I misread "This section [...] illustrate[s] how replication and distributed transactions have been layered onto our Bigtable-based implementation." in the paper as meaning that Spanner is partly layered upon BigTable, but what it really means is that the implementation is based upon (as in, inspired by) BigTable.
Spanner actually has its own tablet implementation as part of the spanserver (stored in Colossus) and does not use BigTable. I've amended the diagram to reflect this.
I misread "This section [...] illustrate[s] how replication and distributed transactions have been layered onto our Bigtable-based implementation." in the paper as meaning that Spanner is partly layered upon BigTable, but what it really means is that the implementation is based upon (as in, inspired by) BigTable.
Spanner actually has its own tablet implementation as part of the spanserver (stored in Colossus) and does not use BigTable. I've amended the diagram to reflect this.