Taxon of the Week: Limusaurus

As a treat, this week’s Taxon of the Week is a dinosaur. Even better, it’s a dinosaur with very small hands. Despite it’s small hands, it’s managed to cement itself amidst a pretty sizeable debate, yes ladies and and gentle, this week’s TotW is Limusaurus inextricabilis.

The facts

Limusaurus was discovered in 2009 by Xing Xu, a Chinese palaeontologist whose seemingly always discovering a new species of dinosaur (over 30 valid species to date). At around 1.7m in length (roughly the size of a large-ish dog), Limusaurus was far from the biggest and most exciting looking new dinosaur of 2009, however, it was certainly a bit weird:

• For starters it’s the first Asian ceratosaur ever described. Yes, you heard right a ceratosaur.
• For your main course, it’s herbiverous.
• To finish off with dessert, it’s got a beak.

Limusurus. Not your average ceratosaur.

True, whilst indeed not your average ceratosaur (or for that matter, theropod) such traits aren’t that weird in primarily carniverous clades. Within Theropoda, there’s many secondarily herbiverous taxa, incl. everyone’s favourite weirdo taxon, the therizinosaurs. Let’s also not forget the well known beaked herbiverous theropods, ornithomimosaurs. Oh, and did you know crocodylomorphs had a crack at beaks and herbivory (of course they did). Even though I keep harping on about its small hands, for frak’s sake, just take a look at alvarezsaurs (with even more pathetic arms than T. rex). So why exactly am I taking the time to blog about Limusaurus?

Mononykus trying, the meme that couldn’t be due to it’s preposterously pathetic forelimbs. Hastily drawn by me.

A debate as simple as 1, 2, 3

If you’re a fan of dinosaurs/palaeontology then you should be aware that birds evolved from theropod dinosaurs. News flash: this isn’t new. This notion really started to gain ground after seminal work by John Ostrom in the late 1960s/early 1970s (including that famous drawing of the ‘naked Deinonychus‘ as Richard likes to call it) who noted a lot of avian-like features in Deinonychus. On top of that, palaeontological records show multiple transitional forms between smaller theropods and birds (e.g. the ever eminent Archaeopteryx), and even some of the larger theropods show avian affinities (e.g. the cranial pneumatic sinuses found in Alioramus). Those are only a select few pieces of evidence that have made the evolutionary link between birds and theropods almost undeniable, there’s a so many more, it’s astounding (incl. inferred behaviour, such as avian sleeping positions in theropods: Xu & Norell 2004, and apparent egg brooding behaviour in Oviraptor: Norell et al. 1995). Watch the video if you need more peer reviewed proof that dinosaurs (even if they did look a bit like birds) were awesome. Try and show a little respect.

However, there’s always new discoveries that just love to get people arguing. The issue revolves around digit homology/identity. Essential, theropods (via evolution) ‘lost’ two fingers (digits) to arrive at the three-fingered hand seen in most theropods (aside from the aforementioned ‘weird taxa’, who lost more than 2). The same is true for birds. However, the debate revolves around which digits are lost, and which 3 form the fingers of the hand. In tetanurans (essentially the more advanced theropods, and by extension birds), it has long been thought that digits IV and V were lost, leaving a three-fingered hand consisting of digits I, II and III. Now, you’d expect birds, by being derived tetanurans to have this digital formula.  Well that would be nice wouldn’t it. Unfortunately the question of I,II,II or II,III,IV in modern birds has been debated for a very long time. New evidence in the mid-late 1990s from developmental and genetic studies showed us that the three digits of the avian hand actually developed from digits II, III and IV. Gasp, a spanner in the works!

This new evidence was then used (rather wrongly) to attempt to oppose the hypothesis that birds evolved from dinosaurs (Feduccia 2002). While I agree, the 90s developmental evidence from modern birds creates some novel evolutionary dynamics to investigate, it cannot be used to deny the whole host of other evidence that links dinosaurs to birds. Following on from the developmental studies, genetic studies in the early 2000s showed that during the ontogeny of some birds, the digit identity would change from the initial II,III,IV to I,II,III. This led to the occurrence of the ‘Frame Shift’ hypothesis, which suggests that certain genetic pathways associated with dinosaur/avian digit identity allow for ‘rapid’ changing of digit homology throughout dinosaurian/avian evolution (at it’s core, and that’s a very simplified summary). Not to be bogged down by detail (genetics isn’t my strong suit), developmental geneticists thought they’d cracked it, and that the change in digit identity over avian/dinosaurian evolution was likely to have been caused by these frame shifts.

