METODOLOGIA UTILIZADA EN LA PROPUESTA DIDACTICA

Maxilla (Figs 4.1D, 4.2 and 4.3; see Appendices I and II for measurements): Both tooth- bearing main bodies of the maxillae are preserved, although slightly displaced, inclined to the left side, and more or less parallel with each other at about 30–45°. The right maxilla consists only of the incomplete tooth-bearing main body. Its dorsal processes and the posterior and anterior ends are broken off.

35

Fig. 4.2:Stereo pairs of the skull BSPG AS I 834 and explanatory sketches. The sketches are relatively enlarged for better resolution. Dark grey illustrates sediment, light grey illustrates inner views of, e.g. the frontals or the dentaries, and hatched areas illustrate broken or corroded surfaces. The label affixes -r and -l stand for right and left of the respective element, where the distinction of each side is difficult to see. A – Left lateral view. B – Outline drawing of the left lateral view. C – Outline drawing of the occipital view. D – Occipital view. See material and methods for a list of the abbreviations. Scale bars: 1 cm.

36

The posterior end, the last tooth, as well as the complete lateroventral tooth edges of the right element are sheared off. The anterior end of the maxilla is narrow, but the body abruptly widens towards the broken attachments of the lateral and medial laminae of the ascending process. A deep, dorsally opening cavern is present in this widened part between the laminae, and was probably connected posteriorly to the antorbital fossa and fenestra. As can be seen in the left element, this cavern seems to pneumatize great parts of the central maxillary body. Behind this cavern, the right maxilla bears the long and deep jugal facet. This facet widens posteriorly and is mainly dorsally directed.

A narrow groove runs along the lateral border of the antorbital fossa from the facet anteriorly, and ends in a foramen leading anteriorly into the base of the lateral ascending lamina, lateral to the cavern described above. The left maxilla lacks the anterior end and the mediodorsal process. Its isolated laterodorsal process is placed on the right maxilla, lying on the medial side and pressed down by the frontals. It is a thin sheet of bone with a pointed posterodorsal corner. As in other basal iguanodontians, except Tenontosaurus tilletti, this process is rather short and did not reach the nasal dorsally (Norman, 2004). The posterior end of the left maxilla is covered by the jugal and sediment, but CT images show that this part is also preserved.

There are nine teeth preserved in the right maxilla and eight in the left element. However, the complete tooth row of the right dentary (which has ten teeth, see below) suggests a similar number for the maxillae. The maxillary tooth row forms a very slightly laterally concave arch, as in many ornithischians. There are a few, irregularly spaced, large foramina on the lateral sides of the maxillae above the tooth row. Five larger foramina are present in the right element, whereas the left bears six smaller foramina. Hypsilophodon is similar, but has more foramina (Galton, 1974). The lateral side of the maxilla extends dorsolaterally from the tooth row, so that the dorsal rim of the maxillary body notably overhangs the tooth row laterally, but this is less obvious compared with, e.g.

37

Jugal (Figs. 4.1D and 4.2A, B; see Appendices I and II for measurements): The left jugal is completely preserved, whereas only the maxillary process of the right element is present, and has been removed from the skull during preparation. The main body of the left jugal has a compact, trapezoidal shape. Parts of the ventral and posterior margins are damaged. The bone is notably convex laterally, both anteroposteriorly and dorsoventrally, indicating a somewhat bulging cheek region in the articulated skull. There is no jugal boss, as present in Zephyrosaurus and Orodromeus

(Scheetz, 1999; Sues, 1980). The long and slender maxillary and postorbital processes are set at an angle of approximately 110° to each other, and form the ventral and posteroventral margin of the orbit. The robust maxillary process tapers anteriorly. The lacrimal facet extends over 8 mm on the anterodorsal margin of the process. It widens anteriorly and is laterodorsally directed. Below this anteriorly descending facet, the anterior end of the maxillary process is broadened ventrally. The orientation and relative extension of the lacrimal–jugal suture is quite variable in ornithopods (see Norman, 2004; Norman et al., 2004; Weishampel, 1984). Posterior to the lacrimal facet begins the smooth and constantly concave orbital margin. The orbital margin of the jugal is broadened, so that the maxillary process of the jugal is triangular in cross section. A sharp rim separates a wider, medioventrally-extending medial surface from the lateral surface. This rim becomes less conspicuous and more rounded posterodorsally on the postorbital process.

