November 6, 2012
If you have been following the Dinosaur Alphabet series so far, you may have noticed a pattern among the first four entries. At one time or another, all the dinosaurs I’ve selected so far were thought to be different animals. The horned Agujaceratops was originally named as a species of Chasmosaurus, the distinctive high-spines of Becklespinax gave Richard Owen’s dopey Megalosaurus its hump, the sauropod Cetiosaurus was originally envisioned as a giant crocodile, and the armored Dyoplosaurus was lumped in with its cousin Euoplocephalus before being split back out again as a distinct genus. I didn’t intend this trend, but it struck me when I came across one of the rejected candidate for yesterday’s entry for the letter D. Had it not shared much of its story with Becklespinax, I would have picked Duriavenator:
Megalosaurus was a mess. Even though this Jurassic carnivore has been a prehistoric icon ever since it was named by William Buckland in 1824, it has been one of the most confounding of all dinosaurs. That’s because generations of researchers attributed dozens of fragments and isolated bones to the dinosaur, creating a monstrous composite of animals from different places and times. Dinosaurs were unfamiliar animals–the name itself only coined in 1842–and 19th-century naturalists didn’t have the kind of geologic resolution their intellectual descendants rely on to properly constrain when particular species lived. Sometimes researchers named too many species on the basis of scrappy, non-overlapping material, and other times they applied the same name ad infinitum to roughly similar fossils.
Eventually, though, it became apparent that Megalosaurus was unstable. No one could say what the dinosaur really looked like or what bones could accurately be attributed to the predator. The situation was so bad that, in 2008, paleontologist Roger Benson and colleagues stripped the name Megalosaurus from everything save for the fragment of jaw originally used to name the animal. Whether the rest of the fossils really belonged to Megalosaurus remained to be seen, and, as Benson demonstrated later the same year, at least one other theropod had been improperly obscured behind the famous name.
In 1883, anatomist Richard Owen described a partial theropod skull found on Dorset, England, as another piece of Megalosaurus “bucklandi.” The sharp-toothed dinosaur was only represented by parts of the upper and lower jaws, but, given how little was known about Megalosaurus to start with, Owen’s assignment was reasonable. Nearly a century later, paleontologist Michael Waldman proposed that these fossils represented a previously unknown species of the dinosaur he called Megalosaurus hesperis. Other researchers weren’t sure that the bones really belonged to Megalosaurus, but it wasn’t until Benson’s reexamination that the fossils were split out as a different dinosaur. While the dinosaur was a close cousin of Megalosaurus bucklandii, Benson was able to pick out subtle anatomical characteristics that distinguished the fragmentary skull. In Benson’s analysis, what once was Megalosaurus took on a new life as Duriavenator hesperis.
Unfortunately, we don’t know very much at all about Duriavenator. The dinosaur lived about 170 million years ago in Jurassic England and was a large carnivore of comparable size to the 20-foot-plus Megalosaurus, but that’s where the evidence gives out. Perhaps other Duriavenator specimens are resting in museum collections, but until the discovery of a nearly complete skeleton allows paleontologists to connect the jaws to a body, the dinosaur will be an enigma. But here Megalosaurus itself gives us reason to hope. The Duriavenator paper was just part of Benson’s effort to rehabilitate Megalosaurus, and in 2010 he published a refined, revised reconstruction of the dinosaur’s skeleton based on material collected from Stonesfield, Oxfordshire–the locality where the original jaw came from. Perhaps, with a little detective work in the lab and in the field, paleontologists might also be able to fill out the form of Duriavenator and other Middle Jurassic mysteries.
Benson, R., Barrett, P., Powell, H., Norman, D. 2008. The taxonomic status of Megalosaurus bucklandii (Dinosauria, Theropoda) from the Middle Jurassic of Oxfordshire, UK. Palaeontology, 51, 2: 419-424.
Benson, R. 2008. A redescription of “Megalosaurus” hesperis (Dinosauria, Theropoda) from the Inferior Oolite (Bajocian, Middle Jurassic) of Dorset, United Kingdom. Zootaxa 1931: 57-67
Benson, R. 2010. A description of Megalosaurus bucklandii (Dinosauria: Theropoda) from the Bathonian of the UK and the relationships of Middle Jurassic theropods. Zoological Journal of the Linnean Society 158: 882. doi:10.1111/j.1096-3642.2009.00569.x.
