Panoplosaurus

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Panoplosaurus
Temporal range:
Ma
↓
Skull of holotype, CMN 2759
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: †Ornithischia
Clade: †Thyreophora
Clade: †Ankylosauria
Family: †Nodosauridae
Subfamily: †Nodosaurinae
Clade: †Panoplosaurini
Genus: †Panoplosaurus
Lambe, 1919[1]
Species:
†P. mirus
Binomial name
†Panoplosaurus mirus
Lambe, 1919[1]

Panoplosaurus is a

scutes
of varying sizes, ranging from large elements along the skull and neck, to smaller, round bones underneath the chin and body, to small ossicles that filled in the spaces between other, larger osteoderms.

Panoplosaurus was originally classified as a

tyrannosaurid Gorgosaurus, as well as other small dinosaurs like Stegoceras, Dromaeosaurus and Ornithomimus, and various fishes, amphibians, crocodiles and pterosaurs
.

Discovery

Outcrops of the Dinosaur Park Formation along the Red Deer River

In

hoplon of Greece, translating as "well -armoured lizard".[4]

Left side view of holotype on display at Canadian Museum of Nature

While the beds of discovery of the

Edmontonia longiceps, all from the Dinosaur Park Formation and similar deposits.[10]

Description

Life restoration

Panoplosaurus was a rather large animal at 1,600 kg (3,500 lb), a comparable size to other ankylosaurs from the same location, and heavier than or approximately equivalent to all predators it coexisted with.

neural spines in the vertebrae, a small and round coracoid fused to the scapula, and a hand that may have only had three digits.[2] There is also a prominent armour plate covering the cheek in the type specimen of Panoplosaurus, which may be unique feature of the taxon,[12] or individual, depending on what additional skulls are referred to P. mirus.[2]

Skull

Skull of Panoplosaurus mirus CMN 2759 in dorsal, ventral, and anterior views

The skull of Panoplosaurus is broad and depressed, narrowing towards the end of the blunt snout to form a triangular shape. At a total length of 335 mm (13.2 in) in a straight line, the skull is curved in a way that across the

ossicles covering the surface between the two lower jaws. The occipital, where the skull articulates with the neck, is short and thick, facing nearly directly downwards, which would have meant the head was held with the snout down in life, about 20 degrees below the horizontal.[1] Unlike in Edmontonia, the groove separating cranial osteoderms in Panoplosaurus never disappear, which show that there is a unique narrow scute across the entire rear of the skull.[2]

Due to fusion and the covering of osteoderms, the only individual bone of the 310 mm (12 in) long mandible than can be identified is the predentary. The rami diverge strongly towards the read of the skull, where they curve inwards at the jaw articulation, and towards the front they bend inwards slightly where the predentary articulates with the dentary, with the thin predentaries meeting at the midline of the jaw. The mandibles are deepest near the rear of the skull, approximately maintaining their depth along the tooth row before narrowing sharply at the front where the predentaries are. Predentaries, which make up the lower portion of the snout, are somewhat horseshoe-shaped, form a sharp beak that fits within the overhang of the premaxillaries in the upper jaw.[1] Both the predentaries and their premaxillary counterparts lack any teeth, a derived feature among nodosaurids where premaxillary teeth are sometimes present.[7] The teeth of the mandible are hidden by armour on the right side, and on the left side of the skull where the cheek plate is not in place, the mandibular teeth are hidden by the teeth of the maxilla in the upper jaw. Eight maxillary teeth are preserved, and though it is not certain that was the full tooth count there is not room for many more in the jaw.[1] All the teeth are similar to those of Edmontonia and Palaeoscincus, with a mild expansion of the crown above the root (cingulum),[2] and denticles formed by prominent ridges on both the front and rear edges of the crown, though there are more on the front than rear edge of the tooth. There are not significant differences between the teeth of Panoplosaurus and those of other armoured dinosaurs.[1]

Postcranial skeleton

Scapulocoracoid, humerus, and tibia and fibula of Panoplosaurus holotype CMN 2759

The number of vertebrae in Panoplosaurus is unknown, as the

sacral vertebrae, and a caudosacral.[3] The identification of four sacrals was questioned by Carpenter in 1990, as Edmontonia only has three true sacrals, but he was not able to definitively identify the first sacral of Sternberg as a dorsosacral.[2] The neural spines of all four sacrals are fused into a single plate-like process. Few caudals are known in Panoplosaurus, but where preserved they are very similar to the corresponding elements of Ankylosaurus.[3]

