Factors in the Rise of Mammals

Teeth have played an important role in the evolution of mammals as they help determine the specific diet organism can have. Those who are born without teeth are often those who grow a single set of baby and eventually adult teeth. Being born without teeth proves advantageous as it allows organisms to nurse on the milk of their mother. By nursing, it allows for faster growth and higher metabolisms which enables activity in cold, darker environments.

The earliest forms of mammals experienced this as having teeth and jaw-closing muscles proved to be more efficient as opposed to the dangerous method of biting and swallowing foods. Cusps on molars as well as tribosphenic molars aided in this as the ability to grind and tear apart foods prevented choking. Those whose teeth best suited the available food sources in various environments had the most fitness and best chances of survival. This led to these traits to be selected for in future mammals.

Along with the variation in teeth and in turn diet, the article mainly attributes the rise of mammals to several environmental occurrences. Mammaliaform forerunners first originated at the start of the Triassic period, following a mass-extinction set off by a volcano which wiped out many giant amphibians and reptiles. The Triassic was eventually superseded by the Jurassic when Pangea split and ecosystems began to collapse.  Certain mammaliaforms survived the event and prospered as they were able to inhabit niches others could not. However, dinosaurs still dominated during the Jurassic and Cretaceous periods until an asteroid triggered extreme, long-term climatic change. Dinosaurs were ill-suited for the change in environment, unlike a few species of early mammals which evolved into those we see today.

Evidence of evolution can be derived from fossil records of early mammals in the Jurassic period. One I found most interesting includes the Agilodocodon as it helped scientists see an early form of arboreal life. Each fossil found from the Jurassic period proved to be interesting as characteristics of each organism can be seen in organisms alive today. For example, the Castorocauda had a beaver-like tail, the Volaticotherium can be likened to a flying squirrel, and the Fruitafossor had teeth like those of an armadillo.

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Ascent of the Mammal

The Rise of Placental Mammals

The pace of evolution that is responsible for the rise of placental mammals is gradualism. Gradualism is defined by small and gradual changes that accumulate into significant phenotypic differences over long periods of time. The earliest known placental mammals have been dated back to the start of the Cretaceous period, about 145 million years ago. They can be traced back to a singular species, the therians, who began to diversify as they developed more robust molars. This was the start of the process known as speciation, the divergance of biological lineages and the emergence of reproductive isolation between those lineages. The therians splintered off into placentals and the metatherians who started down separate evolutionary paths. The placentals began to splinter off into major modern subgroups by the end of the Cretaceous period, which included rodents and primates. We know this is gradualism because of the long periods of time (about 145 million years) for placentals to look completely different from the species they both originated and deviated from. The metatherians developed into marsupials, the common ancestor of koalas and kangaroos. We know primates (descended from placental mammals) are a different species from marsupials because we wouldn’t be able to produce viable, fertile offspring with one another, and therefore a result of speciation.

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Teeth Are Key (to the Rise of Mammals)

Teeth in early mammal forms were revolutionary, they enabled mammal young to nurse on their mother’s milk while they had no (or baby) teeth. This allowed them to grow faster, survive more often and live in cold environments due to higher metabolisms. Teeth (specifically molars) and their corresponding jawbone allowed mammals to consume a wide variety of foods left alone by other organisms that didn’t have the biological tools to do the job. This remarkable evolution is what allowed mammals to survive mass extinction and rise to dominance today. The article The Ascent of the Mammals by Stephen Brusatte and Zhe-Xi Lauo discusses this and much more, touching on all major aspects of the evolution of mammals and why they came to be.

According to the article, scientists have found a wide variety of ancient mammal fossils (having lived around 160 million years ago) most abundantly in China and Colorado. What is most interesting about these finds is that they display many features of modern mammals that scientists had previously thought to have evolved much later on. What drew my attention was the Castorocauda, not only for its status as the “earliest known swimming mammal” but also for its beaver-like description. To me this seems like  evidence that evolution is moving at a much slower rate than I could have ever imagined. My doubts were lessened at the mention of the Volaticotherium which was described as a flying-squirrel without sounding too much different from one. With the evidence of these fossils and many more, scientists have determined that mammals were doing great during the Middle Jurassic with the number of species (and mutations) skyrocketing leading to the beginning of the Cretaceous when the “modern mammal blueprint” was finally created with the icing on the cake, molars.

