Sunday, 20 October 2019

The Alien Prehistoric World Trope: Part 1 - Antediluvia

Hello and welcome to my blog. Since this is my very first post I should probably make some things clear: If you are a reader of Tetrapod Zoology, Love in the Time of Chasmosaurs or any similar paleontology-related blog you can expect content on similar topics. Unlike Darren Naish or Mark Witton however, I am of course not an expert in paleontology and also unlike LITC I will not primarily focus on paleoart. As a student of history (with a minor in earth system sciences) this blog will primarily focus on topics of science-history and history of knowledge, mainly the history of paleontology and related subjects like evolutionary biology and astrobiology. We will therefore not discuss the validity of taxa, anatomy or reconstructions here (although I still may talk about these things, but when I do please keep in mind that I am just giving my non-expert opinion that should not be cited), but will instead examine how humans, be they scientists, painters, filmmakers or even videogame developers, have imagined the prehistoric world over the centuries. The name of this blog is in reference to that, as Manospondylus gigas is an old name for Tyrannosaurus rex remains that fell into obscurity until it became a historic relic (how that came to be is a story for another day). The backstory behind the header- and background-images are, apart from being very amusing, in similar vein. 

Fig. 1: With this blog I can finally put my paleontology book collection to good use. You may notice that some of them have German titles. This is because I am Swiss and those are the books I had growing up as a child. The ones with English titles are those I collected in more recent years. (Image taken by me).
Now that we got this out of the way, let us begin with our topic of the next couple of days: Dinosaurs as aliens. With that title you will probably be either confused or have PTSD-flashbacks to Dino Crisis 3, but what I mean by this is an idea which has been present in depictions of prehistory for a very long time: That the prehistoric world and its inhabitants were so utterly different and bizarre compared to our modern world that they seem extraterrestrial. In 2018 Steve Brusatte described this idea in retrospective in the prologue to his book The Rise and Fall of the Dinosaurs:

“Zhenyuanlong is unlike the dinosaurs I learned about in elementary school, before I became a scientist. I was taught that dinosaurs were big, scaly, scary brutes so ill equipped for their environment that they just lumbered around, biding their time, waiting to go extinct. Evolutionary failures. Dead ends in the history of life. Primitive beasts that came and went, long before humans came on the scene, in a primeval world that was so different from today that it may as well have been an alien planet. Dinosaurs were curiosities to see in museums, or movie monsters that haunted our nightmares, or objects of childhood fascination, pretty much irrelevant to us today and unworthy of any serious study.”

Today dinosaurs are rarely depicted in this way. While we of course have nothing quite similar nowadays to an Apatosaurus or Tyrannosaurus, dinosaurs and other extinct animals are increasingly more often depicted in such a way that they can evoke things the viewer knows in order to make them seem more tangible. Many dinosaurs are depicted with feather-shells and coloring similar to modern birds, have lizard-lips that hide their teeth or are shown engaging in behavior that you would expect of currently living animals, like sleeping, drinking, in-fighting or even playing. A modern Deinonychus looks like a large, flightless hawk and it is not unusual to see it doze off at the edge of a conifer forest, its head tucked under its wing like a duck, while the sky is clear and blue. It is an animal that seems a bit strange, but it and its environment are still familiar enough in their characteristics that we can contextualize them. It may inspire a sense of respect, but also majesty, beauty or even cuteness in us. It is not a complete alien. Go back just one or two decades and it becomes an entirely different story. Deinonychus and its dromaeosaur relatives become the stuff of nightmares and have more in common with Ridley Scott’s xenomorph than with birds. They still are active, fast and walk like birds, but everything else is alien. Their skin is scaly and rubbery, wrapped tightly around the skull and bones to give a skeletal, almost undead, appearance. Their arms and hands are bent in awkward positions like zombies, their eyes have slit pupils like snakes or are black like in sharks and their large claws are their most important features. Most striking is their behaviour, which is monstrous rather than animalistic. Two good examples come to my mind. First there are of course the raptors from Michael Crichton’s Jurassic Park novels and the many movies, comics and games based on them. Not only do they look scary and foreign, they seem to hate humans with a passion, always trying to hunt and kill them. Either that or they are presented as sentient on a near-human level, like the raptors in Jurassic Park 3 or Blue in Jurassic World. In either case they do not come across as ordinary animals that really once existed. 
