CIRCULICHNIS LEOMONTI, A NEW RING-LIKE ICHNOSPECIES (TRACE FOSSIL) FROM THE LATE CAMBRIAN LION MOUNTAIN MEMBER, RILEY FORMATION, BURNET COUNTY, TEXAS

As a simple ring-shaped trace found on bedding surfaces, CirculichnisVialov, 1971, is easily distinguishable from other ichnogenera. As previous workers have noted (e.g., Uchman & Rattazzi 2019), traces that are interpreted as feeding traces should exhibit both entrance and exit openings. However, Circulichnis typically displays neither, casting both interpretations of ethology and trace construction hypotheses into doubt. Known from sediments dating from the Ediacaran period through to the Oligocene epoch, this trace is poorly understood, despite its large range and relative ease of diagnosis. Here we discuss a new occurrence of the ichnogenus from the Cambrian deposits of Central Texas.

Location.–The material for this study is described from an outcrop exposure of the Lion Mountain Member (Cambrian) northwest of Burnet, Texas. The specific outcrop location is 4.8 km north of the intersection of Ranch Roads 2341 and 690 (Fig. 1). The Lion Mountain Member is recognized on the north side of Ranch Road 2341 as it curves westward around the glauconitic bluffs. GPS coordinates for the location are 30.8446° N, 98.3402° W.

Geologic Setting.–The late Cambrian period in Central Texas is represented by the Riley Formation, which was deposited along a passive margin of the Iapetus Ocean. It is composed of three members: the Hickory Sand Member, the Cap Mountain Member, and the Lion Mountain Member. The cross bedded glauconitic sandstones exposed at this locality are typical of the Lion Mountain Member and are in agreement with previously published works (e.g., Comstock 1889; Sellards et al. 1912; Bridge 1937; Cloud et al. 1946; Dekker 1966). The Riley Formation unconformably overlies the Precambrian granitic Town Mountain Formation and displays an overall transgression of glauconitic quartz sandstone to a partially dolomitic limestone dominated system (e.g., Dekker 1966; Cornish 1975).

The Lion Mountain Member and related units have been studied sporadically. Originally described as part of the Potsdam Formation (Shumard 1861; Walcott 1884, 1891), the Potsdam Formation was later renamed as the Hickory Formation by Comstock (1889) due to its well-developed exposure in the vicinity of Hickory Creek. The Cap Mountain Formation, containing what would later become the Lion Mountain Member, was separated from the Hickory Formation by Paige (1912). Bridge (1937), in comparing the Texas units to the better studied Missouri sections, separated the Lion Mountain Member from the Cap Mountain Formation. Cloud et al. (1945) established the Riley Formation as the parent unit, and designated the Hickory, Cap Mountain, and Lion Mountain as members.

Specimens.–The material used for this study and ichnospecies description was found in float, interpreted by lithology, shape, and bedding characteristics to have fallen from approximately 6 m (20 feet) above from an overhanging ledge. Overprints of presumed Circulichnis are exposed in hypichnia on the overhanging ledge mirroring those of the specimens found in float, matching that of the described material. As the source bed was impossible to reach safely, the exact stratigraphic layer is not described in this study.

Eleven specimens of measured Circulichnis leomonti n. sp. are preserved on the surface of a single slab in apparent epichnia. These are easily discernable due to the contrast of the surrounding siltstone with the glauconized sandfill. The slab is roughly triangular in shape and measures 90 by 76 by 68 cm (35.5 by 30 by 27 in) on top and is 22.8 cm (9 in) thick. Circulichnis leomonti n. sp. is preserved along the upper surface of the slab (Fig. 2).

Remarks.–This ichnogenus has had a long and convoluted history, especially considering its improper but accepted naming format (Keighley & Pickerill 1997; Uchman & Rattazzi 2019). While attempts have been made to change the genus name Circulichnis to the more common CirculichnusKeighley & Pickerill, 1997, this name is now considered as junior synonym of Circulichnis and is not recommended for use under Article 33.2 of the International Code of Zoological Nomenclature (ICZN) (Uchman & Rattazzi 2019).

