Body Size Distribution of the Dinosaurs
Authors:
Eoin J. O’Gorman. PhD in Marine Ecology from Cork University College.
Currently a faculty of Natural Sciences at the Imperial College London. His main research interest is on “understanding the role of trophic interactions in mediating ecosystem-level responses to global change”. He works on the effect of anthropogenic disturbances on trophic networks properties across terrestrial, freshwater and marine realms.
David W. E. Hone. Ph.D. in Vertebrate Palaeontology with a thesis titled “The phylogenetics and macroevolution of the Archosauromorpha”
Currently alecturer at the School of Biological and Chemical Sciencesin the University of London. He works on dinosaur biology, with special emphasis on gaining understanding of the ecology and behavior of theropods and pterosaurs. Has written several popular science books about paleontology and has an interesting blog on science and paleontology (https://archosaurmusings.wordpress.com/publications/)
Background:
It is well stablished that for many extant groups of organisms at large spatial and taxonomic scales the body size frequency distribution presents a right skewed shape, with most species being small. Nonetheless, more research on this is needed for extinct faunas. This is especially true for non-avian dinosaurs, a very interesting group from this perspective given that it includes the largest terrestrial animals that have existed. It is of interest to assess if this distribution differs from current distributions, reflecting unique evolutionary pressures and adaptations. With this, the questions asked by the authors are: i) what is the body size frequency distribution of dinosaurs? ii) How it compares to that of other groups and iii) what are the differences among dinosaur groups, Mesozoic eras and sites?
Methods:
- Assemble a database of maximum femur length for 329 dinosaur species using literature reports and measurements from museum specimens.
- Evaluate the relation between femur length and body mass and use this relation to estimate body mass for all the dinosaurs in the database.
- Get body mass information on for all major extant vertebrate groups, Cenozoic mammals and Pterosaurs from literature and published databases.
- Create the body mass distribution (BMD) for dinosaurs as well as the abovementioned groups and compare them using the Kolmogorov-Smirnov test. Here, it was especially important to compare the distribution of body size of dinosaurs against that of the other extinct groups to assess the possible effect of preservation bias of large animals over the results.
- Finally, they compared the distribution among Theropod, Ornithischia and Sauropodomorpha to see if different groups with contrasting ecologies differ in the distribution of body sizes. The mass distribution for different periods, as well as different formations were also evaluated.
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Results
- The body mass distribution of dinosaurs is significantly different from that of the other vertebrate groups analyzed. The distribution is skewed to the left, with most of the species being large.
- Cenozoic mammals and Pterodactyls showed different BMDs from dinosaurs, suggesting that the observed pattern is not the result of taphonomic bias.
- When dinosaurs were analyzed by group, only the herbivores Ornithischia and Sauropodomorpha showed the skewed pattern towards higher body size.
- It seemed that this size distribution pattern appeared toward the end of geological eras during the Mesozoic.
- No difference between formations.
Discussion
- Significant difference in the BMD of dinosaurs against other vertebrate groups indicate differences in the biology of these animals.
- As the pattern is apparent only for herbivore dinosaurs, it is hypothesized that the large size provided an advantage in more effective digestion of plant materials.
- Young large dinosaurs could have occupied the niches of small animals.
- The hypothesis of positive effect of extended stability periods on body size increase is supported by the fact that BMD is left skewed at the end of the Mesozoic periods.
Comments:
I enjoyed reading this paper. It is compelling how from this “simple” analysis, the authors are able to provide that amount of information and provide support for some of the theories explaining the evolution of large bod size not only in dinosaurs, but also in other groups.
The authors don´t talk much about this, but do you think that the more normally distributed body sizes for the dinosaurs at the beginning of
I really enjoyed this paper and find the topic fascinating. So my first thought had to do with homeostasis. Since we are still not in agreement as to what process dinosaurs used to maintain homeostasis, that could influence the body size patterns especially as the dominant group during that time. I'm not sure it would be responsible for the drastic switch to a left skewed body size distribution, but it was a thought. I also wonder what body size distribution was present during the Permian period with mammal-like reptiles.
