October 15, 2012
Out of the scores of non-avian dinosaurs discovered, some get all the love. Almost everyone can rattle off a few of the most famous–Triceratops, Stegosaurus and, of course, Tyrannosaurus rex (the only one we ever feel compelled to call by its whole name). But the Age of Dinosaurs was a 160-million-year reign filled with a startling variety of species that we’re only just beginning to become acquainted with. It’s truly a shame that we continually focus on the same handful when there were so many wonderful forms. Among the unsung dinosaurs is Agujaceratops, a horned herbivore that was only recently recognized for what it truly was.
The story of Agujaceratops goes back the better part of a century. During excavations in 1938 and 1939, a Works Progress Administration crew picked away at a dense dinosaur bonebed in what is now southwestern Texas’ Big Bend National Park. The team pulled more than 340 bones out of the roughly 75-million-year-old Late Cretaceous rock. Although they didn’t know it at the time, most of these bones belonged to a single species of dinosaur that no one had seen before.
Five decades later, Texas Tech University paleontologist Thomas Lehman returned to the skeletal collection. The various pieces came from at least ten individual dinosaurs–from juveniles to adults–that were entombed in the same place. There was no single articulated skeleton, or even a complete skull, but by sifting through the remains Lehman reconstructed several skulls from the new horned dinosaur species. Drawing a comparison with Chasmosaurus, a previously known horned dinosaur found in Canada with similar anatomical motifs among the horns and frill, Lehman called his animal Chasmosaurus mariscalensis.
Not long after Lehman’s paper, other researchers happened upon a lovely specimen that confirmed the southern ceratopsid as a distinct dinosaur. In 1993, ceratopsian expert Catherine Forster and coauthors described a complete Chasmosaurus mariscalensis skull, showing that this dinosaur had much longer brow horns and a more saddle-shaped frill than other Chasmosaurus species to the north.
Yet, even though this study found that Chasmosaurus mariscalensis was more closely related to other Chasmosaurus species than to Pentaceratops–another southern ceratopsid that was a possible candidate for a Chasmosaurus descendant–the southern species didn’t look quite like the northern ones. The northern Chasmosaurus species had shorter brow horns and expanded, V-shaped frills that didn’t curve upwards in the same way. Why was the southern species so different? Perhaps, Forster and colleagues hypothesized, the southern species retained some archaic characteristics while the northern Chasmosaurus underwent greater modifications.
As paleontologists continued to scrutinize ceratopsids, however, the less the southern species looked like a Chasmosaurus. In a 2006 reevaluation of Chasmosaurus and Pentaceratops, New Mexico Museum of Natural History and Science paleontologist Spencer Lucas and collaborators placed “Chasmosaurus” mariscalensis in a new genus–Agujaceratops, named in honor of the Aguja Formation in which the dinosaur is found.
Along with other new discoveries–such as Kosmoceratops and Utahceratops from southern Utah–Agujaceratops changed the big picture of ceratopsid biogeography. As Lehman’s paper hints, some paleontologists used to think there was a kind of faunal continuum between northern and southern swaths of North America. In formations laid down at the same time (about 75 million years ago in this case), you’d expect there to be continuity between the dinosaur genera found down the latitudes. Bits and pieces of dinosaurs found in Utah, New Mexico, Texas and elsewhere were attributed to dinosaur genera discovered about 2,000 miles away in Canada. This didn’t only affect horned dinosaurs. Remains of southern tyrannosaurs, previously attributed to the northern predators Albertosaurus and Daspletosaurus, were recently found to be a previously unknown tyrant called Bistahieversor.
By way of new finds and reexaminations of old material, paleontologists have only just started to become acquainted with Agujaceratops, Bistahieversor and other dinosaurs of the southwest’s Late Cretaceous. At the species and genus levels, the southern dinosaurs are different. The big question is, why? Paleontologists know that a shallow, vanished seaway separated dinosaurs on eastern and western subcontinents for millions of years, but on that western subcontinent called Laramidia, there was apparently some other kind of barrier that isolated northern and southern dinosaur populations.
