Referentes orales

Scapula: In contrast to Tenontosaurus (Forster, 1990a:fig. 7), the end of the scapular blade is not straight in juveniles of Dysalotosaurus, but as gently convex as in the larger specimens. It is much more similar to Camptosaurus (Carpenter & Wilson, 2008:fig.17), especially to C. aphanoecetes. However, a change from a more symmetrically rounded scapular distal blade in young

Dysalotosaurus to a posteroventrally strongly flaring distal blade in larger specimens (Fig. 5.4) is not visible in both Camptosaurus species. This kind of ontogenetic change is very similar to

Hypsilophodon (Galton, 1980:fig. 3C, D). Furthermore, although the relief of the proximal part of the scapula becomes more pronounced in Dysalotosaurus during growth, it never gets such a strong acromion process and adjacent anteroventral ridge (‘deltoid ridge’ in Brett-Surman & Wagner, 2007) as in Camptosaurus or Hypsilophodon. In this feature, it is in turn more similar to Tenontosaurus.

Here, the intermediate phylogenetic stage of Dysalotosaurus is obvious, but the ontogenetic variation in overall scapular shape is interestingly higher than in Camptosaurus and Tenontosaurus

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blade shape of the scapula in Dysalotosaurus does not fully explain the extraordinary differences in the respective shape between two scapulae assigned to Zalmoxes robustus (Weishampel et al., 2003:fig. 19). One specimen (BMNH R3814) is very similar to the scapulae of Dysalotosaurus,

Camptosaurus and Tenontosaurus and fits nicely into the general scheme expected for basal iguanodontians. The other specimen (BMNH R3810) is only about 20% smaller, but the shape of the scapular blade looks very juvenile. Dysalotosaurus shows such a difference only by comparison between the smallest and largest preserved scapulae, where the smallest reaches less than 50% of the represented maximum size. Thus, it is more likely that the scapulae figured by Weishampel et al. (2003:fig. 19) either belong to different taxa, represent a generally high intra-specific variability, or these specimens are another expression of sexual dimorphism, as already suggested by the authors for the ischium of Z. robustus.

Coracoid: The comparison of the coracoid of Dysalotosaurus with Dryosaurus altus is difficult, because only very few specimens are known from the latter. The coracoids of the young juvenile individual are not well preserved for a comparison (Carpenter, 1994; pers. obs.) and the ontogenetic variation described by Carpenter (1994) is based upon a figure of the coracoids of the “dy I” individual of Dysalotosaurus itself (see Galton, 1981:fig. 6M; Carpenter, 1994:fig. 19.6M). The known coracoids of Dryosaurus (CM3392 not described and not adequately figured by Gilmore, 1925 and Galton, 1981; AMNH 834 obviously a younger and overall medium sized individual, see Galton, 1981:tab.2) differ from each other and from Dysalotosaurus mainly in the development of the sternal hook or process. In CM3392, it is longer and distinct due to a deep concavity between this process and the posteroventral beginning of the humeral articular surface (pers. obs. on the mounted skeleton). In AMNH 834, the sternal process is completely absent (Galton, 1981; Shepherd et al., 1977). The coracoids of both individuals resemble only the coracoids of the very young

Dysalotosaurus in the almost equal dimensions of this element anteroposteriorly and dorsoventrally (Fig. 5.5A). The length of the sternal process increases only slightly in Dysalotosaurus during

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ontogeny and the extremes found in Dryosaurus are not visible in any of the preserved coracoids of the former. It should therefore be tested in the future, whether the extraordinary variation in

Dryosaurus is either the result of different preservation or whether it indicates the presence of two distinct North-American species. Ontogeny alone definitely cannot explain this degree of variation.

As already noted, the overall dimensions of the coracoid of Dysalotosaurus are roughly equal in small specimens, whereas it becomes anteroposteriorly longer than dorsoventrally high in large specimens (Fig. 5.5). Similar ontogenetic trends were observed in Hypsilophodon and Orodromeus

(Galton, 1980; Scheetz, 1999:53). It is also probably the case in Camptosaurus, although the typical quadrangular overall shape of the coracoid of the latter is already present in very young individuals (Carpenter & Wilson, 2008:17D; Chure et al., 1994).

