KEY TAKEWAYS Spirit Airlines shares are plunging more than 60% in premarket trading on a report that the budget carrier is getting ready to file for bankruptcy protection after revived merger talks with Frontier Airlines parent Frontier Group Holdings collapsed.According to The Wall Street Journal, Frontier has pulled out of merger talks with Spirit, which has also been in advanced talks with bondholders to work out a bankruptcy plan."A bankruptcy filing is expected to happen within weeks," the report said. ...