In Panoplosaurus the scapula and coracoid are completely fused together, with the only indication of the bone separation being a slight thickening along what would be the suture. The scapular region of the bone is relatively short, but is concave following the curvature of the body and curves down towards its distal end. Because of the curvature of the blade, the coracoid in Panoplosaurus sat at the front of the chest, as in Stegosaurus, Triceratops and Hadrosaurus. The top margin of the scapula forms a shelf as it approaches the coracoid, terminating in a prominent acromion process that projects outwards from the animal, and directly overhangs a large rugose attachment area for the deltoid muscle. The scapula itself is 410 mm (16 in) long, and the coracoid is 255 mm (10.0 in).[3] The complete fusion of the scapulocoracoid is unique to Panoplosaurus.[2] The humerus is a robust, 430 mm (17 in) long bone, with the shape and projection of the head suggesting the upper arm of Panoplosaurus was held in a flexed position in life. The only other portions of the forelimb known are three well-preserved, articulated digits of the hand, which may represent the complete hand suggesting Panoplosaurus had fewer fingers than its relatives.[2][3] The manus was splayed, with two phalanges on the first digit, and three on the second and third. All ungual bones were hoof-shaped, lacking a point and bearing a flat bottom.[3]

The only portions of the

metatarsal and phalanges of the foot resemble the corresponding elements of the hand, but are slightly larger and more robust.[3] Though the tibia and fibula of Panoplosaurus are crushed, they appear to be straighter than the elements in Edmontonia, which are curved along their length.[2]

Armour

Osteoderms and arrangement of neck osteoderms of Panoplosaurus mirus

Armour from the neck and trunk of Panoplosaurus, some of it still in articulation with bones, is known. A gradient of sizes exist from plate-like paired elements through to indistinct ossicles. Where they have a distinct shape, the osteoderms are keeled, with the strength of the keel dependent on location. Lambe identified 7 different categories of osteoderms in his 1919 description of Panoplosaurus. The first kind was large, paired elements with a low keel, which formed bands around the neck leading from the head. On the sides of the neck to the back was a second kind, individual elements that were slightly smaller, suboval, and had a strong keel. Small, keeled scutes with a thick base were identified as the fourth osteoderm type, occurring on the underside of the base of the neck forwards to the chin. A fifth kind of osteoderms was identified as small, polygonal elements that fit together along the underside of Panoplosaurus, slowly grading into the larger rectangular elements of category two on the sides. Small irregular scutes lacking a keel were identified as a sixth form, and were suggested to have been from the limbs, though this was not definitive. The final form of scute were small ossicles, which occurred all over the animal filling in gaps between the larger osteoderms.[1] The scutes along the top and sides of the neck in Panoplosaurus are the most distinct form, differing significantly from the corresponding elements in Edmontonia. Three bands of cervical osteoderms were present in both genera, consisting of rounder plates that united on the midline of the animal, and one narrower element on each side with a sharp keel. In Panoplosaurus both the first and second bands of neck osteoderms had a third pair, lower on the side of the animal, again possessing a sharper keel than the elements on the top of the neck. While Edmontonia possesses lateral spines on the rear neck and shoulders, these are absent in Panoplosaurus. The arrangement of the armour on the torso and tail of Panoplosaurus is unknown, as no elements were found in articulation or association with this region of the skeleton.[2]

Classification

Panoplosaurus was originally named simply as a genus of armoured dinosaur by Lambe in 1919, within the group

Syrmosaurus.[18]

RTMP
, at times considered a species of Panoplosaurus

Coombs reviewed and revised the classifications of Ankylosauria in 1978, which he used as the group to encompass all heavily armoured ornithischians in a similar usage to Nodosauridae of Nopcsa.

Sauropeltinae including Sauropelta and Silvisaurus, Edmontoniinae including Edmontonia and a distinct Chassternbergia, and Panoplosaurinae including only Panoplosaurus.[22]

The first phylogenetic analysis to include Panoplosaurus was that of Yuong-Nam Lee in