The hypothesis relies on basic evolutionary theory. Isolate populations of an ancestor species in different regions, and through factors such as natural selection and genetic drift, those populations will evolve in different ways. The fact that Agujaceratops, Kosmoceratops and Utahceratops are so different from Chasmosaurus and other northern cousins are a sign that such a barrier was in place. No one has found it yet, though, and a great deal of work remains to be done on whether all these dinosaurs were really contemporaries or reveal a much more complex evolutionary pattern. As these investigations continue, though, Agujaceratops will continue to play an important role as a symbol of isolation and evolution.
Author’s note: This is the first entry in a new series of posts, highlighting fantastic dinosaurs that are little known by the public. You won’t find Archaeopteryx, Brachiosaurus, Tyrannosaurus or other classics on this list. Those dinosaurs are famous enough already. Now it’s time to highlight some of their lesser-known cousins and contemporaries, from Agujaceratops to Zalmoxes.
Forster, C., Sereno, P., Evans, T., Rowe, T. 1993. A complete skull of Chasmosaurus mariscalensis (Dinosauria: Ceratopsidae) from the Aguja Formation (late Campanian) of West Texas, Journal of Vertebrate Paleontology, 13:2, 161-170. doi: 10.1080/02724634.1993.10011498
Lehman, T.1989. Chasmosaurus mariscalensis, sp. nov., a new ceratopsian dinosaur from Texas, Journal of Vertebrate Paleontology, 9:2, 137-162 doi: 10.1080/02724634.1989.10011749
Lucas, S., Sullivan, R., Hunt, A. 2006. Re-evaluation of Pentaceratops and Chasmosaurus (Ornithischia: Ceratopsidae) in the Upper Cretaceous of the Western Interior, in Lucas, S. G. and Sullivan, R.M., eds., 2006, Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35.
Sampson, S., Loewen, M., Farke, A., Roberts,E., Forster, C., et al. 2010. New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism. PLOS ONE 5(9): e12292. doi:10.1371/journal.pone.0012292
August 15, 2012
Carnotaurus was a weirdo. Not only did this 26-foot predator of Argentina’s Late Cretaceous have prominent horns jutting from its short, deep skull, but, since the time of the dinosaur’s discovery in 1985, paleontologists have been puzzled by the strange arms of the theropod. Despite having absolutely huge shoulder bones, Carnotaurus had wimpy arms that were even stubbier than those on the oft-ridiculed tyrannosaurs. Stubby forelimbs go all the way back to the beginning of the lineage that Carnotaurus belonged to–the abelisaurids–but this ancient South American predator took the reduction to extremes.
Among the relatively short-armed tyrannosaurs, at least, the evolution of small arms is often associated with developing big, well-muscled heads. As tyrannosaur heads became larger and heftier, their arms became smaller to compensate. The idea is that it’s all about balance–if you have a huge head and beefy arms, you’re going to fall on your face. (Sorry, Trogdor.) So far as I know, no one has actually tracked these evolutionary trends, but it remains the prevailing hypothesis. An in-press Acta Palaeontologica Polonica paper about the neck of Carnotaurus forwards a similar explanation for the puny arms of abelisaurids.
The study, written by paleontologist Ariel Méndez, compares the neck vertebrae of Carnotaurus with the same bones in the dinosaur’s close cousin from Cretaceous Madagascar, Majungasaurus. Both were big, short-snouted predators with strange head ornaments, but, as Méndez points out, the neck of Carnotaurus is much more heavily built. For example, the neck vertebrae of Carnotaurus are much wider, with the last bone in the series being as wide as the dinosaur’s skull. In Majungasaurus, the last neck vertebra is only about half the width of the skull (although it should be noted that the Majungasaurus neck vertebrae were inflated in size by about 20 percent to match the neck of a subadult to an adult skull).
So what do these differences mean? Unfortunately, Méndez does not include a full muscular reconstruction in the study but notes that the bony differences almost certainly indicate different muscle arrangements. In general, it seems that Carnotaurus was a more robust animal than Majungasaurus, although increased power may have come with a cost of reduced flexibility between the base of the neck and the tail. Méndez, referring to previous research, also points out that having more heavily-built skulls and necks may be associated with smaller forelimbs. Indeed, while skulls are often the focus of feeding studies, recent research on a variety of carnivores–such as Tyrannosaurus, the sabercat Smilodon and the modern Komodo dragon–have affirmed the importance of neck muscles to feeding. Even carnivores with relatively weak bites, such as sabercats and Komodo dragons, receive a great deal of extra power from their neck muscles while feeding. Perhaps the same was true of Carnotaurus.
