The latest advances in dinosaur ichnology are showcased in this comprehensive and timely volume, in which leading researchers and research groups cover the most essential topics in the study of dinosaur tracks. Some assess and demonstrate state-of-the-art approaches and techniques, such as experimental ichnology, photogrammetry, biplanar X-rays, and a numerical scale for quantifying the quality of track preservation. The high diversity of these up-to-date studies underlines that dinosaur ichnological research is a vibrant field, that important discoveries are continuously made, and that new methods are being developed, applied, and refined. This indispensable volume unequivocally demonstrates that ichnology has an important contribution to make toward a better understanding of dinosaur paleobiology. Tracks and trackways are one of the best sources of evidence to understand and reconstruct the daily life of dinosaurs. They are windows on past lives, dynamic structures produced by living, breathing, moving animals now long extinct, and they are every bit as exciting and captivating as the skeletons of their makers.
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About the Author
Peter L. Falkingham is Lecturer in Vertebrate Biology in the School of Natural Sciences and Psychology at Liverpool John Moore's University, United Kingdom.
Daniel Marty is a research paleontologist at the "Paleontology A16" (Office de la culture, Canton Jura, Switzerland).
Annette Richter is Senior Custodian of Earth Sciences and head of the Natural History Department at the Lower Saxony State Museum at Hannover.
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The Next Steps
By Peter L. Falkingham, Daniel Marty, Annette Richter
Indiana University PressCopyright © 2016 Indiana University Press
All rights reserved.
Experimental and Comparative Ichnology
Jesper Milàn and Peter L. Falkingham
ONE OF THE MAIN PROBLEMS FACED IN PALEOICHNOLOGY is the delicate relationship between the organism and the sediments it leaves its tracks and traces in. Since the first scientific report of comparisons between fossil and modern tracks, researchers have turned to making experiments and comparing tracks and trackways of modern animals in order to interpret fossil tracks and traces. The easiest experimental approach is simply to make living analogues to the fossil animals walk through soft sediment and directly study the tracks they produce. Modern, more sophisticated experimental procedures include laboratory-controlled settings with sediments of different properties and model feet and indenters impressed into the sediment to various degrees. When cement or plaster is used as a tracking medium in laboratory settings, it is possible to cut vertical sections through the tracks after hardening and to study the formation and morphology of undertracks along the subjacent horizons below the foot. Complementing physical experimentation is computer simulation, in which both substrate- and indenter-specific variables can be precisely, independently, and systematically controlled. Resultant virtual tracks can be visualized completely in three dimensions, together with a time component. Experimental ichnology is an important tool for people working with tracks because the experimental settings are able to provide important data about the variations in track morphologies that can occur as a result of erosion, gait, undertrack formation, ontogeny, and individual behavior of the track maker.
A fossil vertebrate track is much more than just the mere impression of the trackmaker's foot in the substrate. In reality, a track is a complex three-dimensional structure extending into the substrate, the morphology of which is dependent on the local sedimentary conditions, the anatomy of the foot, plus any foot movements exercised by the trackmaker during the time of contact between the animal and the substrate (e.g., Padian and Olsen, 1984a; Allen, 1989, 1997; Gatesy et al., 1999; Manning, 2004; Milàn, Clemmensen, and Bonde, 2004; Milàn, 2006; Milàn and Bromley, 2006; Milàn et al., 2006; Minter, Braddy, and Davis, 2007; Falkingham, 2014). Tracks and trackways are biogenic sedimentary structures, and as such, the taphonomic processes that influence their preservation are different from those that influence bodyfossil preservation. Tracks are therefore likely to be preserved in sedimentary environments where no body fossils are preserved and, furthermore, cannot be transported from the sedimentary environment in which they are made. Tracks are thus very important sources of additional information about past biodiversity and animal behavior. Tracks made in particularly compliant substrates may record soft tissue morphology and distribution in the pedal parts and may even record the motion path of the foot (Gatesy et al., 1999; Gatesy, 2001; Avanzini, Piñuela, and García-Ramos, 2012; Cobos et al., 2016); information that in many instances is unobtainable from the study of skeletons alone.
