<i>Limusaurus inextricabilis</i>

Posted 18 June 2009 by PZ Myers

My previous repost was made to give the background on a recent discovery of Jurassic ceratosaur, Limusaurus inextricabilis, and what it tells us about digit evolution. Here's Limusaurus—beautiful little beastie, isn't it?

(Click for larger image)
Photograph (a) and line drawing (b) of IVPP V 15923. Arrows in a point to a nearly complete and fully articulated basal crocodyliform skeleton preserved next to IVPP V 15923 (scale bar, 5 cm). c, Histological section from the fibular shaft of Limusaurus inextricabilis (IVPP V 15924) under polarized light. Arrows denote growth lines used to age the specimen; HC refers to round haversian canals and EB to layers of endosteal bone. The specimen is inferred to represent a five-year-old individual and to be at a young adult ontogenetic stage, based on a combination of histological features including narrower outermost zones, dense haversian bone, extensive and multiple endosteal bone depositional events and absence of an external fundamental system. d, Close up of the gastroliths (scale bar, 2 cm). Abbreviations: cav, caudal vertebrae; cv, cervical vertebrae; dr, dorsal ribs; ga, gastroliths; lf, left femur; lfl, left forelimb; li, left ilium; lis, left ischium; lp, left pes; lpu, left pubis; lsc, left scapulocoracoid; lt, left tibiotarsus; md, mandible; rfl, right forelimb; ri, right ilium; rp, right pes; sk, skull.

What's especially interesting about it is that it catches an evolutionary hypothesis in the act, and is another genuine transitional fossil. The hypothesis is about how fingers were modified over time to produce the patterns we see in dinosaurs and birds.

Birds have greatly reduced digits, but when we examine them embryologically, we can see precisely what has happened: they've lost the outermost digits, the thumb (I) and pinky (V), and retain the forefinger, middle finger, and ring finger (II-IV), which have been reduced and fused together. This is called Bilateral Digit Reduction, BDR, because they've lost digits from the medial and lateral sides, leaving the middle set intact.

Dinosaurs, when examined anatomically, seem to have a different pattern: they have a thumb (I), forefinger (II) and middle finger (III), and have lost the lateral two digits, the ring and pinky finger (IV-V). This arrangement has been advanced as evidence that birds did not evolve from dinosaurs, since they have different bones in their hands, and getting from one pattern to the other is complicated and difficult and very unlikely.

The alternative hypothesis is that there is no conflict, and that dinosaurs actually underwent BDR and their digits are II-III-IV…but that what has also happened is a frame shift in digit identities. So dinosaurs actually have three digits, which are the index, middle, and ring finger, but they've undergone a subtle shift in morphology so that their forefinger develops as a thumb, and so forth.

Now we could resolve all this easily if only the physicists would get to work and build that time machine so we could go back to the Mesozoic and study dinosaur embryology, but they're too busy playing with strings and quanta and dark matter to do the important experiments, so we've got to settle for another plan: find intermediate forms in the fossil record. That's where Limusaurus steps in.

Limusaurus has a thumb, a tiny vestigial nubbin, and has lost its pinky completely. This is a (I)-II-III-IV pattern, and is evidence of bilateral digit reduction in a basal ceratosaur. In addition, the forefinger has become very robust, and while still distinctly a digit II, has been caught in the early stages of a transformation into a saurian first digit. It's evidence in support of the dinosaurian II-III-IV hypothesis and the frameshift in digit identity! It's almost as good as having a time machine.

Want to learn more? Carl Zimmer has a summary of the digit changes, while one of the authors of the paper, David Hone, also discusses the digits (the story is a little more complicated than I've laid out), and also has more on the rest of the animal—it's a herbivorous ceratosaur, which is interesting in itself.

Xu X, Clark JM, Mo J, Choiniere J, Forster CA, Erickson GM, Hone DWE, Sullivan C, Eberth DA, Nesbitt S, Zhao Q, Hernandez R, Jia C-k, Han F-l, Guo Y (2009) A Jurassic ceratosaur from China helps clarify avian digit homologies. Nature 459(18):940-944.

23 Comments

s1mplex · 18 June 2009

Cool, there's also a crocodilian fossil right next to it!

fnxtr · 18 June 2009

(Cartman)

Heh-heh. Sweeeet.

(/Cartman)

Zach Miller · 18 June 2009

Amazing fossil, wonderful preservation, incredible insight into the diversity of Jurassic theropods. But I'm not convinced of the frame-shift idea. Ceratosaurs generally have pretty weaksauce arms with stumpy fingers anyway (Ceratosaurus, Aucasaurus, Carnotaurus, etc.) so this may simply be an extreme development following that trend. It's kind of like finding a hindwing on Microraptor: it doesn't mean that birds ancestrally had hindwings--it just means that Microraptor did.

It's entirely possible that Limusaurus has nothing at all to do with digit shifting, but that its digit formula is merely an aupomorphy unto itself.

