeding/predatory, filter-feeding/predatory, and arthropod tracks and trails. The lower Blueflower Formation (ca. 560–550 Ma) contains abundant burrows that completely cover bedding surfaces with small (∼1 mm diameter) cylindrical burrows that were strictly restricted to microbial bedding surfaces and exhibited only primitive and inconsistent avoidance strategies. The upper Blueflower contains three-dimensional avoidance burrows and rare filter-feeding or possibly predatory burrows, suggesting increased behavioral responses in food gathering that marked the beginning of the agronomic revolution in substrate utilization. Cambrian strata of the Ingta Formation contain systematically meandering burrows and more diverse feeding strategies, including the onset of treptichnid probing burrows that may reflect predation. These observations imply that Ediacaran burrowers were largely characterized by crude, two-dimensional avoidance meanders that represented simple behavioral responses of individual burrowers to sensory information, and that the subsequent development of more diverse and complex feeding patterns with genetically programmed search pathways occurred during the earliest stages of the Cambrian explosion. These observations further imply that changes occurred in both the food source and substrate during the ecological transition from Proterozoic matgrounds to Phanerozoic mixgrounds.

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Introduction: from snowball Earth to the Cambrian explosion–evidence from China

Geological Magazine ◽

10.1017/s0016756817000644 ◽

2017 ◽

Vol 154 (6) ◽

pp. 1187-1192 ◽

Cited By ~ 5

Author(s):

MAOYAN ZHU ◽

XIAN-HUA LI

Keyword(s):

South China ◽

Critical Time ◽

Snowball Earth ◽

Cambrian Explosion ◽

Time Interval ◽

Doushantuo Formation ◽

Animal Evolution ◽

The Earth ◽

Dengying Formation ◽

Rapid Diversification

The Neoproterozoic–Palaeozoic transition (NPT) around 600 Ma ago was a critical time interval when the Earth experienced fundamental change, manifested as climatic extremes – ‘snowball Earth’ – followed by the emergence and rapid diversification of animals – ‘Cambrian explosion’. How animals and environments co-evolved, and what caused these fundamental changes to the Earth system during the NPT, is a great scientific puzzle, which has been a rapidly developing frontier of interdisciplinary research between bio- and geosciences. South China preserves a complete stratigraphic succession of the NPT developed in various facies ranging from shallow to deep marine realms with extraordinarily well-preserved, successive fossil biotas in various taphonomic settings (Zhu, 2010; Fig. 1), making it a key area and global focus of studies in the field over recent decades. Indeed, the current narrative of early animal evolution has largely been based on the fossil biotas from South China. These include: (1) the world's oldest microscopic animal fossils with cellular details from the early Ediacaran Weng'an biota (Doushantuo Formation); (2) putative macroscopic animal fossils preserved as carbonaceous imprints from the early Ediacaran Lantian, Wenghui and Miaohe biotas (also Doushantuo Formation); (3) typical late Ediacaran faunas, preserved in dark limestone (Shibantan biota) and as large and poorly mineralized tubular animal fossils (Gaojiashan biota), both from the Dengying Formation; (4) phosphatized small shelly and soft-bodied animal fossils from the early Cambrian Meishucun and Kuanchuanpu faunas; and (5) Cambrian fossil Lagerstätten (Chengjiang, Guanshan and Kaili faunas) with typical Burgess Shale-type soft-bodied preservation.

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