Editing Paleozoic (section)

== Boundaries ==
The base of the Paleozoic is one of the major divisions in geological time representing the divide between the Proterozoic and Phanerozoic eons, the Paleozoic and Neoproterozoic eras and the [[Ediacaran]] and Cambrian periods.<ref name=":0">{{Cite journal |last=Geyer |first=Gerd |last2=Landing |first2=Ed |date=2016-11-02 |title=The Precambrian–Phanerozoic and Ediacaran–Cambrian boundaries: a historical approach to a dilemma |url=http://dx.doi.org/10.1144/sp448.10 |journal=Geological Society, London, Special Publications |volume=448 |issue=1 |pages=311–349 |doi=10.1144/sp448.10 |issn=0305-8719}}</ref> When Adam Sedgwick named the Paleozoic in 1835, he defined the base as the first appearance of complex life in the rock record as shown by the presence of [[trilobite]]-dominated fauna.<ref name="Sedgwick-1838" /> Since then evidence of complex life in older rock sequences has increased and by the second half of the 20th century, the first appearance of [[small shelly fauna]] (SSF), also known as early skeletal fossils, were considered markers for the base of the Paleozoic. However, whilst SSF are well preserved in [[Carbonate rock|carbonate]] sediments, the majority of Ediacaran to Cambrian rock sequences are composed of [[siliciclastic]] rocks where skeletal fossils are rarely preserved.<ref name=":0" /> This led the [[International Commission on Stratigraphy]] (ICS) to use [[trace fossil]]s as an indicator of complex life.<ref name=":1">{{Citation |last=Peng |first=S. C. |title=Chapter 19 – The Cambrian Period |date=2020-01-01 |url=https://www.sciencedirect.com/science/article/pii/B978012824360200019X |work=Geologic Time Scale 2020 |pages=565–629 |editor-last=Gradstein |editor-first=Felix M. |access-date=2023-08-24 |publisher=Elsevier |isbn=978-0-12-824360-2 |last2=Babcock |first2=L. E. |last3=Ahlberg |first3=P. |editor2-last=Ogg |editor2-first=James G. |editor3-last=Schmitz |editor3-first=Mark D. |editor4-last=Ogg |editor4-first=Gabi M.}}</ref> Unlike later in the fossil record, Cambrian trace fossils are preserved in a wide range of sediments and environments, which aids correlation between different sites around the world. Trace fossils reflect the complexity of the body plan of the organism that made them. Ediacaran trace fossils are simple, sub-horizontal feeding traces. As more complex organisms evolved, their more complex behaviour was reflected in greater diversity and complexity of the trace fossils they left behind.<ref name=":0" /> After two decades of deliberation, the ICS chose [[Fortune Head]], Burin Peninsula, Newfoundland as the basal Cambrian Global Stratotype Section and Point (GSSP) at the base of the ''[[Treptichnus|Treptichnus pedum]]'' assemblage of trace fossils and immediately above the last occurrence of the Ediacaran problematica fossils ''[[Harlaniella]] podolica'' and ''[[Palaeopascichnus|Palaeopsacichnus]]''.<ref name=":1" /> The base of the Phanerozoic, Paleozoic and Cambrian is dated at 538.8+/-0.2 Ma and now lies below both the first appearance of trilobites and SSF.<ref name=":0" /><ref name=":1" />

The boundary between the Paleozoic and Mesozoic eras and the Permian and Triassic periods is marked by the first occurrence of the [[conodont]] ''[[Hindeodus|Hindeodus parvus]]''. This is the first [[Biostratigraphy|biostratigraphic]] event found worldwide that is associated with the beginning of the recovery following the end-[[Permian–Triassic extinction event|Permian mass extinctions]] and environmental changes. In non-marine strata, the equivalent level is marked by the disappearance of the Permian [[Dicynodon]] [[tetrapod]]s.<ref name=":2">{{Citation |last=Ogg |first=J. G. |title=Chapter 25 – The Triassic Period |date=2020-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128243602000255 |work=Geologic Time Scale 2020 |pages=903–953 |editor-last=Gradstein |editor-first=Felix M. |access-date=2023-08-24 |publisher=Elsevier |isbn=978-0-12-824360-2 |last2=Chen |first2=Z. -Q. |last3=Orchard |first3=M. J. |last4=Jiang |first4=H. S. |editor2-last=Ogg |editor2-first=James G. |editor3-last=Schmitz |editor3-first=Mark D. |editor4-last=Ogg |editor4-first=Gabi M.}}</ref> This means events previously considered to mark the Permian-Triassic boundary, such as the eruption of the [[Siberian Traps]] [[flood basalt]]s, the onset of greenhouse climate, [[Anoxic event|ocean anoxia]] and [[Ocean acidification|acidification]] and the resulting mass extinction are now regarded as being of latest Permian in age.<ref name=":2" /> The GSSP is near [[Meishan]], Zhejiang Province, southern China. [[Radiometric dating]] of volcanic clay layers just above and below the boundary confine its age to a narrow range of 251.902+/-0.024 Ma.<ref name=":2" />