Recent questions over the activities of creationist organisations in the UK, and a public debate in the US have stirred up a host of stories on both sides of the Atlantic over the evidence for evolution. Naturally the same old series of canards, false claims, out of date ideas and profound misunderstandings have been voiced by creationists, and no matter how often they are rebutted they don’t seem to get quelled. Statements such as that Darwin admitted the fossil data was poor and thus there is no evidence from fossils for evolution are all too common and rather incredible even at face value.

After all, should we really judge the fossil record on what Darwin thought it was like more than one hundred and fifty years ago? The study of palaeontology barely existed then, and fossils were both rare and not well understood, though Darwin did live long enough to see the legendary Archaeopteryx discovered and bask in the fact that it fitted his predictions wonderfully. In his own lifetime that argument could be set aside and a century or two of research does wonders for improving our understanding.



Naturally as a palaeontologist, this is the area of biology with which I am most familiar, but it is also the one about which most people are least au fait. While something of a branch of biology, in the UK at least, it is often taught primarily alongside geology and even biology graduates may have never had formal lectures or studies of the fossil record or the older parts of life on Earth. If that’s the case, it’s perhaps not a big shock that it is not well understood or appreciated by the public at large and that can make it a ripe target for misinformation or for people to be misled.

The most common comment is one along the lines of there being no transitional fossils, something utterly absurd. While the fossil record is undeniably incomplete, it is replete with huge numbers of fossils and in some cases magnificent examples of major transitions of single features, and whole lineages.



We have dozens and dozens of hominid fossils for example that allow us to observe changes in hand and foot shape, size, brain size, tooth form and others. The fossil record of early whales and their ancestors allow us to see an incredible transition – from fully quadrupedal and terrestrial animals to those living in the water and unable to survive on land. Take a look at Indohyus, Ambulocetus, Pakicetus, Kutchicetus, Dorudon, Basilosaurus and others in order and you can almost watch the change happening – the nostrils fuse together and move back over the head to form a blowhole, the teeth become more simple and peg-like, the snout elongates, the neck shrinks, the back becomes more flexible to swim better, the hands become more flipper-like, the legs and pelvis reduce and disappear, the bones become more dense.

Facebook Twitter Pinterest The extinct Dorudon. Although some modern whales retain the vestiges of a pelvis, in this ancient form there are still tiny legs present. Compare this to the modern form in the background which also has more flipper-like hands. Photograph: /flickr

Exactly the kinds of transition demanded – one ”kind” of animal (an ancient antelope-like ungulate) has turned into another (a whale). It’s all there and well documented with numerous fossil finds, anatomical characters quite clearly changing, the fossils independently dated to the order in which they would be expected to appear, family trees showing the changes occurred in sequence (and match the ages of the fossils too) and more.

I specialise in dinosaurs, though, and the transformation of non-avian theropod dinosaurs into birds is arguably even better, not least given just how long ago it played out – some 150m years ago. Archaeopteryx, first named in 1861, is still regarded in scientific circles as essentially the earliest bird in the fossil record and this alone can be used to make a grand case as a transitional form. Like many non-avian dinosaurs and unlike any living bird it has a long bony tail, large claws on three fingers of the hand, and teeth in the jaws, and yet it has that most avian of features, large, pinnate feathers, in neat rows, across the arms to form wings. The details of the feathers too are close to those of modern birds, they have a large shaft with small side branches coming off (and branches off of them) and they are also asymmetric (the feathers are larger one side of the shaft than the other).

When discovered it was a sensation, an animal with very obviously reptilian and avian traits – a huge and problematic gap was apparently filled and the birds no longer stood quite so alone compared to other major vertebrate lineages as they had before. Anatomists were quick to identify other features that were held in common with the carnivorous theropods dinosaurs (the shape of the head and structure of the hand and foot in particular) even though the species known at the time were mostly huge hulking beasts like Allosaurus with proportionally small arms and giant heads. However, add 150 years of scientific discoveries and research and the gaps between say Allosaurus or Tyrannosaurus and the Archaeopteryx have shrunk enormously.

The fundamental stance of theropods as upright bipeds is shared by few vertebrates but is common among birds, and actually both have a highly specialised ankle joint not seen in other groups. Look at the groups closest to dinosaurs and one can see a progression of the pubis moving from a forward-pointing position (as in tyrannosaurs) in the early forms to one that points near vertically (in oviraptorosaurs) and eventually backwards (see the dromaeosaurs) as in birds. The shoulder also reconfigures with a change in the orientation of the joint seen in non-avian theropods. And of course there are now numerous smaller theropods known, it’s not just multi-tonne giants but there are a plethora of much smaller animals and indeed they get smaller closer to the origin of birds.

Part of the tail of the tyrannosaur Yutyrannus showing thin brown and black feather filaments. Photograph: Xu Xing

And let’s not forget feathers. For over a century, Archaeopteryx stood alone as the only dinosaur known with feathers, but since the 1990s, dozens of species have been discovered preserving them from fossil beds in China, Brazil, Germany and Canada. Not only that, but as with so many of these features, one can see the feathers changing in size and structure over time in a clear and consistent pattern.



In the first lineages, these are short and simple fibres, as seen in animals like Sciurumimus and Yutyrannus much like the feathers seen in baby birds (indeed, in both Similicaudipteryx and Ornithomimus, we see different types of feathers in juveniles and adults, just as in modern birds), then later on we see larger and longer feathers appearing in taxa like Beipiaosaurus, before we see those with a full shaft in animals like Caudipteryx, and then still larger and longer feathers in the very bird-like Anchiornis, and even asymmetric feathers just as in Archaeopteryx in the gliding dinosaur Microraptor. Transition from one condition to another, through small additions or modifications, over time, building up into major changes, documented at each step.

