Two late Vedic ideas flow into Vishnu. The first is the Purusha of Rigveda 10.90, a giant with a thousand heads and limbs whom the gods offer up; from his body come the four orders of society, from his mind the moon, from his eye the sun, and from him the Vedas. The second is Narayana, whose name the lawbook of Manu explains from the waters, his first resting place. The Narayana Sukta of the Taittiriya Aranyaka names him the supreme reality and speaks of a highest station that later Vaishnavas read as Vaikuntha. Put together, they become the god who lies on the waters at the start of every age.
The idea takes stone form as the reclining Vishnu. At Saranga in Odisha a ninth-century relief more than fifteen metres long, cut in the open air under the Bhauma-Kara kings, shows Narayana lying on the serpent with Brahma seated on the lotus that rises from his navel. Shesha is explained as time; Lakshmi sits by the feet.
What is different here:
An iconographic statement rather than a narrative; it joins the navel-lotus story to the Narayana cosmology in one image.
In the Narayaniya section of the Shanti Parva, Narayana speaks of himself as the beginning and the ending of everything, and the three gods of the later trinity are all placed within him. This is the epic's own statement of Vishnu's supremacy and should be attributed to Vaishnava theology.
What is different here:
Adds the explicit claim that Brahma, Vishnu and Shiva are contained in Narayana; the Vedic hymns make no such ranking.