Yet the stouter neck of Carnotaurus doesn’t actually explain why this dinosaur had tiny arms. After all, Majungasaurus also had the robust shoulder girdle-vestigial arm combination, yet its neck is clearly not as heavily built as in Carnotaurus. More than that, big shoulders and smallish arms seem to go all the way back to early abelisaurids, such as the recently-described Eoabelisaurus. Although the hefty head and neck-small arms idea makes sense, the idea has yet to be rigorously tested against the actual history of dinosaurs such as abelisaurids and tyrannosaurs. Why huge, powerful carnivores had puny arms remains an evolutionary puzzle.
Méndez, A. (2012). The cervical vertebrae of the Late Cretaceous abelisaurid dinosaur Carnotaurus sastrei Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0129
August 3, 2012
Dinosaurs didn’t all live at the same time. Not counting the avian species that have thrived during the last 65 million years, dinosaurs proliferated throughout the world during a span of over 160 million years. As I’ve pointed out before, it’s amazing to think that less time separates us from Tyrannosaurus than separated Tyrannosaurus from Stegosaurus.
Even within specific geologic formations, not all the dinosaurs found in those layers lived side by side. Dinosaur-bearing strata accumulated over millions and millions of years and record both ecological and evolutionary changes. Look closely enough, and you can even see particular communities of dinosaurs give way to different assemblages. In an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper, Jordan Mallon and colleagues have done just that.
Canada’s Dinosaur Park Formation is one of the most spectacular slices of Late Cretaceous time found anywhere in the world. Spanning approximately 76.5 to 74.8 million years ago, the formation has yielded lovely specimens of dinosaurs such as the crested hadrosaur Corythosaurus, the spiky ceratopsid Styracosaurus, the lithe tyrannosaur Gorgosaurus, the heavy-armored ankylosaur Euplocephalus and many others. Not all of these dinosaurs were neighbors, though. Since 1950, at least, paleontologists have recognized that some kinds of dinosaurs are restricted to certain slices of the formation, and the dinosaur community changed over time. Mallon and co-authors decided to have another look at the dinosaur turnover, focusing on the large herbivores and investigating what might have shook up the dinosaur populations during the time the Dinosaur Park Formation was being laid down.
The paleontologists identified two broad divisions in the Dinosaur Park Formation, which they call “megaherbivore assemblage zones.” Each zone lasted roughly 600,000 years each. There are a lot of names here, so bear with me. In the lower zone, the horned dinosaur Centrosaurus and the crested hadrosaur Corythosaurus are found throughout; other dinosaurs restricted to this half of the formation include the ceratopsid Chasmosaurus russelli, the hadrosaurs Gryposaurus and Parasaurolophus, and the ankylosaur Dyoplosaurus.
Yet there are some dinosaurs that first appear in the lower zone and persist into next one. The ceratopsid Chasmosaurus belli, the ankylosaur Euoplocephalus and the hadrosaurs Lambeosaurus clavinitialis and Lambeosaurus lambei show up in the lower zone but pass through into the second zone as well. And, as with the lower swath, there were dinosaurs that were only found in the second zone. The hadrosaurs Prosaurolophus and Lambeosaurus magnicristatus, as well as the horned dinosaurs Styracosaurus, Vagaceratops and a pachyrhinosaur, are only found in the upper zone.
So the big picture is that the lower zone is characterized by Centrosaurus and Corythosaurus, the upper zone is distinguished by Styracosaurus and Prosaurolophus, and there are some dinosaurs–such as Lambeosaurus and Chasmosaurus–that are smeared across the two. As the researchers note, it’s even possible to break down the two halves into even smaller subsets, although the picture gets a little muddier at these levels.
