Mymoorapelta (Meaning "Shield from the Mygatt-Moore Quarry" after a combination of the names of the discoverers of the Mygatt-Moore Quarry that fossils were originally collected from, and the Greek word πέλτα, meaning "shield") is a nodosaurid ankylosaur from the Late Jurassic (Kimmeridgian-Tithonian, around 155 to 150 million years ago) Morrison Formation (Brushy Basin Member) of western Colorado and central Utah, USA. The animal is known from a single species, Mymoorapelta maysi, and few specimens are known. The most complete specimen is the holotype individual from the Mygatt-Moore Quarry, which includes , a partial , , and other bones. It was initially described by James Kirkland and Kenneth Carpenter in 1994. Along with Gargoyleosaurus, it is one of the earliest known nodosaurids.
Mymoorapelta is one of the smaller known nodosaurids, with the estimated length of the largest specimen only reaching . It had a narrow snout and almost triangular skull in dorsal view, with two large horns pointing backwards from the brow and two horns below these that pointed backwards and down on the . Five different armor types have been observed in Mymoorapelta, ranging from elongated, sharp spines on the side of the body to a giant shield composed of tiny osteoderms, called ossicles, that covered the top of the pelvis. In contrast to the club-tailed ankylosaurids, the tail bore spikes that Mymoorapelta likely used for defense.
Mymoorapelta was a low browser in the Morrison ecosystem, feeding on cycads and conifers, in contrast to the high-browsing Apatosaurus known from the same quarries. Other dinosaur groups were also present, including the large theropods Allosaurus and Ceratosaurus and possibly the ornithischian Nanosaurus. Both of the theropods have also been found in association with the animal, and we have evidence that the former preyed on Mymoorapelta.
Discovery and naming
Original discovery
The Mygatt-Moore Quarry was first discovered in March 1981 by hikers JD and Vanetta Moore and Pete and Marilyn Mygatt in Mesa County, west-central Colorado. The quarry's strata come from the Brushy Basin Member of the Morrison Formation, which dates to the Upper Jurassic.Kirkland, J. I., & Carpenter, K. (1994). North America's first pre-cretaceous ankylosaur (Dinosauria) from the Upper Jurassic Morrison Formation of western Colorado. Brigham Young University Geology Studies, 40, 25-42. Fossils of Mymoorapelta were first discovered in the quarry in 1990, and since then over 160 elements have been found scattered over of the quarry.Foster, J., Hunt-Foster, R., Gorman, M., Trujillo, K., Suarez, C., McHugh, J., ... & Schoenstein, H. (2018). Paleontology, taphonomy, and sedimentology of the Mygatt-Moore Quarry, a large dinosaur bonebed in the Morrison formation, western Colorado—implications for Upper Jurassic dinosaur preservation modes. Geology of the Intermountain West, 5, 23-93. A few of these elements are part of a single adult individual and were described by American paleontologists James Kirkland and Kenneth Carpenter in 1994. This specimen (MWC 1815) would be designated the holotype of the species and includes a left (hip bone) with preserved bitemarks. Kirkland and Carpenter named the species Mymoorapelta maysi, the generic name deriving from the names of Marilyn Mygatt and the Moores, who had discovered the quarry, and the Greek root pelta, meaning "shield", due to the preserved armor. The specific name is after Chris Mays, the president of the Dinamation International Corporation and Society, who funded the initial excavation of the Mygatt-Moore Quarry. Mymoorapelta was the first Jurassic ankylosaur named from North America, with the closely related Gargoyleosaurus being named four years later in 1998.Carpenter, K., Miles, C. and Cloward, K. (1998). "Skull of a Jurassic ankylosaur (Dinosauria)." Nature 393: 782–783. Other parts of the type specimen that were excavated from the quarry include several (back) and (tail) vertebrae, , isolated limb bones, and many osteoderms, including the signature triangular cervical spine of basal ankylosaurs.Hunt, A. P., & Scheetz, R. D. (1998). Ankylosaur (Dinosauria) specimens from the Upper Jurassic Morrison Formation The Upper Jurassic Morrison Formation: An Interdisciplinary Study: Denver Museum of Natural History, Denver, USA: May 26–28, 1994, 22, 145-177. Some osteoderms which have been found in the quarry and referred to Mymoorapelta come from a higher stratigraphic layer, suggesting that a second individual is preserved.
