Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA

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.

Figure 1. Geographic distribution of Albertan ankylosaurids.

A) Map of the Canadian province of Alberta. B) Area represented by grey square in (A), showing locations of localities discussed in this paper. Specimens referred to Euoplocephalus tutus have been collected from sediments between Tolman Bridge and Drumheller, within Dinosaur Provincial Park, west of Hilda, and south of Manyberries.

Figure 2. 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.

Figure 3. Skulls in dorsal view.

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 10 cm. Photograph of ROM 832 by C. Brown, and of ROM 1930 by J. Arbour, and used with permission.

Figure 4. Cranial ornamentation patterns compared.

CMN 0210 is the holotype of Euoplocephalus tutus, CMN 8530 is the holotype of Anodontosaurus lambei, and ROM 784 is the holotype of Dyoplosaurus acutosquameus. Abbreviations: asca, anterior supraorbital caputegulum; frca, frontal caputegulum; laca, lacrimal caputegulum; loca, loreal caputegulum; mnca, median nasal caputegulum; msca, middle supraorbital caputegulum; nas apt, nasal aperture; nasca, nasal caputegulum; nuca, nuchal caputegulum; orb, orbit; pnca, postnarial caputegulum; prfca, prefrontal caputegulum; psca, posterior supraorbital caputegulum; qjh, quadratojugal horn; snca, supranarial caputegulum; sqh, squamosal horn.

Figure 5. Skulls in lateral view.

Skulls from Alberta appear above the horizontal line, and skulls from Montana below the line. The left column of skulls from Alberta includes skulls without postocular caputegulae around the base of the squamosal and quadratojugal horns, in right lateral view (AMNH 5337, AMNH 5404, AMNH 5405, ROM 1930, TMP 1991.127.1, and UALVP 31). The right column of skulls from Alberta includes skulls with postocular caputegulae around the base of the squamosal and quadratojugal horns, in left lateral view (AMNH 5238, CMN 8530 (Anodontosaurus lambei holotype), NHMUK R4947, ROM 832, TMP 1997.59.1, TMP 1997.132.1. Below the horizontal line are skulls from Montana (MOR 433 (Oohkotokia horneri holotype), NSM PV 20381, TMP 2001.42.9, and USNM 11892). The anterior rostrum of TMP 2001.42.9 and NSM PV 20381 are heavily reconstructed. AMNH 5404, AMNH 5405, and TMP 1991.127.1 are mirrored left lateral views, and AMNH 5238 is a mirrored right lateral view. Photograph of NSM PV 20381 by T. Miyashita and used with permission. Scale equals 10 cm. Abbreviations: asca, anterior supraorbital caputegulum; co, ciliary osteoderm; laca, lacrimal caputegulum; loca, loreal caputegulum; mx, maxilla; nar, naris; nasca, nasal caputegulum; orb, orbit; pmx, premaxilla; pnca, postnarial caputegulum; poca, postocular caputegulum; psca, posterior supraorbital caputegulum; pt, pterygoid; qjh, quadratojugal horn; snca, supranarial caputegulum; sqh, squamosal horn.

Figure 6. Cranial and mandibular anatomy.

A) AMNH 5337 in ventral view. AMNH 5405 in B) ventral and C) anterior views. D) TMP 1997.132.1 in ventral view. ROM 1930 in E) ventral and F) posterior views. G) ROM 784 (holotype of Dyoplosaurus acutosquameus) in ventral view. H) AMNH 5238 skull in ventral view. Right mandible of UALVP 31 in I) lateral view and J) medial view. Right mandible in lateral view of K) AMNH 5405 and L) AMNH 5403. Scale bars equal 10 cm. Abbreviations: alv, tooth alveolus; art, articular; bas, basioccipital; bpt, basipterygoid process; bs, basisphenoid; ch, choana; co, ciliary osteoderm; cor, coronoid; d, dentary; dpf, descending process of frontal; ee, ectethmoid; endo, endocranial cavity; fm, foramen magnum; inb, internarial bar; ls, laterosphenoid; ltf, laterotemporal fenestra; maca, mandibular caputegulum; meck, Meckelian groove; mnca, median nasal caputegulum; mx, maxilla; mx tom, maxillary tomium; nas apt, nasal aperture; nc, nasal canal; ns, nasal septum; nuc, nuchal crest; oc, occipital condyle; of, olfactory region of nasal canal; orb, orbit; orbs, orbitosphenoid; pal, palatine; pal apt, palatal aperture; para apt, paranasal aperture; parocc, paroccipital process; pmx, premaxilla; pmx n, premaxillary notch; pmx tom, premaxillary tomium; pnca, postnarial caputegulum; preart, prearticular; pro nas pmx, intranasal process of premaxilla; ps, parasphenoid (cultriform process); pt, pterygoid body; ptq, quadrate ramus of pterygoid; ptv, interpterygoid vacuity; ptw, pterygoid wing; q, quadrate; qh, quadrate head; qjh, quadratojugal horn; snca, supranarial caputegulum; socc, supraoccipital; spd, sulcus for predentary; spl, splenial; sqh, squamosal horn; sur, surangular; v, vomer.

