Who were the tracemakers of Protovirgularia – Molluscs, arthropods, or annelids?

Abstract

The ichnogenus Protovirgularia is an old but valid ichnotaxon name with a convoluted taxonomic history and interpretation. Originally erected as body fossil, it is now recognized as a trace fossil with a global distribution throughout the Phanerozoic. A wide range of tracemakers from the phyla Annelida, Mollusca and Arthropoda can produce traces with the significant chevron-like ornamentation, which have been classified in several ichnospecies. Modern analogues have demonstrated that protobranch bivalves are able to make such traces, which led to the general assumption that Protovirgularia represents a characteristic bivalve trace fossil. Examples of three ichnospecies of Protovirgularia from the Middle Triassic of Germany are analysed and show that Arthropoda (e.g., Isopoda) and Annelida (e.g., Polychaeta) must equally be regarded as potential tracemakers of Protovirgularia. This interpretation comes from diverse lines of evidence having varying uncertainties, including functional morphology, modern analogues, and preserved producer. The fact that annelids and arthropods, in addition to bivalves, can be regarded as producers of Protovirgularia offers the possibility of utilizing this ichnogenus for enhanced palaeoenvironmental and evolutionary interpretations.

Introduction

Ichnology has proven to be invaluable for palaeoenvironmental reconstructions and the interpretation of depositional environments (e.g., Pemberton, 1992, Pemberton et al., 2001, Miller, 2007, Seilacher, 2007, Buatois and Mángano, 2011, Knaust et al., 2012). Integrated sedimentological-ichnological studies greatly benefit from the recognition of ichnotaxa, although for some ichnotaxa the taxonomy as well as interpretation of their producers and behaviours remains uncertain. While much progress has been done with a uniform approach to ichnotaxonomy and the treatment of trace-fossil groups (Bertling et al., 2006, Bertling et al., in press), many key ichnotaxa are relatively poorly understood and need revision.

A common ichnological rule states that a trace fossil can originate from the work of different kinds of producers (e.g., Bromley, 1996), and for most invertebrate trace fossils, the producer remains unknown. There are several different ways to deduce information about potential tracemakers of a trace fossil: (1) Functional morphology. A common approach is linking morphological features of a trace fossil to a particular behaviour of its assumed producer (e.g., Seilacher, 1970, Savazzi, 1994). (2) Modern analogues. Recent traces similar to some fossil traces can be produced by known organisms, wherein observations in sedimentary environments and experiments may serve as analogues (e.g., Clifton and Thompson, 1978, Dworschak and Rodrigues, 1997). (3) Preserved producer. Direct evidence comes from instances where the producer became preserved within its trace (e.g., Sellwood, 1971, Neto de Carvalho et al., 2007, Knaust, 2021).

The ichnogenus Protovirgularia M’Coy, 1850 was erected from Silurian (Wenlock) slates of deep-marine origin from the Southern Uplands of Scotland (Benton, 1982). Since then, it has been reported several hundred times and shown to be a cosmopolitan trace fossil in marine and continental Phanerozoic deposits. Despite its wide distribution and several attempts for its classification, the nomenclature and taxonomy of Protovirgularia have not yet been settled. A wide range of trace morphologies and various taphonomic preservations have led to the erection of at least ten nominal taxa at the ichnogenus level, all now regarded as junior synonyms of Protovirgularia (Han and Pickerill, 1994, Rindsberg, 1994, Seilacher and Seilacher, 1994, Uchman, 1998, Mangano et al., 2002 Rindsberg, 2018).

The interpretation of Protovirgularia and its inferred producers has been at least equally debated as its taxonomic treatment. Originally introduced as a sea pen (Octocorallia, Anthozoa), Protovirgularia was subsequently confused with other body fossils (e.g., graptolites, Richter, 1853; annelids, Mayer, 1954), until Häntzschel (1958) clearly stated its trace-fossil nature, although its origin has continued to vex recent workers (e.g., Itano, 2020). An experimental approach enabled Seilacher and Seilacher (1994) to conclude that protobranch bivalves with a cleft foot were the main producers of Protovirgularia. This interpretation has been widely echoed by other workers (e.g., Rindsberg, 1994, Uchman, 1998, Mángano et al., 1998, Gibert and Domènech, 2008, Carmona et al., 2010, Fernández et al., 2010, Nara and Ikari, 2011, Luo and Shi, 2017, Hammersburg et al., 2018, López Cabrera et al., 2019), although it contrasts with earlier interpretations that mainly considered annelids and arthropods as tracemakers (e.g., Richter, 1941, Volk, 1961, Claus, 1965, Greiner, 1972, Han and Pickerill, 1994), but also holothurians (Richter, 1941) and scaphopods (Seilacher and Seilacher, 1994). Moreover, isopods, amphipods and odonatans are recognized as producers of Protovirgularia, particularly in continental environments (Uchman and Pervesler, 2006, Lima et al., 2017, Metz, 2020). Related to this diversity of potential producers and associated uncertainties, any ethological interpretation (i.e., the producer’s behaviour) of individual ichnospecies of Protovirgularia remains subjective and encompasses locomotion (repichnion), grazing (pascichnion), resting (cubichnion) and feeding (fodinichnion) (Seilacher and Seilacher, 1994; Mángano et al., 2002).

The aim of this contribution is to present three ichnospecies of Protovirgularia from the Middle Triassic of Germany that were evidently produced by invertebrates other than bivalves. Each of the three cases arrives at its interpretation by applying one of the three methods for trace-fossil analysis (or a combination of these): functional morphology, modern analogues, and preserved producer. In the first example, Protovirgularia dichotoma M’Coy, 1850, the type ichnospecies of Protovirgularia, is demonstrated to originate from the burrowing activity of an arthropod instead of a bivalve. The second case shows new and well-preserved material of Protovirgularia bidirectionalis Mángano et al., 2002, previously confirmed only in its type area, and therefore described in detail. Key features of this new occurrence allow interpretation of these trace fossils as the burrows of isopod crustaceans. The third example is Protovirgularia bifurcata Knaust, 2021 from a microbialite, which occurs together with its producer, an exceptionally preserved polychaete. The standing interpretation of these (and perhaps other) ichnospecies as bivalve traces is challenged in favour of an interpretation as arthropod and annelid traces.