Mierasaurus was a turiasaurian sauropod dinosaur which lived in Utah during the early Cretaceous Period approximately 139 to 136 million years ago. Mierasaurus lived during a time when the environment of prehistoric Utah became substantially wetter and more humid than what it had been earlier. The dry semi-arid savannah landscape of the Morrison Formation of the late Jurassic Period abruptly changed to a much greener landscape. However, this time of plenty wouldn’t last, because just a few million years later, the landscape of western North America once again reverted to its Morrison Formation-like condition. Mierasaurus existed alongside the iguanodont Iguanacolossus, the therizinosaur Falcarius, and the raptors Geminiraptor and Yurgovuchia. Other animals which inhabited this environment which have not yet been named include a large allosauroidean theropod similar to Acrocanthosaurus and a large ankylosaurian similar to Gastonia, except it was twice as big (1).

In 2010, a nearly-complete skeleton of a sauropod dinosaur was found at a site in east-central Utah called “Doelling’s Bowl” which yielded an abundant quantity of fossils belonging to several dinosaur species. The rock layer where the bones were found dated to the lower part of the Yellow Cat Member of the Cedar Mountain Formation, from 139-136 million years ago. In 2017, a report was published in which this sauropod was officially named Mierasaurus bobyoungi (2).

The 2017 article written by Rafael Royo-Torres and his associates which officially named and described Mierasaurus didn’t give any lengths for the entire animal within the article itself. However, the illustrations and diagrams which accompanied the article depicted the skull and the skeleton. Mierasaurus’ skull is depicted as measuring exactly 24.2 inches (61.5 cm) long, and the skeleton was shown to measure around 35 feet (10.5 meters) long. As far as sauropods go, this was quite small; “normal” size for a sauropod was twice the length of this animal. Yet it must be noted that the skeleton “UMNH VP 26004” which measured 35 feet long is believed to be a sub-adult due to its vertebral centra being unfused. This means that this animal was not yet fully-grown and it could have gotten larger, perhaps to 40 feet or more (3). However, in another 2017 paper describing a related species named Moabosaurus, Brooks B. Britt and his colleagues stated “The degree of fusion of neurocentral sutures does not help in determining ontogenentic stages, because the sutures are fused regardless of vertebral size” (4). Concerning Moabosaurus, partial remains belonging to at least eighteen individuals have been uncovered as of 2017 (there might be more found since then), measuring up to 30 feet (10 meters) long. However, all of these individuals are believed to have been juveniles and sub-adults, so examining the skeletal remains of the related species Moabosaurus isn’t much help when it comes to getting a grasp on how big Mierasaurus could have gotten (5).

Illustration of Mierasaurus skeleton, from Rafael Royo-Torres et al (2017). Estimated length of 35 feet (10.5 meters). Note that this skeleton was judged to belong to a sub-adult, and as such wasn’t fully grown. The light grey areas indicate which bones are missing.

Illustration of the skull of Mierasaurus, from Rafael Royo-Torres et al (2017). Exact length of 24.2 inches (61.5 cm). The grey areas indicate where the bone is missing. Note that the teeth are not depicted within this illustration, even though turiasaurian sauropods definitely had teeth which were similar in appearance to the teeth of Camarasaurus.

Mierasaurus is known from the rock layers that form the lower part of the Yellow Cat Member of the Cedar Mountain Formation, and it was officially named in 2017. The fossils of a closely-related sauropod named Moabosaurus were found within the upper Yellow Cat Member; it’s possible that the latter might be the direct descendant of the former (6).