Humerus: Brett-Surman & Wagner (2007) noted for hadrosaurs the relative ontogenetic increase of the medial (ulnar) condyle of the distal humeri compared to its lateral counterpart. Exactly the same was observed in Dysalotosaurus (Fig. 5.7D-E) and could be an adaptation to increasing body weight or a changing biomechanical input on the medial elbow joint. An ontogenetically increasing body mass, for instance, had to be heaved up after resting on the ground (pers. comm. Remes, 2010). Another reason could be a more forceful grip of larger individuals to hold something in both hands. Anyway, the reason for this ontogenetic change is obviously the same in hadrosaurs and Dysalotosaurus, although the former experienced a shift from bipedality to quadrupedality during growth (Dilkes, 2001) and the latter not (see chapter 5.5.5). However, the strong variation in the distal humerus of Dysalotosaurus, already noted by Galton (1981), is clearly ontogenetic in nature. Similar observations were made in Orodromeus (Scheetz, 1999: tab. 1), where the distal condyles are, as in Dysalotosaurus, more distinct in juveniles than in adults.

Several further ontogenetic changes within the humerus of Dysalotosaurus were observed in other ornithopods. The length of the deltopectoral crest increases compared to overall humeral length also in Orodromeus, Zalmoxes robustus, Tenontosaurus tilletti, and Maiasaura (Dilkes, 2001;

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Forster, 1990a; Scheetz, 1999; Weishampel et al., 2003:fig. 20). There is also an anteroposterior thickening of this crest as in Dryosaurus altus (see Galton, 1981:figs. 6B; 7C) and hadrosaurs in general (Brett-Surman & Wagner, 2007), which is similar to its increased pronouncement in

Orodromeus, Hypsilophodon, Tenontosaurus, and Camptosaurus dispar (Carpenter & Wilson, 2008; Forster, 1990a; Galton, 1980; Scheetz, 1999). However, in the latter four genera, the deltopectoral crest is rather an anterior elevation (pointed in Orodromeus and Hypsilophodon, also in Z. robustus; quadrangular in T. tilletti and C. dispar) than a bulging protuberance as in Dysalotosaurus,

Dryosaurus, and Camptosaurus aphanoecetes. Especially Tenontosaurus is very similar to hadrosaurs in the shape and extant of the deltopectoral crest (Dodson, 1980).

Ulna and radius: Both elements become stouter with relatively larger articular ends and a relatively thicker shaft in Dysalotosaurus during growth. The olecranon process of the ulna never reaches the highly pointed elevation seen in Hypsilophodon, Orodromeus, and Z. robustus (Galton, 1974:fig. 40; Scheetz, 1999:fig. 22; Weishampel et al., 2003:fig.21) or the distinct high shape as in more derived iguanodontians (e.g. Norman, 1980:fig. 58), but reaches a moderately higher elevation during ontogeny than Dryosaurus (Galton, 1981) and is most similar to C. aphanoecetes (Carpenter & Wilson, 2008:fig.21G) in large specimens. However, although the olecranon process expands also in dorsal view in Dysalotosaurus, it is in turn more similar to Dryosaurus, Hypsilophodon, and

Orodromeus than to C. aphanoecetes due to the much more pronounced lateral process in the latter (Carpenter & Wilson, 2008). Hadrosaurs, stegosaurs, and Protoceratops, for instance (Brett-Surman & Wagner, 2007; Brown & Schlaikjer, 1940; Galton, 1982), also show ontogenetically increased robustness and elevation of the olecranon process, but especially hadrosaurs show additional differing ontogenetic tendencies. The olecranon notch for the ulna is larger in older individuals, which is not the case in Dysalotosaurus. Thus, the degree of articular overlap between the humerus and ulna is higher in hadrosaurs. Furthermore, the relative lengths of radius and ulna strongly increase during growth in the latter, which results in more slender elements in older individuals

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(Brett-Surman & Wagner, 2007; Horner & Currie, 1994) and in the special hadrosaur forelimb proportions (long radius and metacarpals compared to relatively shorter humerus; Fig. 5.20; Appendix V) unique within ornithopods.