Fig. 2: This does not look like something you would encounter on Earth, does it? We will discuss Wayne Barlowe's art in a future post more specifically.
The second example is a scene from Stephen Baxter’s 2002 sci-fi novel Evolution, in which a Purgatorius observes a Deinonychus on the hunt (note that the two animals lived almost 40 million years apart). Here the creature is described as something that can neither be called a flightless bird nor a "souped-up crocodile". Apart from looking like its Jurassic Park counterparts, its mind is unfeeling and calculating and it can stand completely still, like a lizard, but then run and jump incredibly fast for short periods of time. This is because the author ascribes it a unique blood-pumping system that allows it to be agile while still technically being cold-blooded. Baxter seems to have made that up, since Deinonychus and many dinosaurs like it were in all likelihood simply warm-blooded similar to modern birds, something which was well-known consensus by the early 2000s. Why did he do this? Because, in his own words, dinosaurs were different. He repeats how dinosaurs were unlike anything ever witnessed by human eyes and frequently ascribes traits to them that make them alien from any modern animals, like all of them neither being really cold- nor warm-blooded. He also makes up speculative prehistoric creatures that seem more at home on the planet Darwin 4 than on Earth, like the Airwhale, a titanic pterosaur with a hundred meter wing-span that lives its entire life in the sky, feeding on aeroplankton and whose bones are so fragile that they never fossilized. All of this under a sky that is described like this:


Red dawn light seeped into the eastern sky. The clouds had a bubbly texture, and the sky was tinged in a peculiar bruised-purple. In this remote time the very air was different – thick, moist, laden with oxygen – even the sky would have looked alien to human eyes.


Baxter may be excused for exaggerating the otherness of the Mesozoic world (and at least he has the decency to give a Troodon proto-feathers), since his novel is written from the perspective of early primates. The bizarre alien dinosaur world that he describes likely exists as a contrast, to make the journey that our ancestors took seem even more adventurous. But Baxter is not the only one who describes the Mesozoic like an alien planet. Let’s go back to Crichton again. While we never get a scene in the Jurassic Park novel directly set in the Mesozoic (apart from a day-dream Alan Grant has in which a Pteranodon is eating algae for some odd reason), in a section about Procompsognathus we get a description of how Crichton imagines the Triassic world. Apart from how the continents were arranged and the climate being hotter, a statement that stands out particularly is how the air-pressure was supposedly higher, as if the Earth used to be like Venus in its past. Baxter and Crichton are far from being the only ones ascribing such peculiarities to prehistoric times. If we go back we find numerous such works, be it movies like Land before Time, Fantasia, King Kong, paleoart like that of Wayne Barlowe, Charles Knight or Rudolph Zallinger or even non-fiction works published by pseudo-scientists and actual scientists, which perpetuate a trope which I will from here on out call the Alien Prehistoric World. I characterize this trope here by several sub-tropes, some or all of which are frequently present in (especially older) works about prehistory:
  • The atmosphere was radically different during prehistoric times. Either the air pressure, the oxygen-content or both were higher and this is what allowed animals like dinosaurs and pterosaurs to grow to huge sizes. The color of the sky itself may have been different. 
  • Volcanism and other tectonic activity was significantly higher. Volcanoes dotted the landscape everywhere. 
  • Prehistoric lifeforms were all either gigantic and/or primitive. 
  • Prehistoric lifeforms might resemble life we could find on other planets. 
