What is the career path from sales development to product marketing? Is it a direct path?

In my experience, I’ve never seen Sales → Product Marketing as a direct path, but like someone else asked about here, it’s definitely one you can pursue and I think many product marketing hiring managers would be open to it if you displayed you had the right skill set and potential. Here’s what my answer to that other question ("What is your advice to someone with a background in software sales who wants to move into Product Marketing?") was with advice for how and what to try to demonstrate to make that transition: https://sharebird.com/what-is-your-advice-to-someone-with-a-background-in-software-sales-who-wants-to-move-into-product-marketing-have-you-hired-someone-with-a-sales-background-for-a-pm-role-if-so-what-convinced-you-to-hire-that-person-what-are-the-top-skills-someone-with-a-background-in-sales-might-be-lacking-that-are-critical-for-a-pm-role-what-about-skills-that-would-translate-well?answer=DPX4gRBxzL

No, it's definitely not a direct path. SDR is the classic entry level tech job. It's good to get your feet wet. The good ones will get promoted to SDR team lead or inside rep. Many wash out and move into other industries. 

It's rare that someone would jump from SDR straight to PMM. I did pull one on to my PMM team once a few years ago, but it was almost coincidence more than anything that he was an SDR. He happened to have really good design skills from his pre-SDR life, so we made him a junior PMM focused on production. He ended up doing a great job, but I don't think his year or two in sales development did anything to help prepare him. 

I would say that SE is probably a better feeder job into PMM than SDR, so if you want an indirect path from SDR to PMM, try to spend a few years as an SE.  

I came really close to hiring an SDR into marketing not too long ago. He was known within the SDR group as "most able to get a response" and one of his key roles was going to be ensuring the messaging we used in campaigns and events was properly translated through to sales. 

In theory, there should be a strong feedback loop between product marketing, campaigns, and SDRs, but in practice it can be hard. My advice to any SDR is to insert yourself in the feedback cycle, offer to help, and weasel your way into marketing that way. Good product marketers crave feedback on the messaging they create and how customers respond to it (or not) Use that craving to make yourself useful.