Creature Feature 20

Well, now that we've covered all of the dinosaur species from Hell Creek, it's time to start getting in to the other types of vertebrates! This week, we're going to look at the stagodont metatherian genus, Didelphodon!

Didelphodon template model, WIP. Texture variants will be used to represent the different species.

Didelphodon was a genus of metatherian mammals - this group includes the marsupials. There are two confirmed species that have been found in Hell Creek - D. vorax (the type species) as well as D. padanicus. There are other remains that may represent new species, but these are either too fragmentary to identify or have not been fully described yet (Kielan-Jaworowska et al., 2004). Didelphodon was a rather large mammal by Mesozoic standards, around the size of a small domestic cat (Fox and Naylor, 2006). Its dentition is indicative of a predatory lifestyle, with distinct bladelike cusps and carnassial notches. In addition, the short, massive jaws bear huge premolar teeth which appear to be well-suited for crushing (Kielan-Jaworowska et al., 2004).

While most mammals are only known from isolated teeth and occasional jaw fragments (such as Alphadon), Didelphodon skeletal material has been found. Its skull is similar to that of the modern Tasmanian devil (Sarcophilus harrisii), while the postcranial anatomy resembles that of a modern otter. It is suggested that it was semiaquatic (Fox and Naylor, 2006; Kielan-Jaworowska et al., 2004), possibly making its home by burrowing into the riverbanks. The diet for Didelphodon may have consisted of crawfish, mollusks, small lizards, plants, and even dinosaur eggs. Given that the skeletal material was found in sediment attributed to a riverbed, it is thought that the reason for the unusual preservation of the fossil (roughly 30% complete) was that it died in its burrow. In addition, a fossil water stain surrounded the specimen, suggesting that its remains were quickly buried by fluctuations in the water table (Rocky Mountain Dinosaur Research Center, 2010). Unfortunately, the paper that will formally describe the new specimen has not been published as of yet, but at some point in the future I hope to get in contact with the authors to see if they can help make sure our model is as accurate as possible.

The mollusk-heavy diet of Didelphodon certainly makes sense, as there was a great diversity of freshwater mollusks - in fact, at my count there are at least 7 genera known from Hell Creek. While as far as I know there are no remains of crawfish, it is not unreasonable to suspect they would have been present. In addition, it may have been preyed upon by Borealosuchus and Brachychampsa, though this relationship could be complicated by the possibility that Didelphodon may have opportunistically preyed upon very young hatchlings as many small opportunistic predators do today (raccoons, for example). Keep in mind that these are just hypotheses that I am proposing based on my own personal knowledge, and should not be taken as fact.

Well, I hope this post has taught you a little bit more about Didelphodon! We're gonna start showing more love to a lot of the lesser-known vertebrates from Hell Creek, so stay tuned!

Acknowledgements:

Kielan-Jaworowska, Z.; Cifelli, R. L.; Cifelli, R.; Luo, Z. X. 2004. Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure. New York: Columbia University Press. pp. 441-462.

Fox, R. C.; Naylor, B. G. 2006. Stagodontid marsupials from the Late Cretaceous of Canada and their systematic and functional implications. Acta Palaeontologica Polonica 51 (6): 13-36.

Didelphodon vorax. Rocky Mountain Dinosaur Resource Center. 2010-12-07. Retrieved 2016-5-17.

Sci-Day 17: T Rex expedition details

Hello again, everyone! This post is a followup to the one from earlier this morning - in this post I will try to give a bit more information about the individual T. Rex in question, and any other information about the locality itself.

