Charles Dondale via email May 2018:
"Your male specimens from the Mojave Desert readily key to Thanatus in “Spiders of North America”. Everything points to an undescribed species. I think you should search for a female to go with the males and describe this species as new. Just to be sure, and to find characters that clearly differentiate your species from any others, I recommend a search of Schick’s 1965 treatment of Thanatus in California. Our old 1972 treatment of Thanatus in Canada includes all 8 species known for North America with diagnostic characters".

I have been trying in vain to find females (or any more specimens) for the past several years. I may eventually need to describe the male only, but I'm still holding out for more specimens... And I still haven't completely ruled out a foreign import...

I spent some time in the literature before I saw you had posted comments from Rod. This spider does seem to straddle the fence between Thanatus and Apollophanes. That tibial apophysis sure looks good for A. margareta but embolus, lack of spotting, and even spines on legs seems to be inconsistent with that ID. Neat spider!

Moved from Running Crab Spiders.

From R Crawford, who was kind enough to take a look at these images:
"Absolutely, positively not A. margareta (unless that species has color forms I've never seen).

Moreover, the palp fails to match anything in the North American revisions. The distinct gap between tegulum and embolus in side view seems unique. The sharply angled type of RTA is found more often in Thanatus than in Apollophanes and I'd be inclined to place it in Thanatus (other features tend this way too).

Not only doesn't it match any North American Thanatus, it doesn't match any European one either. (I always check that since so many European species get introduced).

I'd say you may have found something either new or extralimital".