This diversity can be deceiving, at least when it comes to how mammals create the next generation. Based on how they reproduce, nearly all mammals alive today fall into one of two categories: and . Placentals, including humans, whales and rodents, have long gestation periods. They give birth to well-developed young — with all major organs and structures in place — and have relatively short weaning periods, or lactation periods, during which young are nursed on milk from their mothers. Marsupials, like kangaroos and opossums, are the opposite: They have short gestation periods — giving birth to young that are little more than fetuses — and long lactation periods during which offspring spend weeks or months nursing and growing within the mother’s pouch, or marsupium.

For decades, biologists saw the marsupial way of reproduction as the more “primitive” state, and assumed that placentals had evolved their more “advanced” method after these two groups diverged from one another. But new research is testing that view. In a published July 18 in The American Naturalist, a team led by researchers at the 91̽�� and its present evidence that another group of mammals — the extinct — likely reproduced in a placental-like manner. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials evolved, these findings question the view that marsupials were “less advanced” than their placental cousins.

“This study challenges the prevalent idea that the placental reproductive strategy is ‘advanced’ relative to a more ‘primitive’ marsupial strategy,” said lead author , a postdoctoral researcher at the University of Michigan who conducted this study as a 91̽��doctoral student. “Our findings suggest that placental-like reproduction either is the ancestral reproductive route for all mammals that give birth to live young, or that placental-like reproduction evolved independently in both multituberculates and placentals.”

Multituberculates arose about 170 million years ago in the Jurassic. Most were small-bodied creatures, resembling rodents. For much of their history, multituberculates were the most abundant and diverse group of mammals. But scientists know very little about their life history, including how they reproduced, because of their generally poor fossil record. The last multituberculates died out about 35 million years ago.

Weaver reasoned that the microscopic structure of fossilized bone tissues can house useful life-history information about multituberculates, such as their growth rate. Working under co-author , a 91̽��professor of biology and curator of vertebrate paleontology at the Burke Museum, Weaver and his colleagues obtained cross sections of 18 fossilized femurs — the thigh bone — from multituberculates that lived approximately 66 million years ago in Montana.

All 18 samples showed the same structural organization: a layer of disorganized bone “sandwiched” between an inner and outer layer of organized bone. Disorganized bone, or woven bone, indicates rapid growth and is so named because, under a microscope, the layers of bone tissue are laid out in a crisscrossed fashion. In organized bone, which reflects slower growth, layers are parallel to one another.

The researchers then examined femoral cross sections taken from 35 small-bodied mammalian species that are living today — 28 placentals and seven marsupials, all from Burke Museum collections. Nearly all of the placental femurs showed the same “sandwich” organization as the multituberculates. But all of the marsupial femurs consisted almost entirely of organized bone, with only a sliver of disorganized bone.

The team believes that is stark difference likely reflects their divergent life histories.

“The amount of organized bone in the outermost layer, or cortex, of the femur strongly correlates with the length of the lactation period,” said Weaver. “Marsupials have long lactation periods and a lot of organized bone in the outermost cortex. The opposite is true for placentals: a short lactation period and much less organized bone in the outermost cortex.”

The outermost layer of organized bone was laid down after birth as the femur’s diameter increased. For tiny marsupial newborns, bones must grow much more to reach adult size, so they deposit a greater amount of outer organized bone compared to placentals, according to Weaver.

“This is compelling evidence that multituberculates had a long gestation and a short lactation period similar to placental mammals, but very different from marsupials,” said Weaver.

Based on this correlation, the researchers estimate that multituberculates had a lactation period of approximately 30 days — similar to today’s rodents.

These findings cast further doubt on an old view that marsupials have a “more primitive” and placentals a “more advanced” reproductive strategy. The common ancestor of multituberculates, placentals and marsupials may have had a placental-like mode of reproduction that was retained by placentals and multituberculates. Alternatively, multituberculates and placentals could have evolved their long-gestation and short-lactation reproductive methods independently.

Future studies of multituberculate life history may clarify which explanation is true, as well as other outstanding questions of this, and other, ancient branches of our mammalian family tree.

“The real revelation here is that we can cut open fossil bones and examine their microscopic structures to reconstruct the intimate life history details of long-extinct mammals,” said Wilson Mantilla. “That’s really incredible to me.”

Additional co-authors are former 91̽��undergraduate researcher Henry Fulghum, now a graduate student at Indiana University; 91̽��postdoctoral researcher David Grossnickle; 91̽��graduate students William Brightly and Zoe Kulik; and Megan Whitney, a 91̽��doctoral alum and current postdoctoral researcher at Harvard University. The research was funded by the National Science Foundation, the UW, the Burke Museum, the Society of Vertebrate Paleontology, the Paleontological Society and the American Society of Mammalogists.

For more information, contact Weaver at lukeweav@umich.edu and Wilson Mantilla at gpwilson@uw.edu.

A team of paleontologists from the 91̽�� and its excavated four dinosaurs in northeastern Montana this summer. All fossils will be brought back to the Burke Museum where the public can watch paleontologists remove the surrounding rock in the .

The four dinosaur fossils are: the ilium — or hip bones — of an ostrich-sized theropod, the group of meat-eating, two-legged dinosaurs that includes Tyrannosaurus rex and raptors; the hips and legs of a duck-billed dinosaur; a pelvis, toe claw and limbs from another theropod that could be a rare ostrich-mimic Anzu, or possibly a new species; and a Triceratops specimen consisting of its skull and other fossilized bones. Three of the four dinosaurs were all found in close proximity on Bureau of Land Management land that is currently leased to a rancher.

