Fascinating Cloud Formation

Hearing prolonged rumbles an hour or so ago, I looked out the back door to blue skies and shrugged it off as fireworks being set off by the neighbors. Prairiewolf noticed the noise, too, and after a bit I decided to look out the front door. It was like being in an alternate universe! The sky was very dark, with frequent lightning flashes. There was no blue sky to be seen anywhere.

Even more amazing was the base of the cloud - a really unusal formation that I've never seen before. A circular area was distinctly set off, lower, from the rest of the base of the cloud. This circular formation was attached via a narrow band with a more linear, lowered section that seemed to run along the western side of the base of the storm. Almost a linear roll cloud on the backside of the storm, but much more layered looking and with no visible rotation.

I was able to get several photos. Does anyone know if this formation tells anything special about this thunderstorm? (The cell was an elongated single cell that seemed to arise, unexpectedly, in the middle of otherwise benign looking skies. Nothing was predicted, and it wasn't particularly hot - around 82 degrees F.)