Later finds
In 1998, a partial skeleton referred to Mymoorapelta was described by Kirkland et al from Cactus Park, Colorado, though at the time the fossil was still undergoing fossil preparation. The skeleton preserved natural molds and body fossils of the sacrum, caudal vertebrae, (bones attached to the underside of the tail vertebrae in dinosaurs), (foot), and many pieces of the dermal armor, including parts of the sacral shield, which was a type of armor that would cover the top of the pelvis. Though skull material was mentioned to be preserved it is still in preparation and undescribed.Foster, J. (2007). Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. pp. 327–329. Some of the material, namely from the pes and sacral shield, was preserved in articulation making it the only known articulated specimen of the taxon. In a 2010 abstract, Kirkland and colleagues mentioned the discovery of many more elements of Mymoorapelta at Mygatt-Moore Quarry including a nearly complete skull and every postcranial element except the pubis and femur.Kirkland, J. I., Hunt-Foster, R. K., Foster, J. R., & Loewen, M. (2010). Newly recovered skeletal elements of the Late Jurassic dinosaur Mymoorapelta from its type locality in the Morrison Formation permits re-evaluation of ankylosaur phylogeny. Journal of Vertebrate Paleontology, 30, 116A. All of the fossils found at Mygatt-Moore Quarry and Cactus Park are currently deposited in the Dinosaur Journey Museum of Western Colorado in Fruita.
2015 saw the presentation of an abstract by Katie Tremaine et al that mentions a new Mymoorapelta specimen unearthed at Hanksville-Burpee Quarry near Hanksville, Utah.Tremaine, K., D'Emic, M., Williams, S., Hunt-Foster, R.K., Foster, J., and Mathews, J., (2015), Paleoecological implications of a new specimen of the ankylosaur Mymoorapelta maysi from the Hanksville-Burpee Quarry, latest Jurassic (Tithonian) Morrison Formation (Brushy Basin Member) [abs.]: Journal of Vertebrate Paleontology Program and Abstracts, p. 226. The specimen includes one individual preserving: 24 osteoderms, three ribs, one vertebra, and a femur, though more material has yet to be excavated. The discovery of a single, dorsal osteoderm that had been collected from the Peterson Quarry outside of Albuquerque, New Mexico was described in 2016.Burns, M. E., & Lucas, S. G. (2016). Biostratigraphy of Ankylosaur (Dinosauria: Ornithischia) Osteoderms from New Mexico. Fossil Record 4: Bulletin 67, 67, 9. The osteoderm is morphologically (based on external anatomy) similar to Mymoorapelta, however it is stratigraphically (based on rock layers) closer to Gargoyleosaurus.
Description
Mymoorapelta is one of the smallest ankylosaurs known and the smallest known quadrupedal dinosaur from the Morrison Formation. However, few individuals have been found and only a portion of the known material has been described in detail. The Cactus Park specimen is the largest described individual, with its length being estimated at long in 1998. Body mass estimates based on the type specimen give a weight of around .
Skull and dentition
The specifically has not been described in detail, but the preserved morphology (external anatomy) is very similar to that of Gargoyleosaurus, which also hails from the Morrison Formation. It had a narrow snout and almost triangular shaped skull in dorsal (top) view, with two large horns pointing backward from the back of the brow, and two horns below these that pointed backward and down from the (cheek bone). A preserved jugal described in 1998 has a wedge-shaped, armored morphology and projects ventrolaterally (down and outwards) like in ankylosaurids. The (bones at the tip of the snout) probably possessed a narrow, toothed, and trapezoidal muzzle like in Gargoyleosaurus. In the back of the skull, the -articular area (place of contact between the skull and lower jaw) faces slightly medially (towards the midline of the body), and is subtly lengthened anteroposteriorly (front to back). The distal surface of the quadrate, especially on the medial condyle, is robust and elongated anteroposteriorly. In all ankylosaurs, the jaw joint (joint connecting the skull to the jaw bone) is placed ventrally to the occlusal plane (contact surface between teeth). The (bony projection on the lower jaw) is very well developed in nodosaurids, and the jaw joint is well depressed in turn, which corresponds to a higher relative bite force.
Most ankylosaurs have heterodont dentition (variation in tooth morphology), but not in Gargoyleosaurus, which shows only subtle morphological differences in the premaxillary, maxillary, and dentary teeth. Ankylosaur teeth are compressed labiolingually and are phylliform (leaf-shaped), with an apical cusp and secondary cusps along the mesial and distal edges. Nodosaurid teeth are blade-like, larger, and usually more complex than the basic cusps of ankylosaurids, having a larger crown, a rough enamel surface, confluent fluting with grooves of the marginal cusps, and a crenelated (shelf-like bulge surrounding the base of a tooth crown). The wear facets on Ankylosaur teeth vary greatly, but in nodosaurids they are usually more extensive and steeply inclined than in ankylosaurids, demonstrating underlying patterns of shape-constrained function. In early nodosaurids like Gargoyleosaurus and Gastonia, the (a dermal, paired bone of the palate) is not yet well developed anteroposteriorly and mediolaterally, but in later species they are much wider, with more robust lateral wings, and they are more expanded anteroposteriorly than in basal forms, reaching the level of the distal quadrate condyles. This suggests more developed pterygoid muscles in later nodosaurids than in ankylosaurids, reflecting a more efficient jaw adductor system in nodosaurids. The mandibular adductor chamber and the coronoid process are generally taller in nodosaurids than in ankylosaurs, suggesting differences in the size and attachment of muscles.
Postcranial skeleton