In most cases, those are not “Jain–specific” symbols, but a local / Hindu custom that you are seeing. Let’s separate it clearly:

1. General Indian / Hindu belief about sea shells

At many temples (especially near the sea), real or carved sea shells (shankh) are placed:

• as an auspicious symbol of purity and prosperity
• as a protector of the entrance, believed to keep away negative energies
• because the conch (shankh) is connected with the sacred sound and used in puja, so its form is considered holy and lucky

Sometimes they are simply used as decoration material in coastal areas, because shells are easily available and look beautiful on steps and walls.

2. From a Jain dharmic viewpoint

Jain Dharma is very strict about ahimsā (non‑violence) and avoiding any material that comes from killing living beings.

• A living shell (sea animal) is a trasa jiva (mobile being), so harming or killing it is himsa.
• Therefore, deliberate use of real sea shells as a religious item is not in line with Jain principles, especially if it encourages their collection or trade.

In old or mixed‑tradition temples:

• Sometimes earlier builders followed local (non‑Jain) customs or used whatever material was common in that region.
• If the structure is already built, Jains usually do not damage it, but new Jain constructions ideally avoid such materials.

3. So, why do you see them on the entrance step?

Most likely:

• It is a local decorative or Hindu custom, symbolising auspiciousness and protection,
• Not something specifically prescribed in Jain āgamas or Jain temple–shastra.

In short: Sea shells at the entrance are usually for beauty and local religious belief in good luck and protection, but from a strict Jain point of view, using real shells (taken from killed sea creatures) is not encouraged according to ahimsā.