Fossil Mangroves in Egypt’s Valley of the Whales, as photographed by Erica Hargreave.

Just like the next homo sapien I get excited over fossils, but I’ll admit it, I geek out even more over plant  fossils.  Why?  Well a couple of reasons actually … 1) As plants are typically comprised of softer tissue, than bone, I am always amazed that the right conditions existed to allow for the fossil formation of plants, rather than them decomposing, and 2) As many of these plants still exist today, showing what incredibly resilient organisms they are. Another reason to shake my head when I hear people talk about homo sapiens with an air of omnipotence.  We are really put to shame as a species when you look at the resilience of the plant life that surrounds us.  Organisms that I might add that we require to co-exist with for our own survival on this planet.

Funny, as I write this, I realize that my stumbling into botany (with a high dose of entomology on the side), as oppose studying the sexier beasts out there, like the vertebrates, may not have been the half hazard accident of job offers that I thought it was.  I am fascinated by plants and I always have been. I also chuckle as I finally see in my writing why my cousin called me a tree hugger.

But I digress … I mention this as when I first walked through Wadi Al-Hitan, Egypt’s Valley of the Whales, I was beyond excited to be walking through a 40 million year old seabed and discovering such well preserved fossil specimens of some of the earliest forms of whales, the Archaeoceti, and primitive Sirenia just lying out under the sun on the desert floor. BUT what surprised me the most were these huge fossilized stands of mangrove roots, and fossilized palms that are known to have grown in association with the mangroves, along with fossilized sea grasses. These were not fossils I expected to see, and they were so very plentiful.

Fossilized Mangroves in Wadi Al-Hitan, as photographed by Erica Hargreave.

Now in the case of the mangroves of Wadi Al-Hitan, they date back to the Eocene. However, mangroves as a whole first “evolved soon after the appearance of flowering plants with the earliest records of Nypa in the late Cretaceous to early Paleocene” (Dolianiti 1955).  So mangroves would have, in their early days coexisted with dinosaurs.  Based on palaeogeography, Egypt would have been near the point of evolutionary origin of the mangrove in the Tethys Sea.

As for the mangrove itself, it really is a fascinating organism. These trees and shrubs grow in shallow, saline, sediment based, coastal habitats in the tropics and subtropics—mainly between latitudes 25° N and 25° S.  Ultimately what this means is that the mangroves need to survive the extreme conditions of a low oxygen, high salinity environments with limited access to fresh water. The mangrove does that with the help of some cool adaptations.

Surviving Low Oxygen Environments

To survive low oxygen environments, red mangroves prop themselves above the water level with stilted roots, absorbing air through pores in their bark. Whereas black mangroves live on higher ground and have pneumatophores, which are specialized root-like structures that stick out of the water at low tide, acting like snorkels for breathing.

Managing Salt Intake

Some mangroves, like the black mangrove featured in the above video, can secrete salts directly through two salt glands at the leaf base. While the red mangrove has evolved significantly impermeable roots, excluding between 90% to 97% of salt in the water it sits in.

Preventing Water Loss

Another cool adaptation of the mangrove is it’s ability to limit water loss through it’s leaves by constricting the opening of the stomata, and altering the orientation of their leaves to avoid the harsh midday sun, reducing evaporation from the leaves.

Absorbing Nutrients in the Air

Sitting in waterlogged, anoxic, nutrient depleted soil; mangroves use their pneumatophores to absorb gases directly from the atmosphere. They then store these gases inside the roots, so that they can process them even when their roots are submerged at high tide.

Offspring Survival Advantage

One of the coolest adaptations of the mangrove is that many mangroves are actually viviparous, meaning that they release live young, as oppose to seeds. The seeds germinate while still attached to the parent tree, producing a young seedling or propagule (as they are called).

Once mature, the propagule then drops into the water and allows itself to be transport elsewhere. If swept quite a ways away, the propagule can remain dormant for over a year to survive desiccation. When it is ready to root, its density changes to float vertically rather than horizontally, allowing it to lodge in the mud and root. If it does not root, it can again alter its density to drift again in search of a better place to root.

Perhaps one of the most important things about the mangrove though, from the late Cretaceous to today, is the habit it provides for other organisms to hide, feed and make their homes in. To learn more about the mangrove and the ecosystem it plays an important role in creating, you can tune in to the episode on Mangroves on Bahama Blue on Love Nature.