This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
It says something of the invigoration of dinosaur research that an enormous technical tome, 624 pages long and containing 36 chapters, could be produced on horned dinosaurs (or ceratopsians) alone. New Perspectives on Horned Dinosaurs, published in 2010 and edited by Michael Ryan, Brenda Chinnery-Allgeier and David Eberth, stems from the Royal Tyrrell Museum Ceratopsian Symposium, held at Drumheller, Alberta, in September 2007. The organisers of this meeting were inspired by the major upsurge in horned dinosaur research currently underway – it involves the description of new taxa, a renewed interest in anatomy, behaviour and functional morphology, and the application of new techniques to such areas as the mechanical behaviour of bone, niche partitioning, phylogeny, biogeography and ontogeny.
Despite the popular name, not all horned dinosaurs are horned. Ceratopsia in fact includes psittacosaurids (a group of Asian bipedal forms, the skulls of which are often superficially likened to those parrots), the deep-headed, hornless leptoceratopsids, an assemblage of small- to medium-sized, mostly quadrupedal forms that were once grouped together as protoceratopsids (they have bony frills, and some, like the eponymous Protoceratops, have short or incipient nasal horns), and the members of the large-bodied, horned Zuniceratops + ceratopsid clade*. Ceratopsidae includes both the mostly long-frilled chasmosaurines with their long supraorbital, postorbital or brow horns, and the mostly short-frilled centrosaurines with their long nasal horns and typically short (or absent) postorbital horns. Zuniceratops and several other fossils seem to show that postorbital horns are primitive for the Zuniceratops + Ceratopsidae clade. Protoceratops, ceratopsids and their frill-headed kin (that is, all ceratopsians closer to Triceratops than to Psittacosaurus) are united within Neoceratopsia, while the node-based clade that includes Protoceratops and Triceratops is termed Coronosauria (Sereno 1998). [Highly simplified cladogram below borrowed from Thomas R. Holtz's University of Maryland lecture notes. Version below is low-res, pdf here has far better resolution.]
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* There's no name for the node-based clade that includes Zuniceratops, Ceratopsidae, and all descendants of their common ancestor. However, the names Ceratopsoidea and Ceratopsomorpha both apply to the branch-based clade that includes all taxa closer to Triceratops than to Protoceratops (Sereno 1998, Wolfe & Kirkland 1998). Ceratopsoidea has been used more frequently in the literature than Ceratopsomorpha and this is the best name we have at the moment for the "Zuniceratops + Ceratopsidae clade".
New Perspectives on Horned Dinosaurs consists of five sections: overview; systematics and new taxa; anatomy, functional biology and behaviour; phylogeny, biogeography, taphonomy and palaeoenvironment; and the history of collecting. The bulk of the book is made up of the second and third sections. The first section includes but a single article, and the last section includes just two. Virtually all contributions are very well illustrated and there is even a colour plate section. The book is large-format (c. 29 x 22 cm) and hence very different from most of the other multi-authored dinosaur-themed volumes published by Indiana University Press.
Homage to Prof. Dodson: "40 years of Ceratophilia"
The quality throughout is high and most students of the Ceratopsia were evidently compelled to contribute interesting work. Some articles do stand out as especially interesting, strong, or both.
Peter Dodson’s opening article (the single contribution in section one) – ‘Forty years of Ceratophilia’ – provides an excellent, personal overview of ceratopsian studies of the past four decades. Dodson says that his apparent modern role as “the dean … of ceratopsian studies” (Dodson 2010, p. 3) is a sort of happy accident; we know, in fact, that he is deserving of this role, having led work on the group through innovation, excellent research and brilliant writing (Dodson 1996). The ‘genealogy of Dodson ceratopsian students’ is a really nice touch, paralleling the efforts of other palaeontologists to produce phylogenies of academic ‘relatedness’. [Adjacent image of Dodson from his blog.]
Among other articles that really stand out, Dodson, You and Tanoue’s article on the basicranium and palate anatomy of psittacosaurids and non-ceratopsid neoceratopsians includes some wholly new anatomical information. Scott Sampson and Mark Loewen’s chapter is an excellent review of ceratopsid biogeography and phylogeny. One of their main conclusions – perhaps surprising to those who think that there might now be too many ceratopsids in Late Cretaceous North America! – is that “diversity will increase greatly once less explored geographic regions and temporal intervals are subject to greater sampling” (Sampson and Loewen 2010, p. 423). Indeed, as anyone who’s even vaguely aware of horned dinosaur diversity will know, a surprising number of new (mostly North American) taxa have been named within the last few years.
