How many T. rexes were there? Billions.

By Robert Sanders, Media relations, Berkeley News

T Rex Cover.jpg
Over approximately 2.5 million years, North America likely hosted 2.5 billion Tyrannosaurus rexes, a minuscule proportion of which have been dug up and studied by paleontologists, according to a UC Berkeley study. (Image by Julius Csotonyi, courtesy of Science magazine)

How many Tyrannosaurus rexes roamed North America during the Cretaceous period?
That’s a question Charles Marshall* pestered his paleontologist colleagues with for years until he finally teamed up with his students to find an answer.

What the team found, to be published this week in the journal Science, is that about 20,000 adult T. rexes probably lived at any one time, give or take a factor of 10, which is in the ballpark of what most of his colleagues guessed.
What few paleontologists had fully grasped, he said, including himself, is that this means that some 2.5 billion lived and died over the approximately 2 1/2 million years the dinosaur walked the earth.

Until now, no one has been able to compute population numbers for long-extinct animals, and George Gaylord Simpson, one of the most influential paleontologists of the last century, felt that it couldn’t be done.

Marshall, director of the University of California Museum of Paleontology, the Philip Sandford Boone Chair in Paleontology and a UC Berkeley professor of integrative biology and of earth and planetary science, was also surprised that such a calculation was possible.

“The project just started off as a lark, in a way,” he said. “When I hold a fossil in my hand, I can’t help wondering at the improbability that this very beast was alive millions of years ago, and here I am holding part of its skeleton — it seems so improbable. The question just kept popping into my head, ‘Just how improbable is it? Is it one in a thousand, one in a million, one in a billion?’ And then I began to realize that maybe we can actually estimate how many were alive, and thus, that I could answer that question.”

UCMP_T Rex.jpg
A cast of a T. rex skeleton on display outside the UC Museum of Paleontology in the Valley Life Sciences Building. The original, a nearly complete skeleton excavated in 1990 from the badlands of eastern Montana, is on display at the Smithsonian Institution in Washington, DC. (UC Berkeley photo by Keegan Houser)

Marshall is quick to point out that the uncertainties in the estimates are large. While the population of T. rexes was most likely 20,000 adults at any give time, the 95% confidence range — the population range within which there’s a 95% chance that the real number lies — is from 1,300 to 328,000 individuals. Thus, the total number of individuals that existed over the lifetime of the species could have been anywhere from 140 million to 42 billion.

“As Simpson observed, it is very hard to make quantitative estimates with the fossil record,” he said. “In our study, we focused in developing robust constraints on the variables we needed to make our calculations, rather than on focusing on making best estimates, per se.”

He and his team then used Monte Carlo computer simulation to determine how the uncertainties in the data translated into uncertainties in the results.

The greatest uncertainty in these numbers, Marshall said, centers around questions about the exact nature of the dinosaur’s ecology, including how warm-blooded T. rex was. The study relies on data published by John Damuth of UC Santa Barbara that relates body mass to population density for living animals, a relationship known as Damuth’s Law. While the relationship is strong, he said, ecological differences result in large variations in population densities for animals with the same physiology and ecological niche. For example, jaguars and hyenas are about the same size, but hyenas are found in their habitat at a density 50 times greater than the density of jaguars in their habitat.

damuth-.jpg
A critical part of the analysis was estimating T. rex’s ecological niche using a plot, called Damuth’s Law, of body mass versus population density for living mammals. (Chart courtesy of John Damuth, UC Santa Barbara)

“Our calculations depend on this relationship for living animals between their body mass and their population density, but the uncertainty in the relationship spans about two orders of magnitude,” Marshall said. “Surprisingly, then, the uncertainty in our estimates is dominated by this ecological variability and not from the uncertainty in the paleontological data we used.”

As part of the calculations, Marshall chose to treat T. rex as a predator with energy requirements halfway between those of a lion and a Komodo dragon, the largest lizard on Earth.

The issue of T. rex‘s place in the ecosystem led Marshall and his team to ignore juvenile T. rexes, which are underrepresented in the fossil record and may, in fact, have lived apart from adults and pursued different prey. As T. rex crossed into maturity, its jaws became stronger by an order of magnitude, enabling it to crush bone. This suggests that juveniles and adults ate different prey and were almost like different predator species.

This possibility is supported by a recent study, led by evolutionary biologist Felicia Smith of the University of New Mexico, which hypothesized that the absence of medium-size predators alongside the massive predatory T. rex during the late Cretaceous was because juvenile T. rex filled that ecological niche.

The UC Berkeley scientists mined the scientific literature and the expertise of colleagues for data they used to estimate that the likely age at sexual maturity of a T. rex was 15.5 years; its maximum lifespan was probably into its late 20s; and its average body mass as an adult — its so-called ecological body mass, — was about 5,200 kilograms, or 5.2 tons. They also used data on how quickly T. rexes grew over their life span: They had a growth spurt around sexual maturity and could grow to weigh about 7,000 kilograms, or 7 tons.

T Rexj aw-UCMP.jpg
A T. rex jaw collected in 1977 in Montana from the Hell Creek Formation by the late UCMP paleontologist Harley Garbani. (©2011 University of California Museum of Paleontology)

From these estimates, they also calculated that each generation lasted about 19 years, and that the average population density was about one dinosaur for every 100 square kilometers.

Then, estimating that the total geographic range of T. rex was about 2.3 million square kilometers, and that the species survived for roughly 2 1/2 million years, they calculated a standing population size of 20,000. Over a total of about 127,000 generations that the species lived, that translates to about 2.5 billion individuals overall.

With such a large number of post-juvenile dinosaurs over the history of the species, not to mention the juveniles that were presumably more numerous, where did all those bones go? What proportion of these individuals have been discovered by paleontologists? To date, fewer than 100 T. rex individuals have been found, many represented by a single fossilized bone.
“There are about 32 relatively well-preserved, post-juvenile T. rexes in public museums today,” he said. “Of all the post-juvenile adults that ever lived, this means we have about one in 80 million of them.”

“If we restrict our analysis of the fossil recovery rate to where T. rex fossils are most common, a portion of the famous Hell Creek Formation in Montana, we estimate we have recovered about one in 16,000 of the T. rexes that lived in that region over that time interval that the rocks were deposited,” he added. “We were surprised by this number; this fossil record has a much higher representation of the living than I first guessed. It could be as good as one in a 1,000, if hardly any lived there, or it could be as low as one in a quarter million, given the uncertainties in the estimated population densities of the beast.”

