Ancient Aquatic Flora Was Among the First Flowering Plants

The Conversation

Photosynthesis – the ability to convert energy from the sun into fuel – first appeared on Earth in single-celled organisms, which eventually evolved into algae, then mosses, then ferns. Flowering plants, now such a familiar part of our landscape, didn’t evolve until the Jurassic period, after dinosaurs and mammals had already hit the scene. At this time, insects were diversifying, and the evolving plants used the emerging bugs to carry their own genetic material from plant to plant. Flowering plants, also known as angiosperms, are a product of this early version of sex and its exchange of genetic material – so important in evolution.

A recent discovery and analysis of fossilized plants has opened up the discussion of the nature and relationships of these early plants. First found in the lithographic limestone being mined in the Pyrenees Mountains over 100 years ago, these fossils, with their strange sprawling stems, were little understood. Some thought they were mosses, some considered them to be conifers, but few recognized the fossils as flowering plants.

Now my colleagues and I, on a team of paleobotantists led by Bernard Gomez of Lyon, France,  that this fossil, , which lived as long ago as 130 million years, is the earliest known example of a fully submerged aquatic flowering plant. After careful analysis of hundreds of well-preserved newly collected fossils from northeastern Spain, we believe  flowered underwater and was pollinated underwater, living in a similar fashion to the plant that’s found around the world today.

Flowers are all about sex and getting new genetic material into the breeding line.  is an example of a very early line of evolution that solved this challenge in a new and novel way – relying on water to disperse its pollen, not the wind or animal pollinators.

The plant we see in these new old fossils

We’ve found two forms of this fossil: one form has leaves that are small and closely pressed to the stem. On this form, we frequently saw mature fruits.

The other form has leaves that extend out from the stems and only rarely are mature seeds found attached. We saw the two leaf types associated together at the same fossil localities.

Today, many flowers are made up of petals and then male stamens (with filaments and anthers that produce pollen) and female carpels (which mature into fruits and contain the seeds, like peas in a pod). We didn’t identify any male flowers or remains of where they were borne on the stems in . It appears they had separate flowers to contain pollen organs and carpels.

Fishing around for the first flower

Then the tools of molecular systematics allowed botanists to use DNA and RNA from the nuclei and chloroplasts of plant cells to puzzle out relationships based on molecular characteristics. That’s when a genus called, found living today only in New Caledonia, gained favor as a possible first flowering plant.

Another contender, , was also once thought to be basal to all flowering plants before being displaced by , and its position in the angiosperms has been uncertain since. , at 130 million years old – among the oldest megafossil remains known of any flowering plant – is in the lineage of . This makes this lineage of flowering plants one of the oldest known and suggests that underwater is back in the running to be the original flowering plant.

, modern descendent of first flower

These modern-day descendents of have separate male and female flowers with no sepals or petals, just simple reproductive organs, like stamens and carpels. When they reproduce, the stamens release the anthers that contain the pollen to float up to the water’s surface. Then the pollen is released and begins to slowly sink through the water column. As it descends, being moved by water currents, a branched pollen tube grows out. When the pollen tube makes it into the vicinity of a female, a branch will find a small hole enter and pollinate. This is how the plant fertilizes and creates a seed.

Because of the water dispersal of the pollen there is genetic mixing or outcrossing, just as if an insect had carried the pollen.

The fossil fruits of  also have a similar small pore in the fruit wall, and the seed is positioned similarly as those of  today. This suggests that these very ancient flowers flowered underwater, were very simple in nature (no beautiful petals yet) and were pollinated underwater. This very early and inventive way for flowering plants to manage their reproduction so early in their evolution is impressive.

 places the lineage as one of the oldest of all the flowering plants and suggests that we need to reevaluate the nature of the evolution of the original angiosperm again.

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