Panoplosaurus

''Panoplosaurus'' is a genus of armoured dinosaur from the Late Cretaceous of Alberta, Canada. Few specimens of the genus are known, all from the middle Campanian of the Dinosaur Park Formation, roughly 76 to 75 million years ago. It was first discovered in 1917, and named in 1919 by Lawrence Lambe, named for its extensive armour, meaning "well-armoured lizard". ''Panoplosaurus'' has at times been considered the proper name for material otherwise referred to as ''Edmontonia'', complicating its phylogenetic and ecological interpretations, at one point being considered to have existed across Alberta, New Mexico and Texas, with specimens in institutions from Canada and the United States. The skull and skeleton of ''Panoplosaurus'' are similar to its relatives, but have a few significant differences, such as the lumpy form of the skull osteoderms, a completely fused shoulder blade, and regularly shaped plates on its neck and body lacking prominent spines. It was a quadrupedal animal, roughly long and in weight. The skull has a short snout, with a very domed surface, and bony plates directly covering the cheek. The neck had circular groups of plates arranged around the top surface, both the forelimb and hindlimb were about the same length, and the hand may have only included three fingers. Almost the entire surface of the body was covered in plates, osteoderms and 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 stegosaur related to the similarly armoured form ''Ankylosaurus'', a group that was later divided with ankylosaurs becoming their own group. It was then considered close to ''Edmontonia'' in the subfamily Panoplosaurinae, but then moved into a general placement in Nodosauridae. ''Edmontonia'' was for a time considered the same taxon as ''Panoplosaurus'', making it the only nodosaur from the Campanian of North America, but this was quickly disputed and they are now considered separate. Following consistent placements in phylogenetic analyses close to ''Edmontonia'' and the American taxon ''Animantarx'', ''Panoplosaurus'' was placed into the clade Panoplosaurini, related but not close to ''Nodosaurus'' or '' Struthiosaurus'', which it was considered close to around when it was named. ''Panoplosaurus'' is from deposits slightly younger than ''Edmontonia rugosidens'', and existed alongside hadrosaurids like ''Corythosaurus'' and ''Lambeosaurus'', ceratopsids like ''Centrosaurus'', and the tyrannosaurid ''Gorgosaurus'', as well as other small dinosaurs like ''Stegoceras'', ''Dromaeosaurus'' and ''Ornithomimus'', and various fishes, amphibians, crocodiles and pterosaurs.

Discovery

In 1917, Canadian paleontologist Charles M. Sternberg of the Geological Survey of Canada discovered a complete skull and significant amount of the skeleton of an armoured dinosaur in the sandstone layers of Quarry 69 of the Belly River Group, above sea level. The specimen, designated by the Canadian Museum of Nature accession number CMN 2759, and excavated south of the mouth of the Little Sandhill tributary of the Red Deer River in Alberta, includes a nearly complete skull in articulation, most or all of the cervical vertebrae and the front dorsal vertebrae, and armour plates covering them, a majority of the disarticulated forelimb and three articulated fingers, a fragment of the pelvis and partial sacrum, a few bones of the foot, and multiple hundreds of osteoderms and dermal ossicles. This material was being described by Canadian paleontologist Lawrence M. Lambe of the Geological Society, who completed the description of the skull and osteoderms prior to his death in March of 1919. Lambe's work was published posthumously by the Royal Society of Canada, where he named the new material as ''Panoplosaurus mirus'', and a supplemental description of the vertebrae and limb material, which Lambe had not yet gotten to, was published by Sternberg in 1921. The name ''Panoplosaurus'' derived from the hoplon of Greece, translating as "well -armoured lizard".

While the beds of discovery of the holotype, CMN 2759, were originally described as the Belly River beds, they are now considered part of the upper level of the Dinosaur Park Formation, pertaining to the late middle Campanian, 75 to 76 million years ago. Many additional specimens have since been referred to the genus, including Royal Ontario Museum (ROM) 1215 and Royal Tyrrell Museum of Paleontology (RTMP) 83.25.2 from the Dinosaur Park Formation of Alberta, and an unnumbered Oklahoma Museum of Natural History (OMNH) scapulocoracoid from the Aguja Formation of Texas. A scapula from the Naashoibito member of the Kirtland Formation in New Mexico was referred to ''Panoplosaurus'' in 1981, but as it is from a different age and location from other specimens, may instead represent the ankylosaurid taxon ''Nodocephalosaurus'', although this is uncertain due to lack of overlapping material. 18 other specimens in the ROM, CMN, American Museum of Natural History, and Princeton University were referred to the genus by Walter P. Coombs in 1978, but these were referred on the assumption that ''Edmontonia'' was a synonym of ''Panoplosaurus'', which was considered unjustified by Kenneth Carpenter in a 1990 review on Late Cretaceous nodosaurids. The referral of ROM 1215 to ''Panoplosaurus'' was questioned by Coombs in 1990, and Roland A. Gangloff referred it and the Alaskan specimen DPMWA 90–25 to ''Edmontonia'' in 1995 based on the anatomy of the lack of diagnostic traits, but Victoria Arbour and colleagues retained ROM 1215 in ''Panoplosaurus'' in 2009, limiting the genus to it, the holotype CMN 2759, and specimen AMNH 3072, all from the upper Dinosaur Park Formation. Arbour and Philip J. Currie further restricted ''Panoplosaurus'' to just the holotype in 2016, as a result of uncertainties surrounding the referrals of various specimens between it, ''Edmontonia rugosidens'' and ''Edmontonia longiceps'', all from the Dinosaur Park Formation and similar deposits.

Description

''Panoplosaurus'' was a rather large animal at , a comparable size to other ankylosaurs from the same location, and heavier than or approximately equivalent to all predators it coexisted with. ''Panoplosaurus'' was about long, and was armoured like most ankylosaurs, but lacked prominent spikes anywhere on its body. The characteristics of its armour are one of the features that distinguishes ''Panoplosaurus'' from its relative and contemporary ''Edmontonia'', along with a tapering snout, lumpy cranial armour, a swollen vomer bone in the palate, tall neural arches and neural spines in the vertebrae, a small and round coracoid fused to the scapula, and a hand that may have only had three digits. There is also a prominent armour plate covering the cheek in the type specimen of ''Panoplosaurus'', which may be unique feature of the taxon, or individual, depending on what additional skulls are referred to ''P. mirus''.

Skull

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 in a straight line, the skull is curved in a way that across the the same length is . Behind the , the skull reaches its maximum width of , giving it a very short and broad appearance. As other nodosaurid specimens from the same location that may be referred to ''Panoplosaurus'' lack the distinctively short snout of the holotype, it may be that its unique appearance is due to it being a different age or sex than other individuals. The orbits of ''Panoplosaurus'' are very small, and placed far from the snout, which is very squared and bearing laterally open nostrils. Bony plates are present across the entire exterior of the skull, including large osteoderms on all the sides of the skull and mandible, small scutes within the nostrils, and underneath the head in the form of an aggregation of small ossicles covering the surface between the two lower jaws. The , 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. 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.

Due to fusion and the covering of osteoderms, the only individual bone of the long mandible than can be identified is the . 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 , 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 in the upper jaw. Both the predentaries and their premaxillary counterparts lack any teeth, a derived feature among nodosaurids where premaxillary teeth are sometimes present. 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 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. All the teeth are similar to those of ''Edmontonia'' and ''Palaeoscincus'', with a mild expansion of the crown above the root (), and 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.

Postcranial skeleton

The number of vertebrae in ''Panoplosaurus'' is unknown, as the dorsal and caudal series' are incomplete, and the cervical vertebrae are obscured by armour. The first three cervicals after the skull are fused together into one single ossification, a feature unique to ''Panoplosaurus'' and ''Edmontonia'' among ankylosaurs. Following the fused atlas-axis-third cervical complex, the next three cervicals are all identifiable, but very morphologically conservative, without significant differences along the series, all vertebrae being . Dorsal vertebrae are poorly preserved where present, but the , but the synsacrum, as in other nodosaurids, is partially complete. Sternberg identified that the ilium of ''Panoplosaurus'' was supported by at least six vertebrae in 1921, which he identified as a dorsosacral, four true sacral vertebrae, and a caudosacral. 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. 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''.

