Sunday, 9 October 2022

Sparing a thought for some fossil plants or how I came to love Araucaria

Fig. 1: Giraffatitan brancai struggling to reach a particularly tall araucarian tree (Drawn by Me).

Plants. When was the last time you really put your mind’s focus on them? Despite giving us the food we eat and the oxygen we breathe, their presence around us and our usage of them is such a matter of fact that they do not really register as much more than scenery decoration in the stageplay that is ours, other people’s and animal’s lives. Michael Crichton’s Jurassic Park, of all things, put it quite well:

People were so naïve about plants, Ellie thought. They just chose plants for appearance, as they would choose a picture for the wall. It never occurred to them that plants were actually living things, busily performing all the living functions of respiration, ingestion, excretion, reproduction – and defence. But Ellie knew that, in the earth’s history, plants had evolved as competitively as animals, and in some ways more fiercely. The poison of Serenna vermiformans was a minor example of the elaborate chemical arsenal of weapons that plants had evolved. There were terpenes, which plants spread to poison the soil around them and inhibit competitors; alkaloids, which made them unpalatable to insects and predators (and children); and pheromones, used for communication. […] People who imagined that life on earth consisted of animals moving against a green background seriously misunderstood what they were seeing. That green background was busily alive. Plants grew, moved, twisted, and turned, fighting for the sun; and they interacted continuously with animals – discouraging some with bark and thorns; poisoning others; and feeding still others to advance their own reproduction, to spread their pollen and seeds. It was a complex, dynamic process which she never ceased to find fascinating. And which she knew most people simply didn’t understand.

It was just such a lack of understanding on my part that has recently led to some odd thoughts. You see, I was recently at an Ikea with my girlfriend and there I found a little, less than half a meter tall, plant, whose pot read Araucaria heterophylla. It was cheap, I liked the look of it and I had a vague understanding that araucarians are a very ancient plant group that has been around since the time of the dinosaurs, so I bought it. Once home with my new house organism, I looked up the species and was a bit shocked. A. heterophylla is not a small houseplant, instead I just bought the sapling of what is supposed to become a whole tree, which can potentially grow over 50 meters tall. Furthermore, because it is native to Norfolk Island, a territory of Australia, it can not survive winters outdoors in most European climates, so I will not be able to replant it in my garden should it become too large. Apparently, the saplings were only sold here around this time of the year so they could be used as Christmas trees before being thrown down the woodchipper immediately afterwards, as Christmas trees usually are. It may be “just” a plant, but an odd feeling of responsibility and pity overcame me upon learning this, as I am now taking care of an organism that is supposed to become quite large, while I am also unable to give it the proper conditions to truly thrive in. Sort of like buying a baby orca while you live in the desert.

Fig. 2: The new Araucaria heterophylla I now want to care for. The sauropod toy is by the way one of the original Lego dinosaurs from the 2001 line, which are great for posing. For the fossil in the background, read on.

In the process of looking up how to best care for this plant, I grew a new appreciation not just for the genus Araucaria, but also for the peculiarities and history of the wider family of the araucarians. Here are some things I would like to share with you. If you think some post about fossil plants will be a boring read, I will politely point out that I made a whole post about brachiopods not only work, but also got enough people to read and enjoy it that some are now making speculative evolution posts inspired by it.

Jurassic relicts

The family Araucariaceae, named after the Arauco Province in Chile, is part of the plant order Pinales, which today includes all living conifer trees, such as pines, redwoods and cypresses. All other conifers outside of Pinales are now extinct. Conifers, like ginkgos, cycads and gnetophytes, are part of the gymnosperms, whose cladistic definition is a bit confusing, as they classically also include the extinct seed ferns (pteridospermatophytes) that also gave rise to the angiosperm flowering plants, making the group paraphyletic. When excluding seed ferns and flowering plants, conifers, ginkgos, cycads and gnetophytes do seem to form a monophyletic clade called Acrogymnospermae. Within the conifers, the closest living relatives of the araucarians seem to be the podocarps and the closest relative of those two are the cypresses.

