Der Fossilienbestand ist das wichtigste Archiv der Geschichte des Lebens. Er dokumentiert Prozesse der Evolution, Anpassung und Aussterben und ermöglicht uns zu verstehen, wie Revolutionen in der biotischen Komplexität zur Entstehung der Struktur und Funktion der globalen Biosphäre beigetragen haben.
The fossil record is our primary archive of the history of life, recording processes of evolution, adaptation, extinction, and allowing us to understand how revolutions in biotic complexity have helped to form the structure and function of the global biosphere.
Animals first evolved during the Ediacaran time period, around 600 million years ago. We can gain insights into their early behaviour by studying the feeding traces they left behind.
The Ediacaran and the dawn of animal life
The Ediacaran-Cambrian transition (‘ECT’) ~580-510 million years ago marks several fundamental step-changes in evolutionary history – not only is it where we first see animals appear in the fossil record, but we also see the origin of animal biomineralization (i.e., the first mineralized skeletons), the first evidence for movement, the appearance of sophisticated new animal feeding modes (including predation), and the first time that marine organisms began to burrow deeper into the sediment – a process frequently referred to as the ‘agronomic revolution’. Together, these changes form part of the evolutionary radiation of early animals, known as the Cambrian explosion, the roots of which are now known to lie in the late Ediacaran Period.
Even within this broad context, however, the agronomic revolution has outsized importance – the mixing of sediment by animals (‘bioturbation’) in the present day is an important ecosystem engineering process that creates new habits or niches for other animals and also influences several crucial biogeochemical cycles in the oceans, thus helping to support and maintain seafloor ecosystems. Although this interval has been heavily studied in terms of the origin of new animal groups, comparatively little attention has been paid to the evolution of behavioural complexity. Not only can behavioural complexity sometimes reflect neurology – which has a poor preservation potential – but it can also capture ecology (e.g., movement vs. predation), and how animals interact both with their environment, and each other. The evolutionary history of animal behaviour is thus an invaluable window into the evolution of the biosphere, and one which can yield insights not recorded in body fossils. Fortunately, many facets of animal behaviour can be recorded as trace fossils, which are readily preserved in a large range of ancient sedimentary environments.
In this project focussed on understanding the origins of animal movement, I am working with Dr. Simon Darroch (Curator of fossil cnidarians, Senckenberg Research Institute and Natural History Museum) on some key aspects of the agronomic revolution – specifically, trace fossils referred to as ‘treptichnids’. The term ‘treptichnids’ refers to a group of burrowing behaviours that appear over the ECT, and which are important for 3 reasons: 1) one specific treptichnid trace fossil – Treptichnus pedum – serves as a biostratigraphic marker for the base of the Cambrian Period worldwide; 2) treptichnids mark the first ‘complex’ behaviours known from the fossil record, and thus the very start of the agronomic revolution; and, 3) treptichnids show an increasing variety of burrow morphologies in younger and younger rocks, and thus offer a terrific opportunity to study the evolution of behavioural complexity

During my research stay with Dr Darroch, we visited fossil collections housed at Senckenberg and Tubingen University, taking the first steps in a project that will develop novel quantitative metrics to quantify behavioural complexity. The insights gained during this exchange have stimulated new collaborative projects, with future graduate research planned between our groups.
Documenting early animal movement
We first focused on the collection of Dr. Darroch, who has been collecting treptichnid trace fossil specimens from Ediacaran-aged sediments in Namibia and the southwestern United States over the last ten years, and which are currently housed at the Senckenberg museum. For each specimen we took dozens of photographs from multiple angles so that we could then make 3D models of the fossil specimens, enabling us to see the subtle details of animals probing for food in the sediment. The pattern of the different probe marks are recorded, and so the different approaches to foraging for food are captured by this fossil. These 3D models can then be used to quantify the behavior captured in the resulting trace fossil dataset in two ways. First, the hundreds of specimens in the collections enable us to take a variety of different measurements that describe the morphology and movement of tracemaking organisms, including burrow diameter, depth, burrow sinuosity, probe angles (i.e., measured both laterally, and vertically), and probe density.
Prof. Dolf Seilacher’s collections at Tubingen University
After a fantastic first week in the collections of the Senckenberg Naturmuseum, we headed to Tubingen University to study material amassed by Prof. Dolf Seilacher, considered by many to be one of the founders of ‘palaeoichnology’ – the study of trace fossils. This is an exceptionally large collection (>500 specimens) of Cambrian-aged treptichnids from South Africa, Spain, Pakistan, and Arizona, curated by Dr. Ingmar Werneberg. The Seilacher collection has enabled us to characterize the different treptichnid species – including Treptichnus pedum – and so it is an ideal dataset to begin quantifying how the behavioural complexity of treptichnids evolved over the ECT. A key question in this regard is: did complexity evolve gradually over this interval, or via a series of step-wise ‘pulses’? The answer to this question has significance not only for how we identify T. pedum (and thus the base of the Cambrian) in sequences of rocks but also sheds light on the timing and character of the Cambrian explosion.

References
Cribb, A. T., C. G. Kenchington, B. Koester, B. M. Gibson, T. H. Boag, R. A. Racicot, H. Mocke, M. Laflamme, and S. A. F. Darroch. 2019: Increase in metazoan ecosystem engineering prior to the Ediacaran–Cambrian boundary in the Nama Group, Namibia. Royal Society Open Science 6:190548.
Darroch, S. A. F., A. T. Cribb, L. A. Buatois, G. J. B. Germs, C. G. Kenchington, E. F. Smith, H. Mocke, G. R. O’Neil, J. D. Schiffbauer, K. M. Maloney, R. A. Racicot, K. A. Turk, B. M. Gibson, J. Almond, B. Koester, T. H. Boag, S. M. Tweedt, and M. Laflamme. 2021: The trace fossil record of the Nama Group, Namibia: Exploring the terminal Ediacaran roots of the Cambrian explosion. Earth-Science Reviews 212:103435.
Mitchell, E. G., S. D. Evans, Z. Chen, and S. Xiao. 2022: A new approach for investigating spatial relationships of ichnofossils: a case study of Ediacaran–Cambrian animal traces. Paleobiology 48:557–575.
Mitchell, E.G. and Pates, S., 2025. From organisms to biodiversity: the ecology of the Ediacaran/Cambrian transition. Paleobiology, 51(1), pp.150-173.
Turk, K. A., M. A. Pulsipher, E. Bergh, M. Laflamme, and S. A. F. Darroch. 2024: Archaeichnium haughtoni: a robust burrow lining from the Ediacaran–Cambrian transition of Namibia. Papers in Palaeontology 10:e1546.
Emily Mitchell is an Assistant Professor in the Department of Zoology, University of Cambridge and the Curator of non-insect Invertebrates in the University Museum of Zoology, Cambridge. Her research investigates the role of ecological processes on evolution through deep-time, from the first animal communities of the Ediacaran 600 million years ago, to the present. Deep-time ecology looks at how ecosystems function and have evolved over the history of life on Earth.
@egmitchell.bsky.social
All pictures: Emily Mitchell.



























