Nick,
I don't know for sure what the local wild Brassica is here in Maryland; I've always called it "Mustard". But it often does get a very turnip-like root when it's happy. I've seen them several inches across.
Maybe that's what you're seeing too??
Tom

Tom,
I guess that it might be mustard, though I doubt it. It doesn't look or taste like it at all. I described it to a friend and he said that it sounded sort of like a rutabaga. I haven't grown rutabagas for some 20 odd years or so as no one in my family likes them. Maybe I've forgotten what rutabagas look like, as the leaves don't look like I remember. Eaither way, I'll probably overwinter the plant in a pot in my root cellar and see if it makes seeds next year. Who knows, maybe it'll be a keeper.
-Nick

mustard and collards can't cross.

I have heard that all members of Brassica (the cabbage family ) can crossbreed and produce fertile offspring ! These include :
Kale , cabbage, bokchoy, kholrabi, kholbaga , turnip , rhutabaga ,cauliflower, brocolli, collards,rape,( and yes mustard ). Rhutabagas have a thick stem that grows over 6 inches high . Try crossing collards with cabbage to make a cabbage that would not smell.

I forgot brussel sprouts they are a member of the cabbage family too and there may be others I have ommitted .

Yes, a cabbage turnip cross is possible but not common. That's what a rutabaga is  Brassica napobrassica. The cabbage family (B. oleracea) includes cabbage, collards, kale, broccoli, cauliflower, Brussels sprouts and kohlrabi, plus the Asian gai lohn. The turnip family ( B. napa) includes mustard, broccoli raab, turnips, and all of the "Asian cabbage" family, except for Gai Lohn, aka Chinese broccoli.

You can get a variety of 'cross species' crosses in the Brassica and some of the more interesting types are a direct result of one of the various '3 way' interspecific crosses possible. I've attached a link to a chart that shows them.

B. Juncea (Mustard Greens) are a result of a cross of B. Nigra (black mustard) with B. Rapa (turnips, field mustard, turnip mustard).

B. Napus (rhutabaga, rapeseed)a cross of B. Rapa with B. Olerica (cabagges, kales, broccoli, etc. as above.

B. Carinata (Abyssinian Mustard) a cross of B. Olerica and B. Nigra.

Who knows what other crosses they have had. The Brassicas are a bit "loose" with their genes... ;-)

So while I'd guess what you have is a wild "turnip mustard" it could just as easily be a strange cross...

You had turnips (B. Rapa) and a collard (B. Olerica) so if they crossed you could easily have gotten a new variety of Rhutabaga.

I'd trial it and see if it was any good. Pretty much all the candidates are edible in one way or another...

Here is a link that might be useful: {{gwi:995878}}

I'm linking to an illustrated article on various Brassica hybrids. The plants called "couve" in the article are what we call collards.

Karl

Here is a link that might be useful: Sutton: Brassica Hybrids (1908)