How Therizinosaurs Nibbled and Munched

That new Anchiornis paper is quite something, isn't it? Everyone else is talking about it, so I don't have to, but check it out if you haven't already.

Instead, I will discuss another recent paper about a different group of maniraptors, the unusual therizinosaurs. Since 2012, a series of publications on the functional biology of therizinosaurs, primarily authored by Stephan Lautenschlager, have contributed greatly to demystifying these bizarre herbivorous theropods. (A selection of these papers are linked in the preceding sentence, but it is by no means an exhaustive list.)

These previous papers have largely focused on the therizinosaur Erlikosaurus from the Late Cretaceous Bayan Shireh Formation in Mongolia, a sensible choice given that this genus has the best-preserved skull material of all known therizinosaurs. The new study, also conducted by Lautenschlager, takes the logical next step by asking: how does Erlikosaurus compare to other therizinosaurs?

Phylogeny of therizinosaurs with digital models of the lower jaws of taxa used in the analysis, from Lautenschlager (in press).

To answer this question, Lautenschlager digitally modeled the lower jaws of other therizinosaur taxa, including Falcarius, Jianchangosaurus, Beipiaosaurus, Alxasaurus, and Segnosaurus, and subjected the models to finite element analysis (FEA). Under FEA, stress (force per unit area) experienced by the jaw under different simulated feeding conditions could be calculated, as could relative bite force. The different feeding scenarios tested were biting using one side of the jaw at different tooth positions, biting using both sides of the jaw at different tooth positions, clipping using the tip of the jaw, pulling an object upwards, pulling an object downwards, and pulling an object sideways.

The results of finite element analysis simulating potential feeding behaviors in different therizinosaur taxa, from Lautenschlager (in press).

It is worthy to note that this study did not calculate the absolute bite force of these therizinosaurs, only their relative bite forces. In other words, instead of estimating how strong a bite the therizinosaurs were actually capable of generating, the study estimated which therizinosaur could generate the highest bite force if all of them exerted the same amount of force with their jaw muscles.

With that in mind, what did the results say? It turns out of the taxa tested, Falcarius and Alxasaurus had, on average, the highest relative bite forces. This is consistent with the idea that Falcarius was more omnivorous than other therizinosaurs and may have still fed on some animal prey. Alxasaurus has also been interpreted as a more generalist forager than other therizinosaurs on the basis of its claw morphology, so having a relatively high bite force may have increased the variety of foodstuffs it could feed on.

On the other hand, the results indicated that Falcarius and Alxasaurus would have experienced greater amounts of stress during feeding than other therizinosaurs, whereas Erlikosaurus and Beipiaosaurus would have experienced the least. Additionally, all therizinosaurs would have experienced less stress while pulling items downwards compared to pulling upwards or sideways, suggesting that they habitually fed at or above head level. However, Erlikosaurus and Segnosaurus were more suited to pulling objects sideways than other therizinosaurs. What appears to have made the difference in this case is that Erlikosaurus and Segnosaurus both had a downturned lower jaw.

This is an interesting result considering that a downturned lower jaw has independently evolved in other herbivorous dinosaurs. The unusual ceratosaur Limusaurus even gained one during growth (in addition to losing its teeth)! This study confirms that such a jaw would have been advantageous for herbivores by helping to mitigate stress while feeding. A similar adaptive benefit has been attributed to the widespread presence of a beak in herbivorous dinosaurs (including therizinosaurids).

Among the taxa studied, Erlikosaurus and Segnosaurus were contemporaneous with one another, suggesting there may have been niche partitioning between them. This is supported by the study: Segnosaurus had relatively higher bite forces (as well as probably higher absolute bite forces, considering its larger size) and may have been able to feed on tougher plants, but Erlikosaurus experienced less stress during feeding and may have been able to use a greater variety of feeding methods. Though not discussed in the paper, one wonders whether the same was true of Jianchangosaurus and Beipiaosaurus, both found in the Yixian Formation. Here, however, the differences are less explicit: Beipiaosaurus experienced lower stresses while feeding, but both had similar relative bite forces.

