Dinosauria Owen 32
Ornithischia Seeley 33
Ornithopoda Marsh 34
Hadrosauridae Cope 35
Yamatosaurus izanagii gen. et sp. nov.
Nomenclatural Acts. This published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the International Code of Zoological Nomenclature. The ZooBank life science identifiers (LSID) can be resolved and the associated information viewed by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:act: 1C728683-C3EA-4688-9420-096D13EB6227.
Etymology. “Yamato” refers to the ancient name for a region of the Japanese archipelago, including the western half of the main island (Honshu Island), Shikoku Island, and the northern half of Kyushu Island, ruled by the Yamato Kingdom from the third to the seventh century. “Sauros” means reptiles. The specific name, “izanagi”, refers to a deity in Japanese mythology, which created eight countries of Yamato with another deity, Izanami, based on the oldest history book in Japan, called “Kojiki (Records of Ancient Matters)”, published in 712 CE (Common Era). The first country created was the Awaji Island, followed by the Shikoku, Oki, Kyushu, Iki, Tsushima, Sado, and Honshu islands.
Holotype. MNHAH D1-033516, a right dentary, a right surangular, twelve isolated dentary teeth, four cervical vertebrae, a distal caudal vertebra, three cervical ribs, and a coracoid (Figs. 1F, 2–6). This specimen is stored in the Museum of Nature and Human Activities, Hyogo, Sanda City, Hyogo Prefecture, Japan.
Locality and horizon. Loc. Aw14 (Morozumi 22) of Yura Town, Sumoto City of the Awaji Island, Hyogo Prefecture, Japan; the early Maastrichtian (71.94–71.69 Ma) 29 Kita-ama Formation of the Izumi Group.
Diagnosis. The hadrosaurid with unique characters in having the presence of only one functional tooth/row in the middle of the dentary dental battery and the complete absence of the “branched ridges” on the dentary tooth occlusal surfaces. It is also unique in the combination of the following characters: the small angle between the medial surface of dentary symphysis and lateral surface of the dentary and ventrally facing ventral surface of the surangular.
Description. The right dentary is nearly complete, missing its posterior end and the coronoid process. The lateral surface of the main body of the dentary bears multiple neurovascular foramina. A large foramen is positioned at the level of the seventh dentary tooth, and several smaller foramina are present posteriorly. In lateral view, the anterior portion of the dentary is downturned, and an angle between the ventral edge of the anterior portion and a horizontal plane, or tooth row, is 20 degrees (Fig. 2A). In medial view (Fig. 2B), the long axis of the tooth row is parallel to the ventral margin of the middle region of the dentary main body as in hadrosaurids except for Brachylophosaurini 36. A thin alveolar parapet, covering the medial surface of the dental battery, is dorsoventrally taller posteriorly than anteriorly. The nutrient foramina are organized in a shallow arch ventral to the dental battery. The Meckelian groove extends along the ventral margin of the posterior two-thirds of the dentary. The ventral edge of the dentary main body below the coronoid process is weakly bowed. In dorsal view, the occlusal surface of the dental battery is straight and parallel to the lateral surface of the dentary main body (Fig. 2C).
The dentary diastema is approximately 99 mm. The length is 30% of the length between the anterior end of the dental battery and the posterior margin of the coronoid process (330 mm). Although the coronoid process is not preserved, the latter length was estimated from the base of the coronoid process. The dorsal margin of the dentary, where the lateral ramus of the predentary contacts, is strongly concave in lateral view and is angled by 110 degrees from a horizontal plane (Fig. 2B).
