They were probably drawn to the decaying bodies stuck in the asphaltum and then got stuck themselves.
他们可能被卡在沥青中的腐烂尸体所吸引，然后自己也被卡住了。
英语百科
Gilsonite
（重定向自Asphaltum）
Gilsonite, also known as uintahite or asphaltum, is a bitumen-impregnated rock (asphaltite) mainly found in the Uintah Basin of Utah and Colorado, United States. It is a naturally occurring solid hydrocarbon bitumen. Although it occurs also in other locations, its large-scale production occurs only in the Uintah Basin.