I’m throwing the names out here, not to confuse, but because it shows just how many features can be seen on how many species. Obviously I’m not tracing every change on every lineage, but some are wonderfully clear. Look at wrist flexion, for example. Modern birds have permanently flexed wrists whereas early theropods had straight and almost inflexible ones, but plot this out on a family tree and there’s a progressive and clear trend towards flexion as we move towards birds. The same thing is true of arm length, each successive group having longer and longer arms, and moving away from the stubby ones of basal theropods and the huge and elongate forms of those animals closest to birds.

Not only that, but apparently ”avian” features not originally thought to be present in earlier dinosaurs are now known in numerous taxa. The paravian group, those closest to birds, have both enlarged sterna and also unusual features of the ribs called uncinate processes. Quill knobs on the arms that help support the feathers have been found in Velociraptor. The pneumatic skeleton of birds is clearly present in numerous theropods (and indeed other dinosaurs too) and can also be seen to develop over time, getting larger and more complex as we get closer to the origin of birds.

Facebook Twitter Pinterest The wonderful Berlin specimen of Archaeopteryx, replete with teeth, a bony tail, large claws on the hands and a rather dinosaurian foot, and yet complete with feathers almost indistinguishable from those of modern birds. Photograph: Luis Chiappe/AP

This takes us up to Archaeopteryx, but actually this animal is still quite different to modern birds. However, we can also go the other way. There are plenty of fossil birds out there and we can also bridge the gap between Archaeopteryx and those. We can see the teeth reduce and disappear in a number of lineages and a beak form, that long bony tail reduces to a stub and fuses together to form the classic “parson’s nose” of the modern bird, the bones of the foot fuse to form the avian tarsometatarsus, the back of this expands to produce a hypotarsus (an important attachment for muscles). The pelvis continues to reshape and expand to take up a huge portion of the backbone, the fibula reduces on the leg, the hand fuses, the claws and fingers reduce on the hand, the number of bones in the neck increases, and the sternum adds a keel. One can trace these features across numerous lineages of birds from multiple species. Look at the lost tail in Confuscisornis, the keel in euornithies, or the increased wrist flexion and gripping feet of things like Liaxiornis.

In short, we can run through dozens of species, from the first dinosaurs (or indeed even their ancestors and close relatives like crocodilians and pterosaurs) right the way through the theropod lineage up to Archaeopteryx and then beyond and through to modern birds. Perhaps tens of thousands of specimens (there are dozens if not hundreds of specimens of each of the early birds Confuscisornis and Gansus and perhaps a thousand or more of the paravian Anchiornis) representing hundreds of species can be seen, and their individual features have been examined and placed in context. The transition of individual characters like feather shape, or wrist flexion can, and have been, analysed and found to show a clear and simple progression of increasing magnitude and/or complexity over time. They have evolved, and over time these small changes have accumulated into huge changes and represent the transformation of a group of mostly large, Earth-bound carnivores into a plethora of flying birds.

Facebook Twitter Pinterest A largely complete fossil skeleton of Gansus. This animal shows a pelvis a lot like those of modern birds and the wrists are more strongly flexed than earlier birds and dinosaurs as well as having much reduced claws. Photograph: Hai-Lu You/AP

Even this over-simplified summary barely covers a fraction of the papers and scientific works that have been carried out in this field. I’ve read several books on the subject, I must have hundreds of papers, and have attended conferences, spoken to researchers, studied fossils, collected data, run analyses (both new, and to confirm previous findings) and written papers on the subject, and to be honest it’s really not even my area of speciality. Plenty of researchers have spent far more time and effort on this incredible transition and documented more aspects in more detail than I can recount easily. The sheer volume of work is phenomenal with literally hundreds of papers alone dedicated to the origins and evolution of feathers in dinosaurs.

I could go still further into this as to the studies of preservation of the bones and soft tissues like feathers, the various methods to date fossils and create family trees (phylogenies) with which we can test ideas, the studies showing that actually the two combine beautifully (the order in which we predict species evolved through the phylogenies matches their age in the fossil record), analyses of flight, the origins and changes to muscles as legs and arms change, breathing patterns and physiology, eggshell structure, posture, nest-building behaviours and more. All have been put under scrutiny. There are single papers on things like the wishbone (furcula) or lengths of the arms.



A friend of mine once ended up at some creationist event and when the audience was told there were no transitional fossils, he ventured to ask about Archaeopteryx. The speaker had not even heard of it. If one is ignorant of one of the most famous fossils of all time, one that has been around for a century and a half and even at the time of its discovery was heralded as a specimen that helped reveal the origins of birds, one can hardly expect creationists to be familiar with the dozens of other species and thousands of pages of research that track the origins of birds from their non-avian relatives. It does however demonstrate the stark contrast between the claim and the evidence, and the lack of understanding and even the most basic reading into a subject about which major and dramatic statements are being made.

I hardly expect to have convinced any of those reading who are of the persuasion that the fossil record does not cover such transitions. Still, even those interested in the sciences can struggle to find information on these kinds of things and follow the rapid developments that can appear, with papers on some subjects appearing almost weekly. Science of course is ever progressing and palaeontology has a rather shorter history than many illustrious fields, so there is perhaps some catching up to do, though to put it another way, there is still so much more to find out and great new discoveries and confirmations await.



One thing is quite clear though, as it was even in Darwin’s day: the fossil record is quite simply heaving with transitions, and any statement to the contrary is clearly, and indeed demonstrably, incorrect.