The 20-million-year old lair of an ambush-predatory worm preserved in northeast Taiwan

The feeding behavior of the giant ambush-predator “Bobbit worm” (Eunice aphroditois) is spectacular. They hide in their burrows until they explode upwards grabbing unsuspecting prey with a snap of their powerful jaws. The still living prey are then pulled into the sediment for consumption. Although predatory polychaetes have existed since the early Paleozoic, their bodies comprise mainly soft tissue, resulting in a very incomplete fossil record, and virtually nothing is known about their burrows and behavior beneath the seafloor. Here we use morphological, sedimentological, and geochemical data from Miocene strata in northeast Taiwan to erect a new ichnogenus, Pennichnus. This trace fossil consists of an up to 2 m long, 2–3 cm in diameter, L-shaped burrow with distinct feather-like structures around the upper shaft. A comparison of Pennichnus to biological analogs strongly suggests that this new ichnogenus is associated with ambush-predatory worms that lived about 20 million years ago.

www.nature.com/scientificreports/ The island of Taiwan uplifted during the Penglai Orogeny, which recorded the collision between the Luzon Arc on the Philippine Sea Plate and Eurasian Plate starting about 5 million years ago 21,22 . Along the Northeast Coast of Taiwan, late Oligocene to Pliocene passive margin strata consist of shallow marine to coastal swamp deposits, and record a number of transgressive-regressive cycles [23][24][25] ; these deposits are exquisitely exposed due to a series of imbricated thrust faults (Fig. 1a,b).
The Yehliu Sandstone Member is part of the early Miocene Taliao Formation (~ 22-20 Ma) and is extensively exposed along the Northeast Coast 26,27 . It is interpreted as recording deposition in lower shoreface to upper offshore paleoenvironments 23,[28][29][30] , and is encased between mudstone-dominated members of the Taliao Formation 29 (Fig. 1c). The lower part of the Yehliu Sandstone Member comprises highly bioturbated sandstone with numerous marine trace fossils, and with intermittent scoured sandstone beds containing shell debris (Fig. 1c-e; Supplementary Fig. S1). Within the bioturbated sandstone layers at both Yehliu Geopark and Badouzi promontory (Fig. 1c), large, L-shaped burrows, named herein as Pennichnus formosae n. isp. (the complete systematic ichnology can be found as Supplementary Discussion 1) are locally abundant.
The sandstone beds containing Pennichnus at both Yehliu and Badouzi are at the same stratigraphic position ( Fig. 1c-e); however, Yehliu is considered to preserve deposition of a higher-energy, shallower-water environment that reflects a northwestward shallowing paleo-bathymetry 29 . When Pennichnus co-occurs with Phycosiphon, Schaubcylindrichnus, and Thalassinoides, the number of specimens reaches 1.7 m −2 (between 7.5 and 9.9 m, Fig. 1e); this is the highest density among all Pennichnus-bearing strata ( Supplementary Fig. S2). When Pennichnus co-occurs with Ophiomorpha, Teichichnus, and Skolithos, the number of specimens drops to 0.5 m −2 (17.8-19.