The definition of Circulichnis has been subject to much mistranslation and interpretation without formal amendment for years; however, Uchman & Rattazzi (2019) thoroughly summarized these attempts over the years. Despite this, Uchman & Rattazzi (2019), in bringing the definition of Circulichnis closer to the original description in Russian by Viavlov (1971), missed a character which they indicated as important in distinguishing this ichnogenus from other, potentially similar traces. Specifically, the frequently looping and winding trace of GordiaEmmons, 1844 can be confused with overlapping Circulichnis if the diagnosis does not differentiate them on distinctive features; namely, the lack of apparent entrance or exit to Circulichnis in-plane with the trace is a distinct characteristic of this ichnogenus. Further, their use of the term ‘cylindrical’ is unfortunate, as it would potentially exclude those traces which formed from a non-cylindrical trace maker, e.g., oblong or other burrow shapes. Thus, the generic description above is further modified to reflect these physiognomies.

Derivation of name. ‘Leo-’, from the Latin, for ‘lion’; ‘-monti’ from the Latin, for ‘mountain’. In reference to the Lion Mountain Member of the Riley Formation, from which it was discovered.

Type Material. NPL94419 (Jackson School of Geosciences Non-vertebrate Paleontology Lab, University of Texas at Austin); slab displaying eleven specimens; see Fig. 2 for location of type and paratypes.

Diagnosis. Horizontal trace in the shape of a continuous circular or ovoid ring, constructed by a series of linear to slightly curved connected segments, with wide string diameter, no breaks or deviant branches.

Type Description: The type specimen is one of many examples on the surface of the slab recovered from the Lion Mountain Member (Fig. 2). Type burrow is infilled with coarse sediment, distinguishing the coarse glauconitic sand infill from the fine silt walls. Burrow lining present, constructed of thinly laminated fine silt. The circular structure has a mean diameter of 29.71 mm with a mean burrow diameter of 8.46 mm. Burrow appears to be constructed of multiple lobes, which have been overlapped or chained together to create an elliptical structure. Further, burrows occasionally observed to continue into a looping double ring. No entrance or exit burrow apparent. Associated Skolithos-like vertical burrows and lingulid debris present throughout the material. Description and naming of specimen is in keeping with the recommendations of Bertling et al. (2006).

Paratype Descriptions: On the same slab (NPL94419) there are ten other burrows of Circulichnis present representing paratypes of Circulichnis leomonti. Burrows are variable in width (6–17 mm) and diameter (29–80mm). Burrows are infilled with coarse substrate and have fine-grained lining.

Remarks. Circulichnis leomonti should be considered as distinct from C. ligusticus, C. sinensis, and from C. montanus. Circulichnis leomonti presents a distinct physiognomy in the ratio between its burrow diameter and overall ring diameter, which varies considerably from that of the other three species (Fig. 3, 4). Additionally, this trace is observed to continue into a double ring pattern, a characteristic unreported for other Circulichnis species.

Of the two Circulichnis ichnospecies recognized by Uchman & Rattazzi (2019), C. leomonti is more similar to that of C. montanus. Circulichnis montanus, as described by Viavlov (1971) and redescribed by Uchman & Rattazzi (2019), is diagnosed as a “horizontal, cylindrical burrow, which shows a course along a regular circle or ellipse.” The description is non-specific in regards to ratios of those parts, which greatly affects the overall aspect of the trace, and should be taken into account. Uchman & Rattazzi (2019), in their remarks, suggested that C. montanus should be limited in a scope similar to the type species dimensions, with a ring diameter of 35–41 mm, a string width of 0.7–1.5 mm, and trace vertical height of 1.5 mm (Fig. 4). Confusingly, Uchman & Rattazzi (2019) also suggested that the distinction between C. montanus and C. ligusticus is arbitrary and all other derivations of Circulichnis should be ascribed to either C. montanus or C. ligusticus. This is rejected, as it excludes any other morphology of Circulichnis, including C. leomonti, which shows distinctions from both C. montanus and C. ligusticus.

