Fossilized Flower Places Angiosperms in the Jurassic

1, style branches; 2, dendroid style; 3, sepal; 4, ovarian roof; 5, scale; 6, seed; 7, cup-form receptacle/ovary; 8, bract; 9, petal; 10, unknown organ (staminode?).  [SOURCE]

1, style branches; 2, dendroid style; 3, sepal; 4, ovarian roof; 5, scale; 6, seed; 7, cup-form receptacle/ovary; 8, bract; 9, petal; 10, unknown organ (staminode?). [SOURCE]

Despite their dominance on the landscape today, the origin of flowering plants is shrouded in mystery. The odds of any living material becoming fossilized is extremely rare and when you consider the delicate and ephemeral nature of most flowers, one can begin to understand why their fossils are so special. The last few decades have seen tantalizing evidence emerge from fossil beds dating to the Cretaceous Period but a recent set of fossils from China predate the oldest confirmed angiosperm fossils by 50 million years. That’s right, it would appear that flowering plants were already on the scene by the early Jurassic!

The fossils in question have been coined Nanjinganthus dendrostyla. They were discovered in China in a formation that dates back roughly 174 million years. To most of us they look like a bunch of dark, albeit elaborate smudges on the rocks. To a trained eye, however, these smudges reveal intricate anatomical details. Amazingly, the team of paleobotanists responsible for this discovery had a lot of material to work with. Descriptions were made on a whopping 264 specimens representing 198 individual flowers. This amount of data means that the declaration of angiosperm affinity stands on pretty solid ground.

A single  Nanjinganthus  flower  [SOURCE]

A single Nanjinganthus flower [SOURCE]

Aside from their age, there is a lot about these fossils that surprised researchers. Probably the biggest surprise is their overall appearance. Paleobotanists have long hypothesized that early angiosperm flowers likely resembled something akin to a modern day Magnolia and invoke floral features such as apocarpy, a superior ovary, and a lack of an obvious style as likely features to look for in ancient plant fossils. Surprisingly, Nanjinganthus does not seem to conform to many of these expectations.

One of the most striking features of these fossils are the styles. They are large and branched like tiny trees (hence the specific epithet “dendrostyla”). The tree-like appearance of the style suggests that early angiosperms likely did not rely on insects for pollination. The branches themselves greatly increase the amount of surface area available for pollen capture, which could mean that Nanjinganthus was wind pollinated.

Flowers of  Nanjinganthus  preserved in different states and their details. For specific details on each image, please see   SOURCE

Flowers of Nanjinganthus preserved in different states and their details. For specific details on each image, please see SOURCE

Another surprising feature is the presence of an inferior ovary that, by its very definition, sits below the sepals and petals. It has long been hypothesized that early angiosperms would exhibit superior ovaries so this discovery means that we must rethink our expectations of how flowers evolved. For instance, it suggests we may not be able to make broad inferences on the past based on what we see in extant angiosperm lineages. It could also suggest that the origin of flowering plants was not a single event but rather a series of individual occurrences. It could also be the case that the origin of flowering plants occurred much earlier than the Jurassic and that Nanjinganthus represents one of many derived forms. Only further study and more fossils can help us answer such questions.

Another way in which Nanjinganthus deviates from theoretical expectations is in the presence of both sepals and petals. Up until now, paleobotanists have been fond of the idea that petals arose much later in angiosperms, having evolved over time as leaves became more and more specialized for attracting pollinators. The fact that Nanjinganthus was likely wind pollinated yet had both sepals and petals is a bit of a conundrum and further emphasizes the need to revisit some of our long-held assumptions of flowering plant evolution.

Details of the sepal and petal as seen through different forms of microscopic analysis. For specific details on each image, please see  SOURCE .

Details of the sepal and petal as seen through different forms of microscopic analysis. For specific details on each image, please see SOURCE.

By far the most important feature present in these fossils are the ovaries. For any fossil to unequivocally qualify as an angiosperm, it must have seeds encased in an ovary. This, after all, is the main feature that separates angiosperms from gymnosperms. Indeed, Nanjinganthus does appear to fit this definition. Thanks to the sheer amount of fossils available for study, the team discovered that the seeds of Nanjinganthus were enclosed in a cup-like chamber that was sealed off from the outside world by a structure they refer to as an “ovarian roof.” This roof does not appear to have any sort of opening, which worked out quite nicely for paleobotanists as it prevented sediments from entering into the chamber, thus preserving the seeds or ovules (it is hard to tell where they were in the developmental process) for study. This feature more than all others secures its identity as a flowering plant.

