Heyuannia: beast of the Week

This week we will be checking out a dinosaur they helps us better understand dinosaur eggs, Heyuannia huangi!

Heyuannia was a theropod dinosaur that lived in what is now China and Mongolia during the latest Cretaceous period, between 70 and 66 million years ago. From beak to tail it measured almost 5 feet long (1.5meters) and was likely an omnivore when alive.  It's name translates to "from Heyuan" in reference to the region of China where its bones were first found. 

Watercolor life reconstruction of Heyuannia by Christopher DiPiazza.

Heyuannia was a member of the oviraptorosaur group of dinosaurs, known for being bird-shaped with long necks and short boxy skulls with powerful beaks.  Many of them also sported tall bony crests on their heads, but sadly the whole skull of Heyuannia has not been found so we don't know that part of it's anatomy for sure.  Like its relatives, Heyuannia may have been an omnivore, using its short, but powerful beak to process seeds, fruits, and other tough plant material, but would have also been equally efficient at eating small animals, including hard-shelled invertebrates.  

Heyuannia skeleton on display at the Heyuan City Museum in China.

Heyuannia had proportionally short arms and fingers compared to many oviraptororsaurs.  Each finger was armed with a curved claw.  We don't know why its arms and fingers were so short.  It may have been exhibiting a different feeding style that utilized its head and neck more, but it is almost impossible to predict exactly what that would have been.  Like all Oviraptorosaurs, it is almost certain Heyuannia had feathers in life. 

Photograph of a fossilized Heyuannia nest from the paper by Wiemann et. al. referenced below.  Note how the eggs are in a ring pattern and arranged in pairs.

Heyuannia is also known from many eggs and embryos which were found in the same place as many of the adult skeletons.  The nests of eggs are arranged in a ring pattern and don't appear to have been buried in life.  This implies that the parent would have guarded the nest with its body, likely with its feathered arms spread over the ring of eggs and their body in the middle of the ring.  (We have actual fossils of close relatives that died doing exactly this.) In addition the eggs are arranged in pairs, which implies they were laid two at a time.  This is notable since modern birds do not lay eggs in pairs because they only have one functioning oviduct, an adaptation to lighten their bodies for flight. Oviraptorosaurs, which did not fly, likely had both oviducts in use due to the way we find their eggs.

The most exciting thing about Heyuannia's eggs is the fact that scientists were able to examine the shells closely enough to identify the chemical makeup of some of them, specifically the part that reflects what color the shells would have been.  According to what they found, the eggs of Heyuannia would have been blue-green in color! 

References

Lü, J (2002). "A new oviraptorosaurid (Theropoda: Oviraptorosauria) from the Late Cretaceous of southern China". Journal of Vertebrate Paleontology. 22 (4): 871–875.

Lü, J. (2005). Oviraptorid dinosaurs from Southern China. Beijing: Geological Publishing House. ISBN 7-116-04368-3. 200 pages + 8 plates. (In Chinese: pp 1-83, including 36 figures & 3 tables. In English: pp 85-200, including 5 geological figures.)

Wiemann, J.; Yang, T.-R.; Sander, P.N.; Schneider, M.; Engeser, M.; Kath-Schorr, S.; Müller, C.E.; Sander, P.M. (2017). "Dinosaur origin of egg color: oviraptors laid blue-green eggs". PeerJ. 5: e3706.

Platyhystrix: Beast of the Week

 This week we will be talking about an unusual little amphibian that predates the dinosaurs by about 100 million years.  Check out Platyhystrix rugosus!

Platyhystrix was a kind of prehistoric amphibian that lived in what is now the United States, in Texas, New Mexico, and Kansas, during the early Permian period, about between 300 and 279 million years ago.  From snout to tail measured about 3 feet (1 meter) long.  Like most amphibians it was a predator, likely swallowing smaller animals whole.  The genus name translates from Greek to "Flat Porcupine" (which I find delightful) because of its unusual back ornamentation.  

Watercolor life reconstruction of Platyhystrix by Christopher DiPiazza.

