Many moons ago, while studying for my BSc in Zoology at Anglia Ruskin University, I wrote an essay about whether or not dinosaurs had really gone extinct or not. There are multiple lines of evidence which clearly show that non-avian dinosaurs unfortunately kicked the bucket during the K-T extinction event, but the avian dinosaurs (or birds) survived. I recently came across this essay while recovering some old files and I thought I’d share it with you all. Given that I wrote this over a decade ago, things may be bit out of date, but I hope that it will show how my writing style has developed since then. I also hope you’ll enjoy reading it as rediscovering this essay took me back down memory lane. Thanks to one of my Patrons (James Pascoe) for encouraging me to share this with you all.

Introduction

Most people believe that the last of the dinosaurs went extinct a little after clutching on to survival from the huge asteroid impact, which decimated the planet and changed it forever some 65 million years ago (Steeham et al., 1991). What most people do not know, is that these animals have direct descendants living among us. We see these inquisitive animals in all of their forms every day. This group of animals which have evolved over millions of years from their feathered Theropod dinosaur cousins, are the birds (Milner, 2007).

It is important to say that the group of animals we call dinosaurs, was very varied with around 1000 extinct non-avian species now identified. It is divided into two main groups with the Ornithischia (‘bird-hipped’) dinosaurs on one side and the Saurischia (‘lizard-hipped’) dinosaurs on the other (Organ et al., 2007). Most of the dinosaurs that people think of when you ask them about dinosaurs come from a branch on the Saurischia side called the theropods. This includes all of the big meat-eating dinosaurs such as Tyrannosaurs, Allosaurs, Raptors and Spinosaurs as well as many smaller dinosaurs (Sereno, 1999). This family group is very important as many palaeontologists now believe that birds evolved from the theropods during the Mesozoic Era (Qiang et al., 1998; Organ et al., 2007).

People may look at a chicken and compare it to the image of a Tyrannosaurus rex from the Jurassic Park movies and say they look nothing alike, but the similarities are more than just skin deep and most can’t be seen with a side-by-side comparison. This is partly due to the inaccuracy of the portrayal of the dinosaurs due to our ever changing understanding and also because some people refuse to believe the theory. New discoveries are being made all the time which help show that birds and dinosaurs are even more closely related than we could ever imagine (Zhou, 2004).

Relatives of the dinosaurs, the pterosaurs had already taken to the skies during the Late-Triassic to Early Jurassic (Dyke et al., 2009). Avian dinosaurs and pterosaurs evolved side-by-side through convergent evolution. Birds can be found in Jurassic and Cretaceous rocks (Gregory, 1952). Some see this as evidence that birds did not evolve from dinosaurs, I see it as a more ‘primitive’ being coexisting with a more advanced form, as we see in the natural world everywhere today.

Biological Similarities

Dinosaurs, like all reptiles laid amniotic eggs (Varricchio et al., 1997). This ability to lay a waterproof egg is also seen in birds which, hints at a link between the two. Someone could argue that it is a coincidence as some mammals which have ancient ancestry such as the Echidna and Platypus also lay eggs (Penny & Hasegawa, 1997). This may be the case but there are many more factors which have helped scientists deduce that birds are in fact, dinosaurs.

The bones of theropod dinosaurs are similar in structure to those of modern birds (Rensberger & Watabe, 2000). A femur specimen of Tyrannosaurus rex showed that it had a structure similar to that of medullary bone found in pregnant birds (Schweitzer et al., 2005). If you took the head of a bird and put it next to one from a theropod dinosaur such as a raptor from the Late Cretaceous, you would find that they are not very similar in shape but they are similar in construction.  The dinosaurs have a bony mandible with sharp pointed teeth whereas birds have a beak which contains no teeth. This could be a strong argument against whether or not birds are dinosaurs, if it wasn’t for one fact; rarely and artificially, chickens have the ability to grow teeth (Harris et al., 2006).

