Fossil yucca? plant

One of the rarest fossils I have collected from the Pico Formation south of Newhall, southern California is what I believe is a small base (trunk) of a yucca plant. It is from the upper part of this formation and is of late Pliocene age (about 3 million years old). 

The Pico Formation in this area was deposited in a marine-delta environment, and, as I mentioned in one of my earlier posts (8/15/2014), fossil pine cones can be found (rarely) in these beds. The pine cones were derived from pine trees that grew in the adjacent, ancient San Gabriel Mountains east of the delta. Some of the pine cones eventually floated down a braided river (full of coarse debris consisting of pebbles and cobbles) and were deposited in fine-grained sandstone near an ancient shoreline, and mixed with shallow-marine fossils (e.g., seashells and shark teeth). It seems likely that presumed yucca remains could have also floated into this marine-environment setting.

When I first saw the presumed yucca fossil, I thought it was a pine cone. Upon closer inspection, however, I realized the yucca? fossil is not like the pine cones from this formation. As I walked back to my car, I came across a modern yucca plant (see photo and comments below). I was immediately struck by the fact that the vertical-striations on some of the woody part of the trunk of both the modern and presumed fossil yucca are very similar. If you have knowledge of the bases of fossil yucca plants, please let me know if you think my identification is correct or not.

Late Pliocene yucca? base (trunk), height 12 cm (4.75 in.), width 7 cm (3 in.), from the upper Pico Formation near Newhall, California. The white fossil sticking out along the upper left side is a shallow-marine clam. Notice the cross-section of the high-spired, shallow-marine gastropod shell

of Turritella cooperi near the bottom. 

Modern-day base (trunk) and a few green leaves of a yucca [probably Hesperoyucca whipplei] height 24 cm (9.5 in.), width 15 cm (5 in.), from near Newhall, California. The non-green, hard, woody trunk is 14 cm height. Hesperoyucca whipplei is one of the most common yuccas of the chaparral and coastal-sage scrub plant communities living below 4000 feet in elevation in southern California. The leaves of this yucca are stiff and dagger like.