One of the oldest known mammalian fossils belongs to a small furry species, Hadrocodium wui, that lived 195 million years ago. For the following 100 million years, only a small number of groups of mammalian fossils are found in the fossil record. By 65 million years ago, the non-avian dinosaurs were extinct. By 55 million years ago, there were 130 mammal genera, including 4,000 different species that occupied a wide variety of habitats.

Which of the following best describes the cause of the rapid increase in the number of mammalian species between 65 and 55 million years ago?

Question

One of the oldest known mammalian fossils belongs to a small furry species, Hadrocodium wui, that lived 195 million years ago. For the following 100 million years, only a small number of groups of mammalian fossils are found in the fossil record. By 65 million years ago, the non-avian dinosaurs were extinct. By 55 million years ago, there were 130 mammal genera, including 4,000 different species that occupied a wide variety of habitats.

Which of the following best describes the cause of the rapid increase in the number of mammalian species between 65 and 55 million years ago?

BlackTom AI · Accepted Answer

Mass extinction removed dominant competitors (non-avian dinosaurs), opening many ecological niches and permitting the adaptive radiation of surviving mammal lineages.

1. Relevant knowledge points and concepts
- Mass extinctions remove dominant clades and reduce competition, producing ecological opportunity.
- Adaptive radiation: rapid speciation from a common ancestor into many forms to exploit different niches.
- Speciation rate increases when selection acts on existing variation in a context of new available niches; fossil counts (genera, species) reflect diversification over time.

2. Step-by-step reasoning
- Timeline: Hadrocodium ~195 Ma; few mammal groups persist for the next ~100 million years. Non-avian dinosaurs go extinct by 65 Ma. By 55 Ma (10 million years later) there are ~130 mammal genera and ~4,000 species occupying many habitats.
- The abrupt loss of dinosaurs at 65 Ma removed major competitors/predators, creating vacant niches (herbivore/omnivore/carnivore roles, arboreal, aquatic, burrowing habitats).
- Mammals already had traits (endothermy, dentition diversity, small body sizes) that allowed rapid exploitation of these niches.
- With ecological opportunity, selection favored diversification in form and behavior, accelerating speciation rates consistent with adaptive radiation and producing the observed explosion in genera and species within ~10 million years.

3. Calculations / intermediate steps
- Time window for rapid increase: 65 Ma to 55 Ma = 10 million years.
- Outcome in that window: ~130 genera and ~4,000 species, a dramatic increase compared to the prior 100+ million years of low diversity.

4. Why the correct answer is correct
- The pattern—rapid diversification soon after a mass extinction—is the textbook signature of adaptive radiation due to newly available ecological niches. The timing directly links the dinosaur extinction to the mammal diversification.

5. Why other options are incorrect
- Option A (increased detectability due to larger populations): Detectability/fossil sampling cannot explain a genuine biological explosion to thousands of species in a short interval; the pattern matches diversification, not just discovery bias.
- Option B (slow geographic dispersal leading to allopatric speciation): Allopatric speciation requires barriers and isolation; it is generally slower and cannot account for the rapid, clade-wide diversification immediately after 65 Ma.
- Option D (waiting for random mutations over 100 Myr): Mutations accumulate continuously; rapid speciation is driven by selection and ecological opportunity, not just time for random mutations.

6. Conclusion
The adaptive radiation of mammals following the dinosaur extinction best explains the rapid increase in mammalian species between 65 and 55 million years ago. This matches the provided correct answer: After the dinosaur extinction, many ecological niches became available, leading to the adaptive radiation of mammals.

Similar Questions

There are many large mammals on Earth today. This is because:

Rapid evolutionary diversification within one lineage, producing many descendant species that have adapted to a wide variety of habitats. This is best applied to ….

The phenomenon that causes a single ancestral species to evolve into a wide array of descendant species with diverse habitats, forms, or behaviors that occurs often as a result of colonization of a new area is known as radiation.

Which diagram correctly illustrates the concept of adaptive radiation?

A period of mass extinction is often followed by ______.

Diversity increases rapidly following a mass extinction because...

More Practical Tools for Students Powered by AI Study Helper

Homework AI Solver

Stylized AI Paper Writer

Plagiarism Checker Assistant

Citation AI Academic Writing Tool

In-Class Translation Assistant

AI Note Generator

AI Quiz Answers

Past Exam Questions from University Test Bank

Smart Practice Assistant

Adaptive Practice

Making Your Study Simpler