  • Prehistory and its inhabitants were overall more violent than the modern world, making it an age of monsters. 
  • All prehistoric animals lived roughly at the same time. 
  • Beauty did not exist yet. 
  • A major, often catastrophic event separates the prehistoric world from the modern one and also robbed it of most extraordinary/fantastic elements.
Of course you may say that some of these are not actually tropes, but have at least some basis in scientific fact. You would be right, but only to a degree. Of course prehistoric times were different from our modern times, but most of the works we will be discussing in this series of posts will take often minor or non-existent differences and exaggerate them to an unrealistic degree to where Earth resembles a different planet. Some of these “facts” you might have heard are also not as factual as one would like to think. The first point on the list for example, that the world of the dinosaurs was richer in oxygen and that this is why dinosaurs grew larger than modern animals, is something you might have heard of, but it has always been dubious. First and foremost, there has never been a clear consensus on Mesozoic oxygen levels. There were some studies throughout the years showing that they might have been higher than today, but also some showing that they were roughly the same and increasingly many more showing that they were even lower than today at some points. They all agree however that they never differed from modern levels more than a few percent and definitely never reached the levels present during the Carboniferous period, an age long before the dinosaurs. That the high oxygen content caused the arthropods of the Carboniferous to grow to huge sizes is also something often repeated, but has come under more scrutiny in recent years, as similarly sized arthropods have been found from other, less oxygen-rich, periods. Secondly, this hypothesis ignores that dinosaur and pterosaur anatomy was perfectly capable of supporting itself on this planet’s roughly current conditions. These animals had hollow bones and air sacs like modern birds and unlike mammals, allowing them to increase their volume without significantly increasing their mass. That dinosaurs had complex air sacs, the same ones that birds today use to breathe in high altitudes, also speaks for lower oxygen levels during the Mesozoic. It at least probably allowed them to use oxygen more efficiently than mammals do. Third and last, what this hypothesis also ignores is that the largest currently known animal is an air-breather of modern times, the blue whale. Similar claims that air pressure or even gravity itself was different or that Earth was almost completely covered in swamps or shallow oceans during dinosaur times are also based on a lack of understanding of dinosaur/pterosaur physiology and are often made by people who are not actual paleontologists or geologists (but know how to get a good publicity). Often they are even ideas that are based on already old and disproven concepts that have been forgotten for so long that they can be presented as new ideas again without too many people noticing, but we are getting ahead of ourselves here. What I ultimately want to say is that the only period (or rather eon) of Earth's history that is radically different enough in its conditions from today to be truly called alien would be the Precambrian, but that was a time during which no true animals existed yet and it is rarely depicted in popular media. Non-avian dinosaurs and their world might seem strange, but are in my opinion not different enough from what we know today to be truly called aliens. To quote Andrew H. Knoll: "Dinosaur bones are big and spectacular - they keep you awake at night. But, apart from the size of its inhabitants, the world of the dinosaurs was much like our own". 
Coming back to the point of this post: What I plan to do in this series of blogposts is try to trace back most of the Alien Prehistoric World tropes to their possible origins and follow their development through history. In this very first part I will do something I consider quite novel. I will make the argument that many of the tropes mentioned above do not originate in old paleoart, but can actually be traced back to pre-scientific times. Specifically I think that many of them originate in ancient mythological and religious motives, which then crept their way into Victorian era science, which from thereon out influenced our modern thinking. 
Fig. 3: An 1828 painting by American artist Thomas Cole, depicting Adam and Eve being cast out of the Garden of Eden into the bleak world of the antediluvian times. Take special note of the casually erupting volcano in the background, as this will be a motive we will encounter often in later depictions of dinosaurs. 
