Defining the morphological quality of fossil footprints. Problems and principles of preservation in tetrapod ichnology with examples from the Palaeozoic to the present

Abstract

The morphology of fossil footprints is the basis of vertebrate footprint ichnology. However, the processes acting during and after trace fossil registration which are responsible for the final morphology have never been precisely defined, resulting in a dearth of nomenclature. Therefore, we discuss the concepts of ichnotaphonomy, ichnostratinomy, taphonomy, biostratinomy, registration and diagenesis and describe the processes acting on footprint morphology. In order to evaluate the morphological quality of tetrapod footprints, we introduce the concept of morphological preservation, which is related to the morphological quality of footprints (M-preservation, acronym MP), and distinguish it from physical preservation (P-preservation, acronym PP), which characterizes whether or not a track is eliminated by taphonomic and diagenetic processes. M-preservation includes all the morphological features produced during and after track registration prior to its study, and may be divided into substages (ichnostratinomic, registrational, taphonomic, stratinomic, diagenetic). Moreover, we propose an updated numerical preservation scale for M-preservation. It ranges from 0.0 (worst preservation) to 3.0 (best preservation); intermediate values may be used and specific features may be indicated by letters. In vertebrate footprint ichnotaxonomy, we regard the anatomy-consistent morphology and to a lesser extent the trackway pattern as the only acceptable ichnotaxobases. Only footprints showing a good morphological preservation (grade 2.0-3.0) are useful in ichnotaxonomy, whereas ichnotaxa based on poor morphological preservation (grade 0.0-1.5) are considered ichnotaphotaxa (nomina dubia) characterized by extramorphologies. We applied the preservation scale on examples from the Palaeozoic to the present time, including three ichnotaphotaxa and 18 anatomy-consistent ichnotaxa/morphotypes attributed to several vertebrate footprint producers. Results indicate the utility, feasibility and suitability of this method for the entire vertebrate footprint record in any lithofacies, strongly recommending its use in future ichnotaxonomic studies.

Shape variability in tridactyl dinosaur footprints: the significance of size and function

“…Cursorial ratites and other cursorial taxa tend to shorten or lose non‐central digits (Lull 1904; Patak & Baldwin 1998; Schaller et al . 2011).…”

An integrative physical, mineralogical and ichnological approach to characterize underfilled lake‐basins

“…Mode B represents low fidelity to the trackmaker's pedal anatomy (Preservational Scale 1, Marchetti et al ., .). Tracks are poorly defined and preserved as oval to somewhat irregular outline impressions with a relatively deep shaft (from 0.5 to 6.0 cm) (Fig.…”

“…Mode A represents the highest fidelity of the trackmaker's pedal anatomy (Preservational Scale 3, Marchetti et al ., ). This mode is typified by detailed impression of the digits and palms (Fig.…”

Studying bipedal dinosaur trackways using geometric morphometrics

“…The geometry of the tracks and the trackway bear indirect evidence of the anatomy of the trackmaker, which is why it can be a reliable proxy to interpret, or to infer, important aspects of the anatomy of the animal that produced it. However, the assignment of particular taxa to the trackways is often difficult, sometimes even impossible (Marchetti et al, 2019), because multiple factors influence the final shape of the tracks, including sediment structure and taphonomy, all of which can obscure ichnotaxonomic assignments. In fact, this is the reason why researchers have aimed at parsing subtle features of the footprints with fined-tuned proxies such as geometric morphometrics (Moratalla and Marugán-Lobón, 2009;Castanera et al, 2015).…”

“…Out of the few studies that have been carried out so far with GM, however, most have been performed for the comparison of isolated footprints (Rasskin-Gutman et al, 1997;Rodrigues and San-tos, 2004;Moratalla and Marugán-Lobón, 2009), for ichnotaxonomic purposes (Shell and Boss, 2013), for comparing the variation of the shape of footprints within trackways (Lallensak et al, 2016) or the comparison of footprints between similar trackways (Castanera et al, 2015). The sources of variation between bipedal dinosaur trackways (their measurable geometric differences) might depend mainly on taxonomic (trackmaker identity) and preservational taphonomic factors, much as it does for isolated tracks (see Gatesy and Falkingham, 2017;Marchetti et al, 2019). The fact is that ichnotaxonomic allocation is not always exempt of certain ambiguity (Castanera et al, 2015) especially since a very large source of variation is due to substrate dynamics (Gatesy and Falkingham, 2017).…”