Platyhystrix was a member of the extremely successful and widespread group of ancient amphibians, called the temnospondyls.  Temnospondyls are sadly extinct now, but they persisted from 300 to 120 million years ago. (210 million years total!)  Eryops, Mastodonsaurus, and Metoposaurus are three other examples from this group that have been covered on this blog in the past.  Unsurprisingly for a group that persisted so long, temnospondyls diversified into many different forms during their time on the planet, exhibiting some traits we don't normally associate with amphibians like frogs and salamanders.  One example of this is the presence of bony armor plates, a trait normally associated with reptiles, like crocodilians and dinosaurs.  Platyhystrix not only had this armor, but it evolved a version of it that is completely unlike that of any other animal armor that ever existed, forming a tall semi-circular crest on its back made up of a single row of flat plates growing from the midline attached to the spine.  At first glance this looks like the kind of sail-like structure that has evolved many times in other animals, including but not limited to, the synapsid, Dimetrodon, the dinosaurs, Spinosaurus and Ouranosaurus, the pseudosuchian, Arizonasaurus, as well as many living lizards, like chameleons, basilisks, and sailfin dragons.  These sail structures are all made of extra long top sections of the vertebra, called neural arches.  Despite the resemblance, this is not the case for Platyhystrix, since its crest was made of bony plates that aren't actually part of its vertebrae, but appear to be their own separate body parts that were instead fused to the top of the spine.  The exact function of this structure is still very much a mystery.  Some suggest it had to do with thermoregulation, but others think it was more of a display structure for communication within the species. Maybe both?

Photo of the elongated armor plates of Platyhystrix from Vaughn's paper referenced below.

  The texture on the top of Platyhystrix's skull is rough with many grooves and pits in it, implying there was a tough keratin layer there in life.  This may have been an adaptation for combat within the species, possibly shoving or headbutting each other for dominance.  Modern amphibians, like frogs, can get extremely physical with each other, especially males around breeding time, so maybe Platyhystrix was similar in some ways?  It may also have had to do with controlling the animal's body temperature or maybe even defense against predators.  The teeth of Platyhystrix were small and cone-shaped, ideal for grabbing and holding onto prey that it could swallow whole.  

Illustration of the underside of Platyhystrix's (crushed) skull.  Note the small teeth at the top.  Image from paper by Berman referenced below.

Platyhystrix's environment would have been very different from what the American Southwest is today.  During the early Permian there would have been many rivers intersecting a landscape that would change between seasonally wet and humid to dry.  Despite being a predator, itself, when alive Platyhystrix would have needed to avoid predation from some of its larger contemporaries, including fellow temnospondyl, Eryops, and the large carnivorous synapsid, Dimetrodon.  

References

Berman DS, Reisz R, Fracasso MA. 1981. Skull of the Lower Permian dissorophid amphibian Platyhystrix rugosus. Annals of Carnegie Museum 50:391-416.

Bowler, Neven; Sumida, Stuart S.; Huttenlocker, Adam K. (2022-12-21). "Histological evidence for dermal-endochondral co-ossification of the dorsal blades in the late Paleozoic amphibian Platyhystrix rugosus (Temnospondyli: Dissorophidae)". Journal of Vertebrate Paleontology. 42 (2).

Case, E.C. (1910). "New or little known reptiles and amphibians from the Permian (?) of Texas". Bulletin of the American Museum of Natural History. 28: 163–181.

Mack, Greg H. (2003). "Lower Permian terrestrial Paleoclimatic indicators in New Mexico and their comparison to paleoclimate models". Geology of the Zuni Plateau. New Mexico Geological Society: 231–240.

Vaughn, Peter Paul (1971). "A Platyhystrix-like Amphibian with Fused Vertebrae, from the Upper Pennsylvanian of Ohio". Journal of Paleontology. 45 (3): 464–469.

Diplodocus: Beast of the Week

 This week we will be looking at an iconic dinosaur.  Make way for Diplodocus!

