Herbivorous Dinosaur Jaw Disparity: patterns in biomechanical diversity Jamie A. MacLaren*, Dr. Phil S. L. Anderson & Dr. Emily J. Rayfield School of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, UK, BS8 1RJ *
[email protected]
The radiation of herbivorous dinosaurs was punctuated by ecologically significant events, including the Triassic-Jurassic mass extinction (c.200 Myr ago) and the origin of flowering plants (angiosperms). Assessing biomechanical trait variation within mandibular elements may offer insights into any effects these events may have had on the feeding variation in herbivorous dinosaurs. We conducted, to our knowledge, the first analysis of biomechanical disparity in the jaws of herbivorous (sauropodomorph and ornithischian) dinosaurs. Our results suggest herbivorous dinosaurs rapidly exploited available feeding niches, with discrete ‘functional space’ occupation by different clades occurring by the MidUpper Jurassic. Novel feeding opportunities were exploited in the Upper Jurassic by biomechanically diverse, high-browsing sauropodomorphs. Neither the Tr-J mass extinction nor the proliferation of flowering plants triggered any significant changes in biomechanical disparity. The addition of herbivorous theropods and crocodiles will aid in offering more accurate interpretations of megaherbivore feeding disparity through the Mesozoic. Methodology: A detailed database of 167 lower jaws was compiled for biomechanical traits analyses. 18 continuous functional characters were tested, many of which pertain to important functional outcomes in extant organisms. Functional data were each subjected to multivariate analyses to produce mandibular ‘functional space’, with principle coordinate (PCO) axes describing variation in biomechanical traits. Traits measured included:
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Mechanical Advantage Aspect Ratio (proxy for jaw stiffness) Mandibular Fenestra (% area) Length of Adductor Fossa Offset of Jaw Articulation
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Curvature of Dental Row Length of Dental Row Tooth Procumbency Tooth Height : Jaw Depth Length of Jaw Symphysis
Figure 1. Occupation of 2-D mandibular ‘functional space’ by 5 clades of herbivorous dinosaurs in 5 distinct time bins. Lower Jurassic ‘functional space’ shows the greatest degree of overlap, whereas Middle-Upper Jurassic taxa show greatest disparity, i.e. least ‘functional space’ overlap. (significance ***)
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Jaw disparity through time was calculated across 5 time bins: Upper Triassic, Lower Jurassic, Middle-Upper Jurassic, Lower Cretaceous & Upper Cretaceous, with a starting disparity of 0 in the Middle Triassic (no herbivorous dinosaurs known from this time). In addition, sauropodomorph jaw disparity was tested across vertical feeding envelopes of 3m. Mean pairwise distance (distance based metric) was used to produce all disparity curves.
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Figure 2. Biomechanical disparity of herbivorous dinosaurs through time, based on biomechanical traits of mandibular elements. Disparity is rooted at 0 in the Middle Triassic as herbivorous dinosaurs are not known from this time. Disparity shows no significant change surrounding mass extinction events (T-J & K-Pg). The proliferation of flowering plants (angiosperms) and coincident decline of cycadophytes also show no apparent effect on herbivorous dinosaur biomechanical disparity. A low peak is retrieved in the Upper Jurassic.
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Results: diversity Our results suggest herbivorous dinosaurs exhibit early diversification in jaw mechanical traits to disparity diversity diversity diversity diversity diversity diversity alpha alpha alpha alpha alphadiversity alpha alpha alpha disparity many available niches (Figure 1 & 2). Both sauropodomorph and ornithischian taxa from the 0 5 occupy 10 15 20 5 10 10 15 1520 200.0100 0 5 05 1010 50 15 10 15 50 2015 20 10 5 20 15 10 20 15 20 200 0 5 0.002 0.006
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Figure 3. Sauropodomorph biomechanical disparity across vertical feeding envelopes. Dinosaurs and plants are provided to add perspective (from top: Giraffatitan, Camarasaurus, 0.002
Apatosaurus, Nigersaurus. Plants: Araucariaceae, Magnoliaceae, Cyatheales, Cycadales, Pteridopsida).
Lower Jurassic occupy similar regions of mandibular ‘functional space’ (suggesting uniformity in biomechanical profiles after the Tr-J). Within herbivorous dinosaurs, the five clades showed discrete occupation of ‘functional space’ in only the Middle-Upper Jurassic (Figure 1); ceratopsids and iguanodonts fall within similar regions of functional space, causing a reduction in disparity within these groups through the Cretaceous. The exceptional biomechanical variation of ankylosaurs and leptoceratopsids counteracts the low variation within ceratopsids/iguanodonts, resulting in a Cretaceous functional plateau. Overall, functional disparity plateaus through the Mesozoic with a shallow peak in the Middle-Upper Jurassic coinciding with the presence of high-browsing sauropods e.g. Brachiosaurus, Mamenchisaurus (Figure 2). High-browsing sauropodomorphs display similar levels of functional variation to those feeding at the shrub layer (< 3m), despite a very low taxon count (Figure 3). The origination and proliferation of angiosperms appears to have little to no effect on the biomechanical disparity of herbivorous dinosaurs; the coincident decline in cycads and bennittitales also shows no effect on overall disparity.
In conclusion, the first examination of disparity has revealed:
dinosaurian biomechanical
1. Neither the T-J mass extinction nor the origination of angiosperms affected the biomechanical trait diversity of herbivorous dinosaur jaws. 2. Early functional saturation occurred as herbivorous taxa adapted to available biomechanical roles in the Upper Triassic; these roles were taken over by subsequent taxa, creating a biomechanical disparity plateau. 3. Discrete occupation of ‘functional space’ by all clades in the MiddleUpper Jurassic affect a low peak in biomechanical disparity, coincident with the exploitation of a novel functional niche (high-browsing).
