It's a pumpkin (or some kind of squash) seed and a bunch of other small seeds. Apparently that can happen when you store your car for years with the radiator cap off. I don't know what was living in there, but it had quite a comfortable home. Here's some background history... I saw critter evidence junk in the radiator. Flushed out as much as I could with a hose. Drove the car. and the temps started out fine, but started to creep up the longer I drove. Drained the radiator looked in the top, and there was junk in it again. Apparently some debris was now circulating around the cooling system. So I pulled the radiator and dug/flushed/picked out as much debris as I could and put it back in. This time, however,,, No messing around. I installed a "filter" between the head outlet and the inlet of the radiator with the hope that it would catch debris coming out of the block before it got into the radiator. The filter was a metal screen sock that's used on your washing machine outlet so you don't dump lint into your plumbing. And it just so happens it fit's well into the radiator hose. Looks like this: Chain mail condom: Stuff it into the top radiator hose: Drive around for a couple minutes and I catch this debris before it gets back into the radiator: Rinse and repeat. This is the third time I cleaned out the filter. Probably an hour of engine run time. The amount of debris getting caught is decreasing: I've since pulled it a couple more times and it's almost clean. I can probably remove the filter at this point.