Cambrian Shadows

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Protichnites

Evidence of the earliest land predators?

The fossil record provides scant evidence of the temporal scenario of the transition of metazoans to life on land. The oldest fossil of an animal that putatively lived on land is a euthycarcinoid from the late Ordovician of Australia. The oldest terrestrial faunal assemblages are arachnids and myriapods from the Silurian of UK. Except for some late ichnofossils, the fossil record provides no evidence to set aside the hypothesis that prokaryotes ruled the land to the end of the Cambrian and many millions of years beyond. Of particular importance are ichnofossils of the genus Protichnites, which occur in several North American locations dating from the late Cambrian into the Ordovician.

The Protichnites ichnogenera generally refers to a trackway two rows of imprints (footprints) inside which is a medial impression (such as a telson drag mark). They occur throughout the Paleozoic, throughout the world. Notable Protichnites sites occur in Ontario, Quebec, New York, and Missouri, and all in subtidal, intertidal and/or eolian (i.e., as in dunes above the tide line) sandstones. Among these sites, the Protichnites from the Krukowski Quarry are the oldest at some 510 million years ago. Arthropods underwent enormous change and numerous radiations throughout the periods of the Cambrian. It is likely that the makers of the Protichnites (and Diplichnites) ichnogenus also had many arthropod makers that changed across geologic time. But what animals made the upper Cambrian tracks found in the Krukowski quarry. The main suspects are eurypterids, horseshoe crabs, Euthycarcinoids and the Aglaspids, or some closely related arthropod or stem group among these taxons.

It was a time when evolutionary adaptation was producing predators, and a time when adaptation was producing defense from predation, both powerful selective pressures in part fueling what is known as the Cambrian explosion. It is reasonable then to conjecture that animals found ways to first emerge on marine shorelines for different reasons -- some to graze in safety on abundant bacterial colonies, and possibly others to hunt the grazers.

The animal that made Protichnites tracks could have been one of the Earth's first air-breathing animals. The Protichnites ichnogenus is a Repichnia Ichnofossil associated with vagile locomotion. Sir William Logan described the Protichnites genus in 1863, the time of Darwin, as fossil evidence that animals had evolved more rapidly and much earlier than thought at the time of its discovery. Notable in Protichnites is the prominent markings of a tail or other body part being dragged; footprints, when present, are subtle by comparison. The maker of these fossil trackways might have been an animal resembling the extant but ancient horseshoe crab, except of a stem group that lacked a hard shell to be preserved. Others have posited that Protichnites was a soft-bodied progenitor of the large, now extinct group of early predators, the Eurypterids (Chelicerate "meaning biting claws" Arthropods). The "Eurypterid theory" is particularly intriguing since it would imply that just as animals were first moving to land to feed, hungry new predators were already on the hunt.

Some of the mystery of Protichnites trackways is just being cleared up as, after more than a decade, the very first body fossils of a putative Protichnites track maker have at last been discovered in the Krukowski quarry. The creatures that were discovered in a single dessication zone during the summer of 2003 have affinity to some of the most problematic groups of Arthropods, the Euthycarcinoids and the Aglaspids. The euthycarcinoids that are known from 13 species from Upper Cambrian or Lower Silurian-Middle Triassic from Argentina, Western Australia, Europe (e.g., the Rhynie chert) and the Mazon Creek in Illinois (Vacarri, 2004). But it is only in the Argentina and Wisconsin sites that the Protichnites trackways are also found. The Krukowski arthropods may emerge as the oldest land-based animal fossil in the entire fossil record. Similarly, the aglaspids are arthropods whose taxonomy among arthropods is uncertain.

Also see: Ichnofossil Nomenclature

References