For everyone looking for answers about the spine at the base of the tree:
It is parrallel to Nidhoggr of Norse myhtology. A Serpent/Dragon that gnaws at the root of the great tree of life. Counter acted by an eagle who resides at the top (yet to be seen in AoT) and the squirrel that connects the worlds of those two (likely the beast titan, Zeke).
It is also said that it will rise to aid the giants at the end of the world, Ragnarok. It symbolizes a loss of honor and villainy.
Quote: NÃðhöggr or Nidhogg (Malice Striker) is a powerful dragon that is found in Norse Mythology. The big mighty dragon lives in the roots of the great ash tree Yggdrasil which holds the 9 realms of Norse cosmology together. NÃðhöggr also chews on the roots of the tree for all eternity with hopes to damage it and topple it. He has a rivalry with an unnamed eagle that lives at the top of the World Tree and the two exchange harsh words and messages through Ratatöskr, the giant mischievous squirrel, that bears the words between the two. It is said that the NÃðhöggr will go off to Ragnarok, the Norse Judgement day to aid the giants in their battle against the gods.
Depictions of Nidhoggr: