Glishades, Prieto-Márquez, Albert, 2010

Prieto-Márquez, Albert, 2010, Glishades ericksoni, a new hadrosauroid (Dinosauria: Ornithopoda) from the Late Cretaceous of North America, Zootaxa 2452, pp. 1-17 : 3-7

Glishades gen. nov.

Etymology: “Glis” is the Latin for “mud” and “hades” means “unseen” in Greek; thus, the generic name may be translated as “concealed in mud”, in reference to its being found in sedimentary strata. Also, “Hades” was the dark lord of the underworld in Greek mythology, here metaphorically referring to the “world” beneath the surface where fossils occur.

Type and only species: Glishades ericksoni sp. nov.

Diagnosis: As for the type species (below).

Glishades ericksoni gen. et sp. nov. Fig 1

Etymology. The specific name honors Dr. Gregory M. Erickson, for his important contributions to the knowledge of archosaur paleobiology.

Holotype. AMNH 27414, partial left and right premaxillae.

Type locality and horizon. The specimen was collected by Brown, Kaiser, and Johnson ca. 1915 in South Milk River, 30 miles west of Sweetgrass, Glacier County, Montana ( United States of America), from strata corresponding to the Two Medicine Formation (Campanian, Upper Cretaceous).

Diagnosis. Hadrosauroid dinosaur with the following unique combination of premaxillary characters: absence of recurvature (“reflection”) of oral margin; arcuate oral margin with wide and straight, obliquely oriented, and undeflected anterolateral corner; foramina on anteromedial surface above oral edge and adjacent to proximal end of narial bar; and grooved transversal thickening on ventral surface of premaxilla posterior to denticulate oral margin.

Nomenclatural note. Clade names and phylogenetic definitions follow the comprehensive recent revision of hadrosaurid interrelationships by Prieto-Márquez (in press) ( Figs 2–3 ). Accordingly, Hadrosauroidea consists of Hadrosaurus foulkii and all taxa most closely related to it than to Iguanodon bernissartensis , including Hadrosaurinae (represented solely by Hadrosaurus ) and Saurolophidae . The latter is defined as the last common ancestor of Saurolophus osborni , Lambeosaurus lambei , and all its descendants, which includes the two major hadrosaurid clades: Saurolophinae and Lambeosaurinae .

Description. Only the anterior regions of the articulated left and right premaxillae are preserved. In dorsal view, the oral margins of both premaxillae display an arcuate contour. This contour is not perfectly smooth, but is slightly truncated by the oblique orientation and nearly straight dorsal profile of the anterolateral corners of the oral margin ( Fig. 1 a). The anterior and lateral regions near the oral margin are ventrally deflected. In contrast, the anterolateral region is dorsoventrally compressed and slightly convex dorsally at the oral margin ( Fig. 1 c–d). There are transversal fractures across the lateral oral margin. Each facture separates a posterior and anterior portion, so that the posterior portion is slightly offset ventrally relative to the anterior one. This displacement only affects the lateral half of the mediolateral width of the oral margin. When the posterior segment is restored to its original position by having its anterior edge meet the posterior edge of the anterior portion, the oral margin remains smooth and ventrally deflected. Adjacent and lateral to the interpremaxillary joint, each element is anteroposteriorly expanded to form the anteriormost extent of the narial bar. The anterior surface of this expanded area is triangular and faces anterodorsally and slightly laterally. It is pierced in each premaxilla by two foramina ( Fig. 1 a, d). A larger oval foramen is located a short distance from the narial bar; the second, much smaller foramen is located anteroventrally to the larger foramen. In addition to AMNH 27414, similarly located foramina are only unambiguously present in small (e.g., AMNH 6501 and 6575) to larger (e.g., Godefroit et al. 1998: plate 3, fig 4A) individuals of Bactrosaurus johnsoni , a subadult specimen of Edmontosaurus cf. annectens (CMN 8509), and a subadult Gryposaurus sp. (CMN 8784), along with additional foramina in Brachylophosaurus canadensis (e.g., MOR 794) and Maiasaura peeblesorum (e.g., OTM F138).

