Long-term differences in extinction risk among the seven forms of rarity

Rarity is widely used to predict the vulnerability of species to extinction. Species can be rare in markedly different ways, but the relative impacts of these different forms of rarity on extinction risk are poorly known and cannot be determined through observations of species that are not yet extinct. The fossil record provides a valuable archive with which we can directly determine which aspects of rarity lead to the greatest risk. Previous palaeontological analyses confirm that rarity is associated with extinction risk, but the relative contributions of different types of rarity to extinction risk remain unknown because their impacts have never been examined simultaneously. Here, we analyse a global database of fossil marine animals spanning the past 500 million years, examining differential extinction with respect to multiple rarity types within each geological stage. We observe systematic differences in extinction risk over time among marine genera classified according to their rarity. Geographic range played a primary role in determining extinction, and habitat breadth a secondary role, whereas local abundance had little effect. These results suggest that current reductions in geographic range size will lead to pronounced increases in long-term extinction risk even if local populations are relatively large at present.     

埃迪卡拉纪至寒武纪苗岭世刺细胞动物化石研究现状与展望

刺细胞动物是一类具有刺细胞的水生无脊椎动物,分布在世界各地的海洋和淡水中.作为后生动物最早分化出的一支,刺细胞动物对研究后生动物的起源和早期演化具有极其重要的意义,也为研究后生动物系统发育、地层对比和古地理恢复等方面提供了重要的科研线索.本文简要介绍了刺细胞动物早期(埃迪卡拉纪至寒武纪苗岭世)的化石记录和研究现状,将刺...     

Modelling the Transport of Nutrients in Early Animals

The single-celled ancestors of multi-cellular animals (metazoans) did not need to transport nutrients between cells, but this ability is vital for modern animals. How could intercellular nutrient transport have begun? And how did this influence the early evolution of animals? In this hypothesis, I suggest that nutrients could have passed directly between the cytoplasm of conjoined cells in early compacted cell-balls, along the plane of the closed epithelium. This would have limited early animals to the size and form of modern embryos. The mechanisms that indirectly transport nutrients between discrete cells, via the extracellular fluid within the body-space, are modelled to have evolved sequentially; so comparison of nutrient transport processes could provide evidence of any early divergences of phyla. When the last of the indirect intercellular transport processes for essential nutrients had been developed, the extracellular fluid within the body-space would have contained all necessary nutrients. Then the epithelium could have greatly expanded, and cells lived and divided within the body-space. This development of nutrient transport processes would have enabled animals to greatly increase in size and complexity.     

The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records

Unravelling the timing of the metazoan radiation is crucial for elucidating the macroevolutionary processes associated with the Cambrian explosion. Because estimates of metazoan divergence times derived from molecular clocks range from quite shallow (Ediacaran) to very deep (Mesoproterozoic), it has been difficult to ascertain whether there is concordance or quite dramatic discordance between the genetic and geological fossil records. Here, we show using a range of molecular clock methods that the major pulse of metazoan divergence times was during the Ediacaran, which is consistent with a synoptic reading of the Ediacaran macrobiota. These estimates are robust to changes in priors, and are returned with or without the inclusion of a palaeontologically derived maximal calibration point. Therefore, the two historical records of life both suggest that although the cradle of Metazoa lies in the Cryogenian, and despite the explosion of ecology that occurs in the Cambrian, it is the emergence of bilaterian taxa in the Ediacaran that sets the tempo and mode of macroevolution for the remainder of geological time.     

Onshore–offshore gradient in metacommunity turnover emerges only over macroevolutionary time-scales

Invertebrate lineages tend to originate and become extinct at a higher rate in onshore than in offshore habitats over long temporal durations (more than 10 Myr), but it remains unclear whether this pattern scales down to durations of stages (less than 5 Myr) or even sequences (less than 0.5 Myr). We assess whether onshore–offshore gradients in long-term turnover between the tropical Eocene and the warm-temperate Plio-Pleistocene can be extrapolated from gradients in short-term turnover, using abundances of molluscan species from bulk samples in the northeast Atlantic Province. We find that temporal turnover of metacommunities does not significantly decline with depth over short durations (less than 5 Myr), but significantly declines with depth between the Eocene and Plio-Pleistocene (approx. 50 Myr). This decline is determined by a higher onshore extinction of Eocene genera and families, by a higher onshore variability in abundances of genera and families, and by an onshore expansion of genera and families that were frequent offshore in the Eocene. Onshore–offshore decline in turnover thus emerges only over long temporal durations. We suggest that this emergence is triggered by abrupt and spatially extensive climatic or oceanographic perturbations that occurred between the Eocene and Plio-Pleistocene. Plio-Pleistocene metacommunities show a high proportion of bathymetric generalists, in contrast to Eocene metacommunities. Accordingly, the net cooling and weaker thermal gradients may have allowed offshore specialists to expand into onshore habitats and maintain their presence in offshore habitats.     

Macroevolutionary Processes and Biomic Specialization: Testing the Resource-use Hypothesis

The resource-use hypothesis predicts that generalist species have lower speciation and extinction rates than specialists. In this work we test several subsidiary predictions of the resource-use hypothesis using the biomic specialization index (BSI) for each African large mammal species, which is based on its geographical range within different climate zones. This index can be used globally allowing intercontinental and intertaxa comparisons. Our results are consistent with the axioms of the resource-use hypothesis theory, which predicts (1) a high frequency of stenobiomic species, (2) carnivores are more eurybiomic than herbivore clades (particularly, Artiodactyla and Primates), (3) the higher incidence of these biomic specialists in the tropical rainforest and desert biomes, and (4) the fact that certain combinations of inhabited biomes occur more frequently among species than do others. We also found that the tropical deciduous woodland is an important source of new species, and that there is a macroevolutionary segregation between extreme eurybiomic species (inhabitants of five or more biomes) and ‘semi-eurybiomic’ species (inhabitants of 2–5 biomes). These results can also be explained within the premises of the resource-use hypothesis. Finally, we discuss the relevance of our results to the understanding of the latitudinal gradient in species richness.Co-ordinating editor: N. Chr. Stenseth