Posted by More dinosaury stuff from January 2009: for the background story, please see part I.
Last time we looked at therizinosauroids. Alvarezsaurids have also been the subject of much discussion lately. This is thanks to Ceratonykus oculatus, a new taxon from the Upper Cretaceous of Mongolia (Alifanov & Barsbold 2009). Alvarezsaurids have had a slightly confusing taxonomic history. For many people, it started in 1993 with Mononykus (then called Mononychus, a name preoccupied by a beetle), first described as a bizarre flightless bird closer to modern birds than to Archaeopteryx [reconstructed skeleton of Mononykus shown here]. It was later shown that Alvarezsaurus, described in 1991, was a primitive relative of Mononykus (thereby meaning that the name Mononykidae had to be replaced with Alvarezsauridae). The idea that these weird animals might be birds was contested by several workers (mostly on entirely typological grounds), but data from new taxa, and the creation and testing of larger data sets, has meant that everyone now supports a non-avian position for these dinosaurs. Exactly where they do go within Theropoda is now the subject of debate however: some say they’re close relatives of ornithomimosaurs, while various different positions near the base of Maniraptora have been supported by others.
Alifanov & Barsbold’s paper is therefore pretty surprising, because the authors cast doubt on the idea that alvarezsaurids are theropods. The fact that their paper is in Russian hasn’t helped those of us in the Anglophonic world, but a translation is now available. The authors say that alvarezsaurids are unlike theropods in having strangely large prefrontals, in having a long surangular process and short angular process in the lower jaw (theropods usually have a short surangular process and long angular process), in having a totally weird pelvis, and in apparently lacking gastralia. Mysterious small, spike-like bones are suggested by the authors to be claws from fourth and fifth digits, and they say that the presence of such are inconsistent with a theropod identification. The shape of the brain is also said to be weird and unlike that of theropods [Ceratonykus skull shown below].
The hypothesis that alvarezsaurids aren’t theropods should be ignored: while it’s true that alvarezsaurids are weird, so are lots of other animals, and just because you’re a weird theropod, that doesn’t mean you’re not a theropod. Tens of characters seen throughout the alvarezsaurid skeleton are uniquely theropodan, and they are definitely members of this group. Anyway, several of the ‘non-theropod’ characters mentioned by Alifanov & Barsbold (2009) are incorrect: big prefrontals are not unique to alvarezsaurids but are seen in other theropods (in particular, ornithomimosaurs) and the possible ‘fourth and fifth claws’ in Ceratonykus are totally unconvincing and look nothing like claws.
A new basal bird
One more thing on theropods before we move to ornithischians. Though having come to attention in December 2008, the new basal bird Anchiornis huxleyi Xu et al., 2008 (or is it 2009?) hasn’t yet got the accolade it deserves, so here’s my bit in bigging it up. It’s from Liaoning Province, China, but (unlike so many recently described theropods) is not from the Yixian Formation: rather, it’s from an unnamed unit of uncertain age but from somewhere round about the Jurassic-Cretaceous boundary [Anchiornis holotype shown here: (a) part and (b) counterpart. Scale = 30 mm].
Its arms are long (about 80% the length of the hindlimbs) and it hands are about 130% longer than the femur. While only a few faint feather impressions are associated with the specimen (and they’re not preserved adjacent to the limbs), these long arms and hands suggest that Anchiornis had a functional wing (or ‘functional forelimb airfoil’, as the authors say). A strongly convex semilunate carpal shows that Anchiornis had a highly mobile wrist similar to that of modern birds, and the fact that the semilunate doesn’t cap metacarpal I lends support to the idea that the semilunate carpal does not include a contribution from distal carpal I. In the hindlimb, the tibiotarsus is very long in relation to the femur (150% of its length), the hallux is almost certainly not reversed (it wasn’t in Archaeopteryx either, by the way), and the ungual on the second toe was particularly big compared to the others. The species name honours Thomas Huxley, and I won’t insult your intelligent by telling you why.