Frank B · 18 June 2009

Thanks PZ, for the information. The fossil record has another fascinating story to tell, about how God made Lamusaurus with a different digital arrangement 6000 years ago;) But honestly, this is great stuff.

John Harshman · 18 June 2009

I would like to point out that Archaeopteryx apparently did have hindwings. Though I'm too lazy to find the citation right now. Was it in Paleobiology?

Zach Miller · 18 June 2009

It's a Nick Longrich paper, and I forget the journal. I have the paper, though. And yes, Archaeoptyerx apparently did have a small hindwing running up the thigh and lower leg, but nothing like Microraptor's. More like small symmetrical feathers running up the leg, like in a modern raptor.

John Harshman · 18 June 2009

Zach Miller said: It's a Nick Longrich paper, and I forget the journal. I have the paper, though. And yes, Archaeoptyerx apparently did have a small hindwing running up the thigh and lower leg, but nothing like Microraptor's. More like small symmetrical feathers running up the leg, like in a modern raptor.

No, not at all like a modern raptor. Modern raptors have contour feathers covering the legs. Archaeopteryx had flight feathers on the legs. Modern raptors' legs are tucked into the body in flight. Archaeopteryx would have spread its legs in flight to generate lift, and conceivably flapped them to generate thrust, though that seems unlikely. Much more like Microraptor than like any modern bird.

Henry J · 18 June 2009

about how God made Lamusaurus with a different digital arrangement 6000 years ago;)

Maybe God preferred analog to digital? :) Henry

Dean Wentworth · 18 June 2009

I was wondering if the frame shift would have affected the hindlimbs as well as the forelimbs. This paper Serial Homology and the Evolution of Mammalian Limb Covariation Structure deals exclusively with mammals, but I'm assuming the results probably would apply to dinosaurs and birds as well.

Abstract The tetrapod forelimb and hindlimb are serially homologous structures that share a broad range of developmental pathways responsible for their patterning and outgrowth. Covariation between limbs, which can introduce constraints on the production of variation, is related to the duplication of these developmental factors. Despite this constraint, there is remarkable diversity in limb morphology, with a variety of functional relationships between and within forelimb and hindlimb elements. Here we assess a hierarchical model of limb covariation structure based on shared developmental factors. We also test whether selection for morphologically divergent forelimbs or hindlimbs is associated with reduced covariation between limbs. Our sample includes primates, murines, a carnivoran, and a chiropteran that exhibit varying degrees of forelimb and hindlimb specialization, limb size divergence, and/or phylogenetic relatedness. We analyze the pattern and significance of between-limb morphological covariation with linear distance data collected using standard morphometric techniques and analyzed by matrix correlations, eigenanalysis, and partial correlations. Results support a common limb covariation structure across these taxa and reduced covariation between limbs in nonquadruped species. This result indicates that diversity in limb morphology has evolved without signficant modifications to a common covariation structure but that the higher degree of functional limb divergence in bats and, to some extent, gibbons is associated with weaker integration between limbs. This result supports the hypothesis that limb divergence, particularly selection for increased functional specialization, involves the reduction of developmental factors common to both limbs, thereby reducing covariation.

raven · 18 June 2009

I was wondering if the frame shift would have affected the hindlimbs as well as the forelimbs.

I don't see why, necessarily. Bat wings are certainly different from bat legs. Same for birds. Whales have flukes for front limbs and vestigal hind legs.

danny Satterfield · 19 June 2009

Fascinating. I love learning a piece of new science! Thanks!

Glendon Mellow · 19 June 2009

Great article, and what a gorgeous fossil. I kind of hope it can be left in its matrix like that.

Dean Wentworth · 20 June 2009

As I understand this post and the links, the hypothesized hand development goes as follows:

The basal theropods have hands with five digits, digit I being a robust thumb, digits IV and V being wimpy. Some theropods (e.g., the Coelophysidae and Dilophosauridae) keep this configuration. In the line leading to the ceratosaurs and Tetanura, first the pinky is lost, then the thumb. At this point the first digit (II) is an index finger. Next, in the Tetanura line, a frame-shift happens such that digits II, III, and IV develop as though they were digits I, II, and III, i.e., the first digit is a thumb again. The hand now looks uncannily like a basal theropod's that has lost the last two wimpy digits.

In the line leading to ceratosaurs and Tetanura, thumbs apparently became superfluous and hence were lost. Yet, in the Tetanura line, the digit frame-shift giving them thumbs again was advantageous enough to be selected for and propagated. I do think birds are descended from theropod dinosaurs. That said, the frame-shift scenario seems shaky.

Dean Wentworth · 20 June 2009

The Vargas et al. paper on HoxD-11 expression in the bird wing is very convincing that there has indeed been a frame shift in bird forelimb digit identity.