ReplyDeleteI, too, enjoyed this paper. One intriguing thing about dinosaurs is that they had partly hollow bones much like birds, which undoubtedly allowed them to grow larger than if they had mammal-like bones. The large herbivores also were able to swallow large chunks of food instead of having to excessively chew their food. Both of these adaptations may have decreased size limitations.
ReplyDeleteI'd be interested in seeing the bird body size distribution including all the large, now extinct species like the moa, and the Phorusrhacids (terror birds). Maybe the dinosaur distribution looks so different partly because they used only extant species for their other distributions (apart from pterosaurs).
Everybody loves a good dinosaur story, but I find that so many discussion sections about dinosaur ecology are filled with arm-waving and speculation simply because you cannot prove otherwise.
ReplyDeleteThe further you go back in time, the more biased the fossil record becomes. It's not just geologically biased towards the large and robust, but also socially biased. Early paleontologists like Cope and Marsh were out to find the biggest thing they possibly could and, to a certain degree, many field paleontologists still are. With 65Ma+ yrs passing, I don't know we can reliably dismiss the argument that we may be missing a huge chunk of the diversity at the time.
I certainly think it helps that they compared their distributions to pterosaurs and cenozoic mammals and there may be validity to the pattern they found. Modern faunas are missing most of the megafaunas that used to exist, so perhaps it's a real pattern, but I think it is overconfident to explain this pattern and then produce a mechanism for it in dinosaurs.
Super cool topic- after reading the first article I was surprised by the left-skew distributions that were given, though after reading Willow's comment I'm curious whether the findings are valid.
ReplyDeleteWhile this paper was much easier to read and understand their methods and results, one major thing that I noticed is their choosing the largest femur if there were multiple per species. It seems that this would automatically bias the data.
ReplyDeleteI would like to have seen size distributions from before the Triassic, as some of the synapsids and other "mammal-like" reptiles did grow to be quite large.
Also, I wonder if the discussion on sauropods versus ornithischians vs theropds for body size distribution and phylogenetic history would be different given the evidence for an updated dinosaur phylogeny tree (Baron et al. 2017; https://blogs.scientificamerican.com/tetrapod-zoology/ornithoscelida-rises-a-new-family-tree-for-dinosaurs/).
This pretty cool paper that shows a comparison between extinct and extant species body size distributions. It is quite interesting to see the distribution of the body mass of dinosaurs is different than the rest of the vertebrate groups. Also, the hierarchical pattern of niche occupation is pretty neat. I learned more about how the digestive system affects their energy use and diet preferences. This paper is very informative that I can go over again and carefully read each point to learn more information. Very interesting hypothesis and discussable points were generated. I enjoyed reading this paper. The only problem is that I am not good with extinct vertebrate and dinosaur names. I like their shape. Once again, a simple variable (body size) can reveal cool insight in a research. So much to learn. I also think that studying extinct vertebrate shows the true natural history of different period of time (no human influence). That make me think that what would be the picture look like if humans did not influence the 83% of the wildlife by invading directly and indirectly to their life.
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ReplyDeleteI think this paper's topic was quite interesting. It as smart using dinosaur species from different time periods as well. This paper did a nice job helping understand dinosaurs and their large body sizes. I am slightly worried that using the largest femur measurements when offered multiple femurs for particular species might have skewed data more than originally intended. The paper brought more information to light about dinosaurs than originally known. Overall, this was a good read packed with a lot of knowledge.
ReplyDeleteI was wondering how much was actually known about dinosaur ecology and natural history. After reading Willow's comment it was clarified though: "not much". Even if what they propose is cool, it should be taken carefully. Also, how do they know the gut size of a dinosaur? It is know for many species?
ReplyDeleteIt's interesting that dinosaurs are unique among vertebrates because they have right-skewed distribution. I really enjoy reading the part about digestive efficiency. But then I have to agree with Laura. The author should have listed more data on gut size in the discussion.
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