What does all this evolutionary dinosaur shuffling mean? Other researchers have proposed that the Dinosaur Park Formation represents a series of turnover pulses–after a period of stability, rapid ecological change wiped out some dinosaurs while creating opportunities for a new community. The now-vanished Western Interior Seaway has been invoked as a possible mechanism for this. As this shallow sea, which once split North America in two, expanded and encroached further inland, the area of the Dinosaur Park Formation became a mostly coastal, muddy, swampy habitat. This may have put pressure on some forms of dinosaur while providing opportunities for others. As the seaway fluctuated, the attendant changes would have altered the environment and therefore affected dinosaur populations.
According to Mallon and collaborators, though, there’s no strong evidence for the turnover pulse hypothesis. We simply don’t have the resolution to tell how closely certain dinosaurs were tied to particular habitats or niches, and shifts in ecology would have influenced dinosaur evolution. Other possible influences–such as dinosaurs migrating to the area from elsewhere, or the evolution of one species into another within the formation–are also frustratingly unclear. As the researchers state, “Whether the appearance and disappearance of the megaherbivorous taxa of the [Dinosaur Park Formation] was due to evolution, migration, or to a combination of these factors, is difficult to determine.” We don’t yet know what drove the alterations in the formation’s dinosaur communities.
Aside from the ongoing mystery about what caused the changes between the two zones, the revised look at the Dinosaur Park Formation also raises a few questions about dinosaur ecology. Despite the shifts in dinosaur communities, the paleontologists note, there were about six to eight different megaherbivorous dinosaur species living alongside each other. That’s a lot of big herbivores on the landscape, especially since the hadrosaurs and ceratopsids may have formed huge herds. Such vast, hefty dinosaur communities would have required a large amount of vegetation, and the disparate megaherbivores were in competition with each other for food. In order to live alongside one another, then, we can assume that there was some kind of niche partitioning–the dinosaurs were adapted to have restricted diets or live in particular habitats as a result of their competition for resources. How exactly this happened, though, requires further study into the ecology and evolution of these dinosaurs.
And there was something else that caught my eye. The new study focused on the megaherbivores, but what about the large carnivores? The large tyrannosaur Gorgosaurus was also present in the Dinosaur Park Formation and was rejected by the researchers as a zone marker because this theropod ranges throughout the formation. Think about that for a moment. We can see a significant amount of change and turnover among the big herbivores, but one of the large carnivores stays the same throughout the entirety of the formation. Why should this be so? Perhaps it has something to do with the fact that the ornamentation and headgear of hadrosaurs and ceratopsids changed quite a bit, but their general body plans were conservative–a Gorgosaurus could take down a Corythosaurus just as well as a Lambeosaurus.
Likewise, I wonder if the same pattern might hold true elsewhere. The Kaiparowits formation of southern Utah, laid down around the time of the Dinosaur Park Formation further north, also hosts an array of hadrosaurs, ceratopsids and ankylosaurs, but there seems to be just one large dinosaurian predator, the tyrannosaur Teratophoneus. (The giant alligator cousin Deinosuchus was another megacarnivore in the Kaiparowits.) We need more fossils to be sure, but perhaps, like Gorgosaurus, the short-snouted Teratophoneus remained the same as different large herbivores came and went. If this turns out to be the case, the lack of an arms race between predator and prey would be further evidence that the ornamentation of ceratopsids and other dinosaurs had more to do with decoration and combat among each other than defense.
Indeed, the new study of the Dinosaur Park Formation lays some important groundwork for future studies. Paleontologists are currently investigating and debating why the roughly 75-million-year-old dinosaurs from Alberta are different from the roughly 75-million-year-old dinosaurs from southern Utah. What factors drove the diversity and disparity of these dinosaurs across the latitudes, and who really lived alongside whom? So far, the Dinosaur Park Formation is the best-sampled slice we have, and there is much work to be done. With any luck, and a few more decades of careful sampling, we’ll be able to put together an intricate picture of how dinosaurs lived and evolved during this brief span of Late Cretaceous time.