Per that paper, the frame shift doesn't change the primary axis of cartilage formation, only the ordering of HoxD gene expression in the digits.

A Limusaurus-like hand essentially has a nub for a thumb and a robust finger for digit II, presumably due to HoxD gene expression. Why would a frame shift to such a hand reintroduce a digit with "thumbness?" Wouldn't it just result in the loss of digit I, digit II becoming a nub, digit III a robust finger, and so on?

What if the frame shift happened in a theropod that had four digits (I - IV), digit I being a robust thumb? After the shift, digit I disappeared and new digits II - IV took on the characteristics possessed by original digits I - III. The shifted HoxD gene expression for original digit IV caused previously absent digit V to reappear, now with the characteristics of original digit IV. Before and after the shift, there are still four digits, the first being a robust thumb.

(P.S. It won't hurt my feelings a bit for the knowledgeable people here to tear me a new one over this, so dive in.)

Telamon · 21 June 2009

Wow, what an amazing find!
Though I never would expect them to find a toothless, herbivorous ceratosaur...
Evolution, what have you wrought?

Dean Wentworth · 22 June 2009

I'm curious why nobody has proposed that the homeotic frame shift happened in a basal theropod with 5 digits. Without a frame shift, three-digited theropods have digits I, II, and III. From what I've read about digit reduction, digit I is the easiest to lose, followed by digit V (Morse's Law). Losing digit IV is next to impossible because of its primary axis position.

Figure 2 in the paper An old controversy solved: bird embryos have five fingers compares the hands of, among others, Ceratosaurus (4 digits) and Herrerasaurus (5 digits).

If the frame shift happened in a basal theropod with a Herrerasaurus-like hand, all the digits would shift over one condensation location and digit V would be lost because it has no base from which to grow, right? If so, the resulting hand would be virtually identical to the Ceratosaurus hand.

Make the Ceratosaurus's first digit vestigial and something very like the Limusaurus hand would result.

With this scenario there's no need to postulate that thumbs arose again after becoming vestigial or lost entirely.

Dean Wentworth · 22 June 2009

I can't seem to get those links to work.

The Evolution of HoxD-11 Expression in the Bird Wing

An old controversy solved: bird embryos have five fingers

Dean Wentworth · 23 June 2009

Could anyone suggest a blog where bird-dinosaur digits and how Limusaurus fits in might elicit more discussion?

tresmal · 24 June 2009

Dean Wentworth said: Could anyone suggest a blog where bird-dinosaur digits and how Limusaurus fits in might elicit more discussion?

Try Tet Zoo.

Dean Wentworth · 24 June 2009

Thanks, tresmal.

KP · 10 July 2009

The LYING LIARS at ICR have recenly weighed in on this.

http://www.icr.org/article/4773/

It's so bad the thought of the line-by-line refutation they've opened up here exhausts me...

Jaime A. Headden · 12 July 2009

PZ has his data off slightly, although the paper is clear on the topic.

Not all dinosaurs have lost the outer two digits, and most in fact have only lost the fifth digit, if that. Some sauropods possess a minor fifth digit, although it's a nubbin of a phalanx. It is only theropods that progressively reduce the fourth digit as well, and it's at least a phalanx large in some basal theropods, while the fith metacarpal is still present. This is the condition in Herrerasaurus, whcih may or may not be a theropod, but is likely to be a saurischian (Sauropodomorpha + Therpoda) at the very least. It is therefore improper to imply that dinosaurs (or even theropods) have only three digits.

Limusaurus possess only two functional digits, with three phalanges each, indicating not only bilateral digit reduction, but progressive phalangeal loss. While this occurs in some birds (ostriches have a single functional digit, the major digit), it does so well after the established II-III-IV pattern of digit identity is established. Limusaurus shows loss of the innermost digit, outermost digit, and phalanx reduction. The manus is a mirror immage. Moreover, it appears within a clade of dinosaurs that show similar manual digit and phalangeal reduction, and assuming that Limusaurus has any bearing on the pattern of digit loss for birds and theropods is to assume that digit regrowth apparently also occured -- as Limusaurus' pattern of x-II-III-x-x can definitively imply such regrowth is mandatory -- convergence is a far more simplistic and parsimonious argument that the authors do not entertain as a conflict with their attempt to support a variation of the frame-shift hypothesis.

Alexander Vargas · 6 October 2009

Vargas, AO, Wagner GP, and Gauthier, JA. Limusaurus and bird digit identity. Available from Nature Precedings hdl.handle.net/10101/npre.2009.3828.1

Here is our response to the Limusaurus paper that was recently rejected by nature, not for any technical reason but because it was considered not to be of sufficient interest/importance.

We have uploaded it at the nature precedings citable archive, because we think it is important there is a quick and citable reply that unlike Xu's proposal, is consistent with the view of the larger community of theropod paleontologists, namely, that tetanuran digits still are I, II, III.
We are preparing a longer paper on this topic.