Smithsonian.com

Triceratops was one of the last dinosaurs. What would the descendents of this ceratopsid look like if they were alive today? Photo taken at the Smithsonian National Museum of Natural History by the author.

In Slate’s recent poll for 2011′s “Question of the Year,” dinosaurs came in third. “Why are smart people usually ugly?” was the winner. Spoiler: the answer is, “they’re not.” But my favorite Mesozoic archosaurs were respectable runners-up with the question: “Let’s say that a meteor never hits the earth, and dinosaurs continue evolving over all the years human beings have grown into what we are today. What would they be like?”

There is an easy answer for this. Dinosaurs truly did survive the end-Cretaceous extinction and continued to evolve. Birds, the descendants of one lineage of feathered maniraptorans, carry on the dinosaurian legacy. But I imagine this isn’t what the reader who posed the question had in mind. Birds seem categorically different from the collection of impressive, non-avian dinosaurs that roamed the planet prior to 65.5 million years ago. Had Tyrannosaurus, Triceratops and their ilk been given an indefinite stay of execution, what would their descendants look like?

Pondering the form of future dinosaurs has been a long tradition in paleontology. Charles Lyell, one of the 19th century founders of modern geology, thought that the progression of life through time was so closely tied to certain climatic conditions, in turn created by geological changes to the continents, that one day habitats appropriate for prehistoric organisms might reappear. At some future time, Iguanodon, Megalosaurus and others might return to lush, primordial forests created by a replay of Mesozoic conditions.

Early 20th-century paleontologist William Diller Matthew suggested a different path by which dinosaurs might return. If mammals suddenly disappeared, today’s lizards, turtles and crocodiles might evolve into dinosaur-like creatures. Naturalist John Burroughs disagreed. “Does not the evolutionary impulse run its course? Can or will it repeat itself?” he asked, and he pointed out that evolution does not run according to pre-determined pathways. Even if reptiles someday rise to dominance, we would expect the descendants of modern forms to be distinct creatures substantially different than anything that has come before. It is not as if there is some vacant “dinosaur niche” in the evolutionary ether that reptiles will fill as soon as they get the chance.

Of course, paleontologists batted around these ideas before the full catastrophic magnitude of the end-Cretaceous mass extinction was discovered. The more we learn, the more mysterious the disappearance of the non-avian dinosaurs becomes—how could such a widespread, disparate and successful group be driven to extinction in a geologic instant? Dinosaurs showed no sign of slipping into evolutionary irrelevance or becoming outmoded, as was the traditional 20th century belief. They seemed to thrive right until the end.

The shift in our understanding of dinosaur extinction—as well as a refreshed image of dinosaurs as highly active, behaviorally complex, intelligent animals—generated at least two different thought experiments. In 1982, paleontologist Dale Russell collaborated with artist Ron Séguin to create the “Dinosauroid,” a speculative vision of what the small and relatively smart deinonychosaur Troodon might look like had the dinosaur survived the mass extinction and continued to evolve. The result was similar to the Sleestaks on The Land of the Lost, or the big-headed alien archetype that is ubiquitous in science fiction. Since Troodon was a relatively brainy dinosaur, and Russell believed that the human body was the optimal physical manifestation of a highly intelligent creature, he molded the dinosaur into humanoid form. But there’s no reason to think that our bodies represent the best possible conveyance for smart organisms. Crows, for one, are exceptionally smart, tool-using birds that demonstrate that dinosaur descendants evolved a high degree of intelligence in a body quite different from our own. If dinosauroids evolved at all, they would probably look like the raptor-like, feather-covered beings envisioned by artist Nemo Ramjet.

Dougal Dixon considered a similar idea in 1988 in his book The New Dinosaurs, although he entirely canceled the Cretaceous extinction and played with a wider variety of dinosaurs. Published before paleontologists confirmed that many coelurosaurs were covered in feathers, Dixon’s colorful creatures were often coated with fuzz or similar hair-like coatings, and many were cast as counterparts to modern day mammals. The small “Waspeater” was the dinosaur answer to the tamandua, a tree-dwelling anteater, and the tiny “Gestalt” was effectively one of the dome-headed pachycephalosaurs refashioned to be a naked mole rat. A few of Dixon’s dinosaurs maintained the monstrous forms that we adore, though. Dixon’s “Lumber” was effectively a Diplodocus with a short, fleshy trunk—an idea that was actually kicked around and ultimately discarded by paleontologists—and the “Gourmand” was a tyrannosaur that had entirely lost its forelimbs and took the appearance of a giant, two-legged crocodile.

Many of Dixon’s speculative animals suffered from the same problem as Russell’s dinosauroid—they were dinosaurs molded to fit the natural history of creatures we see around us today. It is impossible to say whether such creatures might have ever existed had history took a different course. As Stephen Jay Gould pointed out in Wonderful Life, we cannot go back to some critical moment in evolutionary history and “replay life’s tape” to see how nature might be altered. We can be certain of one thing, though—modern dinosaurs would be significantly different than anything we know from the fossil record.

As John Burroughs rightly pointed out in his argument with William Diller Matthew, evolution does not proceed along a pre-set course. The major patterns of evolution are not predictable. Contrary to once-popular, non-Darwinian evolutionary mechanisms, there are no internal driving forces that cause evolution to repeat itself or force organisms along ladders of progress toward some ideal type or form. Nor is natural selection so demanding that all lineages are constantly being fashioned into a small handful of forms.

The fossil record clearly shows that the big picture of evolution is a fantastically branching bush of diversity and disparity in which chance, contingency and constraint all have significant roles to play. Some lineages will rapidly and drastically change, and others will remain in relative stasis over millions and millions of years. Perhaps some dinosaur lineages, like sauropods, would remain more or less the same, while horned dinosaurs might undergo dramatic changes into something different. After all, 65 million years is about the amount of time that separated Late Triassic dinosaurs like Coelophysis—a small theropod which lived alongside various other wonderful archosaurs before the onset of dinosaur dominance—from Allosaurus, Stegosaurus, Apatosaurus and other titans of the Jurassic. Sixty-five million years is plenty of time for spectacular changes to transpire.

How lineages might change is squarely within the realm of speculation. But we can expect that new dinosaur species would continue to evolve, just as they had been since the Late Triassic. Dinosaur species did not last very long—even the longest-lived species were around for only about two million years or so—and if we are working from the premise that dinosaurs would have survived to the present, we would expect to see an entirely different cast of dinosaur species. Some might look familiar, and others might be entirely alien to us, but all the surviving dinosaurs would be different from their Cretaceous ancestors.

This is why I’ll be watching Pixar’s upcoming dinosaur film with interest. The film fleshes out the premise that I’ve been prattling on about, although, in the animated fantasy, the dinosaurs live alongside humans. (That’s fine for the movies, but, had non-avian dinosaurs actually survived, mammal evolutionary history would have been severely altered. If the end-Cretaceous extinction was canceled, our species would not have evolved to debate the question of what would have happened in alternate timelines.) I hope that Pixar fashions a new dinosaur cast. Tyrannosaurus, Barosaurus, Centrosaurus and Edmontosaurus do not belong in the alternate present. They would have disappeared long ago, ultimately replaced by different genera and species. Even if we can’t know how non-avian dinosaurs changed during the past 65 million years, we should at least recognize that the survivors would have undoubtedly evolved into new species, and new species would have branched off from those, and so on and so on until the present day.

So, to answer Slate’s question, we don’t know what dinosaurs would be like. All we know for sure is that at least one variety of dinosaurs is still here, and that’s a wonderful thing.