THE T. REX:

The Tyrannosaurus rex specimen has already been partially excavated, and the material that has been cleaned up is on display at the KU Natural History Museum. So far, the consists of the left maxilla, lacrimal, jugal, and postorbital, the nasal, the right dentary, several cervical ribs, the furcula, a few vertebrae, fragments of the pubis and ischium, and several bones from the hind limbs. Based on measurements of the dentary, this individual was between 14 and 16 years old. This age estimate is also supported by the fact that the teeth are shorter in proportion to body size than in older T. rex specimens. This probably means that it would have hunted smaller prey - given that juvenile T. rex had proportionally longer hindlimbs and a lighter overall build than adults, they may have been better equipped for tackling more swift-moving herbivores.

Additionally, a cross-section of the femur shows what may be evidence of medullary bone - though it could also be due to some sort of pathology such as an infection. If upon closer analysis it does turn out to be evidence of medullary bone, this would give us another T. rex specimen of known (ie confirmed) sex. In T. rex specimens, there are two distinct morphotypes, called the 'gracile' and 'robust' morphs. Some have argued that this represents sexual dimorphism, with the robust morphotype representing the females, but due to the fact that it is nearly impossible to determine the sex of a fossil animal in normal circumstances this has been an issue of debate. However, given that [provided that the medullary bone presence is confirmed] both confirmed female T. rex are of the robust morphotype, this could be solid evidence that the morphotypes represent the two sexes (though a sample size of 2 is not going to give a great level of certainty). Since there are many other dinosaur species showing the same pattern of 'gracile' and 'robust' morphotypes, understanding the cause of this dichotomy in T. rex could allow us to apply that knowledge to other species.

T. rex femoral cross-section, showing possible medullary bone (light colored center of the bone).

A secondary reason why the presence of medullary bone in this specimen would be significant is due to its age. Since medullary bone is only found in birds that are gravid (eggs not laid yet) or have recently laid eggs, this would indicate that the animal reached sexual maturity before it was finished growing.

The specimen also shows evidence of past injury - there is a raised, roughened area on the left tibia, providing evidence of possible infection or disease. However, whether or not this played a role in the death of this individual is uncertain. Closer inspection and excavation/cleaning of more skeletal material may give us a better idea, and may even reveal additional pathologies - only time will tell!

THE LOCALITY:

The locality is fluvial mudstone, likely representing a curve in a river - it is likely that the T. rex was carried downstream after death. In the same locality, a femur attributed to an Ornithomimosaur has been uncovered, though it is not known whether or not there is more material from the specimen since the site where the femur was found has not been excavated any further. I do hope that at some point someone does more digging to see if there is more material, as it could potentially be a specimen of Ornithomimus velox, which is currently only known from a few ankle bones. As more work is done on the layer, there may be yet more fossils uncovered.

In the pictures, the fossil-bearing layer is visible as a purplish-brown layer of rock. This summer, we will be working with awls and brushes from the level of the shovel (see attached photo) to the cliff face.

As I've said, I'm super excited for this amazing opportunity, and I hope that this post about the specifics of the expedition has gotten you excited as well!

Sci-Day 16: T-Rex Excavation!!!

Like I've been building up for the past week, today's Sci-Day is very exciting, and is not like any other Sci-Day. Rather than talking about a general concept in science, this is going to be about a very specific paleontological endeavor that is in progress right now: the excavation of a Tyrannosaurus rex skeleton.

I managed to help secure a large portion of the funding necessary for this excavation that is led by the Vertebrate Paleontology team here at the University of Kansas. Additionally, I will be going out to the site (in Montana) for a week sometime this summer to help with the dig. I think that anyone who truly understands what Dinosaur Battlegrounds is about can see why this is very exciting! While I do have some lab experience and have read many papers and published literature on paleontology, I have never actually had the chance to go out into the field and uncover fossils. I hope that this will be only the first of many more such expeditions in the future!

Another reason this is so wonderful is because it gives me a chance to give back to the paleontological community - the reason Dinosaur Battlegrounds has resonated with so many people and has the potential to do so well is because our focus is on accuracy. Our accuracy is based on the hard work and sweat of many smart, experienced people who have dedicated their lives to learning about the ancient earth. If it weren't for them, Dinosaur Battlegrounds would be nothing more than just another dinosaur game. We owe them our gratitude, and it is only fair that we do our part.