Feduccia’s (2002) argument in a nutshell.

Enter Limusaurus. So, finally, we come to Limusaurus’ role in all of this. In 2009, Xu et al. looked at Limusaurus’ small hands and went “you know what, that’s a reduced digit I” (NOT ACTUAL QUOTE), which makes Limusaurus’ digit identity II, III, IV. As we discovered right at the start of this article (before the I made my lack of genetics knowledge crystal clear), Limusaurus is a ceratosaur, which isn’t a tetanuran, but a more primitive theropod, meaning that the II,III,IV digit identity may well be shared by most/all tetanurans, with Limusaurus representing an intermediate of sorts. Thus Xu et al. state that the digit identity of Limusaurus is more in favour of a slower, stepwise acquisition of the digit identity seen in advanced tetanurans, and eventually birds. But, as the famous saying goes, you know what small hands means…(small gloves?)

That’s right, a big controversy.

If you’re confused (don’t worry, so am I), this may help.

The small gloves are off

The Xu et al. (2009) caused a fairly serious debate, with authors such as Vargas et al. arguing that the digit condition seen in Limusaurus is derived, and based on developmental and genetic work that they (Vargas et al.) carried out, suggest that faster genetic shifts occurred in the evolution of birds. Xu et al. quickly responded (and quoted  Arthur Conan Doyle in a somewhat dramatic conclusion) and argued that the shift proposed by Vargas is not likely when the digit (and manus) morphology of fossil tetanurans is considered.

I’d sincerely like to end this post with a succinct conclusion, saying that the debate has, over the last few years been wrapped up. However, such large debates in palaeontology, due to the very nature of our field (i.e. everything we love is dead) are rarely fully resolved. This case is no exception. Researchers from Yale (Bever et al. 2011) and other top world universities have stated time and time again that the frame shift hypothesis is still viable in the context of avian evolution, and in a recent summary by Xu and Mackem, Xu is not so sure, saying neither hypothesis has evidence to topple the other. Not one for revelling in an unnecessarily depressing ending (*coughs* Firefly) I’ll leave you with this: yes, we can’t always find all the answers to big questions in palaeontology and evolution, but by creating a synergistic relationship between palaeontology and biology (genetics, evo devo etc.), future discoveries in both fields are sure to shed some light on even the biggest of debates.

If you’d like to read more on this subject, and weren’t put off by my murdering of the genetics side of things, then I’d highly recommend the aforementioned Xu & Mackem (2013) paper for a recent summary of the field (see references below, it’s in bold).

References

Xu, X. et al. (2009). A Jurassic ceratosaur from China helps clarify avian digit homologies. Nature 459, 940-944.

Xu, X. & Mackem, S. (2013). Tracing the Evolution of Avian Wing Digits. Current Biology 23, R538–R544 (and references therein).

Bever, G., S. et al. (2011). Finding the frame shift: digit loss, developmental variability, and the origin of the avian hand. EVOLUTION & DEVELOPMENT 13:3, 269–279.

Vargas, A.O., Wagner, G.P. & Gauthier, J.A. in Nature Proceedings (2009).

Vargas, A.O. & Wagner, G.P. Frame-shifts of digit identity in bird evolution and Cyclopamine-treated wings. Evolution & Development 11, 163-169 (2009).

Young, R. L. et al. (2011). Identity of the avian wing digits: problems resolved and unsolved. Dev Dyn. 2011 May;240(5):1042-53.

What’s New(s) 31/01/14

So, only four days late putting this up.  I’ll save the things happening this week for Fridays post, but fear not! Plenty occurred in the paleontological world last week.  Some of it’s even non-Mesozoic (gasp!).

