PIN 614 and MPC 100/1305 possess numerous ovoid, keeled, and sometimes teardrop-shaped osteoderms on the anterolateral sides of the upper and lower forelimbs. AMNH 5866 includes numerous small osteoderms with this morphology ( Carpenter 2004 ), at least some of which were probably derived from the forelimbs. It is unclear if the hind limbs also bore osteoderms in ankylosaurines.

The tip of the tail in all ankylosaurines is enveloped by multiple osteoderms that obscure the terminal caudal vertebrae, forming the tail club knob (sensu Coombs 1995 ). AMNH 5214 is the only specimen of Ankylosaurus to preserve a tail club knob ( Fig. 7 ). The knob is 60 cm long, 49 cm wide, and 19 cm high. It is semicircular in dorsal view, similar to the tail club knobs of Euoplocephalus and Scolosaurus, and unlike the pointed knob osteoderms of Anodontosaurus or the narrow and elongate knob of Dyoplosaurus.

In Asian and North American taxa where caudal osteoderms are preserved, such as cf. Pinacosaurus (PIN 614 and MPC 100/1305), Euoplocephalus (ROM 1930), Dyoplosaurus (ROM 784), and Zuul (ROM 75860), they are typically keeled or triangular and arranged in rows as on the rest of the body. Asian ankylosaurines preserve triangular lateral osteoderms along the tail club handle, but in North American specimens triangular osteoderms along the handle are so far only known in Zuul ( Arbour and Evans 2017 ). AMNH 5895 and AMNH 5866 both include compressed triangular osteoderms ( Carpenter 2004 ) that could have been located on the lateral sides of the pelvis or tail.

Pelvic osteoderms in MPC 100/1305 are not present below the lower edge of the ilium. The lateral pelvic osteoderms, unlike the dorsal osteoderms, have a tall keel and take on a triangular profile in dorsal view. The largest of these osteoderms is located at the posterior corner of the ilium, and at least two more osteoderms of this shape, but decreasing in size, are present anteriorly. North American ankylosaurines do not appear to have had huge triangular lateral pelvic osteoderms, but both Dyoplosaurus and Scolosaurus preserve smaller triangular lateral pelvic osteoderms ( Parks 1924 ; Nopcsa 1928 ; Arbour et al. 2009 ). A few osteoderms with this morphology are present in AMNH 5866 ( Carpenter 2004 ), and these were probably located along the edges of the ilium.

MPC 100/1305, cf. Pinacosaurus, preserves in situ osteoderms on the left flank ( Carpenter et al. 2011 ; Arbour and Currie 2013b ). A typical flank osteoderm has a more square outline than those on the dorsal surface, and the keel is sigmoidal or kinked upward either anteriorly or posteriorly. Several osteoderms with this morphology are preserved in AMNH 5895 and AMNH 5866 ( Carpenter 2004 ). MPC 100/1305 includes four longitudinal rows of flank osteoderms, and Ankylosaurus may have been similar.

The osteoderms immediately behind the second cervical half ring generally mimic the shape and arrangement of those in the half ring (e.g., MPC 100/151 and NHMUK R5161). In particular, a huge, roughly triangular-based osteoderm in AMNH 5895 ( Fig. 6L ) bears a strong similarity to the medial osteoderms in the first transverse row behind the second cervical half ring in MPC 100/151 ( Arbour and Currie 2013b ). Other large, generally oval osteoderms with low keels in AMNH 5895 and AMNH 5866 probably represent osteoderms in the dorsal thoracic region, although it is more difficult to place some of these precisely. In Scolosaurus (NHMUK R5161), dorsal osteoderms generally decrease in diameter posteriorly, so smaller flat osteoderms in AMNH 5866 were probably located more posteriorly.

The largest osteoderms (excluding smaller interstitial osteoderms and ossicles), like the cervical half rings, are arranged in transverse and longitudinal rows across most of the length of the body. Scolosaurus (NHMUK R5161) includes four or five transverse segments in the thoracic region delineated by creases in the skin impressions. Although no Ankylosaurus specimens preserve the integument in situ, Ankylosaurus likely had a similar osteoderm pattern consisting of transverse and longitudinal rows of osteoderms, with four or five transverse rows separated by skin creases.

All ankylosaurids that preserve cervical half ring material (e.g., Anodontosaurus (CMN 8530); Euoplocephalus (AMNH 5404, CMN 210, and UALVP 31); Pinacosaurus (IVPP V16854); Saichania (MPC 100/151); Scolosaurus (NHMUK R5161); Shamosaurus (PIN 3779/2); and Ziapelta (NMMNH P-64484)) demonstrate that there were at most two cervical half rings, and that these formed continuous U-shaped yokes over the dorsal part of the neck and anterior part of the shoulders. Ankylosaurus (AMNH 5895) had two large cervical half rings with six keeled, oval-based osteoderms on each half ring ( Figs. 6 , 9 ). The medial osteoderms are unknown in Ankylosaurus cervical half rings, but the lateral osteoderms are flat with a low keel.

