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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Hi Darren, it was indeed an active decision by Xu Xing *not* to promote Anchiornis as new specimens and new work was being done and he felt they were worthy of far greater attention - he basically didn't want to steal his own thunder. The holotype paper spent a long time in review and the new material only came to light once the paper had been accepted, so it was a bit late to incude it all and start over, but equally he knew there was more coming. The new descriptions etc. (with luck) will appear in 2009.
I also know of a big new alvaresaur paper is being worked on so more to come on them this year too with luck.
That's about all I can say really for now on either of these. Sorry to be secretive, but it's not my work to talk about and hopefully it at least generates a bit of interest.
A taxonomic question:
If future analysis would demostate that Archaeopteryx is closer to the dromaeosaurid-troodontid clade (and, so, is a deinoychosaurian following the taxonomic definition of Padian et al., 1999), what would be the better name for the "Deinonychosauria+Avialae node"? Aves or Eumaniraptora? I think the first would be the rigth one, but the second seems more desiderable, given that most of us are "platonically" unable to call a dromaeosaurid "an avian"...
What do Alifanov & Barsbold suggest the Alvarezsaurids are if they aren't theropods?
The way I understand it, when the PhyloCode comes along and codifies clade definitions, Aves is likely going to be restricted to the crown group. So if you don't like calling dromaeosaurids avian, don't worry; they'll be good old fashion non-avian dinosaurs, just like Hesperornis and Ichthyornis. ;)
"...it's just that people take more interest in basal birds than in all those other maniraptoran groups."
Unfortunately, its true. Its the human idea of "things that have ancestors in the past are cooler than things that are now extinct but also have ancestors in the past, unless they are really badass.
"The way I understand it, when the PhyloCode comes along and codifies clade definitions"
What's PhyloCode?
"What do Alifanov & Barsbold suggest the Alvarezsaurids are if they aren't theropods?"
Most likely ornithodirans or some other archosaur...or maybe they'll pull a Dave Peters and suggest they are lizards.
Looking forward to the next few posts, and leaving the far too over-studied theropods, behind.
This should add new inspiration to the bandits ...:-/
Honestly, I'm very surprised to hear Rinchen Barsbold drawing such a conclusion. He's been one of the first to consider the vast similarity between non-avian maniraptorans and birds, and this new idea sounds not just plain wrong, but also very strange.
Darren, you've posted some good information here on dinosaur groups that certainly couldn't be called general, because I haven't found it anywhere else. Your last two postings on the subject (both by you and some of the commentators) have been very enlightening. Dinosaurs may be very popular, but all dinosaur blogs aren't equally informative. Good job!
[from Darren: sorry, delayed by spam filter, and not seen sooner as I've had no internet access]
The International Code of Phylogenetic Nomenclature, or, for short, the PhyloCode, is a nomenclatural code that will regulate the naming and defining of clades. It is currently in draft form, and will probably be implemented in a couple of years. It's intended to exist alongside rank-based codes like the ICZN and the ICBN. The PhyloCode allows phylogenetic definitions for preexisting, rank-based names (e.g., Aves) as well as new names (e.g., Avialae).
The PhyloCode is going to be published with a Companion Volume, which will provide the first names and definitions under the code. Since Gauthier and de Queiroz are editors on the Companion Volume, and since the PhyloCode encourages using crown group definitions for common neontological names when feasible (Rec 10.1B), Aves is almost certainly going to be defined as a crown group (i.e., the clade of modern birds). Avialae will probably get Gauthier and de Queiroz' (2001) definition, which is apomorphy-based, based on the presence of powered flight synapomorphic with that in modern birds.
Under that definition, it's actually ambiguous as to whether Archaeopteryx is an avialan. (It is ambiguous under the branch-based definition as well, but for other reasons.)
If Aves is the crown group, then this leaves Eumaniraptora for that node-based clade.
I have to agree with Gauthier and de Queiroz (2001) and several others that the Archaeopteryx node is not a very interesting clade except for historical reasons, and probably doesn't need a formal name. The branch-based sister clade to Deinonychosauria could possibly use one, though, and a candidate has been discussed elsewhere.
There's a translation available? Where can I get it?
Doing cladistics with one or three characters and consciously ignoring all others seems to be a very common approach in Russia today. For example, I've met someone who actually believes Kurochkin's (1996) crazy, crazy ornithothoracean diphyly hypothesis (Enantiornithes being dinosaurs, Euornithes being ABSRD).
Well, duh. They're AFAIK basically everything except certain fucking champsosaurs in having prefrontals that are so large they meet in the midline. I mean, there's other evidence champsosaurs are not from this planet, but... let's talk about something else.
