It’s now widely accepted amongst paleontologists that birds are living dinosaurs. This idea is actually not nearly as new as it appears, and was thrown around by paleontologists as far back as the late 19th century. It took a long time to catch on though, over 100 years in fact. Yet for the past decade or so at least, the ‘birds are dinosaurs’ theory has been accepted by all but a few fringe dissenters in the world of paleontology, no matter how much of a debate the media often makes it out to be.
Despite this, there’s still a fair bit of confused information misleading people as to why exactly birds are classified as dinosaurs, and how that relationship works. Hopefully this will make whole things a little clearer. It bears repeating that, while paleontologists are about a certain as you can be in this line of work that birds are a type of dinosaur, there’s still some small gaps of uncertainty as to which exact family of nonavian (or non-bird) dinosaurs are the birds’ closest relatives. The line between avian and nonavian dinosaurs is not clear-cut and definite; it’s grey, blurry, and of uncertain position. Many characteristics that seem unique to living birds, such as feathers, air sacs, hollow bones, a furcula (wishbone), long arms with wings, and brooding a nest of young were all gradually acquired from their dinosaurian ancestors or even earlier (Brusatte et al., 2014).
This means that our very definition of what exactly is and isn’t a ‘bird’ is not crystal clear. There’s an entire spectrum of small, feathery dinosaurs throughout the fossil record, and where we draw the line is somewhat arbitrary. Some authors choose a more narrow viewpoint and use the word ‘bird’ to describe all living bird species, their most recent common ancestor, and all of its descendants. This is how the Linnaean class Aves and its synonym Neornithes is defined (Gauthier & de Queiroz, 2001). Others are more inclusive, and use ‘bird’ to encompass all feathered dinosaurs primitively capable of powered flight (the Avialae), which includes a variety of fossil taxa leading all the way back to the late Jurassic. For the purposes of this article, we’ll use the more inclusive definition Avialae for the word ‘bird’, and casually call the Neornithes the ‘crown group’ or ‘modern’ birds (some authors even argue for the inclusion of every primitively feathered archosaur group, or even all those more closely related to birds than crocodiles, as birds. Under these definitions, most if not all dinosaurs and perhaps even pterosaurs would be called birds. We won’t go that far here).
The classification confusion goes even further. If you spend enough time talking to kids (and even some adults) about dinosaurs, you’ll often hear phrases like “the closest relative of T. rex was a chicken” or some variation of this theme repeated (I think kids find it funny that the most fearsome beast of their imaginations is tied to the benign barnyard fowl that often winds up on their dinner table). You’ll often get asked which bird is the closest relative to T. rex or Velociraptor or any other famous dinosaur.
These statements and questions come from a lack of understanding of how biological classification works, but at no fault of the person repeating them whatsoever. The truth is, all living birds, chickens included, are more closely related to each other than any nonavian dinosaur. All modern birds descended from a single common ancestor that probably lived in the Cretaceous Period, and each living species branched off from this shared root more recently than their ancestor did from any nonavian dinosaur. And there are many nonavian dinosaur groups that are more closely related to birds than either group is to Tyrannosaurus.
Birds belong to the lineage of dinosaurs called the theropods- the bipedal, mostly carnivorous, often feathered suborder that contains all the classic villains of dino-themed stories. More specifically, birds are coelurosaurs- a branch of theropods that was likely ancestrally feathered and came in a wide variety of body shapes and niches. Some coelurosaurs were remarkably birdlike in their anatomy and lifestyle, and many have been mistakenly (or, to some authors, properly) included as birds themselves. The tyrannosaurs are in this group, but they went in the direction of becoming large terrestrial carnivores, and it seems that they may have eventually lost their feathers (Bell et al., 2017).
The ostrich-like dinosaur Struthiomimus on display at the Philip J. Currie Dinosaur Museum. Photo by Nicholas Carter
The ornithomomosaurs are also found here, and at first blush most species in this group look exceedingly like big ground birds such as ostriches thanks to their long s-shaped necks, birdlike limbs, and small heads with big eyes and toothless beaks. This resemblance was not lost on paleontologists, who gave them names like Albertan species Ornithomimus (“bird mimic”), Struthiomimus (“ostrich mimic”), and Dromiceiomimus (“emu mimic”). They may have behaved in a similar manner to big ground birds too, subsisting largely on plants and seeds, relying on their speed to avoid predators (Barrett, 2005). Preserved keratinous sheathes (a rhamphotheca as we say in the biz) in some species suggest they might have used their beaks to strain small plants and other organic matter from water like some ducks do today, but this interpretation could be wrong (Norell et al., 2001).
