It is the Carboniferous period, and Yorkshire is a swamp. All around you are enormous tree trunks, their branches far beyond your reach. There are other plants lower down – which look like ferns – and in the background is a buzz of insect life. The air is warm and humid, and what little light filters down makes the murky swamp water sparkle.
Fast forward around 319 million years, to the outskirts of the city of Bradford in West Yorkshire. The coal mining industry is in full boom and our fossil is about to come to light. There are several key figures in this tale. First are those who brought the fossil to the attention of the world: the coal miner, William Firth, who discovered the fossil in the coal mine at Toftshaw Bottom, and Louis Compton Miall, curator of the Bradford Philosophical Society. It was Miall who arranged for the fossil to be removed and transported to London for scientific study. The specimen is preserved in several pieces, all a jumble of disarticulated bones and armoured scutes. And so it came to the attention of one of the most important scientific figures of the Victorian age – Thomas Henry Huxley. Huxley was a great friend and supporter of Charles Darwin, whose book On The Origin of Species had been published 9 years previously (coincidentally… whilst Darwin was in the nearby town of Ilkley). Despite the Bradford fossil's jumbled state Huxley recognised its importance and so this strange fossil tetrapod became another piece of evidence in support of the new idea of evolution! A formal description of the specimen was published in 1869 and at last our creature has a name: Pholiderpeton scutigerum. So what actually IS Pholiderpeton? We say it is a tetrapod, but what do we mean by that? In the millions of years preceding the coal swamps, the only vertebrates on Earth were fish. Some of these fish, a group known as lobe-finned fish, made the evolutionary transition to be the first vertebrates living on land*. These first terrestrial vertebrates had four feet, and hence all their descendents are known as tetrapods (tetra = four, pod = feet). Confusingly, some later tetrapods have lost their feet as part of adaptations to different habitats (e.g. dolphins). We have four feet too, although our front pair have adapted and we call them hands. The transition to life on land was not a quick change but more of a gradual shift, with many early tetrapods using waterways to hunt and reproduce. These early terrestrial forms are often grouped together as ‘amphibians’ as they likely lived alongside and within freshwater. Technically speaking, the animals which we call amphibians in the modern day (frogs, newts, salamanders) are a different lineage which branched off from tetrapods later on, with definite fossils known from the Triassic (70 million years later than Pholiderpeton). Pholiderpeton belongs to a group of Carboniferous tetrapods known as embolomeres (which are sometimes placed in another group called the anthracosaurs). Embolomeres and other early tetrapods have enjoyed a resurgence in research interest in the last fifty years, casting great light on this most important stage of vertebrate evolution! And so, we come across the final key figure: a young preparator named Jennifer Clack. Her work revealed features of the skull of Pholiderpeton which were previously hidden by rock, and she went on to do a PhD redescribing the specimen in detail. The most important feature she uncovered was a middle ear bone named the stapes. In the 1980s, Pholiderpeton was the earliest specimen of a tetrapod with this bone preserved. Later tetrapods have a rod-like stapes which is associated with a wide indent on the skull, called the temporal notch, which holds the ear drum (tympanum). In these animals the stapes conducts vibrations from the ear drum to the inner ear. The robust stapes of Pholiderpeton and the narrow shape of the temporal notch suggests that it did not have an ear drum, and its hearing capabilities were likely similar to lobe-finned fish. This creature may have adapted its limbs to move on land, but its ear bones had yet to catch up! This raises the question of what the temporal notch was actually used for in Pholiderpeton. A clue can be found in a primitive fish still alive today - the bichir. Many types of fish have spiracles: two small openings on the head which play a role in drawing in water over the gills. Bichirs, however, also possess primitive lungs, and the spiracles have been shown to play an important role in drawing air into these lungs (Graham et al 2014). The earliest tetrapod ancestors of Pholiderpeton occupied a similar environment to the modern day bichir (shallow waters which were prone to drying out or having low oxygen levels) where increased breathing capabilities through the spiracle were an advantage. The swamp waters of Yorkshire contained a large quantity of dead plant material which would have rotted and used up the oxygen - so spiracular breathing may have been useful for Pholiderpeton as well! The stapes would have played a role in controlling spiracle movements. It is only in later tetrapods that these bones and structures are adapted for transmitting sound. You may have already made the connection - if you are a tetrapod, does that mean your ear drum had its evolutionary origins in the fish spiracle? The answer is yes! You also have a stapes - in the human middle ear this bone is often called the stirrup.
We return to the coal swamps. One day our Pholiderpeton dies and is buried in the soft swampy mud, underneath the Lepidodendron log whose bark can be seen next to some of the bones (Clack 1987). This is not the end, however. One day the tetrapod relatives of Pholiderpeton will become one of the most successful animal groups of all time, evolving into a wide range of different shapes and lifestyles… including eventually you, the reader! To quote Jenny Clack on the significance of early tetrapod fossils: “We’re lifting the lid on a key part of the evolutionary story of life on land. What happened then affects everything that happens subsequently, so it affects the fact that we are here and which other animals live with us today.” Author: Rebecca Bennion * This is a huge topic which I cannot adequately cover in one blog, but an excellent summary of the lobe-finned fish to tetrapod transition (60-65 million years before Pholiderpeton) can be found here: evolution-outreach.biomedcentral.com/articles/10.1007/s12052-009-0119-2 One of the many obituaries to Prof. Jennifer Clack (1947 - 2020): www.nature.com/articles/d41586-020-01217-8 References Clack, J.A., 1983. The stapes of the Coal Measures embolomere Pholiderpeton scutigerum Huxley (Amphibia: Anthracosauria) and otic evolution in early tetrapods. Zoological Journal of the Linnean Society, 79(2), pp.121-148. Clack, J.A., 1987. Pholiderpeton scutigerum Huxley, an amphibian from the Yorkshire coal measures. Philosophical Transactions of the Royal Society of London. B, Biological Sciences, 318(1188), pp.1-107. Clack, J.A., 1989. Discovery of the earliest-known tetrapod stapes. Nature, 342(6248), pp.425-427. Clack, J.A., 2002. Patterns and processes in the early evolution of the tetrapod ear. Journal of Neurobiology, 53(2), pp.251-264. Graham, J.B., Wegner, N.C., Miller, L.A., Jew, C.J., Lai, N.C., Berquist, R.M., Frank, L.R. and Long, J.A., 2014. Spiracular air breathing in polypterid fishes and its implications for aerial respiration in stem tetrapods. Nature Communications, 5(1), p.3022. Huxley, T.H., 1869. On a new labyrinthodont from Bradford. Quarterly Journal of the Geological Society, 25(1-2), pp.309-311.
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