Both Mierasaurus and its relative Moabosaurus belonged to a family of sauropod dinosaurs called the “turiasaurs”, a middling group of sauropods within the sauropod tree. They were known primarily from Europe (and possibly from Africa) and were found within rocks dating to the middle and late Jurassic Period; only a couple of species, Mierasaurus being one of them, survived into the early Cretaceous. The turiasaurs are a family within the larger sauropod group known as the “eusauropods” or “true sauropods”, which were more advanced than the primitive archaic sauropods that came before during the late Triassic and early Jurassic like Melanorosaurus and Vulcanodon. Other eusauropods include the cetiosaurids as and the mamenchisaurids. However, the turiasaurs still weren’t as advanced as a sub-group of Eusauropoda called the “neosauropods”, or “new sauropods”. This group was composed of the most advanced sauropods within Eusauropoda, and includes most of the sauropod genera which existed from the late Jurassic Period onwards, including the diplodocids, the macronarians, and the titanosaurs (7).

If the turiasaurs were known to inhabit Europe, then how did European sauropods arrive in North America? During the late Jurassic and early Cretaceous Periods, the northeastern part of North America was connected to Europe by a trans-Atlantic land bridge, and this allowed animal species to travel back and forth between the two continents – the scientific term for this is “faunal interchange”. It’s possible that European turiasaurian sauropods migrated into North America across this land bridge before the Atlantic Ocean expanded and separated the two continents permanently (8).

Three specimens are currently referred to the genus Mierasaurus. The first is a nearly-complete skeleton, designated “UMNH VP 26004”, which was found at Doelling’s Bowl. Two other specimens consist of a juvenile-sized dentary “UMNH VP 26010” and a juvenile-sized femur “UMNH VP 26011” (9).

There might be a fourth Mierasaurus specimen, this time not from Utah, but from South Dakota. In August 1962, a large sauropod humerus was found approximately 15 km southwest of the town of Hot Springs, South Dakota within a layer of “clay-pebble conglomerate in a red sandstone”. This humerus was stated to be similar in appearance to that seen in Camarasaurus and Cetiosaurus, and both Mierasaurus and its close relative Moabosaurus bear an outward resemblance to Camarasaurus in their general shape. Based upon the short description of the rock stratum’s appearance, the closest match to what this rock layer is would be the “S-2” sandstone unit of the Chilson Member of the Lakota Formation. This would date this rock layer to the late Berriasian Stage to the middle Valanginian Stage of the early Cretaceous Period, 142 to 136 MYA; specifically, the bone was dated to 139 MYA. This is the same time as the lower half of the Yellow Cat Member of the Cedar Mountain Formation – the same time that Mierasaurus was alive. This fossil humerus is currently housed in the collections of the Field Museum of Natural History in Chicago, Illinois (collections ID code: FMNH PR 487). Unfortunately, the humerus found in South Dakota cannot be compared to the nearly-complete Mierasaurus skeleton that was found in Utah because that skeleton was missing both of its humeri. Since Mierasaurus and the South Dakota specimen lived at the same time, then it’s very likely that the specimen designated as “FMNH PR 487” actually belongs to Mierasaurus. However, until a humerus which can be definitely identified as belonging to Mierasaurus is found and compared to the specimen from South Dakota, the identity of FMNH PR 487 will remain uncertain (10).

There is also a sauropod from the Cedar Mountain Formation named Brontomerus, named in 2011, which was described as being a close relative of Camarasaurus. Unfortunately, the quantity of specimens from this animal is very sparce: one cervical rib, one-half of a shoulder blade, one sternal plate, fragments of three ribs, half of one dorsal vertebra, part of one ilium, and two small tail vertebrae. Also, the specimens of Brontomerus date to about 110 million years ago – the same time that creatures like Deinonychus, Tenontosaurus, and Astrodon roamed North America – 25 million years later than the specimen found in South Dakota. Therefore, they are probably not the same animal.

Below is an illustration which I made of Mierasaurus. The drawing measures 20 inches long, which, if this animal grew to 40 feet long, would make the drawing 1/24 scale. The drawing was made with No.2 and No.3 pencils on printer paper.

Mierasaurus bobyoungi. © Jason R. Abdale (September 30, 2022).

Keep your pencils sharp.