In July 2021, a team of volunteers, paleontology staff, K-12 educators who were part of the and students from 91̽��and other universities worked together to excavate these dinosaurs. The fossils were found in the , a geologic formation that dates from the latest portion of Cretaceous Period, 66 to 68 million years ago. Typical paleontological digs involve excavating one known fossil. However, the Hell Creek Project is an ongoing research collaboration of paleontologists from around the world studying life right before, during and after the that killed off all dinosaurs except birds. The Hell Creek Project is unique in that it is sampling all plant and animal life found throughout the rock formation in an unbiased manner.

“Each fossil that we collect helps us sharpen our views of the last dinosaur-dominated ecosystems and the first mammal-dominated ecosystems,” said , a 91̽��professor of biology and curator of vertebrate paleontology at the Burke Museum. “With these, we can better understand the processes involved in the loss and origination of biodiversity and the fragility, collapse and assembly of ecosystems.”

All of the dinosaurs except the Triceratops will be prepared in the Burke Museum’s fossil preparation laboratory this fall and winter. The Triceratops fossil remains on the site because the dig team continued to find more and more bones while excavating and needs an additional field season to excavate any further bones that may be connected to the surrounding rock. The team plans to finish excavation in the summer of 2022.

Called the “Flyby Trike” in honor of the rancher who first identified the dinosaur while he was flying his airplane over his ranch, the team has uncovered this dinosaur’s frill, horn bones, individual rib bones, lower jaw, teeth and the occipital condyle bone — nicknamed the “trailer hitch,” which is the ball on the back of the skull that connects to the neck vertebrae. The team estimates approximately 30% of this individual’s skull bones have been found to date, with more potential bones to be excavated next year.

The Flyby Trike was found in hardened mud, with the bones scattered on top of each other in ways that are different from the way the bones would be laid out in a living animal. These clues indicate the dinosaur likely died on a flood plain and then got mixed together after its death by being moved around by a flood or river system, or possibly moved around by a scavenger like a T. rex, before fossilizing. In addition, the Flyby Trike is one of the last Triceratops living before the K-Pg mass extinction. Burke paleontologists estimate it lived less than 300,000 years before the event.

“Previous to this year’s excavations, a portion of the Flyby Trike frill and a brow horn were collected and subsequently prepared by volunteer preparators in the fossil preparation lab. The frill was collected in many pieces and puzzled together fantastically by volunteers. Upon puzzling the frill portion together, it was discovered that the specimen is likely an older ‘grandparent’ Triceratops,” said Kelsie Abrams, the Burke Museum’s paleontology preparation laboratory manager who also participated in this summer’s field work. “The triangular bones along the frill, called ‘epi occipitals,’ are completely fused and almost unrecognizable on the specimen, as compared to the sharp, noticeable triangular shape seen in younger individuals. In addition, the brow horn curves downwards as opposed to upwards, and this feature has been reported to be seen in older animals as well.”

Amber and seed pods were also found with the Flyby Trike. These finds allow paleobotanists to determine what plants were living alongside Triceratops, what the dinosaurs may have eaten, and what the overall ecosystem was like in Hell Creek leading up to the mass extinction event.

“Plant fossil remains from this time period are crucial for our understanding of the wider ecosystem. Not only can plant material tell us what these dinosaurs were perhaps eating, but plants can more broadly tell us what their environment looked like,” said Paige Wilson, a 91̽��graduate student in Earth and space sciences. “Plants are the base of the food chain and a crucial part of the fossil record. It’s exciting to see this new material found so close to vertebrate fossils!”

Museum visitors can now see paleontologists remove rock from the first of the four dinosaurs — the theropod hips — in the Burke’s paleontology preparation laboratory. Additional fossils will be prepared in the upcoming weeks. All four dinosaurs will be held in trust for the public on behalf of the Bureau of Land Management and become a part of the Burke Museum’s collections.

For high resolution images, videos and interviews, contact burkepr@uw.edu.

A new published Feb. 24 in the journal Royal Society Open Science documents the earliest-known fossil evidence of primates.

A team of 10 researchers from across the U.S. analyzed several fossils of Purgatorius, the oldest genus in a group of the earliest-known primates called plesiadapiforms. These ancient mammals were small-bodied and ate specialized diets of insects and fruits that varied by species. These newly described specimens are central to understanding primate ancestry and paint a picture of how life on land recovered after the Cretaceous-Paleogene extinction event 66 million years ago that wiped out all dinosaurs — except for birds — and led to the rise of mammals.

, a 91̽�� professor of biology and curator of vertebrate paleontology at the UW’s , co-led the study with of Brooklyn College and the City University of New York. The team analyzed fossilized teeth found in the Hell Creek area of northeastern Montana. The fossils, which are now part of the collections at the University of California Museum of Paleontology, are estimated to be 65.9 million years old, about 105,000 to 139,000 years after the mass extinction event. Based on the age of the fossils, the team estimates that the ancestor of all primates —including plesiadapiforms and today’s primates such as lemurs, monkeys and apes — likely emerged by the Late Cretaceous and lived alongside large dinosaurs.

“It’s mind blowing to think of our earliest archaic primate ancestors,” said Wilson Mantilla. “They were some of the first mammals to diversify in this new post-mass extinction world, taking advantage of the fruits and insects up in the forest canopy.”

The fossils include two species of Purgatorius: Purgatorius janisae and a new species described by the team named Purgatorius mckeeveri. Three of the teeth found have distinct features compared to any previously known Purgatorius species and led to the description of the new species.