Furthermore, in order for us to be as accurate as possible, we need to learn as much as we can about the ecosystems we are working to restore. To do that, we need to fund research and field expeditions as much as possible so that we can continue to improve our simulations. Further field expeditions could yield more complete remains of various species that we had to make lots of approximations for, it could help us reduce the number of 'placeholder models', and things of that nature. As we start expanding to include formations such as the Kem Kem beds (with Spinosaurus), this will become even more critical as even the 'main attraction' species are rather mysterious and not much is known with extreme certainty.

Later today, I will post a second Sci-Day after talking with Dr. Krishtalka (he was the one who proposed this fantastic opportunity, I am forever in his debt), giving a bit more information about the specific T Rex, and other things of that nature. I also plan to have a day-by-day journal during the experience, talking about things as they are going along, so that all of our fans can get a true idea of what field work is actually like. I will also talk a little bit about preparations beforehand, so that people can understand a bit about what it takes. This way, for any of you who are hoping to one day enter the fascinating and wonderful field of paleontology, you know what things you will need!

I hope you all are at least half as excited about this as I am - seeing fossils on display or in a museum collection is one thing, but the experience of actually unearthing them from the rock is another! I'd like to thank Doctor Leonard Krishtalka as well as everyone on the KU Vertebrate Paleontology team for offering me this opportunity, and for being so helpful as resources of information on all things prehistoric!

Creature Feature 19

Hello, fans! This week will be the last dinosaur Creature Feature... For those of you who have been paying close attention, you know what species that is. It is the recently described, massive dromaeosaur Dakotaraptor steini!

 Dakotaraptor steini model, WIP.

Dakotaraptor steini was a truly gigantic dromaeosaur, only exceeded by Utahraptor ostrommaysorum in size. However, Dakotaraptor does not share the proportions or adaptations found in Utahraptor - its proportions more closely resemble those of smaller dromaeosaurids such as Deinonychus (DePalma et al., 2015). The total estimated length of Dakotaraptor is approximately 5.5 meters.

The remains of Dakotaraptor were found in Harding County, South Dakota by Robert DePalma in 2005, and the species description was published 5 years later. The holotype (PBMNH.P.10.113.T), consists of a partial skeleton (lacking the skull) of an adult animal, consisting mainly of forelimb and hindlimb elements, though a single fragmentary dorsal centrum, furcula, and 20 caudal vertebrae were also preserved. The description also included 9 paratype specimens from the same site, though these seem to represent a more gracile morphotype. These paratype specimens consist of hindlimb elements, several furculae, and isolated teeth (DePalma et al., 2015).

One particularly fascinating aspect of the Dakotaraptor remains is that the left ulna clearly shows the ulnar papilli, or quill knobs, like those found in some other dinosaurs as well as in modern birds. Based on the spacing of the knobs, it is estimated that there were 15 secondary wing feathers branching off the ulnar part of the forelimb (DePalma et al., 2015). Unfortunately, remains of the phalanges and other forelimb elements are not well enough preserved to determine the number/arrangement of feathers across the entire wing.

When a phylogenetic analysis of the Dromaeosauridae was conducted, it was found that Dakotaraptor steini was sister to Dromaeosaurus albertensis, and these two taxa in turn were sister to Utahraptor ostommaysorum (DePalma et al., 2015).

Dakotaraptor represents a significant leap in our understanding of the paleoecology of the Hell Creek Formation. Prior to its discovery and subsequent description, there was a strange dichotomy of carnivorous dinosaurs - small maniraptorans, and Tyrannosaurus. However, Dakotaraptor represents a transitional size form between these two groups, and as such requires us to reanalyze our views of the ecological and trophic interactions among dinosaurs and other taxa from Hell Creek. Additionally, the presence of two distinct morphotypes adds to our knowledge of the dynamics of body size in dromaeosaurids (DePalma et al., 2015). In fact, it may actually be evidence for sexual dimorphism - though if this is the case we do not know which morph represents males and which represents females.

