User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130704185128/@comment-3146930-20130704190434

The ones in The Titan's Curse were summoned the same way, using dragon teeth to bring them to life. Gray also seems to be able to pull rib bones out like the ones in TTC. It could possibly be a small difference between Greek Spartus and Roman Spartus.