Ordovician faunas of Burgess Shale type

Abstract

The renowned soft-bodied faunas of the Cambrian period, which include the Burgess Shale, disappear from the fossil record in the late Middle Cambrian, after which the Palaeozoic fauna dominates. The disappearance of faunas of Burgess Shale type curtails the stratigraphic record of a number of iconic Cambrian taxa. One possible explanation for this loss is a major extinction, but more probably it reflects the absence of preservation of similar soft-bodied faunas in later periods. Here we report the discovery of numerous diverse soft-bodied assemblages in the Lower and Upper Fezouata Formations (Lower Ordovician) of Morocco, which include a range of remarkable stem-group morphologies normally considered characteristic of the Cambrian. It is clear that biotas of Burgess Shale type persisted after the Cambrian and are preserved where suitable facies occur. The Fezouata biota provides a link between the Burgess Shale communities and the early stages of the Great Ordovician Biodiversification Event.

A new phyllopod bed-like assemblage from the Burgess Shale of the Canadian Rockies

“…The presence of these taxa within the Marble Canyon assemblage significantly expands their geographical and temporal ranges, and indicates that their absence from younger Burgess Shale-type assemblages was not the result of endemism or extinction, but was controlled by ecological and taphonomic factors. This finding, together with those from the recently described Early Ordovician Fezouata biota 17 , suggests that the longevity and geographical ranges of soft-bodied Cambrian taxa at both high and low taxonomic ranks may be severely underestimated.…”

Silurian horseshoe crab illuminates the evolution of arthropod limbs

“…Synziphosurines are a paraphyletic group low on the stem of Xiphosura with an unfused opisthosoma composed of nine to 11 segments (4). Synziphosurines also first appear as an undescribed taxon in the Lower Ordovician of Morocco, although they likely originated even earlier (5), and range into the Lower Carboniferous (Namurian) (6). The interrelationships of synziphosurines are uncertain: limbs are only known in the Silurian Offacolus kingi from Herefordshire, England (7,8), here shown to be a synziphosurine, and Venustulus waukeshaensis from Waukesha, Wisconsin, USA (9), and the Devonian Weinbergina opitzi from Hunsrück, Germany (10).…”

“…Horseshoe crabs are united by several characters: a prosoma with ophthalmic ridges and a cardiac lobe, and an opisthosoma with a reduced first segment and a defined axial region (3). Among horseshoe crabs the Xiphosurida, which includes the living forms, are characterized by a fused opisthosoma (4); the oldest example is an undescribed taxon with preserved appendages from the Lower Ordovician (Tremadocian-Floian) of Morocco (5). Synziphosurines are a paraphyletic group low on the stem of Xiphosura with an unfused opisthosoma composed of nine to 11 segments (4).…”

A unifying model for Neoproterozoic–Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression

“…While numerous other palaeoenvironmental and diagenetic considerations have been invoked for kerogenization, such as interactions with clays 6,16 or ferrous iron 17 , high alkalinity 15,18 and oxidant restriction (that is, lack of sulfate for BSR) through early diagenetic sealing 18 (though see also ref. 19), the common association of kerogenized fossils with pyrite 1,6,12,16,[20][21][22][23][24] bolsters the interrelationship of these taphonomic processes.…”