Marshall expects his colleagues will quibble with many, if not most, of the numbers, but he believes that his calculational framework for estimating extinct populations will stand and be useful for estimating populations of other fossilized creatures.

Tyrannosaur-tooth.jpg
The tooth of a tyrannosaur – not a T. rex – where Charles Marshall found it in Montana in 2019. While T. rex is an exclusively North American dinosaur, several other tyrannosaur species have been discovered in North America and Asia as well. (UC Berkeley photo by Charles Marshall)

“In some ways, this has been a paleontological exercise in how much we can know, and how we go about knowing it,” he said. “It’s surprising how much we actually know about these dinosaurs and, from that, how much more we can compute. Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analyzing them and better ways of integrating information over the multiple fossils known.”

The framework, which the researchers have made available as computer code, also lays the foundation for estimating how many species paleontologists might have missed when excavating for fossils, he said.

“With these numbers, we can start to estimate how many short-lived, geographically specialized species we might be missing in the fossil record,” he said. “This may be a way of beginning to quantify what we don’t know.”

Marshall’s co-authors are UC Berkeley undergraduate Connor Wilson and graduate students Daniel Latorre, Tanner Frank, Katherine Magoulick, Joshua Zimmt and Ashley Poust, who is now a postdoctoral fellow at the San Diego Natural History Museum.

See: https://news.berkeley.edu/2021/04/15/how-many-t-rexes-were-there-billions/

* The Marshall lab at Berkeley uses paleontological data, typically in conjunction with neontological data and computer simulation, augmented with numerical analysis, to understand the history of life, because the fossil record is evolution's time machine, and the processes that have shaped it.

As a kid I used to devour a comic book titled "Turok, Son of Stone". Turok, an uncanny but wise Native American outsmarting creatures the likes of which mankind had never seen, leads the younger Andar through the Lost Land in search of a way out - or at least a way out of reach of the T-Rex!

turok.jpg

I was thrilled by these stories before Superman and then the paperback Conan series grabbed my attention. For a few summer months, courtesy of my local pharmacy, I imagined a world where the wise Turok ventured through lands populated by dinosaurs before my later readings obliterated those quaint ideas. At least we now know that T-Rex was solely a denizen of North America.
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I can't help speculating "What if that asteroid didn't hit the Earth 65 myo? What kind of world would Earth be?" And since the rock did hit, "just how and why did mammals and eventually great apes and Hominins get so favored by random chance to dominate the Earth"? It has to be just more than a smack from an asteroid and repopulation with other species? ..... Resulting unique climate change stresses? Would the Age of Mammals have happened no matter? I would like to see any theories and references. Thanks.
 
I can't help speculating "What if that asteroid didn't hit the Earth 65 myo? What kind of world would Earth be?" And since the rock did hit, "just how and why did mammals and eventually great apes and Hominins get so favored by random chance to dominate the Earth"? It has to be just more than a smack from an asteroid and repopulation with other species? ..... Resulting unique climate change stresses? Would the Age of Mammals have happened no matter? I would like to see any theories and references. Thanks.


If the dinosaurs had never died out, could one of them have possibly evolved sentient like intelligence similar to us? It’s certainly a fanciful notion, but not altogether impossible, after all if an alien visited Earth in the aftermath of the K/T (Cretaceous/Tertiary) extinction could they have foreseen the evolution of humans from small tiny mammals that mostly resembled modern shrews?

Perhaps the most ‘advanced’ dinosaur known to have been living at the time of the extinction was a small theropod called Troodon. They were small dinosaurs that walked in a bipedal fashion and lived in large groups. Even more compelling was that a detailed analysis of their brain structure seems to suggest that they possessed very good vision and even potentially the ability to solve complex problems.

troodon-reptile-was-carnivorous-small-dinosaur-lived-north-america-cretaceous-period-61036333.jpg
Troodon was a carnivorous small dinosaur that lived in North America during the cretaceous period.

With its large, substantial brain, long grasping hands and big eyes, could Troodonhave wandered down the same evolutionary path as we did, to possess a similar level of intelligence, but even come to resemble us physically?

In 1982, Dale Russell – then at the Canadian Museum of Nature in Ottawa – published a paper proposing that an intelligent ‘dinosauroid’ might one day have evolved. He commissioned a life-sized model, which today looks like an alien from a dated sci-fi show, with green skin and huge eyes. His theory was that the carnivorous dinosaur Troodon had an unusually large brain and might have been the lineage from which brainy dinosaurs evolved.

evolution of troodon .jpg
How Troodon may have evolved if the asteroid had missed.

Some palaeontologists think that it’s likely that at least one kind of dinosaur could have evolved along the same sort of lines as primates or humans. Their argument centres on the fact that we humans are an incredibly successful form of life, and so if intelligence is a good solution for us, then why shouldn’t it be a good solution for dinosaurs?

“Dinosaurs equivalent to crows, parrots or primates, with very complex brains and problem-solving abilities might have evolved,” says carnivorous dinosaur researcher Tom Holtz of the University of Maryland in College Park , but he doesn’t believe dinosaurs could ever have looked like humans. “The pathway to humans was really odd and involved hanging in trees and so forth… dinosaurs got to bipedality and manipulative hands in a much more reasonable approach over a far longer time.”

Humans often develop a certain arrogance in believing that we represent an evolutionary pinnacle, instead we are a seemingly successful one of millions and millions of natural experiments. I find it highly doubtful that the dinosaurs would have evolved to look anything like a person, they would have probably continued to evolve along that of an advanced dinosaur trajectory, getting bigger brains and bigger eyes, but not necessarily evolving the same kind of intelligence as us. Though without any examples anywhere to be found we'll never know.
Hartmann352
 
  • Like
Reactions: Snorrie
If the dinosaurs had never died out, could one of them have possibly evolved sentient like intelligence similar to us? It’s certainly a fanciful notion, but not altogether impossible, after all if an alien visited Earth in the aftermath of the K/T (Cretaceous/Tertiary) extinction could they have foreseen the evolution of humans from small tiny mammals that mostly resembled modern shrews?

Perhaps the most ‘advanced’ dinosaur known to have been living at the time of the extinction was a small theropod called Troodon. They were small dinosaurs that walked in a bipedal fashion and lived in large groups. Even more compelling was that a detailed analysis of their brain structure seems to suggest that they possessed very good vision and even potentially the ability to solve complex problems.

