A team of researchers described some of the best-preserved three-dimensional trilobite fossils ever found.
A Window to the Past
Trilobites are probably the best known denizens of the Cambrian Period. They have really quite characteristic, fossilized external structures that are known from a very complete fossil record from the early Cambrian to the Permian extinction.
What helps make the trilobite one of the best-studied marine animals is its hard, calcified exoskeleton, which becomes well-preserved in the fossil record. In the past two hundred years, paleontologists have worked out more than 20,000 species of trilobites.
Unlike their contemporaries, most fossils of trilobites indicate only external morphology and thus less information about internal structure. Scientific knowledge of the trilobites is also constrained because they are rarely preserved in soft tissues. Further constraints to knowledge of Cambrian animal anatomy resulted from size-biased preservation processes, compaction, and incompleteness.
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Best-Preserved Trilobite Fossils
Many millions of years ago, something unlucky happened to these marine arthropods, which have now given us an unprecedented record of their anatomy in three dimensions. A volcanic eruption showered rain with ash that was dumped on top of a shallow marine environment.
The fossils, which are 500 million years old, were collected at the High Atlas of Morocco. Scientists fondly call it "Pompeii" trilobites, simply because of its awesome preservation in volcanic ash.
The results of this volcanic eruption were described in the paper "Rapid volcanic ash entombment reveals the 3D anatomy of Cambrian trilobites." A team of scientists headed by Professor Abderrazak El Albani from the University of Poitiers managed to study details of the trilobites' fossils that were preserved in nearly ideal condition.
Moroccan trilobites were buried in hot ash in sea water, which brought on ultra-quick fossilization of their bodies as the ash turned to rock. It was almost the same event that ended the lives of inhabitants of Pompeii after the eruption of Mount Vesuvius.
Every segment of their bodies was preserved by the ash molds, even their legs and hair-like structures running along the appendages. The digestive tract of trilobites was also preserved after it had been filled with ash. Apart from this, small "lamp shells" that were attached to their exoskeletons by fleshy stalks, as they are in life, remained attached.
It showed that, indeed, appendages at the edge of the mouth of trilobites had curved spoon-like bases, but they were so minute that they had passed unnoticed in less perfectly preserved fossils; this was revealed by CT scans and computer models of virtual X-ray slices.
For a long time, experts believed that there were three pairs of head appendages behind the long antennae of trilobites. Only the Moroccan species used in this study first revealed four pairs. A fleshy lobe covering the mouth, or labrum, had never been documented in trilobites before.
Co-author Harry Berks of the University of Bristol said, "The results show a clustering of specialized leg pairs around the mouth." That gives the experts an image of how the trilobites ate.
All of this work did turn up one unexpected impact: that volcanic ash really is a very great medium for fossil preservation under shallow marine conditions. According to El Albani, it is pyroclastic deposits that should become the new targets for scientific studies, since they show huge potential for trapping and preserving biological remains. That itself could lead to major discoveries about the evolution of life on Earth.
RELATED ARTICLE: Trilobites Breathing Through Their Legs, Fossil Record Shows
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