Purgatorius mckeeveri is named after Frank McKeever, who was among the first residents of the area where the fossils were discovered, and also the family of John and Cathy McKeever, who have since supported the field work where the oldest specimen of this new species was discovered.

“This was a really cool study to be a part of, particularly because it provides further evidence that the earliest primates originated before the extinction of non-avian dinosaurs,” said co-author Brody Hovatter, a 91̽��graduate student in Earth and space sciences. “They became highly abundant within a million years after that extinction.”

“This discovery is exciting because it represents the oldest dated occurrence of archaic primates in the fossil record,” said Chester. “It adds to our understanding of how the earliest primates separated themselves from their competitors following the demise of the dinosaurs.”

Co-author on the study was the late who was a professor emeritus at the University of California, Berkeley and former director of the UC Museum of Paleontology. Additional co-authors are Jason Moore and Wade Mans of the University of New Mexico; Courtney Sprain of the University of Florida; William Mitchell of Minnesota IT Services; Roland Mundil of the Berkeley Geochronology Center; and Paul Renne of UC Berkeley and the Berkeley Geochronology Center. The research was funded by the National Science Foundation, the UC Museum of Paleontology, the Myhrvold and Havranek Charitable Family Fund, the UW, the CUNY and the Leakey Foundation.

For high resolution images and interviews, contact burkepr@uw.edu.

A new study led by paleontologists at the 91̽�� and its indicates that the earliest evidence of mammal social behavior goes back to the Age of Dinosaurs.

The evidence, Nov. 2 in the journal Nature Ecology & Evolution, lies in the fossil record of a new genus of — a small, rodent-like mammal that lived during the Late Cretaceous of the dinosaur era — called Filikomys primaevus, which translates to “youthful, friendly mouse.” The fossils are the most complete mammal fossils ever found from the Mesozoic in North America. They indicate that F. primaevus engaged in multi-generational, group-nesting and burrowing behavior, and possibly lived in colonies. Study co-authors — including lead author , a 91̽��graduate student in biology, and senior author , a 91̽��professor of biology and curator of vertebrate paleontology at the Burke Museum — analyzed several fossils, all about 75.5 millioin years old, and extracted from a well-known dinosaur nesting site called Egg Mountain in western Montana.

Fossil skulls and skeletons of at least 22 individuals of F. primaevus were discovered at Egg Mountain, typically clustered together in groups of two to five, with at least 13 individuals found within a 30 square-meter area in the same rock layer. Based on how well preserved the fossils are, the type of rock they’re preserved in, and F. primaevus’ powerful shoulders and elbows — which are similar to today’s living burrowing animals — Weaver, Wilson Mantilla and co-authors hypothesize these animals lived in burrows and were nesting together. Furthermore, the animals found were a mixture of multiple mature adults and young adults, suggesting these were truly social groups as opposed to just parents raising their young.

“It was crazy finishing up this paper right as the stay-at-home orders were going into effect — here we all are trying our best to socially distance and isolate, and I’m writing about how mammals were socially interacting way back when dinosaurs were still roaming the Earth!” said Weaver. “It is really powerful, I think, to see just how deeply rooted social interactions are in mammals. Because humans are such social animals, we tend to think that sociality is somehow unique to us, or at least to our close evolutionary relatives, but now we can see that social behavior goes way further back in the mammalian family tree. Multituberculates are one of the most ancient mammal groups, and they’ve been extinct for 35 million years, yet in the Late Cretaceous they were apparently interacting in groups similar to what you would see in modern-day ground squirrels.”

Previously, scientists thought social behavior in mammals first emerged after the mass extinction that killed off the dinosaurs, and mostly in the Placentalia — the group of mammals humans belong to, which all carry the fetus in the mother’s uterus until a late stage of development. But these fossils show mammals were socializing during the Age of Dinosaurs, and in an entirely different and more ancient group of mammals — the multituberculates.

“These fossils are game changers,” said Wilson Mantilla. “As paleontologists working to reconstruct the biology of mammals from this time period, we’re usually stuck staring at individual teeth and maybe a jaw that rolled down a river, but here we have multiple, near complete skulls and skeletons preserved in the exact place where the animals lived. We can now credibly look at how mammals really interacted with dinosaurs and other animals that lived at this time.”

Co-authors are and at Montana State Univeristy, at Yale University and Meng Chen of Nanjing University. The research was funded by the National Science Foundation, Doris O. and Samuel P. Welles Research Fund, the 91̽��and the Burke Museum.

For high resolution images and interviews, contact burkepr@uw.edu.

Scientists have discovered an extraordinary collection of fossils that reveal in detail how life recovered after a catastrophic event: the asteroid impact that wiped out the dinosaurs 66 million years ago at the end of the Cretaceous Period. Described in a published Oct. 24 in the journal Science, the unprecedented find — thousands of exceptionally preserved animal and plant fossils from the first million years after the catastrophe — shines light on how life emerged from one of Earth’s darkest hours.

Scientists unearthed this new record from the first million years after the asteroid impact in the region around Colorado Springs, Colorado, which forms part of a larger geological province known as the . The find includes both plants and animals, painting a portrait of the emergence of the post-dinosaur world.

The team behind this discovery was led by two scientists at the Denver Museum of Nature & Science, lead author , the curator of vertebrate paleontology, and co-author , curator of paleobotany and director of Earth and space sciences. Among the co-authors is , a professor of biology at the 91̽�� and curator of vertebrate paleontology at the , who joined the team as an expert on the end-Cretaceous mass extinction and the evolution of mammals in its aftermath.

