Exercises: The geological time scale

Order these units of the geological time scale from longest to shortest duration: epoch, eon, period, and era. Give a real example of each.

Name the three eras of the Phanerozoic eon, list them in order from oldest to newest, and cite a characteristic group of living organisms for each.

Place each of these events into the corresponding era and, if possible, period: the extinction of non-avian dinosaurs, the appearance of the genus Homo, and the "Cambrian explosion" of animals.

The geologic time scale has intervals of very unequal duration, unlike the hours of a clock. Explain why its boundaries are placed at events (extinctions, changes in faunas) and not at equal intervals of time.

Name the four eons of Earth's history, order them from oldest to most recent, and indicate in which one we live and since when, approximately.

What event marks the beginning of the Phanerozoic eon and of the Cambrian period? Explain what the "Cambrian explosion" was and what change it represents in the fossil record.

Place the Cretaceous period in context by answering the following questions:

1. a) To which geological era does it belong?
2. b) Between what approximate ages (in millions of years) does it span?
3. c) What event brought it to an end?

Order these milestones from oldest to most recent and express their age in millions of years: the formation of the Earth, the appearance of Homo sapiens, the Cambrian explosion, the extinction of the dinosaurs, and the origin of life.

The Phanerozoic eon began $541$ million years ago, and Earth formed about $4\,600$ million years ago. Calculate what percentage of Earth's history the Phanerozoic occupies and explain why the previous eons (the "Precambrian") are much less well understood.

Explain why the boundaries between eras on the geological time scale coincide with mass extinctions, and use the Mesozoic-Cenozoic boundary as an example, stating what caused it.

The diagram summarizes the major intervals of geological time (eons and eras) with their boundary ages. Name the interval marked with "?", state between which ages it extends, and cite an important biological event that occurred during it.

The end-Permian extinction was the greatest in history: the trilobites and the vast majority of marine species disappeared. Explain which era it brought to a close, which groups benefited from the ecological void that followed, and what relationship this crisis has with the beginning of the age of the dinosaurs.

The Mesozoic extends from $252$ million years ago to $66$ million years ago. Calculate its duration and what percentage of the Phanerozoic ($541$ million years) it represents, rounding the result.

If we assign $1$ year to every $100$ million years, the Earth would be $46$ years old. Calculate at what "age" of this planet-person the dinosaurs appear ($230$ million years ago) and Homo sapiens ($0.3$ million years ago), and comment on what the analogy reveals.

Imagine that the history of the Earth ($4\,600$ Ma) is compressed into a one-calendar-year timeline ending at midnight on December 31.

1. a) Calculate at what approximate date and time you would place three fundamental milestones: the Great Oxidation Event (~$2\,400$ Ma), the appearance of the first macroscopic fossils with shells or skeletons in the Cambrian explosion (~$541$ Ma), and the origin of the genus Homo (~$2.5$ Ma). Show the proportional calculations carried out.
2. b) A popular science documentary states: "The history of the Earth is fundamentally the history of land animals and plants, which have populated the planet since its origins, culminating in the arrival of human beings." Based on the temporal results obtained in the previous section, refute this statement point by point and argue why the geological time scale invalidates any anthropocentric or zoocentric reading of Earth's time.

The background extinction rate throughout the Phanerozoic is estimated to have been between 0.1 and 1 extinction per million species-years (E/MSY). During the great end-Permian crisis (~$252$ Ma), around $95\ \%$ of marine species became extinct within an interval of about $60\,000$ years. Currently, estimates place the extinction rate caused by human activity between 100 and $1\,000$ E/MSY.

1. a) Based on these data, compare the average extinction rate in the Permian with the current rate. Explain why the contemporary crisis is classified as a potential "sixth extinction" even though the absolute number of extinct species documented by the IUCN in recent centuries is much lower than those of the five great geological extinctions.
2. b) Analyze the taphonomic biases of the marine fossil record that complicate direct comparison between historical diversity curves (such as Sepkoski's) and modern terrestrial biodiversity censuses.
3. c) Predict what characteristic biofacies and lithofacies a paleontologist would find 50 million years from now in sediments corresponding to our era if the current pace of anthropogenic transformation continues.

In popular science debates, the longevity of non-avian dinosaurs (~$160$ Ma) is often compared with that of our species (Homo sapiens, ~$0.3$ Ma) to evaluate "evolutionary success."

1. a) Identify and deconstruct the conceptual and taxonomic error of comparing the duration of an entire higher-ranking clade (Dinosauria, with hundreds of successive genera and lineages) with the longevity of a single biological species. Compare the current age of Homo sapiens with the average longevity of a mammal species in the Cenozoic fossil record (~$1$ to $2$ Ma) and assess whether our species has reached its expected evolutionary lifespan or not.
2. b) Explain two macroevolutionary mechanisms (adaptive plasticity, trophic diversification, and ecological redundancy) that allowed dinosaurs to recover and radiate after the Triassic-Jurassic mass extinction, and compare them with the intrinsic vulnerability of a current, ultra-homogenized biosphere that relies on a single dominant species.
3. c) Formulate two reasoned evolutionary hypotheses about the stratigraphic fate of humanity over the next 5 million years: one considering cladogenesis/speciation and another considering terminal biological extinction without descendants, detailing in each case the selection pressures and isolation conditions that would make it possible.

The post-mass extinction stratigraphic record shows that, after the end-Permian collapse, the biosphere took between 5 and 10 million years to recover baseline levels of taxonomic richness and structural complexity. Throughout the Lower Triassic, a widespread "Lilliput effect" is observed, along with the massive proliferation of "disaster taxa" such as the synapsid Lystrosaurus and a prolonged "reef gap" of several million years without coral bioconstructions.

1. a) Reason ecologically why the recovery of diversity does not occur in an immediate explosive manner despite thousands of ecological niches being left empty, detailing the role of the degradation of biogeochemical cycles and the breakdown of multilevel trophic webs.
2. b) Explain the physiological and ecological strategy adaptations (r- vs K-strategists, generalists vs specialists) that turn organisms like Lystrosaurus into the dominant species of a planet in crisis and cause the temporary reduction in individual body size (Lilliput effect).
3. c) If the current anthropogenic extinction were to collapse coral reefs and tropical rainforests, estimate how many human generations ($1\ \text{generation} \approx 25\ \text{years}$) would be required for the biosphere to regenerate ecological structures of equivalent complexity based on a recovery scale of 5 million years. Draw a conclusion about why viewing conservation decisions from the perspective of geologic time undermines the viability of short-term management.