Fig. 3: Araucaria araucana growing around the Chilean volcano Llaima in the Conguillio National Park. Yes, this is also where the sixth episode of Walking with Dinosaurs was filmed.

Coniferous trees originated in the Carboniferous with cypress-like forms such as Walchia, gradually replacing the first fern and progymnosperm trees that had dominated in the Devonian. As the world became drier during the Permian, these tough, seed-bearing plants became more successful and went on to survive the catastrophic end of the Paleozoic without much problem, becoming the dominant group of land plants throughout the Mesozoic. A secret to their success compared to earlier plants groups might be their reproduction, as conifers have evolved a much faster and more direct form of fertilization between the male pollen and the female ovule than other gymnosperms.

Fig. 4: Fossil cones of Araucaria mirabilis from the Paleontological Museum of München.

When exactly the first araucarians evolved is a bit of a mystery, as it depends on the exact classification of certain taxa. If the leaf-taxon Brachyphyllum (as whole plants are rarely preserved, paleobotanists give fossils of specific plant parts, such as trunks and leaves, their own name, even if they come from the same organism) is indeed an early araucarian, their origin might also go as far back as the Late Carboniferous. The oldest definitive araucarian fossil, Araucaria mirabilis, “only” dates back to the Middle Jurassic. Throughout the rest of the Mesozoic, araucarians could be found in the whole world and they and their gymnosperm cousins saw wide success. Then they dramatically declined in the Late Cretaceous, a change often attributed to the rise and spread of angiosperms, whose insect-based pollination made their reproduction more efficient.

Fig. 5: Fossil scorpionflies (Mecoptera) may have been important pollinators of Mesozoic gymnosperms.

Today the flowering plants dominate in most tropical environments, while conifers such as pines only seem to remain dominant in harsher environments such as taigas, where the massive boreal forests form one of the world’s largest carbon sinks. Araucarians specifically have gone completely extinct in the northern hemisphere (except for two species in Malesia) and are now restricted to former Gondwanan landmasses such as South America, Australia and the islands that were once part of the sunken continent Zealandia. In some ways this displacement seems to parallel a similar pattern seen in the evolution of marine invertebrates, where once successful groups such as brachiopods and crinoids gradually disappear from the very productive shallows to migrate into the deep sea. However, just like that example, one has to wonder if we are really seeing here a fierce competition between gymnosperms and angiosperms, where the younger group is supposedly displacing the older one. Surely, taigas must have been a lot less common in the warmer Mesozoic than they are today, which would mean that the conifers were actually more successful at exploiting this new and expanding environment than their younger competitors. And while the insect-based pollination of angiosperms may be more efficient than wind-based pollination, many coniferous plants have independently evolved this form of pollination as well, likely even before the appearance of true flowering plants. Much evidence exists that already in the Early-to-Middle Jurassic, long before flowers and bees, the cheirolepidiacean conifers lived in symbiosis with mecopteran flies (Hallett & Wedel 2016) and such relationships may have even been common in various Mesozoic gymnosperms (Penalver et al. 2015).

Sauropod snacks?

One of the defining characteristics of araucarians, which I evidently failed to know beforehand, is that they grow exceptionally tall, with some specimens reaching 80 meters in height. While not as tall, the species Agathis australis can reach a wood volume of 517 cubic meters, making it the third largest tree in the world, only outmatched by redwoods and giant sequoias. Like their cousins, araucarians can also grow exceptionally old, up to a thousand years (Lüning et al. 2019)

Fig. 6: John C. McLoughlin’s (1979) depiction of Diplodocus feeding on what is implied in the text to be an araucarian.