The rounded postorbital facet, starting at the anterior side of the postorbital process with a deep, rounded depression, extends over approximately the dorsal half of this process. The lateral border of the facet ascends steeply to the posterodorsal side of the process, and is slightly twisted in itself. A slender process extends dorsally on the medial side, and was obviously completely covered by the jugal process of the postorbital in the articulated skull. The lateral part of the facet consists of a long, narrow groove along the twist described above, whereas the slender dorsal process is strongly convex and rod-like. The end of the postorbital process thus almost reaches the upper end of the infratemporal fenestra. Weishampel (1984:43) mentioned Dysalotosaurus as an example for a simple scarf joint between jugal and postorbital. The structure of the postorbital process in this

38

newly described specimen rather suggests a combination of a scarf joint ventrally and a hinge-like joint dorsally. The dorsal extension of the postorbital process is also more comparable with the described pattern in lambeosaurine hadrosaurs (Weishampel, 1984:44). The posterior half of the lateral base of the postorbital process bears a very weak depression, where the surface of the bone descends into the lower temporal fenestra.

The posterior (quadratojugal) process of the jugal is short and high. Its posterior end is considerably expanded dorsally into a long, dorsally thinning, slightly posterodorsally inclined process. Together with the postorbital process of the jugal, the quadratojugal process thus forms the entire anteroventral, ventral, and posteroventral margin of the anteroposteriorly narrow infratemporal fenestra. In basal ornithopods, such as Hypsilophodon, Orodromeus, and

Gasparinisaura, as well as in the basal iguanodontian Tenontosaurus, this region is made up by the quadratojugal, so there is no contact between the jugal and the quadrate (Norman, 2004; Norman et al., 2004; see also Butler et al., 2008b: appendix 3, character 47). Zalmoxes robustus, Dryosaurus altus and all more derived iguanodontians share the general condition with Dysalotosaurus (Galton, 1983; Horner et al., 2004; Norman, 2004; Weishampel et al., 2003). Only Ouranosaurus seems to represent an intermediate condition between basal ornithopods and basal iguanodontians (see Taquet, 1976). The body of the posterior process below the infratemporal fenestra is very high, so that the ventral margin of the latter opening is placed considerably dorsal to the ventral rim of the orbit. The posterior rim of the posterior process of the jugal forms a straight, vertical margin. The ventral rim of the jugal also seems to be generally straight over its entire length, although there seems to be a slight ventrally convex extension directly below the postorbital process, although less distinct than in Orodromeus (Scheetz, 1999). The medial side of the jugal is mainly covered with sediment, so only the distinctive ectopterygoid process at the base of the maxillary process is visible in dorsal view.

39

Fig. 4.3:Stereo pairs of the skull BSPG AS I 834 and explanatory sketches. The sketches are relatively enlarged for better resolution. Dark grey illustrates sediment, light grey illustrates inner views of, e.g. the frontals or the dentaries, and hatched areas illustrate broken or corroded surfaces. The label affixes -r and -l stand for right and left of the respective element, where the distinction of each side is difficult to see. A – Right lateral view. B – Outline drawing of the ventral view. C – Outline drawing of the right lateral view. D – Ventral view. See material and methods for a list of the abbreviations. Scale bars: 1cm.

40

Frontal (Figs. 4.1C, D, 4.2 and 4.3A, C; see Appendices I and II for measurements): The paired frontals are still articulated with the remaining fragment of the parietal. However, neither the frontals among themselves nor the frontals and the parietals are fused, as the sutures between the elements are clearly visible. Only the parietals seem to be fused without any visible suture.

The right frontal lacks the lateroposterior and orbital edge, whereas the left frontal is almost complete. Both have a length of approximately 40 mm, and the better preserved left frontal has a width of 11.8 mm at the posterior end of the orbital rim. The frontals become slightly narrower anteriorly, with the orbital rim forming an almost straight lateral margin. The interfrontal suture is visible as a straight median line between the two elements.

