[reprint] Mass extinction and the rise of the dinosaurs

Growing quickly helped the earliest dinosaurs and other ancient reptiles flourish in the aftermath of mass extinction

Eoraptor lunensis lived roughly 230 million years ago, at a time when dinosaurs were small and rare.
Jordan Harris courtesy of Kristi Curry Rogers, CC BY-SA

Kristi Curry Rogers, Macalester College

It may be hard to imagine, but once upon a time, dinosaurs didn’t dominate their world. When they first originated, they were just small, two-legged carnivores overshadowed by a diverse array of other reptiles.

How did they come to rule?

My colleagues and I recently studied the fossilized bones of the earliest known dinosaurs and their nondinosaur rivals to compare their growth rates. We wanted to find out whether early dinosaurs were somehow special in the way they grew – and if this may have given them a leg up in their rapidly changing world.

Before dinosaurs – the Great Dying

Life on Earth was flourishing 250 million years ago. Dinosaurs had yet to evolve. Instead, giant amphibians and sail-backed reptiles called therapsids thrived.

But within a blink of geologic time, in a span of about 60,000 years, scientists estimate 95% of all living things went extinct. Known as the Permian extinction or the Great Dying, it is the largest of the five known mass extinction events on Earth.

Most scientists agree this near total die-off was caused by extensive volcanic activity in modern-day Siberia, which covered millions of square miles with lava. The resulting noxious gases and heat combined to push global temperatures dramatically upward, eventually leading to ocean acidification, a loss of oxygen in ocean waters and a profound ecosystem collapse, both on land and in the ocean.

Only a few lucky survivors made it through.

The survivors and their descendants

In the ecological vacuum after the mass extinction event, on the stage of a healing Earth, the ancestors of dinosaurs first evolved – along with the ancestors of today’s frogs, salamanders, lizards, turtles and mammals. It was the dawn of the Triassic Period, which lasted from 252 million years ago to 201 million years ago.

Collectively, the creatures that survived the Great Dying were not particularly remarkable. One animal group, known as Archosauria, started off with relatively small and simple body plans. They were flexible eaters and could live in a wide variety of environmental conditions.

Archosaurs eventually split into two tribes – one group including modern crocodiles and their ancient relatives and a second including modern birds, along with their dinosaur ancestors.

This second group walked on their tiptoes and had big leg muscles. They also had extra connections between their back bones and hip bones that allowed them to move efficiently in their new world.

Instead of directly competing with other archosaurs, it seems this group of dinosaur ancestors exploited different ecological niches – maybe by eating different foods or living in slightly different geographical areas. But early on, the dinosaurlike archosaurs were far less diverse than the crocodile ancestors they lived alongside.

Slowly, the dinosaur lineage continued to evolve. It took tens of millions of years before dinosaurs became abundant enough for their skeletons to show up in the fossil record.

Aerial shot of a barren, weathered and rocky landscape.
The Ischigualasto Provincial Park in San Juan Province, Argentina, where the earliest dinosaur fossils have been discovered.
Kristi Curry Rogers, CC BY-SA

The oldest known dinosaur fossils come from an area in Argentina now called Ischigualasto Provincial Park. Rocks there date back to roughly 230 million years ago.

The Ischigualasto dinosaurs include all three dinosaur groups: the meat-eating theropods, the ancestors of giant sauropods and the plant-eating ornithischians. They include Herrerasaurus, Sanjuansaurus, Eodromaeus, Eoraptor, Chromogisaurus, Panphagia and Pisanosaurus.

These early dinosaurs represent only a small fraction of animals found from that time period. In this ancient world, the crocodilelike archosaurs were on top. They had a wider array of body shapes, sizes and lifestyles, easily outpacing early dinosaurs in the diversity race.

It wouldn’t be until closer to the end of the Triassic Period, when another volcanism-induced mass extinction event occurred, that dinosaurs got their lucky break.

The late Triassic extinction event killed 75% of life on Earth. It decimated the crocodilelike archosaurs but left early dinosaurs relatively untouched, paving the way for their rise to dominance.

Before long, dinosaurs went from representing less than 5% of animals on Earth to constituting more than 90%.