View attachment 1034
Troodon was a carnivorous small dinosaur that lived in North America during the cretaceous period.

With its large, substantial brain, long grasping hands and big eyes, could Troodonhave wandered down the same evolutionary path as we did, to possess a similar level of intelligence, but even come to resemble us physically?

In 1982, Dale Russell – then at the Canadian Museum of Nature in Ottawa – published a paper proposing that an intelligent ‘dinosauroid’ might one day have evolved. He commissioned a life-sized model, which today looks like an alien from a dated sci-fi show, with green skin and huge eyes. His theory was that the carnivorous dinosaur Troodon had an unusually large brain and might have been the lineage from which brainy dinosaurs evolved.

View attachment 1035
How Troodon may have evolved if the asteroid had missed.

Some palaeontologists think that it’s likely that at least one kind of dinosaur could have evolved along the same sort of lines as primates or humans. Their argument centres on the fact that we humans are an incredibly successful form of life, and so if intelligence is a good solution for us, then why shouldn’t it be a good solution for dinosaurs?

“Dinosaurs equivalent to crows, parrots or primates, with very complex brains and problem-solving abilities might have evolved,” says carnivorous dinosaur researcher Tom Holtz of the University of Maryland in College Park , but he doesn’t believe dinosaurs could ever have looked like humans. “The pathway to humans was really odd and involved hanging in trees and so forth… dinosaurs got to bipedality and manipulative hands in a much more reasonable approach over a far longer time.”

Humans often develop a certain arrogance in believing that we represent an evolutionary pinnacle, instead we are a seemingly successful one of millions and millions of natural experiments. I find it highly doubtful that the dinosaurs would have evolved to look anything like a person, they would have probably continued to evolve along that of an advanced dinosaur trajectory, getting bigger brains and bigger eyes, but not necessarily evolving the same kind of intelligence as us. Though without any examples anywhere to be found we'll never know.
Hartmann352
Thanks; I didn't know about Troodon.
 
What the members of the team* found, to be published in the journal Science, is that about 20,000 adult T. Rex's probably lived at any one time, give or take a factor of 10, which is in the ballpark of what most of his colleagues guessed.

What few paleontologists had fully grasped, Charles Marshall said, including himself, is that this means that some 2.5 billion lived and died over the approximately 2 1/2 million years the dinosaur walked the earth.

* The Marshall lab at Berkeley uses paleontological data, typically in conjunction with neontological data and computer simulation, augmented with numerical analysis, to understand the history of life, because the fossil record is evolution's time machine, and the processes that have shaped it.

Of note is the following paper and article:

Absolute abundance and preservation rate of Tyrannosaurus rex

by CHARLES R. MARSHALL HTTPS://ORCID.ORG/0000-0001-7832-0950, et al, from 16 April 2021

"Although much can be deduced from fossils alone, estimating abundance and preservation rates of extinct species requires data from living species. Here, we use the relationship between population density and body mass among living species combined with our substantial knowledge of Tyrannosaurus rex to calculate population variables and preservation rates for postjuvenile T. rex. We estimate that its abundance at any one time was ~20,000 individuals, that it persisted for ~127,000 generations, and that the total number of T. rex that ever lived was ~2.5 billion individuals, with a fossil recovery rate of 1 per ~80 million individuals or 1 per 16,000 individuals where its fossils are most abundant. The uncertainties in these values span more than two orders of magnitude, largely because of the variance in the density–body mass relationship rather than variance in the paleobiological input variables."

See: https://www.science.org/doi/10.1126/science.abc8300



SCIENCE VOL. 372, NO. 6539
16 Apr 2021

Estimating dinosaur abundance

by Sacha Vignieri

"Estimating the abundance of a species is a common practice for extant species and can reveal many aspects of its ecology, evolution, and threat level. Estimating abundance for species that are extinct, especially those long extinct, is a much trickier endeavor. Marshall et al. used a relationship established between body size and population density in extant species to estimate traits such as density, distribution, total biomass, and species persistence for one of the best-known dinosaurs, Tyrannosaurus rex, revealing previously hidden aspects of its population ecology."

See: https://www.science.org/doi/full/10.1126/science.2021.372.6539.twis

The lack of T. Rex fossils indicates both the depth and severity of the physical processes involved in shaping the Earth's crust over the eons, like atmospheric erosion, vulcanism, plate tectonics and continental drift, earthquakes, rains and flooding, snow and snow melt, meteor and comet strikes, tsunamis, interstellar dust and other items which result in changes to the Earth's terrain. Imagine these processes from 230 million years ago, when dinosaurs first appeared, to the last 66 million years following their extinction.
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I can't help speculating "What if that asteroid didn't hit the Earth 65 myo? What kind of world would Earth be?" And since the rock did hit, "just how and why did mammals and eventually great apes and Hominins get so favored by random chance to dominate the Earth"? It has to be just more than a smack from an asteroid and repopulation with other species? ..... Resulting unique climate change stresses? Would the Age of Mammals have happened no matter? I would like to see any theories and references. Thanks.

Did you know that BEFORE dinosaurs rose to dominate megafauna, mammals were briefly the most dominant animal EVER on the planet? Ever. The dinosaurs rose as a result of a previous mass extinction. After that extinction, there was one burrowing mammal that survived and left so many fossils behind that just its genus (or something like that) appears to have been the vastly dominating megafauna. It had no predators. However, the dinosaurs quickly diverged and reproduced to take over. Just another thought to ponder.
 
The most spectacular event in Cenozoic terrestrial environments has been the diversification and rise to dominance of the mammals.

