Sunday, 25 November 2012

SVP 2012 - Theropods Part 1


 
There is no doubt that theropods are probably the most popular dinosaur group of all and, knowing most of you like I do, then it is the theropods that are of the greatest interest whenever there is new research and information revealed. So without further ado, we can proceed. I always feel that ornithomimosaurs do not get as much attention as they deserve but they are bang in the news at the moment because of the newly revealed feathered ornithomimosaurs from Alberta and they also received some attention at SVP this year.
Ornithomimosaurs have long be known as “ostrich mimics” but how true a comparison is this with today’s extant palaeognaths – especially when it comes to the crania? By digitally reconstructing three ornithomimosaur skulls (Garudimimus, Struthiomimus and Ornithomimus), Andrew Cuff and Emily Rayfield, both of the University of Bristol, have produced some interesting results.
By utilising a combination of techniques including virtual muscle attachment data, adding hypothetical beaks to the rostra and studying finite element models and then, by comparing them to both ostrich and other theropods, has indicated that the sutures of the skull were key in minimising pressure to the skull. The beaks too also help in reducing strain and the different morphologies, especially of those beaks that are more extensive, help compensate for the loads.
More derived ornithomimosaurs reveal lesser muscle loads but still strain similarly to more basal forms. Compared on a like for like basis, the skulls of the advanced forms buckle quicker than the more primitive forms. The authors take this as indication that derived ornithomimosaurs may have been changing or varying their diets.
Not surprisingly, the skulls of ornithomimosaurs differ greatly from other theropods – in this study with Allosaurus and Coelophysis. The authors speculate that this may simply be a feature of herbivory evolving in ornithomimosaurs and in other theropods and the repeated evolution of beaks in Theropoda would simply have been a natural progression.
I mentioned earlier the feathered ornithomimosaurs from Alberta and a bone bed from the Upper Cretaceous Horseshoe Canyon Formation in the same province has been providing interesting histological data to Thomas Cullen, of Carleton University in Ottawa, and colleagues. The bone bed, the first of its kind in North America, contained the remains of three individuals and multiple leg elements from these specimens were histologically examined.
As is often the case with histological studies of non avian dinosaurs, indications are that growth was rapid, lines of arrested growth are equispaced and there is fibrolamellar tissue present.  At time of death, however, the larger individual reveals growth to be slowing up thus indicating the onset of maturity. Age for the two smaller individuals is estimated to be between two and three years old whilst the older specimen may be approaching four years of age.
Because the lines of arrested growth are constant throughout growth the authors suggest that postcranial long bones may be used as good indication of age – particularly in smaller theropods and, if they are correct in their hypothesis, then we may already have a wealth of information in our collections that enables us to determine growth patterns in different taxa - especially if the overall body size of a specimen can be ascertained.
Now I have mentioned on this blog on more than one or two occasions that the Upper Jurassic Morrison Formation of North America has many surprises still left up its sleeve and that there were many new taxa still to be described. However, this next one may seem surprising since it appears that we have yet another large bodied allosauroid in residence in what already appears to be an ecosystem already well represented in that department.
The specimen was recovered from McElmo Canyon in Montezuma County, Colorado and is late Tithonian in age. There are multiple elements represented including cranial, axial and appendicular and these have been examined by Sebastian Dalman, of Fort Hays State University, and colleagues with particular attention made to the cranial bones which were CT scanned and then digitally reconstructed three dimensionally.
This has enabled the authors to highlight characters in the premaxilla, maxilla and quadratojugal that warrant the erection of a new taxon. This animal is a large allosauroid comparable with other sympatric theropods in the same formation such as Allosaurus and Saurophaganax although, when compared with elements of similar sized Allosaurus specimens, is clearly a more massive animal.
Early phylogenetic analysis indicates the specimen is well placed within Allosauroidea and is closely related to Allosaurus, Fukuiraptor and Neovenator.   
 
