#saurischians

Dinosaur Tianyuraptor ostromi, of Late Cretaceous, China!

Tianyuraptor ostromi is part of the Microraptoria clade, a clade of dromaeosaurs known for long feathers on both their legs and arms giving them “four wings”. However, Tianyuraptor had unusually short arms for a microraptorian, paired with unusually long lower hindlimbs. It was also larger than all other known microraptorines. Tianyuraptor was likely not doing any gliding, but was instead adapted as a ground-based pursuit predator.

Found in the Yixian Formation of Late Cretaceous China, Tianyuraptor ostromi would have lived alongside a plethora of other theropods, including other microraptorians like Sinornithosaurus and Zhongjianosaurus, compsognathids like Sinosauropteryx, a variety of avialans like Zhongornis, Yixianosaurus, Shanweiniao, Sapeornis, Longicrusavis, Hongshanornis, Iteravis, Dalingheornis, Archaeorhynchus, and Confuciusornis, oviraptorosaurs like Similicaudipteryx, therizinosaurs like Beipiaosaurus, and tyrannosauroids like Dilong. Tianyuraptor could have also met ankylosaurs like Liaoningosaurus, ceratopsians like Psittacosaurus lujiatunensis, and hadrosaurs like Jinzhousaurus. Pterosaurs also lived here, including ctenochasmatids like Cathayopterus and Elanodactylus, istiodactylids like Luchibang, and other pterodactyloids like Ningchengopterus.

#Archovember Day 19 - Kelenken guillermoi

The Phorusrhacids, commonly known as “terror birds” were the last dinosaurian apex predators to hunt on land. The largest known phorusrhacid, Kelenken guillermoi, lived in the Middle Miocene of Argentina. It is known only from a skull (though it is one of the most complete large phorusrhacid skulls) and a leg and toe bone, so the rest of its anatomy must be inferred from other phorusrhacids. The skull, the largest of any known bird, is 716 mm (2.3 ft) long, allowing us to estimate Kelenken’s height at around 3 m (9.8 ft) tall. Kelenken would have lived in an open, grassland environment. Its long, slender tarsometatarsus suggests that it was a fast runner, likely adapted for chasing down small animals like rodents, marsupials, and lizards. While large, its skull was relatively weak, so it would have had difficulty gripping onto and subduing large, struggling prey. Instead, it likely preyed on animals it could swallow whole and/or, if targetting larger prey, used repetitive, targeted pecks with its beak. It could have also used its feet to restrain prey. While only Kelenken’s phalanx toe bone has been found, other large phorusrhacids (as well as their living relatives, the seriemas) have a “sickle claw” on their second digits, similar to dromaeosaurs. These claws could have also been used to restrain prey, or to kick out and strike.

Tawa hallae possesses attributes that can be assosciated with different dinosaur taxa. For example, its skull morphology resembles that of coelophysoids, and its ilium approximates that of a herrerasaurid. It also possesses an adaption in its neck vertebrae which supports the idea that cervical air sacs predate Netheropoda and may be ancestral in Saurischians.

As we all know, there are two major groups of Dinosaurs: Ornithischians and Saurischians; with Ornithischians composed of things like Stegosaurus and Triceratops; and Saurischians consisting of things like Apatosaurus and of course, the theropods, such as Tyrannosaurus and modern birds. 

This hypothesis has been accepted for centuries and hasn’t really been challenged in the literature. 

Until today. 

In a paper that came out in Nature by Baron et al., a thorough taxonomic sampling of dinosaurs was used to examine the earliest divergence of the group - again, traditionally thought to be the split between Ornithischians and Saurischians 

They found this instead

I know that cladograms can be confusing for some of you so I made a diagram:

Basically, they found that Ornithischians are actually more closely related to Theropods than either are to Sauropodomorphs. 

Proposing the new name Ornithoscelida for this clade; and redefining Dinosauria. 

Because the definition of Dinosaurs is the most recent common ancestor Passer domesticus + Triceratops horridus (or Megalosaurus and Iguanodon, it means the same thing) and all of that ancestor’s descendants; that would thus exclude Sauropodomorphs (and imagine the headache SAND [Sauropodomorphs Are Not Dinosaurs] would cause my dudes). So Dinosaurs were redefined as Passer domesticus + Triceratops horridus + Diplodocus carnegii for everyone’s sanity. 

But is this study robust?

Well, it’s more robust than a lot of things (Ornithischians tend to be poorly sampled), but it comes to a lot of interesting conclusions, and it’s definitely not the last word on the subject. 

They did find some shared characters for their proposed Ornithoscelida that are not found in the new Saurischia, especially in the head; and the similarities between Herrerasaurids and Theropods are thought to be convergent. 

They also found Nyasasaurus, what is now thought to be a very derived almost-dinosaur or perhaps even the first true Dinosaur, to be a Sauropodomorph; which - if Nyasasaurus is as old as thought, aka, from the Middle Triassic (currently that’s being re-examined) - would push back dinosaur origins considerably. 