The fact that araucarians have such incredibly tall-standing canopies, combined with the age they evolved their characteristic morphology in, has always made them prime candidates for co-evolution with sauropod dinosaurs. It is not difficult to imagine the canopies growing ever taller in response to ever-hungry sauropod maws, while the necks of the latter in turn grow even longer, until we reach the ridiculous proportions seen in both groups. There is at least some direct evidence for sauropods feeding on conifers, as in some of the only known coprolites attributed to them could be found the remains of various Mesozoic gymnosperms, including araucarians (Sonuksare et al. 2017). Microwear on the teeth of sauropods is also consistent with a coniferous diet (Hallett & Wedel 2016). Araucarians and relatives with their small, tough needles might seem like a poor diet at first, especially for such gigantic, energy-dependent organisms, but experiments by Hummel et al. 2008 show that through hindgut-fermentation, which sauropods most likely practiced, these plants could actually release a tremendous amount of energy if digested long enough. Basically, sauropods must have had a large hindgut chamber called a caecum in which araucarian and other pre-angiosperm plant matter was collected into a massive internal compost heap, where microorganisms and the heat created by their own decomposition worked tirelessly to turn them into a nutritious mush. This system meant that sauropod digestion worked best the more matter was piled onto the heap and the longer it was allowed to ferment, in turn creating a permanent bioreactor that continuously generated the required energy. This was maybe not just the method by which sauropods fuelled their huge sizes but may have also been a driving force in their gigantism, as a larger body and in turn larger caecum could generate more energy than a smaller one. Other results of the study are also fascinating, as they found that the most energetically rich plants sauropods could have fed on were horsetails, ginkgos, Angiopteris and araucarians. Horsetails and araucarians were especially singled out for their energy-content after longterm digestion, with low-growing horsetails likely having been the prime fodder for sauropod hatchlings while araucarians were likely the main food of adults. They also found that cycads, tree ferns and podocarps would have only made for poor food.

Fig. 7: The immense cone of A. bidwilli.

The strong association between sauropods and araucarians might put a new light onto the gradual decline of the latter. Surely, after over 100 million years of coexistence, the relationship between sauropods and araucarians must have been more complex than one just feeding on the other. One could easily imagine that sauropods sticking their tiny heads into the canopies while feeding and then wandering from tree to tree could have been used as a method of pollination. Furthermore, many modern trees also exploit large animals to disperse their seeds, the most prominent example to many readers probably being Persea americana, whose fruit, the avocado, was likely once adapted for being fed on by Pleistocene megafauna. Indeed, even some modern araucarian seeds, despite being largely carried by wind, are occassionally dispersed by animals, though often by smaller critters such as opossums and possums. The sheer size of their cones, however, seems ridiculously oversized for that purpose. Araucarians have possibly the largest cones among conifers, with the bunya pine (Araucaria bidwillii) having ones about the size of pineapples. Though cones are not the same as angiosperm fruit, could these maybe have originally evolved to be fed on by sauropods? The growing doubt about sauropods having used gastroliths to chew their food (Wings & Sander 2007) only adds further to the possibility of them having been prime seed-dispersers. If so, could the extinction of the sauropods have had a role in the global decline of araucarians?

Fig. 8: Could giant birds like the moa have helped with the dispersal of conifer seeds? It is possible, though there seems to be no evidence so far.

Jumping further off that idea, the continued existence of araucarians in the Gondwanan landmasses is also intriguing. Not too long ago, Australia and New Zealand were home to the large dromornithids and moas respectively. As large, flightless and herbivorous birds (though the latter is still debated in the case of Bullockornis), they would have been hindgut fermenters like the modern ostrich (and sauropods). Unlike ostriches, evidence exists that moas, with their stronger jaws, were not grazers but instead browsers capable of stripping the twigs of tall plants (Paul 1988). In these aspects, they could maybe be thought of as post-Mesozoic mini-sauropods. Could they therefore have coevolved with and helped smaller araucarians survive in the southern hemisphere by dispersing their seeds? The caveat with this idea is that there is no direct evidence from things like coprolites that these birds fed on araucarians or even conifers in general. Araucarians are also widespread in South America, which never had its own version of a giant herbivorous bird (unless some of the famous terror birds had a more flexible diet than previously thought, but there is no evidence for this).