 
Now we are all familiar with the great nest sites of sauropods and oviraptorids but those of other saurischians, particularly large theropods, whilst not unknown, remain relatively rare. The great nest sites of Portugal are an exception and a talk by Octávio Mateus, of the Universidade Nova de Lisboa, and colleagues have brought some amazing insights.
The nest, from the Upper Jurassic Lourinhã Formation in Paimago, Portugal contains the remains of approximately 100 eggs although erosion and the amount of eggshell present indicates the nest held many  more. Also of interest is that the nest also contains crocodile eggs but we can only speculate as to their presence here.
There appears to be no clear structure to the nest but the best preserved eggs and embryonic bones are preserved in the centre suggesting that these were the first laid and were at a more advanced ontogenetic stage than those on the exterior of the nest.  There are around 300 bones that have been recovered so far and this is despite the fact that the vast majority of the eggs are crushed.
The authors tentatively assign these embryos to Lourinhanosaurus antunesi since, although the bones are very similar to those of this theropod, the autapomorphies required to confirm identification were not preserved. However, there are sufficient differences in the remains to confirm that they are neither Allosaurus, Ceratosaurus nor Aviatyrannis. Nor does it seem that these eggs were laid by Torvosaurus, since the known nest, from the same formation, that was designated as belonging to this taxon are dissimilar in as much as the egg shell ornamentation is completely different and the embryonic remains are larger.
The authors point out that these embryos are very much fully loaded miniature versions of the adults and appear ready to become active predators from the moment they hatched. One other point of interest is that there are some very small patches of skin preserved in the nest and that these particular baby theropods display no evidence whatsoever of feathers or fuzz – only clearly defined scales -  again highlighting the danger of assuming that all theropods were feathered to some degree.
Scott Persons and Phil Currie, both of the University of Alberta, have been continuing their research into the cursorial ability in theropods and this time looked at the relative length of limb bones measured against the enormous variance in overall theropod body size as well as the effects of allometry on limb proportions.
Sixty one different theropod taxa were studied that all had complete hind limbs. The authors confirm that distal limb bones are unaffected by allometry and compensate for this fact. This allowed them to recognise adaptions in limb proportions and, as a result, they were able compare theropods despite the extreme differences in size.
The essential result of this is that there is no evidence that limb proportions in theropods that change throughout ontogeny are different in any way from those that would be predicted by simply observing body size. Highlights of this study include that the relative limb proportions of Nanotyrannus, when compared to other tyrannosaurids including Tyrannosaurus, are so extreme that the authors suggest that Nanotyrannus is indeed a valid taxon (here we go again). There is also evidence that there was continuous adaptivity in improving cursorial ability in many theropod groups including troodontids and tyrannosauroids.
 Of particular surprise, however, is that dromaeosaurids actually demonstrate a reduced ability to run which is quite the shock when you consider how we have all assumed them to be amongst the quickest and most agile of all the theropod dinosaurs.

More in Part Two.

References  
Cuff, A.R. & Rayfield, E.J. 2012. Functional Mechanics of ornithomimosaur crania compared to other theropods. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 82.
Cullen, T.M., Ryan, M.J., Evans, D.C., Currie, P.J. & Kobayashi, Y. 2012. Multi-element histological analysis of an ornithomimid (Dinosauria) bone bed from the Horseshoe Canyon Formation, Alberta. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 82- 83.
Dalman, S., Paulina Carabajal, A. & Currie, P.J. 2012. A new large-bodied theropod dinosaur from the Upper Morrison Formation (Late Jurassic, Tithonian) of Colorado. . Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 84.
Mateus, O., Carrano, M.T. & Taquet, P. 2012. Osteology of the embryonic theropods from the Late Jurassic of Paimago, Portugal.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 137.
Persons, W.S. & Currie, P.J. 2012. Adaptive cursorial trends among theropod dinosaurs and an attempt to look beyond allometry. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 156 - 157.

 

 

      

