'Extant architectural remains' in the opening paragraph of the passage refers to

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From the seventeenth century onward India witnessed the development of its classical architecture in stone. Uptill now the extant architectural remains had overwhelmingly been in the form of monoliths, caves, and buildings cut out of living rock. This mode of architecture continued to create its great monuments at Ellora and Elephanta. But the emphasis was now on stone-block construction.
In the fifth century, Bhitargaon in Northern India had represented a philosophy: a brick temple with a tower, but with a true arch over the doorway. It proved, however, to represent a mere passing phase. The arch, vault and dome, as well as mortar cement, remained strangers to Indian masons till the thirteenth century. As a result, when they built on a large scale, they were obliged either to have towering roofs, the walls tapering to meet at top, or to instal thick pillars, in order to bear the weight of heavy beams. Recourse had then to be taken to using larger and larger solid stone blocks. The inevitable effect of the resulting massiveness was sought to be relieved by a profusion of sculpture with which these structures were increasingly endowed.
The Brahmanical temple constitutes the principal product of this tradition of monumental architecture. Its sanctuary comprised the cella, containing the idol or symbol of the deity and the spire over it. A pillared hall was attached to the sanctuary, with sometimes an intermediate chamber, transepts and porch. Jain temples followed a similar plan.
Two styles, the so-called ‘Indo-Aryan’ (Northern) and ‘Dravidian’ (Southern) have been discerned within this broad tradition of temple architecture. The ‘Aryan’ shikarasare simple ribbed spires, the ‘Dravidian’, terraced towers; the ‘Aryan’ roofs tend to be flat or pyramidal, the ‘Dravidian’, more commonly globular. Both these styles are present in early Chalukya architecture of the Deccan. At Ellora, in Maharashtra, was created the greatest building in India ever cut out of living rock. Cutting down vertically a rectangular space of rock, over 90 m. by 53 m. in area, the shikara being so carved as to be about 30.5 m. high, the rock-cutters scrupulously followed the design of a stone-structured building, with ‘Dravidian’ features.