Imagine yourself walking down the high tide zone of a vast, sandy beach that is not littered with scraps of plastic or any other signs of human presence. Ripple marks in the sand constantly remind you of your closeness to the sea. You notice the quiet in the air. As you continue, a series of weird trackways catch your eye. After a few more paces, they are all around you. You try to guess the identity of their makers. They’re certainly not birds or crabs. The trackways are like nothing you’ve seen on other beaches. The most conspicuous ones resemble motorcycle tracks, but with very unusual “treadmarks.” Some of these trackways have an oval depression at one end about the size of a human foot. The animal might have rested there. Rows of paired footprints, some with a furrow from something that obviously dragged between the rows, also decorate the beach. In other areas trackways have odd, shingled imprints down the middle.
These Diplichnites trackways are probably euthycarcinoid footprints that reached a lower layer of sand than the layer on which the animals walked. The tails did not reach this layer to make imprints.
This Protichnites trackway was probably made by a euthycarcinoid. Note the tail drag furrow between the footprints. This trackway is about 4 cm wide.
Your curiosity takes over as you continue your hike with eyes focused on the ground. You try to make sense of the eerie scene. It strikes you that not one shell has come into view. You then arrive at an area having hundreds of circular mounds of sand. Most of them have the same general structure. You recognize them from your walks along the beaches of coastal California. They are infillings made from stranded jellyfish as they pulsated, drawing sand into their bodies while trying in vain to escape their subaerial doom. But many of these are larger than the ones you normally see. A few are nearly the size of manhole covers.
Several jellyfish (scyphozoan) infillings and Climactichnites trackways are shown on this ripple-marked surface. The scale is about 46 cm long.
Continuing on your journey, you notice something odd about the sand. Closer examination reveals that it is textured and the particles appear bound together. In certain places it looks blistered or wrinkled. As you approach a shallow channel, you come to an area of sand that has a greenish tint. You stoop to investigate. The sediment is muddier than in other places, and mud-cracked. You come in for a closer look. Within one of the cracks are the remains of some kind of animal. It might have died there while waiting for the next high tide to keep it from dehydrating. This hand-sized creature looks as if it could be the tailed animal that made the trackways that had the drag marks down the middle. A segmented body with jointed legs and a long segmented tail screams “arthropod,” but unlike any you’ve ever seen running around other beaches – or anywhere else. The animal appears to have a mandible, which means it can’t be a scorpion, spider, horseshoe crab, or related arthropod. You don’t see any pincers, so it is not a crab or any of its relatives. It seems to come closest to the two other major groups of arthropods: the myriapods (centipedes, millipedes and related forms) and insects, but not exactly.
Distracted by the mysteries of this place, you have failed to notice that the sand beneath you is as solid as rock. You look up. You had almost forgotten that all this time you’ve been walking in a sandstone quarry at a place called Blackberry Hill in central Wisconsin!
The sand had not been soft here since the Cambrian Period, around 500 million years ago. This was a pivotal time in Earth’s history. It is believed to be the time when the first multi-cellular animals crawled or walked out of the sea and began experiencing a life on land and all it had to offer – and this was perhaps one of the best places on Earth to learn what complex life was like when it began to inhabit this brand new environment.
What made these tracks and body fossils so exquisitely preserved appears to have been the same thing that lured some of the animals out of the sea: the lowly microbes. The bound sand that you saw as you walked the “frozen beach” is attributed to microbes, such as cyanobacteria and blue-green algae, coating the sand particles and meshing them together. This process still takes place today. These microbial films (also known as biofilms, bioglue, etc.) sometimes continue to develop into microbial mats, which are thicker and can be extremely cohesive. These microbial materials are believed to have served as a preservational agent allowing the delicate fossils to survive the next tide or tsunami that buried them in a layer of sand. These materials, which were present for millions of years before the Cambrian, both within the seas and on land, also seemed to have served as food sources for some of these pioneering landgoers.
