New records of Dictyodora in the middle Paleozoic of Gondwana: ichnospecies intergradation and evolutionary-stratigraphical implications

PAZOS, PABLO JOSÉ; GUTIÉRREZ CAROLINA; FERNÁNDEZ, DIANA ELIZABETH

Alabama

Workshop; International Ichnofabric Workshop 16A (virtual); 2021

The University of West Alabama

Dictyodora is a complex three-dimensional trace fossil composed mainly of a basal section and a striate vertical wall. The mainly horizontal expression of the vertical section of Dictyodora is the most commonly preserved, with a wide range of morphologies used to characterize them at a specific level. Its ichnospecies have historically been considered as reliable stratigraphic indicators for Paleozoic deposits (e.g., Uchman, 2004; Benton and Trewin, 1980).Recently, several studies have featured Dictyodora (e.g., Zhang et al., 2020; Muszer, 2020; Pazos et al., 2015) showing that the presence of this ichnogenus does not exclusively imply deep-marine settings, and extending its geographic and stratigraphic range. Dictyodora is now documented in shallow- and deep-marine deposits from the Cambrian to the Permian (e.g., Zhang et al., 2020). Almost all ichnospecies of Dictyodora were defined and documented first in Laurentia (Canada, United States, United Kingdom, Portugal, Scotland, Spain, Germany, Czech Republic, Poland), and only scarce records are reported from Gondwana, particularly in Siluro-Devonian basins of the western margin of the Rhaetian Ocean (Argentina) and in the Paleotethys (China). The newest ichnospecies (Dictyodora atuelica) was only documented in offshore-shoreface transition(?) deposits of the Late Silurian-earliest Devonian Río Seco de los Castaños Formation (RSCF) in central-western Argentina (Gutiérrez et al., 2021; Pazos et al., 2015).Only a few cases of intergradation between ichnospecies of Dictyodora have been reported before today (e.g., Dictyodora tenuis-D. zimmermanni, Neto de Carvalho, 2001; Pazos et al., 2015: 32; D. tenuis-D. scotica Lehotský, 2016: Tab. II, 3, although described as D. liebeana by the author; D. scotica-D. tenuis, Zhang et al., 2020: fig. 4c, described as D. cf. scotica by the authors). Recently, D. scotica was recognized in intergradation with D. atuelica and D. tenuis in the RSCF. Structures resembling D. liebeana were also recorded (Gutiérrez et al., 2021). D. liebeana has been described mainly from deep-water settings from the Culm facies (Mikulá? et al., 2004; Orr et al., 1996) and is stratigraphically restricted to the Early Carboniferous (Uchman, 2007, 2004). The stratigraphic use of ichnospecies of Dictyodora was discussed by Pazos et al. (2015) and it is currently still argued that they have only slight stratigraphic use (Zhang et al., 2020). In the present study, complex forms resembling Dictyodora liebeana with spiraling, looping and irregular meandering patterns and Dictyodora zimmermanni in intergradation with D. scotica and D. atuelica have been recognized. Additionally, it permits discussion of this ichnospecies as a stratigraphic indicator, as this is its oldest record. D. liebeana is mostly found in the Carboniferous deep-sea turbidite sequences (e.g., Orr et al., 1996; Mikulá? et al., 2004); this record is a contribution to the scarce record of this ichnospecies in the middle Paleozoic and in shallow-marine settings.These records also allow discussion of the evolutionary sequence previously proposed for Dictyodora (Seilacher, 1967: 78), and emphasize that the development of complex foraging strategies (spiraling, coiling, looping, and/or complex and tighter meanders) had already begun by the middle Paleozoic.