Sampling impacts the assessment of tooth growth and replacement rates in archosaurs: implications for paleontological studies.

Jens C.D Kosch,Lindsay E Zanno

doi:10.7717/peerj.9918

Abstract

Dietary habits in extinct species cannot be directly observed; thus, in the absence of extraordinary evidence, they must be reconstructed with a combination of morphological proxies. Such proxies often include information on dental organization and function such as tooth formation time and tooth replacement rate. In extinct organisms, tooth formation times and tooth replacement rate are calculated, in part via extrapolation of the space between incremental lines in dental tissues representing daily growth (von Ebner Line Increment Width; VEIW). However, to date, little work has been conducted testing assumptions about the primary data underpinning these calculations, specifically, the potential impact of differential sampling and data extrapolation protocols. To address this, we tested a variety of intradental, intramandibular, and ontogentic sampling effects on calculations of mean VEIW, tooth formation times, and replacement rates using histological sections and CT reconstructions of a growth series of three specimens of the extant archosaurian Alligator mississippiensis. We find transect position within the tooth and transect orientation with respect to von Ebner lines to have the greatest impact on calculations of mean VEIW—a maximum number of VEIW measurements should be made as near to the central axis (CA) as possible. Measuring in regions away from the central axis can reduce mean VEIW by up to 36%, causing inflated calculations of tooth formation time. We find little demonstrable impact to calculations of mean VEIW from the practice of subsampling along a transect, or from using mean VEIW derived from one portion of the dentition to extrapolate for other regions of the dentition. Subsampling along transects contributes only minor variations in mean VEIW (<12%) that are dwarfed by the standard deviation (SD). Moreover, variation in VEIW with distance from the pulp cavity likely reflects idiosyncratic patterns related to life history, which are difficult to control for; however, we recommend increasing the number of VEIW measured to minimize this effect. Our data reveal only a weak correlation between mean VEIW and body length, suggesting minimal ontogenetic impacts. Finally, we provide a relative SD of mean VEIW for Alligator of 29.94%, which can be used by researchers to create data-driven error bars for tooth formation times and replacement rates in fossil taxa with small sample sizes. We caution that small differences in mean VEIW calculations resulting from non-standardized sampling protocols, especially in a comparative context, will produce inflated error in tooth formation time estimations that intensify with crown height. The same holds true for applications of our relative SD to calculations of tooth formation time in extinct taxa, which produce highly variable maximum and minimum estimates in large-toothed taxa (e.g., 718–1,331 days in Tyrannosaurus).

Highlights

We find little demonstrable impact to calculations of mean VEIW from the practice of subsampling along a transect, or from using mean VEIW derived from one portion of the dentition to extrapolate tooth formation time (TFT) and Tooth replacement rate (TRR) for other regions of the dentition
Our results indicate that the greatest challenges to accurately estimating TFT and calculating TRR for extinct taxa rests in 1) generating a transect perpendicular to VEL orientation for the measurement of VEIW; and 2) measuring VEIW and crown height along a transect that bisects the central axis as opposed to other regions of the tooth
VEIW decreases with increasing distance from the central axis and use of mean VEIW values from these regions will produce inflated TFTs; the greater the crown height, the more intense the impact of using mean VEIW calculations from regions with more compressed VEL

Summary

Introduction

Tooth replacement rate (TRR) provides key information on the function and evolution of the dentition (Edmund 1960, Osborn 1970, 1975, Richman et al 2013, Whitlock and Richman 2013, Schwarz et al 2015, LeBlanc et al 2016, Bramble et al 2017) that can be used to infer aspects of the paleobiology of extinct taxa such as metabolic activity/investment, dietary preferences and behavior (Johnston 1979, Barrett 2014, Salakka 2014, D'Emic et al 2013, 2019a, 2019b, Brink et al 2015, Button et al 2017; D’Emic et al 2019). Paleontological studies have used VEL count, or estimates of VEL count derived from direct measurements of dentin thickness divided by the mean width between VELs (so called “von Ebner Line Increment Width”, VEIW), to estimate tooth formation times (TFTs) and TRR in extinct species (e.g., Erickson 1996b, Sereno et al 2007, D'Emic et al 2013, 2019b, Garcia and Zurriaguz 2016, Erickson et al 2017, Ricart et al 2019) This practice makes the accurate estimation of VEL counts and mean VEIW a critical consideration, as errors in the calculation of either will have cascading effects on estimations of TFT and TRRs, resulting in erroneous paleobiological inferences and macroevolutionary trends. Because VELs are not always visible along the entire transect, mean VEIWs are typically derived from a transect subsample, as opposed to being calculated from the entire transect length (Erickson 1992, 1996a, 1996b, Gren 2011, Gren and Lindgren 2013, Button et al 2017, Erickson et al 2017, Kear et al 2017, Ricart et al 2019, D'Emic et al 2019b)

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Discussion

Conclusion