1 Department of Geological Sciences, Stanford University , Stanford, CA 94305 , USA.
2 Department of Geology and Geophysics, Yale University , New Haven, CT 06511 , USA.
Proc Biol Sci. 2018 Dec 19;285(1893):20181724. doi: 10.1098/rspb.2018.1724.
Ediacaran fossils document the early evolution of complex megascopic life, contemporaneous with geochemical evidence for widespread marine anoxia. These data suggest early animals experienced frequent hypoxia. Research has thus focused on the concentration of molecular oxygen (O) required by early animals, while also considering the impacts of climate. One model, the Cold Cradle hypothesis, proposed the Ediacaran biota originated in cold, shallow-water environments owing to increased O solubility. First, we demonstrate using principles of gas exchange that temperature does have a critical role in governing the bioavailability of O-but in cooler water the supply of O is actually lower. Second, the fossil record suggests the Ediacara biota initially occur approximately 571 Ma in deep-water facies, before appearing in shelf environments approximately 555 Ma. We propose an ecophysiological underpinning for this pattern. By combining oceanographic data with new respirometry experiments we show that in the shallow mixed layer where seasonal temperatures fluctuate widely, thermal and partial pressure ( pO) effects are highly synergistic. The result is that temperature change away from species-specific optima impairs tolerance to low pO. We hypothesize that deep and particularly stenothermal (narrow temperature range) environments in the Ediacaran ocean were a physiological refuge from the synergistic effects of temperature and low pO.
埃迪卡拉纪化石记录了复杂的大型生命的早期进化，与广泛海洋缺氧的地球化学证据同时代。这些数据表明早期动物经历了频繁的缺氧。因此，研究集中在早期动物所需的分子氧 (O) 的浓度上，同时也考虑了气候的影响。一个模型，即寒冷摇篮假说，提出埃迪卡拉生物群起源于寒冷、浅水环境，因为 O 的溶解度增加。首先，我们使用气体交换原理证明，温度确实在控制 O 的生物利用度方面起着关键作用——但在较冷的水中，O 的供应实际上更低。其次，化石记录表明，埃迪卡拉生物群最初出现在大约 571 Ma 的深水相，然后大约在 555 Ma 出现在大陆架环境中。我们提出了这种模式的生态生理学基础。通过将海洋学数据与新的呼吸计实验相结合，我们表明在季节性温度波动很大的浅混合层中，热和分压（pO）效应高度协同。结果是，温度偏离特定物种最适温度的变化会降低对低 pO 的耐受性。我们假设埃迪卡拉海洋中的深特别是狭温（窄温度范围）环境是免受温度和低 pO 协同效应影响的生理避难所。