Reference: Lautenschlager, S. In press. Functional niche partitioning in Therizinosauria provides new insights into the evolution of theropod herbivory. Palaeontology in press. doi: 10.1111/pala.12289

Natural History Museum of Utah

The welcome reception at this year's SVP took attendees to the Natural History Museum of Utah. The main fossil exhibits are arranged roughly in chronological order, with younger fossils exhibited higher up in the building and older ones closer to the ground floor, all threaded through by a sloping boardwalk. I consider it to be among the most impressive fossil displays I've visited, in terms of both content and setup.

A juvenile Columbian mammoth.

Megalonyx says, "Hi."

Uintatherium, one of the bizarre mammals of the early Paleogene.

An apparently unnamed dromaeosaurid.

A couple of juvenile Teratophoneus.

The giant alligatoroid Deinosuchus.

A smaller alligatoroid, Brachychampsa.

The large oviraptorosaur Hagryphus, mainly based on Anzu.

The skull of the saurolophine hadrosaur Gryposaurus.

The skull of the lambeosaurine hadrosaur Parasaurolophus.

An ornithomimid referred to Ornithomimus. A sizeable animal considering that we're viewing it from an elevated walkway.

The famous wall of ceratopsians, arranged phylogenetically. From left to right: Styracosaurus, Centrosaurus, Einiosaurus, Pachyrhinosaurus, Achelousaurus, Nasutoceratops, Diabloceratops, Chasmosaurus, Kosmoceratops, Anchiceratops, Triceratops, and Coahuilaceratops.

Off to the side are Protoceratops and Zuniceratops.

Ceratosaurus and Marshosaurus have a disagreement over a Stegosaurus carcass.

The whiplash tail of a mired Barosaurus passes beneath the walkway and arcs high above the heads of visitors. The sauropod is beset by several daring Allosaurus, but somehow this was the only picture I took of the mount. Others have a more representative record.

Size variation in Allosaurus.

The skull of Tanycolagreus, one of the more obscure Morrison theropods.

It was nice to see mounts of Nothronychus and Falcarius, but for unknown reasons I didn't take many good pictures of their heads. Their skulls are largely unknown, in any case.

This model of a troodont with colors lifted directly from Anchiornis was somewhat eyebrow-raising.

A model and mount of Lythronax.

Various arthropods from the Green River Formation.

Crossopholis, a paddlefish from the Green River.

I'd only just gotten out of the fossil exhibit when I received a Facebook message from Meig Dickson telling me that she'd found something in the biology exhibit upstairs that I absolutely had to see. Upon reaching the exhibit, I encountered an interactive tree of life, incredible in its scope and presentation.

However, what Meig had wanted me to see was the image they were using to represent protorosaurs on the interactive phylogeny. Oh, no...

Playing around with the phylogeny some more, we uncovered a number of other oddities. Most unusual was its apparent contention that a genus of extant brown algae is an extinct group of synapsids. Oops.

Hiccups with the interactives aside, the museum was rather impressive in how up to date the displays were. The wall of hominid skulls even includes the recently described Homo naledi.

A phylogeny of corvids.

In the display area for the geologic history of Utah was some more paleo-goodness: Seitaad, a basal sauropodomorph.

I shall conclude this post with the humble Triops, used as an example of species specialized for life in ephemeral desert pools.

Dinosaurs Among Us at the AMNH

Upon learning the American Museum of Natural History was opening a temporary exhibition on bird origins, my first instinct was that I had to go. Fortunately, Maryland is in close enough proximity to New York that I was able to make that a reality.

A fair amount of space is devoted to the evolution of avian reproduction, including characteristics shared with their closest living relatives (crocodylians) as well as our understanding of reproduction in Mesozoic theropods. Here is the oviraptorosaur embryo "Baby Louie".

A model of a giant oviraptorosaur nest with a restoration of the probable parent, Gigantoraptor. The dinosaur restorations at this exhibit are (thankfully and expectedly) above average, but a few pennaraptors still suffer from primary-eating feather mites. It's time to stop, please.

The famous "Big Mama", a Citipati preserved brooding on its nest.

An oviraptorosaur with unlaid eggs still in its body cavity! The paired eggs indicate that non-avialan theropods still had two functional ovaries, unlike the single one in modern birds.

A gallery of bird eggs, including both extant and extinct species.

