Introduction

The trace fossil Polykampton Ooster, 1869, typified by P. alpinum Ooster, 1869 from the Paleogene flysch deposits of Switzerland, was almost forgotten for more than a century and treated as a monospecific ichnogenus (Häntzschel, 1975; Seilacher, 2007). Discoveries of four new ichnospecies of Polykampton from Cretaceous–Oligocene flysch deposits (Wetzel and Uchman, 1997; Uchman and Rattazzi, 2018; Uchman et al., 2019, 2020) showed that this ichnogenus is more diverse and widespread than previously thought. In this paper, a sixth named ichnospecies of Polykampton is described and interpreted. The new ichnospecies derives from the Maastrichtian–Paleogene deep-sea deposits of the Northern Apennines in Italy. The objectives of this study are to add to the knowledge on: (1) the general increase in diversity of deep-sea trace fossils since the Late Cretaceous, (2) the diversification of their nutritional strategies since the Late Cretaceous (Uchman, 2004), and (3) the competitive behavior of burrowing invertebrates in generally oligotrophic environments.

Geological setting

The study area is characterized by the Monte Antola Unit of the Northern Apennines, specifically, in the Val Borbera region and its surroundings, north of Genova in northwestern Italy (Fig. 1). The Monte Antola unit is probably an allochthonous tectonic slab of Ligurid units (e.g., Marroni et al., 2001 and references therein). The described trace fossil occurs in the stratigraphically higher part of the slab, which is composed of the Bruggi-Selvapiana Formation overlain by the Pagliaro Formation.

The Bruggi-Selvapiana Formation (late Campanian–late Maastrichtian) is a c. 500 m thick succession of thick- and very thick-bedded hybrid turbidites. Their lower part is composed of siliciclastic material, and their upper part consists mainly of marlstones (marlstone understood as hard rock of the composition as in marl) in the upper part, which alternate with subordinate thin-bedded sandstones and shales (Abbate and Sagri, 1967). In the Borbera Valley, this unit is thinner than elsewhere because it pinches out here (Levi et al., 2006).

The Pagliaro Formation (Bellinzona et al., 1971; Marroni et al., 2001, 2002; Levi et al., 2006) is about 300–400 m thick (Abbate and Sagri, 1967) and is dated to nannoplankton NP1–NP5 zones of the early Paleocene to early late Paleocene (Marroni et al., 2001) or to the CC25b–NP5 zones of the late Maastrichtian–early late Paleocene (Levi et al., 2006; Catanzariti et al., 2007). In the lower to middle part, the Pagliaro Formation consists mainly of thin- to thick-bedded turbidites, whose beds are composed of sandstones and/or siltstones becoming gradually replaced by turbiditic marlstone and/or shale. At the top of the beds, the shales represent partly the pelagic and hemipelagic background sedimentation. They are commonly dark gray and noncalcareous. The silty or muddy turbiditic marlstones are massive and light gray, dark gray, or pale rose in color. Some of these beds are up to a few meters thick. The sandstones are rich in plant detritus. Commonly, thinner beds display the Ta–d Bouma intervals. Locally, the beds thicken in upward trends in packages about 10 m thick. In the upper part of the formation, the marlstone beds are less common. The formation is gently tectonically deformed, with folds, faults, and internal detachments, especially in the middle and upper parts (Marroni et al., 2002; Levi et al., 2006). Trace fossils of the Pagliaro Formation are very diverse and locally abundant; they are typical of the deep-sea Nereites ichnofacies (Uchman, 2007; Uchman and Rattazzi, 2018; Uchman et al., 2019).

Figure 1.

A general location map with inserts containing indications of the sections in detail. (1) Campo dei Re section and Fubbiano section. (2) Salata section. (3) Rio Ferré section. Location of the study region is indicated by the gray quadrangle in the upper right insert.

The Pagliaro Formation is overlain unconformably by lower Oligocene fan-delta conglomerates of the Savignone Formation and locally by other Oligocene sediments, mainly the Ranzano Formation (e.g., Gelati and Gnaccolini, 1978; Ghibaudo et al., 1985; Gnaccolini, 1988). The conglomerates pass upward into the kilometer-thick series of turbidites of the Monastero Formation (Bellinzona et al., 1971; Mutti et al., 1995; Marroni et al., in press). In the lower part, the turbidites are locally conglomeratic, while pelitic facies prevail in the upper part. Locally, pebbly mudstones and slump deposits are present. The calcareous nannoplankton matches the NP23 or NP24 zones of late Rupelian to early Chattian (Marroni et al., in press). These lithostratigraphic units belong to the filling of the episutural Tertiary Piemonte Basin.