Unfortunately, it is rarely simple to read all of this information directly from a track. Tracks emplaced in deep, soft substrates may not record the shape of the foot, or do so only poorly, but instead their morphology can be strongly determined by kinematics of the lower parts of the limb, creating an elongation of the track at the tracking surface (Gatesy et al., 1999). In some sediments, the different parts of the foot can also penetrate the sediment to different depths due to differentiated weight loads on the different parts of the foot (Falkingham, Bates, et al., 2011b); this can be particularly evident as part of the step cycle, because toe-down and kick-off phases present a smaller area and thus higher pressure to the substrate than during the weight-bearing phase (Thulborn and Wade, 1984; Thulborn, 1990). A track whose morphology is strongly influenced by foot anatomy and motion, as well as by the sediment collapsing or transmitting force, will vary considerably according to the depth at which it is exposed either by excavation or by weathering and erosion (Fig. 1.1).
One important factor to take into consideration when studying fossil tracks and trackways is erosion. When a track becomes exposed to subaerial erosion, the shape will gradually disintegrate and fine anatomical details will be lost (Henderson, 2006a). Tracks exposed to severe erosion can be hard to distinguish from undertracks (Milan and Bromley, 2006). In fact, the act of erosion may destroy the surface track and instead reveal some blurred fusion of subsurface undertracks at the surface. Furthermore, erosion can alter the total size and morphology of a track. This is especially the case with tracks originally emplaced in deep soft substrates where the trackmaker's foot has sunk to a considerable depth below the tracking surface, as the track may have a longer period of degeneration before being entirely destroyed.
Another factor able to strongly alter the appearance of a track is the phenomenon of undertracks. When a track is emplaced, the weight of the trackmaker's foot can be transmitted down and outward into the surrounding sediment. In cases where the rock is layered and breaks within the volume deformed by the transmitted force, a stacked succession of undertracks can be exposed (Fig. 1. 2). The phenomenon of undertracks was first noted by Hitchcock (1858), who depicts the same track exposed at successive, subjacent sediment surfaces. If not recognized for what they are, undertracks can be a source of confusion and misinterpretation, because they can make the track seem larger, less detailed and more rounded than the true track. Experimental work with track and undertrack formation has helped to illuminate morphologic variation of undertracks (Allen, 1997; Manning, 2004; Milan and Bromley, 2006) (Fig. 1. 3).
When all of these factors are taken into consideration–that the track encountered can be the combined result of (1) the foot morphology of the animal, (2) the foot movements exercised by the animal, (3) the consistency of the substrate at the time of track formation, and that (4) the visible track may not represent a real "surface" track (due to where and how it is exposed)–then it becomes increasingly difficult to interpret and determine the original morphology and origin of the true track and subsequently the likely trackmaker.
In cases with modern animals, individuals can be identified from their tracks by a sufficiently trained eye (Speakman, 1954; Sharma, Jhala, and Sawarkar, 2005). This can be taken to extremes with modern trackers such as neoichnologist Tom Brown (1999), who teaches the tracking skills of Native Americans in his Tracker School, and who claims to be able not only to identify individual animals but also their behaviors, sex, and intentions from subtle variations in the tracks.
When dealing with fossil footprints, however, such exquisite details are only rarely preserved–and even when they are, the trackmaker may remain unknown to science. In most cases it is, at best, only possible to assign tracks to higher taxonomic groups.
In order to obtain a better understanding of the factors affecting the morphology of a track, experimental and comparative work with track formation has been shown to be an important tool. This chapter will give an historical account of experimental ichnology from the scientific literature, explore some results and approaches of recent experimental work with living animals, and end in the modern and future age of computer-aided virtual ichnology.
HISTORY OF EXPERIMENTAL ICHNOLOGY
Throughout the history of ichnology, researchers have used extant animals with an inferred comparable anatomy and lifestyle to help understand and interpret fossil tracks and traces made by long extinct taxa. The first documented example of experimental ichnology where a scientist used the tracks of extant animals to compare with fossil tracks was that of the Reverend William Buckland, who in 1828 made crocodiles and tortoises walk through soft pie-crust, wet sand, and soft clay in order to identify the origin of fossil tracks and trackways from Permian sandstones in Scotland (Sarjeant, 1974). Buckland was also the first to describe some of the potential problems of experimental ichnology as his tortoise got stuck in the drying clay and had to be freed manually (full story transcribed in Tresise and Sarjeant, 1997). Later, Hitchcock (1836, 1858) compared the Triassic tracks and trackways of the Connecticut Valley with tracks of ratite birds and concluded that the fossil tracks had originated from large ground-dwelling birds (an interpretation that in light of modern understanding of bird evolution was not far wrong!), which led him to coin the name Ornithichnites to the tracks. A similar approach was used by Sollas (1879), who used casts of footprints from emus, rheas, and cassowaries to compare with the slender-toed tridactyl theropod footprints in the Triassic conglomerates of south Wales. Based on the close similarities, he suggested that the footprints could originate from ancestors of ratite birds (dinosaurs were only known from very sparse material at that time). He further noticed that the track morphology of the emu changed with the mode of progression, so that in tracks where the emu was accelerating, the metatarsal pad was less impressed into the sediment than when the emu was walking, and thus the morphology of the tracks from the same trackmaker changed with mode of progression.
