A giant Ordovician anomalocaridid

• Published in: Nature (London) 2011-05, Vol.473 (7348), p.510-513
• Main Authors: Van Roy, Peter, Briggs, Derek E. G.
• Format: Article
• Language: English
• Subjects: 631/181/414; Animals; Arthropoda; Biota; Cambrian; Devonian; Digital cameras; Discovery and exploration; Earth sciences; Earth, ocean, space; Exact sciences and technology; Food and nutrition; Fossils; Gills - anatomy & histology; History, Ancient; Humanities and Social Sciences; Invertebrate paleontology; Invertebrates - anatomy & histology; Invertebrates - classification; Invertebrates - physiology; letter; Morocco; multidisciplinary; Natural history; Ordovician; Paleontology; Phylogeny; Predators; Science; Science (multidisciplinary); Shales
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature09920

Ocean predators The anomalocaridids were the giant, jointed-legged predators of the Cambrian seas. Well known as one of the oddities from the mid-Cambrian Burgess Shales of Canada, anomalocaridids have been found in Cambrian strata worldwide, but it was widely thought that they and their kin did not survive beyond the Cambrian. Peter Van Roy and Derek Briggs now extend the record of these bizarre creatures to the Ordovician, with a report of anomalocaridids from the Ordovician of Morocco. They join an assemblage of relic Cambrian animals recently described from that country. One of the Ordovician anomalocaridids was a metre or so in length, perhaps capable of challenging predators such as nautiloids for dominance of the seas. Anomalocaridids, giant lightly sclerotized invertebrate predators, occur in a number of exceptionally preserved early and middle Cambrian (542–501 million years ago) biotas and have come to symbolize the unfamiliar morphologies displayed by stem organisms in faunas of the Burgess Shale type. They are characterized by a pair of anterior, segmented appendages, a circlet of plates around the mouth, and an elongate segmented trunk lacking true tergites with a pair of flexible lateral lobes per segment 1 , 2 . Disarticulated body parts, such as the anterior appendages and oral circlet, had been assigned to a range of taxonomic groups—but the discovery of complete specimens from the middle Cambrian Burgess Shale showed that these disparate elements all belong to a single kind of animal 3 . Phylogenetic analyses support a position of anomalocaridids in the arthropod stem, as a sister group to the euarthropods 4 , 5 , 6 . The anomalocaridids were the largest animals in Cambrian communities. The youngest unequivocal examples occur in the middle Cambrian Marjum Formation of Utah 7 but an arthropod retaining some anomalocaridid characteristics is present in the Devonian of Germany 5 . Here we report the post-Cambrian occurrence of anomalocaridids, from the Early Ordovician (488–472 million years ago) Fezouata Biota 8 in southeastern Morocco, including specimens larger than any in Cambrian biotas. These giant animals were an important element of some marine communities for about 30 million years longer than previously realized. The Moroccan specimens confirm the presence of a dorsal array of flexible blades attached to a transverse rachis on the trunk segments; these blades probably functioned as gills.