One such possible slime-eater was the animal responsible for making the “motorcycle tracks” you observed. Studies of these trackways, including comparisons with trackways from living forms, indicate giant slug-like mollusks to be their makers. Their meandering paths among the microbial materials seem to support this idea. Fossil burrows suggest that these animals also spent time beneath the surface of the sand. It is unlikely that fossils of their bodies will turn up, as the animal apparently lacked hard parts.
You were right when you wondered if the dead animal you found might have made some of the footprints. The animal was from a group of arthropods known as the Euthycarcinoidea. This extinct group might have been ancestral to all of the other mandibulates that followed, such as the insects, crustaceans and centipedes. The species found here was not only one of the oldest euthycarcinoids on record, but the first verified maker of footprints on land. This or a similar species may also have been responsible for the shingled markings referred to above. As recently revealed, a form of these traces may have been produced as the animal mated – evidence of another motive for coming ashore. It appears that they mated by amplexus, with the male holding on to the female and fertilizing her eggs as they were laid. This might be the oldest mating behavior preserved in the fossil record, and foreshadowed the behavior of the horseshoe crabs that still emerge from the sea in the thousands to mate every spring along the New England coast. However, because of major anatomical differences between these two arthropods, this striking similarity in lifestyles more likely would be a convergence in behavior rather than a close genetic kinship.
Mosineia was a euthycarcinoid that apparently came ashore to mate and lay its eggs where they were relatively safe from predators. This is one of only three specimens that have been positively identified. The rock section shown on this image is about 13 cm high.
If you would have looked more closely in the mudcracks or walked closer to shore, you may have run into another type of arthropod whose body and trace fossils have been found in those environments. These small arthropods were primitive crustaceans known as phyllocarids. With some species surviving to the present, this is possibly the oldest occurrence of phyllocarids (and phyllocarid activity) found thus far, and therefore represents one of the first crustaceans. Their traces appear to consist mostly of narrow locomotion and feeding furrows in the muddier sediment at the shoreline. Some dug beneath the sediment surface. Resting traces have also been found. The majority of the traces tend to be rather simple in structure, with a degree of bilobation being a common feature; however, tiny, paired footprints in the fine sediments are seen on rare occasions. These crustaceans could congregate in tremendous masses. Areas large enough to park a bus in have been found covered with thousands of furrows from what appear to have been small members of this crustacean group.
These three “piggybacking” arthropods are the phyllocarid Arenosicaris, which may be one of the first crustaceans. The rock section shown on this image is about 8 cm high.
These crowded furrows in fine sediment appear to have been produced by phyllocarids. Because of their small size, they appear to be either from juveniles or from a second phyllocarid species.
This was not the only life that began inhabiting the land at this time. Remains or traces of several other forms of arthropods, including an unnamed euthycarcinoid, have also been found at Blackberry Hill. Undoubtedly other species will turn up, as some of the quarries are still quite active. In addition, strata of similar age and environment in New York, Quebec, Ontario and Missouri contain some of the same general kinds of trace fossils and beached jellyfish. A locality in Quebec also has at least one additional species of euthycarcinoid. At another locality in Quebec, presumed euthycarcinoid and slug-like mollusk trackways are about twice the size of their counterparts in other localities. In localities in New York and Quebec, trackways are found in dune sand strata that are apparently located even further from the shoreline than the fossils at Blackberry Hill.
Life on land had thus been well established and diverse by this time in the Cambrian Period. The vast, sandy tidal flats of what is now North America were apparently well suited for starting the terrestrialization process. In spite of what our history books might have taught us, it was the euthycarcinoids that first stepped foot on the “New World” – while giant slug-like mollusks slimed ashore and primitive crustaceans fed along the land/water’s edge.
Chris Gass is a paleontologist and researcher from Wisconsin. Find a number of studies authored by him at academia.edu.
I am indebted to my sons, Todd and Joshua, who discovered key material and provided some of the photographs used in this article. My thanks go to Whitey Hagadorn and Joseph Collette, who provided useful comments, and to those gentlemen and Adolf Seilacher for teaching this trilobitologist so much about trace fossils and paleodetective work.
References and Further Reading
Collette, J. H., K. C. Gass, and J. W. Hagadorn. 2012. Protichnites eremita unshelled? Experimental model-based neoichnology and new evidence for a euthycarcinoid affinity for this ichnospecies. Journal of Paleontology 86 (3): 442–454.
