Question

Increased diversity of siphonate bivalves occurred in response to

• A. the Cambrian explosion in the Paleozoic
• B. increased temperature in the Cenozoic
• C. increased predation pressure in the Mesozoic
• D. increased oxygen level in the Proterozoic

Answer 1

The Correct Option is C

The question addresses the increased diversity of siphonate bivalves as a function of environmental factors. Let's explore the given options to identify the correct factor responsible for this increase in diversity:

1. The Cambrian explosion in the Paleozoic:
   • The Cambrian explosion was a pivotal event approximately 541 million years ago characterized by the rapid appearance of most major animal phyla. It occurred in the Paleozoic era. While it marked an increase in biodiversity, it is not specifically linked to the diversity of siphonate bivalves.
2. Increased temperature in the Cenozoic:
   • The Cenozoic era, starting around 66 million years ago, has seen various climatic shifts, but the focus here is not on temperature influencing bivalve diversity.
3. Increased predation pressure in the Mesozoic:
   • The Mesozoic era, spanning from about 252 to 66 million years ago, is noted for the evolution of new predators. This era saw a significant increase in shell-crushing predators and other new threats, contributing to adaptive radiations among bivalves. Siphonate bivalves, with extended siphons, could burrow deeper into substrates as a defense mechanism, leading to their increased diversity as a response to predation pressure.
4. Increased oxygen level in the Proterozoic:
   • The Proterozoic eon, occurring from 2.5 billion to 541 million years ago, witnessed significant atmospheric oxygenation events. However, this eon is much earlier than the radiation of bivalves, and thus not directly linked to siphonate bivalve diversity.

Conclusion:
The correct answer is increased predation pressure in the Mesozoic. This explanation is justified by the evolutionary adaptations bivalves underwent due to novel predation threats during that era, specifically leading to diversity in siphonate bivalves.