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

2020 ◽  
Vol 10 (4) ◽  
pp. 20190110 ◽  
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.

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

2019 ◽  
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.

scholarly journals Molecular clocks and the early evolution of metazoan nervous systems

2015 ◽  
Vol 370 (1684) ◽  
pp. 20150046 ◽  
Author(s):  
Gregory A. Wray
Keyword(s):  
Sensory Processing ◽  
Fossil Record ◽  
Sequence Data ◽  
Divergence Times ◽  
Molecular Clocks ◽  
Sense Organs ◽  
Animal Evolution ◽  
Nervous Systems ◽  
Nervous System Evolution ◽  
Fossil Records

The timing of early animal evolution remains poorly resolved, yet remains critical for understanding nervous system evolution. Methods for estimating divergence times from sequence data have improved considerably, providing a more refined understanding of key divergences. The best molecular estimates point to the origin of metazoans and bilaterians tens to hundreds of millions of years earlier than their first appearances in the fossil record. Both the molecular and fossil records are compatible, however, with the possibility of tiny, unskeletonized, low energy budget animals during the Proterozoic that had planktonic, benthic, or meiofaunal lifestyles. Such animals would likely have had relatively simple nervous systems equipped primarily to detect food, avoid inhospitable environments and locate mates. The appearance of the first macropredators during the Cambrian would have changed the selective landscape dramatically, likely driving the evolution of complex sense organs, sophisticated sensory processing systems, and diverse effector systems involved in capturing prey and avoiding predation.

scholarly journals Trilobite evolutionary rates constrain the duration of the Cambrian explosion

2019 ◽  
Vol 116 (10) ◽  
pp. 4394-4399 ◽  
Author(s):  
John R. Paterson ◽  
Gregory D. Edgecombe ◽  
Michael S. Y. Lee
Keyword(s):  
Fossil Record ◽  
Evolutionary Rates ◽  
Cambrian Explosion ◽  
Molecular Clocks ◽  
Genomic Evolution ◽  
Quantitative Evidence ◽  
Dating Methods ◽  
Marine Biosphere ◽  
Evolutionary Stasis ◽  
Broad Scale

Trilobites are often considered exemplary for understanding the Cambrian explosion of animal life, due to their unsurpassed diversity and abundance. These biomineralized arthropods appear abruptly in the fossil record with an established diversity, phylogenetic disparity, and provincialism at the beginning of Cambrian Series 2 (∼521 Ma), suggesting a protracted but cryptic earlier history that possibly extends into the Precambrian. However, recent analyses indicate elevated rates of phenotypic and genomic evolution for arthropods during the early Cambrian, thereby shortening the phylogenetic fuse. Furthermore, comparatively little research has been devoted to understanding the duration of the Cambrian explosion, after which normal Phanerozoic evolutionary rates were established. We test these hypotheses by applying Bayesian tip-dating methods to a comprehensive dataset of Cambrian trilobites. We show that trilobites have a Cambrian origin, as supported by the trace fossil record and molecular clocks. Surprisingly, they exhibit constant evolutionary rates across the entire Cambrian, for all aspects of the preserved phenotype: discrete, meristic, and continuous morphological traits. Our data therefore provide robust, quantitative evidence that by the time the typical Cambrian fossil record begins (∼521 Ma), the Cambrian explosion had already largely concluded. This suggests that a modern-style marine biosphere had rapidly emerged during the latest Ediacaran and earliest Cambrian (∼20 million years), followed by broad-scale evolutionary stasis throughout the remainder of the Cambrian.

Fossils, molecules and embryos: new perspectives on the Cambrian explosion

Development ◽  
1999 ◽  
Vol 126 (5) ◽  
pp. 851-859 ◽  
Author(s):  
J.W. Valentine ◽  
D. Jablonski ◽  
D.H. Erwin
Keyword(s):  
Fossil Record ◽  
Cambrian Explosion ◽  
Molecular Clocks ◽  
Early Cambrian ◽  
Developmental Systems ◽  
Developmental Control ◽  
Sudden Appearance

The Cambrian explosion is named for the geologically sudden appearance of numerous metazoan body plans (many of living phyla) between about 530 and 520 million years ago, only 1.7% of the duration of the fossil record of animals. Earlier indications of metazoans are found in the Neoproterozic; minute trails suggesting bilaterian activity date from about 600 million years ago. Larger and more elaborate fossil burrows appear near 543 million years ago, the beginning of the Cambrian Period. Evidence of metazoan activity in both trace and body fossils then increased during the 13 million years leading to the explosion. All living phyla may have originated by the end of the explosion. Molecular divergences among lineages leading to phyla record speciation events that have been earlier than the origins of the new body plans, which can arise many tens of millions of years after an initial branching. Various attempts to date those branchings by using molecular clocks have disagreed widely. While the timing of the evolution of the developmental systems of living metazoan body plans is still uncertain, the distribution of Hox and other developmental control genes among metazoans indicates that an extensive patterning system was in place prior to the Cambrian. However, it is likely that much genomic repatterning occurred during the Early Cambrian, involving both key control genes and regulators within their downstream cascades, as novel body plans evolved.

scholarly journals Darwin's dilemma: the realities of the Cambrian ‘explosion’

2006 ◽  
Vol 361 (1470) ◽  
pp. 1069-1083 ◽  
Author(s):  
Simon Conway Morris
Keyword(s):  
Fossil Record ◽  
Current Work ◽  
Cambrian Explosion ◽  
Molecular Clocks ◽  
Burgess Shale ◽  
Cause And Effect ◽  
Evolutionary Event ◽  
History Of ◽  
Wrong Sort