“I’ve been working on this time period and on these mammals for 21 years, and this is truly an exceptional window into this pivotal event in life history,” said Wilson, who was a curator at the Denver Museum of Nature & Science before joining the 91̽��faculty in 2007. “We change over from terrestrial ecosystems dominated by dinosaurs to those that become dominated by mammals in a geological blink of an eye.”

“The course of life on Earth changed radically on a single day 66 million years ago,” said Lyson. “Blasting our planet, an asteroid triggered the extinction of three of every four kinds of living organisms. While it was a really bad time for life on Earth, some things survived, including some of our earliest, earliest ancestors.”

A moment of serendipity pointed the way to these rare fossil finds. Lyson, who had been looking for post-impact vertebrate fossils without success, took inspiration from a fossil that had been sitting in a museum drawer and fossil-hunting techniques used by colleagues in South Africa. In the summer of 2016, he stopped looking for glinting bits of bone in the Denver Basin and instead zeroed in on egg-shaped rocks called concretions.

Cracking open the concretions, Lyson and Miller found fossils such as the skulls of mammals from the early generations of survivors of the mass extinction. Since most of what is understood from this era is based on tiny fragments of fossils, such as pieces of mammal teeth, finding a single skull would be exceptional. Lyson and Miller found four in a single day and more than a dozen in a week. So far, they’ve found fossils from at least 16 different mammalian species.

Wilson participated in excavations at Corral Bluffs, a site just east of Colorado Springs. He and co-author , an assistant professor at the City University of New York’s Brooklyn College, worked to identify the mammal fossil that the team found, calculate changes in body size over time and analyze the diversity of species after the asteroid impact. The researchers determined that just 100,000 years after the cataclysm, mammalian diversity had approximately doubled. At 300,000 years after the impact, the maximum body mass of mammals had increased threefold, and mammals were evolving specialized diets, possibly in response to changes in plant diversity.

These findings illustrate how the Denver Basin site also is adding evidence to the idea that the recovery and evolution of plants and animals were intricately linked after the asteroid impact. More than 6,000 leaf fossils were collected as part of the study to help determine how and when Earth’s forest rebounded after the mass extinction event. Combining a remarkable fossil plant record with the discovery of the fossil mammals allowed the team to link millennia-long warming spells to specific global events, including massive amounts of volcanism on the Indian subcontinent.

These events may have shaped the ecosystems half a world away. For example, the research team saw another increase in body size among mammals at about 700,000 years post-impact, which coincided with the evolution of legumes, then a new type of plant. Additional analyses of these fossils, as well as the unearthing of new specimens, will only deepen scientists’ understanding of this critical period in Earth’s history and the evolution of mammals that came before humans.

“Our understanding of the asteroid’s aftermath has been spotty,” Lyson explained. “These fossils tell us for the first time how exactly our planet recovered from this global cataclysm.”

Additional co-authors include David Krause, James Hagadorn, Antoine Bercovici, Farley Fleming, Ken Weissenburger with the Denver Museum of Nature & Science; William Clyde and Anthony Fuentes with the University of New Hampshire; Kirk Johnson and Rich Barclay with the Smithsonian Institution’s National Museum of Natural History; Matthew Butrim at Wesleyan University; Gussie Maccracken at the University of Maryland; and Ben Lloyd of Colorado College. The research was funded by the Lisa Levin Appel Family Foundation, M. Cleworth, Lyda Hill Philanthropies, David B. Jones Foundation, M.L. and S.R. Kneller, T. and K. Ryan, and J.R. Tucker as part of the Denver Museum of Nature & Science No Walls: Schools initiative.

For more information, contact Wilson at gpwilson@uw.edu.

Adapted from a release by the Denver Museum of Nature & Science.

The myth’s focus isn’t on dinosaurs. Its main characters are ancient mammals and their relatives, which together are known as . According to the myth, a world crowded with dinosaurs left little room for mammaliaforms. As a result, mammals and their kin remained tiny, mouse-like and primitive. The myth posits that mammals didn’t evolve diverse shapes, diets, behaviors and ecological roles until the 66 million years ago killed off the dinosaurs and “freed up” space for mammals.

“This is a very old idea, which makes it very hard to defeat,” said , a postdoctoral researcher in the Department of Biology at the 91̽��. “But this view of mammaliaforms simply doesn’t stand up to what we and others have found recently in the fossil record.”

Grossnickle is the lead and corresponding author of a published June 19 in that summarizes the latest fossil evidence for an alternative view: Mammals and their relatives have actually undergone three significant “ecological radiations” in their history. In evolutionary biology, a radiation occurs when a particular lineage invades and adapts to new ecological niches. In each of the radiations discussed in the review, mammaliaforms diversified from insect-chomping, rodent-like ancestors and adapted to a variety of ecological niches. New species arose that, for example, could climb, glide or burrow — and ate more specialized diets of meat, leaves or shellfish.

Two of these three ecological radiations of mammailaforms occurred during the Jurassic and Cretaceous periods when dinosaurs were thriving, according to Grossnickle and co-authors of in Chicago and , a 91̽��associate professor of biology and curator of vertebrate paleontology at the UW’s .

The co-authors summarize the three ecological radiations, each of which involved different groups of mammaliaforms:

• The oldest mammaliaform ecological radiation ran from 190 to 163 million years ago in the early-to-mid Jurassic Period — amid the breakup of the supercontinent Pangaea — and involved the first true mammals and their closest relatives.
• A second ecological radiation of mammals began 90 million years ago in the Late Cretaceous Period, shortly after flowering plants evolved, and ended at the 66 million years ago.
• The Paleocene-Eocene radiation began 66 million years ago around the time of the K-Pg event and ended about 34 million years ago, and led to the establishment of all the major lineages of placental and marsupial mammals alive today.