Molars allowed mammals to branch out to even more new food sources, specifically angiosperms.  In “an event that would reset the course of mammal history” angiosperms began to populate land and provide food to those who could eat them (therian mammals with molars). Angiosperms allowed mammals to populate while other species were dwindling, an event that culminated at the end of the Cretaceous with the coming of an asteroid and total environmental change. While many mammals with specialized diets were wiped out, more dinosaurs perished allowing the quick evolution of placentals. I believe that without dinosaurs to block the sun’s rays and eat mammals that got to large, they were able to populate faster and freer than ever before. This would account for the quick evolution and following rise of placentals that followed.

Among other things, this article was very enlightening on the progression from early life forms (starting with early mammal-forms rather than the typical chemical particles stance) to present day mammals including us humans. I enjoyed reading about key mutations and being able to see some overarching evolutionary trends present throughout mammals’ time on Earth. I didn’t know that jawbones and molars were such a pivotal evolutionary trait so it was interesting to learn their evolutionary advantage. It is also very weird to think that we are only here because of one small mutation in a past life form. Imagine if past mammals hadn’t been warm blooded, grown fur or even had molars!

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Ascent of Mammals

This was a very interesting piece of writing to read because it really showed  a timeline of how all these mammals and fossils were introduced to us. I have never really had much of an interest in dinosaurs and the mammals of that era but this article really approached the discovery in a way that instantly captured my attention.

I think the thing that really surprised me was that you can actually figure out a lot just by observing the teeth that these mammals had. Looking at things like their chewing movement, or whether they even had teeth leads you to learn so many other things of how they could have lived. You can look at the efficiency of molars and see how they were beneficial in that era. I was not aware that advantageous for youngsters to not have teeth, or have baby teeth, because it allows them to nurse on their mother’s milk. Therefore, they were allowed to grow faster and attain higher metabolisms, which helped them be active in colder environments and overall gave them characteristics to survive in their environment. It’s crazy how all of this can be derived by just looking at teeth when today not many people even stop to think about how we have evolved to have the teeth that we do.

All of these observations just go on to support Darwin’s theory of natural selection. I think that the teeth structure and characteristics that were mentioned, must have proved to be advantageous for these individuals and it is really captivating at the variety of different characteristics one can have just based on their teeth.

This article also got me thinking about all the different eras their have been (Permian, Triassic, etc.) and the mass extinctions that ended them. This is something that I really hope we look further into just because I think most people just focus on the extinction of the dinosaurs but we don’t talk as much about the others.

 

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Ascent of the Mammals: Groundwork by Evolution

The ascent of the mammals was a long process indeed, one that actually contradicts many of the previous theories of how it occurred. Originally, it was believed that mammaliaforms– the closest relatives of true mammals– and eventually primitive mammals stayed low and unimpressive during the “reign” of the dinosaurs (the Triassic, Jurassic, and Cretaceous periods). However, new fossil discoveries have shed light on the fact that they were doing exactly the opposite.

During the Jurassic Period, mammaliaforms were adapting to change by using their sharp senses, fine-motor coordination, and elevated metabolism to survive the environmental conditions, including volcanic eruptions, and they were also diversifying their body types. The studies of Chinese fossil mammals showing this diversity were fascinating to me. In the studies, researchers describe finding Castorocauda, a creature with webbed hands and feet that is the earliest known swimming mammal, and in a completely different ecological niche, Agilodocodon, a tree climber that fed on sap by gnawing through tree bark. I was also amazed by the fact that the new Jurassic mammal fossils exhibit almost every way of life seen in today’s small mammals.

By the Cretaceous period, tribosphenic molars opened new opportunities for mammal diversification within therians. Evolutionary lines that lead to the modern mammal groups began to take shape, and the therians, suddenly better suited for their environment upon the evolution of angiosperms, proliferated. Here, I was struck by how intertwined the rise of therians was with that of angiosperms, as well as how the rise of the angiosperms contributed to the decline of previously-successful mammals of the Cretaceous period.