Perhaps one of the most widely known dinosaurs, Diplodocus was a sauropod that lived in what is now Western United States, most notably Colorado and Wyoming, during the Late Jurassic period, between 154 and 152 million years ago. Like all sauropods, it was a plant-eater, and like many sauropods, it was notably huge.  Diplodocus could grow to between 80 to 90 feet (about 24-27 meters) on average, but some specimens imply it could grow to even longer, exceeding 100 feet (30.4 meters) in some cases.  The genus name translates to "Double Beam" in reference to the upside-down "T" shaped projections growing from the underside of its vertebrae.  Currently two species of Diplodocus are recognized.  Diplodocus carnegii, which was described first, and Diplodocus hallorum, which appears to have been the larger of the two species.

Life reconstruction of Diplodocus (the big one) in watercolors by Christopher DiPiazza. 

Diplodocus is an important dinosaur because although it was not the first ever discovered sauropod dinosaur (that goes to Cetiosaurus, discovered in 1842 in England), it was the first sauropod discovered with enough bones to get a decent idea of what sauropods actually looked like, showing scientists just how bizarre these animals truly were.  No other kind land animal got close this large, let alone had these kinds of proportions.  You could argue giraffes come somewhat close with long necks, but theirs aren't nearly as proportioally long, nor do they have the insanely long, whiplike tails of many sauropods. 

Tail vertebra of Diplodocus on display at the Natural History Museum in London.  Note the upside down "T" projections on the underside of each bone.

 Diplodocus was a member of the more specific family of sauropods called the diplodocidae. (which is named after Diplodocus, itself.) Diplodocids are known for having more elongated faces than those of other sauropods, had peg-shaped teeth concentrated at the front of their jaws, and front limbs that were slightly shorter than their hind limbs.  The teeth were likely adaptations for stripping or clipping leaves off of trees.  The leg proportions means the dinosaur's center of gravity would have been around its hips, and therefore it could have probably stood up on its hind legs at least for short periods, maybe to display, scare predators, or reach higher leaves to eat.  

Probably the most notable trait of diplodocids, however, is their incredibly long, whiplike tails, which consisted of 80 vertebrae and could make up more than half the length of the entire animal's body.  The base of the tail was flexible and backed up by huge muscles, which would have allowed the dinosaur to move the end of this tail around extremely fast and with a lot of concentrated force.  Most experts suspect this would have been an effective weapon against potential predators.  Despite not having any spikes or other bony protrusions on its tail like many other kinds of dinosaurs had, since the tip of Diplodocus' tail could have hit so hard, it likely would have been able to seriously injure, and possibly lacerate the bodies of its enemies. 

Diplodocus skeleton on display at the National Museum of Natural History in Washington DC.

Like all sauropods, Diplodocus had an extremely long neck, made up of vertebra that had huge hollow spaces in them.  These were the attachment sites for air sacs which connected to the dinosaur's respiratory system, which allowed the huge body to be as light as possible as well as more effectively distribute oxygen throughout.  The neck bones also had bony ribs growing out of the bottom which overlapped.  In life these structures would have stiffened and therefore structurally strengthened the neck, making it easier for the dinosaur to keep its head up. 

In addition to huge adults, paleontologists have also uncovered bones from more than one small juvenile Diplodocus.  Experts think it is unlikely huge sauropods, like Diplodocus, cared for their eggs and young, but the skeletons of these small individuals, which were all found in the same area, suggest babies may have banded together for extended periods of time as they grew up.  At smaller sizes sauropods would have been extremely vulnerable to predators when young, so being part of a larger group, with more sets of senses alert for danger could have been a strategy to minimize the likelihood that each baby Diplodocus would be eaten by a predator.  (Fun trivia: During my time working and volunteering at the Academy of Natural Sciences in Philadelphia I helped prep some of the bones from these Diplodocus specimens!)

These young Diplodocus remains also preserved skin impressions, showing us that these dinosaurs had scaly skin that varied in pattern and texture on different parts of the dinosaur's body.  This is not a surprise, since most modern reptiles exhibit this on their skin in various ways, but it's still extremely exciting to get confirmation of it for a dinosaur like Diplodocus, especially since preserved skin is so rare in fossils.  

Fossilized skin impression from young Diplodocus from Gallagher's paper referenced below.  Note how they vary in shape and form.  