Fig. 5.20:Resulting scatter plot of the Principal Component Analysis of all long bone ratios of several ornithopods and some other ornithischians within a 95% ellipse. The first principal component is dominated by the ratios of both the humerus and radius to the third metacarpal (Eigenvalue = 3.552; variance = 82.183%). The second principal component describes the influence of the relative length of the third metatarsal onto the distribution (Eigenvalue = 0.538;variance = 12.437%). Note the close proximity of all hadrosaurs to each other and to Mantellisaurus, whereas Iguanodon bernissartensis

is more closely related to Ouranosaurus and all more primitive and at least facultative quadruped basal iguanodontians. Four taxa, where each is a basal member of its respective clade, are also plotting closely together. All used sources, specimens, values, and ratios are noted in Appendix V. Abbr.: B.c. – Brachylophosaurus canadensis. C.a. – Camptosaurus aphanoecetes. C.c. – Corythosaurus casuarius. C.d. – Camptosaurus dispar. D.a. – Dryosaurus altus. D.l. – Dysalotosaurus lettowvorbecki.

E.r. – Edmontosaurus regalis. H.f. – Hypsilophodon foxii. H.t. – Heterodontosaurus tucki. I.b. –

Iguanodon bernissartensis. M.a. – Mantellisaurus atherfieldensis. O.n. – Ouranosaurus nigeriensis.

P.n. – Psittacosaurus neimongoliensis. S.a. – Saurolophus angustirostris. S.l. – Scutellosaurus lawleri.

S.o. – Saurolophus osborni. T.i. – Tethyshadros insularis. T.n. – Thescelosaurus neglectus. T.t. –

Tenontosaurus tilletti.

Ilium: As in Dysalotosaurus, the main body or blade of the ilium apparently becomes also relatively longer compared to the height in Orodromeus and probably C. dispar (Carpenter & Wilson,

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2008; Scheetz, 1999). Especially in hadrosaurs, the taxonomic variation of the ilium seems to be high, but ontogenetic differences are, apart from increasing robustness and larger muscle attachment sites, almost absent (Brett-Surman & Wagner, 2007; Horner & Currie, 1994). The variation of the orientation of the preacetabular process in Hypsilophodon (Galton, 1974) seems definitely not to have an ontogenetic origin (see measurements of ilia in Galton, 1974:tab. II). The increasing space of the acetabulum in the ilium of Dysalotosaurus resulted in more space for the femoral head (Fig. 5.10C, E). Unfortunately, this cannot securely be checked in other ornithopods at the moment.

Ischium: The increasing robustness of the blade and peduncles of this element were expected, but far more interesting is the medial migration of the depression at the neck of the iliac peduncle, the deepening of the acetabular fenestra (Fig. 5.11), and the slight proximal migration of the obturator process. The first ontogenetic changes are obviously linked to the increasing size of the femoral head (see below), which indicates strongly pronounced ossification of the latter. This change is clearly linked to deal with stronger forces acting on this structure in larger individuals during locomotion. It is of course not quite comparable to the situation seen in the thyreophorans

Euoplocephalus and Stegosaurus, but the more spherical and more clearly delimited femoral head in large individuals of these taxa compared to their juveniles (Coombs, 1986; Galton, 1982) looks similar to the ontogenetic changes seen in Dysalotosaurus. No significant ontogenetic differences are visible between the two known ischia of Dryosaurus (Galton, 1981:figs. 10A, E), where the specimen of AMNH834 corresponds approximately to a large, medium-sized Dysalotosaurus individual and the holotype YPM1876 corresponds to the largest known Dysalotosaurus individuals (derived from femur lengths; Galton, 1981:tab. 2 and own measurements). Thus, the very shallow acetabular depression between the poorly separated peduncles in Dryosaurus does not have an ontogenetic reason and can therefore be treated as a clear taxonomic difference between both dryosaurids.

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Femur: The deepening of the anterior intercondylar groove indicates either a thickening of the patella tendon (consisting of the united tendons of M. iliotibialis, M. ambiens, and Mm. femorotibiales) or a better fixation or guidance of this important knee extensor. This is probably a response to increasing body weight and size (Fig. 5.12B-D).

