minecraft-src/data/minecraft/worldgen/structure_set/nether_fossils.json

{
  "placement": {
    "type": "minecraft:random_spread",
    "salt": 14357921,
    "separation": 1,
    "spacing": 2
  },
  "structures": [
    {
      "structure": "minecraft:nether_fossil",
      "weight": 1
    }
  ]
}