It isn't every day that your friends make the cover of Science magazine. Belated congrats to my friend Randy Irmis and his colleagues Sterling Nesbitt, Kevin Padian and others for their neat work on the dinosauromorph assemblage of Hayden Quarry, New Mexico (Irmis et al. 2007). Exciting stuff. Why? Well...
At Hayden Quarry, Norian-aged sediments of the Chinle Formation preserve temnospondyls, drepanosaurids, aetosaurs and diverse other crurotarsans, and dinosauromorphs. The big deal is this: despite the Late Triassic date of the assemblage, Irmis et al. (2007) have been able to demonstrate the presence of both dinosaurs and non-dinosaurian dinosauromorphs. Everyone knows what dinosaurs are (or at least I hope they do), but what are dinosauromorphs? Dinosauromorpha was coined by Benton (1985) and later defined (Sereno 1991) to include all those ornithodiran archosaurs closer to dinosaurs than to pterosaurs: essentially, it is the clade that includes dinosaurs and all their close 'proto-dinosaur' relatives. The latter animals are best known for little Marasuchus from the Middle Triassic Chañares Formation of Argentina: they were dinosaur-like, but lacked the specialisations used to differentiate dinosaurs proper.
[The Science cover shown at top depicts Donna Braginetz's reconstruction of various of the Hayden Quarry dinosauromorphs: Dromomeron is at bottom left, the Hayden Quarry silesaurid is adjacent to it, at top left the basal dinosaur Chindesaurus has a small crocodylomorph in its mouth, and at top right a coelophysoid theropod approaches. Nice work, Donna!]
By the Norian, the dinosaur radiation was well underway. But it was thought that the non-dinosaurian dinosauromorphs had all but gone; their places filled by the all-new dinosaurs. Hayden Quarry demonstrates that this was not so, and that non-dinosaurian dinosauromorphs were still hanging on and living alongside dinosaurs. This is a big deal because previous ideas about the early evolution of dinosaurs have argued either that dinosaurs were competitively superior to their close relatives, and hence able to push such groups into extinction and take over their ecological roles, or that dinosaurs were 'victors by default' that only rose to ascendance once extinction events had knocked out most of the competition (Benton 1983). If dinosaurs and non-dinosaurian dinosauromorphs actually lived alongside one another for a long time, these models become redundant: early dinosaurian success did not depend on the replacement or extinction of these other forms. Does this simply mean that there was enough room for everyone in the Late Triassic, and that the diversification of dinosaurs was not dependent on the changing fortunes of contemporary groups? No doubt we'll see a lot more on this topic in the future.
What were these late-surviving non-dinosaurian dinosauromorphs? The first of them is the new taxon Dromomeron romeri: said quickly, that name has a certain poetic euphony about it. Shared characters of the hindlimb bones indicate that Dromomeron is a close relative of Lagerpeton chanarensis from the Middle Triassic Chañares Formation of Argentina, and the two group together in a dinosauromorph clade in Irmis et al.'s study. The name Lagerpetonidae Arcucci, 1986 already exists for this clade: Arcucci (1986) spelt it Lagerpetonidae, but it should really be Lagerpetontidae I think. Finding lagerpetonids hanging on as late as the Norian is a big surprise [in the adjacent image, I've enlarged the Dromomeron section of the scene shown on the cover of Science].
Lagerpetonids were unusual little beasts: they had very long hindlimbs and really weird feet [left foot of Lagerpeton shown in adjacent image. Scale bar: 20 mm]. In Lagerpeton, the foot was tetradactyl, but digits I and II were strongly reduced, meaning that the animal only walked on digits III and IV. Sereno & Arcucci (1993) suggested that this foot morphology, combined with the relatively small size of the pelvis and anteriorly inclined neural spines on some of the dorsal vertebrae, indicated that Lagerpeton was a saltator: a jumping archosaur. Unfortunately we don't know what the forelimb anatomy of these animals was like, so we're not sure whether they were quadrupedal or bipedal.