Look at those ichthyosaurs go! Diversity remains roughly constant with the Jurassic throughout their Cretaceous range. From Fischer et al.

The last hurrah of the ichthyosaurs.  This group of iconic Mesozoic marine reptiles, who recently starred in TOTW, didn’t quite make it all the way to the K/T party, and went extinct during the late Cretaceous.  Traditionally the group has been seen as going out on a bit of a whimper after a lengthy decline from the Jurassic.  New work on European ichthyosaurs by Fischer et al has shown that this picture isn’t necessarily correct, with ichthyosaurs showing a pretty stable diversity throughout their existence, at least in Europe.

Polypterus having a breathe. From Graham et al.

Stem tetrapods can breathe easy with the news that Polypterus, a basal ray-finned fish, breathes air through its spiracles.   These large paired openings on its had have previously been argued to have a use in air breathing, but Graham et al have for the first time demonstrated this to actually be the case.  Stem tetrapods (the lineage of fish leading up to terrestrial vertebrates, including our friend Tiktaalik) possessed spiracles which have been linked to the transition to air breathing, and this work supports this argument.

Reconstruction of the size of NHMUK R16303 (grey) in comparison to a more normally sized silesaurid. From Barrett et al.

Giant Silesaurids!  Silesaurids are a group of Triassic archosaurs thought to be somewhere near the base of the dinosaur lineage, and so are important to understanding early dinosaur evolution.  Previously silesaurids have all been fairly small animals, particularly when compared to their later dinosaurian brethren.  Barrett et al have, however, described the femur of an unprecedentedly large silesaurid from Tanzania.  This specimen, with the catchy name NHMUK R16303, shows that silesaurids gained bigger sizes than previously thought, bigger than some early dinosaurs, with implications for ideas of why and how dinosaurs were so successful.

The paper just has pictures of bits of broken femur in, so instead here’s a picture of a penguin undergoing what appears to be some kind of interview. From Wikipedia.

An ancient seabird has been described from the Palaeocene of New Zealand by Mayr and Scofield, and unlike everything else described from this locality it isn’t a penguin!  While I love penguins as much as the next man, this fleshes out the picture of the avian fauna of this area shortly after the K/T extinction.  It also continues to expand the picture of the diversity of birds in the Palaeocene, showing that the dinosaurs were still doing pretty well.  Extinct indeed.

Apologies for the decline in picture usefulness through this post, but it was a either this or a complicated schematic of the action of Hox genes which I’d have done a poor job of explaining.   From Wikipedia.

Fish fingers. For a long time, scientists have been trying to reconcile the digits of tetrapods with fishes’ fins.  These two structures that are superficially similar, but frustratingly different in layout.  While fossil data has been building up a picture of this transition, ‘evo-devo’ studies have also been providing valuable information.  One such recent study, by Woltering et al, suggests that the digits of tetrapods aren’t in fact homologous (ie. evolutionarily the same) to the fin radials in a fishes’fin.  This is based upon the expression of Hox genes (genes that dictate the layout of a developing organ) in zebrafish and mice.  When Hox genes from the fish were expressed in developing mice, they only affected development in the proximal parts (ie. arm) of the limb, suggesting fish don’t use the ‘digit-causing’ part of their genetic toolkit.  This in turn suggests the two structures are not homologous.

• Fischer et al (2014) High diversity in Cretaceous icthyosaurs from Europe prior to their extinction.  PLOS One
• Graham et al (2014) Spiracular air breathing in polypterid fishes and its implications for aerial respiration in stem tetrapods. Nature communications.
• Barrett, Nesbitt and Peecook (2014) A large-bodied silesaurid from the Lifua Member of the Manda beds (Middle Triassic) of Tanzania and its implications for body-size in Dinosauromorpha.  Gondwana Research.
• Mayr and Scofield (2014) First diagnosable non-sphenisciform bird from the Early Paleocene of New Zealand.  Journal of the Royal Society of New Zealand.
• Woltering et al (2014) Conservation and divergence of regulatory strategies at Hox loci and the origin of tetrapod digits.  PLOS Biology.