In order to interpret the rich amphibian ichnofauna of the Permian Coconino Sandstone in northern Arizona, a substantial amount of comparative work with salamander and reptilian trackways has been conducted through time. McKee (1944, 1947) performed a series of experiments in which he made different kinds of reptiles, mostly lizards, walk up the slopes on simulated dune forests, similar to those found in the Coconino Sandstone. By varying the angle of the slope and the water content of the sand, from dry to saturated, McKee (1944, 1947) made convincing analogies to the different track morphologies found in the Coconino Sandstone. Peabody (1959) made detailed research on the trackways of living salamanders for comparison with Tertiary salamander tracks from California. Brand and Tang (1991) used subaqueous salamander trackways to argue for an underwater origin for the Coconino Sandstone, otherwise considered aeolian, but their arguments were heavily disputed (Lockley, 1992; Loope, 1992). Similar experiments with salamanders in substrates ranging from muddy to fine sand, level or sloping and with moisture contents from dry to submerged, clearly showed that the condition of the substrate is an important factor for the trackway morphology (Brand, 1979, 1996). McKeever and Haubold (1996) reclassified several Permian vertebrate trackways by demonstrating that several of the different ichnogenera erected through time were, in reality, sedimentological variations of no more than four valid ichnogenera.
MODERN FIELD AND LABORATORY EXPERIMENTS
The use of comparative and experimental ichnology, established in the early 1800s, remains to this day the ichnologist's most useful tool in the interpretation of dinosaur tracks. Modern experimental ichnology can largely be divided into those studies using extant taxa, building upon the earlier experimental work or more constrained and controlled laboratory-based indenter experiments.
Using Extant Taxa as Analogues
Following the work of Sollas (1879), ratite birds have been used especially for comparison with small bipedal dinosaurs. Padian and Olsen (1989) used the tracks and trackway pattern of a rhea (Rhea americana) to infer stance and gait of Mesozoic theropods, and Farlow (1989) examined the footprints and trackways of an ostrich (Struthio camelus) and compared them with theropod tracks and trackways. Diminutive theropod trackways from Zimbabwe were compared with a trackway from an ostrich chick to demonstrate the juvenile nature of the theropod trackmakers (Lingham-Soliar and Broderick, 2000). Gatesy et al. (1999) compared peculiar, partly collapsed theropod tracks emplaced in deep mud from Jameson Land, East Greenland, with the tracks of a turkey walking in similar deep substrate and found close similarities in the track morphologies. Gatesy et al. (1999) therefore concluded that the foot movement of theropods during walking exhibited close similarities to the foot movements of modern birds. Despite the gross overall similarities, the footprints of large ratites differ significantly from each other when examined in detail. This phenomenon was investigated by Farlow and Chapman (1997) and Farlow, McClain, and Shearer (1997), who used field observations and casts of tracks from emu, ostrich, cassowary, rhea, and the extinct moa to demonstrate how tracks from even closely related forms exhibit differences so significant as to warrant assignation to different ichnotaxa had they been found as fossil footprints.
Excerpted from Dinosaur Tracks by Peter L. Falkingham, Daniel Marty, Annette Richter. Copyright © 2016 Indiana University Press. Excerpted by permission of Indiana University Press.
