March 7, 2018.
Today we are going to begin to examine some of the evidence and data have lead virtually all working biologists to the conclusion that the diversity of life on this earth is a product of evolutionary processes. All too often the debate over evolution is so filled with rhetoric and argument that the actual data in its context isn’t actually addressed, or it is ignored/misconstrued. If we are seeking truth, then this is not how we should deal with any lines of reasoning. At the very least, I ask you to honestly consider the evidence, and even if you still come out on the other side, please realize that (most) scientists do not have personal vendettas against religion, nor do they try to hide and conceal facts in order to uphold evolution in the face of all the contrary evidence. There is no conspiracy theory. There is real, legitimate reasons that they are compelled to agree about evolution. We shall explore a small part of the data now. There is more to be said than just this, but it will be a start. Perhaps I will add to it as time goes on. We will go over early evidence and evidence from the fossil record today, then in a subsequent post we will go over the genetic evidence.
Evidence from Field Observations
The theory of evolution didn’t start with Darwin thinking, ‘I would like to believe that there is no special creation, this world is not designed and all life has formed by natural processes without any intervention of a deity. I should go out and find some evidence to fit what I want to believe.’ No, as we have seen from previous posts, Darwin was actually a fan of Willian Paley’s Natural theology before he went on his voyage on the Beagle. The theory of evolution came from observations in nature, and continues to be bolstered by more and more observations. We will now look at some early evidences and indications of evolution.
1. Fossils in the Strata
Even before Darwin, Geologists had been studying rocks and fossils and began to have reservations about what they had historically held to be true about the creation. We have seen previously that many scientists (and much of the informed public as well) had come to the conclusion that the earth was much older than 6,000-10,000 years, and that it had a complex history. At the turn of the 19th century, geologists were beginning to notice faunal succession in the layers of strata in the rocks of the earth. The principle of faunal succession describes the observation that sedimentary rock strata contain a specific, reliable succession of fossilized fauna across the world. William Smith, one of these early geologists, is remembered as the father of modern geology for his work in this field as he published the first geologic map of England in 1815. Faunal succession is so reliable that geologists even today use their knowledge of fossils to determine which strata they came from and where oil companies should drill for oil.
What do these fossils say about evolution? They were the first observation that indicated that there was a pattern of emergence of life on the planet, an orderly succession of fauna over time. The evidence does not lend itself well to all animal life (or plant life for that matter) being alive at the same time, as the majority of these same geologists believed at the time. As we will see in the next section, paleontologists now use these fossils and their dates determined relatively by faunal succession and absolutely by radiometric dating to directly observe the evolutionary history of the world. However, it was this reliable pattern and observation that first indicated the complex history of life on earth and challenged the assumptions of these early geologists.
2. Geographic Distribution of Animals
Further evidence for the complex history of life on earth came when the naturalists began discovering more and more species, many of which were unknown in Europe but found in other parts of the world. When Darwin set sail on the Beagle, he embarked on a journey that would allow him to observe spices endemic specific locations such as continents or islands that had been geographically isolated for long periods of time. Today, we have much more information than Darwin gathered, and the observations are crucial in beginning to understand the complex history of life on the planet. There are many islands that have species that are specific to those islands and are found nowhere else in the world. However, I would like to use one striking example (though there are more) to illustrate how biogeography, or the study of how life is dispersed across the earth, makes the most sense in an evolutionary context.
Australia is known for its unique presentation of animals, many of which are only found in Australia and nowhere else in the world. When we look at a particular group of mammals, the marsupials, the story gets even more interesting. Mammals are divided into three categories, monotreme, placental and marsupial. Monotremes are egg laying mammals, of which the only known living species are the spiny anteater, or echidna, and the platypus, each endemic to Australia. Placental mammals are the largest group of mammals, who give birth to fully gestated live offspring. Marsupials, such as the kangaroo, give birth to offspring that are not completely developed and finish development in their mother’s pouch outside the abdomen. What is interesting about Australia is that the distribution of mammals is quite opposite from the rest of the world. All other places are dominated by placental mammals. However, Australia is overwhelmingly dominated by marsupial mammals. There are only a few species of placental mammals in Australia, but many species of marsupials, such as the kangaroo, koala and wombat. The only known living marsupial outside of Australia is the North American Opossum.