The prominence or distinction of the medial depression for the M. caudofemoralis longus varies independently of size in Hypsilophodon (Galton 1974). This is in contrast to Dysalotosaurus, in which the ontogenetic change of this depression is clearly size-related. However, within the tendency of decreasing prominence of this depression, especially of its anterior border, the intraspecific variation is too high to make founded interpretations on the reasons for this ontogenetic change. Moreover, taphonomic distortion of this region is rather abundant and complicates secured statements. Apart from that, one can definitely eliminate the insertion of the M. puboischiofemoralis internus (Norman, 1986:348-349). In most femora of all sizes, the inferred main direction of the inserting muscle is posterodorsal by varying angles, which is especially well visible in the specimens R6861 and MB.R.2517 (Fig. 5.12A) for instance. Although I have found a similar condition in the large femur MB.R.2511 (Fig. 5.12E) as described by Norman (1986), a shift of the direction of the inserting muscle from posterodorsal to anterodorsal is very unlikely and should treated as the result of intraspecific variation and general weak prominence of the edges of this depression in large femora.

The absence of the distal migration of the 4th trochanter (in contrast to Alligator [Dodson, 1975] and Zalmoxes [Weishampel et al., 2003]), the rather negative allometry of the height of its base, and the almost isometric growth and position of the medial depression indicates the relative constancy of the strength and lever arm of the M. caudofemoralis of Dysalotosaurus (see also chapter 5.5.5). Furthermore, the slight indication of an increase in the anteroposterior dimensions within the proximal and distal shaft compared to the respective mediolateral dimensions would be the opposite pattern seen in other dinosaurs (e.g. Bonnan, 2004:465; Carrano, 2001) and highlights the possibility that increased eccentricity in the femur takes only place in very large species. The bended femoral shaft in small cursorial dinosaurs, such as Dysalotosaurus, which differs significantly

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from the straight shaft in large graviportal dinosaurs, may have played an important role for this differing morphology.

As noted above, the femoral head increases very strongly compared to most of the other measurements. This fits very well to the ontogenetically increasing size of the acetabulum indicated by the respective increasing dimensions in the ilium and ischium. Thus, the femoral head experienced above-average ossification, because it was the location of strong impact of stress during locomotion. Such large and separated femoral heads are typical for obligate bipedal dinosaurs (see e.g. Brochu, 2003:fig. 95; Chure, 2000:fig.145; Currie & Peng, 1993:fig. 1a; Galton, 1974:fig. 54) and are much more pronounced compared to obligate quadruped dinosaurs, although these animals also experienced a slight increase in pronouncement and separation of the femoral head during growth (Coombs, 1986; Galton, 1982).

The lesser trochanter becomes more prominent in Dysalotosaurus during ontogeny, which was also observed in Zalmoxes (Weishampel et al., 2003), but there is no sign of a closer approximation or even a tendency of fusion of it to the greater trochanter as in Stegosaurus (Galton, 1982), Protoceratops (Brown & Schlaikjer, 1940) and some hadrosaurs (Brett-Surman & Wagner, 2007; Godefroit et al., 1998). However, there is significant variation in the fusional degree between both trochanters among hadrosaur species during growth and sometimes even between the left and right side of a single individual (Brett-Surman & Wagner, 2007; Horner et al., 2004), so that a clear ontogenetic signal is not visible in this group. Hadrosaurs are also not well comparable to

Dysalotosaurus, because at least one distinct muscle scar on the femur decreases its extension and the femoral shaft becomes less robust during growth. This is probably the result of an ontogenetic change from bipedality to facultative quadrupedality in hadrosaurs (Dilkes, 2001) and of the much more extensive cartilage caps on the articular ends in hadrosaur juveniles (Horner & Currie, 1994). This further highlights the possible differences in breeding strategy between both taxa (Horner & Weishampel, 1988; Horner et al., 2001; chapter 6.7.3).