The Hayden Quarry assemblage includes another non-dinosaurian dinosauromorph: it's a silesaurid, a group that were all but unknown prior to 2003 but have fast become poster-children for 21st century dinosauromorph research. First named for Silesaurus opolensis from the Carnian of Poland (Dzik 2003), silesaurids are also known from the Carnian Caturrita Formation of Brazil where Sacisaurus agudoensis was discovered (Ferigolo & Langer 2007). Eucoelophysis baldwini from the Chinle Formation - originally described as a coelophysoid theropod - is another silesaurid, as is Pseudolagosuchus major from the Middle Triassic of Argentina (Nesbitt et al. 2007). The new Hayden Quarry silesaurid (represented by a partial jaw, part of a pelvis, and some hindlimb material) might be referable to Eucoelophysis. These taxa all share a distinctive femoral morphology and, in those taxa for which jaws are known, exhibit subtriangular, coarsely serrated teeth that have swollen crowns marked with striations, a constricted crown-base, and are partially fused to the jaw bones. These teeth - superficially similar to those of ornithischians - suggest that silesaurids were predominantly herbivorous. The elongate forelimbs of Silesaurus suggest that it was quadrupedal: it was a gracile, reasonably long-legged archosaur with a total length of about 2 m.
What has received a lot of attention is the fact that silesaurids sport a toothless prong at the tip of the lower jaw; a structure that bears a superficial resemblance to the ornithischian predentary bone. The predentary is a single, U-shaped structure that has grooves on its posterior surface for reception of the two dentary bones*. The silesaurid dentary prong is strongly up-curved in Silesaurus but straighter in Sacisaurus, and in the latter the prong is reported to be formed from two separate, paired ossifications, both of which are separate from the adjacent dentaries (the rostral end of the dentary is not present in Eucoelophysis and the Hayden Quarry silesaurid). Ferigilo & Langer (2007) went as far as arguing that the paired prong of Sacisaurus was homologous with the ornithischian predentary, and that the ornithischian predentary must have evolved from paired rostral structures like those of Sacisaurus. For this to happen however, silesaurids would have to be basal members of Ornithischia (or at least the ornithischian sister-group), and hence part of Dinosauria. This isn't supported by the character data: silesaurids instead appear to be non-dinosaurian dinosauromorphs (Irmis et al. 2007), so at the moment any similarity that the silesaurid dentary prong might have with the ornithischian predentary must be assumed to be convergent. Furthermore, it's difficult to be confident that the prong of Sacisaurus really was separated from the dentaries and thus similar to an ornithischian predentary: the junction that Ferigilo & Langer (2007) reported looks like a crack [life restoration of Silesaurus shown in adjacent image].
* Predentary bones evolved independently in several fishes and also in some birds, including hesperornithines and teratornithids.
In recent years, the base of the dinosauromorph tree has become populated by some very odd, fascinating archosaurs. They pose new questions about dinosaur ancestry: was the ancestral condition for dinosaurs bipedality (as usually posited for Marasuchus) or quadrupedality (as posited for the silesaurids)? Were dinosaurs ancestrally carnivorous (like - it is assumed - Marasuchus) or herbivorous (like the silesaurids)? But rather than being a flash-in-the-pan, Middle Triassic phenomenon, we now know that non-dinosaurian dinosauromorphs hung on well into the Late Triassic, to survive and thrive alongside their better-known cousins.
And.. huh, so much for that goodbye :) I blame Randy.
Refs - -
Arcucci, A. B. 1986. New materials and reinterpretation of Lagerpeton chanarensis Romer (Thecodontia, Lagerpetonidae nov.) from the Middle Triassic of La Rioja, Argentina. Ameghiniana 23, 233-242.
Benton, M. J. 1983. Dinosaur success in the Triassic: a noncompetitive ecological model. The Quarterly Review of Biology 58, 29-55.
- . 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84, 97-164.
Dzik, J. 2003. A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23, 556-574.
Ferigolo, J. & Langer, M. C. 2007. A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology 19, 23-33.
Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D. & Downs, A. 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317, 358-361.
Nesbitt, S. J., Irmis, R. B. & Parker, W. G. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5, 209-243.
Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11 (Supplement to Number 4), Memoir 2, 1-49.
- . & Arcucci, A. B. 1993. Dinosaurian precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis. Journal of Vertebrate Paleontology 13, 385-399.
Is Benton's theory really out? There was a mass extinction event at the end of the Triassic (often ascribed to the Central Atlantic Magmatic Province (CAMP)). That's pretty close to the Norian.
By the end of the Triassic you already had big dinosaurian herbivores and carnivores, so they were already on their way up. The end Triassic extinction may have been important in confirming the dinosaurs' dominance, but even before then they were a successful group.