This may just seem like an interesting fact, until you start thinking about the implications of it. Why is it that Australia has very few placental mammals? Why are the vast majority of marsupials found only in Australia and lower South America? From a flood paradigm, it would not make sense that the only mammals to disperse to Australia would be marsupials. Further, how would they even get there? However, from an evolutionary point of view, there is a fitting explanation. Plate tectonics has shown that Australia used to be connected to South America long ago, where the fossil record has shown much evidence of marsupials. Once Australia broke off from South America, the ancient marsupials were cut off from the mainland population- as were any placental mammals cut of from migrating to Australia. What is even more interesting is that there is a high level of convergence of Australian marsupials and other placentals in the world. What I mean by this is that many marsupials in Australia look very similar to placental mammals found elsewhere, such as the wolf and the Tasmanian wolf, the flying squirrel and the flying phalanger, and the groundhog and the wombat. Since there were no placental mammals occupying particular niches in Australia, the marsupials evolved convergent features to exploit the unoccupied niches. Though they look very similar, they are actually only distantly related. This is a stunning instance of convergent evolution in action.
3. Comparative Anatomy
Finally, we should say something about comparative anatomy in the study of evolution.
Evidence from the Fossil Record
Now that we have seen the beginning hints at evolution, we will move on to evidences found in the fossil record. However, before we do, we must make some observations about the nature of the fossil record. Fossilization is actually a highly improbable event that only occurs under certain conditions. For the vast majority of animal or plant deaths, all organic material and bones are easily decomposed and do not get covered by the necessary sediment layers. There are only about 250,000 known species of fossil animals and plants today. For comparison, there are somewhere in the neighborhood of 4-5 million species living today. Taken with all the species that have ever lived, the fossil record is but a tiny fraction of one percent of all the species that have lived on this earth. That’s incredibly incomplete- it’s a wonder that we can learn anything from the fossil record at all.
Yet we can and do! When Darwin published Origin, his biggest reservation was the incompleteness of the fossil record and transitional forms. With only a tiny fraction of all the species that ever lived, this is understandable, and in Darwin’s day he didn’t get much help from the fossil record. However, even though the record is still incomplete, it has actually become one of the strongest evidences for Darwin’s theory. Contrary to popular belief, there are actually many transitional fossils that we have found to date. I remember when I was younger, I was very anxious that one day a paleontologist would find a “missing link” and my young-earth creationism (YEC) would simply collapse (along with my faith in the Bible, due to the false dichotomy that I had been taught). Little did I know that transitional fossils had already been found even then, and more have been found now. Why didn’t I know? Because I had been told over and over “there are no transitional fossils in the fossil record.” As we will see shortly, this is simply not true.
There are a few reasons that YEC makes the claim that there are no transitional fossils. The first is because many are simply unwilling to recognize them when they are shown. They will claim “that’s just a bird, not a transition between a reptile and a bird” when staring at Archaeopteryx. Actually, it is probable that no “transitional fossil” could ever satisfy YEC due to a priori commitments. However, many transitional fossils have been found. A second reason that YEC claim that there are no transitional fossils has to do with an exploitation of terminology. As we saw in the previous post, there is a common misconception about evolution: it proceeds in a linear, ladder-link manner (see Fig. 1a). This is simply not how evolution works. The misconception is probably exaggerated by the typical “ape to man” diagram for evolution. Actually, evolution is very bushy, and lineages look more like what is shown in Figure 1b.
If you think of evolution as is depicted in Figure 1a, then it is understandable that you would expect to have found a great many ‘true’ missing links, because every fossil should be a gradual change to the next (i.e., species B is a true ancestor of species C, which is a true ancestor of species D, etc.). However, evolution proceeds in a manner akin to Figure 1b, where ‘true’ ancestors (a fossil of one of the branching points) are much less likely to be found than fossils along the horizontal lines. We must recognize, then, that transitional fossils are not necessarily “ancestors” of a subsequent group, but rather sister groups, such as species B and C. Thus, we may never find the common ancestor of species D that was the progenitor of species D and E, but species B is a sister group of that common ancestor that was quite similar to it, and thus is indeed a transitional line that went extinct. This transitional fossil will likely show many of the characteristics of both species D and E. However, YEC often exploits this subtlety in the language (by quote mining paleontologists) to make it seem like there have been no transitional fossils found. Ask any working paleontologist and they will tell you this is absurd. Even though we only have a very tiny fraction of one percent of all life that has lived on this planet, we have found numerous transitional fossils that link two different groups. In fact, in some areas where we have an almost, if not, complete sampling fossil record (such as marine microfossils) ‘true’ ancestors can and have been recognized. Further, for some of the transitional fossils we will see, there is no rule out the possibility (however slight) that they are ‘true’ ancestors of a subsequent species.
