Around 66 million years ago, a giant asteroid smashed into the Earth, marking the beginning of a mass extinction event during which three quarters of the plant and animal species on our planet are thought to have been wiped out.
Among the animals that disappeared during the extinction were non-avian dinosaurs, pterosaurs, ammonites, as well as many birds and mammals.
But what were some of the last dinosaurs living around the time when the asteroid struck?
The last Age of the Dinosaurs is known as the Maastrichtian, which spanned around six million years and ended with the catastrophic impact. This age was the final part of the Cretaceous Period and broader Mesozoic era.
Scientists know a lot about the Maastrichtian as a whole, but only a few sites around the world preserve the very end of this period on land, skewing our knowledge of the dinosaurs that lived at this time.
"That's why we mostly hear about the last dinosaurs that were alive in North America, because some of the best and only rocks we've discovered for this moment in time are in Montana and the Dakotas," Ashley Poust, a paleontologist from the San Diego Natural History Museum, told Newsweek.
"That's only a very small bit of what was likely a huge, unknown diversity of 'last dinosaurs' across the globe," he said. "Dinosaurs in some places may have been stressed by gigantic volcanic eruptions in what is now India, but we have every reason to believe that dinosaurs and their ecosystems were functional right up until the end, so we'd expect many cool dinosaurs around the world living in all kinds of habitats."
The time just prior to the mass extinction was a period when many of the most famous dinosaur species roamed the Earth. When it comes to the dinosaurs living at the end of the Maastrichtian, among the ones we know best from rocks in North America include the iconic Triceratops and Tyrannosaurus rex.
Triceratops was a hefty, three-horned, herbivorous dinosaur that walked on four legs with a parrot-like beak and a large frill, which could measure around three feet across.
"Triceratops was one of the bulkiest of the horned dinosaurs, being similar in size to a modern elephant, and could be recognized by its distinctive three horns and the extensive bony frill forming the back of its skull," Thomas Cullen, a postdoctoral fellow at Canada's Carleton University and research associate at the Field Museum of Natural History, told Newsweek.
"While Triceratops was not the largest herbivore to ever walk the Earth—that honor would go to the long-necked sauropod dinosaurs—it was a very large animal and no doubt would have been a formidable challenge for potential predators."
Tyrannosaurus rex, which co-existed with Triceratops, meanwhile is one of the largest predators known to science.
"Tyrannosaurus rex was one of the largest bipedal animals to ever walk the Earth, measuring over 12 meters [40 feet] in length as an adult, with a large, heavy skull, reduced forelimbs, and a relatively long counterweight of a tail," Cullen said. "It would have been the top predator of its time, and has been estimated to have had one of the strongest bite forces known among any terrestrial carnivore."
Triceratops and T. rex are among several other dinosaurs that have been found in the Hell Creek Formation—a rock unit stretching across swatches of Montana and the Dakotas that preserves the best evidence of the age just before the asteroid struck, according to Matthew Lamanna, a palaeontologist from the Carnegie Museum of Natural History.
Among other dinosaurs documented in the Hell Creek Formation include the "duck-billed" herbivore Edmontosaurus, the heavily armored plant-eater Ankylosaurus, the thick-skulled plant-eater Pachycephalosaurus, the "raptor" Acheroraptor, and the bizarre, bird-like "Chicken from Hell" Anzu, which was described by Lamanna and colleagues back in 2014.
"These would've been some of the very last non-avian dinosaurs on Earth; most or all were probably in existence when that giant asteroid fell out of the sky 66 million years ago and caused massive environmental disturbances," Lamanna said.
"The ancient environment of the Hell Creek Formation would've been a coastal lowland adjacent to a gradually draining inland sea. In terms of modern analogues, it's often been likened to places like the Louisiana bayou—warm, humid, and chock-full of lush, diverse vegetation."
The dinosaurs above were very specific to western North America and don't appear to have lived anywhere else.
"Dinosaur fossil-bearing rocks from the very end of the Mesozoic are exceedingly rare on other continents, but it's clear that different dinosaur faunas were living on other landmasses when the asteroid hit," Lamanna said.
"For instance, marine rocks that date to the latest Mesozoic in Morocco have yielded fragmentary fossils that indicate the presence of a short-faced, tiny-armed, sometimes horned meat-eating dino group called abelisaurids that very probably weren't found in North America or Asia at the time."
Some species of giant, long-necked sauropod dinosaurs known as Titanosaurs were also living at the time of the mass extinction event.
"One of the last ones is Jainosaurus which would have trampled the reddish rocks of India before it even connected with the rest of Asia," Poust said.
More broadly, around the same time of the mass extinction event and shortly before geologically speaking (so several million years prior) was also when large bizarre theropod species such as Deinocheirus and Therizinosaurus could be found in Mongolia, according to Cullen. The predatory theropod Majungasaurus and the long-necked herbivore Rapetosaurus were present in Madagascar, and the relatively small and potentially aquatic Vegavis could be found in Antarctica.
"Vegavis is particularly interesting in being a representative of the only group of dinosaurs to actually survive the mass extinction—the birds," Cullen said. "It has been interpreted as being fairly similar in appearance to modern waterfowl like ducks and geese, and lived in relatively cool climates along coastal areas in the southern polar region, which although cold by Cretaceous standards, was still considerably warmer than Antarctica is today."
So, perhaps it is not truly accurate to speak of the "last" dinosaurs when birds (i.e. avian dinosaurs) are essentially living dinos, with more than 10,000 species present on Earth today.
