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. 2019 Mar 23;1(1):obz006.
doi: 10.1093/iob/obz006. eCollection 2019.

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

Haley D O'Brien  1 , Leigha M Lynch  2 , Kent A Vliet  3 , John Brueggen  4 , Gregory M Erickson  5 , Paul M Gignac  1
Affiliations

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

Haley D O'Brien et al. Integr Org Biol. .

Abstract
in English, Hebrew, Portuguese

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 R 2≥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.

Alometria de Largura da Cabeça de Crocodilianos e Previsão Filogenética de Tamanho Corporal em Crocodilianos Extintos (Crocodylian Head Width Allometry and Phylogenetic Prediction of Body Size in Extinct Crocodyliforms)Tamanho corporal e mudanças em proporções corporais afetam amplamente os parâmetros da biologia de todos os animais, o que faz de estimativas precisas do tamanho corporal para táxons extintos um componente importante da compreensão de sua paleobiologia. Entre os crocodilianos extintos e seus precursores (por exemplo, os suchianos), vários métodos foram desenvolvidos para prever o tamanho corporal a partir de conjuntos de próteses de tecido rígido. No entanto, vários desses conjuntos têm aplicações limitadas devido à disparidade entre alguns grandes grupos de crocodiliformes e vieses no registro fóssil. Aqui, testamos a utilidade da largura da cabeça como um indicador de tamanho corporal aplicável a Crocodyliformes vivos e fósseis. Utilizamos um conjunto de dados de indivíduos machos e fêmeas sexualmente maduros (n=76), de uma amostra abrangente de espécies crocodílicas viventes, abrangendo quase todos os taxa conhecidos (n=22), para desenvolver um modelo filogenético bayesiano para predizer três morfometrias convencionais para tamanho: massa corporal, comprimento da abertura nasal e comprimento total. Em seguida, usamos o modelo para estimar parâmetros de tamanho para um conjunto específico de crocodiliformes extintos de afinidade filogenética conhecida (Montsechosuchus, Diplocynodon, e Sarcosuchus). Comparamos então nossos resultados com os tamanhos relatados na literatura para exemplificar a utilidade de nossa abordagem para uma ampla gama de fósseis asiáticos. Nossos resultados mostram que a largura da cabeça é altamente correlacionada com todas as outras métricas (R 2≥0.85, para todas) e é comparável a dimensões femorais como um preditor confiável de tamanho corporal. Fornecemos o código R para permitir que outros pesquisadores utilizem o modelo em suas próprias pesquisas.Translated to Portuguese by J.P. Fontenelle (jp.fontenelle@mail.utoronto.ca).

Alometría del Ancho de la Cabeza de Cocodrilo y Predicción Filogenética del Tamaño Corporal en Cocodrilos Extintos (Crocodylian Head Width Allometry and Phylogenetic Prediction of Body Size in Extinct Crocodyliforms)El tamaño corporal y los cambios de tamaño corporal afectan ampliamente los parámetros de la historia de vida de todos los animales, lo que ha hecho que las estimaciones precisas del tamaño corporal de los taxones extintos sean un componente importante para comprender su paleobiología. Entre los crocodilianos extintos y sus precursores (por ejemplo, los suquios), se han desarrollado varios métodos para predecir el tamaño corporal a partir de conjuntos de indicadores de tejido duro. Sin embargo, muchos tienen aplicaciones limitadas debido a la disparidad de algunos grupos importantes de crocodiliformes y sesgos en el registro fósil. Aquí, probamos la utilidad del ancho de la cabeza como un estimador de tamaño corporal ampliamente aplicable en crocodiliformes vivos y fósiles. Utilizamos un conjunto de datos de individuos machos y hembras sexualmente maduros (n=76) de una muestra exhaustiva de especies existentes de cocodrilos que abarcan casi todos los taxones conocidos (n=22) para desarrollar un modelo filogenético bayesiano y predecir tres métricas convencionales para el tamaño: masa corporal, longitud del orificio de ventilación y longitud total. Luego usamos el modelo para estimar los parámetros de tamaño para una serie selecta de crocodiliformes extintos con afinidad filogenética conocida (Montsechosuchus, Diplocynodon, y Sarcosuchus). Luego comparamos nuestros resultados con los tamaños reportados en la literatura para demostrar la utilidad de nuestro enfoque en una gama amplia de tales fósiles. Nuestros resultados muestran que el ancho de la cabeza está altamente correlacionado con todas las otras métricas (todo R 2≥0.85) y es conmensurable a las dimensiones femorales debido a su confiabilidad como predictor del tamaño corporal. Proporcionamos el código R para permitir que otros investigadores empleen el modelo en su propia investigación.Translated to Spanish by C.A. Alfonso (calfonsoc@vt.edu).

