There are at least two panels featuring the Sakyamuni: one at the base of the second gopuram and the other in the main temple. Here, events are depicted in comic-strip style, using small sequential sculptural panels. This was an artistic convention that can be observed in the temples of the medieval period in Tamil Nadu. You can see similar story-telling miniature sculptures in other temples also. It could be an episode from mythology or depicting a historical event. At the Vaikuntha Perumal temple in Kanchipuram, certain events from Pallava history are told in the manner, while at the Gangaikondacholapuram temple, the story of Bhagiratha is depicted similarly.
The second set of Buddha figures is in the body of the main temple, on the right balustrade of the step leading into the sanctum and on the southern side. There are three sculpture pieces here. The first one shows Buddha seated under a tree, flanked by royalty. The gandharvas are depicted on the upper part of the frieze. The next one shows Buddha standing under a tree, and royalty worshipping him. Behind them are the gandharvas, also in a posture of supplication.The Buddha at Thanjavur
In that article there is mention of Buddhism flourishing in the coastal region during the Chola period and Jainism in the delta region. In Kerala there were many Buddhist temples many of which were later believed to be converted to Hindu Temples. The most famous of them is Ayyappa temple at Sabarimala.