Tarchia cf. gigantea Maleev, 1956

Arbour, Victoria M., Lech-Hernes, Nicolai L., Guldberg, Tom E., Hurum, Jørn H. & Currie, Philip J., 2013, An ankylosaurid dinosaur from Mongolia with in situ armour and keratinous scale impressions, Acta Palaeontologica Polonica 58 (1), pp. 55-64 : 56-60

Tarchia cf. gigantea Maleev, 1956

Material.— ZPAL MgD I/113, posterior portion of abdomen including fragmentary ribs, partial pelvis, and in situ osteoderms and skin impressions; nearly complete caudal series including tail club handle and in situ osteoderms, but missing tail club knob.

Description.— ZPAL MgD I/113 consists of a large piece of the left side of the posterior abdomen and most of the tail ( Figs. 1–5). Several ribs from the left side articulate with the dorsal vertebrae, most of which are only preserved as impressions. Both ilia are broken, but there is an imprint of the left ilium. The tail is nearly complete, but lacks the tail club knob. Several blocks of uncertain position include vertebrae, ribs, and a limb element. Numerous osteoderms were recovered with the specimen, including a layer of osteoderms and soft tissue imprints on the trunk ( Fig. 3 ), and in situ osteoderms alongside the caudal vertebrae ( Figs. 4 , 5 ). In addition, more than a hundred loose pieces containing bone elements of unknown origin have been identified.

The section of trunk from the posterior left side of the body is about 92 cm in length, and the body is 86 cm wide. It is split in half along the vertebral column. There are seven poorly preserved ribs, and all except the first and last contact the poorly preserved transverse processes. It is unclear how many of these represent dorsosacral vertebrae. A layer 3–10 cm thick and 45–50 cm wide covers part of the trunk dorsally, and contains numerous flat and oval dorsal osteoderms ( Fig. 3 ). The sacrum includes five almost complete verte−

ARBOUR ET AL.—ANKYLOSAURID DINOSAUR FROM MONGOLIA 57

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brae, including one caudosacral. The centrum is not fused to the last sacral and is thus not part of the sacral rod.

The tail is composed of at least 28 caudal vertebrae, 14 of which are free caudals ( Table 2). The free caudals ( Figs. 1, 4 ) occur in two separate blocks of sandstone (free caudals 1–7 in a block 77 cm long, and free caudals 8–11 in a block 48 cm long), and are visible in right lateral view. Free caudal 8 is split in half and the two halves can be seen in anterior and posterior view in their respective blocks. The total length of the free caudals is 1.245 m. More proximal caudals are of almost the same height and width, while distally the caudals become more elongate. All the neural arches except for the neural arch of the first free caudal are fused to the centra. The neural spines decrease in height posteriorly, and become more inclined posteriorly. The neural spine of free caudal 2 is vertically oriented, while the neural spine of free caudal 7 is tilted some 45 ° posteriorly. The neural arches become progressively lower and more elongate. The prezygapophyses are long, narrow, and horizontal; their length increases posteriorly along the tail. The postzygapophyses are very short for the first three free caudals and disappear completely after free caudal 3. The lengths of the prezygapophyses of free caudals 2, 7, and 10 are 42 mm, 53 mm, and 59 mm, respectively. The transverse processes become reduced posteriorly and are not visible posterior to free caudal 8. The haemal spines are largely obscured by the surrounding matrix.

The 17 caudals (handle vertebrae, following the terminology of Coombs, 1995) making up the handle ( Figs. 1, 4 ) of the tail club are partly embedded in five separate blocks of

http://dx.doi.org/10.4202/app.2011.0081

sandstone. Generally, the ventral sides of the vertebrae are all covered by matrix, except for the block containing the last three vertebrae. The total length of the handle caudals is 1.965 m. These caudals are typical for ankylosaurids, with elongate prezygapophyses and neural spines that form interlocking Vs in dorsal view. The neural spines and prezygapophyses diverge at an average angle of 35 °. Ossified tendons are preserved on the lateral sides of the handle. The total length of the tail is 3.1 m.

Ten different types of osteoderms have been identified in the specimen and these vary in size, shape and surface detail ( Fig. 2). An attempt to organise and generalise armour terminology was made by Blows (2001), although several osteoderms described herein do not correspond to any of these terms, or to the terms initially used by Maryańska (1969).

The large, backswept Type 1 plates of Maryańska (1969) are not preserved in ZPAL MgD I/113, nor are morphologies corresponding to Type 3 (slightly compressed keeled osteoderms with a sharp, slightly backswept tip). Osteoderms corresponding to Types 2 and 4–6 are preserved. However, morphologies not described by Maryańska (1969) are also present, and these are designated as Types 7 through 10 herein.

