DOI: 10.5061/dryad.wh70rxwqb
title: Data from: A new approach for investigating spatial relationships of ichnofossils: a case study of Ediacaran–Cambrian animal traces

description: Abstract — Trace fossils record foraging behaviours, the search for
 resources in patchy environments, of animals in the rock record. 
 Quantification of the strength, density and nature of foraging behaviours
 enables the investigation of how these may have changed through
 time.  Here, we present a novel approach to explore such patterns
 using spatial point process analyses to quantify the scale and strength of
 ichnofossil spatial distributions on horizontal bedding planes. 
 To demonstrate the utility of this approach we use two samples from the
 terminal Ediacaran Shibantan Member in South China (between 551 and
 543 Ma) and the early Cambrian Nagaur Sandstone in northwestern India
 (between 539 and 509 Ma).  We find that ichnotaxa on both
 surfaces exhibited significant non-homogeneous lateral patterns, with
 distinct levels of heterogeneity exhibited by different types of trace
 fossils.  In the Shibantan, two ichnotaxa show evidence for
 mutual positive aggregation over a shared resource, suggesting the ability
 to focus on optimal resource areas.  Trace fossils from the
 Nagaur Sandstone exhibit more sophisticated foraging behaviour, with
 greater niche differentiation.  Critically, mark correlation
 functions highlight significant spatial autocorrelation of trace fossil
 orientations, demonstrating the greater ability of these Cambrian
 tracemakers to focus on optimal patches.  Despite potential
 limitations, these analyses hint at changes in the development and
 optimisation of foraging at the Ediacaran–Cambrian transition and
 highlight the potential of spatial point process analysis to tease apart
 subtle differences in behaviour in the trace fossil record.
description: The Shibantan bed-sole surface was
 chosen because there was a reasonable area of bedding plane with abundant
 ichnofossils and limited evidence of preservational or erosional
 heterogeneity (Fig. 4A; see also fig. 6a of Xiao et al. 2021).  Taxonomy
 of Ediacaran trace fossils can be highly controversial (Darroch et al.
 2021), and so here we recognize two distinct morphogroups consisting of
 relatively large (~1 cm) and small (~1 mm) horizontal traces, with no
 overlap in width.  The smaller ichnofossils are
 <em>Helminthoidichnites</em>-like, and the larger ones are
 characterized by poorly preserved spiral structures reminiscent of
 <em>Streptichnus</em> (Fig 4E; Jensen and Runnegar 2005; Xiao
 et al. 2021).  Importantly, even if this preferred taxonomy is incorrect, the
 statistical analyses outlined below will not be impacted, as these large
  trace fossils represent similar behaviours, likely made by the same
 progenitor. The Naguar
 bed-sole surface similarly contains limited preservational or erosional
 heterogeneity (Fig. 4C).  We mapped four distinct types of trace fossils
 on this bedding plane. Of primary interest are larger horizontal traces,
 similar to <em>Treptichnus</em> <em>pedum
 </em>previously identified from this unit (Pandey et al. 2014), and
 smaller horizontal trails ~1 mm wide (Fig. 4D, white oval). The larger
 trails may not contain all the diagnostic features of <em>T. pedum,
 </em>so we classify them more broadly as <em>Treptichnus
 </em>isp. These exhibit a probing behaviour broadly similar to that
 of <em>Streptichnus </em>from the Shibantan. Smaller trails
 were regarded as <em>Planolites </em>isp. (Pandey et al.
 2014). Circular traces are of similar diameter to the width of the larger
 horizontal traces, and the two are often associated. A fourth group of
 bilobed traces represent <em>Rusophycus </em>and/or<em>
 Cruziana</em> (Fig. 4F; Pandey et al. 2014). The outline of each bed and individual
 trace fossils were marked as vector lines within Inkscape 0.92.3 (Fig. 5).
  Areas with low fossil density, interpreted as artefacts of differing
 erosion and/or poor photographic contrast, were excluded (Fig. 5B).
  Critically, such areas are much larger than the centimetre scale
 heterogeneities investigated here, and thus would not affect this study. 
 Most of the traces can be clearly marked, however two different modes were
 employed for larger horizontal trails (<em>Streptichnus
 </em>and <em>Treptichnus</em>).  Initially, each
 individual trace was marked irrespective of adjacent ones (unconnected
 trails, Fig. 4B, D).  Then, in instances where traces appear to represent
 a series of segments/steps that can be reliably ascribed to the same
 progenitor, they were connected or simplified to a single segment to focus
 on the overall paths of the tracemakers (connected trails, Fig. 4B, D).
  While traces are continuous objects, rather than points, these spatial
 methods have already been established for modern organisms, such as root
 systems, so are appropriate to apply to trace segments (Kitchell 1979; Koy
 and Plotnick 2010). Each single line represents a single action of the
 trace maker so that as long as consistent labelling approaches are applied
 across each surface, this marking up approach results in a representation
 of the behaviours of the trace makers across the bedding planes.
 Line data was extracted from Inkscape
 using a custom script written in Haskell (https://github.com/egmitchell/traces).  Analyses were performed in R (R Core Team 2017) using spatstat (Baddeley and Turner 2005).