Source Citations

1. Kirkland, James I.; Britt, Brooks; Burge, Donald L.; Carpenter, Kenneth; Cifelli, Richard; DeCourten, Frank; Eaton, Jeffery; Hasiotis, Stephen; Lawton, Tim (1997). “Lower to Middle Cretaceous Dinosaur Faunas of the Central Colorado Plateau: A Key to Understanding 35 Million Years of Tectonics, Evolution, and Biogeography”. Brigham Young University Geology Studies, volume 42, issue 2. Page 76; Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 119-123; Tennant, Jonathan P.; Mannion, Philip D.; Upchurch, Paul; Sutton, Mark D.; Price, Gregory D. (2017). “Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover”. Biological Reviews, volume 92, issue 2 (2017). Page 7.
2. Royo-Torres, Rafael; Upchurch, Paul; Kirkland, James I.; DeBlieux, Donald D.; Foster, John R.; Cobos, Alberto; Alcalá, Luis (2017). “Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA”. Scientific Reports, volume 7 (2017): 14311.
3. Royo-Torres, Rafael; Upchurch, Paul; Kirkland, James I.; DeBlieux, Donald D.; Foster, John R.; Cobos, Alberto; Alcalá, Luis (2017). “Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA”. Scientific Reports, volume 7 (2017): 14311.
4. Britt, Brooks B.; Scheetz, Rodney D.; Whiting, Michael F.; Wilhite, D. Ray (2017). “Moabosaurus utahensis, n. gen., n. sp., a new sauropod from the early Cretaceous (Aptian) of North America”. Contributions from the Museum of Paleontology, University of Michigan, volume 32, issue 11 (April 10, 2017). Page 233.
5. Britt, Brooks B.; Scheetz, Rodney D.; Whiting, Michael F.; Wilhite, D. Ray (2017). “Moabosaurus utahensis, n. gen., n. sp., a new sauropod from the early Cretaceous (Aptian) of North America”. Contributions from the Museum of Paleontology, University of Michigan, volume 32, issue 11 (April 10, 2017). Pages 193, 196-197, 233.
6. Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 125-138.
7. Royo-Torres, Rafael; Upchurch, Paul; Kirkland, James I.; DeBlieux, Donald D.; Foster, John R.; Cobos, Alberto; Alcalá, Luis (2017). “Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA”. Scientific Reports, volume 7 (2017): 14311; Royo-Torres, Rafael; Cobos, Alberto; Mocho, Pedro; Alcalá, Luis (2021). “Origin and evolution of turiasaur dinosaurs set by means of a new ‘rosetta’ specimen from Spain”. Zoological Journal of the Linnean Society, volume 191, issue 1 (January 2021). Pages 201–227.
8. Galton, Peter M.; Jensen, James A. (1975). “Hypsilophodon and Iguanodon from the Lower Cretaceous of North America”. Nature, volume 257 (October 23, 1975). Pages 668-669; Galton, Peter Malcolm; Jensen, James A. (1979). “Remains of ornithopod dinosaurs from the Lower Cretaceous of North America”. Brigham Young University Geology Studies, volume 25, issue 3. Pages 1-10; Brikiatis, Leonidas (2016). “Late Mesozoic North Atlantic land bridges”. Earth-Science Reviews, volume 159 (May 2016). Pages 47-57; Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 138, 154.
9. Royo-Torres, Rafael; Upchurch, Paul; Kirkland, James I.; DeBlieux, Donald D.; Foster, John R.; Cobos, Alberto; Alcalá, Luis (2017). “Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA”. Scientific Reports, volume 7 (2017): 14311.
10. D’Emic, Michael D.; Foster, John R. (2014). “The oldest Cretaceous North American sauropod dinosaur”. Historical Biology, volume 28, issue 4 (November 18, 2014). Pages 1-7; Royo-Torres, Rafael; Upchurch, Paul; Kirkland, James I.; DeBlieux, Donald D.; Foster, John R.; Cobos, Alberto; Alcalá, Luis (2017). “Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA”. Scientific Reports, volume 7 (2017): 14311; Britt, Brooks B.; Scheetz, Rodney D.; Whiting, Michael F.; Wilhite, D. Ray (2017). “Moabosaurus utahensis, n. gen., n. sp., a new sauropod from the early Cretaceous (Aptian) of North America”. Contributions from the Museum of Paleontology, University of Michigan, volume 32, issue 11 (April 10, 2017). Pages 192, 196-197, 201, 206-207, 218, 228-229, 236.