With all this in mind, it is time to turn to the fossils and see what they have to say.
Along with the general statement that the fossil record has no transitional forms, some have specifically noted that “There simply are no transitional forms in the fossil record between the marine mammals and their supposed land mammal ancestors.” It may indeed seem fanciful to think of land dwelling tetrapods transitioning into aquatic mammals, and without much study, you might seem to have a good case against evolution. I mean, how could a bear survive at sea? However, over the past few decades, many important fossils have been found that clearly show the transition from land to sea for the Cetaceans, and these transitional fossils have become one of the clearest examples of macroevolution found in the fossil record.
To begin, we will start with something that will be discussed further below: atavisms. Living baleen whales have vestigial pelvic and femur bones in the lower region that form the basis for hind limbs, and sometimes even have lower leg bones and metatarsals (feet bones) that do not protrude from the body wall. These vestigial limbs do not serve a purpose for the animal, but rather show that the whales once had fully formed hind limbs that have now been reduced due to their loss of function in an aquatic environment. Further, whales that actually do have fully formed hind limb atavisms have been documented. These are clear signs of the past history of Cetaceans.
But we can go further. The fossil record for whales has been extensively developed, much of the work done by Philip D. Gingerich, a leading expert of whale evolution. Two of the most striking fossil skeletons that have been found are Maiacetus inuus and Dorudon atrox, shown below (from Gingerich, 2012):
Another discovery of a primitive whale, named Basilosaurus isis, has shown remarkable evidence of feet in primitive whales. A more complete family tree for whale can be found in Houssaye et. al. (2015) as seen below:
An article on biologos by Dennis Venema has a very good outline of the transitional fossil record for whales, showing each transition in relation to one another, along with the other lines of evidence that link these species (click here).
Another transition that might be hard to conceptualize is that which sea turtles underwent to form their shells. Could a turtle have half a shell and survive? Indeed, until 2008 much of the evolution of the turtle body plan was a mystery, as the oldest known turtle that had been found already had a fully formed shell. This is wehre many YEC sites might highlight this gap and emphasize the lack of transitional fossils. However, in 2008, Li et. al. documented an intermediate step in turtle evolution, describing Odontochelys semitestacea, a 220-milllion-year-old turtle with half a shell, the ventral and dorsal views shown below.
The Wikipedia article has a reconstruction of what Ondontochelys might have looked like alive:
From these fossils and more, scientists have been able to construct a timeline and intermidates for the evolutionary history of the turtle shell that until recently was a complete mystery.
3. Dinosaur to birds
The classic transitional fossil that paleontologists have shown (which has been known since 1877, and thus been scrutinized heavily by YEC proponents), is Archaeopteryx. Archaeopteryx is the earliest known bird, but this species has a host of both bird and reptilian features. It was one of the first transitional fossils to have been discovered, and provides evidence for the link between birds and dinosaurs. Since 1877, more Archaeopteryx fossils have been found, some of which are shown below:
Most YEC proponents claim that Archaeopteryx was just a bird and thus dismiss the reptilian characteristics of the specimens, such as the long, bony tail, teeth and maxilla bones, free (unfused) vertebra and claws on three unfused digits. These are features that Archaeopteryx shares with dinosaurs but not with modern birds.
But even with the reptilian features, why does Archaeopteryx have feathers? It doesn’t seem likely that this species would just grow feathers when all the other dinosaurs didn’t, right? Over the past couple of decades, the evolutionary history of feathers has been a lively discussion amongst evolutionary biologists and paleontologists. Interestingly, there have been a number of non-avian dinosaur fossils discovered that have feathers. Remarkably, the tail of a feathered, non-avian theropod has been preserved in great detail in amber, which has directly informed the evolutionary history of feathers. Another discovery in 2004 shows a non-avian dinosaur preserved in a sleeping position that is characteristic of modern birds, further showing a behavioral link between non-avian dinosaurs and modern birds. All of these links provide strong evidence that modern birds are derived from a dinosaurian linage. Remember that evolutionary trees are bushy. This does not mean that all dinosaurs turned into modern birds, or even that these feathered dinosaurs are necessary precursors to modern birds. Only one line of dinosaurs evolved into birds, and these fossils are likely sister taxa. Nevertheless, they do show evidence that birdlike qualities were evolving in non-avian and primitive avian species.