As I read it Benton's model requires a mid-triassic extinction event (is there one?) and I'm not sure we know enough about dinosauromorph ecology to say whether they were being shouldered aside by their "better" relatives. I wouldn't be surprised if it was a combination of the two models (and probably even a third no-one's thought of yet)- the late surviving dinosauromorphs hung on because they were doing something dinosaurs weren't or couldn't, and some of their unlucky relatives died out giving dinosaurs their first breaks.
I noticed the bbc used Randy Irmis' art work with crediting him as the artist. Obviously they haven't learnt there meant to do that still...
Our paper does not address Tr/Jr events - and I still do think that stuff happened around that time. Certainly in the Early Jurassic dinosaurs are everywhere and have diversified quite a bit.
Tommy, I'm not sure what you mean when you say "that's pretty close to the Norian." Based on new work by Muttoni et al (2004) and Furin et al (2006) the Norian is from ~228 ma to 205 ma, so it depends what part of the Norian you're talking about. The Hayden assemblage is probably between 215 and 210 ma. The main period of CAMP volcanism starts in the latter part of the Rhaetian, so I don't think you can invoke Benton's theory or CAMP for changes during the early or middle Norian.
Also, I'd like to point out that our paper is explicit about the fact that the timing and duration of the rise of dinosaurs may have been different on other continents.
Thanks Darren for the plug! I'm glad to cause a temporary decline in your productivity ;)
Er, you can't just correct Lagerpetonidae like that. ICZN Article 29.5 states-
29.5. Maintenance of current spellings. If a spelling of a family-group name was not formed in accordance with Article 29.3 but is in prevailing usage, that spelling is to be maintained, whether or not it is the original spelling and whether or not its derivation from the name of the type genus is in accordance with the grammatical procedures in Articles 29.3.1 and 29.3.2.
So even though it may not be formed correctly, we're to maintain Lagerpetonidae because it is in prevailing usage.
[from Darren: it's my blog, I can do what I like. But seriously... yes, good point. 'Lagerpetonidae' it is (yuck). There are at least a few names out there which have been 'corrected' (e.g., Lorisidae vs Loridae, Indriidae vs Indridae, Galagidae vs Galagonidae) following ICZN rulings in view of Art 29, so maybe we should do the same thing for Lagerpetonidae]
Darren, you're being bad! You're SUPPOSED to be working on your conference papers!
(But thanks-- appreciating technical papers is difficult for us non-specialists, and background like what you have provided is what we need.)
Question for Randy: I can't help wondering if Dromomeron is some sort of clever, cryptic anagram, as I note that you can extract 'romer' from Dromomeron... this leaves d-o-m-o-n left over though. Is this a coincidence or were you playing silly buggers?
Randy, with "that's pretty close to the Norian" I meant that it wouldn't take much of a ghost lineage (or Signor-Lipps effect if you prefer that) to extend the range of the dinosauromorphs to the Tr/J border.
Hell, with the kind of ghost lineages the molecular people insist on for birds and mammals, they might have been around till last wednesday.
I'd like to point out that this isn't the only occurence of a probably basal dinosauromorph and a dinosaur. Agnosphitys from the Norian/Rhaetian of southwest Britain was a contemporary of the early sauropodomorph dinosaur 'Thecodontosaurus' (or Pantydraco as we should now call it). Surprisingly, this isn't mentioned in Irmis et al., as one of the authors of the Science article (and Randy's PhD advisor) is Kevin Padian, one of the original describers of Agnosphitys.
On Agnosphitys, remember that there are a few different opinions. Fraser et al. (2002) concluded that the taxon should be provisionally regarded as a dinosauromorph that might, in future, prove to be part of Dinosauria, but Langer (2004) (while regarding Agnosphitys as a nomen dubium) argued that the type specimen and at least some of the referred material are probably dinosaurian. Regardless, yeah... it might perhaps have warranted a mention.
By the way, convention dictates that you must swear under your breath when uttering Pantydraco. I've opted for 'Panty-bloody-draco', but others have their own more explicit alternatives.
Refs - -
Fraser, N. C., Padian, K., Walkden, G. M. & Davis, A. L. M. 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology 45, 79-95.
Langer, M. C. 2004. Basal Saurischia. In Weishampel, D. B., Dodson, P. & Osmolska, H. (eds) The Dinosauria, Second Edition. University of California Press, Berkeley, 25-46.
Darren: No - Dromomeron does not say "Paul is Dead" when played backwards on a turntable. But we did intentionally pair "Dromomeron" and "romeri" because it was so alliterative.
