The Gheranda Samhita describes shalabhasana as lying face down with the chest and belly against the ground, the palms planted, and the legs lifted behind. Shalabha is the locust or grasshopper, and the raised legs behind a body pressed to the earth are the insect's hind legs.
The posture is not in the Hatha Yoga Pradipika. It belongs to the set of four prone forms that the later manual adds, with the crocodile, the camel and the serpent, and those four together do more than any other group to distinguish the two texts. The earlier manual's fifteen postures are seats with two arm balances and a reclining rest; the later manual's thirty two open the body out onto its front and add kneeling and standing work as well.
That widening is a fact about the texts rather than an explained development. Whether the Gheranda Samhita records practice that already existed when the Pradipika was compiled, or practice that had grown up in the interval, is not settled by anything in the manuscripts, and no evidence has been identified that decides it. The atlas states the distribution and marks the question as open rather than offering a narrative to cover it. Mallinson and Singleton's Roots of Yoga presents the primary material in translation and is the basis on which the comparison can be made.