Including Distorted Specimens in Allometric Studies: Linear Mixed Models Account for Deformation

Wynd, Brenen M.; Uyeda, Josef C.; Nesbitt, Sterling J.

doi:10.1093/iob/obab017

Cited by 4 publications

(10 citation statements)

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“…Taphonomical alterations impact the original shape of a fossil and consequently alter its biological information (Eifremov, 1940; Webster & Hughes, 1999). Accounting for taphonomy is even more relevant when performing geometric morphometric and statistical analyses, such as here on limb bones (Hedrick et al, 2018; Lefebvre et al, 2020; Pintore et al, 2021 in press , Wynd et al, 2021). Thus, only complete femora without clear, visible distortion of the anteroposterior curvature of the diaphysis and of the proximodistal angle between the two epiphyses were considered.…”

Section: Methodsmentioning

confidence: 99%

Femoral specializations to locomotor habits in early archosauriforms

et al. 2021

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The evolutionary history of archosaurs and their closest relatives is characterized by a wide diversity of locomotor modes, which has even been suggested as a pivotal aspect underlying the evolutionary success of dinosaurs vs. pseudosuchians across the Triassic–Jurassic transition. This locomotor diversity (e.g., more sprawling/erect; crouched/upright; quadrupedal/bipedal) led to several morphofunctional specializations of archosauriform limb bones that have been studied qualitatively as well as quantitatively through various linear morphometric studies. However, differences in locomotor habits have never been studied across the Triassic–Jurassic transition using 3D geometric morphometrics, which can relate how morphological features vary according to biological factors such as locomotor habit and body mass. Herein, we investigate morphological variation across a dataset of 72 femora from 36 different species of archosauriforms. First, we identify femoral head rotation, distal slope of the fourth trochanter, femoral curvature, and the angle between the lateral condyle and crista tibiofibularis as the main features varying between bipedal and quadrupedal taxa, all of these traits having a stronger locomotor signal than the lesser trochanter's proximal extent. We show a significant association between locomotor mode and phylogeny, but with the locomotor signal being stronger than the phylogenetic signal. This enables us to predict locomotor modes of some of the more ambiguous early archosauriforms without relying on the relationships between hindlimb and forelimb linear bone dimensions as in prior studies. Second, we highlight that the most important morphological variation is linked to the increase of body size, which impacts the width of the epiphyses and the roundness and proximodistal position of the fourth trochanter. Furthermore, we show that bipedal and quadrupedal archosauriforms have different allometric trajectories along the morphological variation in relation to body size. Finally, we demonstrate a covariation between locomotor mode and body size, with variations in femoral bowing (anteroposterior curvature) being more distinct among robust femora than gracile ones. We also identify a decoupling in fourth trochanter variation between locomotor mode (symmetrical to semi‐pendant) and body size (sharp to rounded). Our results indicate a similar level of morphological disparity linked to a clear convergence in femoral robusticity between the two clades of archosauriforms (Pseudosuchia and Avemetatarsalia), emphasizing the importance of accounting for body size when studying their evolutionary history, as well as when studying the functional morphology of appendicular features. Determining how early archosauriform skeletal features were impacted by locomotor habits and body size also enables us to discuss the potential homoplasy of some phylogenetic characters used previously in cladistic analyses as well as when bipedalism evolved in the avemetatarsalian lineage. This study illuminates how the evol...

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Section: Methodsmentioning

confidence: 99%

Femoral specializations to locomotor habits in early archosauriforms

et al. 2021

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“…In the absence of additional ontogenetic data, the common default is to evaluate the trajectory and patterns of growth via allometric reconstructions ( e.g. , Cheverud, 1982 ; Abdala & Giannini, 2000 ; Blob, 2006 ; Wynd, Uyeda & Nesbitt, 2021 ), rather than infer the developmental stage of a single specimen ( e.g. , juvenile vs adult).…”

Section: Methodsmentioning

confidence: 99%

“…Taphonomic distortion is a critical issue in reconstructing patterns of allometry in extinct animals that often results in measurement omission or estimation ( Brown, Arbour & Jackson, 2012 ; Brown & Vavrek, 2015 ; Wynd, Uyeda & Nesbitt, 2021 ). Fortunately, skull length in E. argentinus is typically preserved and not prone to as much distortion, partly due to regions where many bones contact one another with a relatively large surface area to form resistant structures ( e.g.…”

Section: Methodsmentioning

confidence: 99%

“…1 ). Ordinary least squares (OLS) regression is commonly used for undistorted measurements whereas generalized linear mixed models (GLMM) have been explored on datasets that include undistorted and distorted measurements ( Wynd, Uyeda & Nesbitt, 2021 ). The existing sample of E. argentinus specimens often have distorted features that would lead to dropping those specimens from analysis, which leads to a mean number of available specimens of 16, rounded.…”

