Mymoorapelta

Skull of the similar Gargoyleosaurus, also from the Morrison Formation.

The skull 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.[7] 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 jugal (cheek bone). A preserved jugal described in 1998 has a wedge-shaped, armored morphology and projects ventrolaterally (down and outwards) like in ankylosaurids.[5] The premaxillae (bones at the tip of the snout) probably possessed a narrow, toothed, and trapezoidal muzzle like in Gargoyleosaurus.[12] In the back of the skull, the quadrate-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 coronoid process (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 cingulum (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 pterygoid (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.[12]

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

A large part of the postcranial (behind the skull) skeleton of Mymoorapelta is known. For instance, many cervical vertebrae (vertebrae of the neck) are known, though many of them have not been fully researched or excavated. In the cervicals, the faces (ends) of the centra are wider than they are tall and display a heart-shaped morphology. The centra overall are anteroposteriorly short with lengths that are around equal to their width. The neural spines are short and thick,and possess round, circular neural canals. A deep, tear-shaped fossa (small opening in bone) is present between the postzygapophyses (backwards projections of the vertebral arch).[5] Several dorsal (back, sometimes referred to as thoracic) vertebrae are known from Mymoorapelta, and they are unique when compared to those of Cretaceous nodosaurids. The centra are cylindircal, longer posteriorly, and less laterally compressed than those of other, later nodosaurids like Sauropelta and Dracopelta. The anterior dorsals are unique in that they bear a triangular pit between the prezygapophyses (forwards projections of the vertebral arch), which are also much more elongate at the anterior end of the vertebra. Meanwhile, the postzygapophyses are truncated at the posterior end, with the left and right postzygapophyses divided by a large groove. The mid dorsal vertebrae differ in that they are rounded ventrally and have a weak keel at the bottom. The transverse processes of the dorsal vertebrae do not ascend steeply and are instead gradually heightened, similar to the condition in basal ankylosaurs, and differing from that of nodosaurids. Mymoorapelta has 13 ribs, all of which have triangular cross-sections and a weak T-shape, in contrast to other ankylosaurs. Mymoorapelta is also one of the few ankylosaurs with 4 sacral ribs, a trait it shares with only 3 other taxa.[5][13]

Life restoration

The caudal (tail) vertebrae of Mymoorapelta and other nodosaurids are longer than they are wide, and dorsoventrally (top-bottom) flattened with a reduction of the neural spines when compared to the anterior caudals. The haemal spine takes on a rounded, hatchet-shaped appearance. The prezygapophyses are short and extend past the anterior edge of the centra by only 25% or less. This trait is only present in nodosaurids and basal anklosaurs, but in taxa like Liaoningosaurus and more derived ankylosaurids they overlap by at least 50% of its length, making them more fused together. Mymoorapelta lacks tail club handles or tail club knobs, as in other nodosaurids, giving them less ankylosis. The transition from anterior to posterior caudal vertebrae is less abrupt compared to ankylosaurids, which have an abrupt transition at the midcaudals, the lack of this abrupt transition would give the tail more flexibility, which is unique to nodosaurids when compared to the strongly fused tails of ankylosaurids.[14] The vertebral centra and neural spines are much shorter when compared to Gargoyleosaurus.[5] Mymoorapelta, like other dinosaurs, preserves chevrons along the undersides of the caudal vertebrae, and just like in many other ankylosaurs, the chevrons are strongly fused to the lower posterior part of the centra, and the haemal arches extend parallel to the elongate vertebral centra. The extension of these arches is greater anteriorly than posteriorly, forming a continuous bony structure underneath the haemal canal. One of the preserved chevrons measures 9.1 centimetres (3.6 in) and is strongly recurved posteriorly.[1][5]