Recently a line of new discoveries has been made which shows that theropod dinosaurs such as the Tyrannosaurs and Velociraptor could have been covered in a layer of downy protofeathers. These would have covered most of their bodies with longer pennaceous feathers on their forearms (Appenzeller, 1999; Xu et al., 2004). With feathers being present on some dinosaur species and modern day birds could be a link proving that birds evolved from dinosaurs due to the fact that birds are the only modern day animals with feathers (McKellar et al., 2011). Scales and feathers are made out of the same biological material, keratin.

Soft tissues have been preserved in fossils such as Scipionyx, have been preserved with soft tissue intact (Sasson & Signore, 1998). The vessels found in fossilised Tyrannosaurus rex bone (after treatment) were almost identical to those found in an extant Ostrich when using Scanning Electron Microscopy (SEM) (Schweitzer et al., 2007). Sequencing of proteins from well preserved Tyrannosaurus rex bones show that the peptide chains are 58% in common with a chicken (Asara et al., 2007). There is probably more in common but due to the age of the fossil, only a few chains that could be retrieved.

Biological Differences

Casts taken from exceptionally well preserved dinosaur skulls (and those taken digitally), such as those of Allosaurus, show that it’s brain is similar to that of modern crocodilians (Larsson et al., 2000). In contrasts, Archaeopteryx’s brain is much more bird like and comparisons show that it may have had the same level of social complexity as modern chickens (Burish et al., 2004).

Dinosaurs such as Tyrannosaurus rex had olfactory bulbs which were greatly enlarged (Brochu, 2000). This enlargement gave then a superb sense of smell. Most modern bird species have reduced olfactory bulbs as the sense of smell isn’t that important to them (Cobb, 1968).

 Physical Similarities

In the animal kingdom there is a bird called the Hoatzin (Opisthocomus hoazin) that inhabits the swamps and mangrove swamps of the Amazon Rainforest. This bird has some unusual features which may help us understand how the early ancestors of dinosaurs lived. The birds have had a lot of doubt cast over their current taxonomic position due to their unusual characteristics such as a foregut fermentation, which might have been used by their ancestors (Grajal et al., 1989). In this argument, it is the chicks which help answer some questions about the links between dinosaurs and modern day birds. The chicks of the Hoatzin have a feature which may point back to a dinosaurian ancestry. The attribute in question is the claws on the end of the chick’s first and second digits which they use to climb up trees after diving into a river below its nest to evade predators (Hughes & Baker, 1999).

The chicks only express these claws whilst they are developing their wing muscles, which help them to evade predators during this vulnerable development stage. These residual claws may be a form of atavism which could be a link them and late dinosaurs such as Archaeopteryx, which had similar features.

More birds which show similar ties to their ancient ancestors are the Ratites. Birds like the Common Ostrich (Struthio camelus), Southern Cassowary (Casuarius casuarius), Emu (Dromaius novaehollandiae) and Darwin’s Rhea (Rhea pennata) which have scaly, bald legs. They also have a three-toed foot (with the exception of the Ostrich which only has a two-toed foot), like theropod dinosaurs. These flightless birds also have no keel bone much like dinosaurs, as well as having sharp claws on their feet. Discoveries of exceptionally preserved fossils in formations such as the Solnhofen Limestone in Germany and formations in China (Xu et al., 2012) have been helping discover more about feathered dinosaurs.

Physical differences

There are a number of differences between modern birds and dinosaurs; this is to be expected after over 65 million years of evolution. One characteristic which has been lost is the bony three-clawed hands which were common place amongst the theropod dinosaurs, with the exception of a few species (such as T. rex who had two-fingered hands). The loss of hands was a consequence of evolving wings. Birds such as the chicken still possess the three digits from the hands they once had but they have been highly modified for flight.

