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The Big Muddy: The Mowry Sea of the Early Cretaceous Period

Today the broad expanse of the Great Plains, which forms the central part of North America, is a vast area of farms, prairies, and deserts. Here, immense herds of buffalo once roamed and wild mustangs galloped. However, the land did not always look like a swelling sea of grass. During the prehistoric past this was, quite literally, a sea. In fact, the central part of North America has been inundated underwater at several points throughout prehistory.

For example, during the middle to late Jurassic Period, most of central North America was underneath a shallow body of saltwater called the “Sundance Sea”. Here lived oysters, fish, hybodont sharks, belemnites, and at least three species of marine reptiles. When this sea was at its fullest-size about 165-160 million years ago, North America was shaped like a U, with the sea in the middle. However, as sea levels dropped and as tectonic forces pushed the North American landmass upwards above sea level, the Sundance Sea gradually shrank until it completely disappeared by the end of the Jurassic Period. Please see Prehistoric Times issue #112 (Winter 2015) for an article about this Jurassic seaway.

Many people with knowledge of paleontology will recall that later, during the middle to late Cretaceous Period, central North America lay underneath another body of saltwater called the “Western Interior Sea”. This was much larger and deeper than the earlier Sundance Sea, and it completely split North America in half. Here lived famous marine creatures such as the giant plesiosaur Elasmosaurus, the giant mosasaur Tylosaurus, the enormous sea turtle Archelon, and several species of huge carnivorous fish.

However, the formation of the Western Interior Sea was not a quick overnight process; it took many millions of years for the middle of North America to be re-flooded again. Beginning in the early Cretaceous Period, sea levels started to rise again. Soon, the northern parts of North America, in the areas that now form northwestern Canada, gradually became submerged underwater. Over the course of millions of years, sea levels continued to rise until much of central Canada and most of the central United States were at the bottom of the ocean. This northern shallow water body was called the Mowry Sea, and it formed the earliest stage of the development of the Western Interior Sea.

The Mowry Sea is known from multiple layers of rock which were deposited during the early to middle parts of the Cretaceous Period. Each of these layers signifies a time when the sea expanded and shrank. By about 100 million years ago, the Mowry Sea had taken on much of the area that the Sundance Sea had previously occupied sixty million years earlier (1). It would not be until the sea broke through the southern boundary of North America, and turning the continent into two large islands, that the Mowry Sea would become re-named as the Western Interior Sea.

The Skull Creek Shale is the lowest level of marine sediment deposit, and therefore constitutes the earliest stages of the creation of the Western Interior Sea. It is identifiable by a very dark shale, almost black in color, that stands out from the rest of the sediment layers, and measures about 250 feet thick. There have been very few fossils found within this layer, consisting mostly of brachiopod and pelycopod shells as well as isolated fish bones and teeth (2).

The Newcastle Sandstone, which lies on top of the Skull Creek Shale, is a mixture of sandstone, claystone, and siltstone. Paleontologists hypothesize that this layer represents a time when the sea temporarily shrank and the landscape turned into a shallow brackish water estuary, similar to the Mississippi Delta. Fossils found within this layer are more abundant than in the underlying Skull Creek Shale, and consist of the freshwater snail Viviparus and five species of saltwater clams (Corbula, Mactra, Protocardia, Tellina, and Thracia) (3).

The Mowry Shale, which overlies the Newcastle Sandstone, is a dark gray siliceous shale dated to the middle Cretaceous Period (4). This layer can be found in exposed outcrops throughout much of the northern and central Rocky Mountain region (5).

As you can see, much of the geology of the Mowry Sea consists of shale. Shale is formed from clay and silt particles, which remain suspended in the water column for prolonged periods of time until they eventually settle to the bottom. Other rocks within these layers are siltstone and sandstone. So we can assume that the Mowry Sea had very muddy murky water with very poor visibility. The Mowry Sea was also “hypoxic”, which is a fancy way of saying that the water had very little oxygen in it. Modern-day analogies to such an environment would be the Mississippi Delta, the waters of New York Harbor, and the western half of the Long Island Sound (6).

