Treptichnus pedum is a trace fossil produced by a soft, wormlike creature, a creature that wasn’t necessarily an “animal” in any conventional sense of the word. The traces that the worm left behind have been picked out to represent one of the most important turning points in the geological time scale, between Precambrian time and the Phanerozoic eon. The Precambrian is the entire interval from the formation of the earth to the emergence of macro-scale life (it divides into three geological eons, Hadean, Archean, and Proterozoic). The Phanerozoic eon is the rest: the eon of “visible life,” the last 541 million years. T. pedum, the bearer of the distinction between the two, can be a way to think about the meaning of geological time divisions as such. Geological intervals—the Anthropocene among them—should be understood as figures of difference, not as a series of essences.
In modern geology, time units (and the time-rock units that correspond to them) are defined wherever possible on the basis of boundary stratotypes, or GSSPs. You pick a specific layer of rock somewhere in the world to represent the beginning of the eon, period, era, epoch, or age that you’re interested in, and, if convenient, you hammer a little inscribed golden spike into the relevant spot. The unit is said to have begun whenever that stratum was laid down, and it ends simply when the next one above it starts: it’s beginnings, not endings, to which stratigraphers devote their attention. The GSSP or golden spike for the Phanerozoic is found on the sea-edge of a cape in Newfoundland. Specifically (as the standard textbook, The Geologic Time Scale 2012, eds. Felix Gradstein et al., explains), it’s “located in the lower Mystery Lake Member of the Chapel Island Formation in the Fortune Head Section of the Burin Peninsula.” The earliest epoch/series of the eon is the Terreneuvian—the name is simply from the French name, Terre-Neuve, for Newfoundland, but it’s obviously a richly apposite one.
Within the Mystery Lake Member of the Chapel Island etc., the beginning of the Phanerozoic is marked by the earliest known appearance of Treptichnus pedum trace fossils (ichnofossils) at the time of designation, in 1992. (T. pedum has since been identified lower down in the strata as well.) Those fossils are metonymic for “the appearance of complex sediment-disturbing behaviour by multiple epifaunal [seafloor-dwelling] and infaunal [subsurface-sediment dwelling] animals” (Gradstein et al. again). And on a larger view, those sediment-disturbing habits are themselves representative of a very large evolutionary event: a decisive turn towards complexity in animal behaviour at the beginning of the Phanerozoic, a turn that would shortly include, for instance, the first known predator–prey relationships. That event was Cambrian Explosion, the period of rapid emergence of diversified, multi-niched ecosystems that constitutes probably the great evolutionary leap forward in the history of the earth.
It puts a lot of weight on the soft body of the T. pedum worm to make it represent the beginning of Phanerozoic life as a whole. Nothing survives of the creature itself, which had no mineralized parts; even its name really just refers to its burrows.
Here’s T. pedum, courtesy of the Royal Ontario Museum (the white line is one centimetre):
And here’s a diagram of the burrows, courtesy of the University of Kansas:
The burrow-maker would push through the uppermost region of the matted sediment below shallow sea waters, then probe upwards to gather nutrients at the sediment/water boundary. That turn is the crucial phenomenon. Horizontal locomotion, then an upward turn, a little striving, a swerve—a sort of clinamen. A strike and retreat, a manoeuvre in the shape of a fork or a zigzag. Under the right conditions, the creature’s twisting path could be re-filled by sediments in a way that preserved indefinitely a trace of the disturbance. Those plaited traces are the T. pedum ichnofossils. They’re the record of an articulation, a branching: the creation of a line, like those shown above, that’s both single and varied, one that accommodates both continuity and stoppage, unpredictably angled, a map of voluntary re-routings. And that arborescent line seems to have been—though you can’t be absolute about things like this—something new. Simpler linear scratch-burrows are known from earlier periods, and they do reveal a slowly increasingly complexity through time. But the tunnels and galleries carved by T. pedum are precociously three-dimensional and purposive.
Hence the creature’s name: Treptichnus pedum. The first and most crucial part of the name indicates a trope, a turn, a variation; the second part, a tract in the sense of that which is drawn out or pulled, or which trails or makes a path; the third part, a pedestrian, a moving foot. Thus Treptichnus, the turning path, the worm that turned.
