Crocodylian Head Width Allometry and Phylogenetic Prediction of Body Size in Extinct Crocodyliforms

Abstract

Synopsis Body size and body-size shifts broadly impact life-history parameters of all animals, which has made accurate body-size estimates for extinct taxa an important component of understanding their paleobiology. Among extinct crocodylians and their precursors (e.g., suchians), several methods have been developed to predict body size from suites of hard-tissue proxies. Nevertheless, many have limited applications due to the disparity of some major suchian groups and biases in the fossil record. Here, we test the utility of head width (HW) as a broadly applicable body-size estimator in living and fossil suchians. We use a dataset of sexually mature male and female individuals (n = 76) from a comprehensive sample of extant suchian species encompassing nearly all known taxa (n = 22) to develop a Bayesian phylogenetic model for predicting three conventional metrics for size: body mass, snout–vent length, and total length. We then use the model to estimate size parameters for a select series of extinct suchians with known phylogenetic affinity (Montsechosuchus, Diplocynodon, and Sarcosuchus). We then compare our results to sizes reported in the literature to exemplify the utility of our approach for a broad array of fossil suchians. Our results show that HW is highly correlated with all other metrics (all R2≥0.85) and is commensurate with femoral dimensions for its reliably as a body-size predictor. We provide the R code in order to enable other researchers to employ the model in their own research.

Beyond Endocasts: Using Predicted Brain-Structure Volumes of Extinct Birds to Assess Neuroanatomical and Behavioral Inferences

“…Resulting p-values of the phylogenetic ANCOVA comparing the intercept of each extinct member of the sample to the intercept of the rest of the sample, given a common slope, with the outlier Apteryx excluded. These p-values have been adjusted with the Benjamini and Hochberg [89] procedure to control for false discovery rate; unadjusted p-values are available in Table S3. Significant differences are highlighted in aqua.…”

“…Resulting p-values of the phylogenetic ANCOVA comparing the intercept of each extinct member of the sample to the intercept of the rest of the sample, given a common slope, with the outliers Apteryx and Dinornis excluded. These p-values have been adjusted with the Benjamini and Hochberg [89] procedure to control for false discovery rate; unadjusted p-values are available in Table S3. Abbreviations: BR, brain-remainder volume (brain volume minus optic-tectum volume); ER, endocast-remainder surface area (endocast surface area minus optic-lobe surface area); FMa, foramen-magnum area; FMw, foramen-magnum width; OL, optic-lobe surface area; OT, optic-tectum volume.…”

“…The surface areas of the Wulsts and optic lobes of the extinct birds in the sample were combined with a dataset of these endocast structures and their underlying brain structures in extant birds for analysis with the {BayesModelS} package [88]. R code to perform this analysis was adapted from [89]. The package {BayesModelS} conducts phylogenetic predictions using Bayesian inference to calculate the value of a trait that cannot be directly measured in the taxon of interest (e.g., the brain-structure volume of an extinct bird).…”

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

“…Other authors (9) have used skull length as a proxy for head size (either as a proxy for body size or to directly examine the relationship between labyrinth shape and head size), but this too is difficult for crocodylomorphs because snout length and skull proportions vary drastically within the group (62). Thus, in our cochlear duct length regression, we used the distance between the vestibules of the right and left labyrinths as a measure of specimen size, as head width is highly correlated with measures of body size in extant species (63). We realize, however, that this is a nuanced measure of specimen size, which is why we did not use it more extensively to represent specimen/head/body size to test for relationships with labyrinth size, shape, and linear measurements, as other authors have done (e.g., refs.…”

Giant extinct caiman breaks constraint on the axial skeleton of extant crocodylians

“…We used the dataset and R software code from O’Brien et al (2019) with P. mirandai input into the phylogeny as per Figure 1 and with a branch length of 0.01 (assuming 25% corrected mass as per their methods) to estimate the total body mass and total length of specimen AMU-CURS-541 (head width ~0.88 m). Based on this more conservative approach, our analysis estimated 1686–2637 kg and 7.11–8.01 m for total body mass and length, respectively (from lower to upper interquartile ranges of Bayesian analysis).…”