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 that projects outwards from the animal, and directly overhangs a large attachment area for the deltoid muscle. The scapula itself is long, and the coracoid is . The complete fusion of the scapulocoracoid is unique to ''Panoplosaurus''. The humerus is a robust, 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. The was splayed, with two on the first digit, and three on the second and third. All bones were hoof-shaped, lacking a point and bearing a flat bottom.

The only portions of the pelvis and hindlimb that are known in ''Panoplosaurus'' are the tibia, fibula, and a few isolated bones from the foot, the remainder being unpreserved. The right tibia and fibula were found still in articulation, with the tibia being noticeably longer than the fibula. As in ankylosaurids, the tibia of ''Panoplosaurus'' was shorter than the humerus, which contrasts with ''Stegosaurus'' and basal taxa like ''Scelidosaurus''. This suggests that the forelimb of ''Panoplosaurus'' was only slightly shorter than the forelimbs. Both the metatarsal and phalanges of the foot resemble the corresponding elements of the hand, but are slightly larger and more robust. 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.

Armour

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. 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.

Classification

''Panoplosaurus'' was originally named simply as a genus of armoured dinosaur by Lambe in 1919, within the group Stegosauria, though it was subsequently specified by Sternberg in 1921 that within Stegosauria it was closest to taxa like ''Ankylosaurus'' and ''Euoplocephalus'', and so within Ankylosauridae. The description of the new genus ''Edmontonia'' in 1928 by Sternberg identified it as a very close relative of ''Panoplosaurus'', almost close enough to be considered the same genus, both being closely related to ''Palaeoscincus''. Following further description of the armoured taxon '' Struthiosaurus'' by Franz Nopcsa in 1929, a different classification war proposed. Thyreophora replaced Stegosauria as the clade encompassing multiple families, with ''Panoplosaurus'' falling alongside ''Edmontonia'', ''Dyoplosaurus'', ''Hierosaurus'', ''Scolosaurus'' and ''Stegopelta'' within the new subfamily Panoplosaurinae, which formed the family Nodosauridae with Nodosaurinae, Ankylosaurinae and Acanthopholinae. Nopcsa's classification of Panoplosaurinae was amended slightly by Charles W. Gilmore in 1930, who moved ''Palaeoscincus'' into the subfamily and removed ''Dyoplosaurus'', which was discovered to have the skull of an ankylosaurine. ''Panoplosaurus'' was then placed in the new subfamily Edmontoniinae by Loris S. Russell in 1940, along with ''Edmontonia'' and ''Palaeoscincus'', an equivalent classification to Gilmore's where Panoplosaurinae was used. Disagreeing with the classifications based on the work of Nopcsa, Evgeny Maleev moved ''Panoplosaurus'' into the family Ankylosauridae, which he considered to contain all ankylosaurs except '' Syrmosaurus''.