Paul: Agnosphitys is really tough. I don't think there's enough material at this point to say where it belongs in the tree. I've only seen the ilium and astragalus in person, and they could fall in or out of Dinosauria. What I will say is that it is most definitely a dinosauriform. It didn't get mentioned because of its ambiguous phylogenetic position, and because we already knew basal dinosauriforms creeped into the Late Triassic (e.g., Silesaurus and Eucoelophysis). Oh, and there are also those pesky word limits that Nature and Science have.
While the name is somewhat unfortunate, for the moment Pantydraco is the correct name to use. Although there is a lot of sentiment attached to Thecodontosaurus, the type material is clearly indeterminate and the majority of the referred specimens are generally undiagnostic and unassociated. If people were serious about conservation of the name, a formal proposal for a neotype (perhaps the holotype of P. caducus) should be framed and sent to the ICZN for a decision. I've heard of no such action occuring, however, even though there is a lot of discontent about this, particularly in Bristol (understandably) who have a long association with, and a lot of fondness for, the name Thecodontosaurus. As for Agnosphitys, given that that the occurence of a Norian non-dinosaur dinosauriform is important enough to warrant a paper in Science, it would have been useful if other relevant taxa were also mentioned as these could have an equally important part to play, especially if large scale macroevolutionary events are being debated as a result of the occurence (i.e. the possibility and potential importance of two Norian basal dinosauriforms, rather than just one).
I realise the name Pantydraco is very unpopular, and yes I am one of the authors, but let me state for the record it was not my choice of name. If it makes people feel any better it doesn't sound quite so bad if you give it a welsh pronounciation: with an 'uh' rather than 'ee' sound. Also I wouldn't dismiss Thecodontosaurus just yet. I'm not convinced that Peter's morphs from the original Clifton site are real and a second accumulation (Tytherington) certainly appears to be monospecific. The Tytherington sample is for the most part identical with known pieces from Clifton. So although the neotype of Thecodontosaurus may not be diagnostic by itself, taken as a whole the taxon certainly is. So I continue to use Thecodontosaurus and include in it most, if not all?, sauropodomorph bones from Clifton and Tytherington. Pantydraco is really quite different from these (I am now kicking myself for not naming a new genus when I described 'T.' caducus).
Well, I would say that the main point of the paper was the co-occurrence of non-dinosauriform dinosauromorphs with dinosaurs in the Norian. As I said before, non-dinosaur dinosauriforms were already known from the Late Triassic and the Norian (e.g., Eucoelophysis). Sure, Agnosphitys could have bolstered a multi-continent argument, but as I've said before, its extremely ambiguous as to whether it is a non-dinosaur dinosauriform or within Dinosauria. I'd like better evidence before I hang my hat on the co-occurrence of basal dinosauriforms and dinosaurs in the Norian of the U.K. Thats not to say though that I'd welcome such a finding.
I'd also like to point out that the Hayden Quarry isn't the only place where you find this co-occurrence. In our Supplementary Information we discuss other sites in North America that show this co-occurrence was stratigraphically and geographically widespread. More on this will be published in the future.
Laelaps was kind enough to send me the paper, and I loved it! Dinosaurmorphs are fascinating critters, being so close to dinosaurs yet, somehow, inferior to them (in the long run). After reading Silesaurus' description so many years ago, I became a proponent of the "Silesaurus as basal ornithischian" train o' thought, but after reading in this paper that silesaurs have partiallyl jaw-fused teeth, I'm beginning to doubt that assertation.
(Something that's always bugged me: how is one supposed to pronounce "Silesaurus?" I've always said "SILE-SORE-us.")
If silesaurs developed their predentary bone independantly of ornithischians, that could mean that the first ornithischian dinosaurs were silesaur mimics, given how successful the herbivorous dinosaurmorphs were. Also, are there basal ornithischian remains in the Hayden Quarry? I only ask because, if they are missing, that could mean that the silesaur population was supressing an ornithischian radiation there.
*light bulb goes off over head* Hey--the Eocursor paper said that ornithischians had a tougher time diversifying than saurischians...maybe because of dinosaurmorphs? Just a thought! Great post, Darren! Now I've gotta blog some of these ideas before I forget them...
