Ugrunaaluk kuukpikensis, Mori & Druckenmiller & Erickson, 2016

Mori, Hirotsugu, Druckenmiller, Patrick S. & Erickson, Gregory M., 2016, A new Arctic hadrosaurid from the Prince Creek Formation (lower Maastrichtian) of northern Alaska, Acta Palaeontologica Polonica 61 (1), pp. 15-32 : 20-24

Ugrunaaluk kuukpikensis sp. nov.

ZooBank LSID: urn:lsid:zoobank.org:act:1CAF186F-11A2-4A9E-A8F9-C3789B97459F

Figs. 4–10 .

Etymology: The specific name is derived from the Iñupiaq word kuukpik, which refers to the Colville River, Alaska, USA along which the type material was found.

Type material: Holotype: UAMES 12995 , anterior portion of a size class 1 right premaxilla . Paratypes: All paratypes are of size class 1, unless otherwise specified. UAMES 4271 , posterior portion of the right nasal ; UAMES 13250 , left prefrontal ; UAMES 4245 , left lacrimal ; UAMES 4189 , right jugal ; UAMES 4272 , left quadratojugal ; UAMES 4286 , right quadrate ; UAMES 33308 , right postorbital of size class 3 ; UAMES 4361 , right squamosal of size class 2 ; UAMES 4327 , right maxilla ; UAMES 15284 , left laterosphenoid ; UAMES 4357 , right prootic ; UAMES 4301 , basisphenoid ; UAMES 4276 , basioccipital ; UAMES 4309 , parietal ; UAMES 4291 , supraoccipital ; UAMES 4095 , right exoccipital-opisthotic ; UAMES 4240 , right ectopterygoid ; UAMES 4331 , left palatine ; UAMES 4215 , left pterygoid ; UAMES 4437 , predentary ; UAMES 4946 , left dentary of size class 2 ; UAMES 4457 , right surangular ; UAMES 6646 , dorsal vertebra of size class 3 ; UAMES 23071 , sacrum of size class 3 ; UAMES 4873 , right coracoid ; UAMES 12711 , right scapula ; UAMES 21596 , right humerus ; UAMES 12525 , right ulna ; UAMES 6272 , left radius ; UAMES 6637 , left ilium ; UAMES 22058 , pubis ; UAMES 12955 , left ischium ; UAMES 12515 , femur ; UAMES 12715 , left tibia ; UAMES 15553 , left fibula ; UAMES 21950 , astragalus ; UAMES 21884 , right calcaneum ; UAMES 12545 , right metatarsal IV of size class 3 .

Type locality: Liscomb bonebed, along the Colville River , northern Alaska, USA. The exact location is on file with the Bureau of Land Management Arctic Field Office .

Type horizon: Upper portion of the Prince Creek Formation , lower Maastrichtian (Upper Cretaceous) .

Referred specimens.—See SOM 1.

Diagnosis.—Saurolophine hadrosaurid that differs from Edmontosaurus in possessing the following unique combination of characters: a circumnarial ridge of the premaxilla that projects posterolaterally without a premaxillary vestibular promontory ( Edmontosaurus has anteroposteriorly expanded circumnarial ridge with vestibular promontory); groove lateral to the posterodorsal premaxillary foramen is shallow (deep in Edmontosaurus annectens ); absence of a shallow postorbital fossa ( Edmontosaurus has a distinct, deep postorbital pocket); dorsoventrally short maxilla (relatively taller in E. annectens ); relatively gracile jugal (relatively robust in E. annectens ); the posterior margin of the anterior process of the jugal is strongly angled (less angled in Edmontosaurus ); wide lateral exposure of the quadratojugal (relatively narrow in E. regalis ); short symphyseal process of the dentary that is 30% dental battery length (relatively longer in E. annectens ).

