Children love dinosaurs, of which the most revered is Tyrannosaurus rex. This is for good reason. With its gigantic gait and bone-crushing teeth, T. rex invokes a sense of fear and awe. As the king of dinosaurs, it was an apex predator — the largest strictly meat-eating land animal that is known to have ever existed. T. rex roamed across North America for over two million years until a huge asteroid hit the earth 66 million years ago.
I must confess that like other children, I went through an extended dinosaur phase in my childhood. Back then, T. rex was depicted as a dull-coloured dinosaur with lizard-like scales. At the time, this image of dinosaurs was also etched into the public psyche by Hollywood movies such as Jurassic Park. However, scientific evidence accumulated over the past few years indicates much of what we knew back then might be incorrect.
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All tyrannosaurs, including T. rex, belong to a class of two-legged meat-eating dinosaurs known as theropods. From some remarkable fossils found in China over the past three decades, we know much more about theropods. These exquisite fossils have details of soft tissue, and so they provide a unique window in dinosaur life and evolution. Quite stunningly, we have strong evidence that theropods had feathers.
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In a landmark article published in Nature in 2004, Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and his colleagues described an early tyrannosaur the size of a dog that lived in the early Cretaceous period nearly 130 million years. They named it Dilong paradoxus, and noted that it had fluffy primitive feathers called protofeathers.
Eight years later, in another blockbuster article in Nature, Xing and his team highlighted the discovery of fossilised remains of a tyrannosaur, which they named Yutyrannus huali. It was an incredible discovery of a gigantic dinosaur with feathers — one that must rank as one of the greatest scientific discoveries of the previous decade.
As a class of dinosaurs, tyrannosaurs weren’t always behemoths. Paleontologists now believe tyrannosaurs started small, but over millions of years gave rise to fearsome predators such as T. rex.
T. rex became extinct at the end of the Cretaceous. But when someone says all dinosaurs are extinct, it rankles me. You see, not all dinosaurs are dead. As I type, flying dinosaurs known as birds chirp away outside my window.
Yes, modern birds are descended from theropods. And not just that; they are theropods themselves, in the same way that we are apes. I urge you to remember that the feathered chicken is a distant relative of the mighty T. rex.
That said, extinct theropods such as tyrannosaurs didn’t use their feathers for flying. Their downy feathers were likely for show or insulation. But early feathers of theropods did give way to the more complex ones that birds use today.
Since then, scientists have made discoveries that indicate that feathers, hair, and scales all originated in a single ancestral reptile that lived around 300 million years ago.
Birds tend to mature quickly. Previously, paleontologists that studied tyrannosaurs thought they grew slowly and lived long. The consensus now seems to be that even in the manner of growth, they were like modern birds. Tyrannosaurs matured within three decades. As Steve Brusatte writes in his excellent book, The Rise and Fall of the Dinosaurs, T. rex was “the James Dean of dinosaurs: it lived fast and died young.” At the peak of its growth, a juvenile probably put on over two kilograms of weight a day.
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Researchers have also been looking at modern birds to guess what colour the feathers of dinosaurs might have been. Feathers of birds (and other theropods) contain coloured sacs called melanosomes. By comparing the kinds of melanosomes in living birds with those in fossil specimens of an extinct theropod called Sinosauropteryx, they concluded in an article in Nature that it was covered in reddish-brown feathers. Who said dinosaurs had to be dull and grey?
We know about dinosaurs from their fossils. But can we extrapolate from fossils just how many T. rex ever existed?
Charles Marshall at University of California at Berkeley and his team attempted to answer this question in an article published in Science on April 16. They used the concept that there are few very large modern animals compared to small ones. Marshall and his team plugged in numbers for the likely size and weight of T. rex and came up with a range that varied from around 1,300 to 328,000 adults in North America at one time (with a likely number being 20,000). That’s around 3,800 in an area the size of California.
How does the number of T. rex relate to fossils we have found? If every plant or animal that ever lived had been fossilised, the earth would be covered in mountains of fossils. Clearly, that’s not the case. But just how rare are they?
Since T. rex were on the planet for about two million years, that gives us an estimate of around 2.5 billion of them in total. Given that we have found only a few dozen complete fossilised skeletons of this predator, we can confidently say that T. rex fossils are exceedingly rare.
Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction.
This is the first of his weekly column, to be published every Wednesday, on science, placing scientific discoveries and news in context to give it meaning and understand the world around us
These are his personal views.