Collette, J. H. and J. W. Hagadorn. 2010. Three-dimensionally preserved arthropods from Cambrian Lagerstatten of Quebec and Wisconsin. Journal of Paleontology 84 (4): 646–667.
Collette, J. H., J. W. Hagadorn, and M. A. LaCelle. 2010. Dead in their tracks: Cambrian arthropods and their traces from intertidal sandstones of Quebec and Wisconsin. PALAIOS 25 (8): 475–486.
Getty, P. R., and J. W. Hagadorn. 2008. Reinterpretation of Climactichnites Logan 1860 to include subsurface burrows, and erection of Musculopodus for resting traces of the trailmaker. Journal of Paleontology 82 (6): 1161–1172.
Getty, P. R., and J. W. Hagadorn. 2009. Palaeobiology of the Climactichnites tracemaker. Palaeontology 52 (4): 753–778.
Goldring, R., and A. Seilacher. 1971. Limulid undertracks and their sedimentological implications. Neues Jarbuch fur Geologie und Palaontologie Abhandlungen 137: 422–442.
Hagadorn, J. W. and E. S. Belt. 2008. Stranded in upstate New York: Cambrian scyphomedusae from the Potsdam Sandstone. PALAIOS 23 (7): 424–441.
Hagadorn, J. W., J. H. Collette, and E. S. Belt. 2011. Eolian-aquatic deposits and faunas of the middle Cambrian Potsdam Group. PALAIOS 26 (5): 314-334.
Hagadorn, J. W., R. H. Dott, and D. Damrow. 2002. Stranded on an Upper Cambrian Shoreline: Medusae from Central Wisconsin. Geology 30 (2): 147–150.
Hagadorn, J. W, M. LaCelle, and P. Groulx. 2012. Mirabel’s ancient surfers: Insights from Cambrian trace fossils and sedimentology of the Potsdam Group, Quebéc. Abstract Volume, Canadian Paleontology Conference. University of Toronto. p. 37.
Hagadorn, J. W., and A. Seilacher. 2009. Hermit arthropods 500 million years ago? Geology 37 (4): 295–298.
Hoxie, C. T. 2005. Late Cambrian arthropod trackways in subaerially exposed environments: Incentives to simplify a problematic ichnogenus. Unpublished B.A. Thesis: 1–89.
MacNaughton, R. B., J. M. Cole, R. W. Dalrymple, S. J. Braddy, D. E. G. Briggs, and T. D. Lukie. 2002. First steps on land:Arthropod trackways in Cambrian-Ordovician eolian sandstones, Canada. Geology 30 (5): 391–394.
McDowell, C. and J. W. Hagadorn. 2009. Quantifying the effects of microbial mats on the erosion, transportation, and deposition of sand grains in a unidirectional flow. Geological Society of America Northeastern Section – 44th Annual Meeting 41 (3): 16.
Ortega-Hernandez, J., J. Tremewan, and S. J. Braddy. 2010. Euthycarcinoids. Geology Today 26 (5): 195–198.
Owen, R. 1852. Description of the impressions and footprints of the Protichnites from the Potsdam sandstone of Canada. Geological Society of London Quarterly Journal 8: 214–225.
Seilacher, A. 2007. Trace Fossil Analysis. New York, New York: Springer. p. 226.
Seilacher, A. 2008. Biomats, biofilms, and bioglue as preservational agents for arthropod trackways. Palaeogeography, Palaeoclimatology, Palaeoecology 270 (3–4): 252–257.
Seilacher, A. and J. W. Hagadorn. 2010. Early molluscan evolution: Evidence from the trace fossil record. PALAIOS 25 (9): 565–575.
Yochelson, E. L. and M. A. Fedonkin. 1993. Paleobiology of Climactichnites, an enigmatic Late Cambrian fossil. Smithsonian Contributions to Paleobiology 74, 74 p.
Young, G. A. and J. W. Hagadorn. 2010. The fossil record of cnidarian medusae. Palaeoworld 19 (3–4): 212–221.