Description.—In this section, only taxonomically informative characters that distinguish Ugrunaaluk kuukpikensis gen. et sp. nov. from Edmontosaurus annectens and E. regalis are described, along with a description of ontogenetic trajectories for these features. Characters that are ontogenetically variable were excluded. A more comprehensive and detailed description of all skeletal elements of Ugrunaaluk kuukpikensis gen. et sp. nov., along with an analysis of ontogenetic variation in the taxon, will be presented elsewhere.

Premaxilla: In size class 1 specimens of Ugrunaaluk kuukpikensis gen. et sp. nov., the circumnarial ridge of the premaxilla is narrowly triangular in the parasagittal plane and projects posterolaterally, extending nearly to the lateral margin of the premaxilla. As a result, the circumnarial ridge divides the anterior region of the circumnarial fossa into anterior and posterior premaxillary depressions. In Edmontosaurus regalis and E. annectens , the circumnarial ridge diverges laterally to form an anteroposteriorly expanded vestibular promontory, giving this structure an inverted fan-shape in dorsolateral view ( Fig. 5 ). The vestibular promontory is not present in any specimens of U. kuukpikensis gen. et sp. nov., including a size class 2 specimen (UAMES 4184), which has a poorly preserved circumnarial ridge. In E. annectens and E. regalis , the vestibular promontory also defines the medial margin of an elongate and well defined lateral premaxillary cavity (accessory fossa of Prieto-Márquez 2011), which is absent in U. kuukpikensis gen. et sp. nov. Size class 2 E. annectens premaxillae (AMNH 5046, LACM 23504, ROM 53534) also possess the same fan-shaped vestibular promontory as in other adult E. annectens specimens, suggesting that this is not an ontogenetically variable feature. Additionally, the Alaskan material bears a shallow groove lateral to the posterodorsal premaxillary foramen. This groove is also seen in other Edmontosaurus ; however, in E. annectens , this groove is more recessed anteriorly and is dorsoventrally much taller, resulting in a conspicuously C-shaped posterior outline of the circumnarial septum, even in size class 2 and adult specimens (e.g., CMN 8509, ROM 53526, LACM 23502, UCMP 128374, AMNH 5046).

Maxilla: Compared to the size class 2 maxilla of Edmontosaurus annectens (LACM 23504), the maxilla of Ugrunaaluk kuukpikensis gen. et sp. nov. (i.e., UAMES 4219, 4250, 4327) is dorsoventrally short relative to overall length (height to length ratio of 0.41 and 0.32, respectively; Fig. 6 ). During hadrosaurid ontogeny, the skull becomes relatively elongate anteroposteriorly ( Dodson 1975c; Horner et al. 2004; Campione and Evans 2011), implying a similar pattern in maxillary development ( Horner and Currie 1994; Prieto-Márquez 2011). Thus, the relatively elongate maxilla of juvenile U. kuukpikensis gen. et sp. nov. (size class 2) is not merely an ontogenetic artifact, rather it reflects a true morphological difference when compared to comparably-sized material of E. annectens (LACM 23504).

Postorbital: Several postorbitals of size class 1 are known from the LBB. Additionally, an isolated size class 3 postorbital (UAMES 33308) was also collected near the LBB from nearly the same stratigraphic layer. We regard UAMES 33308 as conspecific with Ugrunaaluk kuukpikensis gen. et sp. nov. for the following reasons: (i) it is nearly identical in morphology, although larger, than the size class 1 materials from the LBB; (ii) it lacks the dorsal promontorium on the frontal process commonly seen in lambeosaurines ( Prieto-Márquez 2008, 2010a) and therefore does not appear to represent an unrecognized hadrosaurid from the formation; and (iii) it is unlikely that two saurolophine taxa co-existed at or near the time the LBB was deposited.

In size class 1 individuals of Ugrunaaluk , the jugal process is both anteroposteriorly short and mediolaterally narrow. The regression analysis indicates that Edmontosaurus regalis has a wider jugal process than E. annectens and that the narrow jugal process of U. kuukpikensis gen. et sp. nov. can not be distinguished from an hypothetical juvenile of E. annectens . It is narrower than a hypothetical juvenile of E. regalis , although this result is not supported when the minimum dentary length is adopted as the proxy for body size ( Fig. 7E ). The jugal process shows positive allometric growth patterns in E. annectens . This is likely true in U. kuukpikensis gen. et sp. nov. as well, because the size class 3 specimen of U. kuukpikensis gen. et sp. nov. (UAMES 33308; not included in Fig. 7E regression analyses) has a relatively wider jugal process than those of size class 1.