Crichtonsaurus benxiensis,[30] the nodosaurid Struthiosaurus,[31] and the supposed ankylosaurid Tatankacephalus.[32] The next novel analysis was that of Richard Thompson and colleagues in 2012, combining previous ankylosaurian analyses into a single one to analyse both Nodosauridae and Ankylosauridae. Panoplosaurus was resolved next to Edmontonia, deep within an unresolved polytomy of all nodosaurids more derived than Animantarx, which included Niobrarasaurus, Nodosaurus, Pawpawsaurus, Sauropelta, Silvisaurus, Stegopelta, and Texasetes.[23] In 2016, the phylogenetic analysis of Arbour and Currie initially meant to test the relationships of Ankylosauridae was expanded to include many of the nodosaurids known at the time, with Panoplosaurus limited to the holotype due to a lack of consensus about referred specimens.[10][33] Following further modifications and expansions, Panoplosaurus was continually found within a group including Edmontonia and also at times Animantarx, Texasetes, Denversaurus (if considered separate from Edmontonia) and ‘’Patagopelta’’. As a result of this consistent support, Daniel Madzia and colleagues decided to name the clade uniting all taxa closer to Panoplosaurus than Nodosaurus or Struthiosaurus, giving it the formal name Panoplosaurini, modifying the suffix -inae from previous uses as it was continually nested within the clade Nodosaurinae.[34] The reference phylogeny for Panoplosaurini designated by Madzia and colleagues was that of Rivera-Sylva et al. (2018),[34] which is a modified version of the Arbour and Currie analysis expanded to include the Mexican taxon Acantholipan. Their results can be seen below.[35]

Mounted skeletons of Denversaurus and Tyrannosaurus, Houston Museum of Natural Science
Nodosaurinae

Paleobiology

Feeding

Ankylosaurs were traditionally viewed as having a generic method of feeding due to their simple teeth, stiff skulls, and unspecialized musculature, comparable to modern

gastroliths, showing that at least it had a diet regularly consisting of almost 85% fern material, along with 3.7% cycad matter, trace elements of conifers, and 11.4% undiagnostic plant remains.[37] Gastroliths may have been found with Panoplosaurus as well, but their identification is uncertain as they were not originally mentioned among the material found as part of the specimen.[36]

Airways and senses

Skulls of Panoplosaurus (left) and Euoplocephalus (right) and their respective sinuses

While nodosaurids were traditionally thought to have had simply sinuses, lacking complicated cavities and paranasal sinuses. While this can be seen in some taxa like Edmontonia, the nasal system of Panoplosaurus is far more complex than previously thought. The complete nasal passage of Panoplosaurus undergoes two complete 360 degree loops in different planes along its length, before entering the olfactory recess for scent processing. However, while the shape of the nasal passages is more complicated, Panoplosaurus does lack the additional parallel sinus tracts that can be found in ankylosaurids like Euoplocephalus.[38] It is possible that the function of these complicated sinuses was to warm incoming air as it passed through the skull. Inflowing air in Panoplosaurus was simulated to undergo a raise of between 17.9 and 18.2 °C (64.2 and 64.8 °F), primarily in the elongate nasal passage, while saturating the air with moisture. This is less heat efficient than the more complicated sinuses of Euoplocephalus, but still shows that the sinus cavities of Panoplosaurus increased the recovery of lost heat and moisture by over 60%.[39]

The brain of Panoplosaurus takes up 33% of the length of the skull, similar to the nodosaurid Pawpawsaurus where the value is 30%, a higher value than in ankylosaurids. Panoplosaurus had a similar sense of smell to both Pawpawsaurus and Euoplocephalus, with the ratio between the length of the olfactory bulb and cerebral hemisphere being 44.0, 46.2 and 54.0 respectively.[40] However, the size of the region of the brain devoted to the sense of smell is smaller in Panoplosaurus than expected for an animal of its size.[41]

Paleoecology

Map of North America and the Western Interior Seaway 75 mya

The Dinosaur Park Formation deposits are a 70 m (230 ft) layer of sediments exposed in the badlands of Alberta, lying on top of the older

fungi, with plentiful algae where there was standing water.[44]

The constant presence of water in the Dinosaur Park Formation led many forms of freshwater or marine animals to enter the otherwise predominantly terrestrial ecosystem.

elasmosaurid Fluvionectes, plesiosaurs are only known from isolated and incomplete elements.[45][51][52] Two or three true crocodilians are known, including the named genera Leidyosuchus and Albertochampsa.[53]

Depiction of megaherbivores in the Dinosaur Park Formation, Panoplosaurus on the far right

A rich and diverse vertebrate assemblage is known from the Dinosaur Park Formation, with the lower region, excluding the

tyrannosaurid Gorgosaurus, although an unnamed species of Daspletosaurus was present in deposits slightly younger than known for Panoplosaurus.[5]

References

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See also
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