Mallon, Jordan C., Evans, David C., Ryan, Michael J., Anderson,, & Jason S. (2012). Megaherbivorous dinosaur turnover in the Dinosaur Park Formation
(upper Campanian) of Alberta, Canada Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2012.06.024
July 5, 2012
On Monday, the world met yet another fuzzy dinosaur. The little theropod – named Sciurumimus albersdoerferi – is beautifully preserved in a slab of roughly 150 million year old limestone found in Germany. (These deposits have also brought us Archaeopteryx and the also-fluffy Juravenator.) And, with a little evolutionary context, Sciurumimus hints that filament-like protofeathers were more common among dinosaurs than we previously expected.
Birds – the only surviving lineage of dinosaurs – are covered in plumage. No surprise there. But since 1996, paleontologists have identified about 30 genera of non-avian dinosaurs with feathers. Most of these dinosaurs are coelurosaurs – the major group of theropod dinosaurs that contains tyrannosaurs, the switchblade-clawed deinonychosaurs, the truly weird therizinosaurs, and, among others, birds. As the discoveries accumulated, it seemed that feathers originated at the base of this group, and were inherited by birds. And feathers were not only present an small, especially bird-like dinosaurs. As the recently-described Yutyrannus shows, even 30-foot-long tyrannosaurs were fluffy.
Up until a few years ago, birds and their closest non-avian relatives were the only dinosaurs known to have feathers. Simple enough. But then two ornithischians crashed the party.You see, the dinosaur family tree is split into two halves – the saurischians on one side, and the ornithischians on the other. The split goes back about 230 million years or so, nearly to the origin of the very first dinosaurs.
The feathery coelurosaurs belong to the saurischian side of the tree, but paleontologists have also discovered dinosaurs on the other side – on the ornithischian branches – with feather-like structures. In 2002, paleontologists discovered that the archaic ceratopsian dinosaur Psittacosaurus had a brush of bristle-like structures jutting from its tail. And in 2009, another team discovered Tianyulong – another ornithischian dinosaur with a row of similar filaments running down its back. The bristles were not just like the fuzz and feathers seen among the coelurosaurs, but they were structurally similar.
Paleontologists were left with two possibilities. Either protofeathers evolved multiple times in different dinosaur lineages, or simple “dinofuzz” was an ancestral dinosaur feature that was later lost in some lineages. We don’t have enough fossils yet to know for sure, but the discovery of Sciurumimus is a significant clue that most, if not all, dinosaur lineages were at least partially decorated with protofeathers.
Even though Sciurumimus is a theropod dinosaur – part of the saurischian side of the family – it isn’t a coelurosaur. Sciurumimus is a megalosauroid, which is a lineage of dinosaurs that’s closer to the base of the theropod group. In other words, Sciurumimus is a relatively archaic theropod that isn’t very closely related to birds, yet it still has dinofuzz.
Paleontologist Thomas Holtz helped provide some context on Twitter shortly after the new dinosaur was announced. Before Sciurumimus, only coelurosaurs were known to have fuzz. (What the bristles on Psittacosaurus and Tianyulong actually are is still unclear, but no one calls their filaments “fuzz.”) After Sciurumimus, fuzz has been moved down a branch to a group called the Carnosauria.
We are still left with two possibilities. The fuzz on Sciurumimus could have originated independently. But as paleontologists add fuzz to lineages of dinosaurs only distantly-related to birds, it seems less and less likely that protofeathers evolved from scratch in each and every lineage. It’s looking more and more like feathers were a common, ancestral feature of dinosaurs. In this case, Sciurumimus indicates that simple feathers were an early, common theropod trait that evolved close to the origin of the group. The diminutive dinosaur also fits in the wide gap between coelurosaurs and their very distant ornithischian dinosaurs, bringing us a little closer to the idea that dinofuzz was an early, widely-shared dinosaur feature.
And there’s something else. Pterosaurs – the flying archosaurs with leathery wings stretched over elongated wing fingers – were the closest relatives to the Dinosauria as a whole. They had fuzzy body coverings, too. No one knows for sure, but this might mean that wispy plumage was present in the last common ancestor of dinosaurs and pterosaurs, and those simple body coverings were subsequently modified or lost in different lineages as both groups evolved.
We need more fossils to test the idea that dinosaurs started out feathery. Additional fossils preserving fuzz – fluffy baby sauropods, maybe? – would help us understand the spread of feathers and their precursors among dinosaurs. And, even then, we’d still need to find exceptionally-preserved specimens of the earliest dinosaurs to see if they had any kind of filament-like body covering. The trouble is that the high-definition deposits that would even have a chance of preserving feathers are rare. It may be a very long time before we ever know for sure.
