The Rise of Mammals: Out with the Old and in with the New

Unlike what may have been thought previous to 1824, mammals were no new creation. At their most primordial form, they have existed since a little over 200 million years ago following the end of the Permian period and into the Triassic. For mammals, it was through essential physiological changes that allowed them to survive each new natural disaster that had ended the existence of many other species.

Fossils found in Fleming Fjord in Greenland shed some light on which and when traditional mammalian traits existed. The fossils showed that the small rodent-like creatures had fur, better chewing ability, and also a shift in tooth development from continual tooth growth cycle to only having one set of adult teeth after losing baby teeth. These traits combined with many others allowed mammals to survive natural disasters such as the end of the Triassic period when the splitting of Pangaea caused volcanoes to to erupt and poison the environment. When events like these occur, killing most of the world’s population, the animals that survive have an automatic advantage for the next period with ample resources. With these new resources, the original mammal populations experienced adaptive radiation and had a burst of evolution. Following the large evolution burst, fossils in China have shown just how varietal mammals had become as they developed in their ecological niches.

Mammal species continued to grow at a fast pace as resources were abundant and evolutionary changes allowed them to take advantage of even more resources. A pivotal change in mammal history was the involvement of angiosperm plants which provided food for mammals all over the world, but mainly only helped the populations which had developed tribosphenic molars, which was just another innovation for chewing. All of these new changes came at a great time as survival ability would be essential to survive the Cretaceous. The catastrophes that so infamously killed the dinosaurs is in reality what has allowed for the modern mammal to develop. Though not all mammalian species survived the period, research shows that the ones that did, the earliest placentals were the species that most closely relates to us today as young are given birth to when they are relatively well-developed.

Many people do not know the many aspects of the rise of mammals. For many it is mystery how and when the earliest mammals existed and how populations were able to to develop into mammalian species today. This article provided a lot of insight for myself and helped me better understand the traits that play the most pivotal role in survival. Though I had learned before that the death of the dinosaurs was what helped the rise of mammals, it was only what allowed them to become the main planetary “players”. It was the advantageous traits that allowed them to survive to that period in the first place and continue developing species that exist today.

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The Ascent of Mammals

I found it interesting when the article explained that young who lack teeth or have baby teeth are better able to nurse from their mothers. I initially did not know that this was the reason behind why some organisms have baby teeth that are shed later on to grow adult teeth. I never put much thought into this quality of some organisms, and when I did, I thought that the reason why some organisms lack teeth or have baby teeth when they are young was simply because full-sized adult teeth would be too large to fit in their jaw and never even considered the advantages that baby teeth would have for the organism. I feel like before I read this article, when I thought about evolutionary fitness, I tended to think more about the qualities of adult organisms that were beneficial. However, after reading about this aspect of evolution, I realized that advantageous traits do not apply exclusively to organisms that are fully developed — fitness applies to an organism and its ability to survive throughout its entire life, including its developmental stages.

I really enjoyed the part of the article where they discussed the different shapes of teeth and their evolutionary advantages. I think that the way teeth have changed as speciation and “branching out” has occurred over time is interesting, because they take on so many different forms in organisms today. For instance, a horse’s teeth are very different from those of a cat, but teeth overall still serve the same function (chewing) for many animals. The article discussed how ancient mammals had different types of teeth that functioned best based on their diet. Agilodocodon had “spade-shaped” teeth that allowed it to be better suited for eating tree sap. Such adaptations benefited mammals because there were a variety of mammals with different phenotypes that allowed them to occupy specific niches, which often meant a reduced level of competition for resources such as food. Though they are not mammals, the idea of selection for phenotypes beneficial for certain niches reminded me of saltwater sheepshead fish, who have “human-like” teeth to better serve the purposes of their omnivorous diet, such as crushing shells or consuming plant matter. Could this possibly be an example of evolutionary convergence, where these fish developed similarly shaped teeth to those of humans, who are also omnivorous? I thought that the part of the article that discussed evolutionary convergence was very interesting. The article mentioned many ancestors of modern mammals and compared them to beavers and rats in order to describe them, but also compared some ancient mammals and modern mammals that were not related to each other. For instance, the article mentions multituberculates, who were bucktoothed and looked like rats, but were not related and “converged on a rodentlike body plan” due to eating a similar diet to that of modern rodents. I feel that being able to draw such a close comparison to unrelated organisms is fascinating because it truly supports the idea of evolution. It shows that organisms really can and do adapt to their environmental conditions, and thus animals with similar roles in their environment can end up with similar physical qualities. Before reading the article, I didn’t put that much thought into the idea that unrelated organisms who occupy similar niches can take on such similar body forms, but after reading the article, I realized how logical it is for this to happen based on the idea of natural selection for traits that are best suited for certain niches in certain environments.

