Most palaeontologists tend not to think about cells too much. Our world is dominated by the parts of living things that preserve well, and on a human scale: teeth, bones, shells, or (in my case) the bits of plants which best survive the processes of decay and preservation. Soft part preservation is highly unusual, and cell-scale detail is incredibly rare. Yet it is what soft parts do that most of biology focuses on, from molecules up to ecosystems. If we want to reconstruct lost worlds, we need to think about all these scales.
Where we do see cellular detail, it is thanks to permineralisation, a mode of preservation where mineral-rich fluids infiltrate the cells of an organism before decay can take hold. If you have seen a polished slice of petrified wood, with its tree rings clearly visible and beautifully preserved, you have seen a permineralised fossil. Microbial fossils, found in some of the oldest rocks on the planet, are the most ancient direct evidence of life on Earth, and by their very nature, show us cell-scale detail.
We often talk about fossils as snapshots of the past, and permineralised specimens are a prime example. The biological limits of ‘fossil snapshots’ were extended in a new paper this month from Alexander Hetherington and colleagues, which identified actively-growing plant stem cells from over 300 million years ago.
Unlike animals, plants retain populations of stem cells, known as meristems, which self-renew and differentiate throughout the life of the plant. Each cell divides to produce two daughter cells, which are both identical to the original cell. These cells go on to differentiate into distinct cell types.
The shoot apical meristem provides cells for organs such as leaves and flowers, while the root apical meristem gives rise to cells for root growth. Modern plant biology has revealed much about how this process works, at the cellular and molecular scales, and even A-level biology students can study cell division in root meristems in the school lab.
Understanding the evolution of plant roots might seem dull, but it is important. Roots provide plants with a mechanical fasthold, and transport water and nutrients from the soil to the rest of the plant. Much time, effort and research funding is put into fundamental research into the ‘hidden half of plant biology’ (which is what we call it when we want root biology to sound really exciting), because of the potential for improving crop yields through better root systems.
The roots described in Hetherington and colleagues’ paper are preserved in coal balls: concretions permineralised by calcite, which formed in ancient peat before it became coal. They are from the Carboniferous period, from the very coal measures which drove the industrial revolution in the north-west of England. These plants grew in a very different world, dominated by spore-producers such as ferns, horsetails and clubmosses, which formed immense tropical forests teeming with over-sized dragonflies, millipedes and other nightmare arthropods.
Where fossil root apices preserved in coal balls have, through sheer luck, been thin-sectioned in the right orientation (a longitudinal slice along the root), the way in which the cells are organised can be compared to the patterns seen in modern root meristems. Active meristems show a gradient in cell sizes from tiny meristematic cells to larger expanding differentiating cells, whereas when meristem growth stops, larger cells end up right next to tiny, inactive initial stem cells. And sure enough, if you quantify the distribution of different cell sizes, an active meristem can be recognised.
The new root meristem has been named Radix carbonica, and has features in the way in which the cells are organised that distinguish it from any other known root meristem, past or present. The significance of this is that root biology in the past may have been much more diverse than we realise.
The worlds of plant root anatomy and palaeontology seldom overlap so directly, but when they do it provides fascinating results. This study also highlights the ongoing importance of the fossil collections held in museums and herbaria: the coal ball thin-sections used in the study were from well-established collections made over a hundred years ago.