Plant evolution is important to consider in the evolution of life on Earth and in the sexier study of vertebrate palaeontology, as of course, animal life is tied to plant life. That is especially true in the case of plants that create habitats, as well as food, for many animals. Such is the case with seagrass beds.
Seagrass also happens to be a marine organism that provided an evolutionary conundrum to me this past week. In watching the recent episode, Sand, of Bahama Blue, it referenced seagrass as having evolved 17 million years ago …
This presented a conundrum in my mind, as I knew that fossilized seagrass had been found in Wadi El-Hitan, a 40-million year old ocean in Egypt, and that that seagrass had been one of the main food sources for the ancient dugongs – sea cows that lived there in the late Eocene. The Eocene dates back to between 56 to 33.9 million years ago, so how could a plant that evolved 17 million years ago in the Miocene be there?
Well, as the case may be, marine seagrasses actually evolved 3 or 4 separate times. Indeed it is rather fitting that seagrass should be found in the ancient Egyptian seabed of Wadi El-Hitan, as this is a place where fossil evidence was found that whales actually evolved from a land mammal returning to the sea – such is the case too with seagrass, as seagrasses evolved from flowering land plants (angiosperms) that similarly returned to the sea.
In the evolution of seagrass there are four families: Hydrocharitaceae, Cymodoceaceae, Posidoniaceae, and Zosteraceae. Cymodoceaceae, Posidoniaceae, and Zosteraceae are exclusively marine organisms; while Hydrocharitaceae includes some genera that are restricted to freshwater habitats and others that are marine. As such seagrasses occur worldwide in different climatic zones and sharing various metabolic features with their terrestrial counterparts that allow them to adapt to these diverse habitats.
Of these families, it is estimated that the Hydrocharitaceae have a crown node age of 75 million years ago and a stem node age of 88 million years ago, the Cymodoceaceae have a crown node age of 61 million years ago and a stem node age of 67 million years ago, for the Posidoniaceae only a stem node age of 67 million years ago can be estimated, and the Zosteraceae appeared more recently with a crown node age of 17 million years ago and a stem node age of 47 million years ago. Crown node age refers to when the presently living members of the group of organisms evolved; whereas stem node age refers to when the first known ancestor of the organism evolved.
In the case of the seagrass meadows mentioned in the Bahama Blue video, they would be referencing a living species of Zosteraceae that evolved 17 million years ago. Whereas the seagrass in Egypt’s Wadi El-Hitan are of the family Cymodoceaceae, aptly known as the manatee-grass family for the Sirenia, like dugongs, found foraging there.
One palaeontology mystery solved, and so many more to crack!