In both postorbital size classes 1 and 3 of Ugrunaaluk kuukpikensis gen. et sp. nov., the posterodorsal wall of the orbital rim (the anterior surface of the jugal process) forms a shallow concave fossa, and most significantly, completely lacks a deep posterior postorbital pocket seen in Edmontosaurus annectens , including overlapping-sized individuals of size class 2 ( Fig. 7A, B ). The absence of the postorbital pocket in size classes 1 and 3 of Ugrunaaluk and its clear presence in size classes 2 and 3 of E. annectens (ROM 53513, ROM 53514) clearly distinguishes the two taxa.

The postorbital morphology also differs from Kundurosaurus nagornyi , which is morphologically similar to Edmontosaurus ( Godefroit et al. 2012) . The size class 3 postorbital of Ugrunaaluk kuukpikensis gen. et sp. nov. (UAMES 33308) has a large concave anterior surface of the jugal process. The anterolateral rim of the jugal process in this specimen is more extensive than in size class 1 U. kuukpikensis gen. et sp. nov. (UAMES 4268, 4983, 12965, 18224) and in K. nagornyi (AENM 2/921-6; Godefroit et al. 2012: fig. 6). Unlike K. nagornyi , the depression on the dorsal surface of the postorbital, dorsal to the jugal process, is not seen in U. kuukpikensis gen. et sp. nov. The articular surface of the frontal in U. kuukpikensis gen. et sp. nov. is identical to that of size class 2 and 3 of E. annectens (ROM 53513, ROM 53514). Dorsal to the laterosphenoid facet on the medial surface of the postorbital is an anteroposteriorly-elongated groove, which ends anteriorly on the dorsal surface of the orbit. The postorbital of K. nagornyi has a corresponding groove, but is anteroposteriorly shorter, and isolated from the anterior surface of the jugal process ( Fig. 7C ).

Jugal: Near the level of the anterior spur, the posterior margin of the anterior process angles anteriorly more strongly in Ugrunaaluk kuukpikensis gen. et sp. nov. than in both species of Edmontosaurus ( E. regalis, CMN 2289 ; E. annectens, ROM 64076, ROM 53518; Fig. 8B ). There does appear to be ontogenetic and possibly some individual variation in this feature, as a similar strong degree of angling is also seen in juvenile specimens E. annectens (size class 2, MOR 601-J.l.1; Nicolás E. Campione, personal communication 2015). However, size class 3 specimens of Ugrunaaluk (UAMES 14174) differ markedly from similar sized material of E. annectens (ROM 53518; Fig. 8B ), suggesting each species follows a different ontogenetic trajectory by this stage of development.

Additionally, the jugal of Ugrunaaluk kuukpikensis gen. et sp. nov. is more gracile than in Edmontosaurus , as measured by the ratio between the posterior constriction depth and the distance between the lower-most points of the infratemporal fenestra and orbit (jugal dorsal length; Fig. 8B 2 ). The jugals of size class 1 Ugrunaaluk kuukpikensis gen. et sp. nov. (UAMES 4922; Fig. 8D , SOM 4) have a much smaller value for the posterior constriction/dorsal length ratio (0.56±0.09, 2σ) than do size class 2 materials of E. annectens (0.76±0.11, 2σ; ROM 53518; Fig. 8B ; SOM 4). The regression analysis also indicates that U. kuukpikensis gen. et sp. nov. does not resemble the juvenile condition of E. annectens and that E. regalis and E. annectens are indistinguishable. However, the regressions ( Fig. 8E ) also fail to reject the hypothesis that U. kuukpikensis gen. et sp. nov. represents a hypothetical juvenile of E. regalis . Thus, these data suggest the jugal of Ugrunaaluk is relatively gracile with respect to E. annectens , but is equivocal with regard to this element in E. regalis .

