The ichnofauna of the Skiddaw Group (early Ordovician) of the Lake District, England

Abstract

Deep-marine strata of the Skiddaw Group, deposited in high southerly palaeolatitudes on the northern margin of Eastern Avalonia in the early Ordovician, contain a diverse ichnofauna dominated by examples of fodinichnia. This includes Alcyonidiopsis pharmaceus, several ichnospecies of Chondrites, Dictyodora zimmermani, Glockerichnus radiatus, Gordia marina, Ixalichnus enodius, two or possibly three ichnospecies of Phycodes, Planolites montanus, Volkichnium volki, an ‘arthropod-produced? repichnion’, ‘paired pits’, ?Gordia aff. marina, ‘thinly-walled, looping, burrows’ and burrows of Planolites/Palaeophycus affinity. Stellascolites radiatus Etheridge, 1876 is reassigned to the ichnogenus Glockerichnus Pickerill, 1982 as Glockerichnus radiatus (Etheridge, 1876).Several of the ichnotaxa are present in other ichnofaunal assemblages of early Ordovician age which were also emplaced at high southerly palaeolatitudes. The relatively high diversity of this ichnofaunal assemblage supports suggestions that models of the rate at which the deep-marine environment was colonized during the early Phanerozoic are in need of revision.

Colonization of the deep‐marine environment during the early Phanerozoic: the ichnofaunal record

“…Pascichnia and agrichnia are rare in some of the well-dated early to middle Ordovician assemblages (e.g. Data Points 16, 18 and 19); which are instead dominated by examples of fodinichnia ( Figure 4C) (Orr 1996); the restructuring may have been delayed until the early Ordovician. The lateral partitioning of agrichnia and pascichnia into the Paleodictyon and Nereites subfacies of the Nereites ichnofacies had occurred by the early Silurian, at the latest (Orr 1995).…”

“…For example, Neonereites occurs as a pascichnion and a simple repichnion in Llandovery strata of west Wales (Orr 1995, Figures 6e and 6f, respectively), and thus was not included. The anomalous occurrence of`shallow-marine' ichnotaxa (those more typical of shelfal, especially inner shelf and nearshore, settings) in early Phanerozoic deep-marine ichnofaunal assemblages has been commented on by a number of authors (Crimes 1976;Pickerill 1992;Orr 1996). Following other authors, the ichnogenera Arenicolites, Bergaueria, Cruziana, Dimorphichnus, Diplichnites, Diplocraterion, Monocraterion, Monomorphichnus, Phycodes, Rhizocorallium, Rusophycus and Skolithos were considered more typical of shallow-marine environments (see Crimes and Droser 1992, p. 341;Crimes and Fedonkin 1994).…”

“…Several of these ichnogenera are key elements of the Arenicolites and Skolithos ichnofacies and references therein), and recur in Palaeozoic shallow-marine environments. Each occurs more than once in the dataset; individual occurrences of other putative shallow-marine ichnotaxa, for example the arthropod-produced Ixalichnus (Orr 1996), were not included. Analyses of the temporal distribution of pascichnia and agrichnia, and shallow-marine' ichnotaxa, in early Phanerozoic deep-marine environments are summarized in Figures 2 and 3 respectively.…”

“…McCann (1990) noted the poor ®t of regression lines to his plots of ichnofaunal diversity versus time for Phanerozoic deep-marine ichnofaunal assemblages and concluded that any increase with time was neither as dramatic nor as consistent as originally suggested by Seilacher (1974). Pickerill (1980Pickerill ( , 1988Pickerill ( , 1992 and Orr (1996) have emphasized that the diversity of the Cambro-Ordovician deep-marine ichnofaunal assemblages they studied is anomalously high compared to that predicted by existing models. A reanalysis of ichnofaunal diversity versus time is therefore warranted using the signi®cantly larger dataset that has become available in the past decade.…”

Trace fossils from the Dividalen Group of northern Finland with remarks on early Cambrian trace fossil provincialism

“…It is therefore not surprising that trace fossils have not been extensively used for palaeogeographic analyses (see Seilacher 1994, andOrr 1996 for examples of possible exceptions). Indeed, the cosmopolitan distribution of Cambrian trace fossils proved very useful for defining the base of the Cambrian (e.g.…”