[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.

New Evidence on How the Dinosaurs Died

Such a cool article from Universe Today, I think it merits a post all to itself!

Devastating Clouds of Dust Helped End the Reign of the Dinosaurs

When a giant meteor crashed into Earth 66 million years ago, the impact pulverized cubic kilometers of rock and blasted the dust and debris into the Earth’s atmosphere. It was previously believed that sulfur from the impact and soot from the global fires that followed drove a global “impact winter” that killed off 75% of species on Earth, including the dinosaurs.

A new geology paper says that the die-off was additionally fueled by ultrafine dust created by the impact which filled the atmosphere and blocked sunlight for as long as 15 years. Plants were unable to photosynthesize and global temperatures were lowered by 15 degrees C (59 F).

Most scientists agree the disaster started with an asteroid impact, where an asteroid at least 10 kilometers wide struck the Chicxulub region in the present-day Yucatán Peninsula in Mexico. The impact released 2 million times more energy than the most powerful nuclear bomb ever detonated.

The devastation created layer of ash sandwiched between layers of rock, known as the Cretaceous-Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) boundary, which is found across the world in the geologic record. It includes a layer of iridium, an element common in asteroids but rare on Earth. It was this ‘iridium anomaly’ that first revealed the extinction event as an asteroid strike to geologists more than three decades ago.

What has been debated is what created conditions for the post-impact winter. The leading candidates were sulphur from the asteroid’s impact, or soot from global wildfires that ensued after the impact. Both would have blocked out sunlight and plunged the world into a long, dark winter, collapsing the food chain and creating a chain reaction of extinctions.  

Overview of the Cretaceous-Paleogene boundary in North Dakota (USA). The sediments indicate a river and swamp-like environment at the end of the age of the dinosaurs. The pink-brown layer yields ejecta debris derived from the Chicxulub impact event and the grain-size data from this interval were used as input parameters for the paleoclimate modeling study (photo: Pim Kaskes).

But in this new research, scientists from the Royal Observatory of Belgium (ROB) studied new sediment samples taken from the Tanis fossil site in North Dakota in the US, which captures a 20-year period during the aftermath of the asteroid impact. Analysis of the samples revealed evidence of silicate dust particles, particles that were ejected into the atmosphere and eventually settled back down on the planet.

“We specifically sampled the uppermost millimeter-thin interval of the Cretaceous-Paleogene boundary layer,” said Pim Kaskes  from the Archaeology, Environmental Changes & Geo-chemistry (AMGC) at the Vrije Universiteit Brussel (VUB) and the Vrije Universiteit Amsterdam (VUA), who was also involved in the study. “This interval revealed a very fine and uniform grain-size distribution, which we interpret to represent the final atmospheric fall-out of ultrafine dust related to the Chicxulub impact event. The new results show much finer grain-size values than previously used in climate models and this aspect had important consequences for our climate reconstructions.”

Based on their findings, the scientists also created a new paleoclimate computer model that evaluated the roles of sulfur, soot, and silicate dust on the post-impact climate.

Conceptual model of the Chicxulub impact plume showing different stages of (a) production, and (b) transport and deposition of the impact-generated ejecta (not to scale). (c) Paleoclimate model simulations showcasing the time evolution of the dust-induced photosynthetic active radiation flux across the planet following the Chicxulub impact 66 million years ago (modified from Senel et al., 2023; Nature Geoscience).

“The new paleoclimate simulations show that such a plume of micrometric silicate dust could have remained in the atmosphere for up to 15 years after the event, contributing to global cooling of the Earth’s surface by as much as 15 °C in the initial aftermath of the impact,” said Cem Berk Senel from ROB, the lead author of the study.

But while the dust was a contributor to the catastrophic conditions, the sulfur and soot were also a factor.

“We suggest that, together with additional cooling contributions from soot and sulfur, this is consistent with the catastrophic collapse of primary productivity in the aftermath of the Chicxulub impact,” the researchers wrote.

The prolonged disruption in photosynthesis would pose severe challenges for both terrestrial and marine habitats and mass extinctions would occur in groups not adapted to survive the dark, cold, and food-deprived conditions for at least two years. The researchers said this matches the paleontological records, which show that any plants or animals that could enter a dormant phase (for example, through seeds, cysts, or hibernation in burrows) and were able to adapt to an omnivorous diet, or weren’t dependent on one particular food source generally better survived the K-Pg event.

Continue reading “New Evidence on How the Dinosaurs Died”

Tyrannosaurus Lips and Other Wonders of Science

Once my science classes progressed beyond “the parts of the cell,” I loved them. So much so that my college degree is in Biology, which entailed many classes in Physics and General and Organic Chemistry. Fast forward many decades, I had the joy to attend Launch Pad Astronomy Workshop, about which I have previously blogged. But I’ve never given up my love of Things Prehistoric. Here are two wonderful new stories:

T. rex had thin lips and a gummy smile, controversial study suggests


Theropod dinosaurs — a group of bipedal, mostly meat-eating dinosaurs that included T. rexVelociraptor and Spinosaurus — may instead have concealed their deadly chompers behind thin lips that kept their teeth hydrated and tough enough to crush bones. 

Paleontologists had already suggested that T. rex may have had lips, and there has been debate whether carnivorous dinosaurs looked more like present-day crocodiles, which don’t have lips and have protruding teeth, or if they more likely resembled monitor lizards, whose large teeth are covered by scaly lips.

Rhino-like ‘thunder beasts’ grew massive in the evolutionary blink of an eye after dinos died off


In the aftermath of the dinosaur-killing asteroid impact, a second explosion rocked the animal kingdom. 

This time, it was the mammals that blew up. Rhino-like horse relatives that had lived in the shadow of the dinosaurs became gigantic “thunder beasts” as suddenly as an evolutionary lightning strike,  new research, published Thursday (May 11) in the journal Science(opens in new tab), shows.

“Even though other mammalian groups attained large sizes before [they did], brontotheres were the first animals to consistently reach large sizes,” study first author Oscar Sanisidro(opens in new tab), a researcher with the Global Change Ecology and Evolution Research Group at the University of Alcalá in Spain. “Not only that, they reached maximum weights of 4-5 tons [3.6 to 4.5 metric tons] in just 16 million years, a short period of time from a geological perspective.”

Last year, weird “bramble snout” fossils were documented at the site called “Castle Bank,” but new research published May 1 in the journal Nature Ecology and Evolution(opens in new tab) describes the whole fossil deposit.

Hosting a myriad of soft-bodied marine creatures nd their organs, which are scarcely preserved in the fossil record, the site resembles the world-renowned Cambrian deposits of Burgess Shale in Canada and Qingjiang biota in China. The rocks of Castle Bank, however, are 50 million years younger and give researchers a unique window into how soft-bodied life diversified in the Ordovician Period (485.4 million to 443.8 million years ago), according to a statement released by Amgueddfa Cymru – Museum Wales.

Researchers believe they’ve recovered more than 170 species from the site, most of which are new to science. These include what appear to be late examples of Cambrian groups, including the weirdest wonders of evolution, the nozzle-nosed opabiniids, and early examples of animals that evolved later, including barnacles, shrimp and an unidentified six-legged insect-like creature. The rocks are also home to the fossilized digestive systems of trilobites and the eyes and brain of an unidentified arthropod, as well as preserved worms and sponges.

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”