Mymoorapelta

Left ilium of the holotype (MWC 1815)

The Mygatt-Moore Quarry was first discovered in March of 1981 by Vanetta Moore and Pete and Marilyn Mygatt in west-central Colorado in Mesa County, the quarry's strata coming from the Brushy Basin Member of the Morrison Formation that dates to the Upper Jurassic.[5] It wasn't until 9 years later that the first fossils referred to Mymoorapelta were discovered in the quarry, with over 160 elements in total coming from the quarry since 1990 scattered over 25 square meters of the 150 square meter large quarry.[2][5] A few of these elements of a single adult individual were described in James Kirkland and Kenneth Carpenter's description of the taxon in 1994, namely a left ilium with preserved bitemarks that would become the holotype specimen (MWC 1815) from the individual.[5] Kirkland and Carpenter gave the fossils the name Mymoorapelta maysi, the generic name meaning "Vanetta Moore and Pete and Marilyn Mygatt's shield" after the discoverers of the quarry and the armored nature of the taxon.[5] The specific name is after Chris Mays, the president of the Dinamation International Corporation and Society who had funded the excavation of Mygatt-Moore Quarry.[5][2] The ankylosaur was also the first Jurassic ankylosaur named from North America, with the closely related Gargoyleosaurus named 4 years later in 1998.[6][5] Other fossils then excavated included several dorsal and caudal vertebrae, ribs, isolated limb bones, and many dermal armor elements, including the signature triangular cervical spine of basal ankylosaurs, from the type adult individual.[4][2][5] Some osteoderms referred to Mymoorapelta come from stratigraphically higher in the quarry, suggesting that a second individual is preserved.[2] In 1998, a partial skeleton referred to Mymoorapelta was described by Kirkland et al from Cactus Park, Colorado, though the specimen has yet to be fully described as it was still in preparation when described.[4] The skeleton preserved natural molds and body fossils of the sacrum, caudal vertebrae, chevrons, pes, and many pieces of the dermal armor of the sacral shield, though skull material was mentioned to be preserved but still in preparation.[7][5] 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.[4] In a 2010 abstract in the Journal of Vertebrate Paleontology, Kirkland and colleagues mentioned the discovery of many more elements of Mymoorapelta, including a nearly complete skull and every postcranial element except the pubis and femur.[4][1][2] 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.[2][5] 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.[3] The specimen includes one individual preserving: 24 osteoderms, three ribs, one vertebra, and a femur, though more material has yet to be excavated.

Although dozens of fossils are known from Mymoorapelta, few individuals are known and there is much material that remains to be described. The Cactus Park specimen is the largest described specimen, with estimates placing it as large as 3 meters (9.8 feet) in 1998.[4] Despite this, Mymoorapelta is possibly the smallest known quadrupedal dinosaur from the Morrison Formation and the small size of other basal Nodosaurids indicates a ecological and size evolution over the course of the Cretaceous.[8][4][9] John Foster estimated the taxon to weigh about 560 kilograms (1,232 lbs), a very small weight for an Ankylosaur, based on Gastonia.[7] The preserved jugal described in 1998 has a wedge-shaped and armored morphology.[4] The vertebral centra are much shorter compared to Gargoyleosaurus and the neural spines are shorter in Mymoorapelta.[4][5] In the distal tail vertebrae, the prezygapophyses do not extend past the anterior edge of the centrum by more than about 25% of the centrum length, allowing much more flexibility compared to the tail clubbed Ankylosaurids.[10] The tail vertebrae of Mymoorapelta also carried dermal spikes on the sides like the Nodosaurids, likely using them for defense.[10] Mymoorapelta is also one of the few Ankylosaurs with 4 sacral ribs, a trait shared with only 3 other taxa.[11]

Armor

Three of the thin, triangular plates on display at the Dinosaur Journey Museum, Fruita.

