The ichnogenus Dictyodora Weiss: an attempt to unify terminology

Abstract

Dictyodora is a three-dimensional complex trace fossil characteristic of marine Paleozoic deposits recorded since the middle Cambrian [1]. According to the latest diagnosis [2], it is composed of a basal structure (lower margin) and a very thin structure (spreite) arranged perpendicular to the basal burrow showing a variable inclination. This structure appears as an irregular spiral or meandering "band", which corresponds to the intersection of the spreite with the planes parallel to the bedding [2]. Both structures were described as such in the last diagnosis [2]; however, they were called by different names in the literature. For example, the lower margin [2,3] has been referred to as the lower section [4] or basal burrow [5-7] or even as the lower part of the spreite [8]. Likewise, the spreite has been named as: striated vertical wall mural structure [9], narrow "wall" structure [6], dorsal vertical sediment curtain [10], crest-like or vertical wall [4,11], mid-dorsal vertical ridge [7], snorkel spreite [12]. Additionally, given the three-dimensional morphology of Dictyodora, the vertical, horizontal, and oblique preservations of each structure, and the different horizontal expressions (lower or higher horizontal sections) have also been given different names. The spreite is the structure that is frequently preserved in different planes and bedding levels (Fig. 1). The horizontal expression of the spreite is the most recorded and has been described with a wide range of morphologies and used as the main ichnotaxobases to determine ichnospecies. Six ichnospecies are currently valid [4]: Dictyodora liebeana (Geinitz), Dictyodora scotica (M’ Coy), Dictyodora silurica Yang and Hu, Dictyodora tenuis (M’ Coy), Dictyodora zimmermanni Hundt and Dictyodora atuelica Pazos et al. The morphology of the horizontal expression of the spreite varies in the ichnospecies from regular to irregular meanders with one or more orders of sinuosity. In other cases, irregular non-meandriform paths, and spiral meandriform or spiral forms are distinguished (Fig. 2). The use of terms such as "wide", "loop", "regular" or "irregular" have different meanings depending on the author. For instance, the word loop has been used to describe a meander [13]. This heterogeneity in the use of descriptive terms (some even used the original descriptions/diagnoses) is another problem when comparing ichnospecies. Moreover, some of the diagnostic features of the ichnospecies are not mutually exclusive and are sometimes present in more than one ichnospecies. For example, D. scotica is distinguished by regular first-order meanders, although they may also be irregular according to the same authors [5]. Also, D. silurica has both regular and irregular meanders [14]. In the case of D. tenuis, the morphology of the meanders is irregular [5], but it has also been described as completely irregular or more or less regular [15]. Another case is that of D. zimmermanni where the shape of the meanders is either quite irregular [13] or is less regular than in D. scotica [16] or may have both morphologies [15]. As used so far, this character is not useful to differentiate ichnospecies. Similarly, the secondary sinuosity is also ambiguous, being described as absent for example in D. zimmermanni but it seems present when illustrated by the same authors [13], or it is not described in D. silurica but it is showed in the type material [14]. This variability in the terminology hampers a proper comparison between specimens and an ichnotaxonomic discussion. Therefore, it is important to find an agreement in the use of these terms. Although it is beyond the scope of this work to discuss and redefine the ichnospecific ichnotaxobases for Dictyodora, it is important to achieve a consensus on the descriptive terms. After the study of hundreds of specimens from Gondwana and Laurentia, to solve some of the problems mentioned above, terms for each component and their respective morphologies are proposed (Fig.1).The term “basal burrow” is proposed because “lower margin” and “tube” have not been used in extenso in the literature. Furthermore, the use of the term "tube" refers to a single cylindrical structure, and in some cases the basal burrow does not have a cylindrical morphology as a whole, but rather lenticular, ovoid and even trilobed. Moreover, the term spreite is preferred for the medial structure arranged perpendicularly and above the basal burrow, partly because it is the term used in the diagnosis [2], but mainly because it is a descriptive term, highlighting characteristics of the structure such as the delicate and fine striations observed in its lateral vertical sections. Furthermore, it is also the term used most extensively in the literature. Finally, this structure has a wide range of inclinations (40º–90°) and is not consistent with being called a 'vertical structure' in the strict sense of the term. Given the morphological range of the spreite depending on which part of its vertical extension is sectioned, to aid in the description three different sections are here proposed. The wider, lower section, where the basal burrow connects to the spreite, is observed only in vertical or oblique preservation of Dictyodora, which is not frequent. The middle and upper sections are defined in relation to the lower section and reflect the horizontal expressions of the spreite that are most commonly preserved. The use of the terms "upper" and "middle" is considered arbitrary and with the sole purpose of allowing comparison of the spreite when this structure changes morphologically vertically within the same specimen [17]. Additionally, different terms to describe the morphology of the trajectories of both the basal burrow and the horizontal expression of the spreite are proposed (Fig. 2).