A Herbaceous Conifer From the Triassic

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It is hard to make broad generalizations about groups of related organisms. There are always exceptions to any rule. Still, there are some “facts” we can throw around that seem to apply pretty well to specific branches on the tree of life. For instance, all of the gymnosperm lineages we share our planet with today are woody, relatively slow to reach sexual maturity, and are generally long-lived. This has not always been the case. Fossil discoveries from France suggest that in the past, gymnosperms were experimenting with a more herbaceous lifestyle.

The fossils in question were discovered in eastern France back in the 1800’s. The strata from which they were excavated dates back to the Middle Triassic, some 247 million years ago. Immortalized in these rocks were numerous spindly plants with strap-like leaves and a few branches, each ending in what look like tiny cones. Early interpretations suggested that these may represent an extinct lycopod, however, further investigation suggested something very surprising - a conifer with an herbaceous growth habit.

Indeed, thanks to even more scrutiny, it is now largely agreed upon that what was preserved in these rocks were essentially herbaceous conifers. The fossils were given the name Aethophyllum stipulare. They are wonderfully complete, depicting roots, shoots, leaves, and reproductive organs. Moreover, the way in which they were fossilized preserved lots of fine-scale anatomical details. Taken together, there are plenty of clues available that allow paleobotanists to say a lot about how this odd conifer made a living.

For starters, they were not very big plants. Not a single specimen has been found that exceeds 2 meters (6.5 ft) in height. The main stem of these conifers only seem to branch a couple of times. Cones were formed at the tips of the upper branches and not a single specimen has been found that depicts subsequent growth following cone formation. This suggests that Aethophyllum exhibited determinate growth, meaning that individuals grew to a certain size, reproduced, and did not continue to grow after that. Female cones were situated at the tips of the upper most branches and male cones were situated at the tips of lower shoots. The smallest reproductive individuals that have been unearthed are only 30 cm (11 in) in height, which suggests that Aethophyllum  was capable of reproducing within a few months of germination.

Artists reconstruction of  Aethophyllum stipulare

Artists reconstruction of Aethophyllum stipulare

Amazingly, researchers were also able to extract fossilized pollen and seeds from some of the Aethophyllum cones. The pollen itself is saccate, much like what we see in many extant conifers. By comparing the morphology of the pollen extracted from the cones to other fossil pollen records, researchers now feel confident that Aethophyllum is the source of pollen grains discovered in sediments from western, central, and southern Europe, Russia, Northern Africa, and China, suggesting that Aethophyllum was pretty wide spread during the Middle Triassic. Aethophyllum seeds were small, ellipsoid, and were not winged, likely germinating a short distance from the parent.

The stems of Aethophyllum are interesting in the own right. Thanks to their preservation, cross sections have been made and they reveal that these plants only ever produced secondary tracheids and primary xylem. The only place on the plant where any signs of woody secondary xylem occur are at the base of the cones. This adds further confirmation that Aethophyllum was herbaceous at the onset of sexual maturity.

Another intriguing aspect of the stem is the presence of numerous large air spaces within the stem pith. Today, this anatomical feature is present in plants like bamboo, Equisetum, and the flowering stalks of Agave, all of which exhibit alarmingly fast growth rates for plants. This suggests that not only did Aethophyllum reproduce early in its life, it also likely grew extremely fast.

1. Smallest fertile plant in the Grauvogel and Gall collections, with two stems extending from the root, and terminal ovulate cone (OC) on one branch (scale bar=10 cm). 2. Cross-section of stem in the Grauvogel and Gall collections showing cauline bundles with scanty wood (at left, top and right) surrounding large pith with large, aerenchymatous lacunae and interspersed pith parenchyma cells. Vascular cambium, phloem, and more peripheral tissues are not preserved (scale bar=200 μm). 3.Seedling in the Grauvogel and Gall collections showing primary root (R), cotyledons (C) and stem (S) with apically borne leaves (scale bar=10 cm).  Quoted from SOURCE

1. Smallest fertile plant in the Grauvogel and Gall collections, with two stems extending from the root, and terminal ovulate cone (OC) on one branch (scale bar=10 cm). 2. Cross-section of stem in the Grauvogel and Gall collections showing cauline bundles with scanty wood (at left, top and right) surrounding large pith with large, aerenchymatous lacunae and interspersed pith parenchyma cells. Vascular cambium, phloem, and more peripheral tissues are not preserved (scale bar=200 μm). 3.Seedling in the Grauvogel and Gall collections showing primary root (R), cotyledons (C) and stem (S) with apically borne leaves (scale bar=10 cm). Quoted from SOURCE

