Chapter 4 CONCLUSIONS
Chapter 4. Conclusions
4.1. Summary The objectives of the present work were to (1) gather information about the South American dinosaur fauna from the Jurassic-Cretaceous boundary (Chapter 2) and (2) investigate the evolution of pedal morphology and function in dinosaurs of the ornithopod clade (Chapter 3). In section 2.1, I described a new tracksite from northern Chile, in the Estación Member of the San Salvador Formation (Kimmeridgian – Early Cretaceous). This trackbed contains narrow-gauge sauropod footprints and three different theropod footprint morphotypes: medium to large sized tetradactyl, small to medium tridactyl, and subaquatic tridactyl impressions. The absence of wide-gauge sauropod trackways suggests that titanosaurid migrations (wide-gauge trackways) to South America may have occurred later during the Cretaceous, thus confirming current palaeogeographical theories on dinosaur distribution. This pattern is also observed in the Termas del Flaco trackbed (Baños del Flaco Formation, Tithonian), central Chile (section 2.2). In this section, the ichnospecies Iguanodonichnus frenki was redescribed and reidentified as a trackway of a narrow-gauge sauropod (not of an iguanodontian). My observations suggest that South American Jurassic sauropods had long and straight claws in the first digit, but ones that were extremely reduced in digits II to V. In section 2.3, I presented concurrent large theropod ichnocoenoses in two new tracksites from Querulpa Chico (Peru) and Chacarilla (Chile), which represent two different palaeoenvironments (tidal basin vs. meandering river). These tracksites show abundant large theropod trackways, two of them indicating a direct relationship between the trackmakers. My observations suggest some degree of grouping behaviour, which was perhaps facilitated by the lack of water in a surrounding arid environment. Also, the Querulpa Chico and Chacarilla tracksites expand the time period and geographic distribution for large-bodied theropod dinosaurs in South America, into the Late Jurassic and toward the western margin of Gondwana. From the data presented in Chapter 2, I concluded that the Late Jurassic – Early Cretaceous fauna in South America was dominated by narrow-gauge sauropods (probably diplocoids), and extremely large theropods.
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Chapter 4. Conclusions
In Chapter 3, I describe and quantify the morphology of pedal phalanges in ornithopod dinosaurs. In this study, I point out that basal ornithopods had short phalanges, great dorsoventral thickness, and high development of the interlocking system (i.e. sagittal ridges and correspondent grooves, great joint curvature, tendon processes and deep collateral ligament fossae). My results show that derived ornithopods (hadrosaurids) evolved a long first phalanx with shorter second and third, and strong reduction of the interlocking system. This indicates that hadrosaurids had a more upright pedal posture (particularly in adults) than basal ornithopods, by aligning the pes to the main direction of the ground reaction force. Also, Finite Element Analysis (FEA) of isolated phalanges and complete digit III suggest that basal ornithopods had digitigrade postures, which are characteristic of high-tension loads. Therefore, well-developed interlocking systems as well as prominent tendons and ligaments were necessary in order to counteract this tension. In contrast, hadrosaurids were more compatible with a subunguligrade posture, which minimizes and homogenizes stresses, and therefore performs better under compressive stresses. This characteristic probably facilitates the evolution of larger sizes. The internal structure of the phalanges is also consistent with FEA loading patterns, confirming its occurrence in real pedal loading conditions. Although this research was focused on ornithopod dinosaurs, the functional morphological issues discussed here are relevant for other terrestrial vertebrates, which present a similar morphology and had also evolved large sizes. My observations indicate that these characteristics represent a common physical solution in order to support higher loads.
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Chapter 4. Conclusions
4.2. Future work The present work opens research possibilities in a wide variability of fields, since it provides a base for more detailed studies on the Late Jurassic-Early Cretaceous ichnofauna from South America, but also gives a fundamental frame for the research on the evolution of pedal functional morphology by using varied techniques, which could be applicable in either modern or extinct terrestrial vertebrates.