Morphology of Pennichnus.
The morphology of Pennichnus is reconstructed using 319 specimens observed in Yehliu Geopark and Badouzi promontory. The overall configuration of Pennichnus is that of an L-shaped, smooth-walled burrow that can exceed 2 m in length. In vertical section, the burrow diameter is typically 2.5 ± 0.6 cm at the top of the burrow, and this tapers down-burrow to 2.0 ± 0.6 cm near the end of the horizontal section ( Supplementary Fig. S3). Most examples of Pennichnus are filled with sediment similar to the host sediment or with sandstone that is less muddy than the host sediment, and this suggests that the burrows were passively filled after the trace maker either abandoned the burrow or died 33 .
Pennichnus consists of three distinct parts ( Supplementary Fig. S4). The upper part is composed of a vertical shaft with a funnel aperture surrounded by inverted, cone-in-cone structures, and which show a feather-like appearance in vertical cross-section (referred to as feather-like structures; Fig. 2a; Supplementary Figs. S5 and S6). The vertical part of the burrow comprises about 40% of the total burrow length, and is connected to a gently curving middle section (10% of the total length), and then a lower horizontal section (50% of the total length; Fig. 2b; Supplementary Fig. S5). Downward along the burrow, the feather-like structures gradually disappear ( Fig. 2a; Supplementary Fig. S5). In bedding plane view, the funnel aperture displays a circular to oval shape surrounded by concentric laminations, and this is similar to the opening around modern Bobbit worm burrows ( Fig. 2c-e; Supplementary Fig. S5).
Among the 157 well-exposed Pennichnus specimens, 82.2% of specimens show no aberrant morphological features, while 17.8% show one or two (Supplementary Fig. S7; Supplementary Table 1). Specifically, 12.1% of well exposed Pennichnus specimens are preserved with a homogenous burrow fill penetrated by an inner tube, 3.2% display evidence of active backfilling in parts of the burrow but without an inner tube, and 2.5% of them display both evidence of active backfilling, and an inner tube. (Supplementary Table 1). The fill of the inner tube is homogenous. Similar thin tubes were observed in the sediment surrounding Pennichnus with no apparent connection to the burrows and this suggest that when found inside the trace fossil, they may represent secondary occupation of an abandoned burrow rather than comprising part of Pennichnus.
Geochemical characterization of Pennichnus. X-ray fluorescence (XRF) core scanning reveals that there is an abrupt increase in iron (Fe) in the burrow lining and the deformed sediment surrounding the burrow (Fig. 3a). The deformed sediment corresponds to the innermost part of the feather-like structures (Fig. 2a). Energy Dispersive X-ray Spectrometer (EDS) analysis done using scanning electron microscope also shows low levels of Fe in the outer feather-like structures and higher Fe in the innermost part (Fig. 3b). Thus, the featherlike structures are divided into: (1) an inner disturbed zone, which is the deformed sediment immediately surrounding the burrow and with a high Fe concentration; and (2) an outer feather-like collapse structure zone with deformed sediment and low Fe concentration.