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. Within this suborder, he divided all taxa into two families, Ankylosauridae and Nodosauridae, distinguished by cranial and postcranial anatomy. Within Nodosauridae, which he considered a senior synonym of Acanthopholinae, Panoplosaurinae, Edmontoniinae, and other equivalent names, Coombs synonymized ''Panoplosaurus'' with ''Edmontonia'', making ''Panoplosaurus'' the only Late Cretaceous nodosaurid from North America. Panoplosaurinae was then revived in 1986 by Paul Sereno, who used it to unite ''Panoplosaurus'', ''Sauropelta'', ''Silvisaurus'' and '' Struthiosaurus'' within Nodosauridae, excluding ''Hylaeosaurus'' and ''Polacanthus''. Robert Bakker then redescribed specimens from the Late Cretaceous of North America in 1988, prompting him to separate ''Edmontonia rugosidens'' into the new subgenus '' Chassternbergia'', remove a specimen from ''Edmontonia longiceps'' making it the new genus ''Denversaurus'', and identify a former specimen of ''Panoplosaurus mirus'' as ''P.'' sp.. To distinguish the ''Panoplosaurus'' species from the ''Edmontonia'' complex, Bakker placed ''Panoplosaurus'' in Panoplosaurinae, and ''Edmontonia'', ''Chassternbergia'' and ''Denversaurus'' in Edmontoniinae, uniting the two subfamilies into the family Edmontoniidae, which he placed in Nodosauroidea, the family Nodosauridae of Coombs' use elevated to superfamily rank. Carpenter further reviewed Late Cretaceous North American nodosaurids in 1990, concluding that Bakker's justification for separation was lacking; placing ''Chassternbergia'' and ''Denversaurus'' as junior synonyms of ''Edmontonia'', and only narrowing down the classification of ''Panoplosaurus'' and ''Edmontonia'' to Nodosauridae. In 1998, Sereno modified his earlier position, using the name Nodosaurinae for the group of derived nodosaurids including ''Panoplosaurus'', and informally defining the subfamily as all taxa closer to ''Panoplosaurus'' than ''Sarcolestes'' or ''Hylaeosaurus''. Tracy Ford published a novel classification of ''Panoplosaurus'' in 2000, using osteoderm anatomy to divide Nodosauridae into Nodosaurinae including only ''Nodosaurus'', Sauropeltinae including ''Sauropelta'' and ''Silvisaurus'', Edmontoniinae including ''Edmontonia'' and a distinct ''Chassternbergia'', and Panoplosaurinae including only ''Panoplosaurus''.

The first phylogenetic analysis to include ''Panoplosaurus'' was that of Yuong-Nam Lee in 1996, where ''Panoplosaurus mirus'', including both CMN 2759 and ROM 1215 nested next to ''Edmontonia'', which included both ''E. longiceps'' and ''E. rugosidens'', to the exclusion of all other nodosaurids. A similar union of ''Panoplosaurus'' and ''Edmontonia'' was recovered in 1998 by Jim Kirkland and colleagues, but the 2001 analysis of Carpenter instead recovered ''Edmontonia'' closest to ''Animantarx'', while ''Panoplosaurus'' nested next to a clade of ''Sauropelta'' and ''Silvisaurus''. The 2003 analysis of Robert Hill and colleagues resolved a group of ''Edmontonia'' and ''Animantarx'', with ''Panoplosaurus'' as the sister taxon, and the analysis of Matthew Vickaryous ''et al.'' recovered ''Panoplosaurus'' and ''Edmontonia'' as sisters, though ''Animantarx'' was not analysed. Expansions on the analysis of Vickaryous and colleagues resolved the same clade of ''Panoplosaurus'' and ''Edmontonia'' with the additions of the nodosaurid ''Hungarosaurus'', the ankylosaurid '' Crichtonsaurus benxiensis'', the nodosaurid ''Struthiosaurus'', and the supposed ankylosaurid '' Tatankacephalus''. 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''. 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. 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 an unnamed ankylosaur from Argentina. 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. The reference phylogeny for Panoplosaurini designated by Madzia and colleagues was that of Rivera-Sylva ''et al.'' (2018), which is a modified version of the Arbour and Currie analysis expanded to include the Mexican taxon ''Acantholipan''. Their results can be seen below.

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 iguanids. However, unlike iguanids ankylosaurs frequently have significant tooth wear, sometimes all the way down to the base of the crown. Nodosaurids normally have oblique wear facets on teeth, in ''Panoplosaurus'' having a single wear facet per tooth which changes angulation across the tooth row, going from sub-vertical at the snout to nearly horizontal at the back of the mouth. This differs from ankylosaurids, but may be due to the alignment of teeth between the upper and lower jaws, instead of a difference in form of mastication. While the shape of teeth in ''Panoplosaurus'' and other ankylosaurs suggests a simple, soft food diet, their variability implies a less restrictive feeding strategy. Nodosaurid teeth are more blade-shaped than those of ankylosaurids, a possible evolutionary response to a tougher, fibrous diet. However, microwear on the teeth of ankylosaurids and ''Panoplosaurus'' instead show that their diets were not significantly different, with regular pitting and abrasion showing a diet consisting occasionally of fruit as well as more abundant foliage. Stomach contents of the closely related taxon ''Borealopelta'' were identified amongst 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. 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.

