The Loom

Dinosaurs: Beyond Cute

i-82017f0ab143fe20fd18bba7493469d9-Mahakala500.jpgAt about a pound and a half, Mahakala omnogovae was certainly a cute dinosaur. But cuteness is not why paleontologists traveled to the remote ends of Mongolia to find it. It’s part of a much bigger story.

Paleontologists have known for a while now that birds evolved from one group of dinosaurs called theropods–the two-legged beasts that include the likes of Tyrannosaurus rex and Velociraptor. But precisely which lineage of theropods birds belong to has been the subject of a lot of debate. These debates, like all debates in science, are fueled by uncertainty. Paleontologists can base their arguments only on the fossils they have already pulled out of the ground. So when a new dinosaur turns up, it provides an opportunity to give these arguments a fresh look.

Today in Science, a team of paleontologists describe the newly discovered Mahakala, which lived about 80 million years ago. They survey the anatomy of this long-tailed, chicken-sized dinosaur. And then they do what paleontologists generally do these days: they compared it to a lot of other dinosaurs. They generated an evolutionary tree, which I’ve reproduced at the bottom of this post. You can get a bigger picture by clicking on it. A new species such as Mahakala sometimes allows scientists to get a clearer understanding of how species are related to one another. It can help show, for example, the order in which certain traits found in a group of species evolved.

Adding Mahakala to the mix produces a tree that shows birds sharing a common ancestry with two groups of dinosaurs–the dromeosaurids and the troodontids. (Update: on the tree below, birds are the upppermost branch, troodontids are the middle, and dromeosaurids are the lower most.) Earlier studies have pointed to those groups as close relatives of dinosaurs birds before, but this new analysis offers a more precise hypothesis: dromeosaurids and troodontids descend from a common ancestor, and that common ancestor in turns shares a common ancestor with birds. Roughly speaking, they’re siblings to each other and first cousins to birds.

Sorting out these family matters allows the scientists to learn about how the bird body plan–so different from any other living animal’s body plan today–gradually evolved. A couple months ago I wrote about how new research shows that the small genomes of birds actually got small in their non-flying dinosaur ancestors. Well, other traits seem to have gotten a head start, too. Scientists have been discovering more and more dinosaur fossils with traces of feathers or feather-like growths on their skin. In this new tree, a species known as Jinfengopteryx–which had originally been considered a bird–ends up on the troodontid branch, complete with contour feathers of the sort you can find on birds today. It’s additional evidence that the common ancestor of birds and troodontids and dromeosaurids already had fairly sophisticated feathers.

What’s particularly interesting about this new study is that the scientists looked at how big the animals all were. In recent years paleontologists have been finding some very big species of bird-like dinosaurs–see Utahraptor, for example at the bottom of the chart. The fuzzy picture of how all these dinosaurs are related to birds has made it difficult to understand how the size of bird ancestors changed. Flying birds are relatively small, although some flightless birds have evolved to big sizes. Did they evolve from some Utahraptor-sized ancestor? A lot turns on this question. If the ancestors of birds shrank down dramatically, they might have done so through an evolutionary change to their growth–stopping early, for example, or growing more slowly.

To answer that question, the scientists first estimated how much each dinosaur weighed, extrapolating from their bones. (This method works successfully when scientists test it on living birds.) They then used a series of statistical methods to estimate how much the common ancestors of these species weighed. Working their way back to the base of the tree, the scientists concluded that the ancestor of birds and their closest dinosaur relatives–a group called Paraves–was about the size of Mahakala. In other words, small.

This finding will have to be taken into consideration in future hypotheses about the origin of flight. The small size of birds was not an adaptation to flight. The reverse may have been true: cute little paravians may have already had a light enough body to experiment with gliding and flying. Their monstrous relatives got large later–on four separate occassions, in fact, marked 1 through 4 on this chart. This finding is part of a pattern that goes way beyond dinosaurs: animals get big again and again. In fact, the really interesting story may lie there, and not in the tiny size of bird ancestors. Cute, it turns out, only gets you so far.

