Hippodraco

Hippodraco, meaning “horse dragon” in a mixture of ancient Greek and Latin, was an ornithopod dinosaur which lived in western North America during the early Cretaceous Period. The fossils were found by Dr. Andrew R. C. Milner in 2004 in east-central Utah at a place now referred to as “Andrew’s Site”. The rocks in this fossil-rich locality date to the upper part of the Yellow Cat Member of the Cedar Mountain Formation, which would date the fossil from 135 to 132 million years ago (1).

But Andrew’s Site was not the only place where Hippodraco fossils have potentially been found. In 2001, Matthew Stikes, a geology graduate student from Northwest Arizona University, was doing field research on the sedimentology of the Cedar Mountain Formation. He had been sent there by his teacher, Jim Kirkland, to examine the rock layers of the Poison Strip Member, and told him to be on the lookout for any bones that he might find there. As he arrived at a steep ridge overlooking Arches National Park, he spotted a small thin bone sticking out of the ground. At first, he thought that it looked like a human arm bone. Could it possibly be Stone Age remains? Well, not quite. When Stikes informed Jim Kirkland and Scott Madsen about what he had found, they knew right away what it was – the limb bone of a theropod dinosaur. However, the site wasn’t surveyed until four years later in 2005 when Scott Madsen, Don DeBlieux, and Jim Kirkland arrived and examined the site with their own eyes. The rock layer that the bone was found in was not dated to the Poison Strip Member, but to the earlier Yellow Cat Member. One of the first rocks that Kirkland split open contained a claw and the front end of the lower jaw of a juvenile Utahraptor. This find in itself was sufficient to warrant further examination of this site, and further digging uncovered even more bones nearby, also probably belonging to Utahraptor. Alright, this was a major discovery, and it needed to be looked at in more detail. The locality was christened “Stikes Quarry” in honor of its discoverer (2).

While Jim Kirkland and other members of the Utah Geological Survey did a quick site inspection, that was really all that they could do at that juncture. The Utah Geological Survey was busy working on many other projects, including a massive quarry chock-full of Falcarius skeletons, and couldn’t add another dig-job onto their already daunting work list, so it could not conduct an excavation of Stikes Quarry. Therefore, they contacted Washington University in Saint Louis, Missouri, which had recently established a paleontology program, and asked if they would like to take a crack at excavating the site. SURE! So in the Summer of 2006, a team from Wash.U. arrived on the scene and conducted the first dig season at Stikes Quarry. The first of several large blocks was collected, hauled out of the site using an old car hood as a sled, and was afterwards transported to Saint Louis for study (3).

Karen Poole, a graduate student from Washington University (left), and Utah state paleontologist Jim Kirkland (right) examine the underside of the first bone jacket after it was turned upside-down. Utah Geological Survey. “Utahraptor Megablock Fossil Project: A Timeline of Excavation and Preparation”.
https://storymaps.arcgis.com/stories/8d63f2a68feb4a64a0ea5eebebdd9eaa.

A team from the Utah Geological Survey spent ten dig seasons working on the site. Like Andrew’s Site, the rock layer at Stikes Quarry also dated to the Yellow Cat member of the Cedar Mountain Formation. The culmination of their efforts was a massive block crammed full of Utahraptor fossils weighting nine tons! Yet the Utahraptors were not the only animals found here. Several bones belonging to at least two individuals of an as-yet-unknown iguanodontian dinosaur were also found mixed into the matrix. These fossils included an ilium (the top of the hip), ischium (the back of the hip), femur, astragalus (ankle bone), several tail vertebrae, and two tail chevrons. That’s what the dig-team had spotted so far, but there might have been more still buried within the rock. A description of the iguanodontian fossils which had been discovered so far was written up by Karen Poole in 2008, but she did not venture to give it a species identification. However, she did note several times that the bones were somewhat similar to Camptosaurus, a plant-eating dinosaur from the late Jurassic Period, and to Mantellisaurus, an iguanodont from the early Cretaceous (4).

NOTE #1: My gracious thanks to Dr. Karen Poole for providing me with a copy of her 2008 Master’s thesis which describes these finds.

NOTE #2: Excavation and preparation of this massive block is a slow methodical pains-taking process, and it’s still ongoing. This is largely due to the block’s sheer massive size, the fact that there are so many small specimens crammed together that need to be very carefully separated, and, at the time that I’m writing this article in late June 2022, only ONE fossil preparator is working on this huge stone block, and funding to carry out the work is very limited. The museum has therefore been reaching out to the public to ask for donations to carry out the work. Please check out “The Utahraptor Project” for updates, and to donate to this important scientific project. Please follow them on Facebook at https://www.facebook.com/The-Utahraptor-Project-1740182959575280. If you want to donate, please check out the Utah Geological Survey’s “Utahraptor Megablock Fossil Project” at https://geology.utah.gov/popular/general-geology/dinosaurs-fossils/megablock/. You can get more specific info about donations by going here: https://geology.utah.gov/docs/pdf/STEM-utahraptor-megablock-letter.pdf.

