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Mammal Milestones

Shundong Bi, professor of biology

Shundong Bi, professor of biology. Photo: Keith Boyer

Fossils Offer Clues to Animal Origins

In more than a decade with the IUP Biology Department, paleontologist Shundong Bi has nurtured countless students in exploring the development of life on Earth.

He has fanned their flames of interest in human anatomy, challenged them to explore how not just humans but all life forms of today came to be.

Some students Bi has counted as teammates and included in his celebrated discovery of a milestone in the evolution of life on Earth.

As a partner in an international study of fossils in his native China, and with support from students at IUP, Bi has reported in research papers published in the journal Nature evidence that pinpoints the distinct developments of some of the earliest mammals in the world.

The fossils, uncovered about 200 miles from Beijing in northeastern China’s Liaoning province, led to discovery of a creature that lived about 126 million years ago and that is now one of the earliest known placental mammals of the Cretaceous Period.

This squirrel-like animal about 10 inches long that is believed to have survived on a diet of insects has been named Ambolestes zhoui.

In his research published in Nature in June, Bi reported that a complete fossilized skeleton of the Ambolestes shows features of the ear and neck that are signatures of placental mammals, giving scientists a better idea when placental and marsupial mammals first developed.

It was Bi’s fourth published report in Nature.

“Every year, maybe six papers from paleontologists are published in Nature,” Bi said. “It’s a top journal. So, a lot of researchers are very impressed, because for 100 years researchers have argued, ‘What did the placental mammals once look like?’ Now, the complete skeleton of Ambolestes affords an amazing window on the early evolution and lifestyle of the mammal group that would become dominant after the demise of the dinosaurs 66 million years ago.”

Bi’s mission to fix the time of the first mammal began in 2012. Since then, he has recruited students with scientific talents to support his ongoing research.

A fossilized skeleton of the Ambolestes zhoui, which lived about 126 million years ago.

A fossilized skeleton of the Ambolestes zhoui, which lived about 126 million years ago. Photo courtesy of Shundong Bi.

During studies that identified the Ambolestes, Bi’s team included Natalie Cignetti, of Indiana, who graduated from IUP in May with a degree in natural science, a pre-med concentration, and minors in chemistry and studio art.

So valued were her contributions to the Ambolestes research that Bi named Cignetti among his five coauthors for the paper published in Nature.

Cignetti joined Bi’s research team as a sophomore, while taking his course in anatomy, and she employed her artistic talents to do anatomical reconstruction.

“It was the first time I had ever seen anatomy in my life,” Cignetti said. “My main job was to analyze these fossils with him and then try and draw them out and try to make them easier for people to look at.

“I think it helped me to see anatomy from a different perspective, and it really forced me to think creatively about anatomy and picture it in different forms. Anatomy is a tough concept for a lot of people. You study it from a textbook, and it’s hard to transfer that information from a 2-D piece of paper to real life.”

After Cignetti finished Bi’s course, she stayed on with the study strictly as an extracurricular activity. She said that in the summer of 2017 she was paid from a grant for 200 hours of work.

Each year, Bi travels to China for fossil digs in cooperation with local museum researchers. The areas they target have yielded a bounty of full-body remnants—complete skeletons of animals that lived in Jurassic times about 190 million years ago and in the earlier Triassic era.

“It’s a hotbed. They call it the ‘treasure box’ for the number of fossils showing the origin of birds and the origin of mammals,” Bi said.

Studying the remnants of ancient animals from IUP, however, presents a distinct challenge.

The fossils found in China have to stay in China. After spending his summers collecting and studying fossils there, Bi returns each year to the US with extensive notes, photographs, drawings, and computed tomography images showing multiple cross-sections of the fossils.

The fossil shows an even older mammal—a euharamiyid from roughly 165 million years ago

The fossil shows an even older mammal—a euharamiyid from roughly 165 million years ago. Photo: Keith Boyer

That information and a digital art program helped Cignetti visualize what the fossilized animals looked like when they were alive.

