ONTOGENETIC

Wosik, Mateusz, Goodwin, Mark B. & Evans, David C., 2019, Nestling-sized hadrosaurine cranial material from the Hell Creek Formation of northeastern Montana, USA, with an analysis of cranial ontogeny in Edmontosaurus annectens, PaleoBios 36, pp. 1-18 : 7-9

ONTOGENETIC ASSESSMENT

Dentary —The robustness of the dentary ramus in Edmontosaurus annectens appears to increase proportionately through ontogeny with the length of the dentary ( Fig. 5). At the edentulous margin, the ramus significantly begins to thin mediolaterally in juveniles (ROM 53529, 53530) and only the ventral half maintains a robust appearance. The symphyseal process deflects anteroventrally more in juveniles (Character 39 [1]; 152–156°) relative to UCMP 235860 (165°) and subadults (ROM 73859) (162°). The anterior terminus of the symphyseal process strongly curves dorsomedially in subadults to form a flat ventral margin that is parallel with the mandibular symphysis and the horizontal long axis of the dentary (Character 46). This recurving of the mandibular symphysis region begins to occur at the juvenile stage because ROM 53530 exhibits the dorsomedial curving whereas ROM 53529 is still more ventrally oriented and is relatively smaller in size and ontogenetically younger based on linear proportions and alveoli counts, respectively. The ventrolateral foramina vary in size and number through ontogeny without any clear pattern beyond an anteroposterior alignment that follows the thin dorsal half of the dentary ramus and cease around the midpoint of the dental battery.

The length of the diastema is longest in adults (ROM 57100) and makes up a quarter of the total dentary length. The ratio between the length of the edentulous margin anterior to the dentary dental battery and the horizontal distance between the first tooth position and the posterior end of the coronoid process progressively increases with ontogeny (Character 38); UCMP 235860 exhibits a ratio of 0.27 (State 1), juveniles vary from 0.31 (State 1; ROM 53529) to 0.37 (State 2; ROM 53530), and subadults are greater than 0.51 (State 3; ROM 73859). The angle made between the horizontal and the slope of the dentary anterior region that contacts the predentary is slightly larger in adults (Character 40 [0]).

The posterior end of the dental battery extends beyond the posterior margin of the coronoid process in the larger of the two juveniles ( ROM 53530 ) (Character 56). The dental battery increases along the major axes to accommodate the addition and enlargement of tooth rows, also demonstrated histologically by Bramble et al. (2017). The posterodorsal deflection of alveoli is main- tained for the anterior region of the dental battery but the larger juvenile dentary also exhibits an anterodorsal deflection of posterior alveoli; the two deflections meet near the coronoid process and alveoli become parallel with the vertical axis of the dentary. Alveoli count also increases through ontogeny from nine in UCMP 235860 to 44 in the subadult (Character 1) .

The coronoid process generally becomes more robust and the base expands further laterally. The slight anterior orientation of the coronoid process relative to the horizontal axis observed in UCMP 235860, juveniles, and subadult becomes more anteriorly inclined in adults (Character 48 [2]). The development of the ridge on the medial side of the coronoid process, which is faintly present although not well preserved in UCMP 235860, becomes more developed and posteriorly expanded through ontogeny (Character 52). The Meckelian fossa is deep in juveniles such that the posteroventral region of the dental battery creates a distinct overhang that tapers posteriorly. The Meckelian groove also becomes deeper and contributes to this overhang gradually tapering to form the ventral margin of the dentary directly dorsal of the anterior most alveolus.