Discussion
Iron variation laterally through the upper shaft ( Fig. 3) suggests that Pennichnus were produced by mucussecreting animals which repeatedly rebuilt the burrow opening through time. When marine invertebrates, especially soft-bodied infauna, construct their burrows, they usually secrete mucus to strengthen the burrow wall.
Organic-rich compounds in the mucus then attract microbes such as sulfate-reducing bacteria that feed on the carbon leading to the formation of reducing conditions which triggers iron sulfide to precipitate in the lining [34][35][36][37] . During uplift and erosion, the diagenetically altered burrows are exposed to oxidizing pore waters, and the iron sulfide is oxidized to iron hydroxides such as hematite, limonite or goethite 37 ; this results in elevated Fe levels in the burrow lining and disturbed zone . In contrast, low Fe concentration in the feather-like collapse structures (Fig. 3) suggests that these features were not affected by mucus, but rather formed through the collapse of sediment surrounding the burrow opening.
There are multiple trace makers that can produce large, lined burrows like Pennichnus and the three most probable animal groups are infaunal shrimp; fast-burrowing, siphonated bivalves; and, large polychaetes. Shrimp are well known for constructing open burrow networks 38,39 and sometimes their burrows contain an inner tube potentially constructed by juvenile shrimp that re-occupy abandoned or infilled burrows 40 . However, shrimp burrow networks constructed in water-saturated sand typically comprise complicated shapes, like mazes or boxworks, and have turn-around chambers 41,42 . As well, shrimp burrow walls in water-saturated sand are normally reinforced with mud pellets 38 . Although previous studies showed that sediment collapse near the opening of abandoned shrimp burrows can form nested funnels which look similar to the feather-like structures of Pennichnus 38,43 , the nested funnels in shrimp burrows only occur above the aperture, rather than along the margin  Fig. S8). As well, Pennichnus specimens lack pelletoidal wall reinforcement, branching structures or turn-around chambers, and this suggests that shrimp are probably not the producers of Pennichnus.
With the presence of a partial meniscate backfill and inner tube in some Pennichnus specimens, another possible trace maker are fast-burrowing siphon-feeding bivalves such as Ensis directus (Atlantic razor clam) and Tagelus plebeius (Stout razor clam) [44][45][46][47] .These bivalves can build burrows up to 70 cm long 46 , and potentially produce collapse structures surrounding the actively backfilled burrows 44,45,48 . The maximum burrowing depth of these organisms is controlled by the strength of the substrate, which increases with depth 44,45 . The increase in sediment strength could explain the horizontal orientation of the burrows at a certain depth. However, the www.nature.com/scientificreports/ circular cross-section of Pennichnus is at odds with the shape of fast-burrowing bivalves, which tend to generate more almond-or keyhole-shaped burrows 49 . Moreover, rapid-burrowing bivalves excavate the sediment through local fluidization around their bodies 44 , and this commonly results in sediment collapse in their wake 49 . When sediment collapses into the cavity left by the bivalves, it blurs the burrow wall, which produces a burrow morphology that is very distinctive from the clear burrow wall preserved in Pennichnus. Furthermore, only few Pennichnus specimens (17.8%) display a backfill and/or inner tube, and no bivalve shells were observed within any Pennichnus even though shell material is common in the surrounding sediment. This suggests that fastburrowing bivalves are not the trace maker. The morphology of Pennichnus suggests that the trace maker was a large, slender organism that supported the burrow with its body, similar to how polychaetes do 47 . In addition, feather-like collapse structures and a disturbed zone surrounding the burrow lining indicates extensive sediment disturbance at shallow depths. These morphological features of Pennichnus are consistent with the activities of an ambush predator, and hence, we hypothesize that giant polychaetes, such as Bobbit worms, are the most probable trace makers (Fig. 2c). Bobbit worms are ambush-predators, and require long burrows to accommodate their 2 to 3 m-long and 2 to 2.5 cm-diameter bodies 3 . Their burrow morphology (Fig. 2d) and hunting behavior in modern environments explains the funnel opening and feather-like structures observed in Pennichnus. After each feed, the ambush-predatory polychaete re-establishes its burrow opening resulting in an accumulation of mucus linings, and this explains the occurrence of a Fe-enriched, disturbed sediment zone around the upper shaft. Sediment disruption after the Bobbit worm's retraction is also consistent with the feather-like collapse structures. The body movements of Bobbit worms, as with all polychaetes, are achieved by controlling hydrostatic skeletons and parapodia, which give them various ways to move, including crawling, undulation and peristalsis 50,51 . Sedentary burrowing polychaetes, including ambush predators, mostly employ peristaltic locomotion when they burrow 52 , which produces a smooth-walled burrow without sinuosity. Since polychaetes respire through their body wall 53 , they might burrow horizontally at depth to avoid low-oxygen pore waters. This may explain the horizontal development of Pennichnus in the lower part of the burrow. Alternatively, the increasing energy required to excavate sediment at depth 44,45 may also prompt the trace maker to burrow horizontally in less compacted sediment.
To summarize, we hypothesize that about 20 million years ago, at the southeastern border of the Eurasian continent, ancient Bobbit worms colonized the seafloor waiting in ambush for a passing meal (Fig. 4). When prey came close to a worm, it exploded out from its burrow, grabbing and dragging the prey down into the sediment. Beneath the seafloor, the desperate prey floundered to escape, leading to further disturbance of the sediment around the burrow opening. The retreat of the ancient Bobbit worm and prey into the sediment caused the sediment to form the distinct feather-like collapse structures preserved in Pennichnus formosae. Upon consumption of its prey, the worm re-established its burrow, leading to the Fe-enriched disturbed zone surrounding the burrow wall. uses its strong jaws to catch the prey (e.g., fish) passing by the burrow opening (see video at https ://www.mmora a.com/video ). (c) As the struggling prey is pulled into the burrow, the sediment collapses around the aperture to form feather-like collapse structures surrounding the upper burrow. Between the burrow and featherlike collapse structures is a disturbed zone caused by the repeated feeding action of the worm and burrow re-establishment that results in an accumulation of mucus lining over time. Scale bar = 30 cm.

Scientific Reports
| (2021) 11:1174 | https://doi.org/10.1038/s41598-020-79311-0 www.nature.com/scientificreports/ In terms of the temporal and spatial distribution of Pennichnus trace makers, the eunicid polychaetes went through an evolutionary radiation in the Ordovician 54,55 and giant jaw-bearing polychaetes have been described as far back as the Devonian 9 . In consequence, the record of Pennichnus, representing the predatory behavior of giant worms, may extend back to the Paleozoic. Pennichnus specimens described from Yehliu and Badouzi sections occur with trace fossil assemblages typical of the Cruziana Ichnofacies in a depositional environment close to storm-wave base level on a broad continental shelf. Thus, a similar offshore to shelf setting is probably where Pennichnus are likely to be found. However, we note that Pennichnus has not been described previously from strata that are interpreted to represent similar paleoenvironments as the Taliao Formation, suggesting that the occurrence of Pennichnus is controlled by a number of coinciding environmental conditions. Furthermore, although eunicid polychaetes are identified as the most probable trace maker in this study, we can't exclude the possibility that other vermiform invertebrates besides eunicid polychaetes could produce Pennichnus when engaging in similar behavior.
In conclusion, the distinctive configuration, geometry and internal structure of the burrows described herein supports the erection of a new ichnogenus, Pennichnus, which records the behavior of giant predatory worms beneath the seafloor (Supplementary Discussion 1). The interpreted activities of the Pennichnus trace maker records a life and death struggle between predator and prey, and indirectly preserves evidence of more diverse and robust paleo-ecosystem than can be interpreted from the fossil and trace fossil record alone.

Methods
Morphological descriptions. The morphology of Pennichnus is reconstructed based on observations of 319 specimens in Yehliu Geopark and Badouzi promontory ( Supplementary Fig. S1a,b). Each Pennichnus specimen was photographed and morphological parameters including vertical extent, horizontal extent, burrow diameter, and width of the feather-like structures were measured. Owing to its large size, nearly all specimens were only partially exposed, so we classified the exposed section as representing the upper (U), middle (M) and/ or lower (L) part of the burrow (Supplementary Fig. S4). Measurements taken from the specimens preserving one of the three parts of the burrow were then compiled to quantify burrow morphology.
Serial grinding. To overcome limited exposure and to better understand the internal structure of the burrow, selected specimens were ground down in the field using a portable sander (Makita DBO180 18v Random Orbit Sander). Specimens were ground layer-by-layer in increments < 1 mm providing serial sections through the trace fossil, and each successive layer was photographed ( Supplementary Fig. S9).
Computed tomography (CT) scanning. Five specimens were cut out of the outcrops for detailed analysis. To assess the 3D geometry of the burrow and surrounding feather-like structures, one specimen was examined by CT scanning (LightSpeed Ultra16; GE Healthcare Japan Corporation, Japan, 120 kV, 100 mA, slice thickness: 0.625 mm) at the Center for Advanced Marine Core Research (CMCR), at Kochi University in Japan. The CT scanning data were then processed using OsiriX imaging software ( Supplementary Fig. S6c,d).
SEM-EDS analysis. The mineral composition and clast arrangement of the burrow wall, fill, and host rock was assessed using petrographic thin sections studied with an optical microscope. However, the feather-like structures and wall-lining were difficult to identify under the microscope and were therefore analyzed in thin sections using a thermal field emission scanning electron microscope (JEOL FE-SEM: JSM-7100F) in the EPMA lab at Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan. Chemical identification was carried out using an Energy Dispersive X-ray Spectrometer (EDS: Oxford Instruments X-max80 with INCA-350), equipped on the FE-SEM. Each sample was mounted, polished, and analyzed with FE-SEM and EDS under low vacuum conditions (50 Pa), using an acceleration voltage of 15 kV and beam current of 0.12 nA. The duration of EDS counting time was 15 s for each spot.

X-ray fluorescence (XRF) analysis. In order to measure elemental variations between different parts of
Pennichnus, two samples were studied with both an Itrax XRF Multiscanner (Department of Physical Geography, Stockholm University), and an Itrax Core Scanner (Department of Geoscience, National Taiwan University). Both machines are manufactured by Cox Analytical Systems.
Construction of the stratigraphic columns. Stratigraphic columns including lithology, sedimentology, ichnology and body fossils were generated for the Yehliu Sandstone Member at both Yehliu and Badouzi. For each bed, grain size, sedimentary structures and bed thicknesses were recorded. Ichnological analysis focused on identifying trace fossils and assessing the degree of bioturbation with the bioturbation index 56 . The density of Pennichnus specimens was quantified by dividing the number of specimens by outcrop area.
Nomenclatural acts. Following the rules of International Code of Zoological Nomenclature (ICZN) 57 , this study and its nomenclatural act are registered in ZooBank, the online registration system governed by ICZN, to validate the new ichnotaxon. By keying LSID (Life Sciences Identifier) in ZooBank website, associated information of this work can be viewed. The LSID for this publication is: urn:lsid:zoobank.org:pub:2B03334F-27D3-4583-B057-2B5C1E321AEF. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.