Airways and senses

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''. 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 , 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%.

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. 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.

Paleoecology

The Dinosaur Park Formation deposits are a layer of sediments exposed in the badlands of Alberta, lying on top of the older Oldman Formation and underneath the younger Bearpaw Formation. ''Panoplosaurus'' was found about above boundary with the Oldman Formation, which is 76.5 million years old, within the "sandy zone" of the formation deposited by alluvial paleochannels. All the deposits of the Dinosaur Park Formation were lain during the beginning of the last significant sea level rise of the Western Interior Seaway. Sediments deposited during in the formation show that the alluvial channels were flowing to the east or southeast, towards the Bearpaw Sea of the Western Interior Seaway, creating meandering paths as the flow or water down and up during tidal cycles across the floodplain was balancing out. The climate of the environment in the Cretaceous was much different than in present day, being warmer and more temperate, with wet and dry seasons allowing for a large variety of flora and fauna. Periodic areas of standing water would occur in the low-lying plains. Where there was dense vegetation, conifers were the dominant plants, with an understory composed of ferns and tree ferns, and flowering herbs and shrubs. The forest floor was covered by decaying vegetative matter, small ferns, lycopods and angiosperms, mosses, lichen, with plentiful algae where there was standing water.

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. In the lower Dinosaur Park Formation, assemblages of crevasse sites show that mollusks were commonly dominated by the freshwater clam ''Sphaerium'', which occurred with abundant gastropods of the genera ''Goniobasis'' and '' Lioplacodes''. This sphaeriid-dominated diversity was present during all of the duration of the formation and its over- and underlying deposits, but the mussel and brackish-water assemblages that can be seen in older and younger layers are absent in the range of above the Dinosaur Park Formation-Oldman Formation contact. A variety of forms of fish are present in the fluvial beds of the Dinosaur Park Formation, including chondrichthyans, teleosts and other ray-finned fishes. The ray ''Myledaphus'' is characteristic of the formation and similar deposits, and lived alongside the less common shark '' Hybodus montanensis'', intermediate paddlefish and sturgeons, the long, slender fish '' Belonostomus'', the gar ''Lepisosteus'', bowfins, and an abundance of small teleosts including '' Paratarpon'' and '' Cretophareodus''. At least nine forms of amphibians were present in the Dinosaur Park Formation, including the salamander-like '' Albanerpeton'', two unnamed forms of frog, and salamanders from the genera '' Scapherpeton'', '' Lisserpeton'', ''Opisthotriton'', ''Habrosaurus'', and two or three unnamed forms. Turtles are commonly well-preserved in the formation, with many taxa known from many specimens. Three genera in the primitive turtle family Baenidae are known, '' Plesiobaena'', '' Boremys'' and ''Neurankylus''. The low-domed macrobaenid '' Judithemys'' was named for a nearly complete skeleton and skull from the deposits, an unnamed but new form of snapping turtle is known, ''Adocus'' and '' Basilemys'' represent the diversity of primitive trionychoids, and the softshelled turtles ''Apalone'', '' Aspideretoides'', and a new form, are also present. Fragmentary material from the choristodere ''Cteniogenys'', and many well-preserved skulls and skeletons of its relative ''Champsosaurus'', is present among the fauna of the Dinosaur Park Formation, representing an extinct group of semi-aquatic animals with crocodilian features that spanned the globe for much of the Mesozoic and Cenozoic. Mosasaurs and plesiosaurs are both known from the Dinosaur Park Formation, though mosasaurs are only represented by the teeth of ''Plioplatecarpus'' from the upper deposits, and apart from the relatively complete elasmosaurid '' Fluvionectes'', plesiosaurs are only known from isolated and incomplete elements. Two or three true crocodilians are known, including the named genera ''Leidyosuchus'' and ''Albertochampsa''.