When alive, Diplodocus would have shared its environment with many other dinosaurs, including fellow sauropods, Apatosaurus, Brontosaurus, Brachiosaurus, Barosaurus, Camarasaurus, and Suuwassea.  Other plant-eating dinosaurs, like Stegosaurus, Gargoyleosaurus, and Camptosaurus were also present.  Predators that would have hunted young Diplodocus include Allosaurus, Ceratosaurus, and Torvosaurus. 

References

Carpenter, Kenneth (2006). "Biggest of the big: a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 131–138.

Foster, J.R. (2003). Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. New Mexico Museum of Natural History and Science:Albuquerque, New Mexico. Bulletin 23.

Gallagher, T; Poole, J; Schein, J (2021). "Evidence of integumentary scale diversity in the late Jurassic Sauropod Diplodocus sp. from the Mother's Day Quarry, Montana". PeerJ. 9: e11202. 

Holland WJ (1915). "Heads and Tails: a few notes relating to the structure of sauropod dinosaurs". Annals of the Carnegie Museum. 9 (3–4): 273–278.

Upchurch, P.; Barrett, P.M. (2000). "The evolution of sauropod feeding mechanism". In Sues, Hans Dieter (ed.). Evolution of Herbivory in Terrestrial Vertebrates. Cambridge University Press.

Wilson JA (2005). "Overview of Sauropod Phylogeny and Evolution". In Rogers KA, Wilson JA (eds.). The Sauropods:Evolution and Paleobiology. Indiana University Press. pp. 15–49.

Khaan: Beast of the Week

 This week we will be checking out a dinosaur that gave us insight into its behavior.  Make way for Khaan mckennai!

Khaan was a small birdlike dinosaur that lived in what is now Mongolia during the late Cretaceous period, between 75 and 71 million years ago.  From beak to tail it measured about four feet (1.2 meters) long and its name means "Lord" in Mongolian.  (Doesn't stop dinosaur nerds who are also Star Trek nerds making jokes about it, though.) When alive Khaan was likely an omnivore, eating seeds and plants, but also small animals.  

Watercolor reconstruction of a female and male Khaan mckenai.  There is actual fossil evidence that males of this species may have exhibited courtship dances. (read on)

Khaan was a member of the oviraptorid family, characterized by their overall very birdlike appearance, with long, slender legs and necks, long arms, tipped with hooked claws, and possessing feathers.  They also are known for having unusually blunt skulls with large eye sockets and very short, almost parrot-like beaks.  Many of them also had tall bony crests growing from the front of their faces.  Khaan was a particularly small example of this group, but many, like Citipati were about human-sized, and others, like Gigantoraptor were huge.  

Skull of Khaan on display at the American Museum of Natural History in New York.

Like all oviraptorids, Khaan had a short, tall beak, which many experts think was an adaptation for eating mostly plants when alive.  Others have suggested it was also eating small animals, and even using their seemingly specialized beak for cracking hard-shelled prey like shelled mollusks and crustaceans.  Khaan was actually part of a study that examined the jaws of several kinds of oviraptorids and compared them to those of other kinds of plant-eating theropods, like ornithomimosaurs and therizinosaurs.  This confirmed that for their size, oviraptorids had very powerful bites for their size, and may have been using their jaws for crunching through particularly tough plant material that other, weaker-jawed dinosaurs couldn't access.  

Skeletons of the two Khaan specimens that are thought by experts to represent a male and female.  They may have have even been a pair in life, since they were discovered together. Casts of these skeletons were on display at the American Museum of Natural History in New York for their temporary "Dinosaurs Among Us" exhibit.

Khaan tells us a lot about dinosaur behavior thanks to a few key details about its anatomy.  Two skeletons of Khaan that were adults when they died were discovered right next to each other.  Normally there would be no way to tell if these were the same or different sexes, except for the fact that one of them had slightly longer and differently-shaped tail chevrons (bony extensions on vertebrae) which implies there were more muscles there in life.  This means one of them was using its tail for something the other wasn't.  This had lead to a lot of experts to hypothesize that Khaan may have engaged in courtship dances, like many modern birds do today, like turkeys or grouse, with the males moving their their tails around (probably with feathers on the end) to entice potential partners.  

