Protichnites are trace fossils that display two parallel lines of tracks with a linear depression at the center. The parallel lines are essentially rows of footprints aligned towards the animal’s direction of travel. Carefully examined, these lines can be used to translate and interpret gait. In the case of the currently examined Elk Mound fossils, “the deeper impressions made by the rear pair of walking legs (i.e., the “pushers”) repeat symmetrically, in the same rhythm as the shell marks. This suggests synchronous movement of leg pairs, similar to modern Limulus and eurypterids, rather than the alternating gait reflected in tracks of crustaceans, scorpions, and insects.”
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The linear depression at the center of the Protichnites fossils is thought to be remnant drag marks from a tail. When turning, the arthropod’s tail swings outward from the direction of the turn, like a pendulum; these “wide turns” can provide biomechanical clues describing the gait of the animal.
One set of fossils studied by the authors, later named Protichnites eremite (eremite = Hermit), displayed a medial depression with irregular characteristics. “Instead of following the midline, its markings consist of oblique impressions that are always offset and shingled to the left side. It is unlikely that this represents an individual or population of individuals characterized by a malformed tail, because similar trackways of different widths occur on the same bedding plane and because such trackways occur on more than one horizon. Because there are no pushback hills on the rear sides of the oblique ‘tail’ impressions, it is also unlikely that this asymmetry reflects a behavioral strategy, in which the tail was bent sideways in order to assist in locomotion.”
If the irregular tail marks don’t represent a morphological malformation or provide evidence for locomotion, then what do they indicate, what’s the diagnosis? According to Hagadorn and Seilacher, “the impressions resemble the touch marks of a high-spired, dextrally coiled shell” similar to that carried by modern day hermit crabs.
The conclusion reached by the researchers is that the arthropods, while in transition to a terrestrial existence, probably “still left the water only for short durations, crawling around on the wet sand flats during low tides.” These intertidal sand flats proved ideal for promoting the growth of thin microbial films on which the light-footed arthropods left tracks and trails that later fossilized.
Using modern hermit crabs as an analog, the authors surmised that, “with their cuticular exoskeletons and stiff appendages, arthropods were particularly well preconditioned for terrestrialization. Nevertheless their early pioneers still required special adaptations, such as large body sizes and the use of foreign shells, to minimize water loss.” Transporting a shell on their back buffered the arthropods from arid conditions, but at the same time altered their gait to such an extent that we can read it in the fossils today.
Hagadorn, J., & Seilacher, A. (2009). Hermit arthropods 500 million years ago? Geology, 37 (4), 295-298 DOI: 10.1130/G25181A.1