Abstract
AbstractSample 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.
Highlights
GEOLOGISTS, palaeontologists, archaeologists and bioanthropologists are pragmatic folk and have to work with what they discover even if it is never enough! This is especially true in vertebrate ichnology
For the purposes of this paper, we restrict ourselves to simple linear distances, such as track length (e.g. Wiseman & De Groote 2018; Wiseman et al 2020), which is commonly used to infer the biometrics like stature and body mass of the track-maker or make minimum number of individuals (MNI) estimates (e.g. Thulborn 1990; Bennett & Morse 2014)
The tracks made in compact sand and silty sand (Fig. 2, numbers 1 and 2) show least variance as one might expect, while the highest variance occurs in a shallow mud, in which the foot tends to skate on the sublayer (Fig. 2, number 4)
Summary
GEOLOGISTS , palaeontologists, archaeologists and bioanthropologists are pragmatic folk and have to work with what they discover even if it is never enough! This is especially true in vertebrate ichnology. The problem for ichnologists is that they rarely sample the full range of this morphological envelope due to being limited by the number of tracks that their excavation reveals or preserves, or in some cases the extent to which they are permitted to excavate (Fig. 1) This puts an intrinsic limit on the reliability of inferences such as the biometrics or biomechanics of the track-maker. MNI estimates are made by reference to determining length Æ5% of the mean; if the two tracks exceed this ‘magic’ 5% they are deemed to belong to two individuals In practice this should be the 95% standard error (SE) of the mean, but an ichnologist is rarely able to sample the true variability of a population due to issues of preservation or exposure. To the trained statistician this may all seem obvious, embedded in the properties of the normal distribution, but we believe it is a timely and important reminder for field scientists who usually have to work with what they have
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
