If I remember correcly the oil pan is a structural part of the engine.
Aluminum, unlike steel has a definate fatigue life. The webs add torsional ridgidity to the pan, without them the pan would have little structural signifigance. They also help to increase surface area helping to dissipate heat.
Thats a fatigue crack, it occured at a change in cross section of the material. It may be linked to a casting flaw.
If you remove the pan and clean it, you should be able to find a shop to weld it for you.