From only a few groups of small mammals in the late Cretaceous that lived in the undergrowth and hid from the dinosaurs, more than 20 orders of mammals evolved rapidly and were established by the early Eocene. Although there is some evidence that this adaptive radiation event began well before the end of the Cretaceous, rates of speciation accelerated during the Paleocene and Eocene epochs. At the end of the Paleocene, a major episode of faunal turnover (extinction and origination) largely replaced many archaicgroups (multituberculates, plesiadapids, and “condylarth” ungulates) with essentially modern groups such as the perissodactyls (which include primitive horses, rhinoceroses, and tapirs), artiodactyls (which include camels and deer), rodents, rabbits, bats, proboscideans, and primates.


evolution of the horse

Evolution of the horse over the past 55 million years. The present-day Przewalski's horse is believed to be the only remaining example of a wild horse—i.e., the last remaining modern horse to have evolved by natural selection. Numbered bones in the forefoot illustrations trace the gradual transition from a four-toed to a one-toed animal.(more)

In the Eocene these groups dispersed widely, migrating via a northern route, probably from Eurasia to North America. In the late Eocene an episode of global cooling triggered changes in the vegetation that converted areas of thick rainforest to more open forest and grasslands, thereby causing another interval of evolutionary turnover that included the disappearance of the last of the primitive herbivores, such as the brontotheres. From the Oligocene Epoch onward, land mammal communities were dominated by representatives of the mammalian groups living today, such as horses, rhinoceroses, antelopes, deer, camels, elephants, felines, and canines.


Moropus
Moropus

Moropus, an extinct genus of the chalicotheres (ungulates with claws instead of hooves) related to the horse. Fossil remains are found in Miocene deposits of North America and Asia.(more)
These groups evolved significantly during the Miocene as the changes to climate and vegetation produced more open grassy habitat. Starting with primitive forms that had low-crowned teeth for browsing leafy vegetation, many herbivorous mammals evolved specialized teeth for grazing gritty grasses and long limbs for running and escaping from increasingly efficient predators. By the late Miocene, grassland communities analogous to those present in the modern savannas of East Africa were established on most continents. Evolution within many groups of terrestrial mammals since the late Miocene has been strongly affected by the dramatic climate fluctuations of the late Cenozoic.

The rapid evolutionary diversification or radiation of mammals in the early Tertiary was probably mostly a response to the removal of reptilian competitors by the mass extinction event occurring at the end of the Cretaceous Period. Later events in mammalian evolution, however, may have occurred in response to changes in geology, geography, and climatic conditions. In the middle of the Eocene Epoch, for example, the direct migration of land mammals between North America and Europe was interrupted by the severance of the Thulean Land Bridge, a connection that had existed prior to this time. Although Europe became cut off from North America, Asia (especially Siberia) remained in contact with Alaska during the late Eocene, and repeated migrations occurred throughout the Oligocene and Miocene epochs.

During the early Miocene, a wave of mammalian immigration from Eurasia brought bear-dogs (early ancestors of modern canines of the genus Amphicyon), European rhinoceroses, weasels, and a variety of deerlike mammals to North America. Also during this time, mastodons escaped from their isolation in Africa and reached North America by the middle of the Miocene. Horses and rodents evolved in the early Eocene, and anthropoid primates emerged during the middle Eocene. Immigration of African mammalian faunas, including proboscideans (mammoths, mastodons, and other relatives of modern elephants), into Europe occurred about 18 million years ago (early Miocene). Climatic cooling and drying during the Miocene led to several episodes where grassland ecosystems expanded and concomitant evolutionary diversifications of grazing mammals occurred.

During the late Pliocene, the land bridge formed by the Central American isthmus allowed opossums, porcupines, armadillos, and ground sloths to migrate from South America and live in the southern United States. A much larger wave of typically Northern Hemispheric animals, however, moved south and may have contributed to the extinction of most of the mammals endemic to South America. These North American invaders included dogs and wolves, raccoons, cats, horses, tapirs, llamas, peccaries, and mastodons.

Cetaceans (whales and their relatives) first appeared in the early Eocene, about 51 million years ago, and are thought to have evolved from early artiodactyls (a group of hoofed mammals possessing an even number of toes). Whale evolution acceleratedduring the Oligocene and Miocene, and this is probably associated with an increase in oceanic productivity. Other new marine forms that emerged in late Paleogene seas were the penguins, a group of swimming birds, and the pinnipeds (a group of mammals that includes seals, sea lions, and walruses). The largest marine carnivore of the period was the megalodon (Carcharocles megalodon), a shark that lived from the middle Miocene to the late Pliocene and reached lengths of at least 16 metres (about 50 feet).

Foraminiferans, especially those belonging to superfamily Globigerinacea, also evolved rapidly and dispersed widely during the Tertiary Period. Consequently, they have proved to be extremely useful as indicators in efforts to correlate oceanic sediments and uplifted marine strata at global and regional scales. Differential rates of evolution within separate groups of foraminiferans increase the utility of some forms in delineating stratigraphic zones, a step in the process of correlating rocks of similar age. For example, conical species of Morozovella and Globorotalia are often used to correlate rock strata across vast geographies because they have wide stratigraphic ranges that vary from one to five million years.

The nummulitids were a group of large lens-shaped foraminiferans that inhabited the bottoms of shallow-water tropical marine realms. They had complex labyrinthine interiors and internal structural supports to strengthen their adaptation to life in high-energy environments. Nummulitids also received nourishment from single-celled symbiotic algae (tiny photosynthetic dinoflagellates) they housed within their bodies. Nummulitids of the genus Nummulites grew to substantial size (up to 150 mm [6 inches] in diameter), and they occurred in massive numbers during a major transgression taking place during the middle of the Eocene Epoch. This transgression produced high sea levels and formed extensive limestone deposits in Egypt, which produced the blocks from which the pyramids were built. Nummulites lived throughout the Eurasian-Tethyan faunal province from the later part of the Paleocene Epoch to the early Oligocene, but it did not reach the Western Hemisphere. Following the extinction of Nummulites, other larger foraminiferans, the miogypsinids and lepidocyclinids, flourished.


Major subdivisions of the Tertiary System​

Paleogene Period in geologic time
Paleogene Period in geologic time
The Paleogene Period and its subdivisions.
Neogene Period in geologic time
Neogene Period in geologic time

Classically, the Cenozoic Era was divided into the Tertiary and Quaternary periods, separated at the boundary between the Pliocene and Pleistocene epochs (formerly set at 1.8 million years ago); however, by the late 20th century many authorities considered the terms Tertiary and Quaternary to be obsolete. In 2005 the International Commission on Stratigraphy (ICS) decided to recommend keeping the Tertiary and Quaternary periods as units in the geologic time scale but only as sub-eras within the Cenozoic Era. By 2009 the larger intervals (periods and epochs) of the Cenozoic had been formalized by the ICS and the International Union of Geological Sciences(IUGS). The ICS redivided the Cenozoic Era into the Paleogene Period (66 million to 23 million years ago), the Neogene Period (23 million to 2.6 million years ago), and the Quaternary Period (2.6 million years ago to the present). Under this paradigm, the Paleogene and Neogene span the interval formerly occupied by the Tertiary. The Paleogene Period, the oldest of the three divisions, commences at the onset of the Cenozoic Era and includes the Paleocene Epoch (66 million to 56 million years ago), the Eocene Epoch (56 million to 33.9 million years ago), and the Oligocene Epoch(33.9 million to 23 million years ago). The Neogene spans the interval between the beginning of the Miocene Epoch (23 million to 5.3 million years ago) and the end of the Pliocene Epoch (5.3 million to 2.6 million years ago). The Quaternary Period begins at the base of the Pleistocene Epoch (2.6 million to 11,700 years ago) and continues through the Holocene Epoch (11,700 years ago to the present).

