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Burgess Shale-type biotas were not entirely burrowed away
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2012 (English)In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 40, no 3, 283-286 p.Article in journal (Refereed) Published
Abstract [en]

Burgess Shale–type biotas occur globally in the Cambrian record and offer unparalleled insight into the Cambrian explosion, the initial Phanerozoic radiation of the Metazoa. Deposits bearing exceptionally preserved soft-bodied fossils are unusually common in Cambrian strata; more than 40 are now known. The well-documented decline of soft-bodied preservation following the Middle Cambrian represents the closure of a taphonomic window that was only intermittently open in marine environments thereafter. The prevailing hypothesis for this secular shift in taphonomic conditions of outer shelf environments is that soft-bodied biotas were literally burrowed away from the fossil record by increasing infaunal activity in muddy substrate environments; this would have affected geochemical gradients and increased the efficiency of organic matter recycling in sediments. New and recently published data, however, suggest a more complex scenario. Ichnologic and microstratigraphic data from Burgess Shale–type deposits indicate that (1) bioturbation exerts a limiting effect on soft-bodied preservation; (2) the observed increase in the depth and extent of bioturbation following the Middle Cambrian would have restricted preservation of Burgess Shale?type biotas in a number of settings; but (3) increasing depth and extent of bioturbation would not have affected preservation in many other settings, including the most richly fossiliferous portions of the Chengjiang (China) deposit and the Greater Phyllopod Bed of the Burgess Shale (Canada). Therefore, increasing bioturbation cannot account for the apparent loss of this pathway from the fossil record, and requires that other circumstances, including, but not limited to, widespread benthic anoxia, facilitated widespread exceptional preservation in the Cambrian.

Place, publisher, year, edition, pages
2012. Vol. 40, no 3, 283-286 p.
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-75465DOI: 10.1130/G32555.1OAI: oai:DiVA.org:su-75465DiVA: diva2:516747
Available from: 2012-04-19 Created: 2012-04-19 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Ocean chemistry and the evolution of multicellularity
Open this publication in new window or tab >>Ocean chemistry and the evolution of multicellularity
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Oxygen has been assumed to be a vital trigger for the evolution of multicellular life forms on Earth, partly based on its power to promote substantial energy flux in cell respiration and partly as biosynthesis of compounds like collagen require oxygen. However, the co-evolution of large life and the Earth’s chemical environment is not well understood at present, and there is particular disagreement in the field about whether the Cambrian explosion of animal life forms was a chemical or biological event. Here, I discuss the evolution of multicellularity, divided in simple or complex forms, in light of the evolution of ocean water column chemistry in both the Proterozoic and the early Paleozoic. Even if the appearance of animals is confined to the Ediacaran, other fossil evidence of complex multicellularity can be argued to occur in the Paleo-, Meso- and Neoproterozic. These finds are, if anything, reason enough to keep searching for early experiments in complex multicellularity. In this search, we may have to expand our toolbox by looking at e.g. trace element aggregations and the isotopic composition of key elements. 

Research over the last couple of years have accentuated that much of the interval between the Ediacaran and the Devonian was dramatic with transitional ocean chemistry at the same time that large forms of animal life experienced dynamic radiation and ecological expansion. Results presented here describe some aspects of this time, including geochemistry from Chengjiang and a mechanism for preserving non-mineralized Cambrian animals that was partly dependent on specific ocean chemistry. Also, geochemical proxies using iron and molybdenum are used to infer a Paleozoic atmosphere with less than 50% of present levels of oxygen. The possibility that the subsequent rise is due to terrestrial plants and linked to the appearance of large predatory fish is discussed. Finally, the first mass extinction in the end-Ordovician is linked to low oxygen concentrations in the water column. It appears that more than oxygen was critical to allow the radiation of large life forms on Earth, but that chemistry and tectonic activity were intimately intertwined to biology, in a dance of permitting and being determined by certain aspects of ecology.

Abstract [sv]

Under lång tid har vi sett atmosfärens syrehalt som avgörande för att stora livsformer skulle börja utvecklas på jorden, delvis eftersom syre är ett energirikt bränsle men också för att det krävs vid sammansättningen av vissa ämnen som djur behöver, till exempel proteinet kollagen. Men, i själva verket, har vi inte lyckats reda ut detaljerna om hur utvecklingen av tidigt, stort liv och miljö satt samman, och om den kambriska explosionen framförallt var en biologisk eller kemiskt händelse. I den här avhandlingen diskuterar jag hur utvecklingen av flercellighet, då uppdelat i enkla och komplexa former, kan vara kopplad till hur havens kemi förändrats både i proterozoikum (2.5-0.5 miljarder år sedan) och paleozoikum (0.5-0 miljarder år sedan. Även om fossil från moderna djur dyker upp runt ediacaran och kambrium, så finns det långt äldre fossil som kan påvisa flercellighet. Dessa fossil ger, om inte annat, anledning att leta vidare efter fler spår av pre-kambrisk flercellighet och kanske kan vi utöka våra sökmetoder till att också tolka ansamlingar, eller isotopsammansättningar, av spårmetaller.

Den kambriska explosinen av djurliv (med startskott för 543 miljoner år sedan) är ett etablerat begrepp, men den senaste årens forskning har satt fokus på att en längre period, från ediacaran till devon, var en dynamisk tid med skiftande havskemi, nya djurarter och experimentella ekologiska nätverk. I den här avhandlingen presenteras några resultat som belyser just denna övergångstid, som geokemin i Chengjiang som beskriver hur havets kemi skiftar från syrefritt till sulfatfritt till syrerikt, och hur djur utan skal och ben kunde bli bevarade genom att flera unika förhållanden sammanföll. En annan studie visar hur molybden använts för att påvisa att atmosfärens syrehalt, under den här perioden, var högst hälften av vår moderna nivå. Vi hävdar att stigningen som skedde i devon, delvis tack vare växternas intåg på land, och att stigningen kan speglas i att fiskar först då hade råd att jaga och växa sig stora. Slutligen visar jag också på hur det första stora massutdöendet kan vara sammankopplat med syrefria hav, snarare än kyla och mer syre än djuren klarade av. Ett komplext samspel mellan flera kemiska ämnen, utöver syre, tektonisk aktivitet och biologi ser ut att höra samman med den dramatiska uvecklingen för stora livsformer på jorden.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2012. 51 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 350
Keyword
Geochemistry, biology, paleontology, evolution, multicellularity
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-75466 (URN)978-91-7447-486-2 (ISBN)
Public defence
2012-06-11, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2012-05-10 Created: 2012-04-19 Last updated: 2012-05-03Bibliographically approved

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