However, in spite of its oxygen-deficient water, the Mowry Sea was not lifeless. Numerous animal fossils have been found within the Mowry Shale. There were at least two species of ammonites named Metengonoceras and Neogastroplites (7). Several species of bony fish have been found here, identifiable mostly through isolated fossil fish scales (8).  One of these was Holcolepis, composed of several species, and possibly resembled a modern-day herring in appearance. Another fish, much larger and much more formidable, was Hypsodon granulosus. Hypsodon was a large bony fish closely related to Xiphactinus, a large carnivorous fish found in the Western Interior Sea, and would have looked similar – so similar, in fact, that Xiphactinus was formerly classified as a species of Hypsodon (9).

Skeleton of Xiphactinus. Photograph by Chip Clark of the Smithsonian Institution (March 8, 2017). Public domain image, Wikimedia Commons.  https://commons.wikimedia.org/wiki/File:Xiphactinus_fossil_SI.jpg.

At least one species of modern shark, Carcharias amonensis, also swam in these waters (10). The genus Carcharias is the same genus as the modern-day Sand Tiger Shark (Carcharias taurus). This shark is distinctive for its ability to gulp air in order to maintain buoyancy (11). However, this habit might have also been helpful if it lived in water with very low levels of dissolved oxygen. Carcharias amonensis existed from the Cenomanian to the Turonian Stages of the Cretaceous Period and its fossils, mostly teeth, have been found in Kansas, South Dakota, Colorado, and Utah (12).

Sand Tiger Shark (Carcharias taurus). Photo by D. Ross Robertson of the Smithsonian Institution (November 11, 2016). Public domain image, Wikimedia Commons.
https://commons.wikimedia.org/wiki/File:Carcharias_taurus_SI.jpg.

The Mowry Sea grew slowly but steadily throughout the Cretaceous Period until, around 90 million years ago, it eventually broke through the southern part of the land and connected itself to the Gulf of Mexico. North America was now split in half, forming the island of Appalachia in the east and Laramidia in the west, and the Western Interior Sea was officially born.

Source citations:

  1. J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  2. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Pages 647-648.
  3. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Pages 649-650.
  4. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Pages 651; J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  5. Douglas A. Sprinkel et al (2012), “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Page 12.
  6. J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  7. Arthur B. Campbell, U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960. Page 652.
  8. J. D. Stewart and Marjorie Hakel (2006), “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Page 161.
  9. T. D. A. Cockerell (1918), “Some American Cretaceous Fish Scales, with Notes on the Classification and Distribution of Cretaceous Fishes”. U. S. Geological Survey. Shorter Contributions to General Geology, professional paper 120. Pages173-176; 177-179.
  10. Douglas A. Sprinkel et al (2012), “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Page 12.
  11. The Ultimate Guide: Sharks. The Discovery Channel, 1996.
  12. Douglas A. Sprinkel et al (2012), “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Page 12; Cretaceous Atlas of Ancient Life: Western Interior Seaway. “Carcharias amonensis”. https://www.cretaceousatlas.org/species/carcharias-amonensis/.

Bibliography:

Books:

  • Campbell, Arthur B. U. S. Department of the Interior Geological Survey Bulletin 1082-1: Geology and Mineral Deposits of the St. Regis-Superior Area Mineral County, Montana. Washington: United States Government Printing Office, 1960.

Articles:

  • Cockerell, T. D. A. (1918). “Some American Cretaceous Fish Scales, with Notes on the Classification and Distribution of Cretaceous Fishes”. U. S. Geological Survey. Shorter Contributions to General Geology, professional paper 120. Pages 165-203. https://pubs.usgs.gov/pp/0120i/report.pdf.
  • Sprinkel, Douglas A.; Madsen, Scott K.; Kirkand, James I.; Waanders, Gerald L.; Hunt, Gary J. (2012). “Cedar Mountain and Dakota Formations around Dinosaur National Monument: Evidence of the First Incursion of the Cretaceous Western Interior Sea into Utah”. Utah Geological Survey Special Study 143. Pages 1-20. https://ugspub.nr.utah.gov/publications/special_studies/SS-143/SS-143.pdf.
  • Stewart J. D.; Hakel; Marjorie (2006). “Ichthyofauna of the Mowry Shale (Early Cenomanian) of Wyoming”. In New Mexico Museum of Natural History and Science Bulletin 35. Late Cretaceous Vertebrates from the Western Interior. Spencer G. Lucas and Robert M. Sullivan, eds. Albuquerque: New Mexico Museum of Natural History and Science, 2006. Pages 161-163.

Websites:

Videos:

  • The Ultimate Guide: Sharks. The Discovery Channel, 1996.