Monday, 19 November 2012

SVP 2012 - Some Tasty Morsels


 
Evidence of interaction and behaviour between extinct animals is always fascinating since it brings the prehistoric world to life. And, in this section, such evidence features heavily. David Burnham, of the University of Kansas, and colleagues presented a poster said to demonstrate evidence of predation by a plesiosaur on a specimen of Hesperornis.
Hesperornis is a well known Late Cretaceous ornithurine that was an abundant component of the Niobrara chalk sea in Kansas. A left forelimb of this bird demonstrates a series of small conical holes that suggest this specimen was attacked but survived since radiological examination reveals osteomyelitis infection had developed in the distal tibiotarsus.
Comparing the depressions and spacing of the tooth holes suggests that the attacker was most likely to be a small polycotylid plesiosaur. The authors interpret this as evidence that the plesiosaur attacked the bird from behind but was unable to maintain a grip on its intended victim and, thus, was able to make its escape. There appears, on the face of it, a little uncertainty about this since there were so many other predatory fish in the oceans with similar teeth that to be certain that a plesiosaur was responsible for the attack seems a little premature.
 But Bruce Rothschild was present and defending the poster and he was adamant that they were correct and since he knows more about bone pathologies than most people I think we can give him the benefit of the doubt for now. Even so, this is a very cool specimen that highlights the dangers that these large flightless birds must have faced every time they took to the ocean.
Sigilmassasaurus brevicollis was another theropod found in Late Cretaceous rocks (Cenomanian?) of North Africa. Known as Spinosaurus “B” by Ernst Stromer, the taxon was formally named Sigilmassasaurus by Dale Russell in 1996. And now Serjoscha Evers, of Ludwig-Maximilians-University, and colleagues have taken on the task of identifying this mystery taxon. The validity of this animal has always been in doubt since the remains are limited to vertebrae but now new vertebrae, consisting of one dorsal and several cervicals, found in museums in both London and Munich, have presented new data.
Firstly there several characters in line with the original descriptions that demonstrate that they are definitely from Sigilmassasaurus. Secondly the cervicals also demonstrate morphological differences that clearly show that they are not from Carcharodontosaurus – which some authors have considered that these remains belong to. Under CT-scanning, they also have internal arched pneumatic chambers whilst those of carcharodontosaurids are the familiar honeycomb-like small chambers.
What they are similar to, however, are the vertebrae of megalosauroids – especially spinosaurids. This suggests that Stromer may have been right (not for the first time) and that Sigilmassasaurus may represent a new spinosaurid taxon. The authors take this is a further indication that spinosaurids were a more diverse clade of dinosaurs than originally thought.
The small ornithopod Oryctodromeus cubicularis first came to our attention back in 2007 as the first dinosaur that potentially could be described as a possible burrowing or denning dinosaur (Varrichio et al 2007). The remains were found in the Mid-Cretaceous Blackleaf Formation in Montana and were found disarticulated in a partially preserved chamber at the end of a tunnel. Further research into the burrowing hypothesis has continued but has proven hard to quantify.
Now Jamie Fearon and David Varrichio, both of Montana State University, have been examining the forelimb morphology of Oryctodromeus to see if there are any specific adaptions for a burrowing lifestyle. This is problematic in as much that there are no extant analogues around to be able to compare the specimens with. 
But by using a combination of traditional and other geometric statistical analyses, including principal components analysis (PCA), of various ornithopods (including Oryctodromeus) the authors have been able to demonstrate some interesting results.   Iguanodontians and hypsilophodontids share similar basic forelimb morphologies but are distinct from hadrosaurids due to the shape of the deltopectoral crest. However, the PCA reveals that hypsilophodontids are distinct from both iguanodontids and hadrosaurids mainly due to the variance in the morphology of the scapula which is broader at both the proximal and distal ends.
So where does Oryctodromeus figure in all this? Well surprisingly not that close to any of the other groups mainly due to a ventral expansion along the posterior border of the scapula blade. The authors hypothesise that since this is the attachment site for the deltoideus muscle there is a chance that this can be seen as an adaption for digging since extant mammals that burrow today use that very same muscle. I asked the other question that has bugged the burrowing dinosaur theory – was there enough flexibility in the tail for the animal to be able to turn in such a tight area? Jamie Fearon believes that there probably was although she took great pains to point out they were only looking at the forelimb in this current study.
Serration scars are vivid on this example
A very popular poster, and another that has received good publicity, was titled “How to eat a Triceratops….” – a title guaranteed to generate interest. Denver Fowler, of the Museum of the Rockies, and colleagues have been looking at traces of tyrannosaurid carnivory by studying trace marks on numerous bones of Triceratops.
As noted here before, the Hell Creek Formation Project (1999-2011) has yielded 100 specimens of Triceratops and, of this number, there are eight individuals displaying tooth marks with another six that may possibly show evidence of carnivory. The real interesting thing here is that all the bones bar an ilium are from partially articulated or disarticulated skulls.
All bones display typical tyrannosaurid predation marks including puncture wounds and the classic puncture /pull drag marks. Such marks and analysis of the tooth spacing clearly indicate tyrannosaurid predation – in this case Tyrannosaurus rex since it was (and still is) the only known tyrannosaurid from the Late Maastrichtian rocks of Hell Creek.
Closer scrutiny of these marks reveal interesting, indeed, surprising feeding techniques. It appears that these carnivores were actually quite partial to the head of Triceratops. There are several drag marks on the parietosquamosal frills of a few specimens which, on the face of it, is surprising since there is apparently very little flesh there. The authors suggest that this may be indicative of the tyrannosaur manoeuvring the head to get to the succulent neck muscles and they quantify this suggestion by highlighting the fact that there are two occipital condyles displaying deep predatory gouges on them.
At the same time there are lighter feeding traces on bones such as a nasal and premaxilla suggesting that tyrannosaurs could be quite delicate in their feeding. This is not surprising since the premaxillaries of tyrannosaurids have often been described as “nibbling” teeth and now we have the evidence to back that up. But big tyrannosaurids, and Tyrannosaurus in particular, were well suited to large volume dismemberment of a carcass, whether killed or scavenged, and this new research highlights their various abilities in processing their prey to maximise the calorific intake.
What you cannot appreciate reading this, and unless you were at SVP, is the attached images with the poster showing these bones with the signs of carnivory. They are awesome and, in particular, the gauges on the occipital condyles are amazing. I am not sure how far away this paper is from publication but the images will be even better. When it is published take a look and be impressed – great work.
Yet more tyrannosaurid scars
 