References

Clark, J. M.; Norell, M. A.; Barsbold, R. (2001). "Two new oviraptorids (Theropoda: Oviraptorosauria), Upper Cretaceous Djadokhta Formation, Ukhaa Tolgod, Mongolia". Journal of Vertebrate Paleontology. 21 (2): 209.

Iv, W. S. P.; Funston, G. F.; Currie, P. J.; Norell, M. A. (2015). "A possible instance of sexual dimorphism in the tails of two oviraptorosaur dinosaurs". Scientific Reports. 5: 9472.

Meade, Luke E.; Ma, Waisum (22 February 2022). "Cranial muscle reconstructions quantify adaptation for high bite forces in Oviraptorosauria". Scientific Reports. 12 (1): 3010.

W. Scott Persons IV; Philip J. Currie; Mark A. Norell (2014). "Oviraptorosaur tail forms and functions". Acta Palaeontologica Polonica. 59 (3).

Glacialisaurus: Beast of the Week

This week we will be looking at a dinosaur that demonstrates how different the world's climate used to be millions of years ago.  Check out Glacialisaurus hammeri!

Glacialisaurus was plant-eating dinosaur that lived in what is now Antarctica during the late Jurassic Period, between 186-182 million years ago.  Although it is only known from a few bones, Glacialisaurus is estimated to have been about 20 feet (6 meters) when from snout to tail.  The genus name translates to "Ice Reptile" in reference to the fact that it was discovered in the now icy Antarctic.  

Watercolor life restoration of Glacialisaurus by Christopher DiPiazza.

Glacialisaurus is considered a basal sauropodomorph, also known as a "prosauropod", which are generally considered the "more primitive" relatives of the long-necked sauropod dinosaurs, like Apatosaurus and Brachiosaurus. Like their more popular relatives, this group of dinosaurs had relatively long necks with small heads.  Unlike them, many of these dinosaurs walked on their hind legs, some being obligate bipeds, and others able to switch between two and four limbs depending on what suited them.  Their robust front limbs ended in five flexible fingers.  The first finger on each hand typically had a particularly long and curved claw, possibly for defense against predators, battling rivals of the same species, grasping branches while feeding, or maybe something completely different nobody's imagined. They had long, somewhat rectangular skulls with slightly downturned jaws, lined with leaf-shaped teeth, ideal for shearing plants.   

All that being said, Glacialisaurus, itself, is only known from a few limb bones, including part of a femur and some of the foot.  Based on these bones, experts can deduct it was a close relative of other basal sauropodomorphs, like Massospondylus.  Its bones were particularly robust compared to many of its relatives, meaning it would have been an overall robust animal for its length.  

Images of Glacialisaurus' foot bones from Smeth Et al. referenced below.

Perhaps the most interesting thing about Glacialisaurus isn't as much about its body but more its habitat, which is now frigid Antarctica.  During the early Jurassic, however, Antarcticta was still attached to South America, Africa, Australia, and India. (India would later separate as an island and eventually crash into southern Asia) It was also closer to the equator at the time, which means it was much warmer than it is today.  This is reflected in the kinds of fossils that have been found, including various plants that are known from warmer climates.  Glacialisaurus also would have coexisted with other animals, including a still unnamed pterosaur related to Dimorphodon, at least one other smaller kind of still unnamed basal sauropodomorph, and the large meat-eating theropod, Cryolophosaurus, which may have hunted it.  

References

Bomfleur, Benjamin; Pott, Christian; Kerp, Hans (2011). "Plant assemblages from the Shafer Peak Formation (Lower Jurassic), north Victoria Land, Transantarctic Mountains". Antarctic Science. 23 (2): 188–208.

Cantrill, David J.; Hunter, Morag A. (2005). "Macrofossil floras of the Latady Basin, Antarctic Peninsula". New Zealand Journal of Geology and Geophysics. 48 (3): 537–553.

Hammer, W. R., & Hickerson, W. J. (1996). Implications of an Early Jurassic vertebrate fauna from Antarctica. The Continental Jurassic, 215–218.

Rauhut, O. W. M.; Holwerda, F. M.; Furrer, H. (2020). "A derived sauropodiform dinosaur and other sauropodomorph material from the Late Triassic of Canton Schaffhausen, Switzerland". Swiss Journal of Geosciences. 113 (1): 8.

