Arbour visited dinosaur fossil collections from Alberta to the U.K. examining skull armour and comparing those head details with other features of the fossilized ankylosaur remains. She made a breakthrough that resurrected research done more than 70 years ago.

CMN 0210 is the holotype of Euoplocephalus tutus, CMN 8530 is the holotype of Anodontosaurus lambei, MOR 433 is the holotype of Oohkotokia horneri, and ROM 784 is the holotype of Dyoplosaurus acutosquameus. AMNH 5337, AMNH 5405, CMN 0210, ROM 784, ROM 1930, TMP 1979.14.74, TMP 1991.127.1, TMP 1997.132.1, and UALVP 31 are from the Dinosaur Park Formation. AMNH 5238 and UALVP 47977 are of uncertain stratigraphic position within Dinosaur Provincial Park. AMNH 5223, CMN 8530, ROM 832, and TMP 1997.59.1 are from the Horseshoe Canyon Formation. NHMUK R4947 is from an unknown stratigraphic position in Alberta. MOR 433, TMP 2001.42.9 (much of the anterior rostrum in heavily reconstructed), and USNM 11892 are from the Upper Two Medicine Formation in Montana. Scale equals 10cm. (Credit: Victoria M. Arbour, Philip J. Currie; Photograph of ROM 832 by C. Brown, and of ROM 1930 by J. Arbour)

Arbour explains that between 1900 and 1930 researchers had determined that small variations in the skull armour and the tail clubs in some ankylosaurs constituted four individual species of the dinosaurs.

“In the 1970s the earlier work was discarded and those four species were lumped into one called species Euoplocephalus,” said Arbour.

“I examined many fossils and found I could group some fossils together because their skull armour corresponded with a particular shape of their tail club,” said Arbour.

Cranial anatomy of ankylosaurids, including terminology for ornamentation patterns.
ZPAL MgD II/1, juvenile Pinacosaurus grangeri in A) dorsal and B) left lateral views, showing boundaries of cranial bones. Boundaries between cranial bones are not visible in most adult ankylosaurids. C) UALVP 31, Euoplocephalus tutus, in dorsal view. D) CMN 8530, Anodontosaurus lambei (holotype), in left lateral view. Cranial ornamentation that is subdivided into discrete polygons (rather than generalized rugosity) are referred to as caputegulae. Abbreviations: asca, anterior supraorbital caputegulum; aso, anterior supraorbital; br, break or plaster; fr, frontal; frca, frontal caputegulum; j, jugal; lac, lacrimal; laca, lacrimal caputegulum; loca, loreal caputegulum; mnca, median nasal caputegulum; msca, middle supraorbital caputegulum; mso, middle supraorbital; mx, maxilla; nar, naris; nas, nasal; nasca, nasal caputegulum; nuca, nuchal caputegulum; orb, orbit; par, parietal; pmx, premaxilla; pnca, postnarial caputegulum; poca, postocular caputegulum; porb, postorbital; prf, prefrontal; prfca, prefrontal caputegulum; psca, posterior supraorbital caputegulum; pso, posterior supraorbital; pt, pterygoid; q, quadrate; qj, quadratojugal; qjh, quadratojugal horn; snca, supranarial caputegulum; sno, supranarial ornamentation; sq, squamosal; sqh, squamosal horn.
doi:10.1371/journal.pone.0062421.g002

Finding common features in fossils that come from the same geologic time is evidence that the original researchers were right says Arbour. “There were in fact four different species represented by what scientists previously thought was only one species, Euoplocephalus.”The four species span a period of about 10 million years. Arbour’s research shows three of those ankylosaurs species lived at the same time in what is now Dinosaur Provincial Park in southern Alberta.

Arbour says this opens the door to new questions.

“How did these three species shared their habitat, how did they divide food resources and manage to survive?” said Arbour.

Arbour will also look into how slight differences in skull ornamentation and tail shape between the species influenced the animals’ long reign on Earth.Arbour’s research was published May 8, in the journal PLOS ONE.

Abstract:

Few ankylosaurs are known from more than a single specimen, but the ankylosaurid Euoplocephalus tutus (from the Late Cretaceous of Alberta, Canada and Montana, USA) is represented by dozens of skulls and partial skeletons, and is therefore an important taxon for understanding intraspecific variation in ankylosaurs. Euoplocephalus is unusual compared to other dinosaurs from the Late Cretaceous of Alberta because it is recognized from the Dinosaur Park, Horseshoe Canyon, and Two Medicine formations. A comprehensive review of material attributed to Euoplocephalus finds support for the resurrection of its purported synonyms Anodontosaurus lambei and Scolosaurus cutleri, and the previously resurrected Dyoplosaurus acutosquameus. Anodontosaurus is found primarily in the Horseshoe Canyon Formation of Alberta and is characterized by ornamentation posterior to the orbits and on the first cervical half ring, and wide, triangular knob osteoderms. Euoplocephalus is primarily found in Megaherbivore Assemblage Zone 1 in the Dinosaur Park Formation of Alberta and is characterized by the absence of ornamentation posterior to the orbits and on the first cervical half ring, and keeled medial osteoderms on the first cervical half ring. Scolosaurus is found primarily in the Two Medicine Formation of Montana (although the holotype is from Dinosaur Provincial Park), and is characterized by long, back-swept squamosal horns, ornamentation posterior to the orbit, and low medial osteoderms on the first cervical half ring; Oohkotokia horneri is morphologically indistinguishable from Scolosaurus cutleri. Dyoplosaurus was previously differentiated from Euoplocephalus sensu lato by the morphology of the pelvis and pes, and these features also differentiate Dyoplosaurus from Anodontosaurus and Scolosaurus; a narrow tail club knob is probably also characteristic for Dyoplosaurus.

Citation: Arbour VM, Currie PJ (2013) Euoplocephalus tutus and the Diversity of Ankylosaurid Dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA. PLoS ONE 8(5): e62421. doi:10.1371/journal.pone.0062421

Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America