Cambrian Shadows

 

 

 

 

Darwin both pondered and worried about the paucity of fossils found that dated before the Cambrian. More than a century later we now know that arthropods lacking external exoskeletons walked in the shallows, on the beaches, and ultimately made their appearance on land. Darwin would today be gratified that science has determined that these transitional creatures had left there traces, if only footprints in the sand, substantiating there overarching of the Precambrian/Cambrian boundary of time. This fossil is such evidence, coming from the history-making Krukowski quarry in central Wisconsin that has only recently come under intense study by Paleontologists. New publications are forthcoming, adding to several in the last year (e.g., see Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin, by Hagadorn, et. al., describing a diverse fauna that apparently walked or floated upon an ancient Cambrian shoreline.

Diplichnites is an ichnogenus normally attributed to arthropoda locomotion. The fossil trackway exhibits two parallel lines of feet impressions. The animal was ostensibly striding on the surface and leaving no other impressions such as from the drag of a tail or other body parts. The spacing of the foot prints along the track would be wider at higher speeds.

The Diplichnites shown to the right is unique owing to the obvious large size of the animal, since the sides of the tracks are some 2 inches apart. Note that this large specimen is the convex half of the fossil (called hyporelief by Ichnologists). This particular plate measures 775 by 140 (30 1/2 by 5 1/2 inches), and a yardstick is shown to give perspective.

Popular conjecture is that Diplichnites is the trace of a wandering Myriapod, which brings to mind an ancient centipede (Chilopoda). Given the spacing of these footprints, the centipede that made the tracks of the fossil to the right would have been an awesome predator - perhaps the terror of the Cambrian beach some 1/2 billion years ago. Diplichites from other locations have been attributed to trilobites. Many ichnofossils, protichnites, from the Mount Elk Group show drag traces putatively made by arthropod tails or other body parts, suggesting taxonomic identity among crustaceans, aglaspids, chelicerates or euryipterid stem group.