In 2010, two years after Karen Poole wrote up her findings, a scientific paper was published by Andrew McDonald and his colleagues concerning the fossils which had been discovered at “Andrew’s Site” in 2004. They were determined to belong to a juvenile iguanodontian dinosaur. The animal was officially named Hippodraco, meaning “horse dragon”. Although the specimen is incomplete, it appears to be closely related to Camptosaurus from the late Jurassic Period and Theiophytalia from the middle Cretaceous Period – Hippodraco, which dates to the early Cretaceous, slots neatly in between the two (5). It’s likely that the bones which were found at Stikes Quarry and described by Karen Poole also belong to Hippodraco.

Skeletal reconstruction of Hippodraco scutodens (collection ID code: UMNH VP 20208). Note: the right scapula, right humerus, right femur, and right tibia have been reversed for the purposes of reconstruction). Scale bar = 1 meter. From McDonald, Andrew T.; Kirkland, James I.; DeBlieux, Donald D.; Madsen, Scott K. Cavin, Jennifer; Milner, Andrew R. C.; Panzarin, Lukas (2010). “New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs”. PLoS ONE, volume 5, issue 11 (November 22, 2010): e14075.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989904/.

Partial skull of Hippodraco scutodens. Natural History Museum of Utah, Salt Lake City. Photograph by Jens Lallensack (October 2016). Creative Commons Attribution-Share Alike 4.0 International license.
https://commons.wikimedia.org/wiki/File:Hippodraco_scutodens_salt_lake_city.jpg.

Due to this animal being known from only partial remains, and also due to it not yet being fully grown, it’s unknown how big this animal grew when it was an adult. The partial skeleton which was found was estimated to have been 15 feet long. As to how large it would have become, most estimates that I’ve seen place it as being between 18 to 25 feet long, which is the same general size that Camptosaurus and Theiophytalia are in.

However, some paleontologists (notably Jim Kirkland) think that Hippodraco could have gotten substantially bigger. A series of nine tail vertebrae which were discovered at “Andrew’s Site” (the same locality that the Hippodraco fossils were found) have been tentatively described to some species of iguanodont dinosaur, but an exact identification is difficult because all of the bones are badly crushed. Jim Kirkland thinks that these bones belong to a large specimen of Hippodraco, which would make the animal 30-35 feet long, but this is just one person’s opinion (6). I have seen these bones ascribed to Hippodraco, Iguanacolossus, Cedrorestes, and to as an as-yet-unknown species.

Tail vertebrae from a large iguanodontid dinosaur from Andrew’s site UMNH 1207, upper part of the Yellow Cat Member (collections ID code: UMNH VP 20207). Note the short stubby neural spines on the top of the vertebrae. In McDonald et al (2010), it was simply referred to as “an indeterminate iguanodontian”. Scale bar = 10 cm. From McDonald, Andrew T.; Kirkland, James I.; DeBlieux, Donald D.; Madsen, Scott K. Cavin, Jennifer; Milner, Andrew R. C.; Panzarin, Lukas (2010). “New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs”. PLoS ONE, volume 5, issue 11 (November 22, 2010): e14075.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989904/.

Below is my reconstruction of what a complete skull of Hippodraco MIGHT look like, based upon the single partial skull seen above, and also upon the skulls of Camptosaurus and Theiophytalia. The drawing was made with black Crayola marker on printer paper.

Hippodraco skull. © Jason R. Abdale (May 18, 2022).

Finally is my reconstruction of the entire animal, based upon the skeletons of Camptosaurus and Theiophytalia. The drawing was made with No.2 pencil on printer paper, and measured 20 inches long from nose to tail (1/12 scale).

Hippodraco. © Jason R. Abdale (July 23, 2022).

As always, keep your pencils sharp.

