The world's biggest trilobite—Isotelus rexnew species from the upper Ordovician of northern Manitoba, Canada

Abstract

The largest known trilobite fossil, a virtually complete articulated dorsal shield of the asaphid Isotelus rex new species, has been recovered from Upper Ordovician (Cincinnatian, Richmondian) nearshore carbonates of the Churchill River Group in northern Manitoba. At over 700 mm in length, it is almost 70 percent longer than the largest previously documented complete trilobite, and provides the first unequivocal evidence of maximum trilobite length in excess of one-half metre. Comparisons with other fossil and extant members of the phylum suggest that in terms of maximum linear dimensions it was among the biggest arthropods ever to have lived. Sediments of the Churchill River Group were deposited in an equatorial epeiric setting and the extremely large size of I. rex n. sp. thus marks a striking example of low-latitude gigantism, in sharp contrast to the widespread phenomenon of “polar gigantism” in many modern marine benthic arthropods. Lack of extensive epibiontic colonization of the exoskeletal surface and the presence of large distinctive trace fossils in the same unit suggest that I. rex n. sp. may have been a semi-infaunal predator and scavenger that employed a shallow furrowing and probing mode of benthic feeding. The extinction of the isotelines (and virtually the entire asaphide lineage) at the end of the Ordovician cannot be related to the near contemporaneous achievement of exceptionally large adult size in some representatives. Failure to survive the terminal Ordovician extinction event was most likely a consequence of a pelagic larval life-style that proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats.

The Hirnantian (Late Ordovician) and end-Guadalupian (Middle Permian) mass-extinction events compared

“…Regarding Ordovician marine life, what was unique is the appearance of giant marine invertebrates for the first time in the Phanerozoic, for example orthocerid cephalopods (up to 9 m long), stromatoporoids (up to 3 m long), trilobites (up to 70 cm long) and gastropods (up 25 cm in diameter) (e.g. Teichert & Kummel 1960;Rohr et al 1992;Rudkin et al 2003;Copper et al 2013). All of them disappeared at the beginning of the Hirnantian, however, marking the first termination of gigantism in the Phanerozoic.…”

Oxygen, animals and oceanic ventilation: an alternative view

“…The Silurian-Devonian radiation of large pterygotid eurypterids, for example, corresponds to a modest peak in the Berner curve ( c . 25% p O 2 ≈ 120% PAL), but fails to account for the relatively diminutive eurypterids that dominated the late Palaeozoic oxygen maximum -or the metre-sized trilobites that thrived during the Ordovician minimum (Rudkin et al ., 2003). Perhaps most problematic is the occurrence of Triassic/Jurassic dragonflies with nearly twice the wingspan of extant Petalura ingentissima , despite the constraints of a putative 10-15% p O 2 ( c .…”

Furrows and firmgrounds: evidence for predation and implications for Palaeozoic substrate evolution inRusophycusburrows from the Silurian of New York

“…Candidate trace fossils for this behaviour have been reported from the Lower Cambrian of Sweden (Jensen 1997, p. 99;fig. 66) and the Ordovician of Canada (Rudkin et al 2003). However, this activity would have required uncompacted sediment and therefore would have likely possessed a low preservation potential.…”