This next one is a belter of a fossil. Finding dinosaurs with associated eggs is always fascinating but a partial oviraptorid recovered from the Upper Cretaceous Nanxiong Formation of Jiangzi in China appears to give us truly remarkable insights into the origins of avian reproduction. Tao He, of the Zhejiang Museum of Natural History in China, and colleagues have been looking at this specimen and made some interesting observations.
First of all, only the posterior half of the animal is preserved but it is well articulated. There are two eggs in association and they are both long and asymmetric and are designated as belonging to the Elongatoolithidae. One is located posterior to the sacrum (below caudals 4 – 7) and the other between the ischia. They are both covered in shell which is 1.0mm thick and is streaked by lineated ridges that run the length of the egg. The egg situated between the ischia also displays its blunt end in an apparent caudal direction.
The authors postulate that the relative position of the eggs suggests that they were partially expelled as the animal decayed – something that has been observed in extant drowned pregnant cattle as the calves were expelled after some decomposition(and there were some graphic images to show this). A blunt ended caudally facing egg suggests that theropods did not lay eggs in bound pairs as originally postulated and is actually opposite to what is found today in extant birds.
Two other points of note. Maniraptora have often been described as being monoautochronic – that is the simultaneous production of a single egg by each of a pair of oviducts -  and this specimen is clearly indicative of this. It is also suggestive that laying eggs two by two in a clutch enabled eggs to be proportionally larger and more robust when compared to the overall body size of the adult. This was a great presentation and the displayed images of the specimen were superb.
 
A typical dromaeosaurid
 
Richardoestesia is an enigmatic theropod taxon of uncertain phylogenetic affinities from the Maastrichtian of North America. It has been referred to dromaeosaurids, troodontids and aves but until more definitive remains are found is best left as status to be determined. However, we may be closer to understanding how this taxa eked out a living – by taking a closer look at its teeth.
Microvertebrate fossil specialist Julia Sankey, of the California State University Stanislaus, has been looking at abundant quantities of teeth belonging to Richardoestesia isosceles – a species she named herself back in 2001. Sankey has recognised that the straight tooth morphology is perhaps indicative that the taxon was a fish eater and quantifies this by highlighting that the teeth are much more prevalent in coastal as opposed to inland deposits. Richardoestesia, therefore, could be the first recognised fish eating theropod on the continent of North America.
And finally, in this post, we look at another fascinating case of interaction between taxa but perhaps not quite the animals that you might have expected. Jason Schein, of the New Jersey State Museum, and colleagues have been looking at a hadrosaurid femur recovered from the Main Fossiliferous Layer (MFL) of the lower Hornerstown Formation in New Jersey. The bone displays multiple straight to slightly curved grooves that are typical predation marks of oceanic sharks and the dinosaur was likely a typical bloat-and-float carcass that had been swept out to sea.
This femur, however, may be quite unique. The MFL has proven difficult to age correctly but it does appear likely that these sediments were deposited almost exactly at the contact point between the Maastrichtian and Danian – in fact the MFL has virtually one leg in each stage. Regardless of the complex taphonomic and depositional permutations, the femur represents what may potentially be the youngest non avian dinosaur fossil ever discovered.

References  

Burnham, D.A., Martin, L.D. & Rothschild, B.M. 2012 Plesiosaurs had a taste for birds.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 71.
Evers, S.J., Rauhut, O.W. & Milner, A.C. 2012. Was Stromer right? The affinities of Sigilmassasaurus brevicollis (Theropoda, Tetanurae).  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 91.
Fearon, J.L. & Varricchio, D.J. 2012. Comparative pectoral and forelimb morphology of Ornithopoda: does Oryctodromeus cubicularis exhibit specialization for digging? Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 92 - 93.
He, T., Varricchio, D.J., Jackson, F.D., Jin, X. & Poust, A.W. 2012. An oviraptorid adult-egg association and the origin of avialan reproductive strategies.   Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 108.
Sankey, J.T. 2012. Something’s fishy: was one of the most abundant Late Cretaceous theropods a fish eater? Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 165.
J.T. Sankey, 2001, "Late Campanian southern dinosaurs, Aguja Formation, Big Bend, Texas", Journal of Paleontology 75(1): 208-215 
Schein, J.P., Poole, J.C. & Lacovara, K.J. 2012. A shark-bitten hadrosaurid femur from the basal Hornerstown Formation, New Jersey, USA.: one of the youngest non-avian dinosaur remains known.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 166. – 167.
D. J. Varricchio, A. J. Martin, and Y. Katsura. 2007. First trace and body fossil evidence of a burrowing, denning dinosaur. Proceedings of the Royal Society of London B 274:1361-1368.

