Everything about petrified wood. Part 3. New types/species from our collection.

Introducing Rhexoxylon, Schinoxylon, Woodworthia and Dipterocarpacea.
Short stories and information about these types of petrified wood.
Let’s start with a rock from a famous location known as Blue Forest. What’s unusual about this particular specimen is that it also contains remnants of the tiny animals, seed shrimp, hidden in the cracks of the wood. The scientific name is ostracods. The vast majority of the logs from this location are covered in petrified algae, an indication that, at one point in time, they were standing in the water. The wood coated with a thick layer of algae eventually dried and shrank, creating spaces that were filled with chalcedony, agate, and calcite. Occasionally, small creatures got trapped in the cracks and turned into stone as well. The algal layers sometimes have distinct stromatolite-like appearance and might have been created by the cyanobacteria, which used to be called blue-green algae.
The calcareous layer of the mineralized microbial coating is often removed during preparation of the samples using oxalic acid. This reveals the underlying agates of famous blue color but, in my view, makes the specimen incomplete.
After studying well-preserved cellular structures, scientists determined that the Blue Forest petrified wood represents pepper tree or Schinoxylon. The trunks are found at the depths of approximately one meter in a semi-arid and dusty Eden Valley. The common tool used by the prospectors to find logs is a metal probe, essentially a rod, which is pushed through the soil until it hits a rock. Then you can start digging hoping that it is the right kind of rock because you may spend quite a bit of time and effort digging that hole. Similar probe, by the way, is also used by Florida’s fossil hunters looking for a right kind of river gravel containing shark teeth.
The trees were growing in a swamp-like environment at the eastern edge of the large ancient lake Gosiute about 52 million years ago. Geologically, Blue Forest is related to famous Green River formation, where fossilized fish imprints are coming from. It was an area with huge lakes. There were volcanic fields nearby and they were the source of volcanic ash falling down on the lakes periodically. This part of the lake was the great habitat not only for fish, predominantly catfish and suckers, but also reptiles like turtles and crocodiles.
The word “blue” refers to the bluish hues of the dark chalcedony that can be found inside of this particular type of petrified wood. The color is due to the traces of elements such as titanium, iron, cobalt, and manganese. It is believed that the black color is associated with the remnants organic matter rich in carbon. The light-brown spots are formed when the carbon was washed out by the ground water. This is what collectors called bleaching.
When you examine the microscopic structures the wood itself, you will notice plenty of regularly spaced large holes among tiny cells. It’s a typical feature of hardwood distinguishing it from the softwood. The holes are vessels and they function as pipes running along the trunk to deliver water and nutrients. In softwood, all cells can do this and therefore they look more uniform. In hardwood, it’s done through the special vessels. The trees were flowering dicots and they had berries like modern day pepper trees.
Even though having ostracods is not exactly a unique feature for Blue Forest petrified wood, it is still quite rare and we are happy to have the piece in our collection.
Let’s move to a seed fern specimen supposedly from Africa. It represents an unusual and cool-looking petrified wood called Rhexoxylon. It was a Mesozoic arborescent seed fern. The botanists use word “arborescent” for plants that have a tree-like appearance and thus have trunks. The slab we have is just a slice of a Rhexoxylon trunk.
This particular type of seed ferns grew large and probably served as food and shade for various reptiles during Triassic period. That’s roughly two hundred and twenty million years ago. As you probably know, the single landmass called Pangaea was the only continent at that time. This is why similar fossils are found not just in Africa but also in South America and even Antarctica.
When cut in transverse cross section and polished, Rhexoxylons reveal an interesting structure of the stem. The cambial rings surrounding the central pith were the source of new cells. They formed centripetal xylem wedges that extend towards the pith and, on the other side, the wedges of centrifugal xylem. The wedges were separated by parenchymatous rays. A bit complicated anatomy, which can be described as polyxylic.
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