Reference doi: 10.1126/science.1144066Source

[Illustration: © Frank Ippolito 2007]

i-ee1ea9cc120c8026112852f4ac3ae9c3-Mahakala small.jpg

Comments

  1. #1 Jim Lemire
    September 6, 2007

    where on the cladogram does the lineage leading to modern birds arise?

  2. #2 Carl Zimmer
    September 6, 2007

    Jim–Birds are the uppermost lineage. Only the primitive ancient birds were included in this study. Living birds would share a common ancestor with the branch that included Yixianornis and Apsaravis.

  3. #3 Zach Miller
    September 7, 2007

    God, I love Paravian evolution. So if this little bugger is the newest basalmost dromaeosauroid, that means that deinonychosaurs were not ancesterally flighted. How does Mahakala differ from Sinovenator? I have yet to read (or even get) the paper, so I’d love to know the details. Actually, if you have the paper, sir, I’d love it if you could email it to me.

  4. #4 Raymond
    September 7, 2007

    “So if this little bugger is the newest basalmost dromaeosauroid, that means that deinonychosaurs were not ancesterally flighted.”

    Well…..it simply means the ur-dromaeosaurid _may_ have not engaged in flight, you still have the
    ur-deinonychosaurid and further-back, the ur-paravid, either which may have been flighted(if I’m reading the tree correctly).Semantics I know :)

    Let’s not even talk about Rahonavis seeming to have re-aquired flight btw *Buitreraptor* and *Uenlagia*

    Dang, maniraptorans are just devious little stick insects they are!

  5. #5 djlactin
    September 7, 2007

    “occassions”? for shame, carl!

    a real comment on the contents: i’m dismayed that the upper (bird) clade in the diagram does not continue to the present. is this an oversight or an artifact of incomplete taaxonomic coverage?

  6. #6 Thomas R. Holtz, Jr.
    September 7, 2007

    djlactin,

    The analysis that Turner et al. used did not include modern birds. What is shown on the figure is only the eumaniraptoran (= Avialae + Deinonychosauria) part of the larger theropod analysis included in the supplementary data.

    By the way, the paper is available here and the supplementary data here.

  7. #7 Zach Miller
    September 7, 2007

    Thomas, you’re my HERO. Boy, the supplementary data is a paper in itself. Got to love Cope’s Rule on this one. One wonders how far back the Paravian lineage goes…

  8. #8 Dave Hone
    September 7, 2007

    Carl, for shame, for shame sir! Where are you italics on the genus and species names?

  9. #9 LT
    September 8, 2007

    I dont think a stegosaurus looks anything like a bird….if dinosaurs evolved into birds, what kind of bird did the huge, bulky dinosaurs develop?

  10. #10 Zach Miller
    September 8, 2007

    Not all dinosaurs evolved into birds. Only one branch of paravian maniraptoran tetanurine neotheropodian saurichischian dinosaurs evolved into birds. The rest went extinct.

  11. #11 kai
    September 9, 2007

    Earlier studies have pointed to those groups as close relatives of dinosaurs before

    Should this actually say “close relatives of birds“?

  12. #12 Marco Ferrari
    September 10, 2007

    In many occasions “modern” birds got big. Moas (New Zealand), elephant birds (Aepyornis, Madagascar), Gastornis (Americas). And, almost always, it was in absence of competitors (I gather mammals, in these cases). Might it be the same for the big deinonychosaurs? Any evidence of absence?

    Marco

  13. #13 Zach Miller
    September 10, 2007

    In fact, they probably DID become hyperpredators thanks to an absence of competition. During the Early Cretaceous in North America, when Deinonychus and Utahraptor were the big predators, tyrannosaurs had yet to migrate from Asia, and allosaurs were in decline (I believe it was just Acrocanthosaurus at that point). Troodontids may have gotten larger based for some other reason, as they were hunting different things and, at only six feet long, never got as large as the big raptors.

