The dinosaur in your back garden (and the one in your fridge)

The chicken in the cold cabinet at your supermarket is, on the most defensible molecular evidence we have, the closest thing to Tyrannosaurus rex you are going to interact with this week.

This is a sentence that wants explaining and I will get to it. But I want you to hold the chicken in mind while we go through the dramatic species, because the dramatic species are the wrong answer to the question this article is named after.

The question is “which birds look like dinosaurs.” The reflex answer is the cassowary, the shoebill, the hoatzin, the ostrich, the cinereous vulture, all the species with horns or casques or daggers or three-metre wingspans. The reflex answer is wrong, or at least incomplete. The reflex answer confuses looks like a dinosaur with is a dinosaur, and on the second question the answer is: all of them. Every bird alive. The starling at your bus stop. The pigeon on your bin. The pekin duck on the takeaway menu. The bird in the cold cabinet.

Birds are dinosaurs. Not in the loose pop-science sense. In the formal taxonomic sense that working palaeontologists use without irony. Birds are the only surviving lineage of theropod dinosaurs, the branch that produced Velociraptor and T. rex. The cladistic case for classifying a House Sparrow as a small feathered theropod is as solid as the case for classifying a wolf as a kind of dog. The mass-extinction event 66 million years ago killed everything in the dinosaur family tree except the small feathered branch that learned, in the late Jurassic, how to fly. That branch survived. That branch is what we now call birds.

Most birds, having survived, have spent the intervening time slimming down, modernising, evolving into the small efficient seed-eating animals that fill modern ecosystems. They do not look like dinosaurs because they have been actively unlooking like dinosaurs for sixty million years. The handful that still look the part are the ones who, for one reason or another, kept the old features instead of replacing them.

The hoatzin is the clearest case. Hatchling hoatzins, in the Amazon basin, are born with functional claws on their wings. They use them to climb out of nests over backwaters before they can fly, escaping snakes and falling into the water if they have to. They lose the claws as adults. The trait is one that Archaeopteryx, the Late Jurassic transitional dinosaur, had. No other living bird does. The hoatzin is therefore the closest living bird, structurally, to the moment when birds and dinosaurs were the same animal. He is also, because of a peculiar rumen-style hindgut, the bird that smells most like a cow. The smell is part of the package. He has decided to keep a great deal of the original.

The Southern Cassowary is the closest a modern bird gets to the visual stereotype. He stands up to 1.8 metres, weighs up to around 60 kilograms or more in large females, runs at speeds widely reported at around 50 kilometres an hour, has a bony helmet casque whose function is still being argued about (thermal regulation, head-shielding during forest crashes, sexual selection - the candidates have been on the same shortlist for thirty years), and carries on each foot a single dagger claw long enough and sharp enough to disembowel a person. He has killed at least one human in the modern record, a Florida breeder named Marvin Hajos, who died in April 2019 of injuries sustained when his own pet cassowary attacked him at his farm in Alachua County, Florida. The bird is, by any reasonable measure, what most people think a small theropod ought to look like. Find him in Queensland or New Guinea, do not approach.

The shoebill is the second clearest case to the eye, and the shoebill is, on closer inspection, a stork. Balaeniceps rex, the king of bills. He stands up to 1.4 metres in a Sudanese swamp, holds completely still for hours, and then closes a bill that can reach around 24 centimetres - Wikipedia’s shoebill article gives the range as 18.8 to 24 cm - on a lungfish faster than the human eye registers. He bows at handlers in zoos, a slow nodding display whose purpose remains unclear. He is the bird whose existence is hard to believe even from photographs. The conservation status is Vulnerable. The IUCN estimates the global population at between 5,000 and 8,000 individuals, with numbers in decline.

The Secretary Bird is the third case I find compelling. A 1.3-metre raptor on crane legs, walking the African savannas hunting snakes by stomping them to death. The kick force, measured in 2016 by Portugal and colleagues at the Royal Veterinary College in London, is around 195 newtons - five times the bird’s body weight - delivered in a strike duration of 15 milliseconds. The kick is faster than a human can blink. The bird does not look the way most people imagine a dinosaur, but in motion, hunting, it is one of the more credible theropod-mode performances any living animal puts on.

I could continue: harpy eagles whose talons are longer than a grizzly bear’s claws and who lift sloths from canopies; sandhill cranes whose oldest known fossil - recovered from the Macasphalt Shell Pit in Florida - is 2.5 million years old and structurally identical to the birds alive today and who bugle a call across a Nebraska river in March that has been delivered in that valley since long before our species had language; the Andean Condor with a wingspan that can reach 3.3 metres, broadly recognised as the largest soaring landbird on the planet by combined weight and wingspan. These are the dramatic dinosaurs. They are the photographs.

Now back to the chicken.

In 2007, a research team led by John Asara at Beth Israel Deaconess Medical Center in Boston extracted protein fragments from a 68-million-year-old Tyrannosaurus rex femur recovered from the Hell Creek formation in Montana. The collagen sequences in the T. rex protein, compared against modern animals, were closer to chicken collagen than to anything else they could match against. A 2008 follow-up paper extended the comparison and got the same result. The chicken’s protein sequences were the closest modern relatives of the tyrannosaur’s. Mass spectrometry, not phylogenetic guesswork.

This was not, exactly, a surprise. Phylogenetic trees built from modern bird DNA already placed chickens and other Galliformes deeper toward the base of the modern bird tree than the more obvious theropod-looking birds like ostriches and emus. The Galliformes lineage shares an internal node with the songbirds that everyone agrees are dinosaurs. The ostrich and emu lineage, the so-called ratites, split off earlier.

The result of this branching is that an ostrich, who looks more like a dinosaur than almost any bird, is actually further from T. rex than a chicken is. The chicken, who has been selectively bred for thousands of years - estimates range from around 3,500 to 8,000 years depending on the method used - into a feathered drumstick farm, is closer.

This is the genuinely interesting thing in the dinosaur question. Looks lie. The bird who lost almost everything to selection for production is the bird who kept the deepest line back to the original. The dramatic species are dramatic because they kept the obvious traits, the casques and claws and crests. The supermarket chicken is a dinosaur because of what is in her bones.

What this changes for a casual person looking at birds is small and quite large at the same time. The small change is that the next time you see a Wild Turkey strutting in late autumn, his head bare, his wattles pulsing, his iridescent plumage caught in low sun, you can recognise that you are looking at the closest thing in your local ecosystem to a small theropod. The large change is that the same recognition applies to the sparrow at the bus stop. It is also true of the gull. It is also true of the chicken on your plate. The dinosaurs did not die. They got smaller. They learned how to fly. They started eating seeds. They came to live in the trees outside your kitchen. They are still here.

The cassowary is the headliner because he never bothered to slim down. The chicken is the lead, because she did, and then we bred most of the rest of her away, and what is left is still, unmistakably, a small efficient dinosaur in a freezer bag.