The second species, C. ligusticus (Uchman & Rattazzi 2019) possesses a ‘knotted’ or broken/winding characteristic to the trace, and is unlike any sample observed of C. leomonti. While C. leomonti is segmented, the trace maintains a continuous course and does not deviate from circumscribing an ellipse, unlike C. ligusticus. In addition, the overall ring diameter and trace ring width of C. ligusticus are typically very small, less than half of the size of the typical C. leomonti. Circulichnis montanus is much more variable and demonstrates a large distribution through time.

Fan et al. (2021a, 2021b) have revived C. sinensisYang 1990 as an ichnospecies, despite its rejection by Uchman & Rattazzi (2019). Circulichnis sinensis provides a differing interpretation of the genus, with multilobate segments to the ring-like burrow. Fan et al. (2021a) recognized that there may be taphonomic factors influencing the discernibility between C. sinensis and C. montanus, but emphasized that they should be kept distinct. Fan et al. (2021b) later demonstrated that C. sinensis may be a linking of complex segments approximating a ring-like burrow or feeding structure. In comparison to C. leomonti, C. sinensis is apparently more fragmented in the figured type specimen; however, this distinction is blurred by the differing morphologies of C. sinensis. As the segmentation of C. sinensis is debated, and the potential overlap between C. sinensis and C. montanus, we recommend that C. sinensis be held as a distinct ichnospecies of the genus.

Ethology.–Circulichnis is identified from a variety of environments and time periods and lacks interpretable features, making ethological determination difficult. It is suggested that this simple trace is made by a variety of organisms. Viavlov (1971) attributed the trace to a simple locomotory action by a worm or worm-like animal. Buatois & Mangano (2011) considered this to be a simple, shallow tier feeding trace and implied that the likely trace maker was an arthropod. While other authors have agreed with the fodichnion assertion (e.g., Uchman & Rattazzi 2019), the lack of definitive scratch traces or joint impressions makes arthropods a less viable candidate over more smooth sided or flexible organisms, such as annelids or polychaetes. Uchman & Rattazzi (2019) attributed the large stratigraphic range (Cambrian–Recent) of this trace as evidence for an organism with an expansive evolutionary history for formation of Circulichnis. Simple vertical shafts, likely Skolithos, occur in the same bed as C. leomonti. There is a high preponderance of lingulid brachiopod and trilobite debris in the beds, and it is likely that the Skolithos-like burrows were formed by these organisms, although that in no way implies that C. leomonti was formed by the same. The variability in burrow width and overall ring construction leaves further room for doubt in ascribing a specific trace-maker or group to this ichnogenus. These observations, in conjunction with the broad stratigraphic range and presumed environments, from deep-marine to deltaic to fresh water, for Circulichnis make designation of ethology incredibly difficult and potentially suggest convergent behaviors among a variety of trace producers.

Taphonomic Considerations.–Fan et al. (2021b) discussed the taphonomic considerations of C. sinensis and related traces in detail. Of particular interest are the many angled burrows which were demonstrated to have interconnected to create the ring-like structure known as Circulichnis. While it is not known if this is how all Circulichnis were constructed by their trace makers, or if this demonstrates a connection to other, more elaborate ichnogenera, this material does demonstrate that taphonomy may have a role in the appearance of Circulichnis traces through time; i.e., the lack of entrance or exit burrows, as well as the overall shape, may be an effect of the taphonomic constraints, rather than purely burrow construction.

Conclusion.–Circulichnis leomonti demonstrates a new and distinct morphology among Circulichnis traces in both size and trace construction. The size of the trace is large, especially considering the trace ring width, which is many times larger than that of the type specimens of C. ligusticus and C. montanus. The segmented path construction is also distinct compared to other Circulichnis species. The occurrence of C. leomonti in the Cambrian is indicative of experimental organism behavior, or indicative of early Circulichnis trace construction techniques. This is important, as it clearly demonstrates that Circulichnis is not limited to simple ellipses or circuitous paths, and may provide further insight into the ethology of the trace maker.

We express our gratitude for the support of Tarleton State University for the transportation and storage of the specimens while preparing this manuscript. We are grateful to the Non-vertebrate Paleontology Lab at the Jackson School of Geosciences for accepting and housing the specimen, and for allowing us to hold the specimen while article preparations were underway. Additionally, this manuscript greatly benefited from commentary by reviewers and editorial assistance.