Those many, many new taxa
The volume itself includes the names of no fewer than six new taxa (one of which represents a renaming, not a wholly new taxon).
Coahuilaceratops magnacuerna Loewen et al., 2010 is a new chasmosaurine from the Cerro del Pueblo Formation of Mexico, notable for its gigantic, massively robust postorbital horncores, among the largest of any ceratopsian. Phylogenetic analysis recovers it as a close relative of Anchiceratops and Arrhinoceratops (Loewen et al. 2010) (though see Sampson et al. 2010). The second new taxon, Diabloceratops eatoni Kirkland and Deblieux, 2010, is a new brow-horned centrosaurine from the Wahweap Formation of Utah, named for a spectacular skull. Its nasal horn is tiny, the epijugal processes on the cheeks are especially long, and slender, outwardly curved epoccipitals are present at the posterior edges of the parietals. As noted above, we would predict that centrosaurines began their history with short nasal horns and long brow horns: nevertheless, it is good to have this confirmed in the form of both Diabloceratops and the also recently described Albertaceratops. The authors report a second specimen that seems to represent a second, as-yet-unnamed Diabloceratops species.
Rubeosaurus McDonald & Horner, 2010 is the new generic name for the centrosaurine previously known as Styracosaurus ovatus. While the inclusion of this Upper Two Medicine Formation taxon in Styracosaurus had never previously been challenged (see Ryan et al. 2007), it is not, in fact, a close relative of Styracosaurus proper (S. albertensis) if new phylogenetic studies are to be believed. In fact it is closer to Einiosaurus within the pachyrhinosaur lineage. Today it seems all too easy to forget that, just 20 years ago, Pachyrhinosaurus canadensis was regarded as the sole representative of a peculiar and otherwise enigmatic centrosaurine lineage.
One of the first papers to report new pachyrhinosaurine taxa – Horner et al. (1992) – made news at the time by proposing that Styracosaurus, Rubeosaurus, Einiosaurus, Achelousaurus and Pachyrhinosaurus belonged to an anagenetic lineage, the evolution of which was ‘forced’ by a marine transgression that caused ‘habitat bottlenecking’ and consequent intense selection pressure and rapid evolution. McDonald & Horner (2010) note that this hypothesis is not redundant altogether since rapid evolution and the displacement effects of the marine transgression both seem to have occurred; nevertheless, new species and new studies seem to show that cladogenesis was occurring instead or in addition. Indeed, yet another member of the pachyrhinosaur lineage is described in another of the volume’s chapters. It’s an unnamed form from the Upper Dinosaur Park Formation. Given that it likely represents a new species of Pachyrhinosaurus, this clade alone (that is, Pachyrhinosaurus) now includes four species.
Ojoceratops fowleri Sullivan & Lucas, 2010 is named for a left squamosal from the Ojo Alamo Formation of New Mexico’s San Juan Basin, identified as that of a Triceratops-like chasmosaurine. Already it has been suggested that O. fowleri may in fact be synonymous with Triceratops (Longrich 2011). One peculiarity of Sullivan & Lucas’s (2010) terminology is their use of ‘crown chasmosaurine’ (p. 177). Another new chasmosaurine – Medusaceratops lokii Ryan et al., 2010 – is named for parietal fragments from the Judith River Formation of Montana, originally referred to the centrosaurine Albertaceratops. Large, robust postorbital horns from the same bonebed are inferred to belong to it. Medusaceratops is the oldest reported chasmosaurine.
Then there’s Tatankaceratops sacrisonorum Ott & Larson, 2010 from the Hell Creek Formation of South Dakota. The existence of a new chasmosaurine in the late Maastrichtian of western North America is interesting in view of debates about dinosaur diversity at this time and place, and it is inevitable that some will immediately identify the type and only specimen as a juvenile Triceratops (as anyone interested in dinosaurs will know all too well, debate continues as to whether some or all late Maastrichtian chasmosaurines represent distinct taxa, or growth stages of the same species). However, Tatankaceratops has proportionally short, subvertical postorbital horns that look different from both the far longer, posteriorly curving horns present in juvenile Triceratops and the even longer, anteriorly curving horns present in adult Triceratops (Horner & Goodwin 2006). The possibility that it might represent a dwarf form of Triceratops (Longrich 2011) appears plausible, as does the suggestion that it could be an aberrant individual. These ideas could perhaps be tested via histological analysis.
