When is enough, enough? Questions of sampling in vertebrate ichnology

Abstract

Sample size is a challenge for most field scientists determined not by the statistically ideal, but by the available. In vertebrate ichnology, track length is an important variable correlating well with the track‐maker’s biology. It is also key to estimating the minimum number of individuals (MNI) present on a trampled horizon. Broad assumptions on biometrics of the track‐makers are often made based on a few prints without consideration for intra‐trackway variability. In this study we use a simple bootstrapping algorithm to explore variance within sample size for a range of trackways with fossil and experimental examples to determine the minimum sample size required to extract linear measurements. Predictably, experimentation shows that inter‐step variability changes with track‐maker and substrate, but the degree of variance is not as marked as previously anticipated. Change‐point modelling suggests that a maximum sample size of 22–25 captures most of the variance present in track length at least; another threshold at 7–10 has been identified, which represents the reasonable sample size minimum. Samples of fewer than seven tracks are subject to large amounts of potential variance and are unlikely to provide reliable and consistent measurements. These sampling thresholds hold across a wide range of depositional environments and track‐makers. We calculate generic standard errors for human track‐makers which may assist the practitioner with small samples to estimate the likely errors, especially when making MNI estimates. The challenge is placed to the wider vertebrate ichnology to explore this issue for other track‐makers and develop similar guidance.

From footprint morphometrics to the stature of fossil hominins: A common but uncertain estimate

“…The use of such experimental frameworks is done under controlled conditions close to those where fossil footprints were found, thus allowing for more precise paleobiological estimates for each site. Some of these experimental studies have highlighted the intraindividual morphological variation of the footprints (Morse et al, 2013, Belvedere et al, 2021, Wiseman and De Groote, 2022 in particular in deformable loose sandy substrates in which several fossil footprints have recently been discovered (Morse et al, 2013, Altamura et al, 2018, Bustos et al, 2018, Duveau et al, 2019, Mayoral et al, 2021. This intraindividual variation must be taken into account when studying fossil footprints and more specifically when only isolated footprints, which cannot be associated within a trackway, are discovered.…”

“…This intraindividual variation must be taken into account when studying fossil footprints and more specifically when only isolated footprints, which cannot be associated within a trackway, are discovered. When such isolated footprints are found, the knowledge of intraindividual variation has been used to estimate minimum numbers of individuals (Webb et al, 2014, Duveau et al, 2019, Belvedere et al, 2021. But this knowledge is more rarely applied to stature estimates whereas because of the intraindividual dispersion, the estimation of stature on an isolated footprint is uncertain.…”

“…Finally, both studies on the relationship between footprint morphometry and body characteristics and those on intraindividual morphological variation are mainly based on footprint length (Dingwall et al, 2013, Belvedere et al, 2021, Wiseman and De Groote, 2022. This variable is not only easily measured but also strongly correlated with individual stature.…”

One size fits all? Stature estimation from footprints and the effect of substrate and speed on footprint creation

“…For example, print length within a trackway belonging to a single individual can vary substantially when the underlying substrate materials are changed (Milan & Bromley, 2006; Morse et al, 2013). Changes in hydrology, particle size, and even particle composition can all influence the resultant footprint (Belvedere et al, 2021; Bennett & Morse, 2014; Wiseman & De Groote, 2018). Stature predictions from just one fossil trackway from Walvis Bay, Namibia have estimated that the individual ranged from 1.35 to 1.73 m tall, with the individual claimed to be either malnourished or clinically obese (Bennett & Morse, 2014; Morse et al, 2013).…”