Dinosaur study challenges Bergmann’s rule
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NANUQSAURUS, STANDING IN THE BACKGROUND, AND PACHYRHINOSAURUS, SKULL IN THE FOREGROUND, WERE AMONG THE DINOSAUR SPECIES INCLUDED IN A NEW STUDY LED BY SCIENTISTS AT THE UNIVERSITY OF ALASKA FAIRBANKS AND THE UNIVERSITY OF READING THAT CALLS INTO QUESTION BERGMANN’S RULE.
view moreCREDIT: ART BY JAMES HAVENS
When you throw dinosaurs into the mix, sometimes you find that a rule simply isn’t.
A new study led by scientists at the University of Alaska Fairbanks and the University of Reading calls into question Bergmann’s rule, an 1800s-era scientific principle stating that animals in high-latitude, cooler climates tend to be larger than close relatives living in warmer climates.
The fossil record shows otherwise.
“Our study shows that the evolution of diverse body sizes in dinosaurs and mammals cannot be reduced to simply being a function of latitude or temperature,” said Lauren Wilson, a UAF graduate student and a lead author of a paper published today in the journal Nature Communications. “We found that Bergmann’s rule is only applicable to a subset of homeothermic animals (those that maintain stable body temperatures), and only when you consider temperature, ignoring all other climatic variables. This suggests that Bergmann’s ‘rule’ is really the exception rather than the rule.”
The study started as a simple question Wilson discussed with her undergraduate advisor: Does Bergmann’s rule apply to dinosaurs?
After evaluating hundreds of data points gleaned from the fossil record, the answer seemed a solid “no.”
The dataset included the northernmost dinosaurs known to scientists, those in Alaska’s Prince Creek Formation. They experienced freezing temperatures and snowfall. Despite this, the researchers found no notable increase in body size for any of the Arctic dinosaurs.
Next the researchers tried the same evaluation with modern mammals and birds, the descendants of prehistoric mammals and dinosaurs. The results were largely the same: Latitude was not a predictor of body size in modern bird and mammal species. There was a small relationship between the body size of modern birds and temperature, but the same was not the case for prehistoric birds.
The researchers say the study is a good example of how scientists can and should use the fossil record to test current-day scientific rules and hypotheses.
“The fossil record provides a window into completely different ecosystems and climate conditions, allowing us to assess the applicability of these ecological rules in a whole new way,” said Jacob Gardner, a postdoctoral researcher at the University of Reading and the other lead author of the paper.
Scientific rules should apply to fossil organisms in the same way they do modern organisms, said Pat Druckenmiller, director of the University of Alaska Museum of the North and one of the co-authors of the paper.
“You can’t understand modern ecosystems if you ignore their evolutionary roots,” he said. “You have to look to the past to understand how things became what they are today.”
ADDITIONAL INFORMATION: Read the paper in Nature Communications.
ADDITIONAL CONTACTS: Lauren Wilson, lnkeller@alaska.edu, 406-223-4762. Jacob Gardner, jacob.gardner@reading.ac.uk. Pat Druckenmiller, psdruckenmiller@alaska.edu, 907-474-6989. Chris Organ, Montana State University, organ@montana.edu.
NOTE TO EDITORS: An illustration is available on the UAF news website.
JOURNAL
Nature Communications
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Global latitudinal gradients and the evolution of body size in dinosaurs and mammals
ARTICLE PUBLICATION DATE
5-Apr-2024
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In the evolution of walking, the hip bone connected to the rib bones
New data from a 375-million-year-old fossil fish gives clues to changes in the axial skeleton that might have helped our ancestors climb out of the water
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NEW RECONSTRUCTION OF THE SKELETON OF THE 375-MILLION-YEAR-OLD FOSSIL FISH, TIKTAALIK ROSEAE. IN A NEW STUDY, RESEARCHERS USED MICRO-CT TO REVEAL VERTEBRAE AND RIBS OF THE FISH THAT WERE PREVIOUSLY HIDDEN BENEATH ROCK. THE NEW RECONSTRUCTION SHOWS THAT THE FISH’S RIBS LIKELY ATTACHED TO ITS PELVIS, AN INNOVATION THOUGHT TO BE CRUCIAL TO SUPPORTING THE BODY AND FOR THE EVENTUAL EVOLUTION OF WALKING.
view moreCREDIT: THOMAS STEWART, PENN STATE
UNIVERSITY PARK, Pa. — Before the evolution of legs from fins, the axial skeleton — including the bones of the head, neck, back and ribs — was already going through changes that would eventually help our ancestors support their bodies to walk on land. A research team including a Penn State biologist completed a new reconstruction of the skeleton of Tiktaalik, the 375-million-year-old fossil fish that is one of the closest relatives to limbed vertebrates. The new reconstruction shows that the fish’s ribs likely attached to its pelvis, an innovation thought to be crucial to supporting the body and for the eventual evolution of walking.