A new species of Archaeoceratops, A. yujingziensis, is also named in the volume. A redescription of the Montanoceratops cerorhynchus holotype and a new report of Prenoceratops from the Oldman Formation add new data points to the growing body of literature on leptoceratopsids.
I should finish this discussion of new taxa by noting that descriptions of new horned dinosaur taxa have not exactly been restricted to this volume. Since this book has appeared, North America has yielded the additional new centrosaurine Spinops sternbergorum Farke et al., 2011 as well as the new chasmosaurines Utahceratops gettyi Sampson et al., 2010 and Kosmoceratops richarsoni Sampson et al., 2010. The new name Vagaceratops Sampson et al., 2010 has been given to the taxon originally described as Chasmosaurus irvinensis, and the somewhat controversial Mojoceratops perifania Longrich, 2010 and Titanoceratops ouranos Longrich, 2011 have also been named. Then there’s the Asian centrosaurine Sinoceratops zhuchengensis Xu et al., 2010, the new leptoceratopsids Zhuchengceratops inexpectus Xu et al, 2010, Gryphoceratops morrisoni Ryan et al., 2012 and Unescoceratopskoppelhusae Ryan et al., 2012 and the protoceratopsid-grade taxa Ajkaceratops kozmai Ősi et al., 2010 and Koreaceratops hwaseongensis Lee et al., 2011.
One of my favourite chapters is Jordan Mallon and Robert Holmes’s description of CMN 8547, a near-complete chasmosaurine specimen (mounted at Ottawa’s Canadian Museum of Nature with a replica Anchiceratops skull), conventionally assigned to Anchiceratops due to the association of some supposedly diagnostic frill fragments. It turns out that those fragments aren’t all that diagnostic and that the taxonomic status of the specimen is uncertain. This is unfortunate (and needs resolving) since CMN 8547 is such a beautifully preserved, near-complete specimen. Its robust build, massive limbs and short tail are peculiar features that might suggest a hippo-like lifestyle.
Horns, frills, herding and lifestyle… a few unorthodox ideas
What of behaviour and ecology? The diverse and often spectacular horns, frills and other cranial structures present in the group have of course invited substantial speculation on social behaviour, sexual selection, the need to deal with contemporaneous theropods, thermoregulation and even acoustics. Needless to say, there is a huge amount of information here that will be of interest to those investigating horned dinosaur ecology, behaviour and palaeoenviromental preferences. Chapters include David Krauss and colleagues’ on the correlation between horn and frill morphology in chasmosaurines (parietal fenestrae are positioned just outside horn reach), Donald Henderson’s on niche partitioning as indicating by skull shape, and Rebecca Hunt and Andrew Farke’s on behavioural information as inferred from bonebeds. They conclude that, while there is good evidence for herding behaviour in some taxa, the evidence is not so good that we should make assumptions about herding across the clade.
Two behaviour-themed articles in particular stand out as unusual, and indeed the editors note in the preface that they are “sure to spark debate” (Ryan et al. 2010, p. xiii). Nick Longrich uses data from sclerotic ring size to propose that the unusually large-eyed Protoceratops might have been scotopic (or nocturnal). Given the abundance of specimens and quality of preservation, surprisingly little has been published on the palaeobiology of Protoceratops. Most work has concentrated on the supposed presence of sexual dimorphism and on the nests and eggs associated (sometimes incorrectly) with this dinosaur; its probable diet and habits have not been well explored. Schmitz & Motani (2011) used a similar technique to Longrich (2010) to analyse possible activity patterns in dinosaurs and other Mesozoic archosaurs, though did not report the same conclusions for Protoceratops. While this angle of analysis looks promising, already there are uncertainties given that Microraptor – inferred from eye form to be scotopic – is known to possess glossy feathers, a feature never present in extant scotopic species (Li et al. 2012). [Adjacent photo by Jordi Payà, licensed under Creative CommonsAttribution-Share Alike 2.0 Generic license.]
