Composite and compound ichnotaxa: A case example from the Ordovician of Quebec, eastern Canada

Abstract

Deep‐water strata of the Lower Ordovician of the Lévis Formation (Middle Cambrian‐lower Middle Ordovician) exposed in and around the Lévis‐Lauzon area of southern Québec, eastern Canada, contain a well preserved ichnofauna dominated by simple horizontal burrows of Planolites Nicholson, Alcyonidiopsis Massalongo and the fecal pellet Tomaculum Groom. The numerical increase in abundance of these ichnotaxa through coarsening upward (Logan) cycles developed within the sequence lends support to previous interpretations of such cycles as records of progressive upward changes (increase) in redox conditions in response to lowstands of sea level. Planolites, Alcyonidiopsis and Tomaculum occur as a continuum of forms developed in one of three ways. First, each may occur as individual and discrete ichnotaxa. Second, they may occur as composite material, comprising Planolites possessing low density, randomly dispersed fecal pellets of Tomaculum or Alcyonidiopsis with a high density fill of Tomaculum. Third, they may occur as compound material, represented by common examples or Planolües intergrading with Aleyonidiopsis and, less commonly, Aleyonidiopsis intergrading with Planolües. With both categories, Planolites may be discrete or composite; Aleyonidiopsis is, by definition, composite. Consistent with their original diagnoses neither Planolües nor Aleyonidiopsis exhibits true branching. Apparent branching results from burrow intersections, false and, particularly, secondary successive branching. Secondary successive branching is particularly developed with both composite and compound material, thereby precluding accurate assessment of the complete original character of older burrow morphologies due to superimposition within them of younger burrow generations. The nomenclature of composite and compound ichnotaxa is addressed. It is recommended that individual ichnotaxa comprising composite specimens should each be assigned their appropriate names, the primary nomenclature reflecting the overall character of the diagnostic ichnotaxon and the secondary name(s) signifying associated forms. Thus, for example, the nomenclature of simple burrows and fecal pellets within them should remain separate. Similarly, as compound material comprises integradational specimens, individual components of such specimens should also be accorded individual names, with the primary descriptor reflecting the dominant and diagnostic ichnotaxon and the subsidiary name its minor but intergradational morphotypes. Such nomenclatural schemes are unambiguous and unequivocal and avoid the need for cumbersome and repetitive descriptive phrases.