  14. #14 J. V. Jackson
    September 17, 2007

    It’s extraordinary that the authors failed to state the obvious message underlying the whole analysis:
    Check the cladogram for yourself, and note that although the top (“bird”) part obviously contains fliers, the bottom part contains Microraptors with not two but four wings, and Rahonavis whose forelimbs, if wings, would be considered long if found in a modern bird (though of perfect wing form). Now ask which of these two options you think it represents:

    1: The entire lot were descended from flying ancestry, some losing it later.

    2: Flight evolved more than once in this group.

    Now ask yourself why the authors choose the latter, and consider it so obvious a choice that they don’t consider it worth defending.

    Also, ask why the authors made the arbitrary, undefended decision to put Archaeopteryx some way away from the root node, when they could have indicated the whole tree was descended from something very close to Archaeopteryx by giving the root Archaeopteryx as one daughter and the rest of the tree as the other. After all, they’ve already opined that the root species was … – matching Archaeopteryx perfectly. Making the earliest type a daughter of the root minimises the total (ghost) lineage of a cladogram; having a cladogram with excessive ghost lineage (which this one has – I tested it yesterday) invalidates it as a tree that could have been produced by evolutionary processes.

    Now ask yourself why we should waste time on deciding whether Jinfengopteryx was a bird or a troodont.

    Now ask what the ancestor of Archaeopteryx was if all the birdlike types shown here are descended from “something extremely similar to Archaeopteryx”. Oviraptors? Allosaurs?

    This paper has convinced people to write stuff like:
    “This finding will have to be taken into consideration in future hypotheses about the origin of flight.” Why just future hypotheses? Why not honour the duty the authors owe to the public and tell them about some of the other theories that have been knocking around for decades? After all science is supposed to be about comparing theories.
    Also:
    “The small size of birds was not an adaptation to flight.” Evidence is observations better accounted for by one theory than another. There’s plenty in this paper that is better explained by the theory that the ancestor of troodonts and dromaeosaurs (the non-”bird” parts of the cladogram) could fly.
    It’s a mistake to choose an alternative, and a scandal to suggest to the public that only those alternatives exist.

  15. #15 Zach Miller
    September 17, 2007

    For reasons I can’t fully comprehent, the paleontology community (for the most part) is very uncomfortable with the idea that Paraves was flighted, and that flight was lost in the Deinonychosauria. The idea has been around for almost thirty years, but despite the numerous anatomical modifications for flight (or loss of flight) in the dromaeosauroids and troodontids, those two groups are always regaled to a “so close, yet so far” outgrouping to Aves proper.

    This bothers me, because in the midst of all the evidence to the contrary, paleontologists steadfastly block out any idea that deinonychosaurs are secondarily flightless. It’s certainly POSSIBLE, but that possibility is rarely brought up.

  16. #16 J. V. Jackson
    September 18, 2007

    Well said, webmaster! (…in comment 15). (I hope it was realised that my attack was directed at the paper rather than the original blog post.)

    Many people think the reason they can’t understand why palaeontologists think as they do is because the palaeontologists are using more sophisticated thinking or more obscure knowledge. Scott Adams blogs about this:
    http://dilbertblog.typepad.com/the_dilbert_blog/2007/09/fossils-still-b.html

    Generating these family trees is an exercise in the use of numerical statistics (on biological entities) to generate belief.

    It truly is a miracle that those such as the majority of palaeontologists, gain a greater insight into such areas from their geology undergraduate degree than those who disagree with them and who have either postgrad qualifications in subjects around information and knowledge processing, or who have done foundation work in developing cladistics algorithms (such as Felsenstein) but who do not endorse the palaeontologists’ conclusions.

    Even more astonishing is the amazing superiority of insight those who merely aim to study palaeontology when they leave school have, over those whose opinions one might otherwise have considered valid due to their background.

    Having said that though… the Turner et al 2007 paper produced the most realistic family tree yet!

  17. #17 Zach Miller
    September 19, 2007

    Yes, it is a very good phylogeny, which is why I like the paper. I like the phylogeny even more than Mahakala! :-D However, like I said before, the suggestion that Deinonychosauria is secondarily flightless is completely ignored, which makes me a sad panda.

The site is undergoing maintenance presently. Commenting has been disabled. Please check back later!