A paper describing the new reconstruction, which used microcomputed tomography (micro-CT) to scan the fossil and reveal vertebrae and ribs of the fish that were previously hidden beneath rock, appeared April 2 in the journal Proceedings of the National Academy of Sciences.
“Tiktaalik was discovered in 2004, but key parts of its skeleton were unknown,” said Tom Stewart, assistant professor of biology in the Eberly College of Science at Penn State and one of the leaders of the research team. “These new high-resolution micro-CT scans show us the vertebrae and ribs of Tiktaalik and allow us to make a full reconstruction of its skeleton, which is vital to understanding how it moved through the world.”
Unlike most fish, which have vertebrae and ribs that are the same along the length of the trunk, the axial skeletons of limbed vertebrates show dramatic differences in the vertebrae and ribs from the head region to the tail region. The evolution of this regionalization allowed the performance of specialized functions, one of which was a mechanical linkage between ribs in the sacral region to the pelvis that enabled support of the body by the hind limbs.
The pelvic fins of fish are evolutionarily related to hind limbs in tetrapods — four-limbed vertebrates, including humans. In fish, the pelvic fins and bones of the pelvic girdle are relatively small and float freely in the body. For the evolution of walking, the researchers explained, the hind limbs and pelvis became much larger and formed a connection to the vertebral column as a way of bracing the forces related to supporting the body.
“Tiktaalik is remarkable because it gives us glimpses into this major evolutionary transition,” Stewart said. “Across its whole skeleton, we see a combination of traits that are typical of fish and life in water as well as traits that are seen in land-dwelling animals.”
The original description of Tiktaalik focused on the front portion of the skeleton. Fossils were meticulously prepared to remove the surrounding matrix of rock and expose the skull, shoulder girdle and pectoral fins. The ribs in this area were large and expanded, suggesting that they may have supported the body in some way, but it was unclear exactly how they would have functioned. In 2014, the fish’s pelvis, discovered in the same location as the rest of the skeleton, was also cleaned of matrix and described.
“From past studies, we knew that the pelvis was large, and we had a sense that the hind fins were large too, but until now couldn’t say if or how the pelvis interacted with the axial skeleton,” Stewart said. “This reconstruction shows, for the first-time, how it all fit together and gives us clues about how walking might have first evolved.”
The researchers explained that, unlike our own hips where our bones fit tightly together, the connection between the pelvis and axial skeleton of Tiktaalik was likely a soft-tissue connection made of ligaments.
“Tiktaalik had specialized ribs that would have connected to the pelvis by a ligament,” Stewart said. “It’s astonishing really. This creature has so many traits — large pair of hind appendages, large pelvis, and connection between the pelvis and axial skeleton — that were key to the origin of walking. And while Tiktaalik probably wasn’t walking across land, it was definitely doing something new. This was a fish that could likely prop itself up and push with its hind fin.”
The new reconstruction of the skeleton also sheds light on specializations for head mobility in Tiktaalik and new details of the fish’s pelvic fin anatomy.
"It's incredible to see the skeleton of Tiktaalik captured in such vivid detail,” said Neil Shubin, Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago and one of the authors of the paper. “This study sets the stage for ones that explore how the animal moved about and interacted with its environment 375 million years ago."
In addition to Stewart and Shubin, the research team includes Justin B. Lemberg, Emily J. Hillan, and Isaac Magallanes at The University of Chicago, and Edward B. Daeschler at Academy of Natural Sciences of Drexel University.
Support from the Brinson Foundation, the Biological Sciences Division of The University of Chicago, an anonymous donor to the Academy of Natural Sciences of Drexel University, and the U.S. National Science Foundation funded this research. Fieldwork was made possible by the Polar Continental Shelf Project of Natural Resources, Canada; the Department of Heritage and Culture, Nunavut; the hamlets of Resolute Bay and Grise Fiord of Nunavut; and the Iviq Hunters and Trappers of Grise Fiord.
JOURNAL
Proceedings of the National Academy of Sciences
ARTICLE TITLE
The axial skeleton of Tiktaalik roseae
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