The big deal, of course, is that – when included in a phylogenetic analysis – Anchiornis is part of the avian lineage, but outside of the clade that includes Archaeopteryx and modern birds. The branch-based clade that includes all birds is called Avialae; the node-based Archaeopteryx + modern birds clade is Aves. Anchiornis is thus a non-avian avialian. However, as we’ve seen before, it isn’t necessarily the ‘only’ animal we know of from this region of the cladogram: some studies find that scansoriopterygids go here as well [Anchiornis details below, from Xu et al. (2008). (a) Caudal vertebrae in ventral view; (b) hand (shown without phalanges); (c) ilium (anterior to right); (d) femur (proximal end at right); (e) foot. Scale bar = 3 mm in a-d and 1 mm for e].
Having just said that thing about this being a ‘big deal’, let’s note that being part of Avialae is in reality no more of a big deal than being part of Deinonychosauria or Oviraptorosauria, or Alvarezsauridae, it’s just that people take more interest in basal birds than in all those other maniraptoran groups. Anchiornis has several unusual characters* which indicate that it was part of a weird little side branch and not ancestral to later avialians, and when alive it would surely have been similar to such things as little troodontids and dromaeosaurids. In fact the large second toe claw led Xu et al. (2008) to note that the specialised deinonychosaur-like second toe now seems characteristic of Paraves (the deinonychosaur + avialian clade), rather than of Deinonychosauria alone. There are two interesting things to note here.
* Namely a particularly short ischium and a coracoid in which the ventral surface is covered with lots of small pits.
Firstly, regarding the distribution of the hyper-extendable second toe, there is already good reason to think that this character is common to Paraves given the morphology of the second toe of Archaeopteryx (if you need the details, please check the sixth paragraph in the Long & Schouten review). As I’ve said here before – and despite what many workers have said and continue to say – Archaeopteryx evidently did have a hyper-extendable second toe, and hence was like a dromaeosaurid or troodontid. Secondly, while it’s true that Anchiornis has a proportionally large second toe claw, this is not the ‘clincher’ when it comes to demonstrating the presence of a hyper-extendable second toe. After all, lots of theropods (including many extant birds, actually) have relatively big second toe claws. The real clincher is that the distal condyles on the first phalanx are convex dorsally (rather than just distally and ventrally), and have a dorsodistal inclination (viz, the condyles are angled slightly upwards). Both features are evident on the first phalanx of Anchiornis, so it does indeed seem to have had a hyper-extendable second toe [in adjacent figure of pedal digits I-IV of Deinonychus, from Ostrom (1969), note that the distal end of phalanx I on digit II is strongly convex dorsally (as well as distally and ventrally) and is angled upwards (the phalanges are the short bones immediately distal to the far longer metatarsals). These features allow hyper-extendability of the remaining phalanges. The same details - though in less extreme form - are seen in Anchiornis and Archaeopteryx].
While the discovery of a new basal bird is big news, the Anchiornis holotype is not particularly pretty, nor is it obvious to non-specialists that it’s a bird. This might explain why Anchiornis has been (so far as I can tell) mostly ignored by the media so far. Dave Hone tells us that several specimens are now known, however, and that fuller, more detailed information on this dinosaur is due to appear later this year or in 2010.
Ok, enough with the theropods already. Dinosaurs of an altogether different sort next.
For previous Tet Zoo musings on alvarezsaurids see Troodontids and owls, oh the irony (part II), and for stuff on Mesozoic birds see the articles on archaeopterygids, Pengornis, on the Crato enantiornithine.
Refs – -
Alifanov, V. R. & Barsbold, R. 2009. Ceratonykus oculatus gen. et sp. nov., a new dinosaur (?Theropoda, Alvarezsauria) from the Late Cretaceous of Mongolia. Paleontological Journal 43, 94-106.
Ostrom, J. H. 1969. Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana. Bulletin of the Peabody Museum of Natural History 30, 1-165.
Xu, X., Zhao, Q., Norell, M., Sullivan, C., Hone, D., Erickson, G., Wang, X., Han, F. & Gui. Y. 2008. A new feathered maniraptoran dinosaur fossil that fills a morphological gap in avian origin. Chinese Science Bulletin (in press) doi: 10.1007/s11434-009-0009-6
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