At the end of the Cretaceous period, an chance asteroid made extinct the once-prevailing dinosaurs, as well as many large mammals with more specialized diets. However, other smaller mammals– particularly placentals– were able to survive under the sudden environmental change and without the competition of the dinosaurs.These mammals split off into the major modern subgroups, including rodents and primates, and evolved quickly to fill available niches. From this, we are able to understand more clearly the role of the extinction of dinosaurs in the rise of the mammals: a placental revolution, one that was a turning point in millions of years of evolution and that has lead to many mammals as we know them today.

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The Ascent of Mammals

Molars, specifically the tribosphenic molars, were considered a game-changer. Because of how the teeth are arranged, the upper and lower molars worked together like a mortar and pestle, making it easier to break down certain foods and widening the mammal’s dietary possibilities. The versatility of the tribosphenic teeth allowed for the therians to branch off and diversify into major mammals groups that are still familiar to us today. The importance of this feature was highlighted during the Cretaceous period, in which the angiosperms appeared. The evolution of the angiosperms is acknowledged as the event that would reset the course of mammalian history; this new feeding opportunity was especially beneficial to the therians and extremely detrimental to the mammals with primitive dentition, which included dinosaurs. The angiosperm continued to spread as did the development of more intricate molars.

Although the presence of teeth was an important factor, the absence of teeth was just as important in a young’s growth and survival. During the early stages of a mammal’s life, for example, humans, babies with no teeth nurse on their mother’s milk. This was and still is especially essential for a young’s survival because it allows for faster growth and a higher metabolism, which aids in living in colder environments, especially during the night.

What I found most interesting about the article was the idea of time. It is impossible for me to imagine what millions of years, let alone thousands of years, may be like, as we humans barely live to witness a hundred years. It is amazing to think that throughout that large period of time a mammal, a size close to a rat, evolved to complex human beings. Thinking about this concept of evolution and time leads me to ponder about the period that will come after and about the next organisms that will reign over the land. Along with time, the environment is also an important deciding factor.

It is evident that the environment is absolutely critical to an individual’s survival. That is obvious in that each period is marked by some significant change in the environment due to some drastic event: the end of the Permian period was marked by a volcano-triggered mass extinction, the Triassic period moved into the Jurassic period as Pangea torn apart and the resulting volcanic eruptions poisoned the atmosphere and destroyed the ecosystem. The next organism that reign next will end up being the organisms that has the ability to adapt to the changing environment, and this idea continues to hold true for all organisms.

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Ascent of the Mammals: Our Teeth and Jaw

It would make sense for mammals to have evolved from something small to the various sizes we have to day. However, I taken by surprise by how our teeth and our jaws played an important role climbing up the biological hierarchy. The size of our jaw and the shape of our teeth were initially so fragile that some mammals could only eat insects while other mammals could only eat worms. Mammals eventually developed a jaw and teeth that could crush plants as well. If a mammal’s teeth and jaw played an important role, it makes you wonder why natural selection didn’t select for more teeth as it is more advantageous. Surprisingly, it is more advantageous to be born without teeth than it is to have the same amount of teeth as sharks. Baby mammals with no teeth or small teeth could feed on their mother’s milk, thus allowing them to become more nourished than those born with a lot of teeth.

Not only did our teeth and jaw come in different shapes and sizes, mammals were found in a variety of ecological niches. In China, they found mammals with webbed feet for swimming, diggers for living underground, and mammals that gnawed on bark for sap. They even found a mammal that flew. Besides a mammal that can fly like birds, I felt particularly interested in the mammal known as Agilodocodon that could gnaw through tree bark. Beavers are also known for gnawing through bark too make their homes, but it is interesting to hear that the Agilodocodon gnawed on bark for tree sap. What confuses me that this mammal went for sap rather than insects, plants, or worms. Instead it went for something much more difficult to attain.

Speaking of diet, the most interesting part of the article was reading about the evolution of plants. Angiosperms evolved to self-fertilize themselves which resulted in fruits, particularly fruits that animals could eat. Mammals changed their diet to fruit because it was easier to seek fruits than hunt different creatures or gnaw through tree bark. However, similar to the Agilodocodon, it amazes me that mammals thought of eating it in the first place. I wonder if they recognized it as food or if it was accidental, but proved to be nutritious food later on.

Although it our jaw and teeth played an important role throughout history– from it determining our in our diets, our ecological niches, and our overall growth– it is completely ironic that the dinosaur bone overshadowed the key to mammalian evolution.

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