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Table of Contents
Introduction / Peter L. Falkingham, Daniel Marty, and Annette RichterPart I. Approaches and Techniques for Studying Dinosaur Tracks1. Experimental and Comparative Ichnology / Jesper Milàn and Peter L. Falkingham2. Close-Range Photogrammetry for 3D Ichnology: The Basics of Photogrammetric Ichnology / Neffra Matthews, Tommy Noble, and Brent Breithaupt3. The Early Cretaceous Dinosaur Trackways in Münchehagen (Lower Saxony, Germany): 3D Photogrammetry as Basis for Geometric Morphometric Analysis of Shape Variation and Evaluation of Material Loss during Excavation / Oliver Wings, Jens N. Lallensack, and Heinrich Mallison4. Applying Objective Methods to Subjective Track Outlines / Peter L. Falkingham5. Beyond Surfaces: A Particle-Based Perspective on Track Formation / Stephen M. Gatesy and Richard G. Ellis6. A Numerical Scale for Quantifying the Quality of Preservation of Vertebrate Tracks / Matteo Belvedere and James O. Farlow7. Evaluating the Dinosaur Track Record: An Integrative Approach to Understanding the Regional and Global Distribution, Scientific Importance, Preservation and Management of Tracksites / Luis Alcalá, Martin G. Lockley, Alberto Cobos, Luis Mampel, and Rafael Royo-TorresPart II. Palaeobiology and Evolution from Tracks8. Iberian Sauropod Tracks through Time: Variations in Sauropod Manus and Pes Morphologies / Diego Castanera, Vanda F. Santos, Laura Piñuela, Carlos Pascual, Bernat Vila, José I. Canudo, and José Joaquin Moratalla9. The Flexion of Sauropod Pedal Unguals and Testing the Substrate Grip Hypothesis Using the Trackway Fossil Record / Lee E. Hall, Ashley E. Fragomeni, and Denver W. Fowler10. Dinosaur Swim Track Assemblages: Characteristics, Contexts, and Ichnofacies Implications / Andrew R. C. Milner, and Martin G. Lockley11. Two-Toed Tracks through Time: On the Trail of "Raptors" and their Allies / Martin G. Lockley, Jerry D. Harris, Rihui Li, Lida Xing, and Torsten van der Lubbe12. Diversity, Ontogeny, or Both? A Morphometric Approach to Iguanodontian Ornithopod (Dinosauria: Ornithischia) Track Assemblages from the Berriasian (Lower Cretaceous) of North Western Germany / Jahn J. Hornung, Annina Böhme, Nils Schlüter, and Mike Reich13. Uncertainty and Ambiguity in the Interpretation of Sauropod Trackways / Kent A. Stevens, Scott Ernst, and Daniel Marty14. Dinosaur Tracks as "Four-Dimensional Phenomena" Reveal How Different Species Moved / Alberto Cobos, Francisco Gascó, Rafael Royo-Torres, Martin G. Lockley, and Luis AlcaláPart III. Ichnotaxonomy and Trackmaker Identification15. Analysing and Resolving Cretaceous Avian Ichnotaxonomy Using Multivariate Statistical Analyses: Approaches and Results / Lisa G. Buckley, Richard T. McCrea, and Martin G. Lockley16. Elusive Ornithischian Tracks in the Famous Berriasian (Lower Cretaceous) "Chicken Yard" Tracksite of Northern Germany: Quantitative Differentiation between Small Tridactyl Trackmakers / Tom HübnerPart IV. Depositional Environments and their Influence on the Track Record17. Too Many Tracks: Preliminary Description and Interpretation of the Diverse and Heavily Dinoturbated Early Cretaceous "Chicken Yard" Ichnoassemblage (Obernkirchen Tracksite, Northern Germany) / Annette Richter and Annina Böhme18. Dinosaur Tracks in Eolian Strata: New Insights into Track Formation, Walking Kinetics, and Trackmaker Behaviour / David B. Loope, and Jesper Milàn19. Analysis of Desiccation Crack Patterns for Quantitative Interpretation of Fossil Tracks / Tom Schanz, Maria Datcheva, Hanna Haase, and Daniel Marty20. A Review of the Dinosaur Track Record from Jurassic and Cretaceous Shallow Marine Carbonate Depositional Environments / Simone D'Orazi Porchetti, Massimo Bernardi, Andrea Cinquegranelli, Vanda Faria dos Santos, Daniel Marty, Fabio Massimo Petti, Paulo Sá Caetano, and Alexander WagensommerDinosaur Track Terminology: A Glossary of TermsList of ContributorsIndex