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 osteoderms, a partial skull, vertebrae, 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 3 metres (9.8 ft). 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 jugal. Five different armor types have been observed in Mymoorapelta, ranging from elongated, sharp spines on the side of the body to a giant sacral 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. Fossils of Mymoorapelta were first discovered in the quarry in 1990, and since then over 160 elements have been found scattered over 25 metres (27 yd) of the 150-square-metre (1,600 sq ft) quarry. 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 ilium (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. Other parts of the type specimen that were excavated from the quarry include several dorsal (back) and caudal (tail) vertebrae, ribs, isolated limb bones, and many osteoderms, including the signature triangular cervical spine of basal ankylosaurs. 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, chevrons (bones attached to the underside of the tail vertebrae in dinosaurs), pes (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. 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. 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. 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. 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 3 metres (9.8 ft) long in 1998. Body mass estimates based on the type specimen give a weight of around 300–562 kilograms (661–1,239 lb).
Skull and dentition
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