The anteromedial border of each premaxilla forms an angle of 65 degrees with the ventral surface. Posterodorsally, this border curves gently, forming a less steep angle with the ventral surface of the premaxilla. Posteriorly and dorsally, the anteromedial region of each premaxilla is extremely compressed mediolaterally to form the anterior terminus of the narial bar and the medial wall of the circumnarial fossa. Only a very small portion of the latter is preserved in each premaxilla, marked by a slightly and medially recessed surface ( Fig. 1 c). Posterodorsally, that surface is bounded by a crescentic edge, which constitutes a small part of the anteroventral terminus of the narial fenestra. The dorsal surface of each premaxilla, posterior to the oral margin and lateral to the narial fenestra, is only preserved anteriorly. There is no evidence of the presence of premaxillary foramina in this surface, as in non-hadrosaurid hadrosauroids and lambeosaurine hadrosaurids.

The ventral surface of each premaxilla is carved with deep anteroposteriorly-oriented grooves ( Fig. 1 b). The prominent ridges that lie between the grooves extend anteriorly into the oral margin of each premaxilla, forming denticle-shaped structures ( Fig. 1 d). Posterior to this denticulate margin, there is another thick and transversal layer of bone. This thickening protrudes ventrally and contains the posterior extent of the series of grooves and ridges that cross most the ventral surface of each premaxilla ( Fig. 1 b–d). A deep transversal sulcus separates the oral denticulate layer from this posterior thickening. This configuration of sulcus and “double denticulate layering” of the premaxilla is typically found in hadrosaurids; among non-hadrosaurid hadrosauroids it is also present in Bactrosaurus johnsoni (e.g., AMNH 6501 and 6575). Outside Hadrosauroidea, a posterior thickening also occurs in the basal iguanodontoidean Ouranosaurus nigeriensis (e.g., cast of GDF 300).

FIGURE 4. Boxplot showing the known range of sizes for the premaxilla for the majority of hadrosauroid taxa for which this element is known. Premaxillary width is used as an indicator of the size of this bone. It is measured perpendicular to the proximal region of the narial bar, as indicated in the inserted photograph of the left premaxilla of Maiasaura peeblesorum (OTM F138). For each taxon, the box plot displays up to five statistical summary measures: smallest value of the sample, lower quartile, median (darkest horizontal line), upper quartile, and largest value. Each box contains the middle 50% of the values; its upper edge indicates the 75th percentile (upper quartile) and the lower edge represents the 25th percentile (lower quartile). The darkest horizontal line inside each box represents the median value of the sample. The ends of the vertical lines (“whiskers”) indicate the minimum and maximum values of the sample for a given taxon. Outliers are indicated by small circles. Taxon abbreviations: Ba, Bactrosaurus johnsoni ; Br, Brachylophosaurus canadensis ; Co, Corythosaurus sp., Coi, C. intermedius ; Cos, C. casuarius ; Ed, Edmontosaurus sp.; Eda, E. annectens ; Edr, E. regalis ; Eo, Eolambia caroljonesa; Eq, Equijubus normani; Gl, Glishades ericksoni ; Gr, Gryposaurus sp.; Grl, G. latidens ; Grm, G. monumentensis ; Grn, G. notabilis ; Hya, Hypacrosaurus altispinus ; Hys, H. stebingeri ; Jz, Jinzhousaurus yangi; Lm, Lambeosaurus sp.; Lmc, L. laticaudus ; Lml, L. lambei ; Lmm, L. magnicristatus ; Ma, M. peeblesorum ; Ol, Olorotitan ararhensis ; Paw, Parasaurolophus walkeri ; Pb, Probactrosaurus gobiensis ; Pt, Protohadros byrdi; Saa, Saurolophus angustirostris ; Sao, S. osborni ; Te, Telmatosaurus transsylvanicus ; Th, Tethyshadros insularis; Ts, Tsintaosaurus spinorhinus ; Ve, Velafrons coahuilensis .

Figures

FIGURE 1. Paired premaxillae from the holotype specimen of Glishades ericksoni (AMNH 27414) in dorsal (a), ventral (b), left lateral (c), and anterior (d) views.

FIGURE 2. Strict consensus cladogram of the 80 most parsimonious trees derived from maximum parsimony analysis of 56 hadrosauroid taxa, showing the phylogenetic position of Glishades ericksoni. Numbers above branches indicate bootstrap proportions, whereas those below indicate decay indices.

FIGURE 3. Maximum agreement subtree derived from maximum parsimony analysis of 56 hadrosauroid taxa, showing the phylogenetic position of Glishades ericksoni. Numbers above branches indicate bootstrap proportions, whereas those below indicate decay indices.