Waldman, M. 1974. Megalosaurids from the Bajocian (Middle Jurassic) of Dorset. Palaeontology 17, 2:325-339.
November 5, 2012
If I started this Dinosaur Alphabet series just a few years ago, I wouldn’t have included Dyoplosaurus. Up until 2009, the dinosaur was hiding within another genus of heavily-armored ankylosaur. But after decades of discovery and debate, Dyoplosaurus is back, and the Cretaceous club-tail has its own role to play in wider discussions about the tempo and mode of dinosaur evolution.
Canadian paleontologist William Parks named the ankylosaur in 1924. Just a few field seasons earlier, in 1920, a University of Toronto crew found the partial skeleton of an armored dinosaur in the Late Cretaceous rock along the Red Deer River in Alberta. “The anterior part of the skeleton had been long exposed and had suffered in consequence,” Parks later wrote, but the team was still able to collect part of the skull, some tooth fragments, ribs and, best of all, the articulated hip and tail. Some of the armor remained in place, and the preservation was delicate enough to include skin impressions and the long ossified tendons that helped support the ankylosaur’s tail. If only the front half had remained intact!
This partial skeleton wasn’t the first ankylosaur to be found in the Late Cretaceous of North America. But, Parks wrote in his report, the animal’s tail club was “distinctly different from any previously described and, as far as I am aware, from any that have been collected.” Based on this slender oval of bone and other features, Parks distinguished the skeleton as Dyoplosaurus acutosquameus. And while the front half of the animal was almost entirely missing, the detail of the back half gave paleontologists a detailed look at how the armor, bones and tendons of ankylosaurids were arranged.
Then researchers sunk Dyoplosaurus. In 1971, in a huge revision of the ankylosaurs, paleontologist Walter Coombs proposed that Dyoplosaurus was not so unique as Parks had proposed. A jaw fragment found with the original Dyoplosaurus specimen was virtually identical to part of a jaw referred to the more famous armored dinosaur Euoplocephalus, Coombs wrote, and therefore Parks’ dinosaur should be considered a Euoplocephalus.
Since this other ankylosaur was named on the basis of even more fragmentary material, the addition of the “Dyoplosaurus” specimen gave paleontologists a new reference for what the hips, tail, and armor of Euoplocephalus looked like. More than that, the find extended the range of Euoplocephalus through Alberta’s Late Cretaceous rock. The “Dyoplosaurus” material was found in a roughly 76-million-year-old park of the Dinosaur Park Formation, and bones referred to Euoplocephalus had also been found in the geologically younger Horseshoe Canyon Formation. Altogether, Euoplocephalus seemed to persist for almost ten million years–quite a feat given that many neighboring genera and species of dinosaur came and went during the same span of time.
As paleontologists found additional ankylosaurs and compared previously discovered material, though, it became apparent that Euoplocephalus had become an osteological umbrella that was hiding more than one dinosaur genus. Indeed, since the original Euoplocephalus material consisted of a partial skull and a half ring or neck armor, it was difficult for paleontologists to compare and accurately refer specimens when there was a lack of overlapping material. As researchers investigated more complete material that was undeniably Euoplocephalus, it became apparent that other specimens from a wide range of time and displaying a wide range of variation had been incorrectly assigned to this dinosaur. Among the incorrectly lumped dinosaurs was Dyoplosaurus.
Ankylosaur expert Victoria Arbour and her colleagues resurrected Parks’ ankylosaur in 2009. While the anatomy of the animal’s skull fragment wasn’t easily distinguishable from the original Euoplocephalus fossils, details of the hips and vertebrae, especially in the tail, distinguished Dyoplosaurus from all other ankylosaurs. From the hips back, Dyoplosaurus was a distinct dinosaur.