Smith, Nathan D.; Pol, Diego (2007). "Anatomy of a basal sauropodomorph dinosaur from the Early Jurassic Hanson Formation of Antarctica". Acta Palaeontologica Polonica. 52 (4): 657–674.

Smith, N.D; Hammer, W.R.; Makovicky, P.J. (2013). "New Dinosaurs from the Early Jurassic Hanson Formation of Antarctica, and Patterns of Diversity and Biogeography in Early Jurassic Sauropodomorphs". Geological Society of America Abstracts with Programs. 45 (7): 405–406.

Unaysaurus: Beast of the Week

 This week we will be checking out an early dinosaur that helps us understand the dinosaur family tree as a whole.  Let's check out Unaysaurus tolentinoi!

Watercolor reconstruction of Unaysaurus by Christopher DiPiazza.

Unaysaurus was a plant-eating, possibly omnivorous, dinosaur that lived in what is now Brazil, during the late Triassic period, about 225.4 million years ago.  From nose to tail it measured about 8 feet (2.5 meters) long.  It's name translates to "black water reptile" from the Tupi language, from near where it was discovered.  This is in reference to the region in which its fossils were found, which is called "Agua Negra" (black water) in Portuguese.  

Unaysaurus is was a member of what is referred to as the prosauropod group of dinosaurs.  Like their generally larger relatives, the sauropods, like Brachiosaurus and Apatosaurus, they possessed long necks and proportionally small heads, but differed in that they often could walk on their hind legs, some of which were actually obligatory bipeds, able to walk on their hind legs only.  It is thought by most experts that the true sauropods evolved from a branch of prosauropods somewhere during the early Jurassic or possibly even the late Triassic period.  Unaysaurus was a particularly small member of the prosauropod group, with other members, like Plateosaurus and Ingentia growing to be quite large in comparison.  What makes Unaysaurus stand out the most, however, is the fact that was especially old for its lineage, predating most other prosauropods by millions of years.  It is considered one of the oldest sauropodomorphs ever discovered alongside its close relative, Macrcocollum.  

Diagram and photograph of Unaysaurus' skull in the paper by Leal et al. referenced below.

Unaysaurus is known from surprisingly a complete skeleton, including an almost complete skull with teeth, arm bones, vertebra, and some leg bones, so we have a decent idea of what it looked like.  Like most prosauropods it had a long, almost rectangular skull, with a slightly downturned lower jaw, which possessed small leaf-shaped teeth.  It likely was primarily a plant-eater but it very well could have eaten smaller animals, as well, like invertebrates or possibly tiny reptiles.  It had proportionally short arms so it was likely an obligatory biped, only able to walk on its hind legs.  Because of its small size and relatively light build, it may have relied on speed or possibly camouflage to avoid predators.  

References

Buffetaut, E.; Suteethorn, V.; Cuny, G.; Tong, H.; Le Loeuff, J.; Khansubha, S.; Jongautchariyakul, S. (2000). "The earliest known sauropod dinosaur". Nature. 407 (6800): 72–74.

Leal, L.A.; Azevodo, S.A.K.; Kellner, A.A.W.; da Rosa, A.A.S. (2004). "A new early dinosaur (Sauropodomorpha) from the Caturrita Formation (Late Triassic), Paraná Basin, Brazil". Zootaxa. 690: 1–24.

Soares, M.B.; Schultz, C.L.; Horn, B.L.D. (2011). "New information on Riograndia guaibensis Bonaparte, Ferigolo & Ribeiro, 2001 (Eucynodontia, Tritheledontidae) from the Late Triassic of southern Brazil: anatomical and biostratigraphic implications". Anais da Academia Brasileira de Ciências. 83 (1): 329–354.

Rauhut, O. W. M.; Holwerda, F. M.; Furrer, H. (2020). "A derived sauropodiform dinosaur and other sauropodomorph material from the Late Triassic of Canton Schaffhausen, Switzerland". Swiss Journal of Geosciences.