Surangular —The periosteal surfaces of juveniles (ROM 53535, 53536) are demarcated with fine striations and maintain a slightly porous exterior, although markedly less than UCMP 235857. The periosteal surface of the adult (ROM 64076) exhibits a high degree of scarring and deep striations,which are occasionally inter-weaving particularly near areas of soft tissue attachment. The lateral lip is asymmetrical in juveniles with a sharper apex that is anteriorly directed. The glenoid facet is also slightly porous in juveniles but begins to develop heavy scarring towards the deepest region of this concavity, similar to the adult condition. The dorsal foramen of juveniles and adults appears to have migrated anterolaterally towards the lateral lip from a central location just anterior of the transverse ridge in UCMP 235857. The anterior end of the dorsal ridge terminates sharply and forms a slight overlap in juveniles and adults, which is significantly different from the gradual decline of UCMP 235857. The splenial and articular grooves are much deeper in juveniles and adults and exhibit a narrow, subtriangular shelf that extends medially and slightly dorsally from the middle of the dorsal surface. This subtriangular shelf is best developed in adults and more elongated anteroposteriorly than in juveniles. In ventral view, the anteroventral V-shaped region is progressively more depressed with adults exhibiting the most extreme condition and exhibits strong anteroposteriorly oriented striations with minimal porosity. In dorsal view, the concavity of the ascending anterior flange in juveniles is significantly more excavated dorsomedially than in UCMP 235857 forming a thin slate of bone for the mandibular adductor fossa. Adults have a similarly deep excavated region but it is bounded with a more gradual transition from the body of the surangular, whereas the rim edges of juveniles are sharper and more pronounced.

Quadrate —In posterior and anterior views, the vertical ramus maintains a straight and vertical orientation through ontogeny( Fig. 6). Juveniles (ROM 53521, 53522) and adults (ROM 57100, 64076, 64623) have a much larger lateral exposure for articulation of the quadratojugal than UCMP 235857, where it is almost nonexistent (Character 136). The quadratojugal notch forms a scarf joint with the quadratojugal along the anterodorsal half of the notch, which is well-developed in juveniles and especially adults but not present in UCMP 235859.

The robustness and posterior deflection of the vertical ramus increases through ontogeny and the pterygoid flange, including its base, becomes proportionately more arcuate. The posteriorly directed deflection of the ventral half is only slightly exhibited in UCMP 235857 and juveniles but is prominent in adults and leads to a wider and more robust lateral condyle. Although the degree of curvature in lateral view cannot be precisely determined in UCMP 2358 because the dorsal half of the dorsal flange is not preserved, the angle between the long axes of the dorsal and ventral flanges of the quadrate range from 145°–162° through ontogeny (Character 129). The lateral condyle, which articulates with the mandibular glenoid of the surangular, expands through ontogeny while the medial condyle appears to undergo a reduction in size relative to the lateral condyle (Character 134). The elevation of the medial condyle relative to the lateral condyle increases through ontogeny where the medial condyle is remarkably elevated in adults but less so in UCMP 235859 and juveniles (Character 135 [1]).

Along the posteroventral margin of the ventral half of the quadrate is a feature that resembles a muscle attachment site; this feature is proportionately largest in UCMP 235859 and reduces through ontogeny until it is com- pletely lost in the adult. The striated markings present in juveniles and adults along the margins of each flange are not present in UCMP 235859. Although the squamosal buttress near the quadrate head is not preserved in UCMP 235859, juveniles and adults have nearly identical morphologies that proportionately increase in size and robustness with the quadrate (Character 133 [0]). The posterolateral spur is rounded and just slightly rises above the surface of the body of the quadrate in UCMP 235857, is well-developed in juveniles with significant detail for soft tissue attachment, and varies in adults between the juvenile condition (ROM 64623) and a more gracile and rounded appearance (ROM 64076). Although the neighboring fossa expands in size through ontogeny, its degree of excavation varies within and between size classes. The ridge extending along the dorsoventral height of the vertical ramus in posterior view exhibits a similar level of variation. Dorsal of the posterolateral spur, the ridge is relatively thinner in some juveniles (ROM 53522) and adults (ROM 64623), and the ventral portion in the same specimens is relatively thicker and less pronounced. This variation does not seem to be related to the degree of excavation of the fossae that neighbor the posterolateral spur.