Rocks versus Clocks

What happened 670 million years ago? Can’t remember? Doesn’t matter, that’s why we have palaeobotanists. Palaeobotanists that is whose science it seems has been much under-appreciated amidst the high expectations, hope and hype surrounding modern methods of inferring evolutionary information from so-called ‘molecular clocks’. Or at least that appears to be the sub-text to the mammoth paper by John Clarke and colleagues that aims to pin down a time scale for plant evolution (New Phytologist 192: 266–301, 2011), a subject that should be dear to the hearts of all who read this column. Challenging phytophylochronologies derived from molecular approaches, the trio extensively analysed fossil and molecular data in the hope of reaching a better consensus and ‘timetree’ for important events – nodes – in plant evolution. With mention of Credibility Intervals, minimum constraints and soft maximum constraints, the paper is quite technical but one of its major conclusions is that the resulting timetree points to substantially earlier origins and diversifications of major plant groups than is evident from the fossil record alone. One of the most interesting and controversial nodes to calibrate is the origin of land plants, but, irritatingly (although the authors’ caution is understandable), you need to go to the commentary on that paper by Paul Kenrick (New Phytologist 192: 3–6, 2011) to pin down that exact date – 670 million years ago. They say that there’s a time and a place for everything; it seems we now have the ‘time’ of plant evolution – what about the ‘place’? However, the article’s summary concludes thus, ‘These conclusions are entirely compatible with current palaeobotanical data, although not necessarily with their interpretation by palaeobotanists’. Let battle be joined! In a fittingly timely manner, we have Philippe Gerienne et al. pushing back the timing on the evolution of wood a further 10 million years (Science 333: 837, 2011). The transatlantic team present evidence that fossils – which are 407 MYO (millions of years old – not an acronym!) from France and 397 MYO (although described as only 397 years old in one press story about this find, as at 9.21am on 5th September 2011) from Canada – show rings of cells radiating outward from the centre, including elongated ‘ray’ cells, which are characteristic of wood. These finds represent the – current! – earliest evidence of secondary growth in plants. Musing on the small size of the plants and the presence of thick-walled cortical cells leads the group to conclude that the early evolution of wood was driven by hydraulic constraints rather than by the necessity of mechanical support for increasing height. The plants described are considered to be precursors of lignophytes (a re-dated node that now needs to be built in to another iteration of Clarke et al.’s new plant timetree?). [Paleobotanists’ beloved ‘morphotaxa’ will soon be a thing of the past (pun intentional?) as these taxa will be eliminated from the revised Code. See ‘Latin is dead (Official!)’ item – Ed.]