Despite what Parks had written, though, Arbour and her coauthors cautioned that the tail club of Dyoplosaurus was not an easy-to-spot difference. As far as paleontologists know now, ankylosaurid dinosaurs were not born with tail clubs. The osteoderms that formed the bludgeon grew later in life, and, since Parks’ Dyoplosaurus specimen was relatively small compared to Euoplocephalus specimens, it’s possible that the dinosaur’s tail club had not finished growing. When comparing dinosaurs, it’s always important to keep the animal’s stage of development in mind–features that may seem to characterize a new species may only indicate immaturity.
Other ankylosaurs are probably hiding within Euoplocephalus. Properly identifying and categorizing them will take years. Studies of hadrosaurs, ceratopsians, tyrannosaurs and other dinosaurs have shown that Late Cretaceous dinosaurs on the western subcontinent of Laramidia–isolated from their eastern cousins by the vanished Western Interior Seaway–that the genera and species differed along the latitudes. Rather than finding the same dinosaurs from Alberta to Utah, paleontologists have found distinct assemblages of dinosaurs that belie isolated evolutionary pockets. And analyses of Canada’s Late Cretaceous species have tracked turnover patterns among the dinosaurs, timing the pulse of evolution and extinction. Splitting out Dyoplosaurus is one more step towards understanding what North America’s dinosaurs can tell us about how evolution works.
Want to know more about other unsung dinosaurs? Check out previous entries in the Dinosaur Alphabet.
Arbour, V. Burns, M. Sissons, R. 2009. A redescription of the ankylosaurid dinosaur Dyoplosaurus acutosquameus Parks, 1924 (Ornithischia: Ankylosauria) and a revision of the genus. Journal of Vertebrate Paleontology 29, 4: 1117–1135. doi:10.1671/039.029.0405
Parks, W. 1924. Dyoplosaurus acutosquameus, a new genus and species of armored dinosaur; and notes on a skeleton of Prosaurolophus maximus. University of Toronto Studies Geological Series 18: 1–35.
October 29, 2012
Sauropods were magnificent dinosaurs. These long-necked, small-headed titans were unlike anything that has evolved before or since, and they were so strange that paleontologists are still debating the basics of how Apatosaurus and kin actually lived. As iconic as their skeletons are now, though, the first sauropod ever described was initially envisioned as a very different sort of creature. The great Cetiosaurus was originally seen as a gargantuan, plesiosaur-crunching crocodile.
In 1841, the British anatomist Richard Owen described a curious collection of limb bones and vertebrae found at various locations in England. The limb elements reminded Owen of the same bones in crocodiles, and the vertebrae were reminiscent of those in whales. The scattered elements seemed to correspond in structure to aquatic animals, and since function was dictated by skeletal form, Owen believed that Cetiosaurus–the “whale lizard”–must have been a marine predator larger than anything that had been found before.
The following year, in his massive Report on British fossil reptiles, Part II, Owen reassessed the various prehistoric reptiles from his country. This was the landmark monograph in which Owen coined the term “Dinosauria,” but he didn’t include Cetiosaurus within the newly named group. The animal seemed vastly different from Megalosaurus, Iguanodon and Hylaeosaurus. Dinosaurs, in Owen’s view, were terrestrial animals with upright limbs, and he saw Cetiosaurus as a marine carnivore. Owen grouped the poorly known animals with crocodiles, instead.
It wasn’t until 1869 that Cetiosaurus was formally recognized as a dinosaur. Thomas Henry Huxley, Owen’s chief academic rival, proposed that Cetiosaurus was a close relative of Iguanodon, although he later changed his mind and suggested that the puzzling animal was an oddball that didn’t belong with crocodiles or dinosaurs. Other researchers were more confident that Cetiosaurus belonged among the dinosaurs. John Phillips, in an 1871 monograph, proposed that Cetiosaurus was an herbivorous dinosaur, and in 1875 Owen conceded that his creature was a huge, aquatic dinosaur.
Like many other early dinosaur finds, the identity of Cetiosaurus was obscured by a lack of material and the unfamiliarity of the Mesozoic curiosities. When O.C. Marsh, E.D. Cope and other North American paleontologists began to uncover relatively complete skeletons of dinosaurs such as Diplodocus and “Brontosaurus” from the American West during the late 19th century, a more accurate vision of Cetiosaurus as a sauropod started to come into focus. All the same, researchers named multiple species of this dinosaur from various sites of different ages. Cetiosaurus became a taxonomic wastebasket for numerous scrappy sauropods found in England.
