Dinosaurs, Plants & Coevolution

In my previous post about Laramidia, and in attempting to explain how the plants and trees were able to provide enough fodder for the large numbers of herbivorous dinosaurs that grazed there, I looked at how a combination of climate, physical and meteorological barriers, and the food value of the plants themselves, could have sustained such a voracious herbivorous assault.

And yet, there was another driving force behind the ability of the plants to, not only survive, but to flourish and quickly recover from sustained grazing – and that was the dinosaurs themselves.  Animal partnerships, relationships and animal/plant relationships were almost certainly as evident in the past as they are today and the term used to describe the change affected on one species that interacts with another is coevolution.

The concept of coevolution was first raised by Charles Darwin in Origin of Species and has been gradually refined through the years but the theory still comes under intense scrutiny as scientists try and decide whether the evidence presented for each individual example is truly coevolution.  In other words, any form of interaction or symbiosis that produces evolutionary change can only be classed as coevolution when substantiated by thorough research and the appropriate phylogenetic analysis.

Having said that, however, there is ample evidence that implied coevolutionary interaction is extremely likely and there are numerous examples around today – bumblebees and flowers are an appropriate example. The effect of one species on another cannot be denied and in its simplest form is very apparent. Entire species have become extinct or virtually extinct because a foreign species was introduced by accident or even on purpose. Examples are legion: grey squirrels driving out the reds in England, possums in New Zealand devastating flightless bird populations and, in Mauritius, even Man eradicated the dodo.

Indirect extinctions are yet another form of interaction that is also far removed from coevolution.   The eradication of a species without any thought can have a massive effect, from the smallest invertebrate through to the biggest apex predator – a domino effect if you will. This form of extinction is most likely to affect different species that are extremely dependent on each other or those taxa that are the cornerstone of many niches thus affecting multiple species.

A couple of very simple examples of this include six species of mite that became extinct when the Carolina parakeet disappeared and a vine plant in Singapore that, when it disappeared, took with it one particular species of butterfly that was dependent on it. To demonstrate a possible future scenario, off the coast of California, there is a delicate ecosystem involving kelp, sea urchins and sea otters. These three species are so totally dependent on each other that the fragility of such ecosystems becomes evident.

Image by Katie Hale

The sea otters keep the sea urchin population under control which, if left unchecked, would completely devour the kelp forests. Without the kelp, a multitude of different organisms would disappear and yet, without a sufficient population of urchins, the sea otter population would quickly crash and decline. This clearly demonstrates the fragility of such ecosystems and dramatically highlights how the extinction of one species can affect so many others.

Partnerships, or mutualism to give it its correct term, are another form of interaction. The most oft cited example of this is the relationship between ants and acacia trees. The ants depend on the acacia for nesting and food whereby  the plant provides specialised thorns that provide shelter, special nectar buds that provide food for the ants and leaf tips that are also of a high food value – especially to the larvae. The ants, in return, remove fungal spores from the tree that may invade the acacia host and provide aggressive defence against any herbivore foolish enough to attempt to feed on the tree. This particular example has often been cited as coevolution so amply demonstrates how hard it can be to classify.

All of the examples highlighted above demonstrate how different species affect one another through various examples of interaction. Despite these all being contemporary examples, it does demonstrate that such relationships have existed, almost certainly, from the earliest forms of life right through to the present day. So how does this all relate to dinosaurs?

Bob Bakker, in his book the Dinosaur Heresies, featured a chapter titled Mesozoic Arms Race in which he postulates how both herbivore and carnivore spurred on the mutual evolution of the two groups – in other words, true coevolution. As theropods got bigger to cope with larger prey, the herbivores got bigger still. If it wasn’t size, it was armour, horns, teeth, claws, agility, and social interaction – a plethora of derived traits continually evolving to keep the different genera ahead of the game and ensure survival. Those that did not evolve became extinct. This is the classic example of survival of the fittest – natural selection. Evolve or die.

I suspect that trees and plants were also part of this life or death struggle and that their ability to be able to sustain large populations of giant herbivorous dinosaurs was a direct response to intense grazing pressure from dinosaurs. As herbivorous dinosaurs got bigger and their ability to crop the plants, masticate and digest plant material became more sophisticated, the plants and trees had to adapt or become extinct. They had to find new ways to be able to withstand the stress of being constantly cropped, be able to reproduce quickly, and grow at inflated rates.

The fact that there are so many different shaped and sized herbivorous dinosaurs is highly indicative that they were successful. Small ornithopods would graze at low levels, mid-sized animals could take advantage of both whilst animals such as brachiosaurs and titanosaurs could feed at higher levels. Whilst appreciating this is a very simplified way of looking at things, the point is well made. Herbivorous dinosaurs evolved into many different and complex varieties to cope with the continually evolving plants.

In the Cretaceous, things moved on to another level with the evolution of much more complex herbivorous dinosaurs.  The well-publicised iguanodont and hadrosaurian dental batteries, which enabled a much more efficient form of food processing, along with ceratopsians, ankylosaurs, sauropods and other ornithopods, pushed plants and trees to new heights of production.

But this time the plants and trees had even more help with an increase in the amount of CO2 in the atmosphere, increased warmth and moisture and this combination enabled the flora to grow at inflated rates. And there was yet one more innovation by the plants to come which aided reproduction, fertilisation and increased diversity and sustainability.

It has been often noted that the dinosaurs may have been responsible for the appearance of flowers during the Cretaceous (eg again see Bakker 1986). Although there is no foundation or scientific evidence to back this assertion, it does appear likely that intense dinosaurian herbivory may have contributed to the evolution of flowering plants (Barrett et al 2001 but see also Lloyd et al 2008).

The truth is almost certainly something in between. The dinosaurs probably prompted rapid development in plants to avoid eradication and extinction. During the Cretaceous conditions were so good for growth that the plants simply took advantage of the situation and evolved extremely rapidly - with flowers simply another tool which allowed further diversity and distribution. This enabled the dinosaurs to feed on an almost never ending supply of fodder and is possibly the chief reason why Laramidia was able to sustain such large populations of big dinosaurs.     

References

Bakker, R. 1986. The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction. William Morrow and Company, New York.

Barrett P.M, Willis K.J. Did dinosaurs invent flowers? Dinosaur-angiosperm coevolution revisited. Biol. Rev. 2001;76:411–447. doi:10.1017/S1464793101005735[PubMed]

Lloyd, G.T., Davis, K.E., Pisani, D., Tarver, J.E., Ruta, M., Sakamoto, M., Hone, D.W.E., Jennings, R., and Benton, M.J. 2008. Dinosaurs and the Cretaceous Terrestrial Revolution. Proceedings of the Royal Society, Series B 275, 2483-2490 (doi:10.1098/rspb.2008.0715).