Mature Aethophyllum aren’t the only fossils available either. Many seedlings have been discovered in close proximity to the adults. Seedlings were also exquisitely preserved, depicting hypocotyl, a primary root system, two two-veined cotyledons, and a short stem with four-veined leaves arranged in a helix. The fact that seedlings and adults were found in such close proximity lends to the idea that Aethophyllum populations were made up of multi-aged stands, not unlike some of the early successional plants we find in disturbed habitats today.

The sediments in which these plants were fossilized can also tell us something about the habitats in which Aethophyllum grew. The rock layers are made up of a mix of sediments typical of what one would find in a flood plain or delta. Also, Aethophyllum aren’t the only plant remains discovered. Many species known to grow in regularly disturbed, flood-prone habitats have also been found. Taken together these lines of evidence suggest that Aethophyllum was similar to what we would expect from herbaceous plants growing in similar habitats today. They grew fast, reproduced early, and had to jam as many generations in before the next flood ripped through and hit the reset button.

Aethophyllums small size, lack of wood, and rapid growth rate all point to a ruderal lifestyle. Today, this niche is largely filled by angiosperms. No conifers alive today can claim such territories. The discovery of Aethophyllum demonstrates that this was not always the case. The fact that pollen has been found far outside of France suggests that this ruderal lifestyle worked quite well for Aethophyllum.

The terrestrial habitats of the Middle Triassic were dominated by the distant relatives of modern day ferns, lycophytes, and gymnosperms. Needless to say, it was a very different world than anything that we are familiar with today. However, that does not mean that the pressures of natural selection were necessarily different. Aethophyllum is evidence that specific selection pressures, in this case regular flood disturbance, select for similar traits in plants through time. Why Aethophyllum went extinct is anyone’s guess. Despite how well they have been preserved, there is still a lot of mystery surrounding this plant.

Photo Credit: [1]

Further Reading: [1] [2] [3] [4]



The Celery-Topped Conifers

I am only just starting to fully appreciate the diversity in form and habit exhibited by the gymnosperm lineages alive today. What I once thought of as a unidimensional group of plants is proving to be wonderfully diverse, despite being overshadowed by the angiosperms. For instance, imagine my surprise when I first laid eyes on a member of the genus Phyllocladus.

At first glance, these weird conifers look more like a broad-leaf angiosperm. This similarity is superficial, of course. Before we get to why they look the way they do, it is worth considering this group from a as a whole. The genus Phyllocladus comprises roughly 5 species spread out among New Zealand, Tasmania, and Malesia. They are somewhat variable in form but usually settle out somewhere between a good sized shrub and a medium sized tree. Where exactly this genus of oddball gymnosperms fits on the tree of life is subject to some debate.

Phyllocladus aspleniifolius

Phyllocladus aspleniifolius

Phyllocladus trichomanoides

Phyllocladus trichomanoides

For many years after its initial description, Phyllocladus was placed in a family of its own - Phyllocladaceae. Subsequent molecular work has only managed to add to the confusion. Despite its unique morphological characteristics, some authors feel this genus fits nicely into the family Podocarpaceae. At least one other study suggests that it doesn’t belong in Podocarpaceae but rather is situated as sister to the family. By the looks of it, this will not be cleared up any time soon. So, for now, let’s focus in on why these plants are so strange.

For starters we have the “leaves.” I place the word ‘leaves’ in quotes because they are not true leaves. The correct term for these structures are phylloclades (hence the generic name). A phylloclade is a flattened projection of a branch that takes on the form and function of a leaf. What we know of as leaves have been greatly reduced in the genus Phyllocladus. If you want to see them, you must look closely at the tips of the phylloclades. Early on in their development, the leaves exist as tiny brown scales. These scales are gradually lost over time as they serve no function for the plant.

Phyllocladus alpinus

Phyllocladus alpinus

Phyllocladus hypophyllus

Phyllocladus hypophyllus

Though no one has tested this directly (that I am aware of), the evolution of phylloclades over leaves likely has to do with energy conservation in one form or another. Why produce stems and leaves when you can co-opt stem-like structures to do the work for you? Oddly enough, some suggest that to consider them stems in the truest sense of the word is erroneous. Morphologically speaking, they share traits that are intermediate between branches and stems. However, I am going to need to do more homework before I feel comfortable elaborating on this point.