Quadratojugal: The posterior portion of the quadratojugal, where it is not covered by the jugal laterally and thus is slightly elevated, is narrower in E. regalis (e.g., CMN 2289, CMN 8744, and ROM 658) than in E. annectens (e.g., CMN 8509, ROM 57100, UMMP 20000; Fig. 9 ). The regression analysis shows that U. kuukpikensis gen. et sp. nov. has a wider area of lateral exposure as in E. annectens , but is not similar to a hypothetical juvenile of E. regalis ( Fig. 9 ).

Dentary: The length of the posterior portion of the symphyseal process (the distance between the posterior-most point of articulation with the predentary and the anterior-most tooth socket) is a character traditionally employed in hadrosaurid cladistic analysis ( Prieto-Márquez 2008, 2010a, 2013; Godefroit et al. 2012; Xing et al. 2014). The regression analysis ( Fig. 10D ) indicates that the length of the posterior part of the symphyseal process changed as Edmontosaurus grew, being longer in E. annectens than in E. regalis ( Campione and Evans 2011) . The dentary from size class 2 Ugrunaaluk kuukpikensis gen. et sp. nov. (UAMES 4946; Fig. 10A ) shows a markedly shorter symphyseal process than E. annectens of comparable size (e.g., ROM 53530, BHI-6218; Fig. 10B ). A similar pattern is reflected in the regression analysis, thereby excluding the possibility that Ugrunaaluk kuukpikensis gen. et sp. nov. is similar to juveniles of E. annectens in this respect ( Fig. 10D ), although the former cannot be distinguished from hypothetical juveniles of E. regalis using this feature.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 4. Cranial reconstruction of Ugrunaaluk kuukpikensis gen. et sp. nov. from the early Maastrichtian Prince Creek Formation,Alaska, in left lateral view. Photograph (A) and bone interpretation (B).

Fig. 5. Premaxilla comparison of a size class 1 Ugrunaaluk kuukpikensis gen. et sp. nov. from the early Maastrichtian Prince Creek Formation, Alaska (A, UAMES 12995, anterior portion of right premaxillae in dorsal view, lacking the vestibular promontory and lateral cicumnarial cavity); adult Edmontosaurus regalis Lambe, 1917 from the late Campanian Edmonton Formation, Alberta, Canada (B, CMN 2289); and size class 3 Edmontosaurus annectens Marsh, 1892 from the late Maastrichtian Lance Formation, Wyoming, USA (C, ROM 53526). Photographs (A1–C1) and explanatory drawings (A2–C2).

Fig. 6. Maxilla comparison of a size class 2 Edmontosaurus annectens Marsh, 1892 from the late Maastrichtian Hell Creek Formation, Montana, USA (A, LACM 23502, right maxilla in lateral view) and a size class 1 Ugrunaaluk kuukpikensis gen. et sp. nov. from the early Maastrichtian Prince Creek Formation, Alaska (B, UAMES 4327, right maxilla in lateral view). U. kuukpikensis has a relatively low maxilla compared to its length. The height (H) to length (L) ratio is 0.32 whereas that of size class 2 E. annectens is 0.41.