Nevertheless, there’s still a possibility that all dinosaur lineages had some kind of bristly or feathery body covering. It’s a hypothesis that needs testing, but not an unreasonable one. Think about this for a moment. Imagine a Stegosaurus with patches of long, stiff filaments covering its body, or a Ceratosaurus with a little splash of brightly-covered fuzz on its already well-decorated head. And I think a huge sauropod – like Apatosaurus – with a partial covering of dinofuzz would look absolutely spectacular. These visions are wholly different than the scaly dinosaurs I grew up with, but they are not so fantastic as to be fiction. We are only just beginning to understand how fuzzy dinosaurs were.
June 20, 2012
What dinosaurs ate, and how they ate it, is an endless source of fascination. Whether it’s the predatory habits of Tyrannosaurus rex or how sauropods managed to horf down enough food to fuel their bulky bodies, the details of dinosaurs’ paleo diets fuel scientific study and dinosaur restorations alike. If basic cable documentaries have taught me anything, it’s that dinosaurs were all about eating.
But dinosaurs were not invulnerable consumers. Even the biggest and fiercest dinosaurs were food sources for other organisms—from giant crocodylians to parasites and bone-boring beetles that took up residence in dinosaur carcasses. Even mammals sometimes dined on dinosaur.
The most famous case is Repenomamus. Hardly a household name, this critter is the exception to everything I heard about mammals in the Age of Dinosaurs. The classic story is that mammals were so stifled by the dinosaurian reign that our furry ancestors and cousins remained small and hid among the shadows. There is some truth to the notion. Mammalian evolution was influenced by dinosaur evolution, and as Mesozoic mammals diversified, most stayed small and became adapted to burrowing, swimming, gliding and other modes of life in the shadow of the dinosaurs.
Repenomamus, on the other hand, was huge for a mammal of its time. This roughly 130-million-year-old carnivore, found in the rich fossil beds of northeastern China, was a badger-like creature a little over three feet long—bigger than some of the feathery dinosaurs that lived at that same time. Repenomamus was big enough to eat dinosaurs, and we know that the mammal definitely did. In 2005, paleontologist Yaoming Hu and co-authors described a Repenomamus skeleton with the remains of a juvenile Psittacosaurus, an archaic ceratopsian dinosaur, in its gut contents. Based on the way the little dinosaur bones were broken up, the researchers said, “the juvenile Psittacosaurus was dismembered and swallowed as chunks.”
We don’t know whether Repenomamus caught the young dinosaur or scavenged it. Those details aren’t recorded in the fossils. Either scenario is possible—Repenomamus was certainly large enough to catch and kill a juvenile Psittacosaurus, but there’s no reason to think that such a large carnivorous mammal would have passed up a dinosaur carcass. While many Mesozoic mammals might have qualified as dinosaur prey, Repenomamus reminds us that the classic narrative of total dinosaur dominance gives the prehistoric archosaurs too much credit.
Of course, mammals didn’t have to be burly carnivores to eat dinosaurs. Dead dinosaurs were rich food resources on the prehistoric landscape, and mammals took advantage of these bonanzas. In a study I wrote about two years ago, paleontologists Nicholas Longrich and Michael Ryan documented several fossils—including dinosaur limb and rib fragments—that displayed toothmarks made by small mammals called multituberculates. These mammals, often restored in opossum-like garb, had large, pointed incisors that helped them gnaw on tough plant foods but that could also be repurposed to scrape at dinosaur carcasses. Given the chance, mammals made the most of dead dinosaurs.
Longrich, N., & Ryan, M. (2010). Mammalian tooth marks on the bones of dinosaurs and other Late Cretaceous vertebrates Palaeontology DOI: 10.1111/j.1475-4983.2010.00957.x
Yaoming Hu, Jin Meng, Yuanqing Wang, Chuankui Li (2005). Large Mesozoic mammals fed on young dinosaurs Nature, 433, 149-152 DOI: 10.1038/nature03102