4. The transition from sea to land
Perhaps one of the biggest superficial criticisms of evolution is the idea that a fish could walk on land and become a land dwelling animal, or that a land dwelling animal could transition to a sea animal. It is an easy criticism because it doesn’t seem intuitive to us. However, many animals living today seem to have adapted (or are adapting!) to do just that. The “walking catfish” can survive extended periods outside water, and can “walk” from one pond to another. The mudskipper actually spends most of its life out of water. Other fish also have evolved ways to survive partially out of water. We now turn to the current understanding of just how some primitive fish took to the land.
In 1983, Marjorie Courtenay-Latimer made a remarkable discovery when she was brought a rather unique fish by a local fisherman. The fish was a coelacanth, or lobe-finned fish, that had been believed to have been extinct for some 70-million years. Since then, more lobe-finned fish have been caught from the sea, and they look similar (though there are notable differences) to their ancient counterparts. What makes these fish unique is that they are one of two types of fish (both lobe finned) that are contenders for the fish that first “crawled out on land”, the other being the lungfish (which have been shown to be more closely related to tetrapods than the coelacanth via phylogenetic analysis). Most fish have fins supported by spiny rays. However, the lobe-finned fish have fins that are supported by robust bones. More evidence for this transition is deduced from the fact that lob-fins have key homologous (bone for bone) structures compared to primitive tetrapods. What is even more remarkable is the fossil record that has shown a relatively robust picture of this sea-to-land transition, as seen below:
5. Snakes and legless lizards
Much like the loss of limbs in whales, there have been a number of discoveries of transitional fossils of snakes that have limbs, some two legged and some four. Also like some extant whales, certain species of living snakes have vestigial structures that once were hind limbs (such as the remnants of a pelvis in boas and pythons). Further, molecular evidence is now shedding some light on the genetic mechanism behind the loss of legs in snakes, giving more evidence to their evolutionary progression. Though the direct line of snake origins is still debated in the literature, it is clear that snakes have evolved from some line of lizards. Actually, the limbless body plan has evolved independently in several different tetrapod species.
6. Other transitional fossils
I have only touched on a few transitional fossils here, though they should be adequate to be indicative of macro-evolutionary changes that have been found in the fossil record. I want to note here that the brevity of the survey of the fossil record is not because these are the only transitional forms, but rather because I am not a paleontologist, and there are a vast amount of resources out there that document many more transitional species. If you would like to see more of the transitional forms, I would recommend Donald Prothero’s book Evolution: What the Fossils say and why it Matters. He documents the extensive record of transitional forms in the fossil record in almost every major group of animals. I will warn you though, Prothero does not have any patience for young-earth creationism and he doesn’t hold any punches. However, you don’t have to agree with his whole philosophy to appreciate his vast experience and detailed record of the evidence of transitional fossils. He talks about transitional fossils that bridge the gap from sea to land, transitional dinosaurs, whales, elephants, mammals, rhinos and even hominoid species. Though many YEC proponents simply deny it, the evidence is out there. I encourage you to look for yourself (in resources that are written by people you may disagree with, but are current experts in the field).
The fossil record serves as our first line of evidence of the evolutionary history of life on earth. Next we will move on to the powerful tales of genetics to show life’s complex history before moving on to the theological aspects of our discussion. Please stay tuned.
 See Ferngren, G. Science and religion: a historical introduction. (The Johns Hopkins University Press, 2002). p. 170
 Ibid. p.179-187.
 See Prothero, D. Evolution: what the fossils say and why it matters. (Columbia University Press, 2017), esp. chps.3-4 for information throughout the paragraph.
 Note here that Henry Morris tries to explain faunal succession in his influential book The Genesis Flood by saying less complex animals were not able to climb as high or get away from the rising flood waters as well as more complex animals. This overt speculation clearly falls apart when examined critically, as insects and other flying animals, or animals that were able to climb would be able to get higher than animals such as the sloth or elephant. Further, there is no reason to assume that all animals would happen to fall in line in the correct strata that would show them as consistent sequence from less complex to more complex.