The described Polykampton occurs in five sections, four in the middle part of the Pagliaro Formation (Fig. 1), the Campo dei Re section (44°39.440′N, 009°02.438′E), the Salata (44°37.063′N, 009°03.042′E) and Fubbiano (44°40.086′N, 009°03.035′E) sections, and one section in the Bruggi-Selvapiana Formation in the Rio Ferré (44°38.283′N, 009°06.217′E). At the first locality, it was found in a marlstone bed outcropping in a few-meters-high escarpment on the right side of a ravine (Fig. 2). Most specimens derive from this bed. In the Salata section, turbiditic deposits a few tens of meters thick are exposed in a steep ravine. The trace fossil was found in a marlstone bed on the left bank of the stream close to the bottom of the ravine (Fig. 2). In the Fubbiano section (an escarpment in the river valley) and Rio Ferré sections (a stream bed in a ravine), specimens of the described Polykampton had been found occasionally in previous years. Those specimens derive from beds that have recently been covered, so detailed sections cannot be presented.

Materials and methods

Sections at Campo dei Re and Salata were measured bed by bed, and collected specimens were hand prepared. For better contrast between host rock and trace fossils, some split surfaces were oxidized with a weak acid. The same effect is obtained after at least a few months of weathering in natural conditions. The size measurements are made by means of mechanical calipers.

Repositories and institutional abbreviations.—Type material and other specimens examined in this study are deposited in the following institutions: The Crocefieschi Museum near Genova in Italy and the Nature Education Centre of the Jagiellonian University (CEP) – Museum of Geology (Kraków, Poland). Specimens of the new Polykampton ichnospecies labeled with four-digit numbers are housed in the Crocefieschi Museum. Specimens in the Nature Education Centre of the Jagiellonian University (CEP) – Museum of Geology are under the acronym INGUJ249P. Several specimens are composed of two pieces of a split bed (labeled as “a” and “b” after the number [in CEP] or as “bis” for one of them, which is the counterpart [in the Crocefieschi Museum]).

Systematic ichnology

Figure 2.

Fragments of sections of the Pagilaro Formation at Campo dei Re and Salata with indication of trace fossils and distribution in the bed bearing Polykampton.

Figure 3.

Types of Polykampton recurvum n. isp. from the Pagliaro Formation in the Campo dei Re section. (1) The holotype, specimen 7335. (2) Paratype 1, INGUJ149P230b. Chi = Chondrites intricatus. (3, 4) Details of (2). (5) Paratype 2, specimen 7333 bis. (6) Paratype 3, INGUJ149P231b.

Figure 4.

Selected specimens of Polykampton recurvum n. isp. from the Pagliaro Formation in the Fubbiano (1, 2) and Campo dei Re (3–6) sections. Cha = Chondrites affinis; Chi = Chondrites intricatus; Pa = Palaeophycus isp.; Phg = Phycosiphon geniculatum. (1) Specimen 6755. (2) 6756bis. (3) 7137. (4) 7213 and 7213 bis. (5) 7145 and 7145bis. Red arrows point to intersection of lobes with the median tunnel. (6) 7136bis.

Figure 5.

Selected specimens of Polykampton recurvum n. isp. from the Pagliaro Formation in the Campo dei Re section (1, 2, 4–8) and from the Bruggi-Selvapiana Formation in the Rio Ferré section (3). Al = Alcyonidiopsis isp.; Cha = Chondrites affinis; Chi = Chondrites intricatus; Pa = Palaeophycus isp.; Phg = Phycosiphon geniculatum; Phi = Phycosiphon incertum; Plbd = Planolites with filling bioturbated with Chondrites (“Bandchondriten”). (1) Specimen 7148bis. (2) 7146. (3) 6718. (4) 7139. (5) Detail of (2), 7146; red arrow points to intersection of lobes with the median tunnel. (6) INGUJ149P244. (7) 7151. (8) INGUJ149P235.

Figure 6.