Treptichnus pedum has been the focus of one paper about the Anthropocene already, an essay by Mark Williams and others in the landmark Stratigraphical Basis for the Anthropocene volume. Their neat argument was that just as the increasing complexity of sea-sediment burrowing by T. pedum can mark the beginning of the Phanerozoic, so too the increasing complexity of urban burrowing by Homo sapiens could mark the beginning of the Phanerozoic’s most recent fraction. So the Anthropocene could be dated to 1861, and the digging of what’s now London Underground’s Metropolitan Line.
There’s another way in which T. pedum can be helpful for thinking about the Anthropocene. It has to do with how we should understand the relationship between geological time intervals themselves on the one hand, and on the other hand the names and the golden spikes that represent them. One of the great obstacles to a clear understanding of the stratigraphic Anthropocene has been commentators’ widespread assumption that that which represents the Anthropocene epoch must be its essence. That is, if it’s called “the Anthropocene” then it must be the Epoch of Humans. The planet in the Anthropocene must be made by humans, belong to humans, and be given all of its identity by humans. The appearance of humans must coincide with the beginning of the Anthropocene; the disappearance of humans (at least in their current form) must coincide with the Anthropocene’s end. Most of that doesn’t make much sense, but it’s a surprisingly tenacious viewpoint.
The reality is that the relationship between slices of earth’s history and the phenomena by which geologists signify them is one of metonymy. The stones of the Jura Mountains are only one example of Jurassic-period geology. Even though they provide the name for the entire period, they don’t hold inside themselves the essence of the Jurassic. Instead, they’re used as a synecdoche for Jurassic strata as a whole. Likewise, the Jurassic begins with the first known occurrence of the ammonite species Psiloceras spelae tirolicum. That’s not because P. spelae tirolicum is the most Jurassic-y of organisms (that would be a dinosaur, I suppose). It’s just that the ammonite arrives in the rock record at the right moment to emblematise the process of transition from the Triassic to the Jurassic—a transition mainly characterised by a mass extinction event.
The same principle applies both to the name of, and to the proposed golden spikes for, the Anthropocene epoch. Taken in its stratigraphic sense, the name doesn’t imply that the epoch begins with the first geological traces of H. sapiens, or that it would necessarily end with their disappearance, or that H. sapiens is the only geologically significant species to exist during the epoch. Dating the Anthropocene to 1952 won’t imply that the earth system of the Holocene was replaced by a new one within the course of those twelve months and no others, or that the detonation of Ivy Mike was the thing that made the new epoch happen. Instead, the global fallout spike of plutonium-239 that began in 1952 is proposed as a synecdoche for the birth of the Anthropocene as a whole.
So in order to understand geological nomenclature, and the periodization of earth history, and even earth history itself, we need to think about metonymy. Metonymy: a figure of speech. A trope.
That’s why it’s so apt that the most fundamental of geological turning points, between the Precambrian and the Cambrian, is emblematised by Treptichnus. Treptichnus pedum is a trope, a synecdoche, for Phanerozoic life. Geologists’ trope for the 541 million years of complex life is this most tropological of organisms, a soft-bodied animal that has long since vanished into the turnings and stitchings of its own burrows. Treptichnus, as the stratigraphers’ trope for the Phanerozoic, can also serve as a trope for the tropology that is implicit within the science of stratigraphy itself. This creature, one that lived by turning up and down through the boundary between ancient water and ancient sediment, has been turned into a marker of how one interval of earth time can turn into another.
Earth-historical intervals succeed to one another on a principle of sheer difference. There is no normal or default state of the earth system: there’s no ordinary baseline condition from which the various eras and periods deviate in particular ways. Instead, the process of change is historical through and through. New configurations of the lithosphere and biosphere emerge as time goes on, with no possibility of an exact return to earlier conditions. Treptichnus pedum, as the metonym for the Phanerozoic, can also represent this historical process of endless turning into something new. Keeping in mind the pattern of serration and sinuosity that T. pedum inscribed into rock is a way of thinking about the very nature of geological time intervals like the Anthropocene.