Time has been viewed differently by different cultures. Some ancient people, like those in Egypt and China, saw the flow of time as a cyclical affair that basically renewed itself with the start of each new ruling dynasty or even ruler. The Ancient Greeks and Romans were more split on the matter, although some like Aristotle or Pliny the Elder also stuck to a cyclical view of an eternal universe. The way we view time today in the Western and Islamic World was largely influenced by ancient cultures of the Middle East and their mythologies. In Mesopotamian, as well as Abrahamic religions, the universe was created once at some definite point and will end again somewhen in the future without being renewed. This creates a linear timeline with clear end-points, which is then able to be split into different sections based on major events. The one you will likely be most familiar with is the separation into a time before the birth of Jesus of Nazareth and a time after his birth (B.C. and A.D.). There are also others, like a time before and after the fall of man and the End Times. One of the most important separations into “eras” that derived from ancient Mesopotamian/Abrahamic religions, and which influenced the thinking of Christians for long into the early modern period, was the distinction into a time before and after a Great Flood that a hero had to build an ark for. The time before the flood was often called the Antediluvian. The word antediluvian is to this day still sometimes used as a synonym for "prehistoric", but for what reason? And what does this have to do with our trope? 
Let us look at how people and texts in the Middle Ages and Antiquity imagined the antediluvian time to be like. The first thing that comes to mind to anyone who has heard the story of Noah is that, after the fall of man, the world was supposedly full of sin, including, presumably, excessive violence. This is of course what caused God to send the flood in order to clean the Earth (Although that is of course just the account from the Old Testament. According to the Sumerian Athrahasis Epic, humans simply became too numerous and loud, keeping the gods from falling asleep). Less well known is the observation that people seem to have lived longer. Noah is described in Genesis to have been around 950 years old, with his relatives reaching similar ages. The Sumerian King List also shows that, before the flood, individual kings could rule for tens of thousands of years. After the flood, the age of kings, and presumably humans as a whole, exponentially declined. While Gilgamesh is said to have still ruled for over 100 years, later kings barely reached 30. Humans were of course not the only inhabitants of the antediluvian world. The Bible describes that the world was inhabited by creatures called Gibborim and Nephilim. What exactly those are meant to be is a matter of debate to this day, but in the past they were often identified as some sort of half-human giants. According to some interpretations of the Dead Sea Scrolls, which tell a slightly different version of Genesis, it was actually these beasts why God had sent the flood. Other monsters also roamed the world, such as the famous sea monster Leviathan, Behemoth, in Jewish Mythology also the giant bird Ziz and the Mesopotamian Tiamat. We summarize that the antediluvian world, as imagined by people in pre-scientific times was:
  • A bleak place full of violence.
  • Roamed by giants and monsters. 
  • Alien and separate from us today (through the long lifespans of antediluvian humans).
  • Destroyed by a gigantic catastrophe that got rid of any brutes, monsters, giants and superhumans.
A lot of these look like the beginnings of the tropes listed above. “But what does this have to do with dinosaurs?”, I hear you say and to answer that we have to turn to the early days of geology. Humans have found fossils probably for as long as humans have existed. Greek philosopher Xenophanes observed that the remains of marine animals, like seashells and snails, can be found on mountains. Of course, not knowing about plate-tectonics or how mountains are built, he assumed that this meant the sea levels used to be way higher in the past. Medieval scholars observed similarly, although they came to the conclusion that such fossils were not evidence of a simple change in sea-level, but of the great flood in the Old Testament. Either that or they regarded fossils as odd stones that just happened to look like seashells, something which was disproven during the Renaissance by none other than Leonardo Da Vinci and his studies on trace fossils. The belief that fossils were the remains of creatures that had perished in the flood continued well into the early modern period. This was a time when science as we know it today was beginning to develop and scholars tried using it to prove (and unintentionally disprove) the historicity of the Bible. A famous example of this are the studies of fellow Swiss Johann Jakob Scheuchzer of the late seventeenth and early eighteenth century. He firmly believed that the existence of God can be proven through scientific means and he was fascinated with studying the world before the biblical deluge. This made him one