Only when it comes time for reproduction does their place among the gymnosperms become readily apparent, that is before the ovules are fertilized. All members of the genus Phyllocladus produce cones. Male cones are tiny, cylindrical structures located at the ends of their side branches whereas female cones are clustered into groups along the axils or margins of the phylloclades. Once fertilized, however, these plants offer another point of confusion for the casual observer.

The fleshy “fruits” of  Phyllocladus aspleniifolius

The fleshy “fruits” of Phyllocladus aspleniifolius

Phyllocladus is yet another genus of conifers that has converged on a fruit-like seed dispersal strategy. As the seed cones mature, the scales gradually swell and become berry-like. Poking out of the bright red and/or white aril is a single seed. These fleshy arils function in a similar way to fruit in that they attract birds, which then consume them, dispersing the seeds later on in their feces.

Another intriguing aspect of their morphology occurs below ground. The roots of this genus form nodules, which provide a home for bacteria that specializing in fixing atmospheric nitrogen. In return for a home and some carbohydrates from photosynthesis, these bacteria pay these trees with nitrogen that would otherwise be unavailable. Pretty remarkable stuff for a such an esoteric group of conifers!

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Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4] [5]

The Japanese Umbrella Pine

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My first impression of the Japanese umbrella pine was that I was looking at a species of yew (Taxus spp.). Sure, its features were a bit more exaggerated than I was used to but what do I know? Trying to understand tree diversity is a recent development in my botanical obsession so I don’t have much to base my opinions on. Regardless, I am glad I gave the little sapling I was looking at a closer inspection. Turns out, the Japanese umbrella pine is most definitely not a yew. It is actually unique in its taxonomic position as the only member of the family Sciadopityaceae.

The Japanese umbrella pine goes by the scientific name of Sciadopitys verticillata. Both common and scientific names hint at the whorled arrangements of its “leaves.” I place leaves in quotes because they are not leaves at all. One of the most remarkable features of this tree is the fact that those whorled leaves are actually thickened, photosynthetic extensions of the stem known as “cladodes.”

Those tiny bumps along the stems are actually highly reduced leaves whereas the whorls of photosynthetic “leaves” are actually modified extensions of the stem called “cladodes.”

Those tiny bumps along the stems are actually highly reduced leaves whereas the whorls of photosynthetic “leaves” are actually modified extensions of the stem called “cladodes.”

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It seems that the true leaves of the Japanese umbrella pine have, through evolutionary time, been reduced to tiny, brown scales that clasp the stems. I am not sure what evolutionary advantage(s) cladodes infer over leaves, however, at least one source suggested that cladodes may have fewer stomata and therefore can help to reduce water loss. Until someone looks deeper into this mystery, we cannot say for sure.

As a tree, the Japanese umbrella pine is slow growing. Records show that young trees can take upwards of a decade to reach average human height. However, given time, the Japanese umbrella pine can grow into an impressive specimen. In the forests of Japan, it is possible to come across trees that are 65 to 100 ft (20 – 35 m) tall. It was once wide spread throughout much of southern Japan, however, an ever-increasing human population has seen that range reduced.

A 49.5 million years old fossil of a  Sciadopitys  cladode.

A 49.5 million years old fossil of a Sciadopitys cladode.

The gradual reduction of this species is not solely the fault of humans. Fossil evidence shows that the genus Sciadopitys was once wide spread throughout parts of Europe and Asia as well. Whereas the current diversity of this genus is limited to a single species, fossils of at least three extinct species have been found in rocks dating back to the Triassic Period, some 230 million years ago. It would appear that this obscure conifer family, like so many other gymnosperm lineages, has been on the decline for quite some time.

Despite the obscure strangeness of the Japanese umbrella tree, it has gained considerable popularity as a unique landscape tree. Because it hails from a relatively cool regions of Japan, the Japanese umbrella tree adapts quite well to temperate climates around the globe. Enough people have grown this tree that some cultivars even exist. Whether you see it as a specimen in an arboretum or growing in the wild, know that you are looking at something quite special. The Japanese umbrella tree is a throwback to the days when gymnosperms were the dominant plants on the landscape and we are extremely lucky that it made it through to our time.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3]