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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Green hills laced with drystone walls, impressive rocky cliffs, and limestone pavements with their unique and delicate flora are all icons of the Yorkshire Dales. It is a landscape unapologetically dominated and influenced by the limestone that underpins it. At around 350 - 330 million years old the lowermost limestone units are up to five times more distant in time from Tyrannosaurus rex than we are from that tyrant lizard, and yet these are still not the oldest rocks in Yorkshire! Lurking beneath the limestone is something far older still, from a time before creatures colonized the land. These rocks tell the story of the death of an ocean and the birth of mountains! Before we dive too deep we need a bit of geographic context, and prepare yourself for a shock. During the time period we are interested in here (the Early Ordovician) Yorkshire was in the south. Very south. The southern hemisphere, in fact. I’ll let you digest that thought for a moment… We find ourselves in the ocean off the northern margin of a tiny little continent named Avalonia. Across the sea to our east lies Baltica, another small continent that will eventually become much of north-west Europe. To our north lies Laurentia (North America, Greenland, Northern Ireland and Scotland). Between these continents lay the Iapetus Ocean. Now you’ve got this picture in your heads let’s look at the rocks! Let us begin in around the village of Ingleton and the Waterfalls Trail along the River Twiss. At around 470 million years old, these are Yorkshire’s oldest rocks. These rocks, known as the Ingleton Group, are a type of rock known as greywackes (pronounced grey wacky), which is not only the best name for a sedimentary rock but it is quite appropriate as they are tilted and folded to as much as 70 degrees at Thornton Force waterfall! Like most of the rocks featured in this blog they were deposited by massive underwater landslides (turbidity currents) cascading off the margins of continents and bringing silt and sand into deep oceanic waters (check out our blog on Pendle Hill for more on turbidity currents!). Remember those three little contents all sitting pretty in the Southern Hemisphere? Well things were on the move: Avalonia (remember that included Yorkshire!) was heading north (yay!). This wasn’t the best thing for the Iapetus Ocean though which was being squeezed between the continents. The sediments of the Ingleton Group were pushed up from the sea bed, bent, buckled and eroded. This all took a considerable amount time, and no rocks were being laid down in the region whilst it was happening. Our story picks up again in the Ashgill (Ordovician, around 440 million years ago), with the deposition of the Coniston Limestone Group. These sediments were deposited in much shallower water and locally is quite fossiliferous. On into the Silurian! And the Iapetus is still closing! The rocks of Yorkshire again record deep water sediments: more turbidite sequences, only this time there are more fossils. In the depths of this closing ocean the low oxygen muds preserved some very delicate fossils.
This type is called Monograptus, ‘mono’ meaning ‘one’ because they have only a single thread of cups in a row. Monograptus was quite abundant during the Silurian and some surfaces can be covered in them, like the above block from a quarry in Horton-in-Ribblesdale. Eventually the Iapetus ocean closed completely, all three continents crunched together in a mountain building event called the Caledonian Orogeny. It’s this mountain building event that gave birth to the Scottish mountains, although they were much larger mountains when they were first formed. Over time the high areas of land produced in Yorkshire by this event were eroded and new seas flooded this ancient land. It is now the Carboniferous, and the classic limestones of the Yorkshire Dales which we all know and love are being formed. The time gap between these rock units has created a type of geological boundary called an unconformity, which can be seen in the valley sides and quarry faces of Ribblesdale, Crummack Dale and Chapel le Dale near Ingleton. At sites like these you can truly get a glimpse of the enormity of geological time, and peel back the layers of the landscape to reveal the dramatic histories of lost oceans! Author: Jed Atkinson
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