Another interesting possibility is that Dakotaraptor may have competed with immature T. Rex (or mature Nanotyrannus lancensis, if it is indeed a distinct species), partially due to the possibility that it hunted in packs (DePalma et al., 2015). If this is the case, it would certainly be an amazing sight to behold!

Well, I hope you all enjoyed the very last dinosaur Creature Feature! Next week we will start to get into the less well-known vertebrates from Hell Creek!

Acknowledgements:
DePalma, Robert A.. Burnham, David A, Martin, Larry D, Larson, Peter L, & Bakker, Robert T. 2015. The first giant raptor (Theropoda: Dromaeosauridae) from the Hell Creek Formation. Paleontological Contributions (14).

Creature Feature 18

Greetings, fans! Today we go over the second to last dinosaur from Hell Creek - the somewhat controversial ceratopsid, Torosaurus latus!

Torosaurus latus model, WIP.

Torosaurus latus was an herbivorous ceratopsid, with a massive, 2.77 metre long frill. In life, it is thought to have reached approximately 7.6 to 9 meters in length, and weighed in at approximately 4 to 6 tons (Holtz, 2011). Torosaurus had two large fenestrae in its frill, long, dorsally concave squamosals, and ten or more epiparietals (small horns on the edges of the frill). Additionally, the nasal horn was rather short (Longrich and Field, 2012). While there is a second species of Torosaurus, though this species is slightly older and is not from Hell Creek - as such, it will not be featured.

Recently, the validity of Torosaurus latus has been disputed. Some authors have argued that Torosaurus latus is actually synonymous with Triceratops, representing the mature growth stage of the latter. Currently, no Torosaurus juveniles have been found, whereas a considerable number of juveniles have been uncovered for Triceratops. Additionally, one distinguishing trait of Triceratops is its short squamosals - these are absent in adult forms of other Chasmosaurines, making this a case of paedomorphosis. Some have argued that this fact is best explained by Triceratops and Torosaurus representing growth stages in the same species (Scanella, 2009). 

A followup analysis of 38 skull specimens from Hell Creek (29 Triceratops, 9 Torosaurus) supported this hypothesis (Scanella and Horner, 2010). The authors placed particular emphasis on the fact that Ceratopsian frills are composed of metaplastic bone, which can lengthen and shorten with time, lengthening and resorbing to form new shapes. They also noted that there is already known to be considerable ontogenetic changes in the skull and frill morphology of Triceratops, with horn orientation changing from backward to forward-facing with age. Furthermore, approximately half of the Triceratops skulls had two thin areas in the frill that corresponded to the placement of the fenestrae in Torosaurus, which were surrounded by mature granular bone. They asserted that this change would help to reduce the weight of the frill as it continued to grow ever larger.

However, this synonymy is not without its problems. In their paper suggesting synonymy, Scanella and Horner also acknowledged that there is data that is not easily explained by the synonymy of the two genera. One such issue is the rarity of Torosaurus remains - if it does indeed represent the mature form of Triceratops, it would be expected to be far more common. However, they also noted that this could be due to higher mortality in subadults, as well as potential preservation biases - it may have been that the older animals preferred to live at higher elevations, where fossilization would be far less common due to erosion. Additionally, some analysis did seem to show the existence of authentic subadult specimens of Torosaurus, though they believed this was actually indicative of individual variation. They also noted the apparent lack of transitional forms showing the formation of the large fenestrae. To address this, they cited the contentious holotype of Nedoceratops as such a transitional form, explaining the problematic traits of the genus as being due to its transitional state. They also cited the variability in position and number of episquamosals within Triceratops as possibly being indicative of an increase in number with age, explaining the higher count found in Torosaurus.

