BILLINGS, Mont. — Animal rights advocates announced a lawsuit against the federal government on Wednesday in a bid to make a Montana mustang population the first group of wild horses to be protected under the Endangered Species Act.

The Pryor Mountain Wild Horse Range just north of the Wyoming border is home to about 155 mustangs believed to be descended from the mounts of Spanish conquistadors who came to North America in the 1500s.

Attorneys for Friends of Animals argued in their lawsuit that the US Fish and Wildlife Service violated federal law by failing to act on a petition filed last June seeking protections for the animals.

The move comes two years after federal wildlife officials rejected a proposal from the group for protections for tens of thousands of mustangs on federal lands across 10 Western states.

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