陆生植物在二叠纪—三叠纪界线存在集群灭绝吗?

Was There a Mass Extinction of Land Plants at the Permian—Triassic Boundary (PTB)?

二叠纪—三叠纪界线(PTB)集群绝灭是显生宙以来海洋生物界发生的规模最大的一次灾变事件,但陆生植物在PTB是否存在集群绝灭却颇有争议。这个争议主要归咎于如何定义陆生植物的集群绝灭。通过考虑集群绝灭定义的4个基本特点(即量值、广度、幅度和时续),本文简要评述了陆生植物穿过PTB的多样性变化,认为它与海洋动物在二叠纪末的多样性变化可能是互相耦合的,这暗示着海洋和陆地生态系统在PTB都遭受了剧烈的扰动。陆生植物穿过PTB发生了全球性的古植代—中植代交替,经历了大的群落重组和新种的演化,这个过程持续了约25Ma。基于孢粉学资料,晚二叠世—早三叠世植物群的演变具有明显的连续性,陆生植物在PTB没有突然发生大规模的绝灭事件。在不同的植物地理区、气候带和保存条件下,陆生植物穿过PTB的多样性变化具有明显的选择性,总体上都显示了危机—残存—复苏的演化式样。基于化石数据库,冈瓦纳植物区和华夏植物区陆生植物属的多样性在PTB显著衰减;而安加拉植物区和欧美植物区,属的多样性在PTB稍许增加。陆生植物中存在的一次高绝灭率事件(中三叠世安尼期)稍微滞后于海洋无脊椎动物和陆生四足动物在PTB同时发生的集群绝灭,这暗示着陆生植物与动物界发生集群绝灭的根本原因及其响应方式可能不尽相同。作为宏体自养型初级生产者的陆生植物拥有更加多样化的地方特有性分子、生存对策和生境,在PTB可能对灾变事件具有更强的缓冲能力和生存能力。因此,笔者等建议在分析全球陆生植物多样性变化式样时,应该综合考虑植物区、气候带、化石采样和保存偏差因素以及大化石和微体化石的相互补充和印证,而基于有限的资料(例如来自一个植物地理区、一个或少数的产地和植物分类群(或分类等级)以及单凭植物大化石的研究)来论证陆生植物在PTB存在集群绝灭极有可能是不可靠的。笔者等倾向于认为陆生植物在PTB不存在全球性的、突然的、多种分类单元的集群绝灭。

The Permian—Triassic Boundary （PTB） mass extinction is the biggest catastrophic event in the Phanerozoic marine faunas, but whether a similar mass extinction event occurred in land plants is still controversial. The debate results primarily from how to define a mass extinction in land plants. By considering the four fundamental characteristics （i.e., magnitude, extent, breadth and duration） used to define a mass extinction, the diversity change of land plants across the PTB is reviewed briefly. The results show that the diversity changes of land plants and marine faunas might be coupled in the end-Permian, suggesting that both marine and terrestrial ecosystems at the PTB suffered strong disturbance. Across the PTB global Palaeo—Mesophytic transition occurred in land plants,which underwent major reorganization of plant communities and evolution of new species, and this process lasted about 25 Ma. According to the palynological data, the evolutionary change of the Late Permian—Early Triassic floras is evidently continuous, suggesting that there is not a major extinction event precipitately occurred in land plants. In different floral provinces,climatic belts and preservation conditions,the diversity changes of land plants are evidently selective,which grossly shows a crisis—survival—recovery evolutionary pattern. Based upon the fossil databases, the genus diversities of the Gondwana and Cathaysian floral provinces decrease markedly while those of the Angara and Euramerica floral provinces increase slightly. Among land plants there is an episode of high extinction rate in the Middle Triassic （Anisian）, which slightly lags behind the PTB mass extinction simultaneously occurred in marine invertebrates and terrestrial tetrapods, implying that the ultimate causes and the responding modes of mass extinctions in land plants and animals may not be entirely same. Land plants as macroscopic autotrophic primary producers with more diverse endemic taxa, survival strategies and habitats may provide a stronger survivability and buffer against the catastrophic events at the PTB. Therefore, writers suggest that analyzing the global diversity pattern of land plants should consider integrative factors：floral provinces, climatic belts, and fossil sampling and preservation biases as well as the reciprocal complementarities and corroborations from megafossils and microfossils. Meanwhile,it is most possibly intenable that a mass extinction is demonstrated to exist in land plants only on the basis of the limited data from those studies of a floral province, one or a few localities and plant taxa （or taxon-levels）, and megafossils alone.Writers are inclined to think that there was not a worldwide, abrupt mass extinction at the PTB in various taxa of land plants.