Tag Archives: not as far as dinosaurs

Shaking dinosaur hips and messing with their heads

by Piter Kehoma Boll

This week brought astonishing news regarding the phylogeny of dinosaurus, as you perhaps have heard or read. New anatomical evidences have completely rebuilt the basis of the dinosaur family tree and I’m here to explain a little bit of what happened.

As we all know, Dinosaurs include a great variety of beasts, from the meat-eating theropods to the long-necked sauropods and from the horned ceratopsians to the armored ankylosaurs, among many others.

largestdinosaursbysuborder_scale

Silhouette of a human compared to the largest known dinosaurs of each major group. Picture by Matt Martyniuk.*

For more than a century now, dinosaurus have been divided into two groups, called Ornithischia and Saurischia. Ornithischia (“bird-hipped”) includes dinosaurus whose pelvic bones are more similar to what is found in birds, with a pubis directed backward. Saurischia (“lizard-hipped”), on the other hand, have a pubis directed forward, as in reptiles in general. This grouped the theropods and the sauropods in the same group as Saurischia while other dinosaurus were grouped as Ornithischia. But birds are actually theropods, thus being lizard-hipped dinosaurus and not bird-hipped dinosaurus! Confusing, isn’t it? So let’s take a look at their hips:

Pelvic_bones

Comparison of the hips of a crocodile (Crocodylus), a sauropod (Diplodocus), a non-avian theropod (Tyrannosaurus), a bird (Apteryx), a thyreophoran (Stegosaurus), and an ornithopod (Iguanodon). Red = pubis; Blue = ischium; Yellow = ilium. Picture by myself, Piter K. Boll.**

As you can see, the primitive state, found in crocodiles, sauropods and early theropods, is a pubis pointing forward. A backward-pointing pubis evolved at least twice independently, both in more advanced theropods (such as birds) and the ornithischian dinosaurus. But could we be so certain that Tyrannosaurus and Diplodocus are more closely related to each other (forming a clade Saurischia) just because of their hips? Afterall, this is a primitive hip, so it is very unlikely to be a synapomorphy (a shared derived character). Nevertheless, it continued to be used as a character uniting sauropods and theropods.

A new paper published by Nature this week, however, showed new evidences that point to a different relationship of the groups. After a detailed analysis of the bone anatomy, Matthew G. Baron, David B. Norman and Paul M. Barrett have found 20 characters that unite theropods with ornithischians and not with sauropods. Among those we can mention the presence of a foramen (a hole) at the anterior region of the premaxillary bone that is inside the narial fossa (the depression of the bone that surrounds the nostril’s opening) and a sharp longitudinal ridge along the maxilla.

skulls

The skulls of both ornithischians and theropods (above) show an anterior premaxillary foramen in the narial fossa (shown in yellow) and and a sharp ridge on the maxilla (shown in green), as well as other characters that are not present in sauropodomorphs and herrerasaurids (below). Composition using original pictures by Carol Abraczinskas and Paul C. Sereno (Heterodontosaurus), Wikimedia user Ghedoghedo (Eoraptor and Herrerasaurus), and flickr user philosophygeek (Plateosaurus).**

In his blog Tetrapod Zoology, Dr. Darren Naish comments the new classification and points out some problems that arise with this new view. One of them is the fact that both theropods and sauropodomorphs have pneumatic (hollow) bones, while ornithischians do not. If the new phylogeny is closer to the truth, that means that pneumacity evolved twice independently or evolved once and was lost in ornithischians.

He also mentions that both ornithischians and theropods had hair-like or quill-like structures on their skin. In theropods this eventually led to feathers. Could this be another synapomorphy uniting these groups? Maybe… but when we think that pterosaurs also had “hairs”, one could also conclude that a “hairy” integumentary structure was already presented in the common ancestor of dinosaurus. In this case, perhaps, we only had not found it yet on sauropods. Now imagine a giant Argentinosaurus covered with feathers!

One concern that appeared with this new organization is whether sauropodomorphs would still be considered dinosaurs. The term “dinosaur” was coined by Richard Owen in 1842 to refer to the remains of the three genera known at the time, Iguanodon, Hylaeosaurus and Megalosaurus, the first two being ornithischians and the latter a theropod. As a consequence, the original definition of dinosaur did not include sauropods. Similarly, the modern phylogenetic definition of dinosaur was “the least inclusive clade containing Passer domesticus (the house sparrow) and Triceratops horridus“. In order to allow Brachiosaurus and his friends to continue sitting  with the dinosaurs, Baron et al. suggested to expand the definition to include Diplodocus carnegii. So, dinosaurus would be the least inclusive clade containing P. domesticusT. horridus and D. carnegii.

In this new family tree, the name Saurischia would still be used, but to refer only to the sauropodomorphs and some primitive carnivores, the herrerasaurids. The new clade formed by uniting theropods and ornithischians was proposed to be called Ornithoscelida (“bird-legged”), a name coined in 1870 to refer to the bird-like hindlimbs of both theropods and ornithopods (the subgroup of ornithischians that includes dinosaurs such as Iguanodon and the duck-billed dinosaurs).

What can we conclude with all that? Nothing will change if you are just a dinosaur enthusiast and do not care about what’s an ornithischian and a saurischian. Now if you are a phylogeny fan, as I am, you are used to sudden changes in the branches. Most fossils of basal dinosaurs are incomplete, thus increasing the problem to know how they are related to each other. Perhaps this new view will last, perhaps new evidence will change all over again the next week.

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ResearchBlogging.orgReferences and further reading:

Baron, M., Norman, D., & Barrett, P. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution Nature, 543 (7646), 501-506 DOI: 10.1038/nature21700

Naish, D. (2017). Ornithoscelida Rises: A New Family Tree for DinosaursTetrapod Zoology.