The Fraser et al (2002)paper unfortunately is a nomenclatural nightmare for several reasons. First off, the actual genus name for VMNH 1745 is uncertain. Under the systematic palaeontology section it is first listed as Agnosphitys gen. nov. and subsequently as Agnostiphys. Then, thoughout the text and figures both spellings are used. The etymology and Fraser's subsequent use of Agnostiphys in his 2006 book would suggest that this is the intended spelling; however, as far as I am aware this taxon has only been discussed in two other publications besides Fraser's book, Langer (2004) who uses Agnosphitys and Nesbitt et al. (2007) who use Agnostiphys. Because none of these subsequent authors discussed the discrepancy between the spellings in detail, none qualify as being the First Revisor (ICZN art. 24) and thus neither spelling is fixed.
Secondly, "dinosauriform" and "dinosauromorph" are used interchangebly throughout the text by Fraser et al (2002) without noting whether they meant to use non-dinosauriform dinosauromorph. The presence of a well-developed brevis fossa, a perferate acetabulum, and a clear antitrochanter of the ilium would suggest placement of Agnostiphys within dinosauriformes. The characters that place is as a non-dinosauriform dinosauromorph are plesiomorphies (e. g., subrectangular deltopectoral crest).
Hopefully future work will clarify the nomenclature used by Fraser et al. (2002). Until then I would agree with Randy Irmis that the phylogenetic position of this taxon is too ambiguous to cite as another specific co-occurrence of non-dinosauriform dinosauromorphs and dinosaurs in the Late Triassic.
I forgot to mention another reference. Sereno (2007) uses Agnostiphys but does not address the nomenclatural problem and thus does not qualify as the First Reviser either.
Bill - the correct name for said taxon has indeed been clarified: I noticed the mistake on publication (see this DML message from Jan 2002) and asked Nick Fraser about it. He confirmed that Agnosphitys was the intended spelling (see this message, dated 8th Feb 2002). This was later confirmed in a corrigendum (Fraser 2002).
As for papers that mention Agnosphitys, there is also Naish & Martill (2007). The submitted draft included the following text, though this got pared down before publication and the relevant bit of text is now missing dammit...
Far more dinosaur-like is Agnosphitys cromhallensis Fraser et al., 2002 from the Upper Triassic fissure deposits of Cromhall Quarry, Avon. Though Fraser et al. (2002) used both Agnostiphys and Agnosphitys, the intended spelling is Agnosphitys (Fraser 2002).
Refs - -
Fraser, N. C. 2002. Corrigendum. Palaeontology 45, 843.
Naish, D. & Martill, D. M. 2007. Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: basal Dinosauria and Saurischia. Journal of the Geological Society, London 164, 493-510.
Shouldn't you be working on our manuscript instead of commenting on a shady blog?
[from Darren: 'shady'?? Harrumph]
Thanks for pointing out the corrigendum to me. I really have no excuse for missing it as I subscribe to Palaeontology, but I am now even more confused. Why would Fraser (2006) continue to use "Agnostiphys" ignoring his own previous correction? Maybe it is an older version that was never corrected? I guess that someone will have to ask him over a beer at SVP.
Nonetheless Fraser (2002) would be the first revisor (ICZN art. 24)and therefore the name is fixed as Agnosphitys. My apologies for missing that 'little' detail, but I would guess that maybe a lot of people have.
Fraser NC (2006) Dawn of the Dinosaurs: Life in the Triassic. Indiana University Press, Bloomfield, 307p.
A couple of possible explanations:
1) The ms for the book was turned in a long time ago. (My copy is at the office, so that I can't check to see how many post-2002 references there are in Dawn of the Dinos).
2) A copy editor who "corrected" the revised spellings based on the original paper.
3) The name is horrendously easy to misspell!!
Oh, and guys: very cool paper!!! But would someone PLEASE get around to formally proposing "Silesauridae" soon...
Nice work Donna!
So Donna is a nice work?
(If you find "A dinosaur with feathery integument from China" worth talking about, that gives me the right to...)
On the issue of Thecodontosaurus: it would be a pity if we lost that name, but not a big one. I mean, in hindsight it's a phenomenally silly name. "Oh, look, how unusual! A lizard with thecodont tooth implantation!!! Oh my!"
(If it were at least a mosasaur...)
Something that's always bugged me: how is one supposed to pronounce "Silesaurus?"
The name is based on Silesia. Make of that what you will.
It goes without saying that the problem evaporates in front of your very eyes as soon as you leave the English language.
I second and third the request for naming and defining "Silesauridae".
Sorry: don't call it "Silesauridae". It would be better if it lacked a suffix that implies a rank; compare Microraptoria.
