Course:EOSC311/2023/What the fossil record tells us about animals on earth

Introduction to animals and geography

Figure 1 Cretaceous fossils showing how animals can be preserved in the rock record

Animals and geography might appear to be on opposite ends of the scientific spectrum. One group involves living creatures, while the other does not necessarily. With that being said, fossils link the two together by showing us what animals were alive during specific times and the conditions of their habitable environments. To elaborate on this, we will be looking at the K-Pg (Cretaceous-Paleogene), formerly known as the K-T (Cretaceous-Tertiary) extinction event and what the relative boundary in the fossil record can tell us about the animals and environment at that time.

In this project, I explore how my major, applied animal biology, relates to geography through the use of fossils. Particularly, how deceased animals can be discovered through fossils and how we can understand their behaviours as well as living habits based off of where they are found within the fossil record. To do this, I focus on the Cretaceous-Paleogene mass extinction, where the extinction of the dinosaurs is thought to have occurred, through the finding of fossils in the K-Pg boundary.

K-Pg/ K-T Extinction Boundary

The Cretaceous-Paleogene extinction took place 66 million years ago between the Cretaceous and Paleogene periods, with the Cretaceous period being the last of the Mesozoic Era, and the Paleogene marking the beginning of our current, Cenozoic era. A study done on fossil bones in North Dakota proposed that this event occurred during spring in the Northern Hemisphere.

This mass extinction is best known for its wipe-out of the dinosaurs, as they were present during the Mesozoic Era, and became extinct in the Cenozoic. Many authors suggest that the evolutionary history of surviving mammals following the extinction event has likely been shaped by the effects of this transition, as it included the altering of the environments land, ocean, and atmosphere. This event was initially recognized over one hundred years ago, as there a significant change in fossil deposits on either side of the geological boundary separating the Cretaceous-Paleogene periods was found.

Figure 1 Cretaceous-Tertiary (Palaeogene) boundary clay depicting a clear difference in substance between the two periods

To understand how science has broken down this extinction event, one must understand the concept of fossilization. Fossilization can be defined as traces of life being preserved in the geological time record (rocks and sediments) and the processes that lead to this. The processes include how their remains, being made up of bones and or tissues, were deposited in sediment. Hard structures such as bones fossilize better longterm than tissues, causing an inherent bias in the record towards hardbodied creatures such as vertebrates. Fossilization of animals can occur, telling us in what period and era they lived, as well as the conditions that they lived in. Moreover, when a fossil ceases to appear in the record, it is likely that the presence of that species ended in the period it was last found. For example, if Bird A fossils were found in Period A, B, and C, and with none in period D, it is likely that Bird A became extinct in period C.

Possible Causes for the K-Pg Mass Extinction Event

In general, while dinosaurs are the best known for becoming extinct due to the K-Pg extinction, other mammals, marine creatures, funghi, bacteria, and plants were also severely affected by the event. With that being said, although a large meteor impact is the leading believed cause of the extinction, other possible or additional causes have been suggested, including volcanic eruptions or tsunamis which may have contributed to the lack of habitable environmental conditions.

To begin, the Chicxulub impact crater is the most prominent evidence for a meteor causing the extinction event. This crater was found off the Gulf Coast of Mexico in a Yukatan Peninsula by a group of scientists including Luis Alvarez, his son Walter Alvarez, Helen Michel, and Frank Asaro. The team discovered the clay boundary was made up of high levels of Iridium, an element primarily found in meteorites and non-earth rocks. This boundary containing Iridium was dated to have formed around 66 million years ago, consistent with the K-Pg extinction timing. Additionally, throughout the last one million years of the Cretaceous period, there has been no other significant impact found to have occurred besides the Chicxulub meteor.

Another respected proposition for the mass extinction event is several successive volcanic eruptions occurring in India, creating the Deccan Traps. These eruptions could have altered the environment by producing excess gasses and soot, making the environment unliveable for many creatures. It has been proposed that the Chicxulub meteor impact in fact triggered the eruptions leading to the Deccan Traps, also known as the deccan basalt larval floods.

Last to mention, sedimentary rocks found off the Gulf Coast of Mexico may have indicated the likelihood of a tsunami event, as the deposits carried clues towards a mangrove-like ecosystem.

Regardless of the primary cause of the environmental change, that lead to the demise of an abundance of species, the effects are thought to be as follows.

The climate and sea level are thought to have been altered immensely. The proposed asteroid effects are that sunlight would have dimmed immediately following the impact, due to the absorption of solar radiation by aerosols and ejected particles within the atmosphere. In addition, this lack of sunlight would have led to a colder environment short term, which would prevent photosynthesis from occurring.

Figure 2: Map depicting the location off the Gulf Coast of Mexico where the Chicxulub crater impact was found.

Several forms of carbon in the atmosphere directly after the impact or eruptions suggest largescale fires occurring, crossing continents. These forms include fusinite, particulate carbon soot, carbon dioxide, aerosols, plant debris, and charcoal. Evidence for these fires, generated by the impact has been recovered from sediments in the boundary layer.