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A Tale of Teeth

For humans, maintaining a set of adult teeth is very important.  Young humans have baby teeth, which are replaced by adult teeth a bit later in life.  If an adult tooth is lost, a new tooth does not grow back in its place.  Teeth are important for chewing food, which fuels daily life.  While still useful today, teeth have played a historically important role in the flourishing of mammals.  For humans, maintaining a set of adult teeth is very important.  Young humans have baby teeth, which are replaced by adult teeth a bit later in life.  If an adult tooth is lost, a new tooth does not grow back in its place.  Teeth are important for chewing food, which fuels daily life.  While still useful today, teeth have played a historically important role in the flourishing of mammals.

In the Triassic period, animals called cynodonts replaced teeth throughout their lives.  On the other hand, cynodonts’ descendants, stem mammaliaforms (the closest relatives of true mammals), began exhibiting the pattern of baby teeth being replaced by adult teeth.  This pattern was advantageous as young stem mammaliaforms could nurse on their mother’s milk with baby teeth.  This allowed them to grow faster, have a higher survival rate, and attain higher metabolisms that allowed for survival in colder environments. Additionally, stem mammaliaforms had cusps on their molar teeth at the backs of their jaws, which made chewing more efficient.

By the Cretaceous period, therians, descendants of stem mammaliaforms, had made a new advancement in terms of teeth.  Tribosphenic molars, pairs of molars that worked like a mortar and pestle, proved advantageous in crushing and chewing food.  With tribosphenic molars, therians had a wider range of foods to eat and began to diversify.  Still, the Cretaceous period was dominated by more primitive mammals with more primitive teeth… until angiosperms evolved.  Angiosperms, which include most of today’s plants, were able to be consumed by therians due to their tribosphenic molars.  More primitive mammals without tribosphenic molars died out in the Cretaceous.Besides therians, other mammal groups were evolving more complex teeth.  Multituberculates, which resembled rodents, had molars built for slicing plants that evolved to be more complex just as angiosperms spread.   A group called the gondwanatherians had teeth that resembled those of modern-day horses and cows: high-crowned molars that had enamel that grew during life.  Gondwanatherians’ teeth were also efficient at chewing plants.

Although teeth weren’t the only factor that helped mammals become prominent, they did help mammals along the course of history.  The pattern of having baby teeth allowed mammals to nurse, which led to new advancements.  The trait of cusps on molar teeth allowed for better chewing.  Then, more improvements to molars allowed greater diversification.  By the time an asteroid struck the Earth and led to drastic environmental changes, mammals were ready for the final part of their glorious ascent.

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Ascent of Mammals

In the article Ascent of Mammals it begins with a breakthrough is evolutionary science that is often a lost one. While Buckland unveiled the first dinosaur, Megolosaurus, the larger evolutionary find that day was two tiny mammal jaws. This changed the viewpoint of mammals because scientist believed that mammals were a recent creation that never would have appeared alongside giant salamanders and lizards of the primeval geologic eras.

From Bucklands discovery of the mammals jaws, others such as Farish Jenkins began to search for fossils that would illustrate the ancestors of mammals. What he found was fossils showing notable differences from cynodonts. While cynodonts had teeth that constantly fell out and regrew, stem mammaliaforms had teeth that were similar to us today. Their teeth followed a similar pattern of baby teeth that later on gets replaced by adult teeth which is one of the signature features of mammals. Young with no teeth or baby teeth are able to nurse on their mother’s milk that is produced by the mammary gland. The evolutionary advantage was allowing for better survival of the young since it allows the young to grow faster.

Studies conducted by Luo and others, including a team led by Jin Meng of the American Natural History Museum in New York City, found that mammals had remarkable variety which allowed them to invade an array of niches. However the most interest of them to me was the Castrocauda it’s features are very similar to the modern semi-aquatic mammals of today such as beavers and platypuses. Their webbed hands and feet stood out to me because this seemed to be one of the earliest mammals that was adaptable to water compared to other mammals during that time that were mostly ground or tree dwelling.