Type 2 osteoderms ( Fig. 2A, B) are sharply keeled and located along the lateral edges of the tail. In the anterior region of the tail, Type 2 osteoderms are triangular in dorsal view with sharp apices ( Fig. 2A). Type 2 osteoderms in the posterior part of the tail have more rounded apices ( Fig. 2B). A Type 4 osteoderm is conical with a circular base ( Fig. 2C). In lateral view, the osteoderm is a tilted cone with a rugose surface. Type 5 is a medium−sized osteoderm with an oval base ( Fig. 2D). A distinct keel extends from the pointed end to meet the longitudinal base line in the rounded end. The keel is strongly angled in dorsal view near the tapered end. Small, flattened osteoderms with circular bases are referred to here as Type 6 ( Fig. 2E). The dorsal surface is rugose but lacks any additional structures. Large, flattened, keeled, osteoderms with slightly concave ventral surfaces are identified as Type 7 ( Fig. 2F). These osteoderms are thin−walled and rugose. Type 8 osteoderms are small, nearly flat, elongate ovals ( Fig. 2G). These are thin−walled, and each has a central or displaced keel and a slightly concave ventral surface. One unusual Type 8 osteoderm is ornamented by ridges and grooves, and has a notched peak ( Fig. 5 ). A Type 9 osteoderm has a teardrop−shaped outline in dorsal view ( Fig. 2H). The keel extends from the tapered end and terminates in a small apex at the centre of the broad end. The base is deeply concave. A strongly keeled osteoderm with an elongate oval base that is pointed at both ends is referred to as Type 10 ( Fig. 2I). Three Type 10 osteoderms have been identified on the dorsal surface of the end of the tail.

The remains of in situ abdominal osteoderms are generally fragmentary and incomplete. A few disarticulated osteoderms identified as being from the trunk (Types 4, 7, and 10) have been recognized. Most impressively, a detached layer containing an almost complete pattern of dorsal trunk integument with imprints from keratinous scales between the larger bony osteoderms is present. This layer is from the dorsal side of the torso, and extends parallel to the vertebral column ( Fig. 3 ). Keratinous scale impressions are large, keeled and approximately rectangular. Single or paired sequences of imprints separate and surround the bony osteoderms.

Note the angled keel and the two parallel grooves on the left side. E. Representative Type 6 osteoderm in dorsal view. This osteoderm is positioned near the posterior end of the tail. F. Representative Type 7 osteoderm in dorsal (F 1), lateral (F 2), and ventral (F 3) views. Note the extremely rugose surface, which is unique to this type. G. Two representative Type 8 osteoderms in; dorsal view in situ (G 1), and dorsal view when removed from tail (G 2). A slightly displaced keel runs in the longest direction. H. Sole Type 9 osteoderm in lateral (H 1), dorsal (H 2), and ventral (H 3) views. A distinct keel is present at the posterior end and terminates in an anterior crest (broken). I. Representative Type 10 osteoderm in dorsal (I 1) and lateral (I 2) views.

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The caudal armour is mostly composed of Type 2 and 5 osteoderms ( Fig. 4 ). Type 2 plates in the specimen originate from the base of the tail. It is reasonable to assume that the Type 2 osteoderms that are still in matrix are in situ (or close to being in situ) because the longest axis of each is parallel to the axis of the tail, they all have dorsal grooves, and each is positioned adjacent to a vertebral centrum. All of the Type 2 osteoderms in the specimen have large grooves on their dorsal surfaces. The last three vertebrae are covered by small osteoderms of Types 6 and 8 ( Fig. 5 ). Unfortunately, ZPAL MgD I/113 does not have a tail club knob, which presumably was lost prior to collection .

Figures

Fig. 3. An ankylosaurid dinosaur Tarchia cf. gigantea, ZPAL MgD I/113 from the Nemegt Formation at Altan Uul III, Mongolia. A. Dorsal view of the trunk. Skin impressions and osteoderms are visible on the trunk. Osteoderms are positioned in situ along the vertebral column. The specimen only represents the posterior part of the trunk. B. Interpretive drawing of osteoderms (white) and scale imprints (grey).

Fig. 4. Tail of an ankylosaurid dinosaur Tarchia cf. gigantea, ZPAL MgD I/113 from the Nemegt Formation at Altan Uul III, Mongolia, compared to tail of Saichania (MPC 100/1305) from the Baruungoyot Formation at Khulsan, Mongolia. Osteoderms found attached to the tail of ZPAL MgD I/113 are marked by numbers according to the Types 1–10 description in the text.

Fig. 5. Dorsal view of the most distal part of the tail preserved in an ankylosaurid dinosaur Tarchia cf. gigantea, ZPAL MgD I/113 from the Nemegt Formation at Altan Uul III, Mongolia. Five relatively small scutes cover the vertebrae (Types 6 and 8). Two large plates fuse to the centra of the three most distal vertebrae. Note the large grooves on both plates (darkened and arrowed), and the shapes of the distal keels. One handle vertebra is darkened to show the interlocking prezygapophyses and neural spines. Anterior is to the right.