Precise stratigraphic positions for the boundaries of the various traditional Tertiary series were not specified by early workers in the 19th century. It is only in more recent times that the international geologic community has formulated a philosophical framework for stratigraphy. By specifying the lower limits of rock units deposited during successive increments of geologic time at designated points in the rock record (called stratotypes), geologists have established a series of calibration points, called Global Boundary Stratotype Sections and Points (GSSPs), at which time and rock coincide. These boundary stratotypes are the linchpins of global chronostratigraphic units—essentially, the points of reference that mark time within the rock—and serve as the point of departure for global correlation.

Several boundary stratotypes have been identified within Tertiary rocks. The Cretaceous-Tertiary, or K-T, boundary has been stratotypified in Tunisia in North Africa. (Increasingly, this boundary is known as the Cretaceous-Paleogene, or K-P, boundary.) Its estimated age is 66 million years. The Paleocene-Eocene boundary has an estimated age of 56 million years; its GSSP is located near Luxor, Egypt. In the early 1990s the Eocene-Oligocene boundary was stratotypically established in southern Italy, with a currently estimated age of approximately 33.9 million years. The Oligocene-Miocene boundary (which also corresponds to the boundary between the Paleogene and Neogene systems) has been stratotyped in Carrosio, Italy; its age has been calculated at roughly 23 million years old. The GSSP associated with the Miocene-Pliocene boundary is located in Sicily and has been dated to about 5.3 million years ago, although the location of this boundary may be repositioned in the future. The boundary between the Pliocene and the Pleistocene, separating the Neogene and Quaternary systems, has been stratotyped in Sicily near the town of Gela and dated to approximately 2.6 million years ago.

With the exception of the vast Tethys seaway, the basins of western Europe, and the extensive Mississippi Embayment of the Gulf Coast region in the United States, Tertiary marine deposits are located predominantly along continental margins and occur on all continents. Miocene deposits are found as far north as Alaska; Eocene deposits are found in eastern Canada; and Paleocene deposits are located in Greenland. Deposits of Paleogene age occur on Seymour Island near the Antarctic Peninsula, and Neogene deposits containing marine diatoms (silica-bearing marine phytoplankton) have recently been identified intercalated between glacial tills on Antarctica itself.

Global sea levels have fallen gradually by about 300 metres (about 1,000 feet) over the past 100 million years, but superimposed upon that trend is a higher-order series of globally fluctuating increases and decreases (that is, transgressions and regressions) in sea level. These fluctuations vary with a periodicity of several million years; where they have occurred along passive (that is, tectonically stable) continental margins, they have left a record of marginal marine brackish accumulations that overlap with continental sedimentary deposits in Europe, North Africa, the Middle East, southern Australia, and the Gulf and Atlantic coastal plains of North America. In most regions, Paleogene seas extended farther inland than did those of the Neogene. In fact, the most extensive transgression of the Tertiary is that of the Lutetian Age (Middle Eocene), roughly 49 million to 41 million years ago. During that interval, the Tethys Sea expanded onto the continental margins of Africa and Eurasia and left extensive deposits of nummulitic rocks, which are made up of shallow-water carbonates. Sediments of Tertiary age are widely developed on the deep ocean floor and on elevated seamounts as well. In the shallower parts of the ocean (above depths of 4.5 km [about 3 miles]), sediments are calcareous (made of calcium carbonate), siliceous (derived from silica), or both, depending on local productivity. Below 4.5 km the sediments are principally siliceous or inorganic, as in the case of red clay, due to dissolution of calcium carbonate.

Nonmarine Tertiary sedimentary and volcanic deposits are widespread in North America, particularly in the intermontane basins west of the Mississippi River. During the Neogene, volcanism and terrigenous deposition extended almost to the Pacific coast. In South America, thick nonmarine clastic sequences (conglomerates, sandstones, and shales) occur in the mobile tectonic belt of the Andes Mountains and along their eastern front; these sequences extend eastward for a considerable distance into the Amazon basin. Tertiary marine deposits occur along the eastern margins of Brazil and Argentina, and they were already known to English naturalist Charles Darwin during his exploration of South America from 1832 to 1834.

Volcanism has continued throughout the Cenozoic on land and at the major oceanic ridges, such as the Mid-Atlantic Ridge and the East Pacific Rise, where new seafloor is continuously generated and carried away laterally by seafloor spreading. Iceland, which was formed in the middle Miocene, is one of the few places where the processes that occur at the Mid-Atlantic Ridge can be observed today.
Two of the most extensive volcanic outpourings recorded in the geologic record occurred during the Tertiary. Near the Cretaceous-Tertiary boundary, some 66 million years ago, massive outpourings of basaltic lava formed the Deccan Traps of India. About 55 million years ago, near the Paleocene-Eocene boundary, massive explosive volcanism took place in northwestern Scotland, northern Ireland, the Faeroe Islands, East Greenland, and along the rifted continental margins on both sides of the North Atlantic Ocean. Volcanic activity in the North Atlantic was associated with the rifting and separation of Eurasia from North America, which occurred on a line between Scandinavia and Greenland and left a stratigraphic record in the marine sedimentary basin of England and in ash deposits as far south as the Bay of Biscay. In both the Deccan and North Atlantic, comparable volumes of extensive basalts in the amount of 10,000,000 cubic km (about 2,400,000 cubic miles) were erupted.

The well-known volcanics of the Massif Central of south-central France, which figured so prominently in early (18th-century) investigations into the nature of igneous rocks, are of Oligocene age, as are those located in central Germany. The East African Rift System preserves a record of mid-to-late Tertiary rifting and the separation event that eventually led to the formation of a marine seaway linking the Indian Ocean with the Mediterranean.

