Tracing the early evolution of animals on Digermulen Peninsula

by Sören Jensen

For most of the Earth’s history life consisted of small, mainly single-celled, organisms. It is only in rocks deposited during the last 570 million years before the present that large, complex, fossils appear, indicating the rise of animals. Digermulen Peninsula, northern Norway, offers the opportunity to study rocks, the Stáhpogieddi and Breidvika formations, laid down during the time of early evolution of animals. The interpretation of the best known of the earliest large complex fossils, the so called Ediacara-type fossils, also found on the Digermulen Peninsula, is controversial; some may bear no relationship to now living animals, some may be related to cnidarians, with there being few candidates for bilaterian animals such as worms or arthropods. Some of the earliest evidence for bilaterians, in fact, comes from burrows and other activity left on, and inside, the sea-floor. Although little can be said about the producers in detail, the complexity of the traces suggest they were made by an organism with features characterizing bilaterian animals. This includes an anterior concentration of the nervous system and an internal body cavity. So, by the beginning of the Cambrian Period, some 542 million years ago, complex branching burrows are found, some of which are called Treptichnus pedum (see image). The argument can be extended to even earlier and more simple trace fossils (see image), on a global scale indicating that bilaterian animals were around at least as far back as 555 million years before the present.

Trace fossils from the Digermulen Peninsula. To the left, Treptichnus pedum, from the basal Cambrian part of the succession; to the right, simple trace fossils from late Ediacaran rocks. Photo Sören Jensen, scale is 2 cm for both images.

After the appearance of complex branching trace fossils, the next major innovation, about 535 million years ago, was the appearance of trace fossil showing evidence for limbs such as those on modern arthropods. This order of appearance of trace fossils has been documented globally, and the first appearance of complex trace fossils such as Treptichnus pedum has been choosen to mark the beginning of the Cambrian Period of time; on the Digermulen Peninsula, placing the Ediacaran-Cambrian boundary in the upper part of the Stahpogiedde Formation. Rusophycus, a burrow made by arthropods, occurs slightly higher in the succession, in the Breidvika Formation. Ediacara-type fossils are known from lower and middle part of the Stáhpogieddi Formation. No trace fossils have been found in these beds, suggesting either an age older than about 555 Ma for these fossils, or that the activity did not get preserved. This is one of the questions that we are currently exploring.

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