The Cambrian ‘explosion’ is widely regarded as one of the fulcrum points in the history of life, yet its origins and causes remain deeply controversial. New data from the fossil record, especially of Burgess Shale-type Lagerstätten, indicate, however, that the assembly of bodyplans is not only largely a Cambrian phenomenon, but can already be documented in fair detail. This speaks against a much more ancient origin of the metazoans, and current work is doing much to reconcile the apparent discrepancies between the fossil record, including the Ediacaran assemblages of latest Neoproterozoic age and molecular ‘clocks’. Hypotheses to explain the Cambrian ‘explosion’ continue to be generated, but the recurrent confusion of cause and effect suggests that the wrong sort of question is being asked. Here I propose that despite its step-like function this evolutionary event is the inevitable consequence of Earth and biospheric change.

scholarly journals Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction

2016 ◽  
Vol 283 (1833) ◽  
pp. 20153026 ◽  
Author(s):  
Thomas John Dixon Halliday ◽  
Paul Upchurch ◽  
Anjali Goswami
Keyword(s):  
Phylogenetic Analysis ◽  
Adaptive Radiation ◽  
Mass Extinction ◽  
Fossil Record ◽  
Morphological Evolution ◽  
Placental Mammal ◽  
Stochastic Methods ◽  
Ecological Niches ◽  
Evolutionary Radiation ◽  
Quantitative Analyses

The effect of the Cretaceous–Palaeogene (K–Pg) mass extinction on the evolution of many groups, including placental mammals, has been hotly debated. The fossil record suggests a sudden adaptive radiation of placentals immediately after the event, but several recent quantitative analyses have reconstructed no significant increase in either clade origination rates or rates of character evolution in the Palaeocene. Here we use stochastic methods to date a recent phylogenetic analysis of Cretaceous and Palaeocene mammals and show that Placentalia likely originated in the Late Cretaceous, but that most intraordinal diversification occurred during the earliest Palaeocene. This analysis reconstructs fewer than 10 placental mammal lineages crossing the K–Pg boundary. Moreover, we show that rates of morphological evolution in the 5 Myr interval immediately after the K–Pg mass extinction are three times higher than background rates during the Cretaceous. These results suggest that the K–Pg mass extinction had a marked impact on placental mammal diversification, supporting the view that an evolutionary radiation occurred as placental lineages invaded new ecological niches during the Early Palaeocene.

scholarly journals Early fossil record of Euarthropoda and the Cambrian Explosion

2018 ◽  
Vol 115 (21) ◽  
pp. 5323-5331 ◽  
Author(s):  
Allison C. Daley ◽  
Jonathan B. Antcliffe ◽  
Harriet B. Drage ◽  
Stephen Pates
Keyword(s):  
Fossil Record ◽  
Trace Fossils ◽  
Cambrian Explosion ◽  
Burgess Shale ◽  
Crown Group ◽  
Total Group ◽  
Stem Lineage ◽  
Coherent Picture ◽  
Insight Into ◽  
Phosphate Deposits

Euarthropoda is one of the best-preserved fossil animal groups and has been the most diverse animal phylum for over 500 million years. Fossil Konservat-Lagerstätten, such as Burgess Shale-type deposits (BSTs), show the evolution of the euarthropod stem lineage during the Cambrian from 518 million years ago (Ma). The stem lineage includes nonbiomineralized groups, such as Radiodonta (e.g., Anomalocaris) that provide insight into the step-by-step construction of euarthropod morphology, including the exoskeleton, biramous limbs, segmentation, and cephalic structures. Trilobites are crown group euarthropods that appear in the fossil record at 521 Ma, before the stem lineage fossils, implying a ghost lineage that needs to be constrained. These constraints come from the trace fossil record, which show the first evidence for total group Euarthropoda (e.g., Cruziana, Rusophycus) at around 537 Ma. A deep Precambrian root to the euarthropod evolutionary lineage is disproven by a comparison of Ediacaran and Cambrian lagerstätten. BSTs from the latest Ediacaran Period (e.g., Miaohe biota, 550 Ma) are abundantly fossiliferous with algae but completely lack animals, which are also missing from other Ediacaran windows, such as phosphate deposits (e.g., Doushantuo, 560 Ma). This constrains the appearance of the euarthropod stem lineage to no older than 550 Ma. While each of the major types of fossil evidence (BSTs, trace fossils, and biomineralized preservation) have their limitations and are incomplete in different ways, when taken together they allow a coherent picture to emerge of the origin and subsequent radiation of total group Euarthropoda during the Cambrian.

Ecdysozoan-like sclerites among Ediacaran microfossils

2015 ◽  
Vol 152 (6) ◽  
pp. 1145-1148 ◽  
Author(s):  
MAŁGORZATA MOCZYDŁOWSKA ◽  
GRAHAM E. BUDD ◽  
HEDA AGIĆ
Keyword(s):  
Fossil Record ◽  
East European Platform ◽  
Cambrian Explosion ◽  
East European ◽  
Stratigraphic Position ◽  
Cambrian System

AbstractWe report the occurrence of organically preserved microfossils from the subsurface Ediacaran strata overlying the East European Platform in Poland, in the form of sclerites and cuticle fragments of larger organisms. They are morphologically similar to those known from Cambrian strata and associated with various metazoan fossils of recognized phyla. The Ediacaran age of the microfossils is evident from the stratigraphic position below the base of the Cambrian System and above the isotopically dated tuff layers at c. 551±4Ma. Within this strata interval, other characteristic Ediacaran microorganisms co-occur such as cyanobacteria, vendotaenids, microalgae, Ceratophyton, Valkyria and macroscopic annelidan Sabellidites. The recent contributions of organic sclerites in revealing the scope of the Cambrian explosion are therefore also potentially extendable back to the Ediacaran Period when animals first appear in the fossil record.

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