4.2.1. Dinosaur footprints From a personal viewpoint, further work needs to be done with the numerous data I collected from the field. The tracksites prospected, including records not mentioned in the present study, reveal a large number of trackway morphologies that have not been described to date: (1) The Chacarilla Formation (Upper Jurassic - Lower Cretaceous) is extensively exposed in numerous canyons along the 1st and
2nd
Regions of northern Chile, in which I
have described an outcrop dominated by theropod dinosaur footprints (section 2.3). But other trackbeds present tri- and penta-dactyl scratch marks. Considering the high frequency of theropod trackways in this formation, the scratch marks could be attributed to theropod dinosaurs; however, the presence of pentadactyl footprints suggests another archosaur as the trackmaker. If this is the case, it would indicate a greater biodiversity for the Chacarilla ecosystem. (2) A new Early Cretaceous tracksite from northern Peru, near Huanzalá and Antamina mines (Chimu Formation, Valanginian) exhibits large ornithopod trackways, preserved in either concave or convex hyporelief. These footprints are exposed in different layers along each side of the Huanzalá-Antamina road. This record corresponds to the earliest confirmed presence of large ornithopods in South America and the first one in the western margin of Gondwana. (3) The Calama tracksite described in section 2.1 shows diverse theropod footprint morphologies, some of which are nicely preserved deep prints that recorded the movement
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Chapter 4. Conclusions
of the foot within the substrate, including some soft tissue impressions. These features could be potentially correlated with foot kinematics and biomechanical studies on digitigrady. The record of Late Jurassic and Early Cretaceous dinosaurs in Gondwana has been significantly increased in the last years (Weishampel et al., 2004), and a better understanding of faunal distribution and relationship is particularly promising as this information continues to be recovered.
4.2.2. Pes biomechanics Human and equid pes remain the best studied in terms of biomechanics, including posture, stress distribution, material properties and kinematics (e.g.; Hinterhofer et al., 2000; McGuigan and Wilson, 2003; Viceconti et al., 2004; Robinson et al., 2005). However, little is known about other animals, modern and extinct, which also present interesting study cases. For example, the bizarre pedal morphology of Megatherium, a large South American edentate; the specialized spring system in the pes of carnivore mammals, and the manner by which plantigrades deal with torsional loads need to be assessed in future investigations. There are three particular questions that emerged from the present research and that I wish to address in future studies: (1) How did large digitigrade dinosaurs support a highly stressed posture? (2) What are the effects of tensional forces exerted by tendons and collateral ligaments on the bone? (3) Will a 3D biomechanical model improve significantly our interpretations of pedal functional morphology? Improvement of biomechanical models, such as the ones presented in this research project will be possible with more accurate material properties (bone, cartilage, tendon, ligament). Also, models would benefit from a better understanding of trabecular and whole bone structural behaviour. The above, in combination with the development of analytic software will facilitate the study of animal functional morphology.
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Chapter 4. Conclusions
4.3. References Hinterhofer C, Stanek C, Haider H. 2000. The effect of flat horseshoes, raised heels and lowered heels on the biomechanics of the equine hoof assed by finite element analysis (FEA). Journal of Veterinary Medicine A47:73-82. McGuigan PM, Wilson AM. 2003. The effect of gait and digital flexor muscle activation on limb compliance in the forelimb of the horse Equus caballus. The Journal of Experimental Biology 206:1325-1336. Robinson JR, Bull AM, Amis AA. 2005. Structural properties of the medial collateral ligament complex of the human knee. Journal of Biomechanics 38:1067-1074. Viceconti M, Davinelli M, Taddei F, Cappello A. 2004. Automatic generation of accurate subject-specific bone finite element models to be used in clinical studies. Journal of Biomechanics 37:1597-1605. Weishampel DB, Barrett PM, Coria R, Le Loeuff J, Xing X, Xijin Z, Sahni A, Gomani EMP, Noto CR. 2004. Dinosaur distribution. In: Weishampel DB, Dodson P, Osmólska H, editors. The Dinosauria. 2nd ed. London, England: University of California Press. p 517-606.
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