of the first people who did not view fossils as simple curiosities, but instead tried to seriously study them. He is today perhaps best known for his biggest blunder, which is rather unfortunate. In 1726 he infamously identified a fossil of a giant Salamander (today known as Andrias scheuchzeri) as being the crushed remains of a human child that was trampled to death in a mass panic during the early stages of the deluge. Of course today it is quite easy to make fun of Scheuchzer for this, as it is obvious for us to see that the fossil in question was not a human, but one has to keep in mind that the concept of extinction had not existed yet in Scheuchzer’s time and since he had never seen a giant salamander before, he had to interpret the remains into a context he was familiar with. I say it is unfortunate that this is the most remembered thing about the man, because it overshadows many of his actual achievements. For example, apart from being the first one to use barometric instruments for altitude-measuring and being one of the founders of crystallography, did you know that Scheuchzer can also be considered one of the first paleobotanists? In his book Herbarium diluvianum he was the first to study fossil plants from the Carboniferous, Permian and Cenozoic (although of course he claimed all of them to come from the antediluvian period) and often drew them in such a detail that the individual species he studied can still be identified today. I believe that his work on fossil plants influenced later artistic representations of the antediluvian world, like those of Thomas Cole, which often show primitive, tropical plants, furthering the association between the mythical pre-flood world and actual prehistory. 
Fig. 4: On the left are sketches of fossil plants by Johann Jakob Scheuchzer, who imagined these being representative of the vegetation of the pre-flood world. On the right is another Thomas Cole painting, showing an antediluvian scene with primitive vegetation, possibly inspired by early paleobotany. Take also note of the foggy atmosphere.
As geology and paleontology evolved further, ideas such as the deluge and the six days of creation still influenced the thinking of scientists. In the early nineteenth century, French naturalist Georges Cuvier, who used fossils of marine reptiles and mammoths to prove that animals could in fact go extinct, coined what is today known as catastrophism. While, like us today, he separated Earth’s geologic history into different time periods based on its inhabitants, such as the Age of Reptiles or the Age of Mammals, he did not believe that lifeforms gradually evolved or became extinct over time (Mind you that it would still be a couple of decades until Darwin would publish On the Origin of Species). Instead he was of the opinion that each geologic age was capped off by a gigantic catastrophe that wiped out nearly all life, forcing it to start anew in the next one, and that these catastrophes are what largely shaped Earth's history and geology. While the Age of Reptiles may have been ended by a comet, he thought that the more recent mammals, such as mammoths and woolly rhinos, were wiped out by a flooding event like that described in the Bible (although he did not take the biblical account literally and instead thought it was simply inspired by a real event). This equated the Pleistocene to the Antediluvian in many people’s minds at the time. For a good part of the Victorian era, Earth’s geologic history was similarly seen through a biblical lens. Our naming and characterization of the geologic periods in part comes from those days. By the time of Sir Richard Owen and Charles Darwin, the geologic column was mainly divided into Primary, Secondary, Tertiary and Quaternary strata, a scheme first laid out by Giovanni Arduino in 1759. Primary describes rocks in which no fossils are present and were thus equated to the first two days of creation, when god had created the Earth but no life yet. Today we know that these are rather rocks of a distinctive type rather than a distinctive age, as they are magmatic and metamorphic rocks, hence why the name Primary isn’t used for any period anymore. The Secondary period was equated to the third to fifth day of creation, as fossils of animals and plants can be found in it, but no humans yet. The Secondary was further divided into the Coal, Lias and Chalk ages. The Coal Age is still a name in use today for the previously mentioned Carboniferous period, as that is where most of our coal deposits come from. The Lias describes what is today better known as the Lower Jurassic, while the Chalk Age became the Cretaceous period (The name deriving from the Latin word for chalk, creta). More periods were eventually recognized and added until the Secondary became obsolete and divided into the Paleozoic and Mesozoic eras of today. The Tertiary period was equated to the sixth day of creation up until Noah’s flood, making it the Antediluvian period, as fossils of modern-looking mammals as well as humans and human artifacts can be found in it. If you are old enough, you may still remember the name Tertiary being used in textbooks to describe the majority of the Cenozoic era, but today