Interesting evidence shows that the rathas were indeed earlier in chronology as compare to the Kailasanatha temple at Ellora. The fact that prominent elephant and lion base at Ellora has been "discovered" here at Mahbalipuram. This of course is restricted only to what is called the Draupadi ratha and the Arjuna ratha.
Here is some details on the Rathas themselves:
Draupadi ratha: Attributed to the goddess Durga, as its re-inforced by the lion vahana on the outside, this ratha follows pure bengal style of rural architecture. It represents a hut, has the goddess enshrined inside the main and only sanctum. Interestingly, the devotees of this goddess, are shown in the act of almost chopping off thier heads as a mark of adouration.
Arjuna ratha: A miniature version of the dharmaraja ratha, and a true picture of dravida style of architecture, this monument is dedicated to shiva. For one, the nandi is carved right next to it, and secondly the iconography of shiva is extremely rich along its walls.
Bhima ratha: An interesting structure, this clearly is a prototype of later gopurams. Judging by the way its been constructed, it was likely to be dedicated to vishnu, as the rock seems to be prepared for carving a reclining vishnu, as seen in other cave temples at Mahabalipuram as well as shore temple. This would indicate that the unfinished rock next to it was either for a boar, as seen in shore temple next to the vishnu shrine or for garuda, the actual vahana.
Dharmaraja ratha: its all too clear. the iconography speaks for itself, so much so that the nandi was not carved next to this big shrine. the vimana, an elaborate dravida style roof holds a whole catalogue on shiva iconography. This was clearly attributed to Shiva.
Nakula shahadeva ratha: built to the style of a buddhist chaitya, but attributed to Indra, since we have the elephant vahana next to it, this temple seems to be greatly influenced by the only cave temple at besra. the style and motifs clearly indicate strong influences from the north.
More coming later.