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Tibia: The most significant ontogenetic changes of the tibia are the much more robust cnemial crest (Fig. 5.13B-C), the extended base of the lateral proximal condyle, and the increasing articulation with the calcaneum at the expense of the articulation with the astragalus. The increasing robustness/thickness of the cnemial crest is also described for Zalmoxes (Weishampel et al., 2003) and hadrosaurs (Brett-Surman & Wagner, 2007). As in the femur, the latter show rather negative allometry of the shaft thickness resulting in less robust tibiae in adult individuals. This is also explained by larger cartilage caps on the poorly ossified articular ends in very young individuals (Horner & Currie, 1994) and by the shift from mainly bipedality in juveniles to mainly quadrupedality in adults (Dilkes, 2001). The consequences of a shift within the articulating surfaces for the proximal tarsals (more calcaneum, less astragalus) is currently unknown, but could probably related to a firmer articulation within the tibiotarsal complex.

Fibula: As in the tibia, the anterior thickness of the proximal end increases most compared to the other measured distances (Fig. 5.14C-D). It opposes the important cnemial crest of the tibia and therefore participates in providing extensive attachment sites for muscles and tendons. No other observation could be made and obvious ontogenetic trends in other ornithopods are not described or known.

Proximal tarsals: Apart from the observed increased ossification and larger and more robust muscle attachment sites, the remarkable lateromedial expansion of the calcaneum and the lateromedial shortening of the astragalus is the main ontogenetic change in the proximal tarsals. One possible explanation is the strengthening of the whole tibiotarsal complex due to increasing body weight during growth. The increasing internal depth and posterior height of the astragalus is indeed a sign of increasing overlap with the tibia. A similar lateromedial widening was found in the calcaneum of Orodromeus (Scheetz, 1999:tab. 1), which indicates a wider taxonomic distribution of this ontogenetic pattern. It is maybe just rarely recognized or described. The fusion of the proximal

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tarsals to each other and to the fibula and tibia, as in thyreophorans (Coombs, 1986; Galton, 1982) or theropods (e.g. Raath, 1990), is never achieved, although almost fused proximal tarsals were observed in Orodromeus (Scheetz, 1999:tab. 1).

Metatarsals: All three elements show the expansion of most of the lateromedial dimensions compared to the anteroposterior dimensions (mt II less than the other two). Among most of the long bones, all three metatarsals even experienced the strongest increase in shaft robustness, especially in lateromedial direction, which implicates a significant higher compactness and connection within the metatarsus. Undoubtedly, this is also related to bear higher stresses during locomotion initiated by larger body mass during growth. The slightly indicated increase of the medial distal condyle of mt III compared to its lateral counterpart was also observed in Orodromeus (Scheetz, 1999:72). However, Scheetz (1999:93) mentioned wider mediolateral proximal dimensions compared to the anteroposterior direction in mt IV in a very young Orodromeus, which would reveal an opposite trend compared to Dysalotosaurus. Hadrosaurs experienced also an opposing trend. As in the other long bones of the hind limb, their metatarsals become less robust and elongated during growth, which was the result of strongly increasing ossification of the articular ends (Horner & Currie, 1994) and of the development of a weight supporting heel pad, which would strongly absorb a large amount of stress otherwise acting on the metatarsals (Dilkes, 2001). The opposing development of metatarsals during growth in Dysalotosaurus and hadrosaurs probably also mirrors the development from a digitigrade foot posture of the former to a sub-unguligrade foot posture in the latter (Moreno et al., 2007).

Phalanges: Combining the results of ontogenetic change in all examined phalanges, the relative length of the toes (the whole foot together with the metatarsals) seems to decrease in

Dysalotosaurus during growth. Interestingly, small extant birds also have relatively longer feet than larger taxa (Gatesy & Biewener, 1991). The other ontogenetic tendencies are strongly influenced by

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the position of the phalanges within the foot, because the toes have different total lengths, the phalanges have different fractional lengths, the proximal end of the toes begin at different relative positions (the third toe starts more distally than the others due to the longer mt III), and the number of phalanges increases from the second to the fourth toe. Nevertheless, some cautious ontogenetic tendencies can be verified. The dorsal condyle facet of the phalanges of the second toe increase stronger during growth than in the phalanges of the other toes, which indicate stronger extension of