Another of the many feathery coelurosaur groups was the oviraptorosaurs of Asia and North America. Like the ornithomimosaurs, these too were toothless herbivores descended from carnivorous ancestors. Oviraptorosaurs had blunt, parrot-like skulls which with often adorned with big crests like those seen on cassowaries. Amazingly well preserved fossils of the dinosaurs from Mongolia tell us that they likely travelled in flocks and brooded over their eggs just like most birds today do (Clark et al., 1999). A relatively basal oviraptorosaur called Caudipteryx was famously discovered with feather traces surrounding its body, including long vaned feathers on the arms and tail (Ji et al. 1998). Oviraptorosaurs had relatively short tails, and the last handful of tail vertebrae were fused into a single unit known as a pygostyle. Birds also have short tails with a pygostyle at the end, and this shared feature has led some paleontologists to toss around the idea that oviraptorosaurs were actually large, secondarily flightless birds (Maryanska et al., 2002). However, most still argue the pygostyle developed in oviraptorosaurs and birds separately as a sort of evolutionary coincidence. There is evidence, however, that this bone anchored large tail feathers in oviraptorosaurs that they may have used for displaying to each other, especially males towards females (Persons et al., 2014).
The last group that contains birds and their nonavian cousins is the Paraves. Alongside birds in this group are the ‘raptors’- the famous dromaeosaurs. This includes species like the small gliding microraptorines which did so much to enhance our knowledge of feathered dinosaurs. There’s also the big-eyed, big-brained troodontids, and possibly another family of bizarre, tiny tree-dwelling dinosaurs with a not so tiny name: the scansoriopterygids. Which of these families is the sister group to the birds is still up for debate, as different authors consider one or the other closer to birds (Godefroit et al., 2013).
Around this point in the theropod family tree, things start to get more than a little fuzzy. Amongst the dromaeosaur, troodontid, and avialan branches are a jumble of small, feathered carnivorous dinosaurs that may be inside or outside the bird group and only blur the line between what taxa are and are not dinosaurs. Examples of these little ‘proto-birds’ are Anchiornis, Serikornis, Caihong, and Aurornis. The best place to find these little theropods is in northeastern China, and slabs of rock bearing squashed skeletons of these animals are still turning up new species to this day, often with immaculately preserved feathers and other soft tissues. Achiornis itself is a remarkable dinosaur- roughly crow-sized, we’ve been able to find colour-producing pigments in its feathers and know what its skin and muscles looked like (Li et al., 2010) (Wang et al., 2017). Some authors have placed these little proto-birds into a family called the Anchiorithidae, though they’re position is uncertain- some have argued they’re close to or within the troodontid family, others say they’re birds and place them in Avialae (Lee & Worthy, 2011) (Xu et al. 2016) (Cau et al., 2017).
The early bird Archaeopteryx contemplates the sunset over the European archipelago. By Nicholas Carter
No discussion of bird origins is complete without mention of Archaeopteryx, and no doubt many readers have wondered when old ‘Arky would come into this. For decades Archaeopteryx was a sort of ‘benchmark of bird-dom’, a hard line where anything more birdlike than it was a bird, and anything less was another nonavian dinosaur. But within the past 20 years or so, a slow but steady stream of little proto-birds have emerged that have somewhat knocked the Urvogel off its lofty perch. While Archaeopteryx might not be the shining example of the ‘first bird’ anymore, it’s still invaluably important to the history of science in that it got people talking about the dinosaurian origin of birds years before the idea was accepted (Ostrom, 1976). Archaeopteryx is usually still recovered near the base of the avialan tree in many evolutionary studies, hanging out near the anchiornithids (Cau et al., 2017). But don’t imagine it zipping through the trees of a vast Jurassic jungle dodging sauropods and escaping the clutches of Ornitholestes as countless examples of paleoart would have you believe- Archaeopteryx inhabited Europe back when that continent was a series of dry, scrubby islands in the midst of an extremely salty ocean.