Paleontologists Paul Upchurch and John Martin sorted out the mess in 2003. Out of 13 different species named from bones belonging to different kinds of sauropods that lived millions of years apart, Upchurch and Martin recognized only one valid taxon–Cetiosaurus oxoniensis. This sauropod trod Jurassic England around 170 million years ago. And even though our knowledge of this dinosaur’s skeleton isn’t yet complete, discoveries both old and new have helped paleontologists outline what this historically significant dinosaur was like.
In 1868, quarry workers at Bletchingdon Station (near Oxford, England) uncovered a Cetiosaurus bonebed containing a trio of skeletons, one being much larger than the others. These bones formed the basis of Phillips’ study of the dinosaur, and, as Upchurch and Martin noted, “potentially represents one of the best preserved sauropods from the Jurassic of Europe.” A century later, in 1968, workers at Williamson Cliffe Brickworks in Rutland discovered bones in their quarry, and some of the remains were briefly described by M.D. Jones in 1970. Upchurch and Martin reexamined the Rutland material as part of their bigger Cetiosaurus project and found that the individual dinosaur is represented by an almost complete neck, various parts of the spinal column and limb elements, making it one of the best-preserved Cetiosaurus ever found.
Altogether, the bones of Cetiosaurus indicate that the sauropod was medium to large in size, though exactly how big this dinosaur was isn’t clear. (Estimating the length and mass of incompletely-known dinosaurs is a difficult task.) What makes Cetiosaurus of special interest to paleontologists, though, is that it was a relatively archaic form of sauropod. Most of the famous sauropods–Diplodocus, Camarasaurus, Brachiosaurus and their ilk–belong to lineages within a big group called the neosauropoda. Cetiosaurus seems to fall just outside this group, and so the dinosaur might clue paleontologists in to what sauropods were like just before the fantastic radiation of neosauropods during the Late Jurassic. It took three decades to change the animal from a crocodile to a dinosaur, and a century more for the sauropod’s identity to be untangled, but, now that the dinosaur has a definite name and evolutionary identity, paleontologists can start to investigate the biological secrets locked inside Cetiosaurus bones.
Check out previous entries in the Dinosaur Alphabet here.
Naish, D. 2009. The Great Dinosaur Discoveries. Berkeley: University of California Press. pp. 30-31
Upchurch, P., Martin, J. 2003. The Anatomy and Taxonomy of Cetiosaurus (Saurischia, Sauropoda) from the Middle Jurassic of England. Journal of Vertebrate Palaeontology 23 (1): 208–231
Upchurch, P., Martin, J. 2002. The Rutland Cetiosaurus: the anatomy and relationships of a Middle Jurassic British sauropod dinosaur. Palaeontology, 45: 1049–1074.
Wilson, J. 2005. Overview of sauropod phylogeny and evolution, pp. 15-49 in Curry Rogers and Wilson (eds.), The Sauropods: Evolution and Paleobiology, Berkley: University of California Press.
October 22, 2012
Poor, neglected Becklespinax. Although this gaudy, sail-backed theropod was an impressive predator at the time it strode across England around 140 million years ago, the fragmentary remains of this dinosaur have a tangled history only recently highlighted by the discovery of a more completely-known relative. In the history of paleontology, Becklespinax the tale is a tragedy.
The bones of Becklespinax were among the earliest spate of dinosaur discoveries in England, before anyone really understand just how many dinosaurs there were and how widely they varied in form. No surprise, then, that when the British anatomist Richard Owen illustrated a strange set of three high-spined vertebrae in 1855, he assigned them to the carnivorous dinosaur Megalosaurus. After all, Megalosaurus was already a hodgepodge of theropod remains from different eras, so it’s no altogether surprising that Owen considered the strange vertebrae as part of the same animal. He was confident enough in his assessment that when Owen schooled the artist Benjamin Waterhouse Hawkins in dinosaur anatomy for the famous Crystal Palace reconstructions, the anatomist instructed the sculptor to give Megalosaurus a hump between the shoulders on account of the elongated neural spines in the one specimen.