K-Pg Boundary Fossils

As mentioned, while dinosaurs are popularly known to have been eradicated during this extinction, they were not the only victims. Research has shown the likelihood of non-avian mammals being more susceptible to the effects of the substantive environmental change, as well as non-marine creatures and arboreal species,  seeing as they have been found more in the boundary. Generally, it has been extremely rare to find any mammalian fossils directly following the K-Pg boundary, up to the first million years after.

To begin, semi-aquatic, as well as fossorial creatures who burrow were predicted through theoretical research to have had better chances of survival due to their protective substrate and living habitat. These habitat shields (water, ground), may have been able to help protect these animals from the short term effects of the Chicxulub meteor, including fires, heat pulses, and shock waves. With that being said, over 90% of plankton in the ocean is thought to have become extinct, leading to a substantial disruption of the oceanic food chain. Furthermore, it is thought that animals living in the water column were more likely to become extinct compared to those who habituated the ocean floor, possibly due to the proximity of the immediate meteor effects. Specifically, there has been an abundance of jawed fish fossils found across the boundary, which strengthens our understanding of extinction patterns within marine vertebrates.

As for arboreal and nonarboreal mammals, it has been found that fossilized arboreal mammals (those who live amongst trees) have not been found after the K-Pg boundary, suggesting their extinction during the main event. On the contrary, nonarboreal mammals are not as prevalent in the boundary, suggesting their likely higher chances of survival. This could be explained by the previously mentioned fires that not only burned through continents, but produced toxic gasses and sediments in the air and contributed to the sudden and short change in temperature.

In regards to the most prominent extinction of the dinosaurs, there are several theories about what occurred up to the extinction event, with some scientists believing dinosaur fossil fragments were already on the decline up to the boundary. Intriguingly, just prior to the Cretaceous-Paleogene extinction, during the late Maastrichtian stage, an abundance of diverse dinosaur fossils were alive. This includes Triceratops, Tyrannosaurus, and Ankylosaurus. This suggests that conditions were favourable, and food was plentiful up until that point. With that being said, palaeontologists agree that following the Cretaceous-Paleogene boundary, dinosaur fossils cease to be found. Evidence for this includes, primarily, dinosaur fossils being found across this boundary, such as in Western North America, and not after. Another interesting finding is that in comparison to prior boundaries such as the Jurassic era, the Late Cretaceous era boundary has included a significant amount (187) of sites holding fossilized dinosaur eggs.

In general, all groups of species were affected no matter their habitat or qualities. To elaborate, evidence that suggests herbivorous species suffered during the extinction event comes from the fact that even 1.7 million years following the impact, plants did not fall victim to predation by insects, meaning plant species thrived and diversified.

All mammals present during the Cretaceous period were affected, with some experiencing extirpation in certain regions, though many did survive. A characteristic found to have aided in the survival of a species was having a reduced body size. Moreover, species that did survive were found to have shrunk, following the impact event. A possible explanation for this is that being smaller (comparable to the size of a rat) would have allowed these animals a better chance of finding shelter.

What Came After?

Fossilization of animals can not only tell us around when a species became extinct, but it can indicate clues of evolution. In fact, the K-Pg extinction had the lowest amount (<10%) of background extinctions compared to the other four major extinction events. As a note, the other well known extinctions occurred at the end of the Trassic, Permian, Devonian, and Ordovician Periods.

Evolution came following the disruption of well established niches which were then, emptied. For instance, adaptive radiation occurred within many mammalian species, mostly smaller insects and omnivores. This is the abrupt and abundant growth into new, morphologically different species(wiki).

Returning to arboreal species, studies done on postcranial morphology have shown that species such as stem primates (Purgatorius, Torrejonia) became specialized to live in trees following the impact event. It is also considered that they may have retained this characteristic across the K-Pg boundary.

Finally, in terms of fossils and what they can tell us about evolution, studies suggest that the growth of modern birds began at the K-Pg boundary, after the largescale extinction eradicated archaic birds and they were no longer found after the event. This may have not only prompted but allowed modern birds to evolve.

In general, fossils provide palaeontologists (those who study fossils) with great insight into the conditions and dynamics in the past environment, although it is difficult work as fossils do not always form perfectly, nor do they form for every species (invertebrates especially).

Conclusion and Final Thoughts

As an applied animal biology student, it has been interesting learning about the difficult and tedious research done in order to understand how dinosaurs and other species became extinct.

I had never actually considered how they knew when dinosaurs went extinct, or any other primitive animal in fact. I thought it was extremely interesting learning about the bias in the fossil records in favour of vertebrate creatures, although this is not intentional, as bones understandably fossilize better than tissues found in invertebrates. It makes me wonder what animals could have been alive that we are simply unaware of due to the gaps in the fossil records, or if there palaeontologists are aware of certain species that had to have lived, but cannot know what exactly they are due to the lack of fossils. I did find it a bit difficult to find a source that described in detail the fossils found at the boundary, other than the species named, and dinosaur eggs being discussed. Overall, learning about the K-Pg extinction was fascinating, especially all of the different theories on what occurred, as it is difficult to determine a singular cause for an event that occurred 66 million years ago.

References

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