In a very large (175 taxa, 228 characters) phylogenetic analysis of the entire Reptilia, taxa such as Silesaurus, Lotosaurus, Chatterjeea, Effigia, and Shuvosaurus all nest within the Dinosauria, not outside of it where Turfanosuchus, Crocodylomorphs and Ticinosuchus + the Aetosauria nest. Deleting all dinos other than Lotosaurus does not change the tree. In other words, Lotosaurus will not nest with Arizonasaurus, a real rauisuchian provided with a similar dorsal frill, even if given no other dinosaurs to nest with. S,L,C,E and S seem to be ornithischia without a fused predentary or a retro pubis because they are all more closely related to Triceratops than to T-rex in this analysis. I look forward to confirmation by someone else inserting similar taxa and whatever characters you want. Results seem to have changed due to the addition of taxa, 10 times more than the average of 40 prior and familiar studies covering smaller subsets of the Reptilia, typically with suprageneric taxa.
In the same analysis BTW, pterosaurs would rather nest with turtles than dinosaurs or any other archosauriformes.
And finally Diadectes, Solenodonsaurus, Casineria, Westlothiana and Limnoscelis all nest within the Reptilia+Amniota, not just outside of it, which was a silly concept all along.
Be glad to share files with anyone interested.
I was quite surprised, some years back, when I discovered that majority opinion held that Dinosauria was a real clade: I had (from adolescence in the 1960s, having read one of Roy Chapman Andrews's popularizations) asssumed that Dinosauria was just a Boolean combination (union) Saurischia and Ornithischia... and assumed it was a priori likely that their resemblances were convergent. (Dinosauria is STILL my second-most-surprising major vertebrate clade: Glires beats it, since a priori I would have thought the convergent evolution of the resemblances between rodents and lagomorphs was even more likely!)
So, if David Peters is right, and a whole lot of critters that are usually thought of as un-dinosaurs are really closer to Ornithischia than to Saurischia... Well, someone could still DEFINE Dinosauria as the l.c.a. of Iguanodon, Cetiosaurus and Megalosaurus and all its descendents, but the class of animals traditionally though of as dinosaurs WOULD be polyphyletic. I can go back to childhood!
Allen: dinosaur monophyly is well supported - the traditional view that ornithischians and saurischians arose from different 'thecodonts' (promoted by workers like Colbert and Charig) was mostly based on the fact that members of the two groups can be easily distinguished, and - supposedly - lacked features that united them to the exclusion of other archosaurs. In other words, these arguments were always weak. And: is David Peters right? I've seen the manuscript in question, and I don't think so.
I know dinosaur monophyly is well supported: the fact surprises me, though. Ditto for Glires.
Sill, I wonder if it might not be-- I don't know a good word for this-- MINIMALLY dyphyletic: las common ancestor a bit earlier than suspected, and not having all the features usually taken to be synapomorphies. (Sort of what Mammalia would be if, say, it turned out that Morganucodon and Kuehneotherium were descended from different Trithelodontid-grade critters, so that their last common ancestor was of a grade traditionally excluded from mammalia.)
David Peters's isn't the only suggestion of this I've seen. In his introduction to Sauropodomorpha on "Palaeos.com"
Toby White says that Saturnalia looks in many ways like a good Prosauropod, but doesn't have an open acetabulum-- suggesting that the last common ancestor of dinosaurs was something which, were we to dig up its hip bone, would look like a non-dinosaurian dinosauromorph.
As an amateur, all I can do is sit back and wait to see what the pros discover. It's always fascinating and very often surprising!
Thanks for the comments Allen. On open acetabula, it's not that Saturnalia is unusual compared to other dinosaurs: instead, new discoveries have shown us that the acetabulum was not fully open in basal dinosaurs (thus negating 'fully open acetabulum' as a dinosaur synapomorphy). In fact a well-developed medial wall to the acetabulum is now known to be present in herrerasaurids and basal sauropodomorphs (like Saturnalia), and some authors have argued for its presence in ornithischians like heterodontosaurids (but this is somewhat arguable). The basal saurischian Guaibasaurus - which exhibits a list of characters in the tibia, ankle and pelvis that are unique to dinosaurs - has a huge medial wall that extends level to the ischial peduncle. If I remember correctly, dinosaurs are still united by an acetabulum that is 'more open' than that of other dinosauromorphs, but it is no longer true that all dinosaurs are united by a fully open acetabulum.
Most of these comments stem from recent work produced by Max Langer: see his chapter in The Dinosauria (Second Edition) for more detail.