In addition, two of the three authors of this paper are responsible for the notorious “Dinosaurs aren’t ancestrally feathered” hypothesis - which my friend John has refuted in this link - and they use this new phylogeny to propose that filamentous integument [the fancy word for feathers] might be just ancestral to Ornithoscelida, and not found in Sauropodomorphs at all. While I will say that that hypothesis has more weight if this is the actual explanation of dinosaur origins; I feel it important to say that the criticisms John, I, and many others had with their ancestral feather paper, still stand (ie, that they don’t consider Pterosaur Pycnofibres to be homologous). 

This isn’t the last word on the subject. They don’t include the new taxa, Ixalerpeton and Buriolestes in their analysis, and as we know, Buriolestes does cause some changes to the traditional Saurischian phylogeny.  They also don’t include many other taxa that I feel would be relevant for such a study, such as Daemonosaurus, Kulindadromeus, and honestly, a more robust sampling from Ornithischia as a whole. 

The analysis is very thorough - and you can trust me to say that, because I’m not entirely the biggest fan of Barret, due to the whole feathered dinosaur argument - but obviously it requires more research, more examining, and replication from other scientists. It does make one thing clear though: the origin of dinosaurs is not set in stone, and we have a lot of work to do to understand it better. 

My Choice: Dinosaur Nanotyrannus lethaeus, from Late Cretaceous, USA!

I was originally going to do some sort of stegosaur for my choice this year, since I unfortunately didn’t have room to include one on the list, but with the recent news about Nanotyrannus I realized I simply had to join the hype.

Nanotyrannus lancensis was first described in 1946 as a species of Gorgosaurus based on a single skull fossil, then re-examined and renamed in 1988 as its own species, Nanotyrannus. For years, the species was intensely controversial. As the skull belonged to (what was thought to be a) juvenile, many paleontologists (and paleofans) debated that Nanotyrannus was merely the juvenile of a Tyrannosaurus rex, not its own species. This debate was finally settled this year, when a new paper was published describing an adult specimen of Nanotyrannus lancensis. Exquisitely preserved, this animal had fewer vertebrae and more teeth than an adult T. rex, along with other anatomical differences that would make Nanotyrannus being a juvenile T. rex impossible. Not only that, but the discovery also shed light on another fossil, the “Jane” specimen, previously argued to be a Nanotyrannus but also thought to be a subadult T. rex. Jane was reassigned as a new species of subadult Nanotyrannus, N. lethaeus. Having been denied species-ship for so long, Nanotyrannus is now up to two.

Of the two species, N. lethaeus was larger, and mature individuals may have reached a body mass of about 1,200 kg (2,600 lb), while N. lancensis may have weighed nearly 704 kg (1,552 lb). They were mid-sized tyrannosauroids, combining characteristics of more basal Appalachian tyrannosauroids and more derived Laramidian tyrannosaurids. It was slender, with longer legs than other predatory theropods of the time, suggesting the niche of a pursuit predator. They would have been significantly faster than T. rex, with an estimated maximum running speed of 45.3–49.8 km/h (28.1–30.9 mph), nearly twice higher than that of adult T. rex with an estimated maximum running speed of 20.9–29.1 km/h (13.0–18.1 mph). Their arms and hands were also proportionally longer and larger than T. rex, suggesting they may have used them to manipulate prey more often than their giant, distant relatives. Jane’s skull has healed puncture wounds, likely the result of another Nanotyrannus bite, which seemed to have been common in larger theropods.

Both Nanotyrannus species lived in the Hell Creek Formation, which spans parts of modern-day Montana, South Dakota, North Dakota, and Wyoming, USA. It would have lived alongside its larger relative Tyrannosaurus, as a mesopredator, being preyed on by adult rexes but also ironically preying on juvenile rexes itself. Nanotyrannus’ long legs would have been very helpful for allowing it to escape this apex predator. In Hell Creek, Nanotyrannus would have lived alongside many of the most famous North American dinosaurs. These included other theropods like the dromaeosaurs Acheroraptor and (possibly) Dakotaraptor, the ornithomimids Struthiomimus and Ornithomimus, the alvarezsaur Trierarchuncus, the caenagnathids Anzu and Eoneophron, and the avialans Avisaurus, Brodavis, Magnusavis, and Potamornis. The most common dinosaur of this formation was of course Triceratops, with two species, but other ceratopsians lived here as well, including Torosaurus, Tatankaceratops, and Leptoceratops. Other ornithischians Nanotyrannus could have come across include the giant hadrosaur Edmontosaurus annectens, the small Thescelosaurus, the pachycephalosaurs Pachycephalosaurus, Platytholus, and Sphaerotholus, and the ankylosaurs Ankylosaurus and Denversaurus. Other animals would have included the crocodylomorphs Borealosuchus, Brachychampsa, and Thoracosaurus, turtles like Axestemys, Aspideretoides, and Neurankylus, the choristodere Champsosaurus, and a variety of mammals. Nanotyrannus lethaeus was collected alongside the vertebrae of the azhdarchid Infernodrakon (which would have been just the right size for the mid-size tyrannosauroid to grab at neck height).

(This is N. lancensis)