Astrodon

If you were to ask people if they know a good place to find dinosaur bones, many of them would say “Go west, young man!” There is a lot of truth to this. There are vast areas of western North America, from Alaska to Mexico, that are renowned for their dinosaur fossils. You can find ceratopsians and hadrosaurs from Alberta to Utah, sauropods and stegosaurs in the Rocky Mountains, and tyrannosaurs and raptors from top to bottom. However, if you want to try your luck out east, then you’re likely to get disappointed. Yes, dinosaur bones have been found in places like Connecticut, New Jersey, Pennsylvania, and Maryland, but the pickings here are very slim.

There aren’t too many dinosaurs known from the eastern parts of the United States. Perhaps the two most famous examples are Hadrosaurus and Dryptosaurus, but there were certainly others. Part of the reason why dinosaur fossils are scarce east of the Mississippi is because there aren’t many places where we have exposed rock layers which date to the Mesozoic Era. Most areas from Maine to Florida have exposed rock formations which date to either the Paleozoic or the Cenozoic, with very little in between. You’re likely to find a lot of brachiopod shells, some sea scorpion exoskeletons, and maybe a mastodon or two, but the likelihood that you’ll find a dinosaur bone is pretty remote. Another reason for the scarcity of dinosaur fossils is due to the fact that the eastern third of the United States is heavily built up with cities, suburban sprawl, and farmland – not a whole lot of room for digging. Even so, dinosaur bones have been found here, and this article concerns one of them – a sauropod dinosaur from the middle Cretaceous Period named Astrodon.

In late November of 1858, a man named Philip Thomas Tyson, who worked as the Maryland state geologist (or “agricultural chemist” as his official title read), was busily engaged in field work as part of an effort to collect information which would be used to create a geological map of the state of Maryland. During his work, he was given two fossil teeth which had been found in Muirkirk, Maryland (Joseph Leidy mistakenly wrote that it was in Bladensbugh, Maryland) within an open iron ore mining pit located on the property of one John D. Latchford. The layer in the mine where these teeth were found dated to the middle part of the Cretaceous Period, about 110 million years ago or thereabouts. Philip Tyson brought these teeth with him to a meeting of the Maryland Academy of Sciences. They were then given to Dr. Christopher Johnston, a dental professor who worked at the Baltimore Dental College, to be examined. For one of these teeth, a cross-section of it was prepared for viewing under a microscope, and Dr. Johnston noted that the structure of the interior of the tooth was quite distinctive – a central bar-like core of dentine, with numerous spoke-like struts extending outwards through the pulp cavity, and then surrounded by a layer of enamel. Viewed from above, it resembled the rays of the sun bursting through a cloud. Doctor Johnston concluded that they belonged to a prehistoric reptile, possibly a new species. In 1859, Dr. Johnston published his findings, in which he named the animal Astrodon, meaning “star tooth”. He did not give it a species name (1).

However, to have his findings confirmed, Dr. Johnston sent the specimens to Philadelphia to be examined by Dr. Joseph Leidy, a well-respected doctor and anatomist and a noted authority on dinosaurs. Doctor Leidy stated that they indeed belonged to a reptile, specifically to a dinosaur. In 1865, Dr. Joseph Leidy christened the animal Astrodon johnstoni, “Christopher Johnston’s star-tooth”. (2).

Cross-section of Astrodon tooth. From “Cretaceous Reptiles of the United States”, by Joseph Leidy. Washington, D.C.: The Smithsonian Institution, 1865. Plate XX, figure 10.

