Tuesday, May 21, 2013

Rot n' Roll in the Mesozoic Muck

Regular readers of this blog ( all 5 of ya') will have picked up on the perseveration I have evinced on several of my recent posts with all things rotting, decaying and dead. Yes detritus, detritivory and detrital food chains have been on my mind as of late. In a manner that parallels the "invisible hand" of parasites in food chains, the importance of detritus and all that subsist on it is too large to be ignored in any consideration of ecological concerns, past and present.

Let's review a couple of key concepts that prove useful for this post. Decomposers- bacteria/fungi- do the actual breaking down of the key nutrients, organic and inorganic, from dead stuff. They chemically liberate the trace elements, proteins. lipids, starches, and carbs that in turn are more easily swallowed up by themselves or other organisms. Detritivores actually ingest internally discrete chunks of detritus. In doing so they are not only ingesting the dead material- but also consuming the decomposers already subsisting on it. And this is an important concept. By the time an earthworm gets to a leaf on the forest floor it has already been colonized by several stages of microbes who have themselves digested the lion's share of sugars, proteins and lipids. But the decayed leaf is not nutritionally void for the earthworm- it gets benefits from eating spore and bodies of the bacteria/fungi themselves which are full of lipids and proteins. Now it is true that many detritivores target the cellulose/lignin in plant stuff- but even here bacteria/protozoans are needed to digest it and are found in the consumers digestive system. But the relationship between decomposers and detritivores is not a one way street- detritivores "shred" material creating more surface area for microbial decomposers to take hold. Detritivores also spread microbes through faecal pellets. So both decomposers and detritivores work hand in hand for maximal nutrient cycling.

If you are an avid gardener or just like dirt you are probably well aware of the "brown revolution" and how intricate and diverse and how alive soil biota is. Check out this cool pic below.

Although you can extend out the putative detrital food chain illustrated above to include some larger critters- loads of birds, reptiles, amphibians- depend heavily on protein extracted from one or more links in the pic above- by and large it is dominated by smaller critters. In contract to the trophic cascades observed when top predators are removed from live plant/herbivore based trophic systems if you get rid of robins, moles and armadillos detrital based ecosystems would likely keep on going with scarcely a hiccup. But although large animals do not figure prominently in detrital food chains,
Purple Snout Mite (c) USD
that is not to imply that there are no monsters in your soil. Predatory mites have some pretty grim looking predatory tackle. Imagine that guy on the left spearing some tasty nematodes in your lawn. And some types of earthworms in Australia get surprisingly large in size. But by and large any investigation into detrital ecosystems is benefited by the use of microscopy. But what if a large animal, say on the order of several tonnes in weight "hijacked" detrital food chains and inserted itself in some link in the chain usually reserved for smaller critters?

It might sound strange and incongruous with what is seen in modern ecosystems but let me make an analogy with baleen whales. Baleen whales, including the largest animals ever evolved, are famous for subsisting on some of the more smaller critters of the ocean. Rorquals actually go after discrete chunks of schooling baitfish/krill. Their "lunge feeding" technique, which has been described as the 'single largest biomechanical action in nature' is best seen as a type of active predation. Something like a bowhead whale shows a more passive processing of large volumes of water. But I am drifting a bit here 'scuse me- the point here is that baleen whales take up ecological space that might otherwise be occupied by small fish, sea birds, squid etc etc.

So if you are following my torturous logic you probably see where I am going with this- were there ever any terrestrial mega-detritivores in earth history that hijacked ecological space traditionally occupied by less robust critters? Something like a giant walking compost bin? Interestingly there is a bit of evidence pointing to dinosaurs occasionally enjoying a bit of detritus.

In 2007 Karen Chin wrote a paper The Paleobiological Implications of Herbivorous Dinosaur Coprolites from Two Medicine Formation of Montana: Why Eat Wood? which addresses some unusual dino shit. The dino dung in question, (there are several), is unusual for being composed mainly of woody conifer fragments. That the conifer wood was already in a state of decay is evinced by the evidence of fungi degradation. And Chin makes the assertion that the dinosaur was seeking to exploit the nutritive benefits from the fungi and possibly other critters in the decaying wood- rather
Nature of Robertson.blogspot
than seeking the nutritionally bereft wood itself. Now this is an interesting and valid interpretation in my opinion. It does take us away from our comfortable view of herbivorous dinos being  "proper" vegetarians. And, as Chin also asserts, in a world largely devoid of easily digestible angiosperms (esp grasses) dinosaur herbivores may have routinely sought out detrital resources to augment their diets nutritionally. Cool and weird. Keep in mind that the modern habitats with the largest amount of biomass on earth are not the tropical rainforests- but cooler termperate rainforests. (Insert discussion on carbon balance, plant respiration, temperatures). And I don't think it is a stretch to suggest that the biomass of many of the more high latitude Mesozoic ecosystems also was also tremendous with ample  opportunities for detritivory (seasonal or full time?) for the herbivorous dinos that lived there.
But before we get swept up with images of armadas of ornithischians falling upon decaying Mesozoic timber lets consider another possibility that I want to put forth...

It should not go without notice that the dino poop in question in the paper is most certainly from Maisaura peeblesorum. The Two Medicine formation is dominated by the famed "Good Mother Lizard" and both Maisaura eggshell and bones are found at this Late Cretaceous dino latrine. Now nesting colonies of large herbivorous dinos must have been special and weird places, we don't really have a proper modern analogue for what these areas were like. If extended parental care was practiced it can well be imagined that the local flora took a profound hit from possibly thousands of megaherbivores in a relatively small area. In this scenario the hadrosaurs were eating decaying wood because everything else in the vicinity had already been consumed.