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Filed under Evolution, Extinction, Paleontology, Systematics

Biological fight: Should we bring mammoths back?

by Piter Kehoma Boll

Everybody knows the amazing large animals that are found in Africa and Southeast Asia. Elephants, giraffes, rhinos, hippos, horses, lions, tigers… such large creatures, mostly mammals, are usually called megafauna, the “large fauna”.

Mammals as big as the African bush elephant once roamed the Americas. Photo by flickr user nickmandel2006*.

Mammals as big as the African bush elephant once roamed the Americas. Photo by flickr user nickmandel2006*.

The Americas once had an astonishing megafauna too, full of mastodons, mammoths, giant sloths, giant armadillos and sabertooth tigers. Nowadays it is restricted to some bears and jaguars. What happened to the rest of them? Well, most went extinct at the end of the Pleistocene, around 11,ooo years ago.

South America once had mammals as big as an African bush elephant. Picture by Dmitry Bogdanov** (dibgd.deviantart.com)

South America once had mammals as big as an African bush elephant, such as the giant sloth. Picture by Dmitry Bogdanov** (dibgd.deviantart.com)

As humans already inhabited the Americas by this time, it was always speculated if humans had something to do with their extinction. It is true that nowadays hundreds, thousands of species are endangered due to human activities, so it is easy to think that humans are the best explanation for their extinction, but 10 thousands years ago the number of humans on the planet was thousands of times smaller than today and our technology was still very primitive, so it is unlikely that we could hunt a species to extinction by that period… if we were working alone.

No, I’m not talking about humans cooperating with aliens! Our sidekick was the famous climate change. Periods of extreme warming during the pleistocene seem to have had a strong impact on the populations of many large mammals and, with the aid of humans hunting them down and spreading like an invasive species, the poor giants perished.

Le Mammouth by Paul Jamin

Le Mammouth by Paul Jamin

This happened more than 10 thousand years ago, TEN THOUSAND YEARS.

In Africa, elephants and large carnivores are well known for their importance in structuring communities, especially due to their trophic interactions that shape other populations. The extinct American megafauna most likely had the same impact on the ecosystem. As a result, suggestions to restore this extinct megafauna has been proposed, either by cloning some of the extinct species or, more plausibly, by introduced extant species with a similar ecological role.

Svenning et al. (2015) review the subject and argue in favor of the reintroduction of megafauna to restore ecological roles lost in the Pleistocene, an idea called “Pleistocene rewilding” or “trophic rewilding”, as they prefer. They present some maps showing the current distribution of large mammals and their historical distribution in the Pleistocene, which they call “natural”. They also propose some species to be introduced to substitute the ones extinct, including replacements for species extinct as long as 30 thousand years ago. Now is this a good idea? They think it is and one of the examples used is the reintroduction of wolves in the Yellowstone National Park. But wolves were not extinct for millenia there, neither are they a different species that would replace the role of an extinct one.

A wolf pack in Yellowstone National Park

A wolf pack in Yellowstone National Park

Rubenstein & Rubenstein (2016) criticized the idea, arguing that we should focus on protecting the remaining ecosystems and not trying to restore those that were corrupted thousands of years ago. They also argue that using similar species may have unintended consequences. Svenning et al. answered that this is mere opinion and that a systematic research program on trophic rewilding should be developed. The reintroduction of horses in the New World and its non-catastrophic consequences is another point used to respond to the critiques.

So what’s your opinion? Should we bring mammoths, mastodonts, giant sloths and sabertooth tigers back? Should we introduce elephants and lions in the Americas to play the role that mastodonts and smilodonts had?

My opinion is no. The idea may seem beautiful, but I think it is actually fantastic, too fabulous and sensational. Horses may have come back to the Americas without bringing destruction, but we cannot be sure with anything, even with several theoretical and small-scale studies. We all know how often introducing species goes wrong, very wrong. Look at poor Australia and Hawaii, for instance. Furthermore, those giant mammals went extinct TEN THOUSAND YEARS AGO. Certainly ecosystems have adapted to their extinction. Life always finds a way. There are worse threats to those ecosystems to be addressed, such as their eminent destruction to build more cities and raise more cattle and crops.

Get over it. Mammoths are gone. Let’s try to save the elephants instead, but without bringing them to the Brazilian cerrado. They don’t belong there. They belong in the African savannah.

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References:

Rubenstein, D. R.; Rubenstein, D. I. From Pleistocene to trophic rewilding: A wolf in sheep’s clothing. PNAS, 113(1): E1. DOI: 10.1073/pnas.1521757113

Svenning, J-C.; Pedersen, P. B. M.; Donlan, C. J.; Ejrnæs, R.; Faurby, S.; Galetti, M.; Hansen, D. M.; Sandel, B.; Sandom, C. J.; Terborgh, J. W.; Vera, F. W. M. 2016. Science for a wilder Anthropocene: Synthesis and future directions for trophic rewilding research. PNAS, 113(4): 898-906. DOI: 10.1073/pnas.150255611

Svenning, J-C.; Pedersen, P. B. M.; Donlan, C. J.; Ejrnæs, R.; Faurby, S.; Galetti, M.; Hansen, D. M.; Sandel, B.; Sandom, C. J.; Terborgh, J. W.; Vera, F. W. M. 2016. Time to move on from ideological debates on rewilding. PNAS, 113(1): E2-E3. DOI: 10.1073/pnas.1521891113

Wade, L. 2016. Giant jaguars, colossal bears done in by deadly combo of humans and heat. Science News. DOI: 10.1126/science.aag0623

Wade, L. 2016. Humans spread through South America like an invasive species. Science News. DOI: 10.1126/science.aaf9881

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Filed under Conservation, Ecology, Evolution, mammals, Paleontology, Zoology