Q.What were the characteristics of life forms that had been fossilised?

The key to understanding which life forms get fossilised lies in the conditions required for preservation. Fossilisation is a rare event, not a common one. Most organisms simply decay or are scavenged after death.

For an organism to become a fossil, it typically needs hard parts like bones, teeth, shells, or woody tissue. Soft-bodied organisms, such as jellyfish or worms, decay too quickly to leave a trace in most environments. The organism must also be buried rapidly by sediment (like mud, sand, or ash) soon after death. This quick burial protects the remains from scavengers, weather, and oxygen, which slows decay. …

The story of fossils is really the story of what survives the long, slow journey through time. When we talk about the characteristics of life forms that became fossils, we are essentially asking: what kind of organism had the best chance of leaving its mark in stone? The NCERT textbook makes it clear that the vast majority of fossils we find come from organisms that lived in water, especially in seas and oceans. Why? Because the process of fossilisation requires rapid burial by sediment — mud, sand, or silt — and that happens most easily under water. A dead organism on land is far more likely to be eaten, decayed, or weathered away before it can be buried.

The single most important characteristic of a fossilised life form is that it possessed hard parts. Think of bones, teeth, shells, or the tough outer covering (exoskeleton) of creatures like crabs or trilobites. Soft tissues — skin, muscle, organs — decay very quickly after death. Unless conditions are extraordinarily rare (like being trapped in amber or frozen in ice), soft parts simply vanish. So the life forms we find as fossils are overwhelmingly those with durable, mineralised structures. This is why we have a rich fossil record of ammonites (with their coiled shells), dinosaurs (with their massive bones), and ancient fish (with their teeth and scales), but almost no fossils of jellyfish or worms.

Another crucial characteristic is that these life forms were abundant and widespread. A single, rare animal living in a remote forest has almost no chance of being fossilised. But a species that lived in huge numbers across a large area — like a common shellfish on a vast seafloor — has many more opportunities for some individuals to be buried and preserved. This is why the fossil record is biased toward marine creatures: the oceans were (and are) teeming with life, and the constant deposition of sediment on the seafloor creates ideal conditions for burial. …

Approach it as a filter: start from 'what happens to a dead organism by default' (decay and scavenging), then ask what conditions must hold to stop that default outcome — a durable body part, fast burial, and low-decay conditions …