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1-2018 Convergent body size evolution of Crocodyliformes upon entering the aquatic realm William Gearty

Jonathan Payne

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This Article is brought to you for free and open access by the Biological Sciences, School of at [email protected] of Nebraska - Lincoln. It has been accepted for inclusion in Posters & Presentations in Biological Sciences by an authorized administrator of [email protected] of Nebraska - Lincoln. Convergent body size evolution of Crocodyliformes upon entering the aquatic realm William Gearty* and Jonathan L. Payne Department of Geological Sciences, Stanford University *[email protected] 1. Introduction 2. Materials and Methods Twenty-four species of populate the globe today, but this • Calculated body masses of 249 crocodyliformes (living and Figure 2.1: Maximum richness represents a minute fraction of the diversity and disparity of extinct) using measurements from primary literature credibility tree of Crocodyliformes Crocodyliformes since their origin early in the . Across this • Assigned habitats based on compilations and primary literature showing invasions of the aquatic • Crocodyliformes (Bronzati et al. 2015) realm within three clade, three major diversification events into the aquatic realm have • Species fossil ranges from compilations and PBDB Summary tree of posterior occurred. Aquatic and terrestrial habitats impose differing selective • Characterless tip-dating analysis using R and MrBayes distribution of trees from MrBayes pressures on body size. However, previous research on this topic in • Macroevolutionary Ornstein-Uhlenbeck (OU) model fitting characterless tip-dating analysis of Crocodyliformes remains qualitative in nature. In this study, our goal • OUwie R package (Beaulieu et al. 2012) the Crocodyliformes supertree. Tips • was to quantify the influence of habitat (terrestrial versus aquatic) Results model-averaged across 17 different models using AIC without habitat and/or size data General Equation of an OU Model: were dropped. Tip labels indicate .on the evolution of body size in Crocodyliformes. We find a history ࢊࢄ ࢚ൌࢻࣂ െ ࢄሺݐሻ ࢊ࢚ ൅ ࣌ࢊ࡮ሺ࢚ሻ terrestrial and aquatic species of repeated body size increase and convergence following shifts to ܺ ݐ : initial body size હ: strength of selection an aquatic lifestyle, suggesting common selective pressures on life in ݀ܺ ݐ : change in body size ો: intensity of random drift ሺݐሻ: random variation ી: body size optimumܤ݀ .water spanning multiple independent aquatic clades 3. Results 4. Conclusions Figure 3.1: Aquatic clades converge on Figure 3.4: Body size governs • All three aquatic clades converge larger body size optima relative time invested in on greater optima, with shorter Weighted means and 2σ confidence temperature regulation intervals of model-averaged body mass Ratios of the time it takes to cool phylogenetic half-lives and optima (θ) as estimated by OUwie analyses down versus the time it takes to smaller stationary variances for terrestrial and aquatic regimes. Aquatic warm up in in air and clades have statistically greater body mass in water (Smith 1976) compared • Lung volume, which has long optima than the terrestrial regime to a stacked histogram of been proposed as the main (p < .001, Mann-Whitney test). terrestrial and aquatic body masses. Larger sizes require less constraint on diving capacity, is warming time with respect to only a constraint at sizes greater Figure 3.2: Aquatic clades converge on cooling time. Living in air is shorter phylogenetic half-lives thermally advantageous at smaller than 10 kg size whereas living in water is Boxplots of model-averaged phylogenetic • half-lives (ln(2)/α) as estimated by OUwie preferable at larger size. The rate of cooling strongly analyses for terrestrial and aquatic regimes. constrains diving capacity at sizes Outliers have been removed. Aquatic clades Figure 3.5: Lung volume and smaller than 10 kg and may be have statistically shorter phylogenetic half- cooling enforce diving capacity lives compared to the terrestrial regime the primary driver of larger body constraints at different sizes (p < .001, Mann-Whitney test). sizes in diving crocodyliformes Lung volume (Wright and Kirshner, 1987; Seymour et al. Figure 3.3: Aquatic clades converge on 2013) and cooling (Smith 1976) References and Acknowledgements smaller stationary variances limits on the diving capacity of crocodiles compared to a stacked Beaulieu JM, et al. 2012. Evolution. Boxplots of model-averaged stationary Bronzati M, Montefeltro FC, Langer MC. 2015. R Soc Open Sci. variances (σ2/(2*α)) as estimated by OUwie histogram of terrestrial and Farlow JO, et al. 2005. J Vert Paleo. aquatic body masses. Cooling Mannion PD, et al. 2015. Nat Commun. analyses for terrestrial and aquatic regimes. Seymour RS, et al. 2013. J Comp Physiol B. Outliers have been removed. Aquatic clades rapidly restricts diving capacity at Smith EN. 1976. Phys Zool. smaller sizes. The smallest aquatic Tennant JP, Mannion PD, Upchurch P. 2016. Proc R Soc B. have statistically smaller stationary Wright JC and Kirshner DS. 1987. J Exp Biol. variances compared to the terrestrial crocodiles are at the smallest size where lung volume is more Thanks to David Bapst for help with tip-dating regime (p < .001, Mann-Whitney test). Thanks to Margaret Deng for collecting crocodile body limiting than heat loss. measurements Silhouettes from phylopic.org