A good overview of the shape of the frontals in ornithopods and some other ornithischians in dorsal view is given by Galton (1997: fig. 9; but note that fig. 9L represents Dryosaurus altus, and fig. 9M represents Dysalotosaurus). Generally, in larger or more derived iguanodontians, such as

Tenontosaurus tilletti (larger but less derived than Dysalotosaurus), Iguanodon, Mantellisaurus, or

Ouranosaurus, the frontals are relatively shorter but transversely wider, and their participation in the orbital rim is shorter (see Galton, 1997; Norman, 1980; 1986; Ostrom, 1970; Taquet, 1976). In some hadrosaurs, the frontals are even completely excluded from the orbital rim (Horner, 1992; Horner et al., 2004).

The anterolateral corner of the frontal bears a deep groove for the contact with the prefrontal. This groove is relatively longer but less deep than in Dryosaurus altus (Galton, 1983, 1997). It slightly widens anteriorly and is entirely laterally directed. Posterolaterally, there is a facet for the contact with the postorbital, which extends posteriorly onto the parietal. This facet is developed as a narrow groove that undercuts the laterodorsal rim of the frontal anteriorly, and widens posteriorly to form a large, dorsally facing surface. At the frontoparietal suture, the lateral rim is slightly raised, probably in the area where the laterosphenoid met the postorbital. The frontoparietal suture forms an interdigitate, anteriorly slightly concave line, with the frontals forming

41

a small, lobe-shaped posterior process laterally. A small, triangular process of the parietals extends approximately 3 mm into the median suture of the frontals.

The dorsal surface of the frontals slopes posteroventrally behind the orbitae towards the supratemporal fossae. However, the supratemporal fossae (of which only the anterior end of the left depression is preserved) do not reach the frontal, but are restricted to the parietal. In contrast, the supratemporal fossa seems to cover the posterior end of the frontals in Dryosaurus altus and in

Lesothosaurus (Galton, 1983; Sereno, 1991), and reaches the posterior margin of the frontal in

Hypsilophodon, Zalmoxes robustus, and Ouranosaurus (Galton, 1974; Taquet, 1976; Weishampel et al., 2003). It seems to be excluded from the frontals in, e.g. Thescelosaurus neglectus, Mantellisaurus atherfieldensis, and Iguanodon bernissartensis (see Galton, 1997; Norman, 1980; 1986), as in

Dysalotosaurus.

A small central dome is located just posterior to the orbital margin on the posterior third of the frontals, similar to a structure found in lambeosaurine hadrosaurs (Evans et al., 2007; Godefroit et al., 2004; Horner et al., 2004). As in the jugal, the orbital rim of the frontal is widened and forms a broad, ventrolaterally facing surface. The medial orbital facets of the articulated frontals form the lateral margin of a narrow, flat median surface on the ventral side of the frontals, in which the bulbi olfactorii of the brain would have been placed. The straight interfrontal suture is visible here as a thin, but deep median groove, similar to Hypsilophodon and Zephyrosaurus (Galton, 1974: fig. 6B; Sues, 1980: fig. 7B).

Parietal (Figs. 4.1C, D, 4.2; see Appendices I and II for measurements): Only the anterior most part of the left anterior wing and the central roof of the fused parietals are preserved. There is no indication of a suture between the parietals, as observed in Orodromeus (Scheetz, 1999) or in the juvenile skull of Dryosaurus altus (Carpenter, 1994). The dorsal face of the preserved left side houses a shallow, oblique depression, which deepens anteriorly. This depression represents the supratemporal fossa, the margins of which are not sharply defined. The fossa ends anteriorly