Friday, 9 November 2012

SVP 2012 - The Ornithischians


 
Victoria Arbour and ankylosaurs go together like two peas in a pod and I’m pleased to say that her love affair with these unusual armoured behemoths continues. This year Victoria, from the University of Alberta, and co-author Phil Currie presented their latest research into the taxanomic affinities of ankylosaurids from the Gobi Desert of Mongolia.
A recently discovered ankylosaurid skull from Upper Cretaceous sediments of the Baruungoyot Formation at Khermeen Tsav displays a number of new cranial characters and two distinct features, one of the squamosal horns and one of ornamentation located posterior to the orbit, that indicate this may be a new taxon since these features are not known in either Saichania or Tarchia.
Although the authors have not formally identified the new skull, they recognised the importance of the specimen and have run a phylogenetic analysis using the updated codings taken from the new skull. They also included a re-evaluated data set from two specimens of Saichania as the only currently known ankylosaurid from Baruungoyot including the holotype (MPC 100/151).
The analysis reveals that Minotaurosaurus, Pinacosaurus grangeri and the new skull are a closely related group whilst Pinacosaurus mephistocephalus and Saichania are also recovered as being closely related but are further removed from the new specimen than the other taxa. Although this latest analysis brings our phylogenetic understanding of Mongolian ankylosaurids to a new height of understanding there are still many question marks and the authors quantify this by stating that the amount of postcranial remains found in association with skulls is scarce and that this data should be viewed with this in mind.
A couple of weeks prior to SVP taking place, Greg Erickson et al presented their findings regarding dental structure and wear biomechanics in hadrosaurids in Science magazine and the same team also presented their research at the meeting.
Hadrosaurids were unique dinosaurs – especially when it comes to the makeup of their teeth. Using three dimensional tribological modelling has revealed some intricate yet amazing detail. Simple reptilian teeth are made up of two dental tissues, mammals have four but hadrosaurid teeth are comprised of six distinct dental tissues. The complexity of this structure is astonishing, especially when you see that this dental tissue arrangement varied both within and across the teeth, in effect allowing a single tooth to perform different functions in different places such as grinding or chewing.
 
 
This unique set of dental characters in combination with the already celebrated hadrosaurid jaw mechanics likely gave the group a significant advantage over other contemporary herbivorous dinosaurs. In effect, these dinosaurs were able to eat almost anything and were able to adapt to other food sources when others became scarce and this almost certainly enabled them to dominate the Late Cretaceous environments right up to their terminal demise at the end of the Maastrichtian.
Parasaurolophus is another of those wonderful iconic dinosaurs with its large tube-like crest that contains part of the nasal passages sweeping back over the skull. Despite being represented by numerous remains the cranial ontogeny of Parasaurolophus is unclear – especially when compared with other lambeosaurines.
But now Andy Farke et al have been looking at an exceptionally well preserved juvenile specimen recovered from the Upper Cretaceous Kaiparowits Formation of southern Utah. The authors presented this data as a poster at SVP and the images they used were life sized! I cannot stress how beautiful this specimen is – a real jewel and extremely important.
The skull is 240mm long and the overall length of the specimen is between 1.9 and 2.5 metres. Data of note includes an extension of the upper beak that extended approximately 30mm beyond the premaxilla which enabled faster harvesting and a larger cropping area due to the expansion. The crest is semi-circular in lateral profile as opposed to the usual tubular configuration found in adults. The crest is also at a more progressive stage than you would expect to find in other taxa at a similar ontogenetic stage.
 
