| Scolosaurus | |
|---|---|
 | |
| Skeletal mount of theS. thronusholotype at theRoyal Tyrell Museum | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Ankylosauria |
| Family: | †Ankylosauridae |
| Subfamily: | †Ankylosaurinae |
| Tribe: | †Ankylosaurini |
| Genus: | †Scolosaurus Nopcsa,1928 |
| Type species | |
| †Scolosaurus cutleri | |
| Otherspecies | |
| Synonyms | |
| |
Scolosaurus is anextinctgenus ofankylosauriddinosaurs within the subfamilyAnkylosaurinae. It is known from the lower levels of theDinosaur Park Formation and upper levels of theOldman Formation in theLate Cretaceous (latest middleCampanian stage, about 76.5 Ma ago) ofAlberta,Canada. It contains twospecies,S. cutleri andS. thronus.[2][1] The type species,S. cutleri, measured up to 5.6 metres (18 ft) in length and 2.2 metric tons (2.4 short tons) in body mass.[3]
Scolosaurus was named byFranz Nopcsa von Felső-Szilvás in1928, based onholotypeNHMUK PV R.5161, a nearly complete specimen that preserves the entire skeleton except for the distal end of the tail, the right forelimb, the right hindlimb, and the skull. The rare preservation ofosteoderms and skin impression are also present. The fossil skeleton was discovered byWilliam Edmund Cutler, an independent fossil collector in 1914 at Quarry 80 of theDeadlodge Canyon locality.[2][4] It was collected from the bottom of theDinosaur Park Formation in fine-grained sandstone and fine-grained claystone sediments that were deposited during theCampanianstage of the LateCretaceous period, approximately 76.5 million years ago.[5] The holotype specimen is housed in the collection of theNatural History Museum inLondon, England.
In 2013,Arbour and Currie reassigned specimen MOR 433, upon which the genusOohkotokia was based, toScolosaurus. This specimen consists of a partial skull, both humeri, a caudal vertebra and several osteoderms and was recovered in the Upper Member of theTwo Medicine Formation, inMontana, which has been dated at approximately 74 million years.[6] The remains were collected in 1986-1987 in grey siltstone that was deposited during theCampanian stage of theCretaceous period.[5] The specimen is housed in the collection of theMuseum of the Rockies inBozeman, Montana.
Thegeneric nameScolosaurus means "pointed stake lizard" and is derived from theGreek wordsskolos (σκῶλος) meaning "pointed stake", andsaûros (σαύρα) meaning "lizard".[7] Thespecific name,cutleri, honours its discoverer and the collector of theholotype, W. E. Cutler,[4] who was seriously injured when the specimen fell on him as he was excavating it.[8]
In 1928, Nopcsa assigned the specimen to the familyAnkylosauridae, and drew morphological comparisons with the fossil material known fromDyoplosaurus. In 1971,Walter Coombs concluded that there was only one species of ankylosaurid during theCampanian age of the Late Cretaceous of North America. He synonymized the speciesAnodontosaurus lambei,Dyoplosaurus acutosquameus, andScolosaurus cutleri withEuoplocephalus tutus but did not provide any justification for these synonymies.[9] The synonymization ofScolosaurus cutleri andEuoplocephalus tutus was generally accepted and thus NHMUK R.5161 was assigned toE. tutus. However, a redescription ofScolosaurus published in 2013 in theCanadian Journal of Earth Sciences by Paul Penkalski and William T. Blows suggested that the genus is a valid taxon. They concluded thatScolosaurus can be distinguished fromEuoplocephalus by theform of theircervical armour, the details of otherarmour and the structure of the forelimb. They also concluded thatScolosaurus andDyoplosaurus are distinct, due to differences noted in thepelvis and armour.[2] Due to its completeness, the holotype ofScolosaurus has formed the basis for mostEuoplocephalus reconstructions since 1971; therefore, most images ofEuoplocephalus actually depictScolosaurus instead.
A 2013 study found that the ankylosaurineOohkotokia was indistinguishable fromScolosaurus, and was therefore considered ajunior synonym.[10] However, this synonymization is contentious asOohkotokia was subsequently recognized as valid.[11] Thus, much of the material illustrated as belonging toScolosaurus may actually pertain toOohkotokia.
The followingcladogram is based on a 2015phylogenetic analysis of the Ankylosaurinae conducted by Victoria Arbour and Phillip J. Currie. The cladogram follows the biogeographical family tree provided by that study, which is a fusion of the study's 50% majority rule tree as well as themaximum agreement subtree. The study's 50% majority rule tree was a cladogram formed by a collection ofclades, although it only included clades that appear in more than 50% of the family trees found during the analysis. The maximum agreement subtree is the cladogram that results from an algorithm which attempts to maximize the amount of taxa included in the result while also retaining the fundamental shape of all other trees in the sample. Some controversial taxa thus had to be omitted by the subtree in order for the resulting cladogram to fulfill the second requirement. The biogeographical tree (i.e. the following cladogram) is basically the 50% majority rule tree, except with some of thepolytomies resolved according to the results of the maximum agreement subtree:[12]
The followingcladogram is based on a 2017phylogenetic analysis of the Ankylosaurinae conducted by Victoria Arbour and David Evans. The cladogram depicts the majority rule (average result) of 10 most parsimonious trees, which each are considered to have the fewest evolutionary steps, thus being the most accurate under the principle ofOccam's razor:[13]
In 1874, G. M. Dawson excavated specimen USNM 7943 at the Milk River locality of theFrenchman Formation in Alberta. It was collected from terrestrial sediments that are considered to be from theMaastrichtian stage of the Late Cretaceous, approximately 70.6 to 66 million years old. The specimen consisted of a partial first cervical ring, which is part of the dinosaur's neck. In 2013, this material was assigned toScolosaurus by Arbour and Currie who conducted a detailed phylogenetic analysis of the ankylosauridae.[14] It is currently housed at the Smithsonian Institution in Washington, DC.
