Vertebrate palaeontologists sometimes become precious after a career peering at old bones, especially when they are as remarkably tiny as those of most Mesozoic mammals – and most of those fossils are teeth. Some defend to death the notion that primates descend from tree-shrews, while others foam at the mouth at the mere suggestion of the ur-shrew. ‘A key feature in primate evolution is reduction of the snout’, is axiomatic to yet others. Again, geneticists have provided extreme selection pressures that will either cause vertebrate palaeontologists rapidly to evolve or to become extinct.
Analysis of living primate genomes produces a phylogeny that links all primates with a group that has been said to be ‘the sort of animals that defy taxonomic categorization, confuse one’s sense of aesthetics, and seem to largely fall under the umbrella of “weird.” ‘ (Janecka, J.E and 7 others 2007. Molecular and genomic data identify the closest living relatives of primates. Science, v. 318, p. 792-794). These are the colugos, or flying lemurs that include the wonderfully named sugar glider.
Planet of the beetles
More than 20% of the known diversity of life on Earth is made up by the order Coleoptera, which includes several hundred thousand species. Although that huge number is largely thanks to beetle collectors, Charles Darwin having been a particularly voracious one, it is difficult believe that any other order or even class of multicelled organisms will prove to be as diverse. Yet there is only a sparse fossil record of these ubiquitous creepy-crawlies. The earliest known beetle fossils date back to the Lower Permian, and the Triassic saw their radiation into wood-eating, predatory and fungus-eating clades – from morphological similarities with living beetles. Their modern diversity depends on the vast range of ecological niches that beetles can fill, many of which are environmentally so subtle that only the beetles exploiting them show that the niches exist at all. Like all organisms the evolution of the beetles has been within the interconnectedness of the whole Earth system, and it through the linkages that such subtlety has emerged and evolved. One of the best known is the sensitivity of different beetle species to small climatic changes, which has allowed their growing use for charting climate change on land: they are far better proxies for temperature than are the foraminifera of the oceans.
Being only sparingly preserved in rocks, how beetles evolved has long been a mystery, considering their overwhelming presence on the planet. Yet again, the rapid rise of molecular phylogeny, including means of timing when mutations took place, is starting to supplant the skills of the traditional palaeontologist (Hunt, T. and 15 others 2007. A comprehensive phylogeny of beetles reveals the evolutionary origins of a superradiation. Science, v. 318, p. 1913-1916). Toby Hunt of London’s Natural History Museum and colleagues from the UK, Czech Republic, USA, Germany and Spain have combined their own RNA sequencing with existing databases of 1880 species from all the beetle suborders, series and superfamilies, 80% of families and 60% of subfamilies, to represent more than 95% of all described species. This establishes a phylogenetic tree for the lineages that they analysed, details of which will excite the coleopterist sororities and fraternities. The general picture, however, presents a more a broadly fascinating surprise. Because a vast number of beetles are associated with plants and fungi, it might seem inevitable that their evolution has parallels with that of plants, especially their explosive diversification once the angiosperms (flowering plants) appeared. The molecular dating clearly shows that is not the case. While the angiosperms emerged in the Cretaceous Period, more than 100 living beetle lineages appeared earlier in the geological record. Unlike the Vertebrata, which diversified after mass extinctions (including the primates), the fundamental beetle lineages were clearly good survivors that were capable of their own diversification whenever opportunities arose. I think we might grow to worry about that…
Mammal evolution makeover
The Cenozoic has been the Era of mammals, and their diversification is the largest recorded adaptive radiation. However, the Linnean names of many mammal clades from the Mesozoic end in ..dont, i.e. they have been defined in terms of their teeth and not much else. Most fossil mammals from the Mesozoic are small and fragile and only survive as teeth and jaw fragments. As a result most of the course of early mammal evolution has been a bit uncertain, to say the least. The view until recently has been that early mammalian evolution was a step-by-step affair in which key innovations accumulated in an orderly manner. However, even on the basis of teeth, developing taxonomic approaches have proved able to reveal that considerably more complicated things happened (Luo, Z-X. 2007. Transformation and diversification in early mammal evolution. Nature, v. 450, p. 1011-1019). For a start, it turns out that mammals, despite their scanty remains, were almost as diverse during the Mesozoic as the dinosaurs that are often said to have driven early mammals underground or into the night (310 mammal to about 550 dinosaur genera). The potential for analysis stems from an explosive growth in fossil discoveries: from 116 genera in 1979 to the present 310, and a 200-fold increase in well-preserved specimens. Clearly, mammal-oriented palaeobiologists have been hard at work.
Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh crams most of the developments into a 6-page review, from which it is possible to learn a great deal, albeit needing quite a firm grasp of cladistic terminology. One of the highlights is how evolution of the mammals before 65 Ma involved repeated evolutionary convergence, i.e. the end products of evolutionary bursts often looked superficially similar. That tendency carried over into the Cenozoic on a grander scale. One example is that of adaptations for burrowing to produce mole-like end products, even some with semi-aquatic habits. Many of the rapid diversifications ended in extinction of the lineage, but all seem to indicate a great deal of ‘experimentation’ with a range of original forms that channelled towards similar functions. The outcome was a vigorous occupation of potential ecological niches in which mammals clearly had the advantage over reptiles, possibly because of their physiologically greater adaptability, partly stemming from warm-bloodedness.
Permian shark bites fish-biting amphibian
It is worth queuing to await the appearance of the 22 January 2008 issue of the Proceedings of the Royal Society B: Biological Sciences. It contains unique evidence of predator-prey relations and the food chain in the Lower Permian Zechstein Sea (Kriwet, J. et al. 2008. First direct evidence of a vertebrate three-level trophic chain in the fossil record. Proceedings of the Royal Society B: Biological Sciences, v. 275, p. 181-186). The object for your amazement is a shark whose gut contains two amphibians. The last meal of one of the amphibians was a small fish.
The paper promises to be reminiscent of the final part of the Monty Python Fish Slap Dance sketch, which can be viewed at http://www.youtube.com/watch?v=d1xfp6Xeu0c&feature=related