 
The authors interpret this as an almost expected ontogentic trait in Parasaurolophus since the crest is extremely large in adults and therefore it would seem likely that it would need to be growing at a faster rate at an earlier age to reach the adult iteration. The crest also demonstrates advanced nasal development at this stage when compared to other lambeosaurines.  The specimen suggests that the increased nasal complexity of hadrosaurid and lambeosaurid crests dictated the early development of the structures in juveniles.
One poster that has certainly seen a lot of publicity since the meeting is that of Joseph Peterson and Collin Dischler, both of the University of Wisconsin, which looked at the frontoparietel domes of pachycephalosaurids. Pachycephalosaurs have often been depicted as head butters but this has been challenged throughout the last decade (eg Goodwin & Horner 2004) since it appeared that the domes are composed of porous bone that is associated with vascular tissue and that it is not congeneric with a butting scenario. And since the domes changed throughout ontogeny it seems reasonable to suggest that the heavy ornamentation was useful for intraspecific communication and recognition.
However, further studies of pathologies and structural models have again lent support to the head butting hypothesis. This latest study involves domes from multiple pachycephalosaurid taxa and looks at lesions, remodelling and pathologies and their distribution on the domes and, by using specific homologous landmarks on the domes, were able to identify some interesting patterns.
All taxa bar one (Gravitholus) demonstrate a greater percentage of lesions on the dorsal surface of the frontal region. This is consistent despite the varying height, shape and size of the domes in the various taxa. The authors interpret this as evidence for both head butting and flank shoving since the locations of the lesions are consistent with just such behaviour. Perhaps the truth lies somewhere between the different hypotheses. Certainly I would not expect pachycephalosaurs to head butt like extant bighorn sheep would do. More sampling required for sure but at least the domes do appear to survive pretty well.
Along these same lines, and equally interesting, Collin Vanburen, of the University of Toronto, and colleagues have been looking at cervical fusion in ankylosaurs. Fusion occurs within the first two cervical vertebrae in three taxa – two nodosaurids and one ankylosaurid. Other taxa do not display this fusion but this may be a sampling issue since anterior cervicals are rarely found.
The morphological arrangement of this syncervical is akin to those of ceratopsians which was thought to be an adaption to cope with the evolution of their massive heads. Despite this similarity no appropriate theory has been formalised to account for the syncervical in ankylosaurs – until now. The heads of ankylosaurs, in comparison with ceratopsians, are actually quite small and this negates the large head support hypothesis.
Instead the authors suggest that this fusion of the anterior cervicals may have been a direct response to ecological pressures or behavioural patterns. This adaption may have been related to a specific feeding response, which kind of makes sense to me as only the three taxa (so far) display the syncervical, or, perhaps, that this may have been related to intraspecific head-to-head behaviour. The authors state that more extant comparisons need to be made with mammalian taxa and birds (eg hornbills) that demonstrate cervical fusion and, of course, so much more material needs to be found for a true comparison to be made. It would be interesting to see a study on ankylosaur skulls similar to the one being performed on pachycephalosaurs by Peterson and Dischler mentioned above.
Maiasaura peeblesorum is one of the best represented and best researched non avian dinosaurs in the world. And now Holly Woodward, of the Museum of the Rockies, and her colleagues have performed a wide ranging histological analysis of forty eight tibiae looking at ontogenetic and individual variation within the taxa.
As we have seen many times with other dinosaurs using histological research, Maiasaura grew very fast until attaining skeletal maturity. Lines of arrested growth (LAGS) show what has been referred to as demonstrating seasonal trends very similar to what has been described for hadrosaurs inside the Arctic Circle (see Chinsamy et al 2012). This apparent similarity in bone structure in animals from the south and the north suggests that this particular “evidence” does not necessarily support the theory that hadrosaurs were endemic residents in the north.
Maiasaura attained half adult size within the first year! They reached adult size around the nine to ten year old mark. Ontogenetic traits and comparisons made between Maiasaura and crocodiles clearly demonstrate that these dinosaurs grew at elevated rates and, because both groups display LAGS, then LAGS should not be used as indicators of slow growth rates and ectothermy.
Body size growth curves vary throughout ontogeny demonstrating variation within individuals and is suggestive of external influences on growth. The amount of tibiae recovered show a marked pattern in the size and age of the individuals and demonstrates that mortality in Maiasaura was high in the first year and, maybe surprisingly, also high in animals reaching skeletal maturity.
In fact over half of the sample examined represents animals of a year old or younger and just shows how important a large data set is and the authors state that forty eight tibiae are probably nowhere near enough anyway. But this is important research since there are very few dinosaurs represented in the quantities that Maiasaura are and gives us essential insights into the ontogenetic history of non-avian dinosaurs.   

