A land of Vegetarian Dragons

There was a time when we could have looked out into a forest and as the gentle breeze wafted through the trees with the leaves rustling, we might have been able to feel the ground shake from a herd of Sauropods coming to graze.  It’s hard to imagine animals so much larger then today’s African elephants, walking on the same planet as us, but it happened.  Museums, Films and Books have all tried to encapsulate the magic of what it was like over 66 million years ago.  The sad fact of the matter is that, in all probability, terrestrial life will never be as big again.  It’s not just the enormous nature of dinosaurs that captures our imagination, the sheer diversity is just incredible.  There were triceratops with meter-long horns sticking up, and an enormous crest. Ankylosaurs can be best described as living tanks, covered in bony armour, with a mighty club at the end of a tail, which could give any wrecking ball a run for its money.  Parasaurolophus had a long head with which it could amplify vocalizations.  Over 50% of all recognised Dinosaur species today were herbivorous.  Herbivorous dinosaurs ruled from the late Triassic 235 million years ago to the Cretaceous when the Dinosaurs disappeared 66 million years ago (Barrett, 2014).  Where did these incredible creatures come from? How did they get so diverse in those 169 million years? Most importantly, did they cause the world to change in a way we can still see today?

 

Illustration by Jon Taylor

Illustration by Jon Taylor

 

The first dinosaurs separated from archisaurs in the late Triassic and it is not quite clear what exactly the ancestor of all dinosaurs looked like, as is the case with any large group of animals and plants.  Fossils can identify potential ancestral species but we will always be unable to say definitively this is the ancestor of all the dinosaurs, because of the small proportion of life that fossils represent.  The actual chance of an organism becoming a fossil is quite small and therefore we only ever really get a snap shot of time.

The fossil evidence so far indicates that the first dinosaur was either a carnivore or an omnivore. We know this from the teeth of early dinosaurs such as Eoraptor. Dinosaurs therefore evolved to become herbivores, which was a complex change, as dentistry, digestion and all kinds of other physiological and anatomical changes occurred between the common ancestor and the end results. What is more incredible, is that herbivory did not evolve just once, it evolved many times.  In the Ornithischia, an order of dinosaurs containing stegosaurs and ankalysaurs (see figure phylogenetic tree), herbivory evolved three times.  The way we can tell this is from the teeth becoming more designed for grinding and chewing similar to the teeth we find in cows today (Barrett,2014).

Dinosaur Phylogeny from  (Barrett, 2014)

Dinosaur Phylogeny from
(Barrett, 2014)

We would think that, dinosaurs having been the dominant class of terrestrial mammals for 169 million years, we would still be able to see the effects of their existence today.  One of the grand theories of dinosaur influence, on life today, is that dinosaurs’ coevolved with the angiosperms or flowering plants.  Angiosperms arose in the early to mid Jurassic and a major sub-group, the eudicots, arose in the late Jurassic to mid Cretaceous (Wikstrom et al., 2001).  This fits in with period when dinosaurs were the dominant form of herbivore species but does this mean they were responsible for the evolution of angiosperms?

A study in 2009 wanted to investigate the relationship between angiosperms and herbivore dinosaurs, to see if the hypothesis had any merit.  They looked at the distribution of angiosperms and major herbivore groups, to see if there were any statistical relationships between their distributions.  It is hard to determine potential ranges for dinosaurs, as we only have the fossil evidence, and fossils only occur where the geology will allow.  Therefore, in order to fully determine dinosaur ranges, certain inferences have to be made. Angiosperms are slightly easier because we can tell their distribution from pollen, which is much easier to preserve.  These inherent flaws however are not sufficient enough to negate the results of the study.  The study found that statistically there was only one correlation between one group of dinosaur ranges and angiosperm ranges, although this was only one statistical model.  If dinosaurs were directly linked to angiosperm success, we would have seen that the ranges of a lot more dinosaurs and plants would be correlated, as one species would be relying on the other. There was positive correlation between two groups of dinosaurs and two other groups of plants.  Ankylosaurs distribution, using one statistical model, was found to correlate with Cycadophytes.  Inguanodontians distribution was found to correlate with Cycadales distribution.  The relationships were only found in one statistical model (Richard J. Butler et al., 2009).

Another study looked to see if there was a relationship, from a different angle, by seeing if the number of species of dinosaurs correlated with the number of specific plant species.  If a number of dinosaur species in a particular group increased and the number species of a particular plant group increased, then co-evolution may have taken place.  There was no relationship found between dinosaurs and angiosperms diversity.  There was a relationship found between stegosaur diversity and cycads but this has very little relevance to the modern day landscape because they were both decreasing in number toward the end of the Cretaceous (R. J. Butler et al., 2009).

It’s hard to believe that such a dominant and diverse group of animals have gone and left almost no recognisable difference to the flora of today.  Dinosaurs did leave a legacy, as birds survived the great extinction and thrive today.  The giant herbivores though, will only ever be a source of awe and wonder of an age now long gone.  An age of vegetarian dragons.

 

References

Barrett, P.M. (2014) Paleobiology of Herbivorous Dinosaurs. Annual Review of Earth and Planetary Sciences. 42(1), 207–230.

Butler, R.J., Barrett, P.M., Kenrick, P., Penn, M.G. (2009) Diversity patterns amongst herbivorous dinosaurs and plants during the Cretaceous: implications for hypotheses of dinosaur/angiosperm co-evolution. Journal of Evolutionary Biology. 22(3), 446–459.

Butler, R.J., Barrett, P.M., Kenrick, P., Penn, M.G. (2009) Testing co-evolutionary hypotheses over geological timescales: interactions between Mesozoic non-avian dinosaurs and cycads. Biological Reviews. 84(1), 73–89.

Wikstrom, N., Savolainen, V., Chase, M.W. (2001) Evolution of the angiosperms: calibrating the family tree. Proceedings of the Royal Society B: Biological Sciences. 268(1482), 2211–2220.

 

 

 

 

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