In 1928, George F. Sternberg, collected specimen USNM 11892, from theMontanazhdarcho holotype locality, high up in theTwo Medicine Formation inGlacier County,Montana.[15] The material, a partial skull, was recovered from channel sandstone sediments that were deposited during the Campanian stage, approximately 74 million years ago. This is also housed at the Smithsonian Institution.
Other referred specimens include FPDM V-31, NSM PV 20381 and TMP 2001.42.9. FPDM V-31 and TMP 2001.42.9 are both skulls, in various states of preservation. NSM PV 20381 includes a skull, dorsal vertebrae, caudal vertebrae, ribs, bothscapulae, bothilia, partialischia, and bothfemora, bothtibiae andfibulae.
A differential diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism.
According to Arbour and Currie (2013),Scolosaurus (including the Two Medicine material) can be distinguished from other ankylosaurines based on the following characteristics:
Argon-argon radiometric dating indicates that the Two Medicine Formation was deposited between 83.5 and 70.6 million years ago, during the Campanian stage of the Late Cretaceous period, in what is now northwestern Montana.[16] IfOohkotokia is the same asScolosaurus it would mean thatScolosaurus existed for around 3 million years. The Two Medicine Formation correlates to the Belly River Group in southwest Alberta, and the Pakowki Formation eastward. The Two Medicine Formation was deposited by rivers and deltas between the western shoreline of theWestern Interior Seaway and the eastward advancing margin of theCordilleran Overthrust Belt. Since the mid-Cretaceous, North America had been divided in half by this seaway, with much of Montana and Alberta below the surface of the water. However, the uplift of theRocky Mountains forced the seaway to retreat eastwards and southwards. Rivers flowed down from the mountains and drained into the seaway, carrying sediment that formed the Two Medicine Formation and the Judith River Group. About 73 million years ago, the seaway began to advance westwards and northwards again, and the entire region was covered by the Bearpaw Sea, now preserved throughout the Western US and Canada by the massiveBearpaw Shale, which overlies the Two Medicine.[17][18] Below this formation are the nearshore deposits of the Virgelle Sandstone. Lithologies,invertebrate faunas, and plant and pollen data support that the Two Medicine Formation was deposited in a seasonal, semi-arid climate with possible rainshadows from the Cordilleran highlands. This region experienced a long dry season and warm temperatures. The extensive red beds andcaliche horizons of the upper Two Medicine are evidence of at least seasonally arid conditions.
Scolosaurus shared itspaleoenvironment with other dinosaurs, such as the duck-billedhadrosaursHypacrosaurus,Acristavus,Gryposaurus,Brachylophosaurus,Glishades,Prosaurolophus andMaiasaura, and theankylosaurEdmontonia.[19]Volcanic eruptions from the west periodically blanketed the region with ash, resulting in large-scale mortality, while simultaneously enriching the soil for future plant growth. Fluctuating sea levels also resulted in a variety of other environments at different times and places within the Judith River Group, including offshore and nearshore marine habitats, coastal wetlands, deltas and lagoons, in addition to the inland floodplains. The Two Medicine Formation was deposited at higher elevations farther inland than the other two formations.[20] A large variety ofceratopsians coexisted in this region, which includedAchelousaurus,Brachyceratops,Cerasinops,Einiosaurus,Prenoceratops andRubeosaurus. Carnivores included an unnamed troodontid, possiblyStenonychosaurus, thedromaeosaursBambiraptor andSaurornitholestes, and the largetyrannosauridsDaspletosaurus andGorgosaurus.[21]
The excellentvertebrate fossil record of Two Medicine and Judith River rocks resulted from a combination of abundant animal life, periodic natural disasters, and the deposition of large amounts of sediment. Many types of freshwater andestuarine fish are represented, includingsharks,rays,sturgeons,gars and others. This region preserves the remains of many aquaticamphibians andreptiles, includingbivalves,gastropods,frogs,salamanders,turtles,Champsosaurus andcrocodilians. Terrestrial lizards, includingwhiptails,skinks,monitors andalligator lizards have also been discovered.Pterosaurs likeMontanazhdarcho andPiksi as well asbirds likeApatornis andAvisaurus flew overhead. Several varieties ofmammals, such as the multituberculateCimexomys coexisted with dinosaurs in the Two Medicine Formation and the various other formations that make up the Judith River wedge. Fossilized eggs belonging to adromaeosaur have been recovered here. When water was plentiful, the region could support a great deal of plant and animal life, but periodic droughts often resulted in mass mortality.[22]