References

Arbour, V.M., Badamgarav, D. & Currie, P.J. 2012 A new ankylosaurid from the Upper Cretaceous Baruungoyot Formation of Mongolia: new cranial characters for ankylosaurine ankylosaurids and a reassessment of ankylosaurid postcranial specimens from Mongolia. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 57.
Chinsamy, A., Thomas, D. B., Tumarkin-Deratzian, A. R. and Fiorillo, A. R. (2012) Hadrosaurs Were Perennial Polar Residents. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology (advance online publication) doi: 10.1002/ar.22428
Erickson, G.M., Krick, B., Norell, M.A. & Sawyer, W.G. 2012 Complex dental structure and wear biomechanics in hadrosaurid dinosaurs.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 90-91.
Farke, A.A., Chok, D., Herrero, A. & Scolieri, B. 2012 Ontogeny in the hadrosaurid dinosaur Parasaurolophus revealed by an articulated skeleton from the Kaiparowits Formation of southern Utah. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 92.
Goodwin, M.B. and Horner, J.R. 2004 Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behaviour. Paleobiology, Spring, 2004, v. 30, p. 253-267.
Petersen, J.E. & Dischler, C. 2012 Distribution of injuries in pachycephalosaurids using frontoparietel landmarks. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 157.
Vanburen, C.S., Arbour, V.M. & Evans, D.C. 2012 Cervical fusion in Ankylosauria: anatomy and function. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 187-188.
Woodward, H.N., Horne, J.R. & Farlow, J.O. 2012 Paleobiological implications of growth history and histovariability in a population of the hadrosaurid dinosaur Maiasaura peeblesorum.  Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 196.

Thursday, 1 November 2012

SVP 2012 - The Ceratopsians


 
So this time I will start off with ceratopsian news and highlight current research, some old and some new, which continues to make the study of these popular dinosaurs so fascinating.
Because morphological and ontogenetic variation within genera, as well as individual taxa, can be extreme in ceratopsians, more and more research is focussing on the processes of stratigraphy, paleogeography and taphonomy. Deanna Brandau and Randall Irmis, of the Natural history Museum of Utah, have been comparing monospecific bone beds of both centrosaurines and chasmosaurines from the middle Campanian Kaiparowits formation in southern Utah.
Chasmosaurine bone beds are uncommon and are often composed of two, maybe three individuals whilst centrosaurine beds can contain many animals – certainly into the hundreds. Typically chasmosaurine bone beds are uncommon and display a low degree of re-working in a low energy depositional scenario while, at the other end of the spectrum, centrosaurine bone beds are more common and comprise of many disarticulated elements that display significant taphonomic modelling – evidence for high energy deposition.
The Kaiparowits bone beds follow the same trend as others in Laramidia, especially those in the north when most of the bones beds are located. Why are chasmosaurines beds less common than their brethren? Because the Kaiparowits has a higher ratio of low energy, fine grained flood plain deposits than the north then you would reason that there should be a greater amount of chasmosaurine bone beds exposed – but there are not.
Because all ceratopsid bone beds are broadly deposited under the same conditions, Brandau and Irmis interpret this as good indication of behavioural and/or paleoecological differences between chasmosaurines and centrosaurines in as much that chasmosaurs may have been solitary animals or only lived in smaller groups as opposed to centrosaurs that, at least part of the time, gathered in large numbers.
Psittacosaurus is a genus of small ceratopsian known from countless specimens that have been recovered from Asia over many years and there are several taxa represented and one of these, Psittacosaurus sinensis, may have been a dwarf. Peter Makovicky et al have examined over a dozen specimens recovered from Shandong Province in China and conducted intensive histological analysis.
This analysis reveals that P. sinensis reached adulthood at small body size – especially when compared with both P. mongoliensis and P. lujiatunensis. Indeed, the authors interpret the adult size of P. sinensis to be at the same size level as a four year old of the other two taxa. Since P. sinensis is closely related to P. mongoliensis the author interpret this as the first histological evidence for dwarfism within ceratopsidae.
Despite currently having no sympatric psittacosaur taxa, the case for P. sinenesis to be a dwarf is reinforced since it is likely that the Shandong Peninsula, where this taxa is from, formed part of an island in the Early Cretaceous and, as we are all aware with other finds worldwide,, may be indicative of island dwarfism. Another taxa, P. gobiensis, is also considered a dwarf but does have a large sympatric taxa living along side and this, in comparison, may suggest niche partitioning - and niche partitioning was a recurring theme in many talks and posters this year of which there will be more later.
The same histological research and analysis used for Psittacosaurus is increasingly used in vertebrate palaeontology – so much so now that is almost becoming the norm to utilise it and so far reaching is it in our science that there is a second international paleohistological symposium next year at the Museum of the Rockies (MOR) in Bozeman, Montana. Julie Reizner, of the very same institution, has also been looking at bone histology – this time of the ceratopsian Einiosaurus procurvicornis.
Tibiae recovered and then sectioned, from a newly discovered monodominant bone bed in the Upper Cretaceous Two Medicine Formation of north western Montana, reveal that, like so many dinosaurian taxa, initial growth rates are fast. They tend to slow up after 3 to 5 years but still maintain a steady rate of growth. 
Since all the specimens tested follow the same trend, this suggests that the bone bed is made up of sub-adults and juveniles. Further research will continue with this bone bed and should reveal further insights into ontogeny and morphology in conjunction with phylogenetic and behavioural implications.
The MOR crew have been working in the Hell Creek Formation (HCF) for many years now and have built up an unparalled collection of fossils and data. Their best known work in recent times involves that well known taxon Triceratops and John Scanella et al reveal yet more data highlighting what a complex stratigraphical and diverse formation Hell Creek is.
There are currently two recognised species of TriceratopsT. prorsus and T. horridus. These taxa have clear morphological differences and are found in different stratigraphical levels. But a new collection of skulls and other elements from exposures located midway between where the two recognised taxa are found suggests the existence of what could be described as a transitional Triceratops morph since the fossils display characters of both.
MOR crews made the decision to concentrate on collecting fossils from this upper middle section of the HCF and have put together an ontogenetic sequence of this transitional animal. The fact these specimens lack clear autapomorphic distinctions lead the authors to suggest that Triceratops clearly demonstrates anagenesis although they also point out that juvenile specimens of the transitional form demonstrate what would be considered mature traits found in in T. horridus which is found immediately below.
This highlights the continuous need to cross reference all necessary data when discussing the taxanomic affinities of all non-avian dinosaurs and reinforces the fact that both stratigraphy and ontogeny are crucial in our overall understanding of evolutionary change – especially when heterochrony is involved as may be the case in Triceratops.   
 
