Is having a master's thesis important for US PhD applications in linguistics? I'm a recent BA graduate. In my field (linguistics), it seems like most successful applicants to PhD programs at top schools have Master's theses, and writing samples in the field are probably the most important part of the application. My writing sample will probably be only 20 pages long at best, even though I think it's really interesting and good.

Am I at a disadvantage? Would it be better for me to get a Master's before trying again? Or do they only just care about the kind of research that you do?
Ph.D. student in linguistics here.

I'd say "probably not," especially if you and your educational background are both American as well. Stand-alone MA programs in linguistics in the U.S. are difficult to find. Where they exist, they tend to be focused on applied linguistics and education. Most domestic applicants will have only undergraduate degrees. Focus on demonstrating that your research interests are well-defined, that you have a good sense of how to pursue them, and that you have a rock-solid understanding of why the University of X would be a great match for you. Don't worry too much about the writing sample; mine was just a small course paper that the professor liked.

That being said, it can't hurt to have an MA from a reputable school, and aiming for that might make sense as a backup plan. I've seen several people move to Canada or Europe to do an MA in theoretical/quantitative linguistics, then come back to the U.S. for a Ph.D. One of these ended up saying that their MA experience north of the border was something they'd wholeheartedly suggest to people having trouble getting admitted to Ph.D. programs in the U.S. I've heard that there are even a few schools up there that provide funding for international MA students in linguistics, though I'm not sure which ones they are. Worth looking into, though!