Each ecological radiation from more primitive, insect-eating, rodent-like ancestors. Many of the diverse forms that arose during the Jurassic and Cretaceous resemble species alive today, such as badgers, flying squirrels and even anteaters. But these dinosaur-era mammaliaforms are not the direct ancestors of their modern counterparts.

“These same ecological adaptations — for gliding, climbing, eating diverse diets — have evolved repeatedly in the history of mammals and their close relatives,” said Grossnickle.

Mammaliaforms that arose during the Jurassic radiation included the semi-aquatic, beaver-like ; , which likely resembled today’s flying squirrels; and the tree-climbing . These lineages died out by the mid-Cretaceous Period — for early mammals and their relatives, likely due to climate change and the relatively rapid turnover of whole ecosystems.

The Late Cretaceous ecological radiation followed this period of decline, and saw the rise of new forms of mammals. These included the badger-sized , a marsupial relative with the strongest pound-for-pound bite force of any known mammal, as well as , a herbivore with some skull features similar to sloths. These diverse groups of mammals perished alongside dinosaurs in the K-Pg mass extinction.

“The presence of this diversity of mammaliaforms in the Jurassic and Cretaceous overturns a classical interpretation of how mammals evolved,” said Wilson. “This new interpretation was really made possible by new fossil discoveries over the past two decades in places like China and Madagascar.”

The Paleocene-Eocene radiation of mammals, which began around the time of the K-Pg event, generated the ancestors of today’s marsupial and placental mammals – from kangaroos and zebras to blue whales and humans. This radiation’s strong connection to today’s mammals may explain how the myth arose that mammals remained static and primitive in the time of the dinosaurs, according to Grossnickle.

“But focusing on the Paleocene-Eocene radiation gives a distorted view of the history of mammals,” said Grossnickle. “It ignores many of the other groups of mammals and their relatives that were diversifying millions of years before then.”

Fossil discoveries over the past quarter century support the view summarized by Grossnickle and co-authors. Dinosaur-era mammaliaforms that were once known by only a single tooth or a few bone fragments are now represented by more-complete skeletons, which show the diversity in body shape, size, locomotion and diet.

“Now we can start to see the huge diversity of mammals and their relatives who lived alongside the dinosaurs,” said Grossnickle.

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For more information, contact Grossnickle at dmgrossn@uw.edu.

A new published April 30 in the identified three factors critical in the rise of mammal communities since they first emerged during the Age of Dinosaurs: the rise of flowering plants, also known as angiosperms; the evolution of tribosphenic molars in mammals; and the extinction of non-avian dinosaurs, which reduced competition between mammals and other vertebrates in terrestrial ecosystems.

Previously, mammals in the Age of Dinosaurs were thought to be a relatively small part of their ecosystems and considered to be small-bodied, nocturnal, ground-dwelling insectivores. According to this long-standing theory, it wasn’t until the about 66 million years ago, which wiped out all non-avian dinosaurs, that mammals were then able to flourish and diversify. An astounding number of fossil discoveries over the past 30 years has challenged this theory, but most studies looked only at individual species and none has quantified community-scale patterns of the rise of mammals in the Mesozoic Era.

Co-authors are Meng Chen, a 91̽�� alumnus and current postdoctoral researcher at Nanjing University; , a 91̽�� biology professor and curator of paleobotany at the UW’s ; and , a 91̽��associate professor of biology and Burke Museum curator of vertebrate paleontology. The team created a Rubik’s Cube-like structure identifying 240 “eco-cells” representing possible ecological roles of mammals in a given ecospace. These 240 eco-cells cover a broad range of body size, dietary preferences, and ways of moving of small-bodied mammals. When a given mammal filled a certain type of role or eco-cell, it filled a spot in the ‘Rubik’s Cube.’ This method provides the first comprehensive analysis of evolutionary and ecological changes of fossil mammal communities before and after K-Pg mass extinction.

“We cannot directly observe the ecology of extinct species, but body size, dietary preferences and locomotion are three aspects of their ecology that can be relatively easily inferred from well-preserved fossils,” said Chen. “By constructing the ecospace using these three ecological aspects, we can visually identify the spots filled by species and calculate the distance among them. This allows us to compare the ecological structure of extinct and extant communities even though they don’t share any of the same species.”

The team analyzed living mammals to infer how fossil mammals filled roles in their ecosystems. They examined 98 small-bodied mammal communities from diverse biomes around the world, an approach that has not been attempted at this scale. They then used this modern-day reference dataset to analyze five exceptionally preserved mammal paleocommunities ― two Jurassic Period and two Cretaceous Period communities from northeastern China, and one Eocene Epoch community from Germany. Usually Mesozoic Era mammal fossils are incomplete and consist of fragmentary bones or teeth. Using these remarkably preserved fossils enabled the team to infer ecology of these extinct mammal species, and look at changes in mammal community structure during the last 165 million years.

The team found that, in current communities of present-day mammals, ecological richness is primarily driven by vegetation type, with 41 percent of small mammals filling eco-cells compared to 16 percent in the paleocommunities. The five mammal paleocommunities were also ecologically distinct from modern communities and pointed to important changes through evolutionary time. Locomotor diversification occurred first during the Mesozoic, possibly due to the diversity of microhabitats, such as trees, soils, lakes and other substrates to occupy in local environments. It wasn’t until the Eocene that mammals grew larger and expanded their diets from mostly carnivory, insectivory and omnivory to include more species with diets dominated by plants, including fruit. The team determined that the rise of flowering plants, new types of teeth and the extinction of dinosaurs likely drove these changes.