Dinosaurs were also a lot larger than their modern descendants. An example of a downsized coelurosaurian is Microraptor, it was smaller than Archaeopteryx and had more bird-like teeth than Archaeopteryx. The decrease in body size in non-avian dinosaurs can be seen as an important factor in the origin flight in birds (Xu et al., 2000).

Powered flight is one characteristic birds do not share with their ancestors. The origin of flight is a mysterious one that can’t just be figured out through only looking at the fossil record. Many people have had different theories on how flight evolved. The main theory accepted today is that flight evolved from arboreal gliding dinosaurs. Recent discoveries such as those like Microraptor gui (Xu et al., 2003), help to reinforce this theory.. Microraptor had pennaceous feathers on all four of its limbs which could combine to make a perfect aerofoil (Xu et al., 2003).

Dinosaurs also possessed a much longer bony tail than modern birds do today. Another feature which dinosaurs shared with each other but not modern birds is teeth. The loss of a body jaw with teeth and evolution of a lightweight beak may have been an adaptation that developed in the goal for flight. The loss of the long bony tail may also have been an adaptation for powered flight.

Behavioural Similarities

It is important when trying to figure out how closely related a group of animals are to another to not only look at physical and anatomical aspects but also at behavioural characteristics. Some species of dinosaurs, like birds made nests and evidence has shown that dinosaurs may have cared for their young (Varrichio et al., 1997). Animals which are related to dinosaurs such as crocodilians and birds show extensive parental control towards their young. These parental instincts may have been present in the dinosaurs, this is not known for sure but it is likely (Meng et al., 2004). Crocodiles and birds are some of the best parents in the Animal Kingdom, both groups are related to dinosaurs, the crocodiles indirectly through the archosaurs and birds directly through evolution. Some dinosaurs even exhibit brooding behaviour, over their nests (Norell, 1995).

Some birds such as the birds of paradise use their extravagant array of feathers to attract a female. A recently discovered dinosaur called Epidexipteryx hiu, shows signs of primitive ornamentation. The four elongate tail feathers may have been used in courtship displays (Zhang et al., 2008).

Some dinosaurs such as Deinonychus hunted in packs to help bring down larger prey (Marwell & Ostrom, 1995). This cooperation between the small-large numbers of carnivorous dinosaurs shows a certain level of social complexity. Many modern day animals such as birds and crocodiles swallow stones to aid them in digestion. These are called gastroliths and there is evidence to suggest that the cousins of crocodiles and birds, the dinosaurs also swallowed stones for the same purpose (Stokes, 1987).

Behavioural Differences

Due to the fact that dinosaurs have been gone so long, we have to use evidence found in rocks to see how they behaved. We can also use some of their closest living descendants as an analogue to gauge how they used to behave (Vergne et al., 2009). It is hard to say how different birds are from dinosaurs, as each new discovery opens our eyes wider to a world of complex organisms, not just some huge slimy, dumb monsters (Norrell, 2011). It is possible that dinosaurs may have used sound to communicate, but they would not have used songs like birds do today, as they lack the ability to do so.

Conclusion

When faced with all of the evidence, it is hard to suggest otherwise that birds are in fact dinosaurs. The fossil record may not paint a perfect picture of the transition between the two, but with modern scientific techniques, new discoveries and the work of eccentrics, the picture is becoming clearer and clearer. Just like us, birds share quite a few features with their long lost ancestors. 65 million years ago, mammals were little rat-like creatures running around the feet of dinosaurs. Like birds, we do not resemble our ancestors visually, but we do anatomically and biologically. Since the discovery of Archaeopteryx, the transitional evolutionary history from dinosaurs to birds has been going from strength to strength.

Modern techniques have helped us show once and for all, that the dinosaurs did not go extinct 65 million years ago but have continued to flourish. Birds have been classified as dinosaurs through cladistics due to all of the scientific evidence. Birds may not come to most people’s mind when you say the word dinosaur, but they are in fact avian dinosaurs. So dinosaurs are still with us, they may be less dominant than they once where but they are more varied and common than they have ever been.

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