“On every layer of the CT scan I would have to draw in and outline the fossil,” she said. “And eventually when I got through all the layers, the computer program put this three-dimensional image together, and it gave us a better idea of what it would have looked like.”

Over the years, Bi said, research has shown the development of three major groups of mammals on Earth today—placentals, marsupials, and monotremes—and how they evolved in relationship to each other.

The groups are distinguished in the way the young of their species develop. Placentals include humans and others whose babies stay in the womb until birth. Marsupials, like kangaroos, koalas, and opossums, keep their young during gestation outside the body in pouches.

“Ninety-nine percent of mammals are either placental or marsupials,” Bi said. The final group, monotremes, represents the duckbilled platypus, the only mammal known to lay eggs and hatch its young after true external gestation.

Explorations have turned up many varieties of reptiles and newer mammals, but Bi’s search has been for the animals with the general appearance of reptiles but with signs that they have completed a change and fit the definition of mammals.

“As a paleontologist, the first question we ask is, ‘Where did the first mammal originate? Where did it come from?’”

The creatures in transition Bi calls near-mammals. “It has some mammalian features, but it has not quite evolved into a mammal yet,” he said.

Fur or hair is a telltale factor. The patterns and formations of teeth also are factors. So is the structure of the ear.

“In the reptile, the inner ear bones, such as the anvil and hammer, are connected to the lower jaw. They don’t have good hearing, because they’re connected, and when they chew, it affects their hearing,” Bi said.

Also, mammals have an abdominal diaphragm, but reptiles do not.

“As mammals, we can run and breathe at the same time,” Bi explained. “But for the reptiles, they cannot run very fast and cannot run a long distance, because it’s too hard to breathe.”

Those characteristics became evident in one of the first mammals identified in Bi’s research, the euharamiyida, which points to the earliest divergence of mammals from reptiles more than 200 million years ago. Bi reported that research in the journal Nature in 2014. He worked with a fossil from Jurassic China that dates back about 160 million years.

Euharamiyida was a category of mammals on the same level as placental and marsupial, but that category has gone extinct. 

“The traditional view was that the first true mammals evolved sometime during the Jurassic. Euharamiyids, while roughly mammal-like, were thought to fall outside the group,” Bi said.

Natalie Cignetti with Shundong Bi at her graduation in May illustration of what fossilized animals, such as the Ambolestes looked like when they were alive

Natalie Cignetti with Shundong Bi at her graduation in May. She illustrated what fossilized animals, such as the Ambolestes (right) looked like when they were alive. Illustration and photo: Natali Cignetti

“With complete skeletons to work from, however, our study found that euharamiyids were true mammals after all. And, given the age of the earliest known haramiyid [the mammal class that included euharamiyids], mammals originated at least in the late Triassic, between 220 million and 201 million years ago.”

While Bi, who divides his time between research and classroom teaching, offers his students in-depth experience in paleontology, he said few who come through his department are looking for careers in his subjects.

“Most of the students go on to nursing school or medical school,” he said.

One of those is Cignetti, now enrolled in North Carolina’s Wake Forest School of Medicine.

“When I was interviewing for different medical schools, I noticed that one of the main things they ask you about is research,” Cignetti said. “They want to see if you have research experience, that you’re learning how to apply principles of research, and that you’re trying to think creatively, too. I think that was one of the strongest parts of my application.

“But this research experience really helped me to think more creatively about anatomy and to understand it at a deeper level.”

While Bi continues his mission to unravel the mysteries of early mammals, he is guarded in describing where his studies will turn next.

“My ongoing research is planning to further calibrate the age of Juramaia [an extinct, shrew-like mammal of 160 million years ago], then test when the placental mammals originated,” he said. To say much more would be to tip his hand to competing researchers.

Count Cignetti among those eager to read his next study.

“I got better at researching as time went on—from watching him do it and getting guidance from him,” she said. “Maybe I won’t be able to apply my knowledge of squirrels in the medical field, but watching Dr. Bi work and seeing how he researched—that’s something that’s going to help me the rest of my life.”