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Figures

Fig. 1
Fig. 1
Likelihood-based molecular phylogeny utilized in the current study; modified from Erickson et al. (2012). Parenthetical values next to each taxon represent the number of individuals sampled; note that not all specimens have available TLs (see Supplementary Information S1). Fossil skull silhouettes are presented on the node to which they were grafted (with near-zero-length branch lengths). Sarcosuchus imperator was estimated from two phylogenetic positions: Sarcosuchus 1 at the base of the phylogeny (conservative phylogenetic placement), and Sarcosuchus 2 at the base of Gavialidae (longirostrine convergent placement).
Fig. 2
Fig. 2
Skull of an American alligator, Alligator mississippiensis, in dorsal view, demonstrating the linear measurement for HW across the quadrates in purple (trans-quadratic width). For incomplete specimens, HW can be measured as twice the distance between the midsagittal plane and the lateral margin of the quadrate. Additional techniques are outlined in Gignac and O’Brien (2016). Skull accessed from Digimorph.org (Rowe et al. 2003).
Fig. 3
Fig. 3
Regression plots quantifying the relationship between HW (cm) and mass (kg). In all plots, the regression line is solid, 95% confidence intervals are the longer dashed lines, and the 95% PIs are the smaller dashed lines. Note the different scales for each plot. Regression plot (a) demonstrates the relationship between HW and raw, uncorrected mass among extant taxa. In plot (b), fossil phylogenetic predictions derived from the regression equation in (a) have been added. Regression plot (c) demonstrates the relationship between HW and a 25% reduction in mass, to account for the mass discrepancy between extant captive and wild crocodylians. In plot (d), fossil predicted values derived from the regression equation in (c) have been added. Sarcosuchus imperator mass has been estimated from two phylogenetic placements: a conservative placement at the base of the phylogeny and a convergent ecomorphological placement at the base of Gavialidae, representing longirostrine forms. Abbreviations: HW, head width; M, mass; LR, longirostrine.
Fig. 4
Fig. 4
Regression plots quantifying the relationship between HW (cm) and TL (cm). In both plots, the regression line is solid, 95% confidence intervals are the longer dashed lines, and the 95% PIs are the smaller dashed lines. Note the different scales for both plots. Regression plot (a) represents the relationship between HW and TL among extant taxa. Plot (b) demonstrates the phylogenetic predictions of TL for extinct crocodyliforms, as estimated from the regression equation in (a). Sarcosuchus imperator has had TL estimated from two phylogenetic placements: a conservative placement at the base of the phylogeny and a convergent ecomorphological placement at the base of Gavialidae (with other longirostrine forms). Abbreviations: HW, head width; LR, “longirostrine”; TL, total length.
Fig. 5
Fig. 5
Regression plot quantifying the relationship between HW (cm) and SVL (cm). The regression line is solid, 95% confidence intervals are the longer dashed lines, and the 95% PIs are the smaller dashed lines. Abbreviations: HW, head width; SVL, snout–vent length.
See this image and copyright information in PMC

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