 
No SVP meeting has been complete over the last few years without more discussion on the issue of synonymising Triceratops and Torosaurus – and this year was no exception. This time Torosaurus utahensis was the focus of a poster presented by Jelle Wiersma et al, from the Natural History Museum of Utah. As I approached the poster Jack Horner was finishing up a conversation with two of the authors and I could sense an element of defiance in them as Jack walked away – not unpleasant you understand but a sense of belief in themselves and that they believe in their research. This is the meat and drink of science – two competing hypotheses that continually test the others opposing premise and this is exactly as it should be.
The Upper Cretaceous (Maastrichtian) North Horn Formation of central Utah have revealed five chasmosaurine specimens which have been diagnosed as T. utahensis based on a combination of two autapomorphies and a unique combination of character states. Other specimens can only be identified at the generic level so remain as Torosaurus sp. The thing to bear in mind here is that Triceratops is not known from the North Horn – not that I am aware of.
To remind you, John Scanella and Jack Horner in 2010 suggested that Torosaurus was the adult morph of Triceratops and that Triceratops underwent dramatic morphological change throughout ontogeny. Torosaurus is rare in the HCF – indeed only one in ten specimens can be attributed to the taxon. In assemblages that are not so heavily sampled only Triceratops is ever recovered. However, since only Torosaurus is recovered from the North Horn the authors suggest that this is indicative that T. utahensis is indeed a valid taxon.
Otherwise synonymy of Torosaurus and Triceratops suggests that only adult animals are preserved in the North Horn – but this would seem extremely unlikely. However, sampling remains the biggest issue when discussing Torosaurus and the North Horn. Compared to the intensive collecting and sampling of the Hell Creek Formation (as mentioned earlier) then it can be deemed difficult to compare the two localities. It is also worth pointing out that the two formations are geographically separate by quite a distance.
But the authors point out that statistically the chances of only finding Torosaurus in the coeval North Horn are very slim indeed – especially if the two taxa are one and the same and they take this as powerful evidence that T. utahensis is a valid taxon and, by implication, so is T. latus.
Excellent work and keeps the debate alive and I have to say that I, for one, do not find this particular issue tiresome but, on the other hand, find it extremely stimulating and look forward to the next chapter with some relish.

References

Brandau, D.L. and Irmis, R.B. 2012 Comparative taphonomy of ceratopsid bone beds: implications of new data from Southern Laramidia. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp 67-68.
Makovicky, P.J., Erickson, G.M., Gao, K. and Zhou, C. 2012 Ceratopsians didn’t just get bigger: evidence for dwarfism in Psittacosaurus.   Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 133-134.
Reizner, J.A. 2012 An ontogenetic study and population histology of the ceratopsid dinosaur Einiosaurus procurvicornis. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 160-161.
Scanella, J.B. and Horner, J.R. 2010 Torosaurus Marsh, 1891 is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae) Synonymy through ontogeny. Journal of Vertebrate Paleontology, 30 (4), 1157 – 1168.
Scanella, J.B., Fowler, D.W., Goodwin, M.B. and Horner, J.R. 2012 Transitional Triceratops: details of an ontogenetic sequence from the upper middle unit of the Hell Creek Formation, Montana.    Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 166.
Wiersma, J.P., Loewen, M.A. and Irmis, R.B. 2012 A re-evaluation of Torosaurus utahensis: implications for Maastrichtian ceratopsian diversity in western North America. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2012, pp. 193.