Before the rise of flowering plants, mammals likely relied on conifers and other seed plants for habitat, and their leaves and possibly seeds for food. By the Eocene, flowering plants were both diverse and dominant across forest ecosystems. Flowering plants provide more readily available nutrients through their fast-growing leaves, fleshy fruits, seeds and tubers. When becoming dominant in forests, they fundamentally changed terrestrial ecosystems by allowing for new modes of life for a diversity of mammals and other forest-dwelling animals, such as birds.

“Flowering plants really revolutionized terrestrial ecosystems,” said Strömberg. “They have a broader range of growth forms than all other plant groups ― from giant trees to tiny annual herbs ― and can produce nutrient-rich tissues at a faster rate than other plants. So when they started dominating ecosystems, they allowed for a wider variety of life modes and also for much higher ‘packing’ of species with similar ecological roles, especially in tropical forests.”

Tribosphenic molars ― complex multi-functional cheek teeth ― became prevalent in mammals in the late Cretaceous Period. Mutations and natural selection drastically changed the shapes of these molars, allowing them to do new things like grinding. In turn, this allowed small mammals with these types of teeth to eat new kinds of foods and diversify their diets.

Lastly, the K-Pg mass extinction event that wiped out all dinosaurs except birds 66 million years ago provided an evolutionary and ecological opportunity for mammals. Small body size is a way to avoid being eaten by dinosaurs and other large vertebrates. The mass extinction event not only removed the main predators of mammals, but also removed small dinosaurs that competed with mammals for resources. This ecological release allowed mammals to grow into larger sizes and fill the roles the dinosaurs once had.

“The old theory that early mammals were held in check by dinosaurs has some truth to it,” said Wilson. “But our study also shows that the rise of modern mammal communities was multifaceted and depended on dental evolution and the rise of flowering plants.”

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For more information contact Andrea Godinez with the 91̽��Burke Museum at burkepr@uw.edu.

Burke Museum story .

Starting Aug. 12, the public can watch fossil preparation of the 91̽�� ‘s Tyrannosaurus rex skull “live.” Over the next several months, Burke paleontologists will carefully remove the rock surrounding the skull, slowly exposing the 66-million-year-old specimen. in summer 2016 in the Hell Creek Formation in northeast Montana, the skull is 4 feet long, weighs 3,000 pounds in its field jacket, is the first in Washington and one of only 15 reasonably complete T. rex skulls ever discovered.

This is one of only a handful of times the public has had the opportunity to see preparation of a T. rex, and it is even rarer to be able to see the process on a T. rex skull.

“The bones we’re seeing so far are among the best I’ve seen,” said Michael Holland, Burke Museum fossil preparator, who has worked on T. rex specimens at museums across the country.

A team of paleontologists and trained volunteers will begin this intricate work in a lab that is part of the Burke’s special exhibit. The Testing exhibit features three working labs and an imaging room that showcases the work happening behind the scenes at the Burke every day, and is a prototype of the new “See Through” experiences the public can enjoy daily in the , opening 2019.

at 11 a.m., 12:30 p.m. and 2 p.m., members of the fossil preparation and excavation crews will give talks about the dinosaur — named “Tufts-Love Rex” in honor of Jason Love and Luke Tufts, the two volunteers who discovered it. There will also be dinosaur crafts and activities for kids every weekend through Labor Day, and more and more of the skull will be exposed as Burke paleontologists work.

Prior to working on the skull, the team spent the last year preparing a lower jaw bone and ribs from the “Tufts-Love Rex.” The finished bones are also on display in Testing, Testing 1-2-3.

“This is going to be one of the most complete T. rex specimens in the world. And it’s gorgeous in terms of its preservation — the bone is spectacular,” said , 91̽��professor of biology and Burke Museum curator of vertebrate paleontology. “I’m super excited to be able to bring this to the Burke, the Pacific Northwest and the 91̽��.”

Removing rock from around such a rare and large fossil requires skill and patience. The Burke’s team of fossil preparators and trained volunteers will spend the next several months uncovering the T. rex skull. Once all of the rock surrounding the bone has been removed and the fossils have been stabilized, the museum plans to display the skull in the New Burke Museum.

Custom-designed equipment was created to hold the massive skull, which would break ordinary lab tables. Holland, who is leading the Burke’s Hell Creek preparator team, collaborated with Crucible, a worker’s cooperative based in Montana, to create a specially engineered cage dubbed the “T. rex Rotisserie Rack (TR3).” The equipment’s appearance lives up to its name, but instead of roasting a chicken, this rack is designed to hold up to 6,000 pounds of fossil. The TR3 consists of two heavy-duty steel wheels connected by structural steel rods filled with high-strength concrete, to reduce flexion from the massive weight of the skull. The device sits on a wheeled frame made of 2-inch tube steel, which allows the Burke’s paleontology team to safely rotate the fossil as the work progresses. Each rod can be individually removed so the team can easily access any part of the skull.

Each time the skull is rotated in the rack, the team will apply rigid urethane foam between the rack’s bars and the fossil in order to create a custom cradle, distributing the weight across the bars and relieving pressure points that could potentially damage the skull. The areas of the fossil that are bearing weight will continue to be covered in a plaster jacket that looks similar to a cast used to set a broken bone.

After applying a consolidant to harden them, the bones can then be handled for research or exhibit mounting. The portions of the skull that the prep team can see emerging so far suggest the individual bones that make up the skull are still articulated (connected). The team’s initial work will focus mainly on the removal of as much rock matrix as possible from the exterior surfaces of the skull, leaving the bones in place.