The symphysial process of the dentary curves medially at nearly a right angle (Fig. 2E) as in non-hadrosaurid hadrosauroids (e.g., Plesiohadros djadokhtaensis; MPC-D 100/745) and some members of Lambeosaurini, including Olorotitan arharensis 37 and Parasaurolophus tubicen (NMMNH P-25100). The maximum mediolateral width of the dentary symphysial region in dorsal view is 81 mm, which is approximately twice as wide as the minimum breadth of the dentary posterior to the dentary symphysial region (Fig. 2C). The medial margin of the symphysial process is largely divergent from the lateral surface of the main body of the dentary by an angle of 20 degrees as in Eotrachodon orientalis 38 but unlike a nearly parallel alignment in hadrosaurids such as Edmontosaurus regalis 39 and Parasaurolophus tubicen (NMMNH P-25100). The medial surface of the symphysial process bears a shallow groove for contact with its counterpart (Fig. 2B). The ventral surface of the symphysial process has a large neurovascular foramen (Fig. 2D).
The base of the coronoid process is laterally expanded as in hadrosaurids and some non-hadrosaurid hadrosauroids such as Plesiohadros djadokhtaensis, Nanningosaurus dashiensis, and Adynomosaurus arcanus 40–42 and is separated from the dental battery (Fig. 2E). The posteromedial surface of the coronoid process is largely excavated, forming the mandibular adductor fossa.
The dorsolateral portion of the right surangular is preserved (Fig. 3). The preserved region of the anterodorsal process is thin and bears a well-defined ridge for the attachment of m. adductor mandibulae externus superficialis 43. The ridge is posteriorly continuous with the dorsolateral flange, which is gently arcuate in dorsal view. The lateroventral surface of the surangular bears no foramen as in derived hadrosauroids including Bactrosaurus johnsoni 44,45. The dorsal surface of the anterior half of the surangular is concave to form the mandibular adductor fossa, whereas the posterior half forms a glenoid fossa that receives the mandibular condyles of the quadrate. The ventromedial portion of the surangular is lost before collection thus the morphology of the caudal process and the retroarticular process is unavailable.
At least 34 tooth positions are present, and each tooth position bears a maximum of four teeth in the right dentary, and at least twelve isolated dentary teeth from the left side are preserved. Tooth crowns are diamond-shaped and bear a prominent straight primary ridge in the middle teeth, while the primary ridge is sinuous in mesial and distal teeth (Figs. 2B, 4A, 4H). The secondary ridges are faint, wrinkled, and in some cases branched as in Eotrachodon orientalis 38 and Hypacrosaurus altispinus 46 but unlike the straight secondary ridges of Corythosaurus casuarius (ROM 868) and Lambeosaurus lambei (CMN 361, CMN 2869). The largest tooth crown, located in the middle portion of the dental battery, is approximately 41.94 mm high and 14.66 mm wide. The tooth crowns become shorter and narrower mesially and distally (Fig. 2B). The height/width ratios of the dentary tooth crowns are slightly less than three on average although vary largely by position. The marginal denticles along the mesial/distal edges of the coronal half of the tooth crown are proportionately smaller than those of Eotrachodon orientalis 38 and Lambeosaurus lambei (CMN 351) and resemble those of Hypacrosaurus altispinus in size 46. The marginal denticles of Yamatosaurus izanagii are formed of multiple small papillae (Fig. 4B-C), which are unorganized unlike in Eotrachodon orientalis 38 and Corythosaurus casuarius (AMNH FARB 8527), but resembling Gryposaurus notabilis (AMNH FARB 8526). The angle between the crown and root of the dentary teeth is 132 degrees (Fig. 4B).
An occlusal surface of an isolated tooth changes from a flat pentagonal surface (Fig. 4D-E) to a concaved oval-shaped surface (Fig. 4F-G) as it wears. At most two teeth are functional per tooth position throughout the dental battery unlike hadrosaurids (e.g., Edmontosaurus regalis, CMN 2289; Parasaurolophus tubicen, NMMNH P-25100), which have more than two functional teeth in the middle of the dental battery. Only one functional tooth in 9th, 11th, 14th, 16th, 19th, 21st, and 23rd tooth positions (Fig. 4I-J), which is unique to Yamatosaurus izanagii. The occlusal surface of the dental buttery is steeply inclined as in Prosaurolophus maximus (CMN 2277, CMN 2280), the distal area of the dental battery of Brachylophosaurus canadensis (CMN 8893), and the mesial area of the dental battery of Gryposaurus (Gryposaurus notabilis, ROM 873; Gryposaurus latidens, AMNH FARB 5465). The occlusal surface of Yamatosaurus izanagii bears low ridges that are associated with cementum and longitudinal giant tubules that fill the pulp cavity 13,47. The occlusal topography is much lower than in Corythosaurus 13 but resembles that of Brachylophosaurus canadensis (CMN 8893). The strong inclination and the low topography of the occlusal surfaces may suggest the pulp cavity of teeth of Yamatosaurus izanagii is filled mainly with transverse giant tubules, which are less wear-resistant than longitudinal giant tubules 13.