The circum-Pacific “Ring of Fire,” an active tectonic belt that extends from the Philippines through Japan and around the west coast of North and South America, was subject to seismic activity and andesitic volcanism throughout much of the Tertiary. The extensive Columbia Plateau basalts were extruded over Washington and Oregon during the Miocene, and many of the volcanoes of Alaska, Oregon, southern Idaho, and northeastern California date to the Late Tertiary. Active volcanism occurred in the newly uplifted Rocky Mountains during the early part of the Tertiary, whereas in the southern Rocky Mountains and Mexico volcanic activity was more common in the mid- and late Tertiary. The linear volcanic trends, such as the Hawaiian, Emperor, and Line island chains in the central and northwestern Pacific, are trails resulting from the movement of the Pacific Plate over volcanic “hot spots” (that is, magma-generating centres) that are probably fixed deep in Earth’s mantle. The major hot spot island groups such as the Hawaiian (which has been active over the past 30 million years), Galapagos, and Society (which were active during the Miocene) islands are volcanoes that rose from the seafloor. Central America, the Caribbean region, and northern South America were the sites of active volcanism throughout the Cenozoic.

In contrast to the passive-margin sedimentation on the Atlantic and Gulf coastal plains, the Cordilleran (or Laramide) orogeny in the Late Cretaceous, Paleocene, and Eocene produced a series of upfolded and upthrusted mountains and deep intermontane basins in the area of the Rocky Mountains. Deeply downwarped basins accumulated as much as 2,400 metres (about 8,000 feet) of Paleocene and Eocene sediment in the Green River Basin of southwestern Wyoming and 4,300 metres (about 14,000 feet) of sediment in the Uinta Basin of northeastern Utah. Other basins ranging from Montana to New Mexico accumulated similar but thinner packages of nonmarine fluvial and lacustrine sediments rich in fossil mammals and fish. In the Oligocene and Miocene the influences of the cordilleras, or mountain chains, on what is now the western United States had ceased, and the basins were gradually filled to the top by sediments and abundant volcanic ash deposits from eruptions in present-day Colorado, Nevada, and Utah. These basins were exhumed during the old Pliocene-Pleistocene boundary (about 1.8 million years ago) with renewed uplift of the long-buried Rocky Mountains, along with uplift of the Colorado Plateau, producing steep stream gradients that resulted in the cutting of the Grand Canyon to a depth of more than 1,800 metres (about 6,000 feet).

Volcanism along the Cascade mountain chain has been active since the late Eocene, as evidenced by the major eruption of Mount St. Helens in 1980 and subsequent minor eruptions. This volcanism was gradually shut off in California as the movement of plate boundaries changed from one of subduction to a sliding and transform motion (seeplate tectonics: Principles of plate tectonics). With the development of the San Andreas Fault system, the western half of California started sliding northward. The Cascade–Sierra Nevada mountain chain began to swing clockwise, causing the extension of the Basin and Range Province in Nevada, Arizona, and western Utah. This crustal extension broke the Basin and Range into a series of north-south-trending fault-block mountains and downdropped basins, which filled with thousands of metres of upper Cenozoic sediment. These fault zones (particularly the Wasatch Fault in central Utah and the San Andreas zone in California) remain active today and are the source of most of the damaging earthquakes in North America. The Andean mountains were uplifted during the Neogene as a result of subduction of the South Pacific beneath the South American continent.

Complex tectonic activity also occurred in Asia and Europe during the Tertiary. The main Alpine orogeny began during the late Eocene and Oligocene and continued throughout much of the Neogene. Major tectonic activity in the eastern North Atlantic (Bay of Biscay) extended into southern France and culminated in the uplift of the Pyrenees in the late Eocene. On the south side of the Tethys, the coastal Atlas Mountains of North Africa experienced major uplift during this time, but the Betic region of southern Spain and the Atlas region of northern Morocco continued to display mirror-image histories of tectonic activity well into the late Neogene. In the Middle East the suturing of Africa and Asia occurred about 18 million years ago. Elsewhere, India had collided with the Asian continent about 40 million years ago, initiating the Himalayan uplift that was to intensify in the Pliocene and Pleistocene and culminate in the uplift of the great Plateau of Tibet and the Himalayan mountain range. Major orogenic movement also occurred in the Indonesian-Malaysian-Japanese arc system during the Neogene. In New Zealand, which sits astride the Indian-Australian and Pacific plate boundary, the major tectonic uplift (the Kaikoura orogeny) of the Southern Alps began about 24 million years ago.

Northwestern Europe contains a number of Tertiary marine basins that essentially rim the North Sea basin, itself the site of active subsidence during the Paleogene and infilling during the Neogene. The marine Hampshire and London basins, the Paris Basin, the Anglo-Belgian Basin, and the North German Basin have become the standard for comparative studies of the Paleogene part of the Cenozoic, whereas the Mediterranean region (Italy) has become the standard for the Neogene. The Tertiary record of the Paris Basin is essentially restricted to the Paleogene strata (namely, those of Paleocene–late Oligocene age), whereas scattered Pliocene-Pleistocene deposits occur in England and Belgium above the Paleogene. The strata are relatively thin, nearly horizontal, and often highly fossiliferous, particularly in the middle Eocene calcaire grossier (freshwater limestone) of the Paris Basin, from which a molluscanfauna of more than 500 species has been described. The Paris Basin is a roughly oval-shaped basin centred on Paris, whereas the Hampshire and London basins lie to the southwest and northeast of London, respectively. The London Basin and the Anglo-Belgian Basin were part of a single sedimentary basin across what is now the English Channel during the early part of the Paleogene.
fossil-containing strata
fossil-containing strata

Fossils help geologists establish the ages of layers of rock. In this diagram, sections A and B represent rock layers 200 miles (320 km) apart. Their ages can be established by comparing the fossils in each layer. (more)