Fig. 7. Postorbital comparison of a size class 3 Ugrunaaluk kuukpikensis gen. et sp. nov. (A, UAMES 33308) from the early Maastrichtian Prince Creek Formation, Alaska, right postorbital in medial (A1) and anterior (A2) views; size class 2 Edmontosaurus annectens Marsh, 1982 (B, ROM 53513, reversed) from the late Maastrichtian Lance Formation, Wyoming, USA, left postorbital in medial (B1) and anterior (B2) views; and adult Kundurosaurus nagornyi C, AENM 2/921-6) from the Maastrichtian Udurchukan Formation, Kundur, Russia, left postorbital in medial view (from Godefroit et al. 2012: fig. 6). Note that size class 3 of U. kuukpikensis does not possess a deep posterior orbital pocket seen in the size class 2 specimens of E. annectens, although the articular surface with the frontal is nearly identical. D. Left postorbital of E. annectens (CMN 8509) from the late Maastrichtian Lance Formation, Saskatchewan, Canada, showing how the jugal process width is defined. E. Postorbital regression analyses. Biplots of the width of the jugal process measured at one-quarter distance from its dorsal end versus dentary length. U. kuukpikensis has a narrower jugal process than Edmontosaurus regalis. For E. regalis and U. kuukpikensis the numbers in the parentheses are those calculated when the minimum dentary length, not average, is adopted as the body size proxy. When the regression lines have statistically indistinguishable slope values but show statistically significant differences in elevation, P values are marked with an asterisk. The 95% confidence interval of U. kuukpikensis dentary length is also shown by the bi-directional arrow.

Fig. 8. Jugal comparison of Ugrunaaluk kuukpikensis gen. et sp. nov. (A, UAMES 14174, right, size class 3; D, UAMES 4922, left, size class 1) from the Prince Creek Formation, Maastrichtian, Alaska; Edmontosaurus annectens Marsh, 1892 (B, ROM 53518, left, size class 2) from the late Maastrichtian Lance Formation, Wyoming, USA, in medial (B1) and lateral (B2) views; and adult Edmontosaurus regalis Lambe, 1917 (C, CMN 2289, left, reversed) from the late Campanian Edmonton Formation, Alberta, Canda. A, B1, C. Anterior processes in medial views, the dashed lines delineate the posterior border of the anterior process. In U. kuukpikensis this border is angled more strongly than in Edmontosaurus. D, B2. Left jugal in lateral view. Note that U. kuukpikensis has a shallower posterior constriction compared to E. annectens. B2 shows how the dorsal length and the posterior constriction are defined. E. Jugal regression analyses. Dorsal length versus posterior constriction of jugal. This suggests the shallower posterior constriction of U. kuukpikensis is not attributable to ontogeny.

Fig. 9. A. Quadratojugal regression analyses. Biplot of the lateral exposure width of the quadratojugal versus dentary length and quadratojugal shown in lateral view of Ugrunaaluk kuukpikensis gen. et sp. nov. Ugrunaaluk kuukpikensis is similar to Edmontosaurus annectens in the lateral exposure width of the quadratojugal, but different from Edmontosaurus regalis. The 95% confidence interval of U. kuukpikensis dentary length is also shown by the bi-directional arrow. When the regressions show statistically indistinguishable slope values but statistically significant differences in elevation, P values are marked with an asterisk. B. Right quadratojugal of U. kuukpikensis (UAMES 4298) from the early Maastrichtian Prince Creek Formation, Alaska, showing how the lateral exposure width is defined.

Fig. 10. Dentary comparison of size class 2 Ugrunaaluk kuukpikensis gen. et sp. nov. (A, UAMES 4946, left; C, UAMES 12941, right) from the late Maastrichtian Prince Creek Formation, Alaska, dentary in lateral view; and size class 2 Edmontosaurus annectens Marsh, 1892 (B, BHI-6218, cast, reversed) from the late Maastrichtian Hell Creek Formation, South Dakota, USA, right dentary in lateral view. Note that U. kuukpikensis has a shorter edentulous process than E. annectens. C. Right dentary of U. kuukpikensis showing how the proximal edentulous process length and dental battery length are defined. D. Dentary regression analyses. Biplot of posterior symphyseal process length versus dental battery length. U. kuukpikensis has a shorter symphyseal process than in E. annectens. When the regressions show statistically indistinguishable slope values but statistically significant differences in elevation, P values are marked with an asterisk. Because the lines for E. regalis and U. kuukpikensis are statistically indistinguishable, only one is shown.

Fig. 2. Temporal distribution of Edmontosaurus species and Ugrunaaluk kuukpikensis gen. et sp. nov. from the Prince Creek Formation in the Late Cretaceous.