In the second ‘unusual’ chapter, Tracy Ford and Larry Martin put forward the surprising (but not wholly novel) proposal that Psittacosaurus was amphibious. The evidence used to support this idea is underwhelming; none of the features they point to are reliable indicators of aquatic habits. They make vague and unconvincing comparisons between the psittacosaurid forelimb and that of sea-turtles, cetaceans and other swimmers, imply that flexible hindlimb joints provide evidence for a swimming habit, suggest that gastroliths may have been present because of a role in buoyancy control, and draw attention to the presence of a laterally compressed tail skeleton and dorsally placed nostrils and orbits. [Adjacent photo by Christopher, Tania and Isabelle Luna]. Most or all of these features either do not link consistently with swimming habits, or are clearly present in other animals that do not, or did not, regularly swim. Their suggestion that the long bristle-like tail structures seen in one psittacosaurid specimen “may have supported a caudal fin that was somewhat analogous to the caudal fin in modern amphibians, such as the Hellbender … and tadpoles” is surprising (Ford & Martin 2010, p. 335). I'm not sure that it's completely untenable, but the very slender, often overlapping fibres on the psittacosaurid tail clearly seem to be external to the epidermis, not sandwiched within it (as they'd have to be if they were within a continuous skin frill or fin). And, so... we have this vision (below, courtesy of Zach Miller of When Pigs Fly Returns).
As David Eberth points out in his review of palaeoenvironmental associations and taphonomy, the unusually high number of fully articulated psittacosaurid specimens suggests that their carcasses did not endure a lengthy ‘bloat-and-float’ phase prior to burial, an observation apparently at odds with Ford and Martin’s model. Nevertheless, the idea is not so bizarre that it can be dismissed entirely without consideration. As the authors note, the proposal may not apply to all psittacosaurid species, and we have to keep in mind the ecological and behavioural diversity seen in certain extant ‘genera’. Furthermore, amphibious habits should not be ruled out entirely for all ceratopsians. The good news is that we have so many psittacosaurid specimens that testing this hypothesis would be very simple, should someone feel it worthy of proper investigation.
An accompanying CD ROM includes two lengthy contributions that would not have worked well as published articles but are worth having for completists: Tracy Ford’s stratigraphically arranged specimen list, and Darren Tanke’s substantial, date-arranged compilation of discoveries, events and biographies relevant to horned dinosaur research in Alberta.
All in all, New Perspectives on Horned Dinosaurs is arguably the most significant dinosaur book to appear in recent years, and this is against a lot of competition. It demonstrates – just in case there was any doubt – that feathered maniraptorans and tyrannosaurids are not the only sections of the dinosaur tree where exciting research and discoveries are happening. In term of density, significance, novelty and sheer volume of content, and in the quality of the text and illustrations, it’s outstanding – certainly head and shoulders above the many other dinosaur-themed volumes published by Indiana University Press. It does a superb job of capturing the status of horned dinosaur research at it was in the first decade of the present century. Surely, with this much exciting research and investigation underway, there are many surprises yet to come.
Ryan, M. J., Chinnery-Allgeier, B. J. & Eberth, D. A. (eds) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Indiana University Press, Bloomington. Hardback, index, refs, pp. 624. ISBN 978-0-253-35358-0.Here on Amazon, here on Amazon.co.uk.
There's now quite a lot on Tet Zoo about horned dinosaurs. For previous articles, see...
Tet Zoo picture of the day # 11 (or, speculations on omnivory in ceratopsids)
SVPCA 2007: dinosaurs attack (includes stuff on Psittacosaurus)
A month in dinosaurs (and pterosaurs): 3, Minotaurasaurus and giant chasmosaurines
Zuniceratops and the early acquisition and alleged dimorphism of ceratopsian brow horns
Did dinosaurs and pterosaurs practise mutual sexual selection?
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Ford, T. & Martin, L. D. 2010. A semi-aquatic life habit for Psittacosaurus. In Ryan, M. J., Chinnery-Allgeier, B. J. & Eberth, D. A. (eds) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Indiana University Press, Bloomington, pp. 328-339.
Horner, J. R. & Goodwin, M. B. 2006. Major cranial changes during Triceratops ontogeny. Proceedings of the Royal Society, London B 273, 2757-2761
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Li, Q., Gao, K.-Q., Meng, Q., Clarke, J. A., Shawkey, M. D., D’Alba, L., Pei, R., Ellison, M., Norell, M. A. & Vinther, J. 2012. Reconstruction of Microraptor and the evolution of iridescent plumage. Science 335, 1215-1219.
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