{"id":4486,"date":"2017-06-01T04:00:10","date_gmt":"2017-06-01T04:00:10","guid":{"rendered":"https:\/\/www.palaeontologyonline.com\/?p=4486"},"modified":"2017-07-11T10:14:10","modified_gmt":"2017-07-11T10:14:10","slug":"patterns-palaeontology-cambrian-environments","status":"publish","type":"post","link":"https:\/\/www.palaeontologyonline.com\/?p=4486","title":{"rendered":"Patterns in Palaeontology: Environments of the Cambrian explosion"},"content":{"rendered":"

by Thomas W. Hearing<\/a>*1<\/sup><\/p>\n

Introduction:<\/h2>\n

Shimmering curtains of sunlight stream down through the waters of a shallow sea that has been advancing landwards for several million years. This transgression has formed wide areas of shallow continental shelf seas. The sea bed teems with life \u2014 some of it familiar, some much less so. The oddities begin on the floor of this tropical sea: a reef built not of corals, but by carbonate-producing microbes and the strange archaeocyathan<\/a> sponges, alongside creatures that look more conventionally sponge-like but probably aren\u2019t. Streams of seaweed drift on the currents; closer examination reveals small, snail-like shelled molluscs<\/a> on some of the tendrils. A trilobite<\/a> scuttles for cover, startled by the flickering shadow passing overhead, and narrowly avoids a stab by the inverting mouthparts of a pouncing priapulid<\/a> worm. The shadow-casting terror glides off in search of slower prey.<\/p>\n

This is what life might have been like 508 million years ago on what is now the middle Cambrian<\/a> Cathedral Escarpment, a rock deposit in the Rocky Mountains of British Columbia, Canada (Fig. 1, Video 1). The fossils found there are classic examples of Cambrian explosion<\/a> taxa: a multitude of animal body and trace fossils<\/a> in the layers overlying Precambrian<\/a> rocks that seem to contain no large, complex lifeforms. The sudden appearance of these fossils was a profound concern for Charles Darwin<\/a> when he was formulating his thoughts on evolution. More than 150 years later, through the discoveries of several famous Konservat-Lagerst\u00e4tten<\/a><\/em>, we now have a much clearer understanding of the biological changes that occurred during the Cambrian explosion. The \u2018shadow-casting terror\u2019 (Fig. 1) could be Anomalocaris<\/em> \u2014 it\u2019s name meaning \u2018strange shrimp\u2019, owing to the appearance of its frontal appendages \u2014 which now seems like a weird evolutionary experiment. But others like the priapulid worms can still be found in their seafloor burrows today, living much as Ottoia<\/em> (Fig. 1) did 508 million years ago; their generalist diet, for example, is different only because the surrounding animals on which they prey have changed. However, there remain many questions to answer \u2014 particularly concerning the environments in which these early animals evolved.<\/p>\n

\"Figure
Figure 1 \u2014 Reconstruction of the Burgess Shale ecosystem: a priapulid (Ottoia<\/em>) burrows in the foreground as Anomalocaris<\/em> hunts overhead. Illustration: John Sibbick.<\/figcaption><\/figure>\n

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