During the spring and summer of 2014, Peter Van Roy, an Associate Research Scientist at the Department of Geology and Geophysics funded by the Yale Peabody Museum, conducted high-resolution photography of large anomalocaridid arthropod fossils in the IPCH Digitization Lab. The fossils that Peter imaged were uncovered during expeditions in southeastern Morocco, a region to which the Yale Peabody Museum has been conducting expeditions since 2009. These anomalocaridid fossils were discovered in the Fezouata formations, which are muddy deposits that date back to the Early Ordovician (ca 480 million years old).
The Fezouata deposits consist of several thousand feet of shales and siltstones that accumulated in relatively shallow waters on the shelf off the ancient paleocontinent of Gondwana over a period of some 8 million years. During the Early Ordovician, the area where the Fezouata formations formed was situated close to the South Pole. The sediments contain an exceptionally well-preserved and diverse fauna, which provide unparalleled insights into the composition and functioning of Ordovician marine ecosystems. The animals that are preserved were rapidly entombed by storm-generated mudflows and include many delicate soft-bodied forms that under normal circumstances would have no chance of fossilization. Because swimming animals could more easily escape these mudflows, the fauna is mainly composed of benthic, or bottom-dwelling animals.
Among the swimming forms that have been discovered are several anomalocaridid fossils. Anomalocaridids are very early representatives of the Arthropoda, which is the most successful and diverse animal group on the planet, and includes, among many others, familiar creatures like horseshoe crabs, scorpions, spiders, millipedes and centipedes, crabs, lobsters, butterflies, ants, beetles, etc. The Fezouata specimens are the youngest unequivocal anomalocaridids that have been found to date; all other anomalocaridid fossils date back to the Cambrian period, with the oldest material being around 530 million years in age. Because they are such ancient creatures, they are of critical importance for understanding the origins and early evolution of Arthropoda.
To our modern eyes, anomalocaridids look very alien: they have a head with a pair of spinose grasping appendages and a circular mouth surrounded by toothed plates; their elongate, segmented bodies carry lateral flaps which they used for swimming. It was long believed that they only had one set of segmentally arranged flaps on each side of the body, but the Moroccan material has shown they actually possessed two sets, with gills attaching to the upper set – a finding which has important implications for our understanding of how modern arthropod limbs evolved. While most anomalocaridids were predators, the biggest Moroccan specimens were filter-feeders, gently harvesting plankton from the ocean. With a size of at least up to 7 feet, they are true giants, and rank among the very biggest arthropods to have ever lived. Interestingly, they foreshadow the appearance of giant filter-feeding whales and sharks much later, and provide a much older example of massive filter-feeders originating from among a predatory group at the time of a diversification of plankton.
Peter’s high-resolution photography of the Yale Peabody Museum specimens has had an impact beyond documentation. His images facilitated study of the large specimens and have led to discoveries about the construction and morphology of the creature’s lateral flaps and gills. The images also helped inform renowned natural history illustrator Marianne Collins, who worked closely with Peter to bring this ancient, extinct giant back to life through her stunning artistic renderings!
For more information about Peter’s findings, please see the Yale News feature http://news.yale.edu/2015/03/11/giant-sea-creature-hints-early-arthropod-evolution Peter’s popular science article https://theconversation.com/fossils-of-huge-plankton-eating-sea-creature-shine-light-on-early-arthropod-evolution-38520#comment_619019 or delve deeper by reading his team’s recent Nature publication http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14256.html !