Since the 2010 publication, other paleontologists have expressed their doubts as well. A 2011 paper noted that the characters in Nedoceratops that were originally interpreted as being indicative of a transitional form are actually pathological in nature (Farke, 2011), also noting that while there is individual variation in the number of episquamosals in many Ceratopsids, there are no known species in which this number changes with age. Furthermore, it was noted that in other Ceratopsids that possess large fenestrae, these holes are present even in very young juveniles, suggesting that formation of fenestrae is not related to ontogeny. Further issues were also addressed, such as the possibility that the areas on the frill of Triceratops actually were indicative of fenestrae formation, but I will not get into the nitty-gritty details here. For those who are interested in learning more, I would suggest reading the original source material.

Several more papers have also raised issues with this idea. One of these used morphometric analysis to examine the various ages of Torosaurus and Triceratops specimens, and while it did find a general trend of Triceratops juveniles and Torosaurus adults, there were several notable exceptions. Two specimens of Torosaurus appeared to be quite young, somewhere approaching the age of some known Triceratops individuals. Conversely, it was also found that ten of the Triceratops skulls had reached a level of maturation equal to that of most aged specimens of Torosaurus (Longrich and Field, 2012). This paper also raised further objections, but due to time and space constraints I cannot get into all of them here.

A third paper published in 2013 used a statistical morphospace analysis (actually something I described in the Sci-Day discussing Species Concepts) to describe the variation of Torosaurus, both species of Triceratops, and Nedoceratops correlated with maturation. They found that Torosaurus specimens still retained a distinct anatomy from that of both Triceratops species, even when frill shape isn't included. While they admitted that the low number of Torosaurus specimens makes the analysis a bit less reliable than it otherwise might be, it still seems sufficient to refute the synonymy of the two genera (Maiorino et al., 2013).

In Dinosaur Battlegrounds, we may try to make it a player choice as to whether or not Torosaurus will be a distinct population of its own, or will simply represent an adult growth form of Triceratops. This would be something that would take a while to implement, as there is a lot of work that would have to go into the population ecology and whatnot, but it could help us understand the issue better.

Well, I hope you have enjoyed this somewhat controversial Creature Feature! We do not want to take sides here at Dinosaur Battlegrounds - our goal is to simply give our fans the evidence, and let you practice your critical thinking skills so you can make your own informed decisions!

Acknowledgements:

Holtz, Thomas R. Jr. 2011. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2010 Appendix.

Longrich, N. R.; Field, D. J. 2012. Torosaurus is not Triceratops: Ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy. PLoS ONE 7 (2): e32623.

Scanella, J. 2009. And then there was one: synonymy consequences of Triceratops cranial ontogeny. Journal of Vertebrate Paleontology 29: 177A

Scannella, J.; Horner, J.R. 2010. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny. Journal of Vertebrate Paleontology 30(4): 1157 - 1168.

Farke, A. A. 2011. Anatomy and taxonomic status of the chasmosaurine ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, U.S.A. PLoS ONE 6 (1): e16196.

Maiorino, L.; Farke, A. A.; Kotsakis, T.; Piras, P. 2013. Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs. PLoS ONE 8(11): e81608. 

Sci-Day 15: Speciation

Happy Sci-Day, everyone! This week will be somewhat related to last week's topic, as well as topics that I will be covering related to evolutionary biology in future posts. The topic for today is speciation - basically, the formation of new, distinct species.

The reason this relates to last week's post is because in order to understand how a 'species' is formed, we have to have some idea of what a species actually is. Speciation basically involves the splitting of an original, single population of organisms into two new populations, each of which is reproductively isolated from the other. Through some process, whether that is biological or abiotic, this isolation means there is no gene flow between the two populations, leading to independent evolution in each new population. Over time, mutations build up in each lineage, and they become more and more distinct (This could be purely on a genetic level - there don't necessarily have to be any evident morphological differences).