We propose a new revision of the osteoderm arrangement in Ankylosaurus based on comparisons with its close relatives Anodontosaurus, Euoplocephalus, Dyoplosaurus, Scolosaurus, Zuul, and the more distant Mongolian relatives Saichania and Pinacosaurus ( Fig. 9 ). Major changes in this restoration compared with those presented by Ford (2003) and Carpenter (2004) include revisions to the cervical half rings, the arrangement of the thoracic and pelvic osteoderms, and general body proportions. Our restoration is broadly congruent with that presented by Ford (2003) .

Brown (1908) included a restoration of the osteoderm arrangement in his original description of Ankylosaurus. Very few ankylosaur specimens were known at the time; Brown (1908) gave Ankylosaurus a suit of armour of closely packed thoracic osteoderms, interpreted the cervical half rings as originating from the pelvis and tail (partly inspired by the caudal rings in glyptodonts), and gave it a tapering, flexible tail lacking a tail club ( Fig. 9 ). The 1910 discovery of AMNH 5214 provided the first evidence for a tail club in Ankylosaurus, and this structure was incorporated into later popular depictions of this ankylosaur, such as the 1947 Age of Reptiles mural by Rudolph Zallinger at the Yale Peabody Museum, or the 1964 World’s Fair Sinclair Dinoland sculpture. Popular depictions of Ankylosaurus also often conflated its armour with that of “Palaeoscincus” (now Edmontonia, a nodosaurid), giving it the large pectoral spikes known in the genus. Indeed, “Palaeoscincus” was also sometimes reconstructed with an Ankylosaurus-inspired tail club, including in a 1930 mural by Charles Knight at the Field Museum, most likely after the publication of an article showing two alternate interpretations of the tail of “Palaeoscincus” by Matthew (1922) . Both Ford (2003) and Carpenter (2004) provided updated, modern restorations incorporating new knowledge about ankylosaur postcranial anatomy and osteoderm arrangements. Ford’s (2003) restoration shifted the caudal osteoderm rings to the cervical region (based on the presence of cervical half rings in many other ankylosaurids), included a tail club knob, increased the spacing between osteoderms in the thoracic and pelvic regions, varied the osteoderm size between the thoracic and pelvic regions, and hypothesized that a pelvic shield (coossified pelvic osteoderms) may have been present ( Fig. 9 ). Carpenter (2004) similarly included cervical half rings and a tail club knob, and included a new arrangement of varying osteoderm shapes in the thoracic region ( Fig. 9 ). Fig. 9. Hypotheses of osteoderm morphologies and placements in Ankylosaurus magniventris. Brown (1908) proposed an arrangement in which the osteoderms are closely spaced and relatively uniform, with pelvic osteoderms united in coossified bands. Ford (2003) provided more space between the osteoderms overall, included a tail club knob, and hypothesized that a pelvic shield was present. Carpenter (2004) suggested that the cervical armour was arranged in quarter rings. We suggest that the cervical armour was united into more typical half rings, and provide an updated osteoderm arrangement based on preserved osteoderms in AMNH 5214, AMNH 5895, and AMNH 5866, and comparisons with related species.

Size of Ankylosaurus

Like many other dinosaur taxa from the Hell Creek Formation, Ankylosaurus is substantially larger than its closest Campanian–Maastrichtian relatives (Russell and Manabe 2002; Longrich 2011; Lamanna et al. 2014; Brusatte and Carr 2016). Brown (1908) did not explicitly state any body size estimates for Ankylosaurus, but an illustration of the skeleton at 1/34 natural size was 22 cm long, and so Brown must have considered AMNH 5895 to be about 7.5 m in length. Carpenter (2004) proposed lengths of up to 6.25 m for CMN 8880 and up to 5.40 m for AMNH 5214 based on a review of the known skeletal material. Ford (2003) reconstructed Ankylosaurus with a length of approximately 7.6 m. We revisit these estimates with new data from more complete ankylosaurine skeletons here.

The skull of CMN 8880 is the largest ankylosaurid skull known: with a width across the supraorbitals of 66.2 cm, it is more than 70% larger than the largest known skulls of Anodontosaurus, Dyoplosaurus, Euoplocephalus, Scolosaurus, or Ziapelta (Table 2). AMNH 5895, a smaller individual, has a supraorbital width of 52.5 cm, making it about 35%–64% larger than the largest other ankylosaurins. The smallest individual, AMNH 5214, has a supraorbital width of 49.0 cm and premaxilla–occipital condyle width of 55.5 cm, and is therefore about 26%–50% larger than the largest skulls of other ankylosaurins.