Dinosaurian acetabula (including Guaibasaurus, Saturnalia, etc.) are at least open along what would be the pubic and ischial contributions. This is not the case in the immediate outgroup (silesaurs), which show the ancestral fully-walled condition. Reports that Marasuchus had a partially-opened acetabulum seem to be in error, as Novas discussed in the 1999 dinosaur monophyly paper in JVP.
I'm actually rather partial to Allen's idea of a dyphyletic Dinosauria. The question of Ornithischian + Saurischian monophyly is something I've covered quite often on my own blog. Aside from the acetabula detail, something that's always bugged me is the seemingly enormous differences between the skeleton of, say, Lesothosaurus, and Eoraptor. We're talking about the two most primitive (known) taxa of their respective groups (unless Eoraptor really is a theropod, in which case I should say Saturnalia), and the skulls and post-cranial skeletons are just wildly different. I mean, just look at the reconstructed skulls of Lesothosaurus and Eoraptor. They look like completely different animals! One would expect (= I would expect) to see more similarities in the overall structure of two animals that so recently diverged.
The ornithischian pelvis, which even in early representatives of the clade (Lesothosaurus, Eocursor) is incredibly derived, is also a point of contention for me. Such a specialization, seemingly in response to a larger gut volume, implies a fairly lengthy period of specialization that we just don't see in the known fossil record. There's no "omnivorous" ornithischian--they're like bats: the first ornithischians are well-equiped to eat plant matter, teeth and guts to spare.
There's no "connecting" ornithischan, and that bugs me. Given the fact that Eoraptor and Saturnalia look a lot like dinosaurmorphs, one would expect to see an ornithischian dinosaur that's a little closer to the ancestral condition. Heterodonty, tri-radiate pelvis, fewer sacral vertebrae...but there's nothing. The earliest known ornithischian is very different from the earliest known saurischian in form and function which, to me, implies either an older origin for the Dinosauria or a dyphyletic ancestry.
That's my rant, and I'm stickin' to it!
P.S. I mean, we've got an omnivorous therizinosauroid now! Com'on! Falcarius, man!
Thanks for the comments.
Very quickly: are basal dinosaurs really so different from one another? My gut feeling is that Eoraptor isn't close to theropods: compare its skull to that of Panty-bloody-draco [reference]. Remember that basal ornithischians like heterodontosaurids and Eocursor have long hands that would have been moderately good at grasping (in other words: are saurischian-like). Pisanosaurus, which is potentially the most basal known ornithischian, might have a propubic pelvis.
Anyway, the bottom line is that saurischians and ornithischians are united by a number of characters (epipophyses on cervical vertebrae, elongate deltopectoral crest, partly open acetabulum) that are not present in other dinosauromorphs, so dinosaur monophyly remains the most robust hypothesis for now. New data might overturn that in the future, but scientific conclusions are based on what we know now, not on what might be.
Yes, I understand that, but given that quite a few traditional synapamorphies for the Dinosauria have been knocked down in recent years (examples: open acetabulum, at least three sacral vertebrae), I remain open to the idea that the Dinosauria as we currently understand it could be somewhat paraphyletic.
I suppose we need to find more complete dinosauromorphs to know for sure, and a better Pisanosaurus specimen would help.
It has been argued quite convincingly by Paul Barrett and others that some basal ornithischians, notably Lesothosaurus and heterodontosaurids, may well have been facultively omnivorous, based upon the retention of features such as recurved premaxillary teeth and serrated caniniforms (in heterodontosaurids) and the absence of numerous specialisations for herbivory seen in later ornithischians.
Although there are numerous differences between basal saurischians and basal ornithischians, there are also numerous derived similarities, some of which Darren has listed above. And let's not forget that, as Randy Irmis and colleagues have demonstrated recently, the Triassic ornithischian record is appalling, and we know virtually nothing about the anatomy of Carnian ornithischians - Pisanosaurus remains a riddle wrapped in a mystery inside an enigma (although, there is some new evidence that might change that...).
Until a comprehensive phylogenetic analysis suggests otherwise, dinosaur monophyly remains strongly supported - I cannot comment on the unpublished results of David Peters, but personally I would be gobsmacked if they are accurate.
Also don't forget the time element here. Millions of years separate Pisanosaurus on the one hand and the Argentine heterodontosaur and Eocursor on the other. If you want to see it to scale, here is a phylogeny from my upcoming revisions to my dinosaur course website.