It is totally weird, but it doesn't ally them to anything else either, it's completely unique and therefore parsimony-uninformative. Again: duh.
It's really interesting how many people were right for totally wrong reasons! :-)
No, like the infamous Daohugou Bed, it's from somewhere between the Middle-Late Jurassic boundary and the Yixian Fm. It could be very far from the J-K boundary -- on either side.
That's not going to happen for Amphibia, though...
This. If you have any questions left after you've read that site, tell me. :-)
Squamate jealousy speaking :-)
Really, nothing is overstudied. I'll immediately grant that squamates in general are understudied and that lots of large squamate clades are way understudied, but nothing is overstudied. Not even, probably, us.
I'd also be interested where I can find the translated paper.
Agree with David. Paraphyletic groups are also still in use by many workers from the USSR (Tereschenkoi, 2007 for instance), though I doubt this is a matter of where a paleontologist comes from, especially in the days of the internet. Arguing that alvarezsaurids are not theropods because some autapomorphies are nowhere else to be found in the theropod tree is close to the exact opposite of parsimony (and thus scientific) thinking. Still weird to find such a way of reasoning (confirmation bias? I intend no disrespect with Alifanov, but might it be he's a BANDit and this paper would go into that direction?) in a paper Rinchen Barsbold co-authored.
Granted, alvarezsaurids are still better bird ancestors or sister groups in a non-parsimony-based field like BAND than Cosesaurus, drepanosaurs and the like ;-)
Are you kidding? It's obvious that Longisquama is a stem-bird. It has hockey stick-shaped feathers. What more do you want? And pterosaurs are lepidosaurs! And humans are ancestrally aquatic!
Well, that's a whole other kettle of fish. :) Actually I was referring to the general bias towards the large toothy carnivores of Dinosauria (especially the birdiest ones) while generally ignoring sauropods, ornithopods, pachycephalosaurs, stegosaurs, ankylosaurs and ceratopians that make up the rest of the group.
You know, the same thing that causes the DML to tend towards the TML a lot of the times. :)
I love (all) dinosaurs, but there is only so many times that one can hear about another new theropod and its relationship to birds, before the eyes start to glaze over. :)
ZM: And humans are ancestrally aquatic!
yup!
Looking at that skeleton, the question has to come to mind... what did Mononykus actually *do*?
It reminds me of a ratite, but with a much shorter neck, far too short to reach food on the ground, unless it bent over in a really awkward, tail-in-the-air way that would leave it ridiculously exposed to predators... and maybe with difficulty in getting up again.
And those digging-looking claws... why on such short forelimbs on a biped with such long hindlimbs? The only thing i can see them being useful for is digging in some sort of vertical or near-vertical surfaces (rotten trees? ant/termite hills?) - but that would be really extreme specialisation, and make it incredibly dependent on one food source... ok, things like pandas and koalas exist, but still, it feels unparsimonious...
Are we sure this isn't a "pathological" specimen, a disabled dinosaur (someone tell Mat Fraser... no, not Mark, Mat!), and that the hindlimbs and the rest of the skeleton really are from the same animal? It just seems so... improbable...
"Looking at that skeleton, the question has to come to mind... what did Mononykus actually *do*?"
Think of a ratite-anteater crossbreed, and you've come pretty close. Feeding on ants and termites (which also appeared in the Cretaceous, perhaps leading to the rise of the alvarezsaurids) is a very specialized lifestyle, but its one that has some measure of success. Look at the modern world, we have anteaters, armadillos, pangolins, a species of hyena, a species of bear, and more, all of which specialize in feeding on ants and termites. Ant-eating groups tend to be widespread, and somewhat diverse, because ants live just about anywhere.
"I'd also be interested where I can find the translated paper.
Agree with David. Paraphyletic groups are also still in use by many workers from the USSR (Tereschenkoi, 2007 for instance), though I doubt this is a matter of where a paleontologist comes from, especially in the days of the internet. Arguing that alvarezsaurids are not theropods because some autapomorphies are nowhere else to be found in the theropod tree is close to the exact opposite of parsimony (and thus scientific) thinking. Still weird to find such a way of reasoning (confirmation bias? I intend no disrespect with Alifanov, but might it be he's a BANDit and this paper would go into that direction?) in a paper Rinchen Barsbold co-authored.
Granted, alvarezsaurids are still better bird ancestors or sister groups in a non-parsimony-based field like BAND than Cosesaurus, drepanosaurs and the like ;-)"
Agreed, there is so much evidence now that birds are little more than a group of dinosaurs, like how bats are little more than a group of mammals, that it would be near impossible to imagine anything else.