In 1888, Prof. Othniel Charles Marsh of Yale University was writing a study of dinosaur fossils that had been found within the rock layers of the Potomac Group of the eastern United States, and he came in person to Dr. Christopher Johnston’s private house in order to request that the teeth be loaned to him for research purposes – the evidence suggests that he showed up completely unannounced. However, Johnston was not home at the time that Marsh appeared on his doorstep, so another man named Philip R. Uhler of the Maryland Academy of Sciences acted in his place and sent the holotype tooth to Professor Marsh. In Uhler’s letter to Marsh dated to January 23, 1888, Uhler wrote “Dr. Johnston asks me if the tooth can be returned within a few weeks” (3). Marsh never gave it back. At some point, Dr. Johnston changed his mind and said that Marsh could keep it – whether this was genuine generosity, or if he finally grasped that Marsh had no intention of giving it back to him, I don’t know. In a letter dated to May 11, 1888, Johnston not only said that Marsh could keep the tooth, which he himself described as “a unique specimen”, but he also mailed the second specimen, the cross-sectioned tooth which had shown its star-like interior structure, to Marsh as well, stating that he took great pleasure in presenting these specimens to him. One wonders if he meant those words. These two teeth are now in the collections of the Yale Peabody Museum (collections ID code: YPM 798). Uhler also made a copy of Philip T. Tyson’s record of the discovery of the teeth and sent it to Marsh as well. Subsequently, Tyson’s original record of the discovery disappeared. Hmmm. In his 1888 survey, O. C. Marsh makes no mention whatsoever of Astrodon. It is unlikely that Marsh was unaware of the discovery, since it had been known to the scientific community for over two decades and surely Marsh would have been keeping up with the literature written by eminent scholars such as Joseph Leidy. It would appear that Marsh was attempting to erase Astrodon’s discovery from the records, and he very nearly succeeded (4).

Marsh wrote in his 1888 survey that the most common fossils in the Potomac Group (written in his report as the Potomac Formation) belonged to sauropod dinosaurs. Many of these fossils came from an open iron ore mining pit located on a farm owned by one William Coffin near Bladensburgh, Maryland. These fossils were collected by one of O. C. Marsh’s chief fossil hunters, John Bell Hatcher (5). Marsh noted that the bones most closely resembled Morosaurus, which has since been recognized as another name of Camarasaurus. However, they were different enough from Camarasaurus to warrant an identification as a new genus, which he then christened as Pleurocoelus, meaning “hollow side”, referring to the deep indentations in the sides of its vertebrae. In fact, Marsh believed that he had identified the remains of two different species of that genus, one slightly larger than the other. In 1888, Prof. Marsh named the specimens Pleurocoelus nanus and Pleurocoelus altus (6).

In one way Marsh had a distinct advantage, aside from his unethical practices. Both Christopher Johnston and Joseph Leidy were basing their findings on just two teeth, whereas Marsh had the advantage of working with at least two partial skeletons. Since there was so much more material to work with, more research and analysis could be conducted, and as a result, Pleurocoelus became more and more well-known to the scientific community, while Astrodon gradually faded into obscurity (7).

Partial skeleton of Pleurocoelus nanus. Caption reads as follows: “Restoration of the Sauropod (herbivorous) dinosaur Pleurocoelus nanus Marsh from Maryland. 1/16 natural size (original). Known portions shaded, remainder from Morosaurus”. Lull, Richard Swann; Clark, William Bullock; Berry, Edward Wilbur (1911), “Systematic Paleontology of the Lower Cretaceous Deposits of Maryland”. Maryland Geological Survey: Lower Cretaceous. Baltimore: The Johns Hopkins Press, 1911. Page 510, plate XI.

Partial skeleton of Pleurocoelus in a more modern anatomically-accurate pose. Illustration by Tracy Ford. From The Dinosaur Society Dinosaur Encyclopedia, written by Don Lessem and Donald F. Glut. New York: Random House, Inc., 1993. Page 378. Image used with permission.

In his 1888 paper, Othniel Charles Marsh stated that Pleurocoelus was unusually small for a sauropod, stating that P. nanus measured “not more than twelve or fifteen feet in length” (8). However in 1903, John Bell Hatcher stated that the bones which Marsh examined belonged to a juvenile, and that the fully-grown adult would have been much larger (9). This was re-affirmed by Donald Glut in his 1997 dinosaur encyclopedia (10). Most secondary sources which I have seen place Astrodon’s / Pleurocoelus’ length at 60 feet, which is still fairly small for a sauropod, but nowhere near as small as the pygmy-sized long-neck that Marsh was describing.

In 1903, John Bell Hatcher claimed that Astrodon and Pleurocoelus were actually the same animal. He argued that the teeth of Astrodon and those of Pleurocoelus were virtually identical. He also stated that the two teeth and the partial skeletons “were found in essentially, or perhaps identically, the same locality and [geological] horizon” (11). They were found in virtually the same location in the same rock layer. This, coupled with the fact that the teeth were so similar to each other meant that, in Hatcher’s eye, “there appears no good reason for considering them as pertaining to either different genera or species. Astrodon johnstoni Leidy having priority should therefore be retained, while Pleurocoelus nanus would become a synonym of that genus and species” (12).