There are two main modes of speciation, defined by the process that split the original population. The first mode, and the one that is the easiest to identify in paleontology, is called allopatric speciation. In allopatric speciation, the ranges of the two new populations do not overlap - one example would be a large mountain range rising within the range of some species, where populations of that species on one side of the range are isolated from the population on the other side due to geographical factors - the species cannot cross that new barrier and as such the population is split.

A second type of speciation that is a bit harder to examine, especially from a paleontological standpoint, is called sympatric speciation. Unlike allopatric speciation, the ranges of the two new populations are not separated geographically - the two populations at least partially overlap at some part of their range. Since observing sympatric speciation relies primarily on genetic/molecular data, it is not exactly an issue that paleontologists can investigate with long-extinct creatures. For that reason, we will focus on allopatric speciation.

There are two main subtypes of allopatric speciation - these are allopatric and peripatric. The difference is that for peripatric speciation, rather than an existing population being split by the creation of some barrier to gene flow, some portion of a population enters a new area that is geographically isolated from the rest of the population. One very good example of this mode of speciation is island colonization by various organisms. Many islands across the world are formed by volcanism, rising up over many thousands of years to finally break the surface of the ocean. This means that no terrestrial organisms exist there to start out with. Basically, through some process, plants and animals from other islands (or perhaps even the mainland) manage to colonize the new island - since said island is geographically isolated from the original population, over time there will be a divergence.

Out of the two subtypes, allopatric is the most easy to observe in paleontology. This is because we have a good understanding of the movement of tectonic plates through geological time, and thus we can understand how continents moved, mountains formed, sea levels changed, etc. There are mounds of easily observable evidence of this type of speciation throughout organisms in history.

As many of you all know, all of the continents were joined into a single landmass called Pangea during the Triassic period. This meant that any population of terrestrial organisms had relatively continuous gene flow - there were no seas to split up these populations. This is why we see very closely related organisms from Triassic rocks across continents that today are thousands of miles away from each other. An example would be Coelophysis - while Coelophysis bauri is found in the Southwest United States, there are closely related species (sometimes even classified in the same genus) in Africa - this is because at that time, there was little to no isolation.

Throughout the rest of the Mesozoic, the continents began to split up, and we can actually see the effects it had on diversity and speciation over time by looking at species from different stages. In the Late Jurassic, we find species of Allosaurus, Ceratosaurus, and Torosaurus from both the Western United States and from Portugal, because during that time those regions had only just started to split up - the populations had only recently become isolated and as such had not diverged all that much.

Another very cool pattern we see is due to the fact that there was a clear North-South divide in the way the continents split, and this is reflected in the types of Theropods we find on different continents. In the Northern continents such as Asia and North America, Tyrannosauroids dominated as the largest carnivores during the Late Cretaceous, whereas the Abelisaurs dominated the Southern continents at that time. We do not find any Abelisaurs in those Northern continents at that time, nor do we find Tyrannosaurs in the South. However, another note is that we see closely related Tyrannosaurs [and dromaeosaurs] in both Mongolia and Western North America. This is because Asia and North America had only split relatively recently, much more recently than the split that resulted in the North-South divide.

Another interesting thing is that North America was split in two during much of the mid to late Cretaceous by a shallow sea, creating two subcontinents called Laramidia [on the west side] and Appalachia [on the East side]. While we have a rich collection of fossil organisms from Laramidia, there is very little material of that age from Appalachia. Since this sea would have isolated any species that ranged across the continent prior to the rise of sea levels, it is certainly plausible, if not probable, to assume that Appalachia would have been home to organisms somewhat similar to those in Laramidia, but still somewhat distinct. However, until more remains from Appalachia of the relevant age are recovered, we will not know to what extent this is true, if it is at all.

Well, I hope this has helped you learn about the fascinating topic of where species come from! Have a wonderful weekend, everyone!