Figure 7. Cervical and dorsal vertebrae.

AMNH 5403 axis (A1) in anterior, left lateral, and dorsal views; cervical (B1) in anterior, right lateral, and dorsal views; cervical (C1) in posterior, left lateral, views; cervical (D1) in anterior, left lateral views. AMNH 5337 cervical (A2) in posterior, right lateral views; dorsals (B2,C2) in anterior, right lateral, and dorsal views. UALVP 31 axis (A3) in left lateral view, and cervical (B3) in posterior view. Abbreviations: c, centrum; cr, cervical rib; di, diapophysis; ns, neural spine; na, neural arch; nc, neural canal; od, odontoid; pa, parapophysis; poz, postzygapophysis; prz, prezygapophysis.

Figure 8. Dorsal vertebrae of AMNH 5337.

AMNH 5337, D in anterior and left lateral views; E in posterior and right lateral views; F in anterior and right lateral views; G in anterior and right lateral views; H in posterior and left lateral views; I to L in anterior and left lateral views. Abbreviations: c, centrum; di, diapophysis; na, neural arch; nc, neural canal; ns, neural spine; pa, parapophysis; poz, postzygapophysis; prz, prezygapophysis; r, rib; tp, transverse process.

Figure 9. Dorsosacral, sacral, caudosacral, and caudal vertebrae.

Partial sacrum of ROM 1930 in A) left lateral and B) dorsal (with anterior to the right) views. Sacrocaudal of AMNH 5245 in C) anterior, D) posterior, E) left lateral, and F) dorsal views. Sacrocaudal of ROM 1930 in G) anterior and H) posterior views. Sacrocaudal of ROM 1930 in I) anterior and J) posterior views; distal end of transverse process is partially reconstructed. Anterior free caudal vertebra of CMN 8530 (holotype of Anodontosaurus lambei) in K) anterior and L) right lateral views. M) Block of articulated anterior free caudal vertebrae of ROM 1930, in right lateral view. Penultimate free caudal vertebra of ROM 1930 in N) right lateral and O) anterior views. Transitional caudal vertebra (last free caudal vertebra before first handle vertebra of the tail club) of ROM 1930 in P) right lateral and Q) anterior views. AMNH 5404 free caudal vertebra in R) dorsal and S) right lateral views. Next most posterior AMNH 5404 free caudal vertebra in T) right lateral and U) anterior views. Scale equals 10 cm. Abbreviations: c, centrum; hs, haemal spine; nc, neural canal; np, notochordal prominence; ns, neural spine; poz, postzygapophysis; prz, prezygapophysis; prz-h, prezygapophysis of the first handle vertebra; r, rib; tp, transverse process.

Figure 10. Pectoral and pelvic girdles.