No postcranial material is known for CMN 8880, but AMNH 5895 and AMNH 5214 preserve elements that can be compared with other ankylosaurins. AMNH 5337, a large Euoplocephalus, includes 10 free dorsal vertebrae (i.e., those not fused into the dorsosacral rod), as does AMNH 5895. Surprisingly, the dorsal vertebrae of AMNH 5895 are not noticeably larger than those of Euoplocephalus (Table 3). Only four caudal vertebrae are preserved in AMNH 5895 and their positions in the caudal sequence are unknown, but the largest of these is probably at least the fourth or fifth caudal vertebra based on the proportions of the transverse processes relative to the centrum and neural spine. The largest caudal vertebra in AMNH 5895 is much larger than any preserved in ROM 784 (Dyoplosaurus), and is similar in size to the largest caudals in ROM 1930 (Euoplocephalus). In the tail club handle, the vertebrae of AMNH 5214 are about the same length as those of AMNH 5245 (Anodontosaurus), ROM 784 (Dyoplosaurus), or ROM 788 (Euoplocephalus or Scolosaurus), but are about twice as wide. The femur of AMNH 5214 is between 12% and 25% longer than those of Dyoplosaurus, Euoplocephalus, and Scolosaurus, and the humerus is about 30% longer than UALVP 31 (Euoplocephalus) (Table 4).

Type Position AMNH 5895 Ankylosaurus magniventris AMNH 5337 Euoplocephalus tutus Centrum length (mm) Centrum width (mm) Centrum height (mm) Centrum length (mm) Centrum width (mm) Centrum height (mm) Cervical 2 61 95 82 — — — Cervical 3 55 — 88 — — — Cervical 4 68 — 93 — — — Cervical 5 64 112 99 — — — Cervical 6 73 135 109 — — — Dorsal 1 105 98 102 104 113 103 Dorsal 2 123 — — 117 108 106 Dorsal 3 127 113 98 120 109 105 Dorsal 4 132 116 107 125 109 112 Dorsal 5 134 105 108 121 116 110 Dorsal 6 127 120 114 115 116 107 Dorsal 7 126 121 111 112 120 112 Dorsal 8 120 120 110 110 106 108 Dorsal 9 121 125 111 108 124 113 Dorsal 10 120 131 114 113 128 107 Caudal ? 62 116 108 — — — Caudal ? 66 113 109 — — — Caudal ? 67 106 81 — — — Caudal ? 62 100 81 — — —

Specimen Width across supraorbitals (mm) Scapula length (mm) Humerus length (mm) Femur length (mm) Ankylosaurus magniventris CMN 8880 662 — — — AMNH 5895 525 615 — — AMNH 5214 490 — 542 / 536 670 Euoplocephalus tutus ROM 1930 388 — 380 — TMP 1979.14.164 385 — — — AMNH 5405 372 — — — AMNH 5337 365 — 415 — AMNH 5404 — — 403 535 UALVP 31 313 428 377 515 Anodontosaurus lambei TMP 1997.132.1 355 — 399 — Dyoplosaurus acutosquameus ROM 784 355 — — 562 Scolosaurus cutleri NHMUK R5161 — 560 440 600 TMP 2001.42.1 263 — — 430

In relatively complete ankylosaurs, such as PIN 614 and MPC 100/1305 (both cf. Pinacosaurus), ZPAL MgD I/113 (Ankylosaurinae indet.), ROM 784 (Dyoplosaurus), and ROM 75860 (Zuul), the tail club represents about 55%–64% of the total length of the tail (Table 5). In PIN 614 and MPC 100/1305, the tail represents about 49%–57% of the total length from the anterior of the cervical series to the distal end of the tail; both specimens lack skulls so our comparative lengths for Ankylosaurus are for the length of the body excluding the skull. AMNH 5214 preserves a portion of the tail club handle that is 1.21 m long, and in which seven vertebrae are present; ankylosaurins such as Dyoplosaurus have about 12 visible vertebrae in the tail club, so we can estimate that the complete tail club may have been about 2 m long, the tail may have been 3.12–3.63 m long, and the total body length (excluding the skull) could have been in the range of 5.47–7.40 m (with the skull included, 6.02–7.95 m). Given that the skull of CMN 8880 is 26% wider across the supraorbitals than AMNH 5214, and using the same proportion estimates as for AMNH 5214, we estimate that the length of CMN 8880 was between 6.89 and 9.32 m excluding the skull, and between 7.56 and 9.99 m including the skull (Table 5). We also attempted an estimate of the length of AMNH 5895 by drawing the lengths of the preserved elements to scale. Using measurements of the preserved skull and vertebrae of AMNH 5895 and the skull and tail club in AMNH 5214, estimating the length of the pelvis based on AMNH 5409 (a large pelvis from the Dinosaur Park Formation), and conservatively estimating the gaps between vertebrae and missing cervical and caudal vertebrae, we illustrated a length of approximately 6.5 m for AMNH 5895. Given that the vertebrae in AMNH 5895 do not differ substantially in size from other large ankylosaurin skeletons, a body length of nearly 10 m for a large Ankylosaurus is probably too long, but a length of up to 8 m is probably within reason.