In 1911, the paleontologist Richard S. Lull also proposed that Astrodon and Pleurocoelus might be the same animal. However, there was little evidence to go on, and he stated that the issue of whether or not these two genera were in fact the same could not be conclusively determined until more fossils from both were collected and a proper compare-and-contrast analysis was done. (13). Charles W. Gilmore also believed that Astrodon and Pleurocoelus were synonymous, and that Astrodon held historical priority (14). However, the lack of physical evidence meant that these claims could not be conclusively proven, and the similarities between the two might be purely coincidental. Eventually, the idea that Astrodon and Pleurocoelus were the same was dismissed. There was much more physical evidence attributed to the latter than to the former, and consequently, it was Pleurocoelus which garnered the majority of scientific study and attention. For the next few decades, the name Pleurocoelus held priority. During the subsequent decades, more isolated fossils were uncovered, giving us more information about what these animals looked like. It wasn’t until the middle 2000s when it was again claimed that Astrodon and Pleurocoelus were the same animal, and the name Astrodon has historical priority (15). Attempts made by people in subsequent years to refute this claim don’t hold any water, at least in my opinion. To me, the evidence seems pretty conclusive: Astrodon is here to stay.

So much for the dispute regarding Astrodon’s identity, but now we enter a different argument: where did Astrodon fit onto the dinosaur tree? We know that it was a sauropod, but what kind of sauropod was it? Most sources which I have seen say that it was a brachiosaur, and if so, it would have looked similar to Brachiosaurus in many respects. However, a few others are a bit more liberal and state that it was a member of the clade Titanosauriformes, which includes the family Brachiosauridae, the family Titanosauridae, and a few others. Until more fossils are found, preferably a complete skull, I don’t think that we can narrow the family identification down any further.

On October 1, 1998, Astrodon johnstoni was officially declared to be the state dinosaur of Maryland (16). However, while it’s true that the first fossils of this animal were found within “the Old Line State”, Astrodon fossils are not confined solely to the east coast. In the very early 1900s, fossils of Astrodon were found in Wyoming (17), and it has been proposed that it was Astrodon / Pleurocoelus which made the famous dinosaur footprint trackway in the Paluxy River near Glen Rose, Texas as it was fleeing for its life from a hungry Acrocanthosaurus (18). And they not only inhabited North America, either. Fossils which have been attributed to Astrodon or Pleurocoelus have also been found in Normandy, France and in southern England (19). However, whether or not they belong to the same animal is still subject to debate.

Below is a drawing that I made of Astrodon. The drawing was made with No. 2 pencil, No. 3 pencil, and Crayola colored pencils.

Astrodon johnstoni. © Jason R. Abdale (June 4, 2021).

Source citations:

  1. Christopher Johnston (1859), “Note on odontography”. The American Journal of Dental Science, volume 9, issue 3 (July 1859). Page 341; Joseph Leidy, “Cretaceous Reptiles of the United States”. Washington, D.C.: The Smithsonian Institution, 1865. Page 102; Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz.
  2. Joseph Leidy, “Cretaceous Reptiles of the United States”. Washington, D.C.: The Smithsonian Institution, 1865. Page 102.
  3. Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz.
  4. Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz.
  5. John Bell Hatcher (1903), “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Page 11.
  6. Othniel Charles Marsh (1888), “Notice of a New Genus of Sauropoda and other new Dinosaurs from the Potomac Formation”. The American Journal of Science, volume 3, issue 35 (January 1888). Pages 90-92.
  7. Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz.
  8. Othniel Charles Marsh (1888), “Notice of a New Genus of Sauropoda and other new Dinosaurs from the Potomac Formation”. The American Journal of Science, volume 3, issue 35 (January 1888). Pages 92.
  9. John Bell Hatcher (1903), “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Pages 12-13.
  10. Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz.
  11. John Bell Hatcher (1903), “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Page 12.
  12. John Bell Hatcher (1903), “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Page 12.
  13. Richard Swann Lull (1911), “The Reptilia of the Arundel Formation”. Maryland Geological Survey: Lower Cretaceous. Baltimore: The Johns Hopkins Press, 1911. Page 175; Richard Swann Lull, William Bullock Clark, and Edward Wilbur Berry (1911), “Systematic Paleontology of the Lower Cretaceous Deposits of Maryland”. Maryland Geological Survey: Lower Cretaceous. Baltimore: The Johns Hopkins Press, 1911. Pages 203-204.
  14. Kenneth Carpenter and Virginia Tidwell (2005), “Reassessment of the Early Cretaceous Sauropod Astrodon johnsoni Leidy 1865 (Titanosauriformes)”. In Thunder-Lizards: The Sauropodomorph Dinosaurs, edited by Virginia Tidwell and Kenneth Carpenter. Bloomington: Indiana University Press, 2005. Page 81.
  15. Kenneth Carpenter and Virginia Tidwell (2005), “Reassessment of the Early Cretaceous Sauropod Astrodon johnsoni Leidy 1865 (Titanosauriformes)”. In Thunder-Lizards: The Sauropodomorph Dinosaurs, edited by Virginia Tidwell and Kenneth Carpenter. Bloomington: Indiana University Press, 2005. Pages 78-114.
  16. Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz; Maryland Geological Survey. “Maryland’s State Dinosaur (Astrodon johnstoni)”, by J. P. Reger (1998).
  17. John Bell Hatcher (1903), “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Pages 9-14.
  18. The Dinosaurs! Episode 1 – “The Monsters Emerge” (PBS, 1992); Dinosaur Attack! (ITV, 1999).
  19. John Bell Hatcher (1903), “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Page 13.

