320 million years ago there lived a giant. This giant could grow to heights of over 50 metres, and was covered in scales. You might imagine that I’m talking about a dinosaur, but the titans of the Jurassic would not thunder across the Earth for another 120 million years. This giant was a tree, or at least, it looked like one. Travelling back in time to the Carboniferous Period of Earth history, we find the north of England a very different place to today. Rolling green hills, drystone walls and grouse moors give way to steaming, swampy forests at a time when what would eventually become the United Kingdom lay close to the equator. It is in these primordial quagmires that we find the topic of this month’s blog, the fossil tree: Lepidodendron (or is it Sigillaria? Or Stigmaria? Is this thing even a tree at all?! Whatever, we’ll worry about that later!) The swamps this… plant… called home sat on the margin of a large continent, atop river deltas that were periodically inundated as sea levels rose and fell in cycles (see our blog on cyclothems for more on that!) A plethora of other plants grew alongside them, from giant relatives of horsetails, which are still the bane of gardeners today, to ferns that wouldn’t look out of place growing among the undergrowth of a modern forest. And then there are the animals that lived there, from small lizard-like creatures that would eventually evolve into the diversity of mammals, reptiles and birds we enjoy today, to giant dragonflies the size of hawks, and millipedes as long as cars. And looming over all of it were our floral giants. But back to the first of the two thorny issues I eluded to earlier. What is the name of this tree-like plant we’re talking about? If you dig through the literature or, if you’re a normal person, read the plaques that often sit alongside the fossilised stumps of these monsters, there are two names that frequently crop up: Lepidodendron and Sigillaria. These names are genera (the term used to group living things that’s one up from a species, like Homo or Tyrannosaurus). So that one’s easy enough, we have two closely-related but distinctly different types of “tree” living together, which can both be found as fossils across northern England. But from there things get a bit more confusing. The most common part of these plants to find are their roots, which does make some degree of sense. After all, you’ve got to be buried underground to become a fossil, and so roots are already half way there. And these fossils are very easy to find. If there’s Carboniferous sandstone in the nearby hills, odds are you’ll come across them in the beds of nearby streams and rivers (just look out for brown or buff-coloured rocks with regular, circular indentations). These fossils are so common, in fact, that they have their own name: Stigmaria. Stigmaria is what palaeontologists refer to as a ‘form taxon’, which is a fancy way of saying the roots of Lepidodendron and Sigillaria are virtually identical, and I’ve got to call this fossil I just found something! Alright, so we’ve resolved the naming issues (for the most part), but what’s all this about them not being trees? Surely that’s what you call a 50 metre tall plant with a trunk and a leafy canopy at the top, right? Well, no. At least, not in this case. Both Lepidodendron and Sigillaria belong to a group of plants called lycopsids. Examples of lycopsids like clubmosses and quillworts can still be found in the boggy upland areas of northern England today, although they’re much smaller than their prehistoric relatives, and rarely reach more than a few inches off the ground. These types of plants are much more primitive than the trees that form today’s forest though. Their enormous trunks weren’t made of wood like modern trees, but rather were soft and spongy, and they reproduced through spores, rather than forming seeds. But it’s not just roots we find. Their bark also fossilises, and when it does it resembles intricate scales covering the skin of some primordial lizard. In fact, the name Lepidodendron literally means ‘scale tree’. Their leaves occasionally show up too, as do the cones from which they released spores. In many cases, these various bits and pieces are also indistinguishable between species. However, the most impressive of these fossils are their giant stumps, of which a handful have been ground across the UK, and many of them are on display in parks and museums. I’d be lying if I said I didn’t have a favourite though… In the graveyard of St. Thomas Church, in the sleepy Weardale village of Stanhope, County Durham, sits one such tree stump. This one is a Sigillaria (it’s often easier to tell when you’ve got more of the plant), and it’s been here since 1964. It was originally found in a sandstone quarry, along with a number of other stumps, a few miles north of Stanhope back in 1914. In researching for this blog, curiosity got the better of me, and I decided to head out to see if I could find the old quarry. Records from the period are sketchy at best, but low and behold, the quarry does still exist. It can be found near Edmundbyers Cross, on the roadside just south of the junction between the B6278 and the small road leading north-east toward Waskerley. There’s not much left of the original quarry (given that it hasn’t been active for a century) but there are still exposures of sandstone to be found, and a decent amount of scree where I managed to pick up a few scrappy plant fossils (pictured below). The site is on open access land, so feel free to go check it out for yourself. Who knows? Maybe there are still fossil trees to be unearthed there! These ancient behemoths give a fascinating insight, not only into how the north of England has changed over millions of years, but how living things have evolved and changed too. What once were titans of a swampy forest, now scarcely warrant a second glance, yet their fossils remind us of their glory days long past. Author: Jake Morton References:
Thomas, B. A. and Seyfullah, L. (2015). Stigmaria Brongniart: a new specimen from Duckmantian (Lower Pennsylvanian) Brymbo (Wrexham, North Wales) together with a review of known casts and how they were preserved. Geol. Mag. 152 (5), pp. 858–870. Plant Fossils of the British Coal Measures. Cleal, C. J. and Thomas, B. A. Palaeontological Association, London, 1994
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