Uttana kurmasana follows the cockerel directly in the Hatha Yoga Pradipika, and the text describes it as coming out of it. Holding the lifted lotus of kukkutasana, the practitioner takes hold of the neck with the hands and rolls back onto the spine, so the body ends up turned over with the legs still bound.
Uttana means stretched out, extended, or turned face upward, and the last sense is the operative one: this is the tortoise inverted. Both surviving manuals carry the posture, which places it in the shared stock rather than among the later additions.
The chain is what makes the entry unusual. Padmasana becomes kukkutasana, kukkutasana becomes uttana kurmasana, and each step is stated by the text rather than inferred. That is three postures in a row where the arrangement of the legs never changes and the relationship to the ground does, and it is the longest such chain anywhere in the classical corpus. Reading the three entries together shows a compiler working from performance: nobody assembling a list from written sources would organise it that way, and nobody who had not done the postures would notice that one converts into the next by a single movement.
The word uttana appears twice in the classical posture vocabulary, here and in the stretched frog of the Gheranda Samhita, and in both cases it qualifies an animal name already in use rather than introducing a new image. That consistency is a small sign of a vocabulary being maintained deliberately: a reader who has learnt what uttana does to one name can read the other without further explanation, which is exactly the economy an orally transmitted technical list needs.