Rodrigo Temp Müller; Max Cardoso Langer; Sérgio Dias-da-Silva (2018). "An exceptionally preserved association of complete dinosaur skeletons reveals the oldest long-necked sauropodomorphs". Biology Letters. 14 (11): 20180633.

Priconodon: Beast of the Week

 This week we will be checking out the coolest armored dinosaur most people never heard of.  Make way for Priconodon crassus!

Priconodon was an armored dinosaur that lived in what is now Maryland, United States, during the early Cretaceous period, about 115 million years ago.  It's name translates to "Pine Saw Tooth" in reference to its distinctive teeth, which it used to eat plants when alive.  Priconodon shows evidence of being the largest armored dinosaur ever discovered, possibly growing to 45 feet (13.7 meters) long from beak to tail.  

Watercolor of an adult Priconodon with two roughly human-sized Deinonychus in the foreground.  Fossil evidence suggests Priconodon may have been a behemoth of an armored dinosaur.

Despite being barely known even among dinosaur fans, Priconodon has been known on the fossil record for a very long time, being one of the first dinosaurs ever to be formally described from North America back in the late 1800s.  The first fossils from this dinosaur were in the form of teeth, which experts at the time were able to deduct were from some sort of armored dinosaur, and as time went on and more dinosaur bones were discovered elsewhere, more specifically a nodosaur.  Nodosaurs were armored dinosaurs in the ankylosaur group that were often adorned with large spikes but lacked tail clubs. (Europelta and Gargoyleosaurus are two examples of other nodosaurs that have been covered here)  Some of the Priconodon teeth unearthed were incredibly large, dwarfing the teeth of any other armored dinosaur.  Based on the size of the teeth compared to other more completely-known nodosaurs, it was known even back then that Priconodon was huge, likely the largest armored dinosaur known, but it was difficult to envision to what extent based on only teeth.  Another interesting trait of the teeth is that most of them show evidence of being water-worn, implying Priconodon may have preferred to feed on plants in or near the water.  This makes sense considering all of Priconodon's fossils have been found in what would have been a slow-moving riverbed or swamp during the early Cretaceous.

Priconodon tooth.  Image from the Smithsonian Museum of Natural History.

Priconodon tooth compared to the teeth of related nodosaur, Peloroplides.  Peloroplides measured about 20 feet from beak to tail.  Based on the difference in tooth size, Priconodon could likely have grown to be much larger.

Finally in 2023, over a century later, several fossils other than teeth from Priconodon were discovered from the same fossil site as its teeth in what is considered the first true dinosaur bone bed on the eastern part of the United States.  Several limb bones and osteoderms (armor pieces), including a large curved spike were uncovered.  Most impressive, however, was a GIGANTIC tail vertebra.  Combined with the largest teeth, when compared to the same parts of more completely known ankylosaurs, this vertebra implies that Priconodon may have grown to 45 feet long from beak to tail, making it the largest armored dinosaur ever discovered by a huge margin. (At the time of me writing this paper there is no formal paper describing any material from Priconodon other than teeth.  Bones from this dinosaur are still being found and prepped from the site in Maryland where it was found and a formal paper is still at least a few years away.  This is why there are no sources for the 45 foot estimate nor the bones below.  Please understand that estimate is a rough number and is of course subject to change.)

Priconodon tail vertebra (with one of the side processes missing) next to my hand, discovered in 2023.  Stay tuned for a formal paper on this and possibly more bones from this amazing armored giant in the future.

Priconodon would have shared its environment with a number of other dinosaurs known from the same fossil bed in Maryland where it was discovered, like Astrodon, Aquilops, Deinonychus, and Acrocanthosaurus, to name a few.  

References

Carpenter, K., and Kirkland, J.I. (1998). Review of Lower and middle Cretaceous ankylosaurs from North America. In: Lucas, S.G., Kirkland, J.I., and Estep, J.W. (eds.). Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science Bulletin 14:249-270.

Coombs, Jr., W.P. (1978). The families of the ornithischian dinosaur order Ankylosauria. Palaeontology 21(1):143-170.

Marsh, O.C. (1888). Notice of a new genus of Sauropoda and other new dinosaurs from the Potomac Formation. American Journal of Science 135:89-94.