And agreed: a more compelete Pisanosaurus is something greatly to be desired.
(Oh, now Darren's going to think I'm a closet ornithischiphile again... :-).
[from Darren: it is too late for you, my son]
Wow--Pisanosaurus is AT the base of the Ornithischia, time-wise. Thanks for that tree, Thomas.
Well, the phylogenetic position of Pisanosaurus is still problematic (but thanks to Irmis, Parker, Nesbitt, and Liu for actually providing photos of the holotype in their Historical Biology paper!) In the Eocursor paper, Butler et al. found a trichotomy at the base of Ornithischia (Pisanosaurus, Heterodontosauridae, and (Eocursor + Genasauria)). If Pisanosaurus does indeed have a propubic-oriented pubic shaft, that would certainly strengthen its position at the base of the tree.
As for the age: the Ischigualasto Fm. is radiometrically date at 228 Ma, at the base of the Carnian Age. This puts Pisanosaurus, Herrerasaurus, and Eoraptor as the oldest well dated dinos. There are footprints from the Middle Triassic that might be dinosaurian (but of course, they might be some near-dinosaur outgroup). The Santa Maria Formation (containing Saturnalia, Staurikosaurus, and Teyuwasu seems to be of comparable age to the Ischigualasto. Guaibasaurus in the Caturrita Formation is a tad younger than these (the Caturrita is superposed on the Santa Maria).
Thanks about the figures. If you want to see the other phylogenies I'm using in my course, they are uploaded to http://www.geol.umd.edu/~tholtz/G104/figures/; these are provided to give more detail and the stratigraphic information that the simpler cladograms (yet to be uploaded) will contain.
I'm back from my grandparents...
The MTr footprints could be dinosaurian or near-dinosaurian, but on the other hand they could be poposaurid, mwahah.
Langer & Benton published a large paper with a phylogenetic analysis of dinosaur origins late last year. It contains detailed character descriptions and illustrations. The only problem is that it's in the Journal of Systematic Palaeontology and thus rather inaccessible. Some kind soul sent me the pdf.
Pisanosaurus remains a riddle wrapped in a mystery inside an enigma
(although, there is some new evidence that might change that...)
My gut feeling is that Eoraptor isn't close to theropods: compare its skull to that of Panty-bloody-draco
Langer & Benton (again) find Eoraptor as the sister-group of Eusaurischia ( = Theropoda + Sauropodomorpha).
In a very large (175 taxa, 228 characters) phylogenetic analysis of the entire Reptilia, taxa such as Silesaurus, Lotosaurus, Chatterjeea, Effigia, and Shuvosaurus all nest within the Dinosauria, not outside of it where Turfanosuchus, Crocodylomorphs and Ticinosuchus + the Aetosauria nest.
Forgive my stubbornness -- I can't take the results of an analysis at face value that barely has more characters than taxa. Below 3 times as many characters as taxa, don't bother trying to get it published.
However, your taxon sampling is impressive. I would greatly appreciate your NEXUS file, if you can send it; I'd like to find out (...someday...) why you get so different results than anyone else, and why you make such confident pronouncements about animals like the very, very poorly preserved Casineria.
Actually, when I mused on the position of Eoraptor in relation to the Theropoda on my original blog, Paul Sereno was kind enough to basically call me a doofus and set me straight. His post is incredibly long, but here's the link to that fateful day:
Tom Holtz wrote:
>Well, the phylogenetic position of Pisanosaurus is still problematic (but thanks to Irmis, Parker, Nesbitt, and Liu for actually providing photos of the holotype in their Historical Biology paper!)
You are welcome. When we were putting the figures together for the paper we decided to fully figure Pisanosaurus, more as a service (since photos of all of the material had never been published) to colleagues rather than for the actual necessity of having it all figured for the paper. I'm glad that it is useful.
what is a synapsomoriphie?
Synapomorphies (note spelling: nothing to do with synapSes) are properties a group of organisms have in common that are taken to be evidence of common ancestry. "Syn" is Greek for together, "apo" for away from. An apomorphy is a property an organism has that is different ("away") from the form of the ancestor. If just one descendant has a certain new property, it's called an autapomorphy (a by-itself-apomorphy), but if several do -- suggesting that they are descended from an intermediate ancestor different from the one(s) the organisms that DON'T have the new property are -- it's a synapomorphy.
For some reason phylogeneticists (= the scientists who draw up genealogical trees of organisms) LOVE Greek words.