The ‘Goldilocks’ effect: preservation bias in vertebrate track assemblages

Abstract

Finite-element analysis was used to investigate the extent of bias in the ichnological fossil record attributable to body mass. Virtual tracks were simulated for four dinosaur taxa of different sizes (Struthiomimus, Tyrannosaurus, Brachiosaurus and Edmontosaurus), in a range of substrate conditions. Outlines of autopodia were generated based upon osteology and published soft-tissue reconstructions. Loads were applied vertically to the feet equivalent to the weight of the animal, and distributed accordingly to fore-and hindlimbs where relevant. Ideal, semi-infinite elastic-plastic substrates displayed a 'Goldilocks' quality where only a narrow range of loads could produce tracks, given that small animals failed to indent the substrate, and larger animals would be unable to traverse the area without becoming mired. If a firm subsurface layer is assumed, a more complete assemblage is possible, though there is a strong bias towards larger, heavier animals. The depths of fossil tracks within an assemblage may indicate thicknesses of mechanically distinct substrate layers at the time of track formation, even when the lithified strata appear compositionally homogeneous. This work increases the effectiveness of using vertebrate tracks as palaeoenvironmental indicators in terms of inferring substrate conditions at the time of track formation. Additionally, simulated undertracks are examined, and it is shown that complex deformation beneath the foot may not be indicative of limb kinematics as has been previously interpreted, but instead ridges and undulations at the base of a track may be a function of sediment displacement vectors and pedal morphology.

The evolution of ornithischian quadrupedality

“…Dinehichnus) consistently lack manus impressions (Lockley and Wright 2001). Computer modelling of trackmaker/substrate interactions suggests that the absence of manus impressions from large 'bipedal' ornithopod trackways might result from differential loading, whereby the forelimb supports a smaller proportion of the total body mass than the hind limb: consequently, the manus indents the sediment less frequently and/or less conspicuously than the pes (Falkingham et al 2011). This suggestion is supported by tracksites preserving both pes-only and manus-pes ornithopod trackways, with this differential preservation reflecting the mechanical properties of the sediment, rather than representing genuine locomotor differences (Castanera et al 2013).…”

A new dinosaur tracksite with small footprints in the Urbión Group (Cameros Basin, Lower Cretaceous, La Rioja, Spain)

“…Both hypotheses explain the absence of small footprints because of ecological biases against them. On the other hand, Falkingham et al (2011) and Kim et al (2012) proposed an alternative and complementary interpretation for the absence of small footprints at tracksites that takes preservation bias into consideration. Kim et al (2012) affirmed that in suitable facies, like fine grained lake margin sediments, large number of small footprints can be found.…”

“…Nevertheless, in more coarse grain sediments, the footprints would be very poorly preserved, showing no detailed anatomical features (Kim et al, 2012). Falkingham et al (2011) proposed the 'Goldilocks' effect, and discussed the different preservations among tracks formed in homogeneous cohesive substrates with and without the presence of a firmer subsurface layer, and their relationship with the size of the trackmaker. In homogeneous cohesive substrates, only a narrow range of loads produce tracks (small animals failed to indent the substrate, and larger animals would be unable to traverse the area), but if a firm subsurface layer is assumed, a more complete assemblage is possible, though there is a strong bias towards larger, heavier animals (Falkingham et al, 2011).…”

“…Falkingham et al (2011) proposed the 'Goldilocks' effect, and discussed the different preservations among tracks formed in homogeneous cohesive substrates with and without the presence of a firmer subsurface layer, and their relationship with the size of the trackmaker. In homogeneous cohesive substrates, only a narrow range of loads produce tracks (small animals failed to indent the substrate, and larger animals would be unable to traverse the area), but if a firm subsurface layer is assumed, a more complete assemblage is possible, though there is a strong bias towards larger, heavier animals (Falkingham et al, 2011). Finally, Henderson (2006) noted that the small and shallowly impressed tracks were the most susceptible to the effects of modern weathering and erosion processes.…”

The birth of a dinosaur footprint: Subsurface 3D motion reconstruction and discrete element simulation reveal track ontogeny

“…Footprints can be a major source of information about an animal or group of animals (4-6), and this is particularly true for extinct taxa that cannot be observed directly (7)(8)(9). Indeed, the only movements that have been recorded in the fossil record were necessarily over/through suitably compliant substrates (10).…”