Ammonite Crabs are hands down the best npc to afk on pures at the moment.
FYI anything underlined in this post brings you to the related Wiki Page if you want more info on something.
I. Info about the crabs
Here's a comparison between Sand Crabs and Ammonite Crabs
Their stats almost exactly the same except for HP 60 vs 100
Sand Crabs have some 4-spawn locations (4 x 60) = 240 HP to train on.
Ammonite Crabs have some 3-spawn locations (3 x 100) = 300 HP to train on.
Another benefit is that since there are only 3 crabs per spawn you will be tanking less crabs than at the Sand Crab 4 spawns. This means you take less damage overall and can stay longer than at Sand Crabs.
Bone Voyage QuestMust have sailed to Zeah at least once (if you were training at sand crabs that is already done)
Dig Site Quest10 Agility
III. How to Get to Fossil Island
When getting your kudos you should find the "clean necklace" if you turn this in to the archaeologists you unlock the ability to make the Digsite pendant which has (5) charges. Simply buy some Ruby Necklaces and cast "Lvl-3 Enchant" on it to make the Digsite pendants.
When you rub this it brings you to the barge that takes you to fossil island.
There are 4 3-spawns on the island that I know of they are marked on the map below in purple.
These crabs have a larger agro radius than rock/sand crabs so it can be hard to see exactly where to stand, use these pictures to see where to stand.
IV. Bonus Info
Rock Crabs / Sand Crabs / Ammonite Crabs all attack with crush.
Having as high as possible crush defense will help you stay longer without needing to eat as often.
Helpful Melee Tops
Varrock Armor 1 Castle Wars Top Iron Chainbody Iron Platebody
> > >
Helpful Melee Helms
Castle Wars Halo Bearhead Coif Iron Fullhelm
> > >
Helpful Shield Slots
Unholy Book Book of War Iron Kite shield
* - Book of war is only better when the +2 Str bonus gives you an extra max hit.
I recommend checking your max hit at either a POH combat dummy or using a max hit calc.
I hope this was helpful if you want me to add something or remove something let me know