I’ll answer in two ways: 1) according to modern science (geology / biology), and 2) briefly how Jain Dharma looks at time and “beginnings”.

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1. “When was Jainism founded?” (Which era?)

From the Jain point of view:

• Jain Dharma is anādi – beginningless, eternal. It is not “founded” at one point in time.
• In every cosmic half‑cycle, 24 Tirthankars appear. In our current descending half‑cycle (avasarpinī), Bhagwan Ṛṣabhdev (Ādināth) is the first Tirthankar and is said to have lived in the distant past, long before recorded human history.
• So, in Jainism, we do not say “Jainism started in X era” the way we say for some historical religions. It is considered eternal dharma which appears again and again in each time‑cycle. ( jainknowledge.com)

From a modern history viewpoint (not the Jain theological view):

• Scholars usually place historically visible Jain community at least by around 1st millennium BCE (roughly Iron Age in India), but this is only about the traceable community, not the Jain scriptural claim of beginningless dharma. ( en.wikipedia.org)

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2. When did the first signs of life appear on Earth? (Scientific era)

According to modern science:

• Earliest simple life (like bacteria) is thought to have appeared in the Archaean Eon, roughly 3.5–4.0 billion years ago.
• This is long, long before any complex plants, animals, or humans.

Jain view is different: time is cyclic and beginningless, and there is no single first moment when jīvas (souls) or life “started.” Life is always present in some form somewhere in the universe. ( jainknowledge.com)

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3. When did the first humans (Homo erectus) appear? (Scientific era)

In modern paleoanthropology:

• Homo erectus appears in the Pleistocene Epoch of the Quaternary Period, roughly about 1.8–2.0 million years ago, and survives until a few hundred thousand years ago in some regions.

Jain scriptures do not speak in terms of “Homo erectus” and modern species names; instead they talk of aras (eras) in the time‑cycle, with humans of very different lifespans and body sizes in different aras.

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4. When did dinosaurs first walk on Earth? (You mentioned Cretaceous)

In modern geology:

• Dinosaurs first appear in the Triassic Period (late part of it), around 230 million years ago.
• They then dominate through the Jurassic Period and much of the Cretaceous Period.
• So, they did not start in the Cretaceous; by the Cretaceous they were already long‑established.

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5. How Jain time‑eras compare (very briefly)

Jainism does not use “Archaean / Triassic / Pleistocene” etc. It uses:

• An endless time‑wheel (kālacakra) with

- Avasarpiṇī (descending half) - Utsarpiṇī (ascending half)

• Each half has six eras (ārā), from Sukhama‑sukhama (maximum happiness) to Dukhama‑dukhama (maximum misery). ( jainknowledge.com)

According to Jain cosmology:

• We are currently in the 5th āra (Dushamā / Dukhama) of the Avasarpiṇī in our region of the universe. ( jainknowledge.com)

You can read a very simple explanation of Jain time‑cycles and eras here