 They realized that there were too many species to fit on a single ark. Modern young earth creationists have tried to address this problem by saying that the animals on the ark were representatives of each “created kind” that subsequently evolved (via microevolution, of course) into the diversity of life we see today. The problem with this is that even if it is granted that each animal is a representative of a “kind”, there are still too many “kinds” to fit on the ark unless some of the “kinds” have undergone macroevolution to become very different creatures today. Further, there hasn’t been sufficient time, assuming the flood was approximately 4,000 years ago (since it was long after the creation of Adam and Eve according to the genealogies, see https://answersingenesis.org/bible-timeline/timeline-for-the-flood/), for this kind of evolution, nor for the migration of these species across the globe. But perhaps the most glaring problem with this theory is that Scripture itself never makes this claim.
 See Ferngren, G. (2002). p.208-218 and Darwin, C. On the origin of the species by natural selection. (1859).
 See Prothero Evolution (2017). Ch. 4.
 See Warren, W. et al. Genome analysis of the platypus reveals unique signatures of evolution. Nature 453, 175–183 (2008). The platypus is an interesting organism that has been shown to have both mammalian and reptilian elements in their genome, indicating an early split in the mammalian clade.
 See Prothero, D. The Princeton Field Guide to Prehistoric Mammals (Princeton Field Guides). 240 (Princeton University Press, 2017) p. 37-46.
 For information throughout this paragraph, see Prothero, D. Evolution (2017), chs. 4, 13.
 Information about the fossil record from Prothero, D. Evolution (2017). ch. 3.
 See http://apologeticspress.org/apPubPage.aspx?pub=1&issue=517&article=2557. See http://www.talkorigins.org/faqs/archaeopteryx/info.html for a detailed list of Archaeopteryx’s bird and reptilian features.
 I encourage you to read Prothero, Evolution (2017) for a detailed account of many transitional fossils.
 See Prothero, DR & Biology, L.-D. Planktonic microfossils and the recognition of ancestors. Systematic Biology (1980). doi:10.1093/sysbio/29.2.119 and Prothero, D. Evolution (2017). ch. 8.
 For more information about whale evolution in the fossil record, see Gingerich, P. D. Land-to-sea transition in early whales: evolution of Eocene Archaeoceti (Cetacea) in relation to skeletal proportions and locomotion of living semiaquatic mammals. Paleobiology 29, 429–454 (2003), Fordyce, R. E. The evolutionary history of whales and dolphins. Annual Review of Earth and Planetary Society 22, 419–455 (1994), Bajpai, Thewissen, J. & Sahni. The origin and early evolution of whales: macroevolution documented on the Indian Subcontinent. Journal of Biosciences 34, 673–686 (2009), Gingerich, P. D. Evolution of whales from land to sea. Proceedings of the American Philosophical Society 156, 309–323 (2012), Gingerich, P. Elwyn Simons: A Search for Origins. (2008) and https://biologos.org/blogs/dennis-venema-letters-to-the-duchess/understanding-evolution-theory-prediction-and-evidence-1.
 Bejder, L. & Hall, B. Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss.Evol Dev 4, 445–458 (2002).
 Andrews. A remarkable case of external hind limbs in a humpback whale. American Museum novitates; no. 9. (1921).
 Gingerich, P. D. Evolution of whales from land to sea. Proceedings of the American Philosophical Society 156, 309–323 (2012), p. 317-18.
 See Gingerich, P., Smith, H. & Simons, E. Hind Limbs of Eocene Basilosaurus: Evidence of Feet in Whales. Science 249,154–157 (1990).
 Houssaye, A., Tafforeau, P., de Muizon, C. & Gingerich, P. Transition of Eocene Whales from Land to Sea: Evidence from Bone Microstructure.Plos One 10, e0118409 (2015).
 See Li, C., Wu, X.-C., Rieppel, O., Wang, L.-T. & Zhao, L.-J. An ancestral turtle from the Late Triassic of southwestern China. Nature 456, nature07533 (2008) and Prothero Evolution (2017), chapter 11.
 By Nobu Tamura (http://spinops.blogspot.com) – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=19462661
 See especially Lyson, T., Bever, G., Scheyer, T., Hsiang, A. & Gauthier, J. Evolutionary Origin of the Turtle Shell. Current Biology 23, 1113–1119 (2013). Also see Lyson, T., Bever, G., Bhullar, B.-A., Joyce, W. & Gauthier, J. Transitional fossils and the origin of turtles. Biol Lett-uk 6, 830–833 (2010) and Schoch, R. & Sues, H.-D. A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature 523, 584–587 (2015).
