The Cambrian explosion

Assembled from 60 blocks · 2 evidence · 31 related

1. Story
2. The interval in which almost all animal body plans that exist today first appear in the fossil record, together with mineralised skeletons, complex burrowing, eyes and predation. The word "explosion" is doing two jobs and only one of them is safe. As a description of the fossil record it is accurate: the change from the Ediacaran to the Cambrian assemblages is abrupt and it is worldwide. As a description of evolutionary history it is contested, because molecular clocks place the divergences of the major animal lineages substantially earlier, in the Ediacaran or the Cryogenian, implying a long cryptic history of small, unskeletonised, rarely preserved animals. Several developments could each make an existing lineage suddenly visible without any new lineage arising: biomineralisation, an increase in body size, the onset of deep burrowing, and the appearance of predators driving the evolution of hard parts. The base of the Cambrian is defined at Fortune Head in Newfoundland and dated on recent ICS charts at 538.8 ± 0.2 Ma, revised down from the 541 Ma figure cited widely through the 2010s.
3. Knowledge
4. Molecular clock dating
5. Fortune Head GSSP, Newfoundland
6. Konservat-Lagerstätte (exceptional preservation)
7. Burgess Shale, Yoho National Park
8. Anomalocaris
9. Chengjiang fossil site, Yunnan
10. The Cambrian explosion
11. The end-Ordovician mass extinction
12. Connections
13. Molecular clock dating
14. Ediacara Hills, Flinders Ranges
15. Fortune Head GSSP, Newfoundland
16. Konservat-Lagerstätte (exceptional preservation)
17. Burgess Shale, Yoho National Park
18. Anomalocaris
19. Chengjiang fossil site, Yunnan
20. The Great Oxidation Event
21. The end-Ordovician mass extinction
22. Fortune Head GSSP, Newfoundland
23. Burgess Shale, Yoho National Park
24. Chengjiang fossil site, Yunnan
25. Anomalocaris
26. Konservat-Lagerstätte (exceptional preservation)
27. Molecular clock dating
28. The Cambrian explosion
29. Mass extinction
30. The Late Devonian crisis
31. International Commission on Stratigraphy
32. The Cambrian explosion
33. Global Boundary Stratotype Section and Point
34. The Cambrian explosion
35. The numerical age of the Cambrian base is revised to 538.8 Ma
36. The Cambrian explosion
37. Konservat-Lagerstätte (exceptional preservation)
38. The Cambrian explosion
39. Walcott locates the Burgess Shale beds
40. The Cambridge re-description of the Burgess fauna begins
41. The Cambrian explosion
42. Konservat-Lagerstätte (exceptional preservation)
43. The Cambrian explosion
44. Hou Xianguang finds the Chengjiang fauna
45. The Cambrian explosion
46. The Cambrian explosion
47. The Cambridge re-description of the Burgess fauna begins
48. The Cambrian explosion
49. Burgess Shale, Yoho National Park
50. Chengjiang fossil site, Yunnan
51. The Cambrian explosion
52. Hou Xianguang finds the Chengjiang fauna
53. The Cambrian explosion
54. LUCA — the last universal common ancestor
55. Radiometric dating
56. The Cambrian explosion
57. LUCA is dated to about 4.2 Ga by phylogenomic analysis
58. Evidence
59. Supports the stage and period boundary ages used throughout this pack, including the Cambrian base at 538.8 ± 0.2 Ma on recent editions against 541.0 ± 1.0 Ma on earlier ones. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked. No chart edition was opened, so the boundary figures are quoted from standing knowledge of the chart rather than read from it, and the edition in which the Cambrian base changed is not identified.
60. Supports the narrative of Walcott's discovery and classification, the Whittington–Briggs–Conway Morris re-description, and the reassembly of Anomalocaris from three separately named fossils. Gould's own thesis about disparity and contingency is a contested interpretation and is not asserted as fact in this pack. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked.