• Invertebrate trace fossil, ichnogenus Cochlichnus are described from the Jinju and Haman Formations.

• RTI (Reflectance Transformation Imaging), a new technology were applied to Cochlichnus characterized by tiny and shallow relief.

• It is expected that the application of RTI technology to more and diverse fossils will increase the possibility of using RTI technology for paleontology as a whole.

Entering the 21st century, various methods (e.g., 3D scanning) have been developed to record and preserve cultural properties. Among them, the recently developed RTI (Reflectance Transformation Imaging) is a technology that allows three-dimensional observation of objects using virtual light sources, and is currently being used in various fields centered on museums, forensics, and cultural heritage (Frank, 2015; Harris and Piquette, 2015; Hughes-Hallett et al., 2021). Because this technology is a computer-based photography method that can illuminate a subject from all directions and angles, it is possible to obtain threedimensional visual information that cannot be obtained with ordinary photography. Currently, the Cultural Heritage Administration (CHA), Korea, uses this technology to record and preserve cultural heritage, especially in the fields of cultural heritage preservation, analysis, monitoring, and experimental archeology (Jo et al., 2021; Kim et al., 2023). For natural heritage, the CHA, Korea, first applied RTI technology to dinosaur footprint fossils from Ulsan earlier this year (manuscript in preparation).

Unlike trace fossils of vertebrates such as dinosaur or bird footprints, trace fossils of invertebrates are small in size and very low in relief, so there have been many difficulties to record and observe the surface information of the invertebrate trace fossils (Bertling et al., 2006, 2022; Uchman et al., 2009). To overcome such difficulties, many studies using photography and 3D precision scanning technology have been attempted, but they are not suitable for research on trace fossils of invertebrates.

We applied RTI technology to invertebrate trace fossils identified as ichnogenus Cochlichnus found from the Jinju Formation and the Haman Formation, Korea. This paper is the first attempt to apply RTI technology to the geoheritage field and discuss the applicability of this technology. As in the case of very small invertebrate trace fossils, RTI technology is highly likely to be applied to fossils characterized by small in size and low in relief, like leaf fossils. Therefore, it is expected that the application of RTI technology to more and diverse fossils will increase the possibility of using RTI technology for paleontology as a whole.

The fossils described in this paper were found from a fossil site of Namhae Gain-ri and the construction site of Jinju Aviation Industrial Complex. The Gain-ri fossil site was designated as a natural monument (No. 499) in 2008, now being managed by the Cultural Heritage Administration, and fossils were collected from the Haman Formation of the site (Fig. 1). The Gain-ri area is the middle to upper part of the Cretaceous Haman Formation and is mainly composed of sandstone and dark gray mudstone with alternating layers of shale and sandstone. Calcareous nodules were sporadically observed (Lockley et al., 2008). In this area, a large number of fossils were reported, including sauropod, ornithopod and theropod dinosaurs footprint fossils (Seo, 1997; Kim et al., 2006), webbed bird footprint fossils (Kim et al., 2006; Lockley et al., 2008) and diverse trace fossils of invertebrate. Sedimentary structures such as raindrop imprints and mud cracks are also observed on the bedding planes of fossil-bearing shale to mudstone. The fossil Cochlichnus reported herein (Fig. 3D, H, I) were found on the bedding surfaces of sandstone. The detailed geology and stratigraphic relationships of the Gain-ri fossil sites were already documented in several papers (Kim et al., 2006; Lockley et al., 2008).

Figure 1. Geologic map and fossil localities.

Figure 3. Ichnospecies Cochlichnus anguineus Hitchcock, 1858 from the Jinju Fm. and the Haman Fm. (A~D) entangled assemblages of the species from the Jinju Fm. (E, F) individual specimen showing a full sine curve. (G) large specimen identified as Cochlichnus sp. (H, I) specimens on coarse rock of the Haman Fm. (collection # A~C: JAC 2209-1, 2, 3, D: JAC 2207-1, E~F: JAC 2206-1, G: JAC 2309-7, H~I: HG1, 2).

A large numbers of the specimens of Cochlichnus fossils were collected from the Jinju Formation of the construction site of Jinju Aviation Industrial Complex (Fig. 1). The Gyeongnam Aviation National Industrial Complex Jinju District, where fossils were found, belongs to the Cretaceous Jinju Fm. The Jinju Fm. is the representative lake sedimentary sequences of the Gyeongsang Supergroup (Chang, 1975; Paik et al., 2019; Chough et al., 2000), and is distributed for about 300 km in the eastern part of the Gyeongsang Basin, from Gunwi to Sacheon. The Jinju Fm. is mainly composed of grey to black mudstones, shales and sandstones. Choi (1986) documented that channel facies deposited in shallow marginal lake are gradually transitional upwards to deeper lake deposits. The lower boundary of the formation is based on the level at which the red layer of the Hasandong Formation ends, while the upper boundary is based on the level before the appearance of the lowermost red bed of the Chilgok Formation (Chang, 1975; Choi, 1986; Chough et al., 2000). Therefore, the Jinju Fm. is characterized by the absence of the red layer, unlike the lower Hasandong Fm. and the upper Chilgok Fm. The thickness of the formation varies with the region: about 1,200m in Jinju, 1,000m in Hapcheon, and 600m in Uiseong.

Since Tateiwa (1929) first began geological investigations on the Gyeongsang Basin, a great number of diverse fossils have been discovered from the Jinju Fm. They include both body and trace fossils. The body fossils include vertebrate (fishes, dinosaurs, pterosaurs) and invertebrate (bivalves, estherids, gastropods, insects, ostracods), and lacustrine micro-domal stromatolite were also reported (Yang, 1975; Yang et al., 2003; Paik, 2005). Trace fossils, especially dinosaur and bird footprints, have been reported consistently (Kim and Lockley, 2016; Kim et al., 2019). Vertebrate (crocodile, lizard, bird) footprint fossils, plant fossils, and various trace fossils were also found at the site of the Gyeongnam Aviation National Industrial Complex Jinju District, where the Cochlichnus fossils were discovered.

Most of the fossil specimens identified herein as Cochlichnus were collected from the Gyeongnam Aviation National Industrial Complex Jinju District, and Gain-ri specimens of Cochlichnus were also used for species classification. The Aviation Industrial Complex construction site is mainly composed of alternation of sandstone and black layers, where most of fossils such as lizard footprints, plant fossils, and stromatolites were found (Fig. 2). The trace fossil Cochlichnus was also found on the bedding surface of the black shale.

Figure 2. Construction site of the Gyeongnam Aviation National Industrial Complex Jinju District. (a) Wide view of fossil bearing strata. (b) Close-up view of the strata. The strata are composed of alternating layers of shale and sandstone beds. (c) Plant fossils of Equisetites sp. (d) Lizard footprint fossils. (e) Cochlichnus anguineus Hitchcock, 1858.

3.1. Cochlichnus Assemblage

The fossil Cochlichnus were observed on the bedding plane of more than 20 rock fragments that fell off during the construction of Jinju district (Fig. 2E, 3A~C, 3E~G). The fossils are continuously and regularly meandering interface burrows, characterized by a typical sine-curved wavelength (Fig. 3.) Most specimens resemble a full sine (Fg. 3H, I) or clothoid wave, however nearly stretched specimens with very low amplitude were also observed (Fig. 3C arrow). They are preserved both as positive epirelief (Fig. 3B, E) or negative epirelief (Fig. 3C, F). Some of rock fragments contain very entangled Cochlichnus assemblages (Fig. 3A~B), while others have a few specimens (Fig. 3E~I). The abundance varies from site to site, but seems to be related with rock types; thin mud layers tend to contain more specimens (mostly from the Jinju Fm.) including a dense assemblage, while coarser rocks (mostly from the Haman Fm.) contain very few specimens (Fig. 3H~I). These fossils are herein classified as ichnospecies Cochlichnus anguineus Hitchcock, 1858.

A total of 64 Cochlichnus individuals (41 from the Jinju Fm. and 23 from the Haman Fm.) were identified, and width, wavelength, and amplitude were measured for statistical analysis (Table 1). Width of the grooves of the species varies between 0.2 and 5.6 mm; individuals with an average thickness of 0.64 are the most dominant (Fig. 4). The Jinju specimens tend to be rather wider than the Haman ones, which probably related to the rock type containing the fossil. The wavelength of the sinuous meanders ranges between 1.5 and 28 mm, and the amplitude between 0.9 and 7.9 mm; the majority has wavelength between 5 and 10 mm and amplitude between 1 and 5 mm. The ratio of wavelength to amplitude was approximately less than 1, showing a linear correlation. The ratio is more or less regular regardless of size of the species and fossil collection sites. Such linear correlation of the ratios shows that the Jinju specimens fit better than the Haman specimens. Some of internal structures in this species are also observed: median lines (Fig. 3H arrow) and point bar type internal structures (Won, 2015); meniscate structure documented by Uchman et al. (2009) are not present.

Figure 4. Morphometric parameters and statistical analysis of the 64 specimens of Cochlichnus found from the Jinju Fm. (red circle) and the Haman Fm. (black triangle).

Table 1 . Morphometric information of Cochlichnus specimens.

no.	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
Jinju Formation	amp.	2.4	2.6	2.7	1.0	3.1	1.0	3.4	7.2	6.1	7.4	4.6	2.2	2.5	2.6	2.4	7.7	7.9	2.6	3.2	2.5
w/l	5.8	6.1	6.0	2.6	7.6	2.3	7.5	17.8	16	23.5	11.8	6.7	7.1	6.3	4.2	17	17.3	6.2	8.0	5.7
wid.	0.6	0.8	0.7	0.4	0.8	0.4	0.8	2.3	2.0	2.7	1.3	1.1	1.0	0.8	0.8	3.0	3.5	0.6	1.0	1.1
Haman Formation	amp.	0.9	1.55	0.95	2.1	0.9	1.3	3.3	4.6	2.1	3.5	7.5	3.2	3.6	1.1	1.2	6.5	6.1	6.1	7.2	4.5
w/l	1.5	4.1	2.2	6.2	2.8	4.4	12	15	9.6	11.6	28	10.5	18	1.6	1.9	23	20	11	23	16
wid.	0.2	0.55	0.65	0.9	1.2	1.5	1.6	2.1	2.4	2.5	3.4	3.9	4.5	1.4	1.0	2.8	2.7	3.6	3.8	2.0

no.	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41
Jinju Formation	amp.	2.9	2.2	1.1	2.6	6.2	7.9	3.0	5.5	2.5	7.7	7.4	2.7	4.5	1.1	6.6	7.8	2.9	11.2	3.8	2.3	3.6
w/l	7.1	6.5	2.4	5.4	14.5	16.6	6.2	13.6	6.2	13.3	15.2	6.6	11.2	2.3	16.7	15.8	6.4	23.6	8.0	4.8	9.5
wid.	1.2	1.3	0.4	1.1	2.2	2.5	1.0	1.8	0.7	4.0	3.0	0.9	2.0	0.2	2.6	2.4	0.5	5.6	1.0	0.9	1.8
Haman Formation	amp.	4.8	4.6	7.2	○. average amplitude of Cochlichnus in the Jinju Formation : 4.16 mm, Haman Formation : 3.69 mm. ○. average wavelength of Cochlichnus in the Jinju Formation : 9.69 mm, Haman Formation : 12.07 mm. ○. average width of Cochlichnus in the Jinju Formation : 1.53 mm, Haman Formation : 2.26 mm.
w/l	21	28	6.2
wid.	2.5	2.6	4.1

amp.: amplitude, w/l: wavelength, wid.: width.

3.2. Systematic Discussion

The Haman specimens illustrated in this paper are from the Ms. thesis of the first author Won (2015), and these specimens are housed temporarily in the paleontology lab of Kyungpook National University. All other specimens were collected in the construction site of Jinju Aviation Industrial Complex, and housed in the Jinju Pterosaur Footprints Museum.

Diagnosis. – Unbranched, continuously and regularly meandering grooves or ridges that resemble at least one full sine wave. Successive waves may gradually diminish in amplitude.

Description. – The specimens are preserved both in positive and negative relief and have characteristic feature of a genus Cochlichnus, showing a more or less regular sine-curved wavelength. The species varies in morphology: width between 0.2 and 5.6 mm, wavelength between 1.5 and 28 mm, and amplitude between 0.9 and 7.9 mm. The ratio of wavelength to amplitude was approximately less than 1, showing a linear correlation. Internal structures such as median lines are observed in some specimens. In some specimens, stretched parts are often observed with very low amplitude.

Tracemaker. – The ichnogenus Cochlichnus has been generally interpreted as feeding activity (grazing traces) or locomotion, but there is still debate about the reliable tracemaker (Michealis, 1972; Buatois et al., 1997; Gibert and Ekdale, 2002; Uchman et al., 2009). Hitchcock (1858), when he established the genus, suggested annelids as a tracemaker. Nematodes worms also has been traditionally regarded as the most likely producers of this trace (Moussa, 1970). The nematodes, when swimming and crawling, use only longitudinal muscles in body wall, resulting in characteristic sinusoidal shaped traces that is very similar to ichnogenus Cochlichnus (Cronin et al., 2005; Malvar et al. 2017). The numerous experimental studies of swimming nematodes (Caenorhabditis elegans) also support these results (Koelle and Horvitzet 1996; Cronin et al., 2005; Ulrich, 2012). The traces produced by nematodes, however, are generally much smaller (wavelength < 0.5 mm) than Korean Cochlichnus specimens (Table 1, Fig. 4). Several authors also suggested that the insect larvae living in the shallow water leave similar structures (Toula, 1908; Metz, 1987). Indeed, several insect larvae (family Therevidae and Ceratopogonidae) have been shown to produce similar structures, but no further experiments or observations have been sustained to support this. We, here in this paper, cannot determine the exact tracemaker of the Cochlichnus, but at least the legless nematodes and insect larvae seem to be the most appropriate producers.

Natural heritage including dinosaur fossils has traditionally been recorded through direct and indirect techniques such as visual observation, field survey, point-by-point survey, and photography (Hughes-Hallett et al., 2021). In recent years, the advanced technologies based on various hardware and software including drones, 3D scanning, RTI, and image processing have been actively used for research and exhibitions (Sohmura et al., 2000; Kong et al., 2011). Such technologies in Korea have recently been successfully applied to dinosaur footprints (Kong et al., 2010, 2011). Although recent successes on dinosaur footprints using RTI technique, it has not been attempted on very small trace fossils. This is because, unlike vertebrate traces, invertebrate trace fossils are difficult to study due to their low relief and small size. We tested, in this paper, RTI (Reflectance Transformation Imaging) technique, in order to enhance the image quality of trace fossil Cochlichnus.

RTI, a new imaging technique developed by Hewlett Packard Labs, is a computational photography using multi-lighting conditions that can observe objects surfaces more clearly in three dimensions. With a range of computational enhancements, this method allows the viewer to examine the visual appearance of an object in various lighting conditions, which highlights and reveals characteristics of the imaged object (Harris and Piquette, 2015; McEwan, 2018). In order to obtain image set, two different photography methods are used: highlight RTI and dome RTI. Highlight RTI, a technique in which the researcher manually adjusts lighting and camera angles, is commonly applied to outdoor immovable objects, while dome RTI is automatic photography using a dome-shaped lighting equipment and camera in a laboratory, which is more suitable for very small and movable objects.

The dome RTI is more appropriate in the case of a trace fossil Cochlichnus characterized by a very small size and low relief. However, the surface information of the fossil is too meager (i.e., relief is too shallow), so a small number of lights (40-60) placed in our dome equipment does not seem to produce good quality. Therefore, we photographed the two Cochlichnus specimens manually in a laboratory in order to obtain a photometric image by directly irradiating light at a low angle. This is because the RTI technology developed recently by the Cultural Heritage Administration, Korea has enabled photography of objects smaller than 2 cm. For photography of the Cochlichnus trace fossils, 50 to 80 images were taken per set, and many images were taken with photometric lighting close to the surface and horizontally (Fig. 5).

Figure 5. Photographing trace fossils using RTI (Reflectance Transformation Imaging). (a) surface vector of an object. (b) vector value depending on the direction of light reflected off an object. (c) numerical information of surface vectors per pixel. (d, e) highlight RTI suitable for outdoor objects. (f) dome RTI suitable for indoor objects. (a, b, c from https://culturalheritageimaging.org)

Figure 6 shows a series of improved images of the small trace fossil Cochlichnus by RTI technology. The pictures of A1 and A2 were taken with a regular camera, while the B1 and B2 are RTI images using a filter that enhances surface contrast by applying a specified coefficient to color data and normal data. Comparing photographs of A1 and A2 with B1 and B2 respectively, it is clear that the surface contrast became sharper and messy information disappeared. The pictures of C1 and C2 are RTI rendering images obtained using filters to control RGB color, reflectivity, and the intensity of light. That is, the image is seen more clearly by the digital rubbing effect. The surface contrast of the image became clearer. When an image visualized in color for exhibitions is necessary, color images such as Fig. 6 D1 and D2 can be obtained by visualizing the normal vector information for the surface shape of the fossils.

5.1. Taxonomic Consideration

Since the ichnogenus Cochlichnus were established in 1858, many authors have debated the reliability of the genus and type ichnospecies C. anguineus (Pickerill, 1981; Kim et al., 2005; Chen et al., 2012; Uchman et al., 2004, 2009). For example, Stanley and Pickerill (1998) regarded Cymatulus a junior synonym of Cochlichnus based on the fact that there is little difference between the two genera. Our specimens illustrated also have characteristic features of the genus: a more or less regular sine-curved ridge. However, each specimen is slightly different from each other in width, wavelength, amplitude and internal structure (Table 1, Fig. 3, 4). Such morphometric parameters including a general shape are diagnostic factors that differentiate species, specially trace fossils characterized only by morphology (Bertling et al., 2022).

In classification of trace fossils, morphology (specially general shape) is one of the most important factors because trace fossils are initially and easily recognized by their morphology. In this respect, the trace fossils illustrated in this paper are consistent with the ichnogenus Cochlichnus. The specimen (Fig. 3C arrow) with a straight stretched part in the middle of the sine curve also meets the condition of this genus. The internal structure is another factor in classification of trace fossils. Diverse internal structures have been reported, which includes presence or absence of median line and wall structure. They are, in general, regarded as movement of organisms who built the traces (Bertling et al., 2006). Several internal structures including a median line are also observed in our specimens (Fig. 3H), but it is not certain that these structures distinguish the species.

Size, that is, width is even more important for species-level ichnotaxonomy, and indeed many ichnospecies tend to be defined within specific size ranges (Nielsen et al., 2003; Bertling et al., 2006). The majority of Cochlichnus illustrated in this paper is 2.5 cm in average width (Jinju specimen 2 cm, Haman specimen 4 cm). Large Cochlichnus specimens with a thickness of more than 4.5 cm (Fig. 3G) are occasionally found. In the large one, all characteristics except size also fit well with a ichnogenus Cochlichnus. In particular, the ratio of wavelength to amplitude of the large one correlates with the species as a whole (Fig. 4). Although, as Bertling et al. (2006) suggested that it may be necessary to reconsider the role of size as an ichnotaxobase, we temporarily identified specimens in all size ranges (0.2 – 4.5 cm) as Cochlichnus anguineus.

5.2. Application of RTI Methods to Study the Trace Fossils

RTI is a recently developed technology, and the Cultural Heritage Administration, Korea has been developing a technology more suitable for Korean natural heritage. Recently, it has begun to be used for recording natural heritage, especially dinosaur footprints. Using RTI technology, one may obtain clearer images and remove unwanted information of the natural heritage, so it is considered to be very useful for exhibitions and scientific researches (Hughes-Hallett et al., 2021). In particular, it is expected to be very useful in the case of natural heritage that has difficulties in research and exhibition due to its small size and very low relief as seen in tiny trace fossils. In this paper, we tested the effectiveness of image improvement by applying RTI technology to two specimens of Cochlichnus (Fig. 5, 6). The tests seem to be successful in many respects, because image contrast became sharper with a decrease in messy information. We tested only two specimens, and thus it is not yet clear whether our method is the best one. In particular, techniques to obtain color images by visualizing normal vector information on surface shapes seem incomplete.

RTI is a computer-based photography method that captures the surface topography information and color of a subject and re-illuminates the subject from all directions and angles. Through this, it is possible to obtain three-dimensional visual information that cannot be obtained with general photographs. Indeed, photography and 3D scanning that are used in the field of natural heritage have difficulties in three-dimensional observation of shallow surface information such as intaglio and relief. It would be effective, therefore, to use the RTI technology known as digital tapering, in order to obtain the clearer 3D images. In addition, compared to the normal photography method, RTI can obtain a three-dimensional effect by the way to observe fine curves or traces using light and shadow, rather than the result taken at a specific lighting angle. Because the camera's image sensor-based data in RTI technology is acquired unlike a 3D scanner that uses a laser light source, more detailed and accurate photography of the image is also possible. RTI is a new technology in the beginning stage and is currently being used in many fields, such as preservation and analysis of cultural properties, monitoring, and research on experimental archeology-based production techniques (Harris and Piquette, 2015; Frank, 2015; Hughes- Hallett et al., 2021). Therefore, for the study of fossils that are not easily observed with the naked eyes, it is necessary for paleontologists to consider the methods to apply RTI techniques.

In this study, a total of 64 specimens of trace fossils were documented, which were collected from the Jinju Formation (41 specimens) and the Haman Formation (23 specimens). The fossils are generally in the shape of more or less regular sine-curved ridge, which is a characteristic feature of the ichnogenus Cochlichnus. The fossil varies in morphology: width between 0.2 and 5.6 mm, wavelength between 1.5 and 28 mm, and amplitude between 0.9 and 7.9 mm. The ratio of wavelength to amplitude is approximately less than 1, showing a linear correlation. Taking all morphometric parameters, specimens in all size ranges are temporarily identified as ichnospecies Cochlichnus anguineus.

In order to obtain more distinct and clearer images of trace fossil Cochlichnus, we selected two specimens and applied a new imaging technology RTI. The samples selected for RTI were photographed manually in a laboratory in order to obtain a photometric image by directly irradiating light at a low angle. For photography of the trace fossils, 50 to 80 images were taken per set with photometric lighting close to the surface and horizontally. RTI technology clearly showed that the images of tiny fossils could be improved. When comparing images with RTI technology and images from normal cameras, the digital rubbing effect was obvious: the surface contrast become sharper and messy information disappeared. Currently, RTI technology is used in many fields including preservation of cultural properties and archaeology. As a consequence, we hope to apply this technique to the field of paleontology, especially to the study of trace fossils of very small size.

We would like to thank Theo Sohn, CEO of the Technology Research Institute for Culture & Heritage (TRIC) and Dr. Min Ji-hyun, who helped us with the outdoor and indoor RTI work for this study. We are also grateful to Prof. Seong-Joo Lee, Kyungpook National University for helping with English proofreading, and to the anonymous reviewers who improved the quality of the manuscript. A special thanks goes to officials of the Cultural Heritage Administration, Jinju City Hall, and Jinju Pterosaur Tracks Museum. This study was carried out as part of a research project on exhibition and utilization of maritime cultural heritage in the central part of the West Sea of the National Research Institute of Maritime Cultural Heritage.