#archosauromorphs

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.

A Patreon request for rome.and.stuff (Instagram) - Pachyrhinosaurus perotorum… that I went a bit overboard with lol. I’ve been waiting for an excuse to draw my favorite ceratopsian, and to digitally adapt my old Pachy marker drawing design.

So! Pachyrhinosaurus! As seen above, there were three known species of Pachyrhinosaurus, living in different locations and eras in Late Cretaceous North America.

The oldest, P. lakustai, was native to the Wapiti Formation of Alberta and British Columbia, Canada. It’s known for the extra spikes it has at the center of its frill.

The slightly younger P. canadensis was native to the lower Horseshoe Canyon Formation and the St. Mary River Formation of Alberta and northwestern Montana. It was the largest of the three.

The youngest, P. perotorum, was native to the Prince Creek Formation of Alaska. As this ceratopsid seemingly stayed put during the long, dark, cold Alaskan Winters, it likely had adaptations for keeping warm.

BoYS

Pterosaur Hamipterus tianshanensis, of Early Cretaceous, China!

Our final pterosaur of this year’s Archovember is the impressive pteranodontoid, Hamipterus tianshanensis, from Early Cretaceous China. Hamipterus is known from dozens of fossils in a bonebed, some with well-preserved soft tissues and some associated with leathery eggs, representing a nesting colony that seems to have been buried by a tragic mudslide. While a travesty for the colony, the Early Cretaceous devastation allowed us a rare peak into the life of these strange animals. Since we have fossils of Hamipterus from egg to adult, we can observe how they grew and changed over the course of their lives. As they grew their snout tips would become relatively wider, the crest on their snout would become more robust and expand its base towards the front, the grooves and ridges on the crest would become more prominent, the snout tip would straighten, and their dentary groove would deepen and lengthen. Both females and males are assumed to have crests, with size being the distinguishing feature between the sexes, as large adult specimens and small adult specimens, both bearing crests, are present. Scans of unhatched Hamipterus eggs show that flaplings would have had well developed thigh bones, and would have been able to walk upon hatching, but had weak chests, so may have needed time before learning to fly. Hamipterus could have practiced parental care, and likely nested in large colonies to keep their young safe during this vulnerable time.

Due to the nature of its discovery, Hamipterus tianshanensis is the most well-known paleofauna discovered in the Shengjinkou Formation. It would have lived alongside other pterodactyloid pterosaurs such as Dsungaripterus and Noripterus, sauropods like Hamititan and Silutitan, and indeterminate theropods. Few other fossils are known from this formation, most of them indeterminate pterosaur and dinosaur footprints.

Dinosaur Brachytrachelopan mesai, of Late Jurassic, Argentina!

In a clade known for their long necks, Brachytrachelopan mesai is known for having the shortest neck. This (comparatively) small sauropod comes from a family of oddball sauropods with already shorter-than-average necks, Dicraeosauridae, known best for the famous sail-necked Amargasaurus and Bajadasaurus. Even though Brachytrachelopan was the smallest of its family, with the shortest neck, it was still a large animal, at 10–11 metres (33–36 ft) in length and 5 metric tons (5.5 short tons) in body mass. Brachytrachelopan’s shorter neck was likely an adaptation for lower browsing, allowing it to occupy a niche similar to iguanodontian ornithopods, which seemed to be absent from Late Jurassic Gondwana. Likewise, while North America at the time was abundant with iguanodontians, they lacked Dicraeosaurids. The two clades may have been ecological analogs, resulting from parallel evolution in two distantly related dinosaurian lineages to fill a similar niche.

Brachytrachelopan mesai is known from the Cañadón Calcáreo Formation of Late Jurassic Argentina. It would have lived alongside other sauropods such as Tehuelchesaurus, which would have filled the niche of high browser instead, an indeterminate stegosaur, and crocodylomorphs like Almadasuchus. It could have been preyed on by the enigmatic theropod Pandoravenator, as well as an indeterminate abelisaurid.

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)