Some of the important fossils which have been studied are those of organisms X, Y and Z. X were marine arthropods which were common between 400 to 600 million years ago. Y were the invertebrate animals (molluscs) with a flat, coiled, spiral shell which lived in the sea about 180 million years ago. Z are the extinct carnivorous or herbivorous reptiles which appeared on the earth about 250 million years ago and became extinct about 65 million years ago. What are X, Y and Z ?

Answer

To solve the question regarding the identification of organisms X, Y, and Z based on their fossil records, we can follow these steps:

### Step-by-Step Solution:

1. **Identify Organism X**:
- The description states that X were marine arthropods that existed between 400 to 600 million years ago.
- Based on this information, we can conclude that X refers to **Trilobites**. Trilobites were prevalent during the Paleozoic era and are well-known marine arthropods.

2. **Identify Organism Y**:
- The question states that Y were invertebrate animals (molluscs) with a flat, coiled, spiral shell that lived in the sea about 180 million years ago.
- Given the characteristics of having a flat, coiled, and spiral shell, we can identify Y as **Ammonites**. Ammonites are a group of extinct marine mollusks that had distinctive spiral shells.

3. **Identify Organism Z**:
- The description for Z indicates that these were extinct carnivorous or herbivorous reptiles that appeared on Earth about 250 million years ago and became extinct about 65 million years ago.
- This timeline and the classification as reptiles point to Z being **Dinosaurs**. Dinosaurs dominated the Earth during the Mesozoic era and are known for their diverse forms, including both carnivorous and herbivorous species.

### Final Answers:
- **X**: Trilobites
- **Y**: Ammonites
- **Z**: Dinosaurs