“Some of the most amazing fossils I’ve ever worked with are from the Hell Creek Formation, and this T. rex definitely lives up to those standards. The sandstone is so soft that it can be scraped away with a fingernail, although we prefer dental picks or other small tools,” Holland said. “Even though the rock is soft, the bones are exquisitely preserved — to such an extent that if they were white instead of brown in color, they would look like they came from an animal that just died a year or two ago.”

It has taken two summers to excavate the “Tufts-Love Rex” from the field. In summer 2016, 91̽��and Burke paleontologists discovered the skull along with ribs, vertebrae and parts of the jaw and pelvis. Suspecting more of the dinosaur remained in the field, the team returned to the site this summer and found a number of new bones, including a belly rib, another piece of the lower jaw discovered last summer and parts of the shoulder blade. Wilson suspects they also may have found another rare part of the T. rex — the notoriously small humerus arm bone. Rock will need to be removed from around the fossil to confirm this discovery.

In total, about 30 percent (90 bones) of the dinosaur has been found — making the “Tufts-Love Rex” one of the top 10 most complete T. rex skeletons ever discovered.

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For more information, contact Godinez at 206-616-7538 or burkepr@uw.edu.

Paleontologists picking through a bounty of fossils from Montana have discovered something unexpected — a new species of lizard from the late dinosaur era, whose closest relatives roamed in faraway Asia.

This ancient lizard, which lived 75 million years ago in a dinosaur nesting site, is described from stem to stern in published Jan. 25 in the . Christened Magnuviator ovimonsensis, the new species fills in significant gaps in our understanding of how lizards evolved and spread during the dinosaur era, according to paleontologists at the 91̽�� and the who led the study.

“It is incredibly rare to find one complete fossil skeleton from a relatively small creature like this lizard,” said , lead author and postdoctoral research associate in the 91̽��biology department and the Burke Museum. “But, in fact, we had two specimens, both from the same site at Egg Mountain in Montana.”

Right out of the gate, Magnuviator is reshaping how scientists view lizards, their biodiversity and their role in complex ecosystems during this reptile’s carefree days in the Cretaceous Period 75 million years ago.

Based on analyses of the nearly complete fossil skeletons, Magnuviator was an ancient offshoot of iguanian lizards — and they’re actually the oldest, most complete iguanian fossils from the Americas. Today, iguanians include of the Old World, and in the American tropics and even the infamous water-walking — or “Jesus Christ” — lizards. But based on its anatomy, Magnuviator was at best a distant relative of these modern lizard families, most of which did not arise until after the non-avian dinosaurs — and quite a few lizards and other creatures — went extinct 66 million years ago.

The team came to these conclusions after meticulous study of both Egg Mountain specimens over four years. This included a round of CT scans at Seattle Children’s Hospital to narrow down the fossil’s location within a larger section of rock and a second round at the American Museum of Natural History to digitally reconstruct the skull anatomy. The fact that both skeletons were nearly complete allowed them to determine not only that Magnuviator represented an entirely new species, but also that its closest kin weren’t other fossil lizards from the Americas. Instead, it showed striking similarities to other Cretaceous Period iguanians from Mongolia.

“These ancient lineages are not the iguanian lizards which dominate parts of the Americas today, such as anoles and horned lizards,” said DeMar. “So discoveries like Magnuviator give us a rare glimpse into the types of ‘stem’ lizards that were present before the extinction of the dinosaurs.”

But Magnuviator‘s surprises don’t end with the Mongolian connection. The site of its discovery is also eye-popping.

Egg Mountain is already famous among fossil hunters. Over 30 years ago, paleontologists discovered the first fossil remains of dinosaur babies there, and it is also one of the first sites in North America where dinosaur eggs were discovered.

“We now recognize Egg Mountain as a unique site for understanding Cretaceous Period ecosystems in North America,” said senior author , 91̽��associate professor of biology and curator of paleontology at the Burke Museum. “We believe both carnivorous and herbivorous dinosaurs came to this site repeatedly to nest, and in the process of excavating this site we are learning more and more about other creatures who lived and died there.”

The team even named their new find as homage to its famous home and its close lizard relatives in Asia. Magnuviator ovimonsensis means “mighty traveler from Egg Mountain.”

Through excavations at Egg Mountain led by co-author at Montana State University and meticulous analysis of fossils at partner institutions like the 91̽��and the Burke Museum, scientists are piecing together the Egg Mountain ecosystem of 75 million years ago. In those days, Egg Mountain was a semi-arid environment, with little or no water at the surface. Dinosaurs like the duck-billed and the birdlike, carnivorous nested there.

Researchers have also unearthed fossilized mammals at Egg Mountain, which are being studied by Wilson’s group, as well as wasp pupae cases and pollen grains from plants adapted for dry environments. Based on the structure of Magnuviator‘s teeth, as well as the eating habits of some lizards today, the researchers believe that it could have feasted on wasps at the Egg Mountain site. Though based on its relatively large size for a lizard — about 14 inches in length — Magnuviator could have also eaten something entirely different.

“Due to the significant metabolic requirements to digest plant material, only lizards above a certain body size can eat plants, and Magnuviator definitely falls within that size range,” said DeMar.

Whatever its diet, Magnuviator and its relatives in Mongolia did not make it into the modern era. DeMar and co-authors hypothesize that these stem lineages of lizards may have gone extinct along with the non-avian dinosaurs. But given the spotty record for lizards in the fossil record, it will take more Magnuviator-level discoveries to resolve this debate. And, unfortunately, part of the excitement surrounding Magnuviator is that it is a rare find.