Fig. 9: All the trunked reconstructions over the years really make one underappreciate how truly bizarre the skull of Macrauchenia was, in many ways resembling more a sauropod head than a mammal’s. I imagine all the arguments against trunks on sauropods should also apply here and indeed more recent research suggests this animal had more of a moose-like nose.

The continent did have other high browsers in the form of giant ground sloths like Megatherium, though their dentition seems to indicate more a diet of leaves, as in modern sloths, rather than conifer needles. Intriguing are instead the very sauropod-like skulls of long-necked South American litopterns such as Macrauchenia, which, by analogy, could mean that these mammals may have fed on coniferous plants, much like modern moose (here meaning Alces alces to avoid that old moose-elk mix-up). Data corroborating this is a bit lacking however. Older isotopic studies done on its tooth enamel resolved Macrauchenia as a mixed feeder (MacFadden & Shockey 1997), which would mean that, indeed like a moose, it could have fed on a variety of plants, including conifers like Araucaria. A newer study on the other hand suggests that it and other litopterns were more grazers instead (Oliveira et al. 2020). That last study was however based on tooth microwear, not isotopic data, and there is some evidence from sauropods that a diet of coniferous plants can produce wear-patterns that mistakenly suggest a grazing diet (Hallett & Wedel), so this could be a false positive.

The incredible Turkish araucarian gemstones

Sauropods (and possibly those that came after them) were not the only ones feeding on araucarians. Various human cultures have and still continue to eat the seeds of various species of Araucaria, such as A. araucana, A. angustifolia and A. bidwillii, including the Mapuche people of Chile and Argentina and the Aboriginal Australians. Beyond the culinary, these trees have also had more material uses. From their timber are made various utensils and vehicles and the Maori have used the resin of the genus Agathis to construct weapons (Neich 1966).

Fig. 10: Prayer beads made of oltustone.

The most extraordinary thing I found out though is the peculiar use of fossil araucarians in art. In the Erzurum Province of eastern Turkey, more specifically in the town of Oltu, mines have for centuries been excavating a special type of black gemstone. This so-called olutstone is globally only found in this region of Turkey and its special property is that upon excavation it is actually quite soft and therefore very easily carveable, only beginning to harden when exposed to air. It has thus been formed into a variety of utensils from jewellery all the way to prayer beads and smoking pipes. Upon closer examination, oltustone turns out to be a type of lignite (a sort of proto-coal) formed entirely out of the stems of Late Cretaceous araucarians. Even though they have been extinct in this part of the world since the age of dinosaurs, the araucarians thus managed to have an impact on the cultures of the northern hemisphere long before western explorers would rediscover their living members.

Fig. 11: If you are wondering about the fossil, it is a genuine Chirotherium footprint gifted to me by Torsten Scheyer from the Zoological Museum of Zürich. Apparently, it was found by a former student of his, but because that student did not keep notes nobody knows anymore where it came from, the lack of context now making it scientifically useless. He asked around the museum for anyone to take it home, because otherwise he would have had to throw it away for more storage. Moral of the story: If you find anything, write all the details down and take care of your notes! “The only difference between screwing around and science is writing it down”, to quote Adam Savage.

I now hope you have enjoyed this little journey through the world of araucarians and have grown more appreciation for fossil plants, as well as those that ate and used them. As for the little A. heterophylla, I will try my best to care for it. It will likely not grow tall into a sauropod feeder, though its seeds are edible, so one day it might feed me. The problem here is that, unlike most plants, Araucaria are not hermaphrodites but actually have split male and female sexes and can therefore not self-fertilize. I would therefore need a second plant of the opposite sex to make it produce seeds. The fact that I am even considering this means the plant is already manipulating a big, dumb animal into helping it with its reproduction. Maybe I am a sauropod?

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1 comment:

  1. I guess that in a sense garden plants have domesticated us.