Bibliography

Articles

Brikiatis, Leonidas (2016). “Late Mesozoic North Atlantic land bridges”. Earth-Science Reviews, volume 159 (May 2016). Pages 47-57.
https://www.sciencedirect.com/science/article/abs/pii/S0012825216300794.

Britt, Brooks B.; Scheetz, Rodney D.; Whiting, Michael F.; Wilhite, D. Ray (2017). “Moabosaurus utahensis, n. gen., n. sp., a new sauropod from the early Cretaceous (Aptian) of North America”. Contributions from the Museum of Paleontology, University of Michigan, volume 32, issue 11 (April 10, 2017). Pages 189-243.
https://deepblue.lib.umich.edu/handle/2027.42/136227.

D’Emic, Michael D.; Foster, John R. (2014). “The oldest Cretaceous North American sauropod dinosaur”. Historical Biology, volume 28, issue 4 (November 18, 2014). Pages 1-9.
https://www.researchgate.net/publication/268449338_The_oldest_Cretaceous_North_American_sauropod_dinosaur.

Galton, Peter M.; Jensen, James A. (1975). “Hypsilophodon and Iguanodon from the Lower Cretaceous of North America”. Nature, volume 257 (October 23, 1975). Pages 668-669.

Galton, Peter Malcolm; Jensen, James A. (1979). “Remains of ornithopod dinosaurs from the Lower Cretaceous of North America”. Brigham Young University Geology Studies, volume 25, issue 3 (1979). Pages 1-10.

Kirkland, James I.; Britt, Brooks; Burge, Donald L.; Carpenter, Kenneth; Cifelli, Richard; DeCourten, Frank; Eaton, Jeffery; Hasiotis, Stephen; Lawton, Tim (1997). “Lower to Middle Cretaceous Dinosaur Faunas of the Central Colorado Plateau: A Key to Understanding 35 Million Years of Tectonics, Evolution, and Biogeography”. Brigham Young University Geology Studies, volume 42, issue 2. Pages 69-103.
https://www.researchgate.net/publication/259021967_Lower_to_middle_Cretaceous_dinosaur_faunas_of_the_central_Colorado_Plateau_a_key_to_understanding_35_million_years_of_tectonics_sedimentology_evolution_and_biogeography.

Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 101-228.
https://giw.utahgeology.org/giw/index.php/GIW/article/view/11.

Royo-Torres, Rafael; Upchurch, Paul; Kirkland, James I.; DeBlieux, Donald D.; Foster, John R.; Cobos, Alberto; Alcalá, Luis (2017). “Descendants of the Jurassic turiasaurs from Iberia found refuge in the Early Cretaceous of western USA”. Scientific Reports, volume 7 (2017): 14311.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5662694/.

Royo-Torres, Rafael; Cobos, Alberto; Mocho, Pedro; Alcalá, Luis (2021). “Origin and evolution of turiasaur dinosaurs set by means of a new ‘rosetta’ specimen from Spain”. Zoological Journal of the Linnean Society, volume 191, issue 1 (January 2021). Pages 201–227.
https://academic.oup.com/zoolinnean/article/191/1/201/5900936?login=false.

Tennant, Jonathan P.; Mannion, Philip D.; Upchurch, Paul; Sutton, Mark D.; Price, Gregory D. (2017). “Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover”. Biological Reviews, volume 92, issue 2 (2017). Pages 776-814.
https://www.academia.edu/64719955/Biotic_and_environmental_dynamics_through_the_Late_Jurassic_Early_Cretaceous_transition_evidence_for_protracted_faunal_and_ecological_turnover.

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