Saturday, January 19, 2013

Hungry, Hungry Bronto

ttopple toppleOver at SVPOW Brian Engh posted a beautiful pic of a Brontomerus amidst a background of semi-toppled trees.

Brian Engh

He actively solicited thoughts on this topic and well, I just had to chime in. Long time readers will know that I don't fall in line with the ubiquitous assumption that larger dinosaurian herbivores, especially sauropods, were the large scale vegetative destructors they are often assumed to be. Here is a brief review of my arguments in favor of sauropods having a lighter touch on vegetation than generally assumed.

1) Weaker jaws and dentition than elephants- which routinely strip bark and girdle massive trees outright killing them.
2) Tree toppling is dangerous, even for large sauropods. An elephant has a much more compact build than a sauropod and is better equipped to topple trees. A sauropod, like camels, ratites, moas, and  giraffes (all of which do not tree topple) would have simply reached up to grab the food it wanted with its neck.
3) Overbrowsing by sauropods on a massive extent was inhibited by their constant movements. In Camarasaurus migratory patterns have been found through isotopic evidence, But all sauropods would have been kept on the move by a combination of predators, parasites, water, food, and reproductive factors. Large tress were subjected to pressure at sporadic intervals but not constant pressure.
4) The largest sauropods were rare and even among more modest sized species the reproductive strategy of dinosaurs suggests a wide spectrum of sizes in a population at any one time- the vast majority of which would be less than full grown.
5) Very little in the adaptive arsenal of modern gymnosperms suggests that they sustained rampant overbrowsing in the past on a scale unsurpassed today. On the contrary gymnosperms tend to grow slower than angiosperms. Many, such as cycads and tree ferns, will outright die if their growing tip is severed.
6) African Elephants serve as ecosystem engineers whose actions of tree killing and brush clearing benefit a myriad of other, smaller critters. Many sauropods, on the contrary, existed within diverse browsing guilds of other sauropods. It does not make ecological sense that multiple species were all acting simultaneously as agents of tree/brush clearing in their environment. No, some species were much softer handed than generally assumed.
7) Keep in mind that sauropods generally lived on smaller island continents than African Elephants. And they did so in greater diversity and at greater size than elephants.Yet they did not eat themselves out of house and home.
8) Uric acid is less water soluble than urea. Mammals can simply "piss themselves to death" and are highly dependent on water compared to uric acid producing reptiles/archosaurs/birds. Elephants, beholden to water, will heavily browse vegetation surrounding water resources during dry times.

If you want to look at analogues for how sauropods interacted with the plants around them look at their closest herbivorous archosaur relatives- ratites and moas. These long necked browsers browse(d) across a wide swath of vegetation on a wide variety of plants. The nature of their feeding discourages heavy browsing. Camels and giraffes offer perhaps the next best analogy to sauropod browsing habits. While they do create browse lines, they are not known for tree toppling despite certainly having the strength and size to do so. While sauropods certainly had the capacity and strength to tree topple, i am dubious that it would have been an excessively common occurrence- especially since it would have been a  lot easier to have used their necks to reach the food that they wanted.

Tree killing, slow reproducing, intelligent, water dependent, and resource intensive- Elephants offer perhaps the roughest approximation for sauropod behavior and ecological role in my view.

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Anonymous said...

There's a critical mistake here: plants adapt *FAST* to the lack of herbivory. This is a classic problem with island plants. They shed defenses and invest more in growth, but that makes them exquisitely vulnerable to introduced herbivores. This happens at the scale of thousands to millions of years.

Modern conifers have not dealt with non-avian dinosaurs for the last 65 million years, and it's silly to assume that they stopped evolving at the KT boundary.

We can see some relicts of anti-sauropod defenses in things like redwoods' ability to readily regrow, the rather interesting toxins of cycads and ferns. Still, I seriously doubt we're seeing the whole panoply of tricks they used against dinosaur browsing.

The other thing to remember is that the trick for plants isn't to repel all herbivores, it's to better survive herbivory than your neighbors do. This is the trick that grasses use to take over the world. This strategy only works when herbivores are a major force. Get rid of them, and every plant that uses this strategy gets overshadowed by those that don't. That undoubtedly happened after the K-T.

Duane Nash said...

Thanks for comment heteromeles!!!

Although I have used arguments involving island endemicism in the past the main thrust of my argument here is that I see no evidence that sauropods were any more, and in fact may have been less, stressful on vegetative communities than modern African elephants. For me we should consider both options- were sauropods massive tree killers, habitat modifiers and brush clearers on a scale unsurpassed in modern ecosystems? or were they in fact "softer" on the vegetation than generally assumed? I argue the latter and provide arguments in favor of my view.

Just because sauropods were absolutely larger than elephants does not prove, imo, that they were absolutely more destructive than elephants.

I am well aware that it gets a bit tricky in trying to infer past plant/herbivore relationships based on extant plants. And yes many conifers can "stump-sprout" and regrow from knarled conditions- but this may be due to surviving fires, landslides, and competition from other trees in the canopy, wind fall etc etc.- we don't have to automatically invoke sauropods. It is interesting to me that many cycads, tree ferns and palms (pretty common in Cretaceous) will out right die once their growing tip is severed. I do not see why, even after 65 million years of evolution, that these plants would ever lose the ability to regrow from chomped growth tips if they ever even had the ability to begin with. Even if mammalian herbivores generally do not target these plants to eat, the ability to resprout from damaged tips proves useful in many instances such as fire, natural breakage, insect damage. I just don't see why that ability would be lost unless it was never there to begin with.

Again thanks for the comments!!

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