AMNH 5406 right scapula in A) medial and B) lateral views, left scapula in C) lateral and D) medial views. TMP 2001.42.19 left scapulocoracoid in E) lateral and F) ventral views. G) UALVP 31 right scapula in medial view. H) AMNH 5406 left scapula in anteroventral view. I) ROM 813 right coracoid in lateral view. J) AMNH 5404 left coracoid in lateral view. K) AMNH 5245 right ilium in ventral view, anterior is up. TMP 2001.42.19, L) right ischium and M) left ischium in medial views. N) CMN 8530 (Anodontosaurus lambei holotype) right ischium in medial view. O) UALVP 31 associated right ilium, sacrum, right femur and right tibia, with ilium in ventral view (anterior is up), and femur in medial view. P) TMP 1982.9.3 pelvis in dorsal view (the right half of the pelvis is reconstructed), anterior is up. Photograph of AMNH 5404 coracoid by R. Sissons and used with permission. Scale bar for A–H and J is 5 cm, scale bar for I is 10 cm. Abbreviations: ace, acetabulum; acr, acromion; cf, coracoid foramen; ds, dorsosacral; fem, femur; gl, glenoid; glf, glenoid fossa; il, ilium; ip, iliac peduncle; is, ischium; ost, osteoderm; medr, medial ridge; mt, metatarsal; mtlc, enthesis of M. triceps longus caudalis; ns, neural spine; posta, postacetabular process; pp, pubic peduncle; prea, preacetabular process; r, rib; s1–3, sacrals 1–3; scb, scapulocoracoid buttress; scor, surface for coracoid; stp, sternal process; sscap, surface for scapula; tib, tibia.

Figure 11. Forelimb elements.

AMNH 5406 right humerus in A) posterior view. UALVP 31 right humerus in B) anterior view. ROM 1930 right humerus in C) posterior view. AMNH 5337 right humerus in D) posterior, E) anterior, and F) proximal and G) distal views. H) TMP 1997.132.1 left humerus and associated osteoderms with humerus in anterior view and osteoderms mostly in interior view. AMNH 5404 right humerus in I) posterior and J) anterior views. K) ROM 47655 left humerus in posterior view. AMNH 5337 right radius in L) medial view. AMNH 5406 M) right radius in medial view and N) right ulna in medial view. Scale bar equals 10 cm. Abbreviations: dpc, deltopectoral crest; hh, humeral head; hum, humerus; it, internal tuberosity; lsr, lateral supracondylar ridge; of, olecranon fossa; op, olecranon process; os, osteoderm; rc, radial condyle; uc, ulnar condyle.

Figure 12. Hindlimb elements.

TMP 1982.9.3 left femur in A) anterior, B) medial, and C) posterior views. D) AMNH 5266 right femur in posterior view. E) AMNH 5266 right tibia in anterior view. F) AMNH 5266 right fibula. ROM 813 left tibia in G) lateral and H) posterior views. AMNH 5404 right femur in I) anterior, J) medial, and K) posterior views, and right tibia in L) lateral and M) anterior views. N) AMNH 5404 right femur in distal view. O) TMP 1982.9.1 left femur in distal view. P) AMNH 5404 right tibia in distal view. Abbreviations: 4^th, fourth trochanter; as, astragalus; ca, calcaneum; cn, cnemial crest; fh, femoral head; im, inner malleolus; lc, lateral condyle; le, lateral epicondyle; mc, medial condyle; om, outer malleolus.

Figure 13. Cervical half rings.

CMN 0210 (Euoplocephalus tutus holotype) first cervical half ring in A) anterior view; B) left medial osteoderm in superficial view; C) right lateral osteoderm in superficial view; D) right distal osteoderm in superficial view and E) dorsal view. F) First cervical half rings of AMNH 5406 in anterior view. UALVP 31, first cervical half ring in G) anterior and H) dorsal views, and second cervical half ring in I) dorsal view. AMNH 5337 first cervical half ring in J) anterior and K) dorsal views. L) AMNH 5404 first cervical half ring in dorsal view. M) CMN 8530 (holotype of Anodontosaurus lambei) first cervical half ring in dorsal view. N) Fragment of first cervical half ring of TMP 1982.9.3 in superficial view. O) Partial first cervical half ring of TMP 1997.132.1 in dorsolateral view. P) NHMUK R5161 in situ cervical rings in dorsal view, anterior is to the right. TMP 2001.49.2 partial first cervical half ring in Q) posterior and R) left lateral views. USNM 7943 partial first cervical half ring in S) anterior and T) dorsal views. U) TMP 2007.12.52 second cervical half ring in anterior view. V) UALVP 45931 partial second cervical half ring in anterior view. Scales in A, G–R equal 10 cm, scales in B–F equal 5 cm. Abbreviations: b, band; dos, distal osteoderm; ios, interstitial osteoderm; los, lateral osteoderm; mos, medial osteoderm; rp, resorption pit on medial osteoderm; s, suture between band segments.