Along with teeth and other bits, the strange sting of vertebrae were thrown together into the species Megalosaurus dunkeri by researchers such as Richard Lydekker. No one found any complete skeleton–just scattered pieces. Then, in 1926, paleontologist Friedrich von Huene proposed that the spines and teeth of this “Megalosaurus” were so different from others of its type that it deserved its own genus–”Altispinax.” So scientists kicked the name Altispinax around for awhile, but this was another hodgepodge dinosaur consisting of various specimens from different places and time periods. In 1991, dinosaur fan George Olshevsky suggested that the set of three vertebrae carry the name Becklespinax altispinax, and, so far, that name has stuck.
But just what sort of dinosaur was Becklespinax? Paleontologist and prolific blogger Darren Naish addressed this question a few years back. The dinosaur was clearly a relatively large theropod, probably over 20 feet long. But, during the late 19th and early 20th centuries, there was no other dinosaur quite like it. Without a more complete skeleton, it was impossible to tell. And even after other big theropods with elongated spines on their backs were discovered–such as the croc-snouted Spinosaurus from the Late Cretaceous of Africa and the deep-skulled Acrocanthosaurus from the Early Cretaceous of North America–the anatomy of Becklespinax didn’t match those forms.
Even worse, the extremely limited material confounded paleontologists who attempted to figure out what the back of Becklespinax looked like. Were those elongated spines a sign of a high sail that ran most of the length of the dinosaur’s back, as in Spinosaurus? Or did it indicate a short, high ornament near the hips? Naish illustrated both possibilities in a 2007 paper he wrote with colleague David Martill. The first vertebral spine contained yet another puzzle. This bone was shorter than the following two. This might have been a pathology, or even because the bones came from the front part of the sail as it was building to its full height. No one knew for sure.
Then along came Concavenator. In 2010, paleontologist Francisco Ortega and colleagues named this carnivorous dinosaur on the basis of a gorgeous, 130-million-year-old skeleton found in Spain. A cousin of the high-spined Acrocanthosaurus from North America, Concavenator also had a weird backbone–the carcharodontosaur had a high, shark-fin-shaped sail just in front of the hips.
In over a century and a half, no one has ever found a better or more complete specimen of the English dinosaur, yet Concavenator offered a glimmer of what Becklespinax might have looked like. Both were sail-backed theropods that lived in the Early Cretaceous of Europe. And while our knowledge of Becklespinax is frustratingly incomplete, the resemblance of the dinosaur’s known remains to the corresponding parts in Concavenator suggest that Becklespinax, too, was a sail-backed carcharodontosaur. Their relationship may even go deeper. While the two dinosaurs lived about 10 million years apart, Naish pointed out, it’s possible that both dinosaur species belong to the same genus. Concavenator corcovatus might, in fact, be rightly called Becklespinax corcovatus. Without a fuller view of what the skeleton of Becklespinax looked like, though, it’s impossible to tell.
Whatever Becklespinax is, paleontologists have almost certainly found other scraps from this dinosaur. The trick is correctly identifying and assembling the scattered pieces. It takes years to untangle the history and form of dinosaurs found during the 19th century, as paleontologist Roger Benson did with Megalosaurus. A skeleton–even a partial one–would be even better. Such a discovery would go a long way towards outlining the nature of the frustratingly-incomplete Becklespinax, although other questions would certainly remain.
Between Acrocanthosaurus, Becklespinax and Concavenator, the massive carcharodontosaurs of the Early Cretaceous were apparently well-decorated predators that bore distinctive ridges and sails on their backs. Why? What good would such ornaments be to large predators? Were they signals of dominance, advertisements of sexual desirability or even just easily-seen markers that an individual belonged to this species and not that one? No one knows. As debates about sexual selection and dinosaur ornamentation heat up, even rapacious carnivores will have a role to play.
Previous posts in this series:
A is for Agujaceratops
Naish, D., and Martill, D. 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, 164 (3), 493-510 DOI: 10.1144/0016-76492006-032
Ortega, F., Escaso, F., and Sanz, J. 2010. A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain Nature, 467 (7312), 203-206 DOI: 10.1038/nature09181
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