Source Citations

  1. McDonald, Andrew T.; Kirkland, James I.; DeBlieux, Donald D.; Madsen, Scott K. Cavin, Jennifer; Milner, Andrew R. C.; Panzarin, Lukas (2010). “New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs”. PLoS ONE, volume 5, issue 11 (November 22, 2010): e14075. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989904/.
  2. Poole, Karen Elaine (2008). “A New Specimen of Iguanodontian Dinosaur from the Cedar Mountain Formation, Grand County, Utah”. Master’s thesis, Washington University, Saint Louis, Missouri (August 2008). Page 6; Utah Geological Survey. “UGS Paleontologists Collect Dinosaur Megablock”, by Don DeBlieux. https://geology.utah.gov/map-pub/survey-notes/collect-dinosaur-megablock/. Accessed on June 6, 2022; YouTube. Utah Friends of Paleontology. “Preparation of the Utahraptor Megablock” (January 7, 2020). https://www.youtube.com/watch?v=S0rXWLHwK0E. Accessed on June 23, 2022; Utah Friends of Paleontology. “Utahraptor Ridge: A Utah Landmark Named for a Landmark Discovery in Paleontology”. https://utahpaleo.org/2021/03/19/utahraptor-ridge/. Accessed on June 6, 2022; YouTube. Utah Friends of Paleontology. “The Utahraptor Project: A Progress Report by Scott Madsen” (May 2, 2017). https://www.youtube.com/watch?v=DZXKTH3gy7M. Accessed on June 23, 2022; YouTube. TheNMSR. “March 10th, 2021 NMSR Meeting – ‘Feathering Utahraptor: History of Dromaeosaur Discoveries’, hosted by Dr. Jim Kirkland” (March 15, 2021). https://www.youtube.com/watch?v=mY5lWGC-ogA. Accessed on March 14, 2022.
  3. Poole, Karen Elaine (2008). “A New Specimen of Iguanodontian Dinosaur from the Cedar Mountain Formation, Grand County, Utah”. Master’s thesis, Washington University, Saint Louis, Missouri (August 2008). Page 6; Utah Geological Survey. “UGS Paleontologists Collect Dinosaur Megablock”, by Don DeBlieux. https://geology.utah.gov/map-pub/survey-notes/collect-dinosaur-megablock/. Accessed on June 6, 2022; Utah Friends of Paleontology. “Utahraptor Ridge: A Utah Landmark Named for a Landmark Discovery in Paleontology”. https://utahpaleo.org/2021/03/19/utahraptor-ridge/. Accessed on June 6, 2022.
  4. Utah Friends of Paleontology. “Utahraptor Ridge: A Utah Landmark Named for a Landmark Discovery in Paleontology”. https://utahpaleo.org/2021/03/19/utahraptor-ridge/. Accessed on June 6, 2022.
  5. Poole, Karen Elaine (2008). “A New Specimen of Iguanodontian Dinosaur from the Cedar Mountain Formation, Grand County, Utah”. Master’s thesis, Washington University, Saint Louis, Missouri (August 2008). Pages 1-56; Utah Geological Survey. “UGS Paleontologists Collect Dinosaur Megablock”, by Don DeBlieux. https://geology.utah.gov/map-pub/survey-notes/collect-dinosaur-megablock/. Accessed on June 6, 2022.
  6. McDonald, Andrew T.; Kirkland, James I.; DeBlieux, Donald D.; Madsen, Scott K. Cavin, Jennifer; Milner, Andrew R. C.; Panzarin, Lukas (2010). “New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs”. PLoS ONE, volume 5, issue 11 (November 22, 2010): e14075. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989904/; YouTube. TheNMSR. “March 10th, 2021 NMSR Meeting – ‘Feathering Utahraptor: History of Dromaeosaur Discoveries’, hosted by Dr. Jim Kirkland” (March 15, 2021). https://www.youtube.com/watch?v=mY5lWGC-ogA. Accessed on March 14, 2022.

Bibliography

McDonald, Andrew T.; Kirkland, James I.; DeBlieux, Donald D.; Madsen, Scott K. Cavin, Jennifer; Milner, Andrew R. C.; Panzarin, Lukas (2010). “New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs”. PLoS ONE, volume 5, issue 11 (November 22, 2010): e14075.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989904/.

Poole, Karen Elaine (2008). “A New Specimen of Iguanodontian Dinosaur from the Cedar Mountain Formation, Grand County, Utah”. Master’s thesis, Washington University, Saint Louis, Missouri (August 2008). Pages 1-56.

Utah Geological Survey. “UGS Paleontologists Collect Dinosaur Megablock”, by Don DeBlieux.
https://geology.utah.gov/map-pub/survey-notes/collect-dinosaur-megablock/. Accessed on June 6, 2022.

Utah Friends of Paleontology. “Utahraptor Ridge: A Utah Landmark Named for a Landmark Discovery in Paleontology”.
https://utahpaleo.org/2021/03/19/utahraptor-ridge/. Accessed on June 6, 2022.

YouTube. TheNMSR. “March 10th, 2021 NMSR Meeting – ‘Feathering Utahraptor: History of Dromaeosaur Discoveries’, hosted by Dr. Jim Kirkland” (March 15, 2021).
https://www.youtube.com/watch?v=mY5lWGC-ogA. Accessed on March 14, 2022.

YouTube. Tate Geological Museum. “Tate Geological Museum’s Spring Lecture Series 2021: Cretaceous Dinosaurs- part 3 – The Cedar Mountain Formation of Utah: North America’s Most Complete Early Cretaceous Record, hosted by James I. Kirkland” (May 11, 2021).
https://www.youtube.com/watch?v=Thhb6Jy-Acw. Accessed on March 14, 2022.



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