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The Scale of the Ascent of Mammals

What interested me the most about the rise of mammals is the pace at which evolution can occur. Throughout the article I was baffled by the sheer scale of time- 12,000 years ago humanity began to develop civilizations, so even one million years is an unfathomable amount of time for humans, let alone hundreds of millions of years. Thus, I was amazed by how long it took for the early proto-mammals to develop what I considered some of the most basic traits of mammals. For example, traits such as the tribosphenic molars made for grinding plant material is one of the reasons mammals flourished even in the time of the dinosaurs, as they benefited from the rapid explosion of flowering plants. Yet, this type of tooth developed roughly 145 million years ago, and scientists know that the stem mammaliaforms existed roughly 210 million years ago. I had never considered the fact that seemingly small adaptations would take so long to develop and be selected for, or the fact mammals could have even existed before flowering plants covered the Earth.

The point in the article that shocked me the most was the explanation of how the diversity of mammals exploded after the extinction of the dinosaurs roughly 66 million years ago. Nearly every other descriptor of time had been on the scale of tens, if not hundreds, of millions of years. In addition, before this time every early mammal looked vaguely shrew-like (with some notable exceptions, such as volaticotherium, which looked like an ancient sugar glider). In class we learned that as competition in an environment decreases, the selective pressure on the members of that environment decrease, leading to more diversity. However, the 500,000 years after the death of the dinosaurs still managed to astound me; with the dramatic competition reduction, mammals managed to split into different sizes, with creatures ranging from additional shrew-like insectivores, to cow sized plants eaters, to carnivores with saber-like teeth. This occurred in half of a million years, while the adaptation for molar teeth did not appear until sixty five million years into the development of mammals.

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Rise of the Mammals

It was very interesting to read mammals had not started out post-dinosaur age like we had thought before. Instead, mammals have been around since the time of the dinosaurs. Granted, those back then were super small and didn’t have much diversity; but now we know that due to angiosperms, the dinosaur-killing asteroid, mammalian ability to adapt to difficult environments and the development of teeth all led to drastic changes that led to us, mammals, dominating for the last 60+ million years since the dinosaurs last roamed the Earth. It was also very interesting to see how the evolution of teeth and angiosperms went hand-in-hand to influence how we developed. The way that our teeth developed to chew on those angiosperms for energy was cool to see. And yet, to see how the main reason that we flourished was because the dinosaurs couldn’t adapt quickly enough to the environment altered by the asteroid crash. By looking at history such as this shows how everything can change so quickly! We evolved so fast after the fall of dinosaurs and that led to our such great diversity! This leads me to believe how one day we humans might not around at all might suffer the same fate as the dinosaurs and some other group of species would rise, ready to take on the challenges of the new environment, or perhaps by then we have conquered the science of space travel and have expanded on to other planets. Only the future will tell us which one of my guesses are correct.

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 What role did “teeth” play in the evolution of mammals? What are the evolutionary advantages of being born without teeth, adult teeth vs baby teeth, molars, etc…?

From the very first mammaliaforms from the Triassic to modern day humans, teeth have been found to serve wide range of motions that have allowed our ancestors to forage and feed on food sources previously unavailable to them. Molars, combined with a hinge joint and strong jaw muscles, allowed for stem mammaliaforms to chew more efficiently. Yet, each of the mammalianforms had distinct differences in the function of their teeth. Morganucodonts had large jaws that were able to crush through the exoskeletons of large beetles, while Kuehneotheriids has delicate teeth better suited for worms and other soft insects. The function of protomammals teeth also allowed them to survive during the Mesozoic era by feeding on small insects and other available food sources in a dinosaur-dominated era.

As the Cretaceous Period displayed the beginnings of the modern mammal, the development of tribosphenic molars allowed the lower and upper molar to work together in crushing food, which proved advantageous when angiosperms evolved. Tribosphenic molars allowed mammals to bite and chew fruits, flowers, and insects, while mammals with teeth made for eating dinosaurs went into extinction. After the asteroid that decimated the dinosaurs in the Cretaceous Period, Mammals with more specialized diets and teeth unfit for the change in the environment caused their extinction as they could not adapt to a new diet.

Being born without teeth or baby teeth leaves offspring to reliant on their mother’s milk for nutrition, which provides nutrients to their offspring in a more efficient way that also increases their chances of survival. Later on, adult teeth prove to be more advantageous than baby teeth as they tend to be stronger and better suited for crushing and biting food sources. The formation of molars allows food to be broken into smaller particles, which increases surface area and thus the energy source is digested at a faster rate. Canines allow an organism to rip and tear food, with carnivores having large canines for tearing muscle. Herbivores have incisors – flat straight-edged teeth – to cut through plants. As omnivores, humans have all three tooth types, allowing for a greater variety of food sources.

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