Bibliography:

Carpenter, Kenneth; Tidwell, Virginia (2005), “Reassessment of the Early Cretaceous Sauropod Astrodon johnsoni Leidy 1865 (Titanosauriformes)”. In Thunder-Lizards: The Sauropodomorph Dinosaurs, edited by Virginia Tidwell and Kenneth Carpenter. Bloomington: Indiana University Press, 2005. Pages 78-114. https://archive.org/details/thunderlizardssa00tidw/mode/2up.

Dinosaur Attack! ITV, 1999.

Hatcher, John Bell (1903). “Discovery of remains of Astrodon (Pleurocoelus) in the Atlantosaurus beds of Wyoming”. Annals of the Carnegie Museum of Natural History, volume 2. Pages 9-14. https://dinodata.de/dinothek/pdf_d/1902/Discovery_of_remains_of_Astrodon_dd.pdf.

Johnston, Christopher (1859). “Note on odontography”. The American Journal of Dental Science, volume 9, issue 3 (July 1859). Pages 337-343. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6087344/pdf/amjdentsci80644-0037.pdf.

Dinosaur Fund: Dinosaur Research in the Capital Region. “Astrodon Rediscovered: America’s First Sauropod”, by Peter M. Kranz. http://terpconnect.umd.edu/~gdouglas/articles/astrodon.html.

Leidy, Joseph (1865). “Cretaceous Reptiles of the United States”. Smithsonian Contributions to Knowledge, volume 192 (May 1865). Washington, D.C.: The Smithsonian Institution, 1865. Pages 1-135. https://www.biodiversitylibrary.org/title/39830#page/7/mode/1up.

Lull, Richard Swann (1911), “The Reptilia of the Arundel Formation”. Maryland Geological Survey: Lower Cretaceous. Baltimore: The Johns Hopkins Press, 1911. Pages 173-178. https://books.google.com.gt/books?id=6-MYAAAAYAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false.

Lull, Richard Swann; Clark, William Bullock; Berry, Edward Wilbur (1911), “Systematic Paleontology of the Lower Cretaceous Deposits of Maryland”. Maryland Geological Survey: Lower Cretaceous. Baltimore: The Johns Hopkins Press, 1911. Pages 183-596. https://books.google.com.gt/books?id=6-MYAAAAYAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false.

Marsh, Othniel Charles (1888). “Notice of a New Genus of Sauropoda and other new Dinosaurs from the Potomac Formation”. The American Journal of Science, volume 3, issue 35 (January 1888). Pages 89-94. http://marsh.dinodb.com/marsh/Marsh%201888%20-%20Notice%20of%20a%20new%20genus%20of%20Sauropoda%20and%20other%20new%20dinosaurs%20from%20the%20Potomac%20Formation.pdf.

Maryland Geological Survey. “Maryland’s State Dinosaur (Astrodon johnstoni)”, by J. P. Reger (1998). http://www.mgs.md.gov/geology/fossils/maryland_state_dinosaur.html

The Dinosaurs! Episode 1 – “The Monsters Emerge”. PBS, 1992.