Specimen Tail length (cm) Tail club length (cm) Length from anterior end of cervical series to tail terminus (cm) Skull width across supraorbitals (cm) % of tail length represented by tail club % of body length represented by tail Estimated body length (excluding skull) (cm) Estimated total body length (including skull) (cm) Ankylosaurus magniventris CMN 8880 — — — 66.2 — — 689–932 756–999 AMNH 5895 — — — 52.5 — — 586–793 645–851 AMNH 5214 — ~200 — 49.0 — — 547–740 602–795 Zuul crurivastator ROM 75860 ∼300a 210 — 393 ≤70a — — ∼600 Nemegt Formation Ankylosaurinae indet. ZPAL MgD I/113 310 196 — — 63 — 543–632 580–670 cf. Pinacosaurus MPC 100/1305 161 104 327 — 64 49 — ∼360 PIN 614 210 116 366 — 55 57 — ~390 Dyoplosaurus acutosquameus ROM 784 204 127 — 22.4 62 — 358–416 395–453 Anodontosaurus lambei AMNH 5245 — >131b — — — — — >400

Using similar proportion estimates for other relatively complete ankylosaurines indicates that Ankylosaurus was the largest ankylosaurine (and possibly even the largest ankylosaur), but a few other species approach our lowest length estimates for Ankylosaurus. PIN 614 and MPC 100/1305 (both cf. Pinacosaurus) have neck-to-tail lengths of 3.27 and 3.66 m, respectively, and we estimate a length of between 3.58 and 4.16 m for ROM 784 (Dyoplosaurus). Based on a relatively complete tail club (AMNH 5245), we estimate that Anodontosaurus reached at least 4 m in length. The holotype of Scolosaurus (NHMUK R5161) lacks the skull and tail club but still measures nearly 4 m in length (Nopcsa 1928), and most likely approached 6 m in total. No specimens of Euoplocephalus are complete enough to make a confident length estimate (Arbour and Currie 2013a), but based on comparisons with skull and limb proportions for Anodontosaurus and Scolosaurus, a large Euoplocephalus may have been 5–6 m long. A large indeterminate ankylosaurine taxon from Mongolia (ZPAL MgD I/113) has a 3.10 m long tail and may also have exceeded 6 m in total body length. Finally, Zuul is estimated to have been about 6 m long, based on the length of the nearly complete, partially articulated skeleton in the field (Arbour and Evans 2017). Large individuals of Ankylosaurus almost certainly exceeded 6 m in length.

To calculate the mass of Ankylosaurus, we used the QE function in the MASSTIMATE v. 1.3 package of Campione (2016) in R version 3.2.3 (R Core Team 2015). This package estimates body mass from combined stylopodial shaft circumferences using a scaling equation described by Campione and Evans (2012). For AMNH 5214, the smallest known Ankylosaurus (humeral shaft circumference = 315 mm, femoral shaft circumference = 363 mm), the estimated mass is 4.78 ± 1.22 t. For comparison, the estimated mass of AMNH 5404, a large specimen of Euoplocephalus (humeral shaft circumference = 244 mm, femoral shaft circumference = 278 mm), is 2.33 ± 0.60 t. Thus, Ankylosaurus appears to have been a substantially bulkier animal, even if its total body length did not greatly exceed those of its closest relatives (which is consistent with standard allometric scaling relationships of the vertebrate skeleton).

How heavy was the largest known Ankylosaurus, CMN 8880? With only an isolated skull available, it is difficult to be certain. However, it is possible to hazard a first approximation by isometrically scaling up stylopodial shaft circumferences from AMNH 5214 using width between the supraorbitals as a common reference. (Stylopodial shaft circumference very likely scaled allometrically in the genus, but there are too few specimens to determine a scaling coefficient.) In this way, we estimate that CMN 8880 might have weighed approximately 7.95 ± 2.04 t—about as massive as a large male African elephant (Loxodonta africana) (Colbert 1993)—but these values must obviously be regarded with due scepticism. Other published mass estimates for Ankylosaurus (Paul 1997), presumably using the smaller and more complete AMNH 5214 as reference, place the animal at approximately 6 t, although methodological details in these studies are lacking. Seebacher (2001) estimated a mass of just ∼1.7 t using his polynomial technique, which strikes us as excessively small for an animal of otherwise elephantine proportions.