On April 19th, Darwin packed up and prepared for the trip back to Rio de Janeiro. His entry today was very brief:
“Left Socêgo, crossed the Rio Macaè & slept at the Venda de Matto: in the evening walked on the beach & enjoyed the sight of a high & violent surf.” (Apr 19)
So I thought I’d take this opportunity to add a little more to the discussion on tree ferns from a couple of days ago.
After posting about tree ferns, I was pleasantly surprised yesterday afternoon to come back to my office to find that my friend Pat B. (who has an amazing collection of petrified wood) had brought in a 290 million-year-old (Permian) fossil tree fern from the Pedra de Fogo Formation in Brazil. (Yes – after many years as a geologist, I still can’t help but marvel at the concept of holding a tree from 300 million years ago.)
Permian tree fern fossil from Pedra de Fogo Formation:
Cross section of the same sample:
Notice the detailed structures in the cross section of the tree fern trunk. The center part of the trunk is called the “vascular cylinder” – the actually stem of the fern. Ferns are vascular plants (some of the earliest on Earth). All vascular plants contain vascular tissue – tubes that carry water and nutrients up (and sometimes down) the stem of the plant (including xylem and phloem). They are an important adaptation for land plants, as the vascular tissue has the ability to draw water upward, overcoming the pull of gravity. Before that tissue evolved, plants had to get all of their water from the surrounding air, and were typically not very tall. You might say vascular tissue made forests possible.
The central vascular cylinder is surrounded by what is called the “root mantle”. This is basically a mass of thin roots that support and strengthen the trunk of the tree fern. Each of these roots also contains its own small bundle of vascular tissue in its center.
Curiously, just 2 days ago there was an interesting article from the NY Times Science section on a group of scientists starting to conduct field work on the animal fossils of the Pedra de Fogo Formation. Apparently there have been a lot of fossils discovered in these Permian rocks, including large crocodile-like amphibians, mammal-like reptiles called dicynodonts, and even sharks. Check out the NY Times article here.
Permian (~290 million year old) mudcracks preserved in the rocks of the Pedra de Fogo Formation (from the NY Times link above) – an indication of an arid climate:
As Darwin was soon to discover in the South American rock record (later this year), there is an important third dimension to the diversity of life on Earth. In addition, to the changes that occurred in species diversity as one moved geographically over the face of the Earth (2 dimensions), life also changed through time. The diversity of life was ever changing. (RJV)