The total Paleogene stratigraphic succession in these basins is less than 300 metres (about 980 feet), and it is made up of clays, marls, sands, carbonates, lignites, and gypsum. These layers reflect alternations of marine, brackish, lacustrine, and terrestrial environments of deposition. The alternating transgressions and regressions of the sea have left a complex sedimentary record punctuated by numerous unconformities (interruptions in the deposition of sedimentary rock) and associated temporal hiatuses, and the correlation of these various units and events has challenged stratigraphers since the early 19th century. The integration of biostratigraphy, paleomagnetic stratigraphy, and tephrochronology (respectively, using fossils, magnetic properties, and ash layers to establish the age and succession of rocks) has resulted in a refined correlation of rock layers in these separate basins.
In North America, by contrast, extensive Tertiary sediments occur on the Atlantic and Gulf coastal plains and extend around the margin of the Gulf of Mexico to the Yucatán Peninsula, a distance of more than 5,000 km (about 3,100 miles). Seaward these deposits can be traced from the Atlantic Coastal Plain to the continental margin and rise and in the Gulf Coastal Plain into the subsurface formations of this oil-bearing province of the Gulf of Mexico. During the Paleocene the Gulf Coast extended northward roughly 2,000 km (about 1,200 miles) in a feature called the Mississippi Embayment, which reached as far as southwestern North Dakota and Montana; there marine deposits known as the Cannonball Formation can be seen as outcrops of sandstone. Although eroded between northwestern South Dakota and southern Illinois, marine outcrops continue southward to the present coastline and continue in the subsurface of the Gulf of Mexico. Tertiary sediments with a thickness in excess of 6,000 metres (about 20,000 feet) are estimated to lie beneath the continental margin along the northern Gulf of Mexico. In the Tampico Embayment of eastern Mexico, thicknesses of more than 3,000 metres (about 10,000 feet) have been estimated for the Paleocene Velasco Formation alone, which developed under conditions of active subsidence and associated rapid deposition. Exposures in the Atlantic Coastal Plain and most of the Gulf Coastal Plain are of Paleogene age, but considerable thicknesses of Neogene sediment occur in offshore wells in front of the Mississippi delta, where thicknesses in excess of 10,000 metres (about 33,000 feet) have been recorded for the Neogene alone. Sediments are dominantly calcareous in the Florida region and become more marly and eventually sandy to the west, reflecting the input of terrigenous matter transported seasonally by the Mississippi River and its precursors. Because of general faunal and floral similarities, it is possible to make relatively precise stratigraphic correlations in the Paleogene between the Gulf and Atlantic coastal plain region and the basins in northwestern Europe.

The name Tertiary was introduced by Italian geologist Giovanni Arduino in 1760 as the second youngest division of Earth’s rocks. The oldest rocks were the primitive, or “primary,” igneous and metamorphic rocks (composed of schists, granites, and basalts) that formed the core of the high mountains in Europe. Arduino designated rocks composed predominantly of shales and limestones in northern Italy as elements of the fossiliferous “secondary,” or Mesozoic, group. He considered younger groups of fossiliferous sedimentary rocks, found chiefly at lower elevations, as “tertiary” rocks and the smaller pebbles and gravel that covered them as “quaternary” rocks. Although originally intended as a descriptive generalization of rock types, many of Arduino’s contemporaries and successors gave these categories a temporal connotation and equated them with rocks formed prior to, during, and after the Noachian deluge. In 1810 French mineralogist, geologist, and naturalist Alexandre Brongniart included all the sedimentary deposits of the Paris Basin in his terrains tertiares, or Tertiary. Soon thereafter all rocks younger than Mesozoic in western Europe were called Tertiary.

The subdivision of the Tertiary into smaller units was originally based on fossil faunas of western Europe that were known to 19th-century natural scientists. These faunas primarily contained mollusks exhibiting varying degrees of similarity with modern types. At the same time, the science of stratigraphy was in its infancy, and the primary focus of its earliest practitioners was to use the newly discovered sequential progression of fossils in layered sedimentary rocks to establish a global sequence of temporally ordered stages. Scottish geologist Charles Lyell employed a simple statistical measure based on the relative percentages of living species of mollusks to fossil mollusks found in different layers of Tertiary rocks. These percentages had been compiled by Lyell’s colleague and friend Gérard-Paul Deshayes, a French geologist who had amassed a collection of more than 40,000 mollusks and was preparing a monograph on the mollusks of the Paris Basin.

In 1833 Lyell divided the Tertiary into four subdivisions (from older to younger): Eocene, Miocene, the “older Pliocene,” and the “newer Pliocene.” (The latter was renamed Pleistocene in 1839.) The Eocene contained about 3 percent of the living mollusk species, the Miocene about 20 percent, the older Pliocene more than one-third and often over 50 percent, and the newer Pliocene about 90 percent. Lyell traveled extensively and had a broad and comprehensive understanding of the regional geologyfor his day. He understood, for example, that rocks of the Tertiary were unevenly distributed over Europe and that there were no rocks of the younger part of the period in the Paris Basin. He used the deposits in the Paris, Hampshire, and London basins as typical for the Eocene. For the Miocene he used the sediments of the Loire Basin near Touraine, the deposits in the Aquitaine Basin near Bordeaux in southwestern France, and the Bormida River valley and Superga near Turin, Italy. The sub-Apennine formations of northern Italy were used for the older Pliocene, and the marine strata in the Gulf of Noto, on the Island of Ischia (also in Italy), and near Uddevalla (in Sweden) were used for the newer Pliocene.

The limits between Lyell’s Tertiary subdivisions were not rigidly specified, and Lyell himself recognized the approximate and imperfect nature of his scheme. Indeed, in their original form, Lyell’s subdivisions would today be termed biostratigraphic units (bodies of rocks characterized by particular fossil assemblages) rather than chronostratigraphic units (bodies of rocks deposited during a specific interval of time).

Subsequent stratigraphic studies in northern Europe showed that deposits were included variously in the upper Eocene or lower Miocene by different geologists of the day. This situation led German geologist H.E. Beyrich, in 1854, to create the term Oligocene for rocks in the North German Basin and Mainz Basin and to insert it between the Eocene and the Miocene in the stratigraphic scheme. As originally proposed, the Oligocene included the Tongrian and Rupelian stages as well as strata that subsequently formed the basis for the Chattian Stage. The Tongrian is no longer used as a standard unit, its place being taken by the Rupelian.

