Early fossil record of Euarthropoda and the Cambrian Explosion

Abstract

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., ) 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.,, ) 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.

Anoxia can increase the rate of decay for cnidarian tissue: Using Actinia equina to understand the early fossil record

“…The timing and tempo of animal diversification is one of the greatest controversies in palaeobiology. The Cambrian Explosion-a dramatic increase in the abundance of animal fossils during the Cambrian Period (Marshall, 2006)-is variably interpreted to reflect a real biological diversification (Antcliffe et al, 2014;Budd & Jensen, 2000;Daley, Antcliffe, Drage, & Pates, 2018) or merely an explosion of fossils with a very deep root for the origin of animals in the Precambrian (Erwin et al, 2011). The problem of identifying the earliest cnidarian fossils in many ways encapsulates the difficulties of resolving the first appearance of the Metazoa and are just as crucial as the debate taking place about the earliest sponge fossils (Antcliffe, 2013(Antcliffe, , 2015Antcliffe et al, 2014;Gold, Grabenstatter, et al, 2016;Gold, O'Reilly, Luo, Briggs, & Summons, 2016;Love et al, 2009;Muscente, Michel, Dale, & Xiao, 2015;Nettersheim et al, 2019).…”

Quantifying intra- and interspecific variability in trilobite moulting behaviour across the Palaeozoic

“…This is in contrast to the moulting behaviours of other arthropod groups, both extant and in the fossil record, which have morphologies and behaviours more specialised to one or two moulting methods . Trilobites, which appear earliest in the fossil record amongst crown-group arthropods (Daley et al, 2018), are therefore unique in their moulting. Previous studies of trilobite moulting, despite their clear importance, have been limited in scope or entirely qualitative observations (e.g., Henningsmoen, 1975;Whittington, 1980Whittington, , 1990McNamara and Rudkin, 1984;Busch and Swartz, 1985;Speyer, 1985;Speyer and Brett, 1985;McNamara, 1986;Brandt, 2002;Bruthansová, 2003;Clarkson et al, 2003;Budil and Bruthansová, 2005;Hunda et al, 2006;Paterson et al, 2007;Cederström et al, 2010;Rustán et al, 2011;Drage and Daley, 2016;Drage et al, 2018a), leaving many unanswered questions about the evolution of this behaviour.…”

Evolution: The Battle of the First Animals

“…Cambrian origination of animals [19,20], in line with these detailed paleontological studies [8,[15][16][17]. The ultimate goal in this field is to understand the mechanisms underlying the Cambrian Explosion, which requires both a clear timeline of the sequence of events and resolution of the interrelationships of the earliest diverging animal phyla.…”

“…Zhao and colleagues [8] suggest a plausible evolutionary scenario for the emergence of the highly derived ctenophores by taking a comprehensive approach drawing on multiple fossils from several localities and styles of preservation. This approach follows other recent studies focused on the palaeontological chronicle of different animal phyla, in particular sponges, cnidarians and arthropods [15][16][17]. These studies are converging on a stable timeline for the Cambrian explosion.…”

“…These studies are converging on a stable timeline for the Cambrian explosion. Stem lineages diversified during the Cambrian, leading to the construction of the modern crown groups, and originated no earlier than the very end of the preceding Ediacaran period, approximately 550 million years ago [8,[15][16][17][18]. As is the case with the arthropods, cnidarians and sponges, putative ctenophore fossils from the Ediacaran period were reexamined, and their ctenophore affinities rejected by Zhao and colleagues [8].…”