Based on the sediments in which these flowers were fossilized, it appears that this plant grew close to water. Also, despite its abundance in this particular fossil layer, it very likely was not a common component of this Jurassic landscape. In reality we still have a lot to learn about Nanjinganthus. What we can say with some certainty at this point is that the presence of Nanjinganthus in the early Jurassic likely means that flowering plants arose even earlier. Nanjinganthus is most definitely not the first flower. We will probably never find the first of anything. It is an ancient flower though, predating all other discoveries by at least 50 million years. This is why paleontology is so incredible. Who knows what the next blow of a rock hammer will turn up!

 

EDIT (10/27/2018): Since writing this post it has come to my attention that there is quite a bit of controversy attached to the description of this fossil. Many have reached out informing me that these fossils may actually be a gymnosperm organ rather than a flower. Despite all of the outcry I have yet to see any published critiques on this particular controversy. I anxiously await more professional input on the subject but for now I have decided to keep the content of the original piece as is. Of course extraordinary claims require extraordinary evidence and not being a paleobotanist myself, I cannot trust hearsay on the internet as fact, no matter how vociferous, until I see it published in a peer reviewed outlet of some sort. Please stay tuned as this story develops! 

Photo Credits: [1]

Further Reading: [1]

Aquatic Angiosperm: A Cretaceous Origin?

It would seem that yet another piece of the evolutionary puzzle that are flowering plants has been found. I have discussed the paleontological debate centered around the angiosperm lineage in the past (http://bit.ly/1S6WLkf), and I don't think the recent news will put any of it to rest. However, I do think it serves to expand our limited view into the history of flowering plant evolution.

Meet Montsechia vidalii, an extinct species that offers tantalizing evidence that flowering plants were kicking around some 130–125 million years ago, during the early days of the Cretaceous. It is by no means showy and I myself would have a hard time distinguishing its reproductive structures as flowers yet that is indeed what they are thought to be. Detailed (and I mean detailed) analyses of over 1,000 fossilized specimens reveals that the seeds are enclosed in tissue, a true hallmark of the angiosperm lineage.

On top of this feature, the fossils also offer clues to the kind of habitat Montsechia would have been found in. As it turns out, this was an aquatic species. The flowers, instead of poking above the water, would have remained submerged. An opening at the top of each flower would have allowed pollen to float inside for fertilization. Another interesting feature of Montsechia is that it had no roots. Instead, it likely floated around in shallow water.

This is all very similar to another group of extant aquatic flowering plants in the genus Ceratophyllum (often called hornworts or coon's tail). Based on such morphological evidence, it has been agreed that these two groups represent early stem lineages of the angiosperm tree. Coupled with what we now know about the habitat of Archaefructus (http://bit.ly/1S6WLkf), it is becoming evident that the evolution of flowers may have happened in and around water. This in turn brings up many more questions regarding the selective pressures that led to flowers.

What is even more amazing is that these fossils are by no means recent discoveries. They were part of a collection that was excavated in Spain over 100 years ago. Discoveries like this happen all the time. Someone finds a interesting set of fossils that are then stored away on a dark shelf in the bowels of a museum only to be rediscovered decades or even centuries later.

All in all I think this discovery lends credence to the idea that flowering plants are a bit older than we like to think. Also, one should be wary of anyone claiming to have found "the first flower." The idea that there could be a fossil out there that depicts the first anything is flawed a leads to a lot of confusion. Instead, fossils like these represent snapshots in the continuum that is evolution. Each new discovery reveals a little bit more about the evolution of that lineage. We will never find the first flower but we will continue to refine our understanding of life on this planet.

Photo Credits: Bernard Gomeza, Véronique Daviero-Gomeza, Clément Coiffardb, Carles Martín-Closasc, David L. Dilcherd, and O. Sanisidro,

Further Reading:
http://www.pnas.org/content/112/35/10985.abstract