But there is another possibility for what was going on here and this one really stretches the weirdness factor and involves some unabashed speculation on my part (but that has never stopped me before): Maisaura peeblesorum was a fungi farmer.

Consider that leaf-cutter ants are often the most important herbivores in many Central/South American ecosystems. But since the ants are not exactly eating the leaves they are cutting, but instead mincing them up for their fungal gardens, they are better described as mycophagists (fungi eaters)
rather than classic herbivores. In a sense they let the fungi digest the plant and the ants consume the more easily digestible fungi.

So what I am I suggesting here- that Maisaura built huge underground fungi colonies? No of course not. What I am suggesting goes like this: We know that Maisaura showed strong site fidelity for nesting colonies and that these colonies may have been exceptionally large. Likely they returned during a specific time of year, most likely coincident with the rainy season and abundant plant growth. Based on crushed eggshells and the preservation of infants several months old in the nest it appears that the nestlings hung out around the nest for some time and were acquiring nutritious food either by foraging or some type of parental provisioning or a combination of the two. From faecal remains we can surmise that adults were consuming fungi-degraded wood and depositing faeeces adjacent to nesting areas. At this point is where the speculation comes in; Maisaura adults were targeting fungi infected wood, most likely of a specific species of fungi, in order to inoculate their own faeces with the spore of the fungi. They would then deposit said faeces in proximity to the nesting colony. The hatchling Maisaura which were somewhat precocial would then have a regular food supply delivered in proximity to the safety of their nest. The faeces would provide an ideal substrate for fruiting bodies (mushrooms) to form, protein rich insects/inverts would be attracted, and the faeces themselves would provide a larder of beneficial digestive microbes that the hatchlings would inoculate themselves with. This provisioning may well have lasted several
Paneolus antillarum on Elephant dung
months as the hatchlings grew in the relative safety of the colony. Although not exactly mother's milk, the faeces along with the insects and fungi growing onto and in them would have been generally a better more protein rich food supply (fungi offer a more complete protein profile than plants) than whatever the hatchling would have foraged for on their own or even regurgitated plant material from the parent- although there is no ruling that out as an additional source of food. But keep in mind that the size of the dino poops reported by Chin was up to 7 liters- a good sized source of food for those cute little  hatchlings.

And if you think coprophagy is weird and unusual behavior- go investigate further- its prabably easier to compile a list of the animals that don't do it rather than those that do, especially among herbivores.

Cute Bunny Chills and Eats Poop.


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heteromeles25 said...

I have my own favorite take on Dr. Chin's work, and it has to do with the rise of ectomycorrhizal fungi.

As a jack mycologist, one of the interesting puzzles to me is ectomycorrhizal evolution. The syndrome evolved convergently at least a dozen times in the plant kingdom, at least five times in the fungal kingdom, and almost no one studies their evolution. Why is this again? Oh yeah, fungi are icky decomposers, and who cares if they're the biggest example of convergence on the planet. I forgot.

Ectos don't show up in the fossil record until the Eocene, but plants (e.g. oak and pine ancestors) that host them start becoming common in the Paleocene. Still, many of the Paleocene forests that I've seen data for were dominated by non-ecto-forming tree species.

Here's my take on it: ectomycorrhizae work by recycling dead woody debris directly into mycorrhizal nutrients that the trees use (it's actually more complicated, but the simple version is that ectos keep nutrients out of mineral soil). This short-circuits the old system, which more-or-less required two steps, where the dead organic debris gets broken down into simple mineral nutrients, the mycorrhizae take up the these nutrients for the plants (not quite true, but pretty close).

Since the ecto trick evolved independently so many different times, one huge question is why it didn't evolve much earlier, say in the Jurassic. My hypothesis has to do with Dr. Chinn's hadrosaurs. Up until the K-T boundary, ectomycorrhizal fungi (assuming they existed in the Mesozoic, which is probably true) were limited to areas where hadrosaurs and other animals weren't processing all the dead woody debris into dung--high elevation pine-oak forests and the like. After the non-avian dinosaurs disappeared, the ectomycorrhizal fungi worked with their host plants to gradually take over forests all over the planet. The hadrosaurs and other huge dinosaurs were no longer present to keep them away from rotting logs, and the fungi, in concert with their host trees, were able to bind up more nutrients in their tissues than previous forest types were. As a result, they gradually took over the northern hemisphere and much of the south, resulting in the taiga conifer forests, oak forests, and southern beech forests we all know and love.

Ultimately, it's possible that the reason so few mammals have made it to dinosaur size is that ectomycorrhizal fungi, working with their host trees, have locked away most of the nutrients that used to support giant, log-chewing dinosaurs. I don't imagine any zoologist will seriously want to think about fungus/dinosaur competition, but then again, I'm not a zoologist.

SciaticPain said...

Interesting take on possible ecto/dino competition for resources. I know hadrosaurs were predominantly a Laurasian phenomena but that does not preclude derived Iguanodonts from performing the same task in Gondwana- where we do have good footprints of very large ornithopods in South America.

Many of the more compelling contemporary arguments gaining traction on how herbivorous dinos got so massive focus on their reproductive strategy. Adults don't have to provision their young and can concentrate on growing big themselves. But I believe a more holistic approach would also involve seeing just where these guys plugged into the flow of nutrients in the ecosystem. It also must take into account the unique botanical, atmospheric and geological characteristics of the Mesozoic. So much room for interpretation, conjecture, and speculation...its frustrating but addictive.

I am however starting to see a shift in the paradigm of imagining herbivorous dinos as simply overgrown archosaurian rhinos, elephants, and cows- and am convinced it proves useful to look across different phyla of verts and inverts and fungi for possible clues as to how these ecosystems functioned.

Thanks for the insightful and useful comment Heteromeles!!

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