Creature Feature 17

Hello, Dinosaur Battlegrounds fans! Today's creature feature is going to be a grab bag - I'm going to cover most of the remaining species of dinosaur from Hell Creek that have not been addressed in previous Creature Features! This is because the majority of the remaining dinosaurs are known from relatively fragmentary remains (or as in the case of Struthiomimus, are so similar to creatures that have already been featured that there is very little to add), so there is not much I can really say about any of them individually. Thus, I would not want to take a whole week to write just a tiny amount about one species. Without further adieu, here are some of the last few dinosaurs from Hell Creek!

Sphaerotholus buchholtzae

 Sphaerotholus buchholtzae model, WIP. Model primarily based on Stegorceras due to extremely fragmentary nature of Sphaerotholus material.

Sphaerotholus buchholtzae was a highly derived Pachycephalosaur, initially described from a partial skull. The genus Sphaerotholus was quite long-lived, with the earliest species S. goodwini from the Late Campanian showing that this genus lasted for at least 7-8 million years (Carr and Williamson, 2002).

Some researchers believe that this taxon is actually synonymous with Prenocephale edmontonensis (Sullivan, 2003), though recent research based on new S. buchholtzae material (a complete postorbital) seems to support its status as a distinct taxon (Mallon et al., 2015).

"Leptorhynchos" elegans

[Model for this taxon has not been made yet, as I need a copy of the Anzu model to make it]

The reason the scientific name is in quotation marks in this case is because the remains from Hell Creek are assigned to this species but most likely represent a unique taxon (whether this is a new genus or just a new species of Leptorhynchos is uncertain), and thus this name is being used until a new name is proposed and accepted. Leptorhynchos elegans was a species of Caenagnathid Oviraptorosaur (belonging to the same family as Anzu), known from the Late Campanian of Western North America. Distinct characters for this genus include small size, a short, robust mandible, and an upturned tip of the beak (Longrich et al., 2013). In the future I will try to communicate with these authors and perhaps others so that I can figure out the best way to go about making a model of the similar taxon from Hell Creek.

Avisaurus archibaldi

 Avisaurus archibaldi model, WIP. In-game version will be feathered.

Avisaurus archibaldi was one of several avian theropods from the Hell Creek formation, and unfortunately both A. archibaldi and its sister taxon A. gloriae are known only from the tarsometatarsus (a single bone in the foot) (Varrichio et al., 1995). Avisaurus was a genus of enantiornithine - the most abundant group of avian dinosaurs in the Mesozoic. They were extremely similar to modern birds, though most retained teeth and clawed fingers on their wings. The tarsometatarsus of A. archibaldi measures 73.9mm, which is the longest known in any enantiornithine (Varrichio et al., 1995). It was one of the largest volant [flying] dinosaurs from the Late Cretaceous, with an estimated mass of 5kg (Longrich et al., 2011).

Cimolopteryx maxima

[Remains too fragmentary to reconstruct a model without thorough collaboration with paleontologists]

C. maxima was a species of Charadiiforme bird, known from several late Maastrichtian formations. It was a rather small bird, approximately the size of a small gull (Hope, 2002). It is known almost exclusively from isolated coracoids, though their anatomy is distinct enough for several species to be identified (currently four species are recognized). C. maxima was an estimated 2kg in weight (Longrich et al., 2011).

Brodavis

[Remains too fragmentary to reconstruct a model without thorough collaboration with paleontologists]

B. baileyi was a species of freshwater hesperornithiform bird, known only from the holotype - a single left metatarsal. The genus Brodavis belongs to its own family, Brodavidae. Interestingly, while the marine members of the Hesperornithoform order appear to have lost volant abilities by the end of the Cretaceous, the minimal amount of pachyostosis in Brodavis suggests that it may have had at least a limited ability to fly (Martin, 2012). One of the unnamed Hesperornithiforms from Hell Creek was also attributed to a second species in this genus, Brodavis americanus.

