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Journal of Paleontology 92(2):272–288
chasmosaurines (Currie et al., 2016; Lehman et al., 2016) and Zuniceratops (Wolfe et al., 2010). ROM 73834 (Fig. 6.3, 6.4, 6.13).—This specimen only
preserves the horncore portion of a right postorbital. The palpebral contact is completely open (Fig. 6.4). The frontal suture is only open anterior to the horncore and the frontal is indistinguishably fused posteriorly. The supracranial sinus system is represented by two depressions on the medioventral surface of the specimen (Fig. 6.4) that do not invade the shaft of the horncore, as is the typical condition for Centrosaurini (Farke, 2010). The length of the horncore (139.7mm) is ~20% longer than that of TMP 2002.069.0010 (Fig. 6.12). The cross section of the horncore is more flattened and triangular than that of TMP 2002.069.0010, with its flat surface projecting poster- olaterally rather than laterally as for juvenile Centrosaurus apertus (Ryan et al., 2001, figs. 10C–10F) and Coronosaurus brinkmani (Ryan and Russell, 2005). WDC-DJR-003 (Fig. 6.9–6.11).—This specimen preserves
the supraorbital region (fused lacrimals, palpebrals, postorbitals, and frontals) with the almost complete right (the tip is reconstructed in the figure) and incomplete left horncores. The preserved horncore length is 296mmwith a basal circumference of 383mm. Unlike TMP 2002.069.0010 and ROM 73834, the cross-section of these horncores is circular (although they have been slightly distorted taphonomically). The horncore is strongly laterally oriented, but this is exaggerated due to the deformation (Fig. 6.9, 6.10). The basal overgrowth of the contact surface for the keratinous sheath, which is prominent on ROM 73831, is only poorly developed on the dorsoposterior two-thirds of the basal circumference of this
specimen.Aunique shallow, broad depression occurs on the anterior and ventral side of the horncore base, which is different from small round deep pit on the ROM 73831 (Fig. 6.11). The frontal fontanelle margin is intact anteriorly, and it
seems to have been open at the time of death. The supracranial sinus system is restricted to the base of the horncores. The antorbital buttress is swollen, making an oval-shaped eminence in lateral view (Fig. 6.10). ROM 73831 (Fig. 6.5–6.8, 6.14).—ROM 73831 is a large
(the basal circumference is 500mm) left postorbital fused with its complementary lacrimal, palpebral, and frontal. The massive horncore is similar to those of Albertaceratops. The anterior region of the orbital rim is thickened, creating a prominent antorbital buttress (Fig. 6.7). The medial margin of the frontal does not reach the midline, indicating an open frontal fontanelle. The supracranial sinus (Fig. 6.8) partially invades the horncore at its base, similar to that seen in WDC-DJR-003. The tip of the horncore is broken and missing, and therefore the total length of the horncore is unknown, but the circumference of the horncore at its base is large (500mm) and comparable to the largest horncore size of the non-Triceratopsini chasmosaurines (510mm in Pentaceratops sternbergii,Wiman, 1930; Fig. 7; Table S2). The cross-section of the horncore is circular throughout the entirety of its preserved length. The horncore has a strong lateral inclination (~80˚ from the sagittal plane in the dorsal and ventral view),with only a limited projection dorsally (~20˚ from the horizontal plane in the anterior and posterior view). The horncore also has a slight curvature in dorsal and ventral views, but far less than that of Nasutoceratops (Sampson et al., 2013; Lund et al., 2016b).
500
300
100
non-Triceratopsini chasmosaurine
Triceratopsini
Figure 7. Comparison of postorbital horncore basal circumference between Medusaceratops, Centrosaurinae, non-Triceratopsini Chasmosaurinae, and Triceratopsini. In the box plots, mean values are represented by lines in the boxes, lower and upper bounds of the boxes represent the first and third quartiles, and the ends of the dashed lines indicate minimum and maximum values of the data. Data used for this plot are provided in Table S2.
The horncore has an unusual overgrowth that creates the
step at the base of the horncore (Fig. 6.5, 6.6), which is also seen in the postorbital horncores of Albertaceratops nesmoi Ryan, 2007. Spiclypeus shipporum Mallon et al., 2016 also has a step at the base of the right postorbital horncore, but the step is restricted to the posterior side of the horncore, and it appears to be formed by extensive pitting rather than overgrowth. The holotype horncores of ‘Ceratops montanus’ (Marsh, 1888) from the Judith River Formation (Mallon et al., 2016) do not have this overgrowth. On the ventral side of the horncore, there is a round foramen that is suggestive of resorption (Fig. 6.5), which is known to occur in postorbital horncores of various eucentro- sauran centrosaurines, such as Coronosaurus (Ryan and Russell, 2005), Spinops (Farke et al., 2011), Centrosaurus (Ryan et al., 2001; Tanke and Farke, 2006), Styracosaurus (Ryan et al., 2007), and Einiosaurus (Sampson, 1995), but not in non-Eucentrosaura centrosaurines.
Nasal.—The nasals of Medusaceratops lack a distinct vertical horncore but instead have ornamentation that is in the form of a low, elongate, rugose
ridge.No newnasalmaterial can be added to the sample described by Ryan (2007). We re-examined the available nasals from the bonebed, but have no significant mor- phological observations beyond those in Ryan (2007); we there- fore direct the reader to the appropriate section of that publication.
Osteohistological ontogenetic assessment
In order to assess the potential that thematerial fromtheMansfield bonebed represents the full ontogenetic sequence of this taxon, the distal end of a left tibia from the bonebed (ROM 67873) was histologically sampled at its minimum diaphyseal circumference. This specimen represents one of the largest individuals among the bonebed material (minimum circumference=300mm). The mid- diaphysis is very robust and the lateral half of the distal end,where the calcaneum articulated, is extended distally compared to the medial half, which is characteristic of ceratopsids. Although it cannot be unequivocally referred to Medusaceratops,itis
horncore circumference (mm)
centrosaurine
Medusaceratops
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