Figure 14. Tail clubs.

Tail club knobs from the Horseshoe Canyon Formation in A–E: AMNH 5245 in A) dorsal and B) anterior views; C) TMP 1994.168.1 in dorsal view; D) AMNH 5216 in dorsal view; E) USNM 10753 in dorsal view. Tail club knobs from the Dinosaur Park Formation in F–Q: F) ROM 788 in ventral view; G) MACN Pv 12554 in ventral view; H) CMN 349 in ventral view; TMP 1983.36.120 in I) posterior and J) dorsal views; K) UALVP 16247 in dorsal view; L) CMN 135 in dorsal view; M) TMP 2001.42.9 in dorsal view; ROM 784 (holotype of Dyoplosaurus acutosquameus) in N) posterior and O) dorsal views; P) UALVP 47273 in dorsal view; Q) ROM 7761 in dorsal view. Scale bar equals 10 cm. Photograph of MACN Pv 12554 taken by E. Snively, photograph of CMN 349 taken by M. Burns, used with permission. Photograph of AMNH 5216 courtesy of the American Museum of Natural History. Abbreviations: hs, haemal spine; maj os, major osteoderm of the tail club knob; min os, minor osteoderm of the tail club knob; ns, neural spine; ot, ossified tendon; prz, prezygapophysis.

Figure 15. Stratigraphic distribution of ankylosaurid specimens in the Dinosaur Park and Horseshoe Canyon formations of Alberta.

A) Distribution of ankylosaurid specimens within the Dinosaur Park Formation. Megaherbivore Assemblage Zones after Mallon et al. . Specimens marked with green stars have GPS coordinates and accurate elevation data, specimens marked with yellow dots have elevations estimated from field notes, and the location of TMP 1991.127.1 (marked by a blue hexagon) was estimated from Alberta Township System coordinates. The elevation of some specimens with GPS coordinates was also estimated using field notes and Google Earth; the vertical lines associated with these illustrate the maximum elevation from using field note data only. Specimens marked by yellow dots, as such, could vary in elevation anywhere from three to seventeen meters. ROM 784 is the holotype of Dyoplosaurus acutosquameus and NHMUK R5161 is the holotype of Scolosaurus cutleri. Although the exact locality and elevation for the holotype of Euoplocephalus tutus (CMN 0210) is unknown, AMNH 5406 and UALVP 31 can be confidently referred to that taxon. B) Stratigraphic column showing Upper Cretaceous formations in southern Alberta. Nomenclature for the Horseshoe Canyon Formation follows Eberth and Braman . CMN 8530, the holotype of Anodontosaurus lambei, occurs in the Horsethief Member.

Figure 16. Results of phylogenetic analysis 1, retaining character state codings from Thompson et al. .

Strict consensus and 50% majority rule trees shown, with frequencies, bootstrap supports, and Bremer supports.

Figure 17. Results of phylogenetic analysis 2, modifying character state codings from Thompson et al. .

Figure 18. Results of phylogenetic analysis 3, with new characters added to modified character state codings from analysis 2.

Figure 19. Stratigraphic distribution of Campanian-Maastrichtian ankylosaurid species in Alberta and northwest Montana.

Indeterminate ankylosaurid material is known from the Foremost and Oldman formations in southern Alberta. The holotype of Scolosaurus cutleri may be from the Oldman Formation or the lower Dinosaur Park Formation; ankylosaurid specimens from the Upper Two Medicine Formation of Montana are referred to Scolosaurus. Dyoplosaurus acutosquameus is known from Megaherbivore Assemblage Zone 1 in the Dinosaur Park Formation. Euoplocephalus tutus has been identified from both Megaherbivore Assemblage Zone 1 and Megaherbivore Assemblage Zone 2 of the Dinosaur Park Formation, but is more common in Zone 1. Anodontosaurus lambei is rare in Megaherbivore Assemblage Zone 2 of the Dinosaur Park Formation, with most specimens identified from the Horsethief, Morrin, and Tolman members of the Horseshoe Canyon Formation. In Alberta, Ankylosaurus magniventris was present in the Scollard Formation.