M1M-316 was a moon in the Pegasus Galaxy, which held a Stargate. This world was intended to be the Alpha Site, as the Atlantis database said that it was safe enough. However, when Sgt. Stackhouse and Major John Sheppard's team came to investigate, they discovered that the planet was inhabited by Tyrannosaurus-Rex creatures. So, they abandoned this world. (SGA: "The Siege, Part 1")
An abandoned Ancient lab was eventually discovered on this moon along with various plant life. Dinosaurs weren't the only creatures there and Dr. Geisler known that the creatures are all transplanted from Earth and were indicative of an Ancient presence in the Pegasus Galaxy millions of years before Atlantis traveled there. It was later determined that the Ancients had done so to study telepathy and empathy genes in the dinosaurs as a way to Ascension but abandoned the experiment. As it was the only known source of ginkgo trees which could activate a Wraith gene in humans, it was ordered destroyed by the International Oversight Advisory. Desperate not to let this world be destroyed, the expedition faked its destruction and its survival remains a closely guarded secret. (SGA: "Blood Ties")