At least four cervical vertebrae are preserved, and three of them are fairly complete (Fig. 5A-O). Although the exact position of these vertebrae is ambiguous, two are identified as anterior (fourth or fifth and fifth or sixth) and one as a middle (seventh to ninth) cervical vertebra based on comparisons with Gobihadros mongoliensis (MPC-D 100/746). The anterior cervical vertebrae are nearly complete other than postzygapophyses, whereas the middle cervical vertebra is missing the ventral half of its centrum. The anterior centra are strongly opisthocoelous. The anterior convexity and the posterior concavity are the most pronounced in the anteriormost cervical vertebra. The lateroventral surface of the centrum is deeply excavated and bears several foramina. The ventral surfaces of the centra are slightly convex and do not form a distinct keel. The neural canal is slightly wider than its height in the anteriormost cervical vertebra and becomes wider posteriorly. The transverse process is the shortest in the anteriormost cervical vertebra and becomes longer posteriorly. The dorsal surfaces of the prezygapophyses are slightly inclined mediodorsally. The diapophyses become longer posteriorly relative to the articular facets of the prezygapophyses. The neural spine is faint and lacks significant dorsal projection in the anteriormost cervical vertebrae and becomes larger posteriorly. Both sides of the base of the neural spine are shallowly depressed in the anteriormost cervical vertebra. The depressions are shallower and smaller in posterior cervicals. The postzygapophyses are less than three times as long as the anteroposterior length of the neural arch unlike hadrosaurids, including Maiasaura peeblesorum (ROM 44770) and Hypacrosaurus stebingeri (MOR 549) but resemble non-hadrosaurid hadrosauroids such as Gilmoreosaurus mongoliensis (AMNH FARB 6551). The postzygapophyseal articular surface is anteroposteriorly longer than mediolaterally wide in the anterior cervical vertebrae, whereas the articulation surface is subcircular in the middle cervical vertebra.
A distal caudal vertebra is nearly complete, missing a right prezygapophysis and a distal portion of the neural spine (Fig. 5V-Z). The centrum is amphiplatyan and is hexagonal in anterior and posterior aspects. The lateral surfaces of the centrum bear a ridge in the middle, whereas the ventral surface is gently convex. The transverse process is absent as in the 38th and more posterior caudal vertebrae of a lambeosaurine indet. (CMN 8330). The neural canal is a mediolaterally wide ellipsoid. The rod-shaped prezygapophysis lacks a distinct articular facet. The postzygapophysis is reduced and indistinguishable from the neural spine.
One right and two left cervical ribs are preserved (Fig. 5P-U). The capitulum is longer and more massive than the tuberculum in all preserved cervical ribs. The lateral crest is dorsoventrally flat and well-developed in the anterior-most preserved cervical rib and gradually diminishes posteriorly.
The right coracoid is complete (Fig. 6). The articular facet for the scapula is shorter than the glenoid and angled 114 degrees from the glenoid in the external view. The coracoid foramen is ellipsoid and does not intersect the scapulocoracoid contact. The anterior margin of the coracoid is straight. The biceps tubercle is absent. The ventral process is only half as long as its base as in non-hadrosaurid hadrosauroids (e.g., Gilmoreosaurus mongoliensis, AMNH FARB 30722), yet the ventral process is recurved caudoventrally as in hadrosaurids (e.g., Brachylophosaurus canadensis, MOR 1071 8-10-99-541; Hypacrosaurus altispinus, AMNH FARB 5272).