The scapular spine forms a vertical ridge opposite the glenoid in Mymoorapelta, as in other ankylosaurids and in an indeterminate ankylosaur scapula found in the nearby Dry Mesa Quarry. In the pelvic region, Mymoorapelta displays an intermediate between some features of basal and more derived ankylosaurs. The ischium is bent as in other nodosaurids and polacanthines, indicating that a straight ischium is a derived feature in ankylosaurids. In the forelimbs, the ulna has a broad, massive olecranon process that preserves rugosities. The coronoid process develops about 1/3rd of the way down the ulna’s shaft, compared to 1/4th of the way down in taxa like Stegopelta and Polacanthus. In the pes (foot), the genus preserves short, wide metapodials with long phalanges, with the distal phalanges looking triangular and pointed in side view, contrary to other nodosaurids like Edmontonia.[1][5]

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

A prominent feature of Mymoorapelta was its armor, which mostly consisted of large spiked osteoderms (bony armor) 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 other nodosaurids and ankylosaurs. In Mymoorapelta, there are five different preserved armor types: elongated spines with a large, hollow base; thin triangular plates with a narrow, asymmetrical base; small bladelike spines with a rounded, solid base; isolated, flat, keeled scutes; and scutes fused into a single plate of armor. The large spine is assumed to have extended from the base of the neck outward laterally, with the point curving posteriorly.[5][1] Evidence from Borealopelta also supports the extension of the spike by a large keratin sheath.[15] The aforementioned thin triangular plates, of which eight are known, are preserved similarly to those in Polacanthus further down the body.[16][1] It was theorized by Kirkland and Carpenter that the largest of these plates were attached to the proximal part of the tail, and protruded laterally along the sides of the body. A specimen of Mymoorapelta preserves one of these at 11 centimetres (4.3 in) in length.[5][1] A single, small triangular plate was reported in 1994 and it preserves a thin, bladelike morphology, though its position is unknown.[1] Twelve of the previously mentioned keeled, flat scutes were recovered from a specimen of Mymoorapelta in 1994, they preserve a keeled oval shape and an external surface that bears uniform, weak pitting, and a privation of neurovascular grooves (the system that controls blood flow and homeostasis) and foramina (small openings within bones).[1] In the internal structure of the scutes, half of the space is made up of trabecular bone, while the rest is made up of thick external and basal cortices.[17]

Lastly, a small fragment of dermal armor preserves a rosette (a large osteoderm surrounded by smaller osteoderms, forming a flower-like arrangement) formed by a large central scute surrounded on all sides by smaller ossicles.[5][1] This piece of armor comes from the sacral shield, which is 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 centimetres (28 in) in width for a pelvis that is 50 centimetres (20 in) wide.[5][1] The sacral shield also preserves two middle rows of larger scutes, with each row preserving seven.[5] Mymoorapelta preserves a category 2 sacral shield, which is characterized by bosses (keeled osteoderms) that border tubercles (elevated knobs) as well as rosettes of co-ossified osteoderms. This category of sacral shield is mostly found in the "polacanthids",[18][19] but the status of this group is generally unstable, and the trait is also known in some ankylosaurids like Shamosaurus.[20][21][19] The category 2 shield is most similar to that of category 3, though the latter preserves polygonal osteoderms that lack rosettes.[19] However, Victoria Arbour et al. (2011) suggested that category 1 shields could simply be an ontogenetic (growth) stage of category 2, as they have very similar morphology but are not co-ossified, which is an ontogenetic trait present in extant armored animals like armadillos and alligators.[19][22][23] The current categorisation of sacral shields into distinct categories finds its origins in a 2011 paper by Arbour, Michael E. Burns and Philip J. Currie, and gives three distinct characteristics, one for each of the three categories. Category 1 shields are characterized by non-coosified osteoderms, category 2 shields by coossified rosettes, and category 3 by uniformly sized coossified polygons.[24]

On the upper surface of the back and tail, the skin was covered in small osteoderms, which are sometimes referred to as ossicles, in-between larger plates of armor as inferred by Sauropelta,[25] though ossicles themselves are not known confidently outside of the sacral shield in Mymoorapelta.[5][1] The armor of Mymoorapelta differs from that of Gargoyleosaurus in that the former has solid conical armor compared to the thin-walled armor of the latter.[4]

