Petrified Wood is a fossil however, the word petrified is derived from the Greek root word petro, which means rock.
The Triassic Era started after the Permian era and before the Jurassic era. The earth's biosphere was a bit of a disaster with lots of species facing extinction. It was very hot and dry. Real mammals started showing up though along with flying vertebrate animals. Dinosaurs showed up in the late Triassic period and, of course, became dominant in the Jurassic period.
When a tree or a big shrub was buried in a landslide, volcanic lava or ice, it is turned into a fossil instead of just decaying. That's because there was no oxygen to create decay where it was buried. Over time, everything alive in the tree or organic, was replaced by minerals from the mud or the in the ice. Quartz is very common, so Petrified Wood usually contains quartz. And other minerals that were in the water that rained on the area or ran through the area joined the petrification process. For instance, this sphere is red petrified wood because there was a lot of oxidized iron in the water where the tree was buried. The iron oxide made it red. There can be all kinds of colors of petrified wood though depending on what was in the water. If there was carbon, the fossil would have a lot of black. If copper was present, it tends to be green or blue. If manganese was in the water, the petrified wood is orange or pink! Chromium turns the fossil green or blue - same as copper.
Because the wood didn't decompose until after it had petrified, it's not like other fossils where you see an impression of the life form. Instead, you are seeing the exact same structure that was there millions of years ago, but now it is crystallized. The process of the exchange of the organic cells of the tree mass for minerals is called per-mineralization. This whole process goes on for millions of years!