Section: Methodsmentioning

confidence: 99%

“…Of our sample, 11 specimens were collected from Argentina and brought to Harvard in 1965, and 13 are currently housed in Argentina. To account for the presence of deformation in a relatively low sample size, we implement a Generalized Linear Mixed Effects model (see Wynd, Uyeda & Nesbitt, 2021 ), to estimate allometric relationships used to infer ontogenetic change. With this reconstruction of ontogeny, we hypothesize a shift in diet from smaller to larger individuals.…”

Section: Introductionmentioning

confidence: 99%

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Ontogenetic growth in the crania ofExaeretodon argentinus(Synapsida: Cynodontia) captures a dietary shift

Wynd

Abdala

Nesbitt

2022

PeerJ

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Background An ontogenetic niche shift in vertebrates is a common occurrence where ecology shifts with morphological changes throughout growth. How ecology shifts over a vertebrate’s lifetime is often reconstructed in extant species—by combining observational and skeletal data from growth series of the same species—because interactions between organisms and their environment can be observed directly. However, reconstructing shifts using extinct vertebrates is difficult and requires well-sampled growth series, specimens with relatively complete preservation, and easily observable skeletal traits associated with ecologies suspected to change throughout growth, such as diet. Methods To reconstruct ecological changes throughout the growth of a stem-mammal, we describe changes associated with dietary ecology in a growth series of crania of the large-bodied (∼2 m in length) and herbivorous form, Exaeretodon argentinus (Cynodontia: Traversodontidae) from the Late Triassic Ischigualasto Formation, San Juan, Argentina. Nearly all specimens were deformed by taphonomic processes, so we reconstructed allometric slope using a generalized linear mixed effects model with distortion as a random effect. Results Under a mixed effects model, we find that throughout growth, E. argentinus reduced the relative length of the palate, postcanine series, orbits, and basicranium, and expanded the relative length of the temporal region and the height of the zygomatic arch. The allometric relationship between the zygomatic arch and temporal region with the total length of the skull approximate the rate of growth for feeding musculature. Based on a higher allometric slope, the zygoma height is growing relatively faster than the length of the temporal region. The higher rate of change in the zygoma may suggest that smaller individuals had a crushing-dominated feeding style that transitioned into a chewing-dominated feeding style in larger individuals, suggesting a dietary shift from possible faunivory to a more plant-dominated diet. Dietary differentiation throughout development is further supported by an increase in sutural complexity and a shift in the orientation of microwear anisotropy between small and large individuals of E. argentinus. A developmental transition in the feeding ecology of E. argentinus is reflective of the reconstructed dietary transition across Gomphodontia, wherein the earliest-diverging species are inferred as omnivorous and the well-nested traversodontids are inferred as herbivorous, potentially suggesting that faunivory in immature individuals of the herbivorous Traversodontidae may be plesiomorphic for the clade.

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Dots on a screen: The past, present, and future of morphometrics in the study of nonavian dinosaurs

Hedrick

2023

The Anatomical Record

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Using morphometrics to study nonavian dinosaur fossils is a practice that predates the origin of the word “dinosaur.” By the 1970s, linear morphometrics had become established as a valuable tool for analyzing intra‐ and interspecific variation in nonavian dinosaurs. With the advent of more recent techniques such as geometric morphometrics and more advanced statistical approaches, morphometric analyses of nonavian dinosaurs have proliferated, granting unprecedented insight into many aspects of their biology and evolution. I outline the past, present, and future of morphometrics as applied to the study of nonavian dinosaurs zeroing in on five aspects of nonavian dinosaur paleobiology where morphometrics has been widely utilized to advance our knowledge: systematics, sexual dimorphism, locomotion, macroevolution, and trackways. Morphometric methods are especially susceptible to taphonomic distortion. As such, the impact of taphonomic distortion on original fossil shape is discussed as are current and future methods for quantifying and accounting for distortion with the goal of reducing the taphonomic noise to biological signal ratio. Finally, the future of morphometrics in nonavian dinosaur paleobiology is discussed as paleobiologists move into a “virtual paleobiology” framework, whereby digital renditions of fossils are captured via methods such as photogrammetry and computed tomography. These primary data form the basis for three‐dimensional (3D) geometric morphometric analyses along with a slew of other forms of analyses. These 3D specimen data form part of the extended specimen and help to democratize paleobiology, unlocking the specimen from the physical museum and making the specimen available to researchers across the world.

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Including Distorted Specimens in Allometric Studies: Linear Mixed Models Account for Deformation

Cited by 4 publications

References 58 publications

Femoral specializations to locomotor habits in early archosauriforms

Femoral specializations to locomotor habits in early archosauriforms

Ontogenetic growth in the crania ofExaeretodon argentinus(Synapsida: Cynodontia) captures a dietary shift

Dots on a screen: The past, present, and future of morphometrics in the study of nonavian dinosaurs

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