"Squamate jealousy speaking :-)
Really, nothing is overstudied. I'll immediately grant that squamates in general are understudied and that lots of large squamate clades are way understudied, but nothing is overstudied. Not even, probably, us."
You think you have it hard? I work with Oligocene-Miocene mammals of North America. When you mention large squamates, like venomous snakes, giant constrictors like Titanoboa or the more modern anaconda, or giant monitors like the Komodo Dragon or Megalania, people immediately are all going "wow". People love poisonous snakes. But start talking about ANY Cenozoic mammal that isn't hominid related, and people's eyes glaze over. I have had that so many times, even when I'm talking to them about giant carnivorous peccaries or brontotheres!
There's a .pdf of the Ceratonykus paper at http://www.springerlink.com/content/119887/, probably not open access though.
Darren, while the distal condyles of pdII-1 phalanx seems dorsally inclined and this condition is not present in pdII-2, this condition appears to also be present in pdIV-1 as well. Does this mean the fourth toe is also hyper-extensible? This also appears to be the case, but to a lesser degree, in pdIII-1, in which the dorsal extension of the condyles is greater than the ventral, if a line is drawn along the dorsal and ventral aspects of the phalanges, and determining where they bisect the condylar margin.
The proximal end of pdII-2 lacks the ventral heel (apparent to a limited degree in some Archaeopteryx) and the distal end of the phalanx is not ventrally declined (as in all other phalanges) but limited and even dorsally and ventrally marginalized, while the ungual is straight and not devurved. This is more reminiscent of the condition in Avimimus and Casuarius, both of which possess the same extremely long second claw but lack the ability to hyperextend the toe as in dromaeosaurids. This still argues that deinonychosaur hyperextension is a unique condition, while other animals possess features relating to a particularly large second claw.
Zach Miller
Re: Longisquama
I realize that was said in jest, but the issue of these feathers existing on a lizard before showing up on theropods has some interesting genetic implications, ie. a lizard "invented" feathers albeit not flight feathers but more than "down" (yes very Lamarckian language). Convergent evolution or a common ancestor? Too bad we can't get a little DNA from one.
Metalraptor--
O.k., I'm much MORE interested in fossil mammals than I am in dinosaurs: I end up reading the "Dinosaur Mailing List" in the hope that someone will go off-topic! Do you know of anything like "Tertiary Mammal Mailing List" I could go to for a fix?
---
Was looking at the tabular appendix to Christine Janis's paper on dental evolution in paleogene ungulates, listing taxa with different molar forms for each early middle and late segment of a NALMA in the relevant time: under Perissodactyls you'd get a Rhinocerotidae list with numerous genera, all marked "lophodont," followed by a Brontotheriidae list with a smaller number of genera all marked "bunolophodont" ... until you got to a time period with a bunch of Rhinocerotidae and NO Brontotheriidae. It felt like seeing a tragedy written in tabular form, the "hero" being brought down by the "tragic flaw" of failing to evolve teeth suitable for coping with the tougher vegetation of drier times!
First, the Internet is in English, at which many Russian academics suck for obvious reasons. And second, there are traditions that you don't suddenly throw overboard when you notice (all of a sudden) that people in faraway lands do things differently and get weird, weird results (in comparison to what you're used to).
Open termite burrows (be those termite mounds or just rotting wood). With force. And without a lightsaber.
Is that on an accessible page? Because I can't find it. But anyway, why do you use a popularization as evidence? What about the actual paper -- is there one?
Yes. Several skeletons, especially of the close relative Shuvuuia which has slightly less extreme hands, were found in articulation.
No, because evolving such teeth would have brought them into competition with the rhinos. Competition is best avoided.
David Marjanovic--
Re: Competition is best avoided.
That was something else that suggested itself when looking at the same tabular appendix. In the Phenacodontid entries. The bunodont Phenacodus seems to have survived a bit later than the bunolophodont Ectocion and the lophodont Meniscotherium. Suggesting that the Phenacodonts that did best were the ones that didn't TRY to compete with Perissodactyls. (Maybe stayed in the darkest and moistest woodland?)
(This is a bit closer to seriously menat than the bit about Titanotheres as tragic heroes, though I don't know how seriously to take either impression.)
David wrote:
No, because evolving such teeth would have brought them into competition with the rhinos. Competition is best avoided.
One might be able to cite that convergent competition usually occurs when two organisms who developed similar diet in isolation from one another come into contact. They compete until one diverges or dies.
"No, because evolving such teeth would have brought them into competition with the rhinos. Competition is best avoided."