it is divided into the Paleogene and Neogene periods. Quaternary, originally used to describe the post-flood world, is still in use today to describe the Pleistocene and Holocene epochs. As time marched on, the natural sciences became more and more divorced from religious ideas. Geologists soon found that there was no evidence for the deluge of the Bible ever having occurred and that many features once attributed to it could more easily be explained through the long-term processes of rivers, glaciers and slow sea-level changes over time. Cuvier’s catastrophism was soon abandoned in favor of James Hutton's and Charles Lyell’s uniformitarianism, which stated that past geologic features can all be explained through slow processes that we can still see today and that the succession of geologic periods and species was a gradual, long-term development (today’s view of geologic history is a mix of the two, as Lyell’s principles of geology still generally apply, but we recognize that catastrophic events can and did sometimes happen on occasion). Catastrophism still went on to inspire many creationist authors who tried imagining mechanisms for the global flood, such as comets or vapor canopies. The latter encompasses ideas like the antediluvian Earth having icy rings like Saturn that once crashed down or that there was no rain and instead the atmosphere was constantly laden with fog. The vapor canopy theory may sound like a good precursor to some of the sub-tropes of the Alien Prehistoric World trope, but the problem here is that most of these were proposed by young earth creationists from the 1940s and I therefore consider them too recent and too niche to have had any major influence on classic paleoart. What I think is more notable is that many nineteenth century paintings of antediluvian times, like those of Thomas Cole, James Tissot or John Martin, show a very foggy atmosphere. This was likely an artistic decision to show how murky and mysterious this time was and as we will see in future posts, this fog carried over into classic paleoart. Over time the symbolic meaning of the fog was likely lost on many viewers and taken literally.
Fig 5.: Duria Antiquior, an 1830 watercolour painting by Henry De la Beche, often considered the first true attempt at reconstructing lifeforms from Deep Time. Depicted are several of the species discovered by famous fossil hunter Mary Anning at Lyme Regis. Almost all are shown as bizarre and violent.
While geology largely abandoned its religious roots, paleontology seems to have still equated the real prehistory with the mythological antediluvian times, at least in popular writing. For example the 1863 book La Terre Avant le Deluge, written by Louis Figuier, illustrated by Édouard Riou and released over 30 years after Lyell’s Principles of Geology, was the first book that extensively described and illustrated prehistoric life, but still told the story of Earth’s geologic history from a biblical perspective (as you may have noticed by the title). Before it, antediluvian tropes had already crept into paleoart as can be seen by the arguably first artistic reconstruction of prehistoric life, Duria Antiquior. The marine and flying reptiles depicted here are not only shown as quite bizarre antediluvian monsters, but are also engaging in excessive violence, seemingly only concerned with eating each other. One of the first depictions of dinosaurs, The country of the Iguanodon, also shows them in a very foggy and violent world. They are depicted as whale-sized lizards partially living in water, likely invoking descriptions of biblical monsters like the Leviathan and medieval dragons and seamonsters. In the average person of the time, these must have surely evoked associations with biblical narratives of the antediluvian world. What I found fascinating is that even Richard Owen, in his 1854 tour-guide for the Crystal Palace Dinosaur Park, describes pterosaurs as dragons. Not just as “similar to dragons”, but literally as synonyms, like they are the same creature. He's even casual about it:

"Nos 2 and 3 are restorations of a flying reptile or dragon, called Pterodactyle,..." (Owen 1854, p. 11)

Given how it was in a tour-guide, the wording, while confusing, may have just been intended to help the average park-visitor better imagine these animals. However, if we consider Owen's own creationist views, it may have well been meant to be taken literally. The Crystal Palace Park and its impressive sculptures of the then current vision of dinosaurs were to the nineteenth century what Jurassic Park is today, as it introduced to a lot of general audiences a relatively wide range of prehistoric creatures in an entertaining manner, even if it was quickly outated (also like Jurassic Park). They even sold smaller replicas of the statues as merchandise. While the sculptures were a lot more calm and animalistic than monstrous, their design still portrayed them as intimidating and bizarre brutes. While paleontology would eventually also go on to lose its connection to mythological beliefs, the cultural memory of dinosaurs and their relatives as antediluvian monsters living in a distant world would still haunt paleoart for decades to come. 