A gallery of pennaraptor skulls and endocasts, showing the evolution of the avian brain. I was glad to see Dr. Eugenia Gold as one of the talking heads on a nearby video screen, considering that her recent research has focused on avian endocranial anatomy. Notice the "bird-o-meter" next to the labels of the fossil dinosaurs, indicating how closely related each one is to modern birds.

A wonderfully-preserved pair of Khaan that were found in close proximity to one another. The wording on the signage was sensibly cautious about interpreting them as a male and female. (Hint: dimorphism cannot be demonstrated with a sample size of two...)

Another excellent fossil, this time of Velociraptor. This may be the "sleeping" Velociraptor specimen I've heard rumors about.

A gallery of furculae. The largest one belongs to Tyrannosaurus.

I was impressed by how up to date some of the displayed information was. Here, recent discoveries of one-way breathing in crocodylians, monitor lizards, and iguanas are used to explain the evolution of the avian respiratory system.

There were a number of life-sized models of representative feathered dinosaurs, including this Beipiaosaurus.

In fact, a virtual parade of life-sized models and partial mounts made up the centerpiece of the exhibit. Here is a fairly nice Velociraptor. (Additionally, I overheard more than a few expressions of surprise from other visitors who were learning that non-avialan dinosaurs had feathers, both a reminder that this fact is not common knowledge outside of the paleontology community as well as real-time demonstration of the value of exhibitions like this one.)

As the largest dinosaurs with direct preservation of feathers, Yutyrannus stole the show.

A fossil cast of Yutyrannus in all its glory.

Some interesting speculation on the presence of feathers in large dinosaurs.

Fluffy ornithischian Tianyulong. Shame about the pronated hands.

An Archaeopteryx that looks like a real animal and not a half-lizard freak. Good job.

A cast of the Berlin Archaeopteryx and the original holotype feather.

An oviraptorid with young. The oviraptorids (there are others not pictured here) were the best models on display, in my opinion. I should have taken more pictures of them.

On the origin of flight, here is an Anchiornis performing wing-assisted incline running.

A cast of "Dave" the (possible) Sinornithosaurus.

A cast of Caudipteryx.

A cast of Tianyulong.

Feathers did not evolve in an aerodynamic context.

This... is one of the few parts of the exhibition that I had issue with. It's fair enough to point out that the developmental homologs of feathers originated long before the evolution of birds, but it does not thus follow that feathers themselves arose at the base of Archosauria (as implied by the accompanying restoration). On top of that, picking Effigia, a pseudosuchian that happens to have several other convergently bird-like characteristics, to clothe in feathers may come across as more than a little misleading. It would be cool if crocodylians were ancestrally feathered. However, we do not have the data to claim they were as of yet.

A lithornithid with fossilized feathers (and Effigia skull at top left).

A cast of Anchiornis. Are we seeing some of the original color pattern on those wings?

Juravenator shows that feathers and scales are not mutually exclusive.

Microraptor represents early experimentation with aerial locomotion in paravians.

Confuciusornis (left) and Xiaotingia (right) may have also had varying degrees of flying ability.

Look, it's Yi!

Jeholornis shows that feathers can function in display, too.

I liked the fact that some space is dedicated to Cenozoic birds as well. Here is another lithornithid.

That's stem-roller Paracoracias on the left and... another lithornithid on the right? My memory fails me on this one.

The skull of Gastornis.

A comparison of flying vertebrates.

An entire wall showing the stepwise acquisition of "avian" traits and the nebulousness of the term "bird". Unfortunately, some workers were repairing the interactive "test how well each dinosaur can fly" game, so I could neither get a better picture of the wall nor observe how the game worked.

The final display case contained a currently unnamed troodont, reportedly soon to be described.

A small gift shop directly followed the exhibition. One of the creatures on this shirt is not a dinosaur. (Surely, it would have made a better tie-in to the exhibit if they had included at least one bird on the shirt?)

Half of these titles belongs. The other half... doesn't.

Despite some minor quibbles about life restorations and feathered pseudosuchians, I found "Dinosaurs Among Us" to be excellent. It presents our understanding of avian origins in an incredibly current and multifaceted manner, and there is little I would add or change if I were in charge of making such decisions. Check it out if you have the chance! (It is open until January 2nd of next year.) It was well worth the long train ride to and from New York, and that's a trip I wouldn't make again in a hurry.

... What do you mean, there is also a pseudosuchian exhibit now?