Going up the avialan family tree, you’ll encounter all sorts of wacky prehistoric birds, some of which looked surprisingly modern. We’ll touch on just a few here. There’s Confuciusornis from the Early Cretaceous of China which evolved a toothless beak long before the ancestors of the modern neornithines developed a similar trait, though it still had individual fingers and claws. Confuciusornis is known from many wonderfully preserved specimens showing feather impressions and even outlines of soft tissue surrounding the bones (Falk et al., 2016). This species is one of the most easily recognizable Mesozoic birds because some specimens (which might represent males) had two long, ribbon-like tail feathers (Chinsamy et al., 2013). This bird wasn’t at the top of its food chain either, as bones from Confuciusornis bones have been found as fossilized gut contents inside a specimen of the giant compsognathid dinosaur Sinocalliopteryx (Xing et al. 2012).
Birds more skeletally adapted for flight than Confuciusornis and its relatives are split between two major groups: the enantiornithes (“opposite birds”) and euornithes (“true birds”). The enantiornithes were the most common and diverse birds during the Mesozoic, and came in a variety of shapes and lifestyles (Chiappe et al., 2002). Aside from the fact that they had clawed wings and teeth, they would’ve looked remarkably like modern birds in life, and may have been equally as good at flying despite differences in the way their wings worked in the shoulder joint (Walker, 1981).. Many of the best enantiornithean fossils come from China (unsurprisingly), but their remains have been found all over the world. Despite the dominance of these birds during the Cretaceous, the meteorite drove them to extinction just as it did their nonavian brethren. This might be due to the fact that they relied on trees for shelter and live prey for food much more than the first modern birds, and when both fell into short supply at the end of the Cretaceous, the enantiornithes were gone (Larson et al., 2016)(Field et al., 2018).
The euornithes contains all modern birds (neornithes), plus a variety of odds and ends below them. A neat example of a basal euornithean is Patagopteryx, a chicken-sized critter from Cretaceous South America that’s one of the earliest examples of an authentic flightless bird (Alvarenga & Bonaparte, 1992). Some of the first well-known Cretaceous birds from North America are also some of the closest relatives to the modern neornithes. There’s Ichthyornis– a toothed gull-like bird known from Alberta and Saskatchewan down to Texas. Ichthyornis lived here during a time when the Western Interior Seaway had drowned central North America, and may have lived in and around this sea like a prehistoric seabird. It shared this habitat with Hesperornis, another toothed bird that grew to over 6 feet in length. Unlike Ichthyornis, Hesperornis was completely flightless, its wing bones almost totally lost in the course of evolution. It was built more like a giant loon, with a long, low body and powerful feet which it used to propel itself through the water with. Since its body was long and low, and its legs bent in positions unsuitable for walking, Hesperornis was highly unsuited to life on land and likely spent the vast majority of its life swimming (Reynaud, 2006).
Hesperornis surfaces with a catch from the waters of the Western Interior Seaway, disturbing an Ichthyornis. By Nicholas Carter
Finally we come to neornithes- the common ancestor of modern birds and all its descendants. Modern birds are distinguished, among other things, by their completely toothless beaks and mobile tail-fans to assist in flying. Many of these early neoritheans may have looked similar to ducks and grouse- small to medium sized ground-nesters who fed on plants and seeds. Neornithean birds have a relatively sparse fossil record prior to the Paleocene, but some evidence suggest they may have branched off as early as the mid-Cretaceous and underwent a dramatic increase in diversity just after the meteor impact (Lee et al., 2014). Perhaps with their toothed enantiornithine and nonavian dinosaur competitors gone, the early modern birds could suddenly branch out and exploit all the newly opened niches in the post-meteor world. The evolution of neornitheans is determined mostly through DNA, and thanks to their less than stellar fossil record, we don’t know terribly much about the earliest modern birds yet.
We’ll save discussing all the individual branches of the modern bird family tree for the ornithologists out there. There’s a variety of matters we’ve skimmed over that would drag this article past being far too long as it is. We barely touched on the evolution of feathers, a tragedy yes, but a topic that deserves its own article one day. As does the history of the ‘birds as dinosaurs’ theory and all the twists and turns that idea has taken. And then there’s the history of flight, and all the ideas as to how dinosaurs first figured out how to use their feathers to get around. So comment if you’d like more on bird evolution!
It might seem strange to imagine contemporary-looking birds during the Mesozoic, hanging around and interacting with their dinosaurian relatives and all those other prehistoric creatures, but that’s how it was. Birds didn’t just evolve from dinosaurs, they arose within that group of animals and existed as a part of their ancient ecosystem. Hopefully this article has shown how there isn’t a strict divide between birds and dinosaurs. Despite being such colourful animals, they’re all just shades of grey to each other.
By Nicholas Carter
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