42

approximately 1.5 mm behind the frontoparietal suture. The anterolateral end of the parietal wing is slightly bilobate where it meets the postorbital. The posterior part of the postorbital facet of the left frontal extends 2 mm onto the dorsal surface of the parietal wing. At the medial margin of the postorbital facet, the anterior end of the parietal forms a small, pointed process between the median part of the frontal and the lateral, lobe-shaped process described above. In comparison with the main parts of the frontals, the parietals are somewhat thickened. As mentioned above, a median parietal process extends forwards into the suture between the two frontals. This process is more finger-like and much more slender than the respective process in Hypsilophodon. There is no anteromedian process in, e.g. Zalmoxes robustus, Tenontosaurus, Mantellisaurus, and Iguanodon, but there is a slight process-like protuberance in Ouranosaurus (Norman, 1980; 1986; Ostrom, 1970; Taquet, 1976; Weishampel et al., 2003; Winkler et al., 1997). The parietal of Ouranosaurus is also more similar to that of Dysalotosaurus than those of the other taxa in its dorsal shape, because it also possesses distinct anterolateral wings.

Postorbital (Figs. 4.1C, D, 4.2A, B): The left postorbital is completely preserved, although the jugal process is covered by matrix. It is completely displaced from its original position, and is positioned upright between the maxillae, approximately in the centre of the skull, as preserved. It has an anteroposterior length of 23 mm and a dorsoventral height of 22 mm (measured from CT images). The main body, the squamosal process, and the anterodorsal extension for the articulation with the frontal and parietal are visible laterally. The lateral side of the postorbital is separated from the dorsolateral part by a slight horizontal swelling, which extends from the posterior squamosal process to the orbital edge. At the point where this swelling meets the orbital margin, the latter forms a small, wide-angled process that extends 1.2 mm into the orbit. This is not as extensive as in

Dryosaurus altus, but might be the result of ontogenetic differences, as the orbital edges of the other known postorbitals of Dysalotosaurus are not as smooth in this area (Galton, 1983; Janensch, 1955). A similar condition is found in Zephyrosaurus (Sues, 1980: fig. 7C).

43

Along the horizontal swelling the lateral side of the postorbital is anteroposteriorly concave and dorsoventrally convex. The stout, slightly ventrally flexed, triangular squamosal process possesses a smooth and flat dorsal facet for the contact with the anterior process of the squamosal. The ventral side bears a flat groove, which is continuous with the muscle attachment site for the M. adductor externus superficialis (Ostrom, 1961) on the squamosal, and borders the dorsal edge of the infratemporal fenestra. In Zalmoxes robustus, this muscle attachment is considerably larger and more anterolaterally placed (Weishampel et al., 2003).

The quadrangular dorsolateral plate of the postorbital that contacts the frontal and parietal borders the supratemporal fenestra anterolaterally. In horizontal CT slices, the deep medioposterior groove for the contact with the postorbital process of the jugal is visible in the long and slender jugal process.

Squamosal (Figs. 4.1C, D, 4.2): The preserved left squamosal lacks all of its medial part, including the parietal facet and most of the paroccipital facet. The bone is rotated about 90°, so that the almost complete ventral processes are removed underneath the left frontal. The anterior most part bears the lateroventrally placed postorbital facet. Behind this facet, the squamosal forms the posterodorsal margin of the infratemporal fenestra. From this area, a posteriorly directed triangular depression for the attachment of the M. adductor externus superficialis (Ostrom, 1961) extends onto the lateral side of the bone. In Dryosaurus altus, this depression seems to be much smaller and the degree of overlap between the postorbital and squamosal is larger than in Dysalotosaurus (Galton, 1983; Hübner & Rauhut, pers. obs.). In other ornithopods, including hadrosaurs, the postorbital– squamosal connection, as well as the size and shape of the laterodorsal depression, is quite variable, but the most unusual morphology is visible in Zalmoxes robustus, in which the depression is located on the postorbital and not on the squamosal (Weishampel et al., 2003).

The precotylar (ventral) process is a long, flat, and slender rod of bone that tapers anteroventrally, indicating that the quadrate was excluded from the infratemporal fenestra, as in

44

Gasparinisaura and in contrast to Hypsilophodon and Orodromeus (Norman et al., 2004). The head of the left quadrate is still articulated with the squamosal in the groove behind the precotylar process. The partly preserved postcotylar process frames the quadrate head posteriorly. The dorsal surface is convex anteroposteriorly. It forms an elongate, anteriorly tapering triangle in dorsal view. A distinct but flat depression is placed above the quadrate articulation on the otherwise smooth dorsal surface.