Fig. 6: The Country of the Iguanodon by John Martin from 1837. Before Jurassic Park, King Kong, the Bone Wars and even before the famous sculptures of the Crystal Palace Park, this is what dinosaurs were imagined to be like by Victorian era scientists/artists. Martin is perhaps better known for his paintings of biblical scenes and as a devout Christian he was very likely influenced by imaginations of antediluvian times and mythological monsters while painting this piece. As a side-note, only while writing this article did I notice that you can very faintly (under better resolution than here) see the outline of a mammoth and perhaps some other extinct mammals in the background. This likely means Martin imagined all these animals living at the same time, a belief which, as far as I am aware, was already outdated during this painting's making.
In the next part of this series we will explore how the Alien Prehistoric World Trope evolved into the early twentieth century through the writings of Marsh and Cope and in the art of Charles R. Knight, as well as movies like King Kong and Fantasia. We will also look how prehistoric times were linked to alien planets for the first time through early speculations about the planet Venus. Thank you very much for reading my very first blog-post and please leave your critique and suggestions in the comments below. 

Literary sources:
  • Barrett, Paul/ Naish, Darren: Dinosaurs. How they lived and evolved, London 2016 (2. Edition).
  • Baxter, Stephen: Evolution, London 2002.
  • Brusatte, Steve: The Rise and Fall of the Dinosaurs. A New History of a Lost World, New York 2018.
  • Conway, John/Kosemen, C.M./Naish, Darren: All Yesterdays. Unique and Speculative Views of Dinosaurs and Other Prehistoric Animals, UK 2012.
  • Crichton, Michael: Jurassic Park, New York 1990.
  • Davidson, Jane: A History of Paleontology Illustration, Bloomington 2008.
  • Davies, Paul: The Eerie Silence. Searching for ourselves in the universe, New York 2010.
  • Jacobsen, Thorkild: The Sumerian King List, Chicago 1939.
  • Knoll, Andrew: Life on a Young Planet. The First Three Billion Years of Evolution on Earth, New Jersey 2003 (Second Paperback Edition).
  • Miller, William: Trace Fossils. Concepts, Problems, Prospects, Arcata 2007.
  • Owen, Richard: Geology and Inhabitants of the Ancient World, London 1854 (Facsimile Edition).
  • Rotziger, John: Allgemeine Geologie, Heidelberg 2008 (7. Edition).
  • Scheuchzer, Johann Jakob: Herbarium Diluvianum, Zürich 1723.
  • Scheuchzer, Johann Jakob: Physica Sacra, Zürich 1731.
  • Witton, Mark: Pterosaurs, New Jersey 2013.
  • Witton, Mark: Recreating an Age of Reptiles, Marlborough 2017.
  • Witton, Mark: The Paleoartist's Handbook. Recreating prehistoric animals in art, Marlborough 2018. 
Online sources:
Images:
  • Fig. 2: Dodson, Peter/Barlowe, Wayne: An Alphabet of Dinosaurs, New York 1995, under the letter O (the book curiously has no numbered pages).
  • Fig. 3: Wikimedia
  • Fig. 4 left: Scheuchzer, Johann Jakob: Herbarium Diluvianum, Zürich 1723, page 17.
  • Fig. 4 right: Wikimedia
  • Fig. 5: Wikimedia
  • Fig. 6: Wikimedia