February 18th, 2016 
Riffin @WFS,Riffin T Sajeev,Russel T Sajeev,World Fossil Society
Evidence of predatory activity can be observed in the fossil record in the form of drill holes, repair scars, bite marks, and recognizable skeletal fragments in coprolites and preserved gut tracts. It is less common, however, to find fossil snapshots of predators caught in the act of feeding on their prey. Such interactions are preserved in recurring associations of the ichnogenera Rusophycus and Cruziana, most commonly attributed to trilobites, with burrows of likely vermiform (worm-like) organisms. In this study, we examine the Cambrian (Furongian Epoch, Steptoean Stage) Davis Formation, near Leadwood, southeastern Missouri, USA. In the lower to middle Davis Fm., several silty shale beds are extensively burrowed, from which we report a new occurrence and large number of Rusophycus traces associated with burrows of vermiform organisms. Within these beds, Rusophycus traces intersect vermiform burrows more often than expected by random chance and display a positive correlation in size between paired tracemakers. The median diameter of Rusophycus-associated vermiform burrows is significantly smaller than that of the non-intersected burrows. These results suggest that the paired traces record size selective predatory behavior. Moreover, low angle predator–prey trace intersections, though few in number, occurred more frequently than expected by random chance, supporting previous hypotheses that low angle attacks are preferred as they may improve prey handling success rates.
The Cambrian Period, which occurred between 541 million and 485 million years ago, is an important point in evolutionary history where most of the major groups of animals first appear in the fossil record. Often called the “Cambrian explosion,” fossils from this time provide glimpses into evolutionary history as the world’s ecosystems were rapidly diversifying. Most fossils preserve the physical remains of organisms and their structure; however, geologists and paleobiologists at the University of Missouri recently collaborated to study fossils that reveal the behaviors of predators preserved as traces in ancient sediments. Thus, fossils from southeast Missouri are helping scientists unlock clues about the behaviors of these predators and their interactions with their prey. Evidence shows that these ancient organisms were behaviorally sophisticated, tailoring their attacks for effectiveness.
A trilobite detects a lumpy worm burrow by sight and perhaps smell, then burrows down and grasps its prey with its many legs. Credit: Stacy Turpin Cheavens of the Department of Orthopaedic Surgery, University of Missouri
“The Saint Francois Mountains in southeastern Missouri have been the focus of geological research for decades and were once islands in the Cambrian ocean,” said Kevin Shelton, professor of geological sciences in the MU College of Arts and Science. “I’ve worked as a geologist studying ore deposits in the area for more than 30 years. In that time, I’ve run across thousands of fossilized trilobite burrows. It is rare that we get to study the activities of 500 million-year-old organisms, yet the fossils in this locality are helping us determine how these organisms behaved.”
The field area near the mountains is home to an abundance of trilobite trace and body fossils. James Schiffbauer and John Huntley, both assistant professors of geological sciences in the MU College of Arts and Science, worked with Shelton and Tara Selly, a graduate student in Schiffbauer’s research group, to collect slabs of rocks from the site. Selly, then analyzed them in the lab at MU.
Using sophisticated three-dimensional laser scanning and digital photograph analyses, sections of the rocks revealed burrows or trails left behind by trilobites and their prey — often worm-like creatures — in ocean sediments. To the scientists, these intersecting trails show how the predators caught their prey. Additionally, previous studies by former MU geology professor, James Stitt, revealed that the trilobites had very large eyes, so the researchers were looking for clues as to how their anatomy played into their feeding habits.
Tracks from the site showed that the predators attacked from above, moving alongside to use their many legs for more effective grappling of their prey. Further, predators preferentially selected smaller prey, indicating that they attacked their food rather than randomly bumping into it.
“Predation, or the action of attacking one’s prey, is a significant factor in evolution; this discovery is extremely important in the study of how organisms evolved in the Cambrian Period,” Schiffbauer said. “In this study, we provide evidence that these trilobites were likely visual predators, displaying selectivity in seeking and hunting their food.”
“Because we had an abundance of samples from the site, we were able to conduct more rigorous statistical analyses” Huntley said. “Our findings are important not only because of the large sample size, but because these early arthropods displayed such sophisticated predatory behavior.”
Courtesy:University of Missouri-Columbia. “500 million-year-old fossils show how extinct organisms attacked their prey: Missouri-based scientists unlock clues to predatory behavior, a significant factor in evolution.” ScienceDaily.
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