The term Paleocene was proposed by German paleobotanist Wilhelm P. Schimper in 1874 on the basis of fossil floras in the Paris Basin that he considered intermediate between Cretaceous and Eocene forms. Typical strata include the sands of Bracheux, the travertines of Sézanne, and the lignites and sandstones of Soissons. The problem of the Paleocene is that, of all the chronostratigraphic units of the Tertiary, it alone is defined on the basis of nonmarine strata, making recognition of its upper limit and general correlation difficult elsewhere. Acceptance of the term Paleocene into the general system of stratigraphic names was irregular, and only in 1939 did the United States Geological Survey, general arbiter of standard stratigraphic nomenclature in North America, formally accept it. The Danian Stage was proposed by the Swiss geologist Pierre Jean Édouard Desor in 1846 for chalk deposits in Denmark. It was assigned to the Cretaceous by virtue of the similarity of its invertebrate megafossils to those of the latest Cretaceous elsewhere. However, since the late 1950s, micropaleontologists have recognized that calcareous marine plankton (foraminiferansand coccolith-bearing nannoplankton) exhibit a major taxonomic change at the boundary between the Maastrichtian (uppermost Cretaceous) Stage and the Danian(lowermost Tertiary) Stage. The Danian is now widely regarded as being the oldest stage of the Cenozoic.

In 1948 the 18th International Geological Congress placed the base of the Pleistocene at the base of the marine strata of the Calabrian Stage of southern Italy, using the initial appearance of northern or cool-water invertebrate faunas in Mediterranean marine strata as the marker. Subsequent studies showed that the type section was ill-chosen and that the base of the Calabrian Stage was equivalent to much younger levels within the Pleistocene. A newly designated stratotype section was chosen at Vrica in Calabria, and for a time the base of the Pleistocene was found comparable to a level dated to nearly 1.8 million years ago. In 2009 the IUGS ratified the decision by the ICS to align the base of the Pleistocene (and thus the top of the Neogene System) with the base of the Gelasian Stage.

Alcide d'Orbigny
Alcide d'Orbigny Engraved portrait of French paleontologist Alcide d'Orbigny.

The boundaries of the Tertiary were originally only qualitatively estimated on the basis of the percentages of living species of (primarily) mollusks in the succession of marine strata in the western European basins (see above). The need for more precise correlations of Mesozoic and Cenozoic marine strata in Europe led to the concept of stages, which was introduced in 1842 by French paleontologist Alcide d’Orbigny. These stages were originally defined as rock sequences composed of distinctive assemblages of fossils that were believed to change abruptly as a result of major transgressions and regressions of the sea. This methodology has since been improved and refined, but it forms the basis for modern biostratigraphic correlation. Early attempts at global correlations of strata were made by direct comparisons with the faunas in the type areas in Europe; however, it was soon realized that faunal provincialization led to spurious correlations. In 1919 an independent set of percentages for the Indonesian region was proposed, which was subsequently modified into the so-called East India Letter Stage classification system based on the occurrence of taxa of larger foraminiferans.

Learn how the radiometric rubidium-strontium dating technique is used to determine the age of rock particles

Learn how the radiometric rubidium-strontium dating technique is used to determine the age of rock particles

The process of rubidium-strontium dating, a radiometric dating technique used to determine the absolute age of rock particles.(more)

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Since about the mid-1900s, increasing efforts have been made to apply radioisotopic datingtechniques to the development of a geochronologic scale, particularly for the Cenozoic Era. The decay of potassium-40 to argon-40 (see potassium-argon dating) has proved very useful in this respect, and refinements in mass spectroscopy and the development of laser-fusion dating involving the decay of argon-40 to argon-39 have resulted in the ability to date volcanic mineral samples in amounts as small as single crystals with a margin of error of less than 1 percent over the span of the entire Cenozoic Era.
seafloor spreading and magnetic striping

Rising magma assumes the polarity of Earth's geomagnetic field before it solidifies into oceanic crust. At spreading centres, this crust is separated into parallel bands of rock by successive waves of emergent magma. When Earth's geomagnetic field undergoes a reversal, the change in polarity is recorded in the magma, which contributes to the alternating pattern of magnetic striping on the seafloor.(more)

Also, since the mid-1960s, investigators have demonstrated that Earth’s magnetic field has undergone numerous reversals in the past. It is known that most rocks pick up and retain the magnetic orientation of the field at the time they are formed through either sedimentary or igneous processes. With the development of techniques for measuring the rock’s original orientation of magnetization, a sequence of polarity reversals has been dated for the late Neogene. In addition, a paleomagnetic chronology has been built for the entire Cenozoic. This work is based on the recognition that the magnetic lineations detected in rocks on the ocean floor were formed when basaltic magma had been extruded from the oceanic ridges. Earth’smagnetic polarity undergoes a reversal roughly every 500,000 years, and newly formed rocks assume the ambient magnetic polarity of the time. As a result, strips of normal and reversed polarity that reflect these magnetic reversals can be observed in deep-sea cores. The calibration of the composite geomagnetic polarity succession to time and the relation of this chronology to the isotopic time scale, however, have proved to be the greatest source of disagreement over various current versions of the geologic time scale. Calibrations of a time scale must ultimately be based on the application of meaningful isotopic ages to the succession of polarity intervals and geologic stages. A geochronologic scheme is thus an integration of several methodologies; it makes use of the best attributes of seafloor-spreading history (that is, the pattern of seafloor magnetic anomalies), magnetostratigraphy, and biostratigraphy in the application of relevant isotopic ages to derive a high-resolution and internally consistent time scale. The recent application of cyclical components driven by astronomical phenomena into the stratigraphic record, such as lithological couplets of marl and chalks and fluctuations in the ratios and percentages of fossil taxa, has resulted in fine-tuning the geologic time scale to a resolution of about 5,000 years in the late Neogene.

Micropaleontologists have created a number of zones based on the regional distribution of calcareous plankton (foraminiferans and nannoplankton) and those of the siliceous variety (radiolarians and diatoms), making it possible to correlate sediments from the high northern to high southern latitudes by way of the equatorial region. The resulting high-resolution zonal biostratigraphy and its calibration to an integrated geochronology provide the framework in which a true historical geology has become feasible.

See: https://www.britannica.com/science/Tertiary-Period/Correlation-of-Tertiary-strata

The astonishing diversity of mammalian species today stems in part from the continuing breakup of the continents that began some 200 million years ago and sent different landmasses moving apart. Australia and South America were isolated from other continents during much of the Tertiary, and marsupial mammals thrived and diversified there, while placental mammals took over similar roles on the other continents.
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“How many T. rexes were there?”​

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