Reconstructed skeleton shown from the side, WDC

Mymoorapelta was a low-browsing herbivore like its relatives, likely feeding on the ferns, cycads, and conifers that dominated the flora of its time due to the lack of complex grasses in the Jurassic.[26] Nodosaurids like Mymoorapelta had narrow snouts, an adaptation seen today in animals that are selective browsers as opposed to the wide muzzles of grazers.[27][12] In ankylosaurs, the Jurassic and mid-Cretaceous forms with narrow and pointed muzzles were apparently the most selective feeders, akin to extant mammalian browsers. This is in stark contrast to the later ankylosaurs that were adapted to bulk feeding on food with lower nutritional value. The preservation of complete a hyoid in taxa like the ankylosaurid Pinacosaurus and the nodosaurid Edmontonia demonstrate that ankylosaurs had fleshy, muscular tongues that could have assisted with feeding on plants that grew low to the ground.[28][12] Tongue protrusion and prehension is not confidently known, but lingual food manipulation could have been used in later ankylosaurids to crop food, like in giraffes. The preservation of cheek plates in Edmontonia and Panoplosaurus provide evidence for fleshy cheeks and chewing, with the cheek covering the tooth rows both for defense and to prevent food loss when eating.[29]

Tooth occlusion is not directly preserved in Mymoorapelta or its close relative Gargoyleosaurus, but it has been found in other nodosaurids from the Cretaceous. The earliest evidence of nodosaurid dental occlusion is in Sauropelta, which demonstrated a basic motion, in that the power strokes moved vertically. In contrast to later nodosaurids, tooth-to-tooth contact was incidental or local when present and a biphasial jaw mechanism was lacking in both Mymoorapelta and Gargoyleosaurus.[12] Ankylosaurs generally demonstrate more variable wear patterns than the contemporary ceratopsids and hadrosaurids of the Cretaceous. Even though ankylosaurs may not have fed on fibrous and woody plants, they may have had a varied diet which included tough leaves and pulpy fruits.[28]

Due to competition with Diplodocid sauropods, Mymoorapelta seems more adapted for open environments.[30] Mymoorapelta has a very limited known distribution, having only been found at two sites in Western Colorado, in addition to the Hanksville-Burpee and potential Albuquerque specimens.[1][5][8][9] This could be due to Mymoorapelta preferring to feed on a specific variety of plant, constricting its possible distribution. The Hanksville-Burpee specimen's taphonomy (how and where it decomposed to end up as a fossil) suggests that the individual had died inland, contrary to the coastal distribution previously inferred for nodosaurids.[8][31][32]

Reconstructions of ankylosaur forelimb musculature made by Coombs in 1978 suggest that the forelimbs bore the majority of the animal's weight and that they were adapted for high force delivery on the front feet, possibly for food gathering. In addition, Coombs suggested that ankylosaurs may have been capable diggers, though the hoof-like structure of the manus would have limited fossorial capibilities. Ankylosaurs were likely to have been slow-moving and sluggish animals,[33][34] though they may have been capable of quick movements when necessary.[35] Mymoorapelta preserves very short limbs with even shorter distal limb elements, which is in contrast to the longer distal limb elements known in other nodosaurids but similar to the condition found in ankylosaurids and Polacanthus.[7]

Reconstructed tail and hip armor, WDC

The scientific literature mostly considers the armor of ankylosauroids to have been used for defensive purposes, mainly to fend off any attacking carnivores.[36] The tails of these animals have a complex evolutionary history, with basal taxa like Mymoorapelta possessing a flexible tail with pointed osteoderms on either side and on top. The aforementioned flexibility of the tail would allow for a greater range of motion than the stiff tails of later ankylosaurids and nodosaurids.[37] It is thought that the tail could also have been used in defense against predators, though more recent findings by Arbour et al. in 2022 also consider the possibility that the tails of ankylosaurids were used for intraspecific combat and might have primarily evolved to fill a function here, which might indicate a similar use in more basal taxa like Mymoorapelta and Gargoyleosaurus.[38]