Potamornis skutchi

[Remains too fragmentary to reconstruct a model without thorough collaboration with paleontologists]

Potamornis was another hesperornithiform, described from remains collected from the Lance Formation - remains from Hell Creek have been attributed to this taxon. While it was almost certainly a member of the Hesperornithes clade, its precise relationships within the group are not certain. Though, it does share the unique pterygoid articulation with the family Hesperornithidae (and poorly defined [or even absent] division of the head), the hinge-like temporal articulation, exceptionally small orbital process, and prominent attachment site for the deep layers of the protractor pterygoidei et quadrati muscle as well as several other details set it apart. These unique characters, combined with its smaller size (roughly 1.5-2kg), seem to suggest a feeding specialization differing from that of Hesperornithidae (Elzanowski et al., 2001). 

Unnamed taxa:

There are three ornithurine taxa that currently lack formal names, though they are informally referred to as "Ornithurine B", "Ornithurine C", and "Ornithurine D". Each of the three are known only from partial coracoids, and as such there is not enough material to publish a sufficient description for a new taxon. Size estimates based on a graph in Longrich et al. (2011) estimates masses of around 500-600g for "Ornithurine B", 700-800g for "Ornithurine D", and around 2.9-3kg for "Ornithurine C". Since the data was presented as a bar graph rather than a table of mass estimates, these numbers are my estimates based on what I can see, so I could certainly be wrong. I would encourage anyone to look at the original literature and make your own informed decision.

I hope this Creature Feature has filled your brains with more knowledge about some of the more mysterious taxa from Hell Creek! There are one [possibly two] more dinosaur taxa for me to cover, but after that I will be doing more of these grab-bag Creature Features due to many of the other vertebrate fauna being rather fragmentary.

Acknowledgements:

Carr, T. E.; Williamson, T. D. 2002. A new genus of highly derived pachycephalosaurian from western North America. Journal of Vertebrate Paleontology 22 (4): 779-801. 

Sullivan, Robert M. 2003. Revision of the dinosaur Stegoceras Lambe (Ornithischia, Pachycephalosauridae). Journal of Vertebrate Paleontology 23 (1): 181-207.
Mallon, Jordan C.; Evans, David C.; Tokaryk, Tim T.; Currie, Margaret L. First pachycephalosaurid (Dinosauria: Ornithischia) from the Frenchman Formation (upper Maastrichtian of Saskatchewan, Canada. Cretaceous Research 56: 426-431.
Longrich, N. R.; Barnes, K.; Clark, S.; Millar, L. 2013. Correction to Caenagnathidae from the Upper Campanian Aguja Formation of West Texas, and a Revision of the Caenagnathinae. Bulletin of the Peabody Museum of Natural History 54 (2): 263.
Varrichio, David J., Chiappe, Luis M. 1995. A New Enantiornithine Bird From the Upper Cretaceous Two medicine Formation of Montana. Journal of Vertebrate Paleontology 15 (1): 201 - 204.
Longrich, Nicholas R.; Tokaryk, Tim; Field, Daniel J. 2011. Mass Extinction of Birds at the Cretaceous–paleogene (k-pg) Boundary. Proceedings of the National Academy of Sciences of the United States of America 108 (37): 15253-15257.

Hope, S. 2002. The Mesozoic radiation of Neornithes. 339-388 In: Chiappe, L.M. and Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs.

Martin, Larry D.; Kurochkin, Evgeny N.; Tokaryk, Tim. 2012. A new evolutionary lineage of diving birds from the Late Cretaceous of North America and Asia. Palaeoworld 21 (1): 59-63.

Elzanowski, Andrzej; Paul, Gregory S.; Stidham, Thomas A. 2001. An avian quadrate from the Late Cretaceous Lance Formation of Wyoming. Journal of Vertebrate Paleontology 20(4): 712-719