A prominent feature of Mymoorapelta was its armor mostly consisting of large spiked osteoderms protruding from the lateral and dorsal sides of the body. These have not been found in articulation, so their exact placement on the body is unknown, though their position can be surmised based on the well preserved skeletons of Nodosaurids and other Ankylosaurs. In Mymoorapelta, there are five different preserved armor types: an elongated spine with a large, hollow base; a thin triangular plate with a narrower, asymmetrical base; a small bladelike spine with a rounded, solid base; isolated, flat, keeled scutes; and scutes fused into a single plate of armor.[4][5] The large spine is assumed to have extended from the base of the neck outward laterally and the point curving posteriorly.[4][5] Evidence from Borealopelta also supports the extension of the spike by a large keratin sheath.[9] Further down the body, the aforementioned thin triangular plates, of which eight are known, are preserved similarly to those in Polacanthus.[12][5] It is theorized that the largest of these plates were attached to the proximal part of the tail, and protruded laterally along the sides of the body and a specimen preserved one at 11 centimeters.[4][5] A single, small triangular plate was reported in 1994 and it preserves a thin, bladelike morphology, though its position is unknown.[5] Twelve of the keeled, flat scutes known in other Nodosaurids were recovered in Mymoorapelta in 1994, preserving an oval shape and these scutes were likely the most common on the body. Lastly, a small fragment of dermal armor preserves a rosette formed by a large central scute surrounded on all sides by smaller ossicles.[4][5] This piece of armor comes from the sacral shield, a large plate of small, connected armor across the sacrum and pelvis for protection. A more complete sacral shield was preserved in the Cactus Park specimen, with estimates placing the complete sacral shield at 70 centimeters for a 50 centimeter wide pelvis.[4][5] The sacral shield also preserves two middle rows of larger scutes with seven large scutes preserved in each row.[4] On the upper surface of the back and tail, the skin was covered in small osteoderms, sometimes referred to as ossicles, in-between larger plates of armor as inferred by Sauropelta,[13] though ossicles themselves are not known confidently outside of the sacral shield in Mymoorapelta.[4][5] The armor of Mymoorapelta differs from those in Gargoyleosaurus in that the former has solid conical armor compared to the thin-walled armor of the latter.[6]

When Mymoorapelta was originally named, James Kirkland and Kenneth Carpenter placed the taxon as a potential Nodosaurid, though at that time there were few taxa to compare the material to. The phylogenetic placement was changed in 1998 when it was considered closer to the European Polacanthidae,[4] though the phylogeny of Polacanthids is in a constant flux.[14] Vickaryous et al. (2004) considered it Ankylosauria incertae sedis,[15] while a cladistic analysis performed by Thompson et al., 2011 suggests that Mymoorapelta is a basal nodosaurid as originally theorized by Kirkland and Carpenter.[16] A 2010 phylogenetic analysis utilizing undescribed material resulted with Mymoorapelta as the sister taxon to Gargoyleosaurus in their own group outside of Polacanthidae, Nodosauridae, and Ankylosauridae.[1] Along with Gargoyleosaurus parkpinorum, Mymoorapelta is one of the earliest known ankylosaurs, providing a look at the early evolution and diversification of this group of dinosaurs. The 2018 phylogenetic analysis of Rivera-Sylva and colleagues is used below, placing it as a basal nodosaurid, with relationships within Panoplosaurini and Struthiosaurini collapsed for simplicity.[17] Alternatively, Mymoorapelta may be an ankylosaur outside both Nodosauridae and Ankylosauridae.[18]

Tail anatomy of ankylosaurs, F is Mymoorapelta

A dorsal vertebra in anterior view at the Dinosaur Journey Museum, Fruita.

Mygatt-Moore Quarry has been interpreted as an attritional accumulation of dinosaur fossils at a permanent waterhole, allowing it to become one of the most fossil abundant sites in the United States. The site notably preserves a menagerie of Jurassic fossils, with plants, pollen, coprolites, and even skin impressions preserved alongside dinosaur fossils. Plant fossils are abundant in the quarry, with horsetails, cycadophytes, gingkoes, Czekanowskiales, and many different conifers represented by leaf, wood, and pollen fossils.[2] Invertebrates represented in the quarry include gastropods and most notably, fossils of fossil crayfish.[2] As for vertebrates, three different kinds of fish have been described from the quarry in astonishing quality, and the dinosaur fossils are the most common in the quarry. Allosaurus fragilis, a large carnivorous theropod, and Apatosaurus, a large diplodocid sauropod, are the two most common dinosaurs in the quarry, with Mymoorapelta being the third most common at the site.[2] The Morrison Formation in total is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains.[7] Vegetation varied from river-lining forests of conifers, tree ferns, and ferns (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.[7]

Mymoorapelta, like its relatives, was a low browsing herbivore in its ecosystem that likely fed on the cycads and conifers of the time due to the lack of complex grasses in the Jurassic.[19] Due to competition with Diplodocid sauropods, Mymoorapelta and its ecological group seems more adapted for open environments.[20] Evidence of predation, likely from a large carnivore like Allosaurus is known on the holotype ilium of the taxon and drilling by insects and parasites has been discovered on some fossils.[21] Mymoorapelta has a very limited known distribution, known only from two sites in Western Colorado and both have few individuals preserved of the genus.[5][4] The Hanksville-Burpee specimen's taphonomy suggests that the individual had died inland, contrary to the coastal distribution previously inferred for Nodosaurids.[3][22][23]

• Timeline of ankylosaur research

• Carpenter, K.; Kirkland, J. I.; Birge, D.; Bird, J. (2001). "Disarticulated skull of a new primitive ankylosaurid from the Lower Cretaceous of Utah". In Carpenter, K. (ed.). The Armored Dinosaurs. Indiana University Press.
• Kirkland, J. I.; K. Carpenter (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.
• Vickaryous; Maryanska; Weishampel, D. B.; Dodson, P.; Osmólska, H., eds. (2004). "Ankylosauria". The Dinosauria. University of California Press.