 Refer to footnote 13.
 See Prothero Evolution (2017), chapter 12.
 H. Raab (User: Vesta) – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8066320, SA 3.0, https://commons.wikimedia.org/w/index.php?curid=406199 and H. Zell – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11597542, respectively.
 Again, see http://www.talkorigins.org/faqs/archaeopteryx/info.htm for a full list and sources.
 See Qiang, J., Currie, P., Norell, M. & Shu-An, J. Two feathered dinosaurs from northeastern China. Nature 393, 31635 (1998), Norell, M. A. & Xu, X. Feathered Dinosaurs. Annual Review of Earth and Planetary Sciences 33, 277–299 (2005), Hu, D., Hou, L., Zhang, L. & Xu, X. A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus. Nature 461,640–643 (2009), Zelenitsky, D. et al. Feathered Non-Avian Dinosaurs from North America Provide Insight into Wing Origins. Science 338, 510–514 (2012), Longrich, N., Vinther, J., Meng, Q., Li, Q. & Russell, A. Primitive Wing Feather Arrangement in Archaeopteryx lithographica and Anchiornis huxleyi. Curr Biol 22, 2262–2267 (2012), and Xu, X. et al. A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature 484, nature10906 (2012).
 Xing, L. et al. A Feathered Dinosaur Tail with Primitive Plumage Trapped in Mid-Cretaceous Amber. Curr Biol 26, 3352–3360 (2016).
 Xu, X. & Norell, M. A new troodontid dinosaur from China with avian-like sleeping posture. Nature 431, 838–841 (2004).
 See Carroll, S. B. The making of the fittest: DNA and the ultimate forensic record of evolution. (WW Norton & Company, 2006). p. 117-124 for information throughout.
 See Prothero Evolution (2017). Ch. 10 for information throughout.
 Amemiya, C. et al. The African coelacanth genome provides insights into tetrapod evolution. Nature 496, (2013).
 Prothero. D. Evolution (2017). Ch. 10. (p. 224-225 in first edition).
 Figure from Clack, J. The Fish–Tetrapod Transition: New Fossils and Interpretations.Evol Educ Outreach 2, 213–223 (2009).
 See Rieppel, Zaher, H., Tchernov, E. & Polcyn, M. J. The Anatomy and Relationships of Haasiophis terrasanctus, a Fossil Snake with WellDeveloped Hind Limbs from the Mid-Cretaceous of the Middle East.Journal of Paleontology 77, 536–558 (2003), Martill, D., Tischlinger, H. & Longrich, N. A four-legged snake from the Early Cretaceous of Gondwana. Science 349, 416–419 (2015), Apesteguía, S. & Zaher, H. A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature 440, 1037–1040 (2006), and Tchernov, E., Rieppel, O., Zaher, H., Polcyn, M. & Jacobs, L. A fossil snake with limbs. Science 287, 2010–2012 (2000).
 See, for example, https://www.amnh.org/exhibitions/darwin/evolution-today/how-do-we-know-living-things-are-related/vestigial-organs/ and https://en.wikipedia.org/wiki/Snake_skeleton#Vestigial_limbs. Also see Caldwell, M. ‘Without a leg to stand on’: on the evolution and development of axial elongation and limblessness in tetrapods.Can J Earth Sci 40, 573–588 (2003), section “modern snakes”.
 Kvon, E. et al. Progressive Loss of Function in a Limb Enhancer during Snake Evolution. Cell 167, 633–642.e11 (2016) and Kaltcheva, M. & Lewandoski, M. Evolution: Enhanced Footing for Snake Limb Development. Curr Biol 26, R1237–R1240 (2016).
 See Prothero Evolution (2017), ch. 11.
 See Wiens, J. J. & Slingluff, J. L. How Lizards turn into Snakes: A Phylogenetic Analysis of Body-form Evolution in Anguid Lizards.Evolution; international journal of organic evolution 63, 2303–18 (2001), Caldwell, M. ‘Without a leg to stand on’: on the evolution and development of axial elongation and limblessness in tetrapods. Can J Earth Sci 40, 573–588 (2003).