Bones tell the story of growth

My collaborators from the Universidad Nacional de San Juan, Argentina, and I wondered whether the rise of dinosaurs may have been underpinned, in part, by how fast they grew. We know, through microscopic study of fossilized bones, that later dinosaurs had fast growth rates – much faster than that of modern-day reptiles. But we didn’t know whether that was true for the earliest dinosaurs.

We decided to examine the microscopic patterns preserved in thigh bones from five of the earliest known dinosaur species and compare them with those of six nondinosaur reptiles and one early relative of mammals. All the fossils we studied came from the 2-million-year interval within the Ischigualasto Formation of Argentina.

Microscopic image of a crosssection of bone tissue with many details present.
Eoraptor bone tissue under a polarizing light microscope shows evidence of rapid, continuous growth – common to both the earliest dinosaurs and many of their nondinosaur contemporaries.
Kristi Curry Rogers, CC BY-ND

Bones are an archive of growth history because, even in fossils, we can see the spaces where blood vessels and cells perforated the mineralized tissue. When we look at these features under a microscope, we can see how they are organized. The more slowly growth occurs, the more organized microscopic features will be. With quicker growth, the more disorganized the microscopic features of the bone look.

We discovered early dinosaurs grew continuously, not stopping until they reached full size. And they did indeed have elevated growth rates, on par with and, at times, even faster than those of their descendants. But so did many of their nondinosaur contemporaries. It appears most animals living in the Ischigualasto ecosystem grew quickly, at rates that are more like those of living mammals and birds than those of living reptiles.

Our data allowed us to see the subtle differences between closely related animals and those occupying similar ecological niches. But most of all, our data shows that fast growth is a great survival strategy in the aftermath of mass destruction.

Scientist still don’t know exactly what made it possible for dinosaurs and their ancient ancestors to survive two of the most extensive extinctions Earth has ever undergone. We are still studying this important interval, looking at details such as legs and bodies built for efficient, upright locomotion, potential changes in the way the earliest dinosaurs may have breathed and the way they grew. We think it’s probably all these factors, combined with luck, that finally allowed dinosaurs to rise and rule.The Conversation

Kristi Curry Rogers, Professor of Biology and Geology, Macalester College

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Continue reading “[reprint] Mass extinction and the rise of the dinosaurs”

The Return of the Brontosaurus

Remember the brontosaurus vs apatosaurus debate? Turns out both sides were right…we think…so far.

Here’s the skinny: The skeleton of a long-necked, long-tailed dinosaur was unearthed in Wyoming by paleontologist Othniel Charles Marsh in 1879, according to the Natural History Museum in London. At the time, scientists dubbed the giant plant eater, which lived during the Jurassic period about 150 million years ago, Brontosaurus excelsus, according to Yale University.

However, in 1903, paleontologist Elmer Riggs found that B. excelsus was very similar to another dinosaur, Apatosaurus ajax, which Marsh discovered in Colorado in 1877, the Natural History Museum noted. The differences between the dinosaurs appeared so minor that scientists decided it was better to place them both in the same genus, or group of species. Because Apatosaurus was named first, the rules of scientific naming kept its name, leading scientists to retire the name Brontosaurus.

More than 100 years later, researchers suggested reviving Brontosaurus as its own genus. A 2015 study of sauropods in the journal PeerJ found that the original Apatosaurus and Brontosaurus fossils may have been different enough to classify them as separate groups.

The nearly 300-page study examined 477 physical features of 81 sauropod specimens. The initial aim of the research was to analyze the relationships between the species making up the family of sauropods known as Diplodocidae, which includes Diplodocus, Apatosaurus and, now, Brontosaurus.

All in all, the scientists found that Brontosaurus’ neck was higher-set, narrower and smaller than Apatosaurus’, study lead author Emanuel Tschopp, a vertebrate paleontologist now at the University of Hamburg in Germany, told Live Science. They suggested three known species of Brontosaurus: B. excelsus, B. parvus and B. yahnahpin.

“They call Brontosaurus ‘resurrected,'” Jacques Gauthier, curator of reptiles at the Yale Peabody Museum of Natural History, who did not participate in this study. “I like the ring of that. ‘Restored’ is a perfectly correct term, but ‘resurrected’ is the official description of what they have done.”

Tschopp noted that they could not have made this discovery 15 or more years before their study; only recently did findings of dinosaurs similar to Apatosaurus and Brontosaurus help reveal what made these groups unique.

