You’re touching two different things here:

1. “Why no dinosaurs in Jain scriptures?”
2. “Why are all Tirthankars in the same kind of human shape if they lived in such long time‑spans – shouldn’t evolution change them?”

Let’s take them one by one, only from the Jain point of view.

---

1. Why no mention of dinosaurs?

In Jainism, the purpose of scripture is not to give a scientific history of the earth or a list of all species. Its main goals are:

• to explain soul (jīva)
• bondage of karma
• and the path to moksha.

So, Jain texts speak in broad categories of life, like:

• one‑sensed beings (plants, earth‑bodies, water‑bodies, etc.)
• two‑sensed, three‑sensed, four‑sensed beings
• five‑sensed beings (including animals, humans, celestial and hellish beings).

“Dinosaurs” would fall under five‑sensed tiryañcha (animal) beings. Jainism accepts countless species and life‑forms, but it doesn’t name every type in modern scientific terms.

So, from a Jain view:

• Dinosaurs = just another kind of animal body (tiryanch gati, panchendriya).
• Scriptures don’t deny them; they simply don’t bother to list every variety, because that’s not the spiritual focus.

---

2. Why are Tirthankars always in human form, and what about “evolution”?

(a) Jain view of time, not evolution

Jainism does not use the idea of biological evolution like modern science. Instead, it speaks of an eternal time‑cycle (kāl‑chakra) with:

• two halves: Utsarpiṇī (ascending) and Avasarpiṇī (descending)
• each half has six eras (ārās), with very different conditions of life, height, age, etc.

We are now in:

• Avasarpiṇī (descending half)
• 5th āra (Duḥṣamā) – a sorrow‑dominant period, very limited spiritual capacity.

(You can read a simple overview here )

In earlier eras of this same half‑cycle:

• People had much taller bodies
• Much longer lifespans
• More strength, more purity, etc.

This gradual decline is not called evolution; it is just the fixed law of time plus the effect of karmas.

(b) Why Tirthankars have human shape

According to Jain cosmology:

• In our region (Bhārata‑kṣetra of Madhya‑loka), Tirthankars always appear as humans.
• They arise only in specific eras (mainly 3rd & 4th āra) of each half‑cycle.
• Their outer form is human, but very ideal, symmetrical, and radiant.
• Earlier Tirthankars had huge height and very long age (measured in enormous units), but still a human‑type body – two legs, two arms, human face etc.

So:

• Form (jāti): human
• Size, age, strength, lifespan: changing according to era.

Modern idols and pictures show them with today‑scale human proportions, but that is for our understanding and worship. Scriptures themselves clearly say that the height and lifespan of earlier Tirthankars were vastly greater than ours.

(c) “If they lived for so long, shouldn’t evolution change them?”

From a Jain standpoint:

• Change in bodies over vast time is governed by karmas + era (kāl), not by natural selection as in Darwinian theory.
• For Tirthankars specifically, there is a special Tirthankar‑nāma‑karma that gives:

- human birth - perfect bodily features - certain fixed marks and qualities.

So Jainism is saying:

> In this world‑region and these eras, the law is that Tirthankars must be born as humans with a particular ideal form.

Hence, no matter how long ago R̥ṣabhdev, Ajitnath, etc. lived (according to Jain time‑scales), they are still described as human‑form beings, though with very different size and lifespan from us.

---

In simple summary

• Jain scriptures are not zoology books; they talk in big categories (animal, human, etc.), not in modern names like “dinosaur”.
• Dinosaurs, if we align with Jain categories, are simply five‑sensed animal beings (tiryanch) that existed in some period of the vast time‑cycle.
• Tirthankars always take human birth in our region, by a fixed karmic law, in specific eras of the time‑cycle.
• Their shape is human, but their height, lifespan, and powers differ greatly across eras.
• Jainism explains change through kāl‑chakra (time‑cycle) and karma, not through the scientific theory of evolution.

This way, Jainism keeps its own consistent spiritual framework, without aiming to be a detailed scientific history of all species on earth.