A rich and diverse vertebrate assemblage is known from the Dinosaur Park Formation, with the lower region, excluding the Lethbridge Coal Zone, being formed by terrestrial and coastal deposits. Microfossil sites are common, but a taphonomic bias is present in the formation towards the better preservation of large-bodied animals like ''Panoplosaurus'', which is 63% complete. Six small lizards are known, representing five different families, with the teiids '' Socognathus'' and '' Glyptogenys'', the xenosaurid '' Exostinus'', the helodermatid '' Labrodioctes'', the necrosaurid '' Parasaniwa'', and the varanid '' Palaeosaniwa''. No snakes are known, which, while it contrasts with the modern diversity of the group, does correspond with the rarity of the group during the Cretaceous. Two pterosaurs have been found, the azhdarchid ''Cryodrakon'' and an unnamed pterodactyloid. All three major types of mammal present in the Cretaceous have been found in the Dinosaur Park Formation, Multituberculata, Marsupialia and Placentalia. The named multituberculates include ''Cimexomys'', ''Cimolodon'', ''Cimolomys'', '' Meniscoessus'', and ''Mesodma''. Marsupials are represented by the taxa ''Alphadon'', ''Eodelphis'', '' Pediomys'', and '' Turgidodon'', and placentals by ''Cimolestes'', ''Gypsonictops'', and '' Paranyctoides'', though all the material represents isolated partial skull or skeletal bones, or isolated teeth. While no other dinosaurs were recovered at the same sites as any ''Panoplosaurus'' specimens, correlation and comparisons of localities throughout the Dinosaur Park Formation show it coexisted with a large variety of animals. The herbivorous fauna of the Dinosaur Park Formation can be divided into two Megafaunal Assemblage Zones, defined as the lowest directly above the Oldman Formation, and the deposits above those, where the common ceratopsid ''Centrosaurus'' and hadrosaurid ''Parasaurolophus'' are absent. ''Panoplosaurus'', which was found above the contact, is from layers just younger than those bearing ''Edmontonia'', but still corresponding to the upper portion of the older MAZ-1, and would have coexisted with the ankylosaurid ''Euoplocephalus'', ceratopsids ''Centrosaurus'' and ''Chasmosaurus belli'', and hadrosaurids '' Corythosaurus intermedius'', ''Lambeosaurus lambei'', '' Lambeosaurus clavinitialis'', and ''Parasaurolophus''. As well as herbivorous megafauna, unnamed ornithopods, the primitive ceratopsian ''Unescoceratops'' and the pachycephalosaurians ''Stegoceras validum'', '' Stegoceras sternbergi'' and '' Stegoceras brevis'' are known from the Dinosaur Park Formation, though their stratigraphic correlations are uncertain. Among theropods, the dromaeosaurids ''Dromaeosaurus'' and ''Saurornitholestes'' are known, the former from a single specimen of uncertain provenance, and the latter from many specimens such as teeth throughout the entire formation, and troodontids from the MAZ-1 are limited to ''Stenonychosaurus''. Three genera of oviraptorosaurs are known, all of which likely coexisted in the Dinosaur Park Formation, ''Caenagnathus'', ''Chirostenotes'' and ''Citipes''. It is possible that a therizinosaur was present based on a single frontal bone, and three ornithomimosaurs are preserved in the MAZ-1, '' Rativates'', ''Ornithomimus'', and ''Struthiomimus''. The only large theropod to coexist with ''Panoplosaurus'' was the tyrannosaurid ''Gorgosaurus'', although an unnamed species of ''Daspletosaurus'' was present in deposits slightly younger than known for ''Panoplosaurus''.

References

See also

* Timeline of ankylosaur research

{{Taxonbar, from=Q131652

Nodosaurids
Late Cretaceous dinosaurs of North America
Fossil taxa described in 1919
Taxa named by Lawrence Lambe
Dinosaur Park fauna
Paleontology in Alberta
Campanian genus first appearances
Campanian genus extinctions
Ornithischian genera