Other co-authors are the late Jack Conrad of the New York Institute of Technology and the American Museum of Natural History and of the University of Cambridge. The research was funded by the National Science Foundation and the American Museum of Natural History.

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For more information, contact DeMar at ddemar@uw.edu or 206-543-4832 and Wilson at gpwilson@uw.edu or 206-543-8917.

DOI: 10.1098/rspb.2016.1902

Grant numbers: 0847777, 1325674.

Move over, hyenas and saber-toothed cats; there’s a mammal with an even stronger bite. A new study by paleontologists at the and the 91̽�� describes an early marsupial relative called Didelphodon vorax that lived alongside dinosaurs and had, pound-for-pound, the strongest bite force of any mammal ever recorded.

in the journal , the team’s findings suggest mammals were more varied during the age of dinosaurs than previously believed. Didelphodon was able to eat a variety of foods and was likely a scavenger-predator who could eat prey ranging from snails to small dinosaurs.

In addition, the team re-traced the origins of marsupials. Previous theories attribute South America as the origin of marsupials, but anatomical features of the Didelphodon point to marsupials originating in North America 10 to 20 million years earlier than originally thought and later dispersing and diversifying in South America.

“What I love about Didelphodon vorax is that it crushes the classic mold of Mesozoic mammals,” , Burke Museum adjunct curator of vertebrate paleontology and 91̽��associate professor of biology. “Instead of a shrew-like mammal meekly scurrying into the shadows of dinosaurs, this badger-sized mammal would’ve been a fearsome predator on the Late Cretaceous landscape — even for some dinosaurs.”

All of these findings are made possible by four fossil specimens recently discovered in the 66 to 69 million-year-old deposits of the Hell Creek Formation in Montana and North Dakota. Prior to these discoveries, the 60 known species of metatherians (marsupials and their closest relatives) from the Cretaceous of North America — including Didelphodon — were almost all identified through fragments of jaw bones or teeth, providing a limited glimpse into marsupials’ closest relatives. These four fossils include a nearly-complete skull from the North Dakota Geological Survey State Fossil collection, a partial snout and an upper jaw bone from the Burke Museum’s collections, and another upper jaw from the Sierra College Natural History Museum.

By analyzing never-before-seen parts of Didelphodon‘s anatomy, Wilson and his colleagues were able to determine these marsupial relatives were about the size of today’s and were the largest metatherian from the Cretaceous. With a nearly complete skull to measure, they were able to estimate the overall size of Didelphodon, which ranged from 5.3 to 11.5 pounds.

To test the bite force of Didelphodon, , then a 91̽��Biology research technician working with 91̽��biology professor and Burke Museum curator , CT-scanned the fossils and compared the gaps in reconstructed skulls where jaw muscles would go to those of present-day mammals with known bite forces. Bite force measurements indicate that, pound-for-pound, Didelphodon had the strongest bite force of any mammal that has ever lived. In addition to the bite force, Didelphodon‘s canines were similar to living felines and hyenas — suggesting they could handle biting into bone, biting deep and killing prey. Its shearing molars and big rounded premolars, combined with powerful jaws and jaw muscles, indicate it had a specific niche in the food web as a predator or scavenger capable of crushing hard bone or shells, and was capable of eating prey as big as it was — even possibly small dinosaurs.

“I expected Didelphodon to have a fairly powerful bite based on the robust skull and teeth, but even I was surprised when we performed the calculations and found that, when adjusted for body size, it was capable of a stronger pound-for-pound bite than a hyena,” said Vander Linden, who is now a graduate student at University of Massachusetts Amherst. “That’s a seriously tough mammal.”

Co-author , former 91̽��biology graduate student and now a visiting assistant professor at Bucknell University, also examined “microwear” patterns, or tiny pits and scratches on the specimens’ teeth, to indicate what the animals were eating as their “last suppers” a few days before the animals died. By comparing the microwear patterns from Didelphodon to the teeth of other fossilized species and current-day mammals from the Burke’s mammal collection, Calede found Didelphodon was an omnivore that likely consumed a range of vertebrates, plants and hard-shelled invertebrates like mollusks and crayfish, but few insects, spiders, earthworms or leeches.

“The interesting thing about these fossils is that they allowed us to study the ecology of Didelphodon from many angles,” said Calede. “The strength of the conclusions come from the convergence of microwear with bite force analysis, studies of the shape and breakage of the teeth, as well as the shape of the skull as a whole.”

The newly-described skull features on these fossils also provided clues that help clarify the origin of all marsupials. The team found five major lineages of marsupial ancestors and marsupials themselves diverged in North America 85 to 100 million years ago. Marsupial relatives also got larger and ate a wider variety of foods, which coincides with an increase in diversity of other early mammals and flowering plants. Most of this North American diversity was then lost gradually starting 79 million years ago, before abruptly dropping during the 66 million years ago that also killed the dinosaurs. Around this time, marsupials’ diversity and evolution shifted to South America.

“Our study highlights how, despite decades of paleontology research, new fossil discoveries and new ways of analyzing those fossils can still fundamentally impact how we view something as central to us as the evolution of our own clade, mammals,” said Wilson.

Other co-authors were Eric Ekdale with San Diego State University and John Hoganson with the North Dakota Geological Survey. The research was funded by the 91̽�� and the National Science Foundation.

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For more information, contact Andrea Godinez at burkepr@uw.edu.

Adapted from by the Burke Museum.