It has been nearly a decade since the paper published, and Tschopp noted that “not everybody accepts such proposals immediately. There have been — and still are — researchers who don’t trust the results quite yet and continue to use the name Apatosaurus for what I call Brontosaurus.”

Mike Taylor, a vertebrate paleontologist at the University of Bristol in England who did not take part in the 2015 study, told Live Science in an email, “you rarely get consensus from paleontologists on these matters, so the answer you get will depend on who you ask. There’s been no pushback in the formal literature, but I’ve heard a bit of grumbling.”

Still, to Taylor, the call to “resurrect” Brontosaurus “just feels like a reasonable thing to do.” He noted that the 2015 study “made a solid argument that most specialists found pretty persuasive and not especially surprising.” Taylor and his colleagues have mentioned B. excelsus and B. parvus in their own studies a number of times.

Romancing the Prehistoric

I was – note the past tense – going to write a post about re-entry after Covid-19 vaccination and how awesome it was to give my younger daughter a hug after over a year, but then I saw this story from Science magazine and could not resist.

Did you ever wish you could see a living dinosaur? I sure did! (I still do…but from a safe distance.) As a child I loved movies with stop-action animation of dinosaurs, like the original King Kong or the Ray Harryhausen movie, The Valley of Gwangi. In high school I wrote a short novel about two teenagers and their horses who discover a hidden valley where dinosaurs still roam. Jurassic Park and its sequels blew me away, the movies even more so than the novels. The novels were longer on explanation, the movies far more powerful in vividness. The moment when Alan Grant, upon learning that Professor Hammond has created a T. rex and almost faints,  that’s how I would have felt. Great acting and directing aside, these books and films spoke to a universal or near-universal human longing to see amazing charismatic animals from the distant past.

The earlier stories, at least the ones I read and watched, made no effort at a scientific basis for the present-day existence of prehistoric animals. It was all “Land That Time Forgot” hand-waving. Crichton took a different tack: dinosaurs did not persist in some undiscovered corner of or beneath the Earth: humans re-created them using DNA preserved in amber. We’ve been able to recover DNA from Pleistocene mammals, but never anything as old as 65 million years. Many scientists doubt that DNA could survive that long, no matter how preserved. When an animal dies, its DNA begins to decay. A 2012 study on moa bones showed that genetic material deteriorates at such a rate that it halves itself every 521 years. This speed would mean paleontologists can only hope to recover recognizable DNA sequences the past 6.8 million years. In 2020, Chinese Academy of Sciences paleontologist Alida Bailleul and her colleagues proposed they had found a chemical signature suggestive of DNA in a 70 million year old baby hadrosaur fossil. If confirmed, this material would be so degraded into components, not sequences. It’s also possible the chemical signature was that of bacteria, not the dinosaur itself.

The Siberian permafrost that has yielded mammoth DNA is about 2.6 million years old, but freezing turns out to be a pretty good preservative of DNA. Scientists have now been able to sequence DNA from extinct mammoths 1.2 million years ago. That’s a world record. The previous record, in 2013, was from a 750,000-year-old horse. The new study includes DNA from three species of mammoth from three time periods (1.2 million, 1 million, and 700,000 years ago) and there are all kinds of reasons to be excited about it, not just the age but the evolutionary relationships and a previously unknown type.

Which brings us to the question we’re all asking: Once we’ve sequenced this DNA, whether from mammoths, saber-toothed cats, ground sloths, or whatever – what do we do with it? What we can do now is better understand the evolution and relationships of these amazing animals. What popular media want, however, is to use the material to create living extinct species. The process of de-extinction can proceed either by cloning – taking material from a recently extinct species and replicating it – or by using ancient, fragmentary DNA. We’ve got a long way to go with either technique. Many extinct species lack contemporary surrogates to carry the artificially created embryos to term. For others, suitable habitat no longer exists (really? Where would you turn a giant ground sloth loose? A saber-toothed cat? Or would these animals exist only in the unnatural environment of zoos?) Back in 2009, Spanish scientists cloned a newly extinct Pyrenean ibex, although the clone died within a few hours of birth.

There are, however, a few good candidates for which possibly viable DNA sources exist. Species like the passenger pigeon and Carolina parakeet might fare well, given the human responsibility for their disappearance, although they might turn out to be temporally invasive species. Continue reading “Romancing the Prehistoric”