Competition is usually avoided because either features of the newcomer's anatomy allows them to triumph over those who are already established in that niche, or the newcomers fail to establish themselves and those creatures with adaptations towards that niche do not prosper.
And anyway, if competition is best avoided, then why are there so many grazers on the African savannah? I know some of them feed on differing lengths of grass, but when you look at how many of them there are; wildebeest, zebras, impala, thompson's gazelles, cape buffalo, rhinos, and more, it doesn't look like they got the memo.
Probably the best reason for brontothere extinction is that they were adapted to feeding on leafy vegetation in forests, while the rhinos were taking on new niches in the opening scrublands. While the brontotheres were having their heyday in the forests, the climate was slowly drying out. Eventually, it began to dry more rapidly, and the rhinos were better adapted to the new conditions. Eventually, there were only a handful of brontothere genera which just barely survived into the earliest Oligocene (Brontops, or Megacerops, depending on who you talk to).
Minor point: redating has meant that all North American brontotheres are now Eocene, none are Oligocene (though don't ask me about the Mongolian taxa, as I'm still confused). And, according to recent revisions by Mader and Mihlbachler and colleagues, it is indeed Megacerops, not Brontops. Incidentally, Megacerops is horrendously over-split. Janis et al. (2008) included about 31 'species' within Megacerops coloradensis.
Ref - -
Janis, C. M., Hulbert, R. C. & Mihlbachler, M. C. 2008. Addendum. In Janis, C. M., Gunnell, G. F. & Uhen, M. D. (eds). Evolution of Tertiary Mammals of North America, Vol. 2. Cambridge University Press (Cambridge), pp. 645-693.
Yes, and grass of different dryness, and so on. They also differ in their emergency strategies (what they do first when grass becomes scarce): some migrate, some shift to high-browsing, some scavenge...
Sounds reasonable.
(Being unfamiliar with the Cenozoic in general, I can't say anything stronger.)
DM: Is that on an accessible page? Because I can't find it. But anyway, why do you use a popularization as evidence? What about the actual paper -- is there one?
Evidence is needed for arguments, but consensus for agreements. I agreed with ZM, and linked to an interesting article in Science News, on page 6 in both the online and paper version IIRC. Okidoki?
DDeden, please learn to use the <blockquote> tag. I first thought the paragraph that follows "DM:" consists of questions that you ask me...
Sorry I didn't find the article (it's indeed right in the top left corner of the page you link to, under the confusing headline "Scientific Observations"), I was too tired that evening... Anyway, you have yet to explain how our innate swimming abilities are evidence for a recent aquatic ancestry. For example, you need to find out if it only works with humans or also with other mammals.
Worse yet, using that article to argue for the Aquatic Ape Hypothesis is quote-mining. That's because it's a quote from Desmond Morris's book, which does explicitly not support the AAH.
No. As a scientist, you must not (dis)agree with a hypothesis unless you can explain why you (dis)agree -- unless you can, in other words, explain the evidence (against/)for the hypothesis. There is such a thing as ethics of belief.
David:
I haven't read Amazing Baby so I don't know what, if anything, it says about the Aquatic Ape Hypothesis. But in some of his previous books, Desmond Morris has actually been quite supportive of the AAH. Ever since The Naked Ape (1967), he's kept on bringing the subject up in his books on human biology. In this 2008 interview he says: "The problem with the aquatic theory is that we still have no direct fossil evidence to support it. But it does explain some of our special features." So I think it's fair to say that Morris is at least mildly sympathetic to the AAH.
Thank goodness; I thought I'd just wasted a good 20 minutes.
Of course, by now this post is buried.... Oh well.
DM, what I said.
Dartian, thanks, interesting interview.
Darren, amazing article, as always!
Darren, there are some occurences of Megacerops within the very earliest sections of the Oligocene in the White River Badlands. However, perhaps these were just Dead Clades Walking, members who just managed to limp across the Eo-Oligo boundary before the whole group kicked the bucket.
Hi - you're right that the White River Group spans the Eocene-Oligocene boundary. But, so far as I can tell from the literature, the youngest White River brontotheres are late Chadronian and didn't make it to the Oligocene boundary. This is confirmed by Prothero's 2006 After the Dinosaurs, and he's particularly good at keeping up to speed with re-datings and so on.
That's the problem with the White River Badlands, there's all sorts of bizzare critters there; entelodonts, a myriad of camels, my favorites the oreodonts, brontotheres, chalicotheres, horses, nimravids, hesperocyonid dogs, some of the earlier osteoborid dogs, and more. The problem is that there is no good guide on them. The last definitive guide to the White River Badlands was in 1920, and it lists oreodonts as suines, entelodonts as obligate herbivores, and nimravids as cats.