Selected specimens of Polykampton recurvum n. isp. from the Pagliaro Formation in the Campo dei Re (1–6) and the Salata (7) sections. Cha = Chondrites affinis; Chi = Chondrites intricatus; Cht = Chondrites targionii; Phg = Phycosiphon geniculatum; Pl = Planolites isp.; Th = Thalassinoides isp. (1) INGUJ149P237a. (2) INGUJ149P233. (3) INGUJ149P238. (4) INGUJ149P239a. (5) INGUJ149P231b. (6) INGUJ149P390. (7) INGUJ149P388b.

Figure 7.

(1) Morphometric parameters of Polykampton recurvum n. isp. (2) Width of subsequent lobes on both sides in specimen 7213. (3–8) Selected statistical relationships between some morphometric parameters.

Table 1.

Morphometric parameters of Polykampton recurvum n. isp., n = 45. For the parameter definitions, see Figure 7.1.

Distribution and associated trace fossils

Polykampton recurvum n. isp. occurs in single beds of beige marlstone. In the Campo dei Re section, the bed is 5 cm thick and covered by a 0.7 cm thick layer of gray noncalcareous mudstone (Fig. 2). In the Salata section, P. recurvum is present in a 22 cm thick marlstone bed having a 3 cm thick layer of marly siltstone at the base and a 2 cm thick layer of gray marlstone at the top (Fig. 2). After splitting of the beds approximately along a horizontal plane, P. recurvum is usually seen on both the lower and upper slit surface. The color contrast between the burrow filling and the host rock is greater after oxidized.

In the Campo dei Re section, Polykampton recurvum occurs in the same bed as Chondrites intricatus (Brongniart, 1823) (Figs. 3.1, 4.5, 5.1, 6.2, 6.4, 6.5, 6.7), Chondrites affinis (Sternberg, 1833) (Figs. 5.2, 5.5, 6.1, 6.3–6.6), Palaeophycus isp. (Figs. 4.5, 5.1, 5.7, 5.8), Planolites isp. (Fig. 6.2), Alcyonidiopsis isp. (Fig. 5.8), Phycosiphon geniculatum (Sternberg, 1833) (Figs. 4.6, 5.7, 6.1–6.5), and possibly Thalassinoides isp. (Fig. 6.2). Considering the penetration depth of these trace fossils from the top of the beds, Polykampton recurvum occupies a middle tier; only Chondrites intricatus more deeply and distinctly crosscuts P. recurvum.

In the same bed in the Salata section, Polykampton recurvum co-occurs with Chondrites intricatus (Fig. 6.7), Chondrites targionii (Brongniart, 1828) (Fig. 6.7), Chondrites affinis, Tubulichnium rectum (Fischer-Ooster, 1858), Phycosiphon geniculatum, and a simple, tubular burrow that is 8–9 mm wide (?Planolites isp.). The base of the bed contains Protopaleodictyon isp. Here, all the associated burrows descend below P. recurvum. In both sections, P. recurvum occurs 2–2.5 cm below the top of the marlstone.

The slabs bearing Polykampton recurvum from the Fubbiano section contain Palaeophycus isp. (Fig. 5.2). The slab with P. recurvum from the Rio Ferré section contain Chondrites intricatus (Figs. 4.4, 5.3), Chondrites affinis (Fig. 4.4), Phycosiphon geniculatum (Fig. 5.3), and Planolites isp., which was preferentially reworked by the Chondrites producer forming “Bandchondriten” sensu Ehrenberg (1942) (Fig. 5.3).

Discussion

The dark mudstone of the lobes and their meniscate structure prove that Polykampton recurvum n. isp. is actively filled with mud of similar lithology deposited above the top of the marlstone beds. This implies that the burrow has had an open connection to the seafloor and was formed when the mud had already accumulated as background sediment after deposition of the marly turbidite (Fig. 10). The connection to the seafloor was not detected in the rock although some effort was made for that purpose. The lighter color of the median tunnel suggests that it was an open burrow, which was passively filled in most specimens. However, meniscate filling in one of the specimens (Fig. 5.8) suggests that the tunnel could also be actively filled, probably only in the part that served occasionally as a lobe. It is not clear whether the small pellets in the fill of some specimens were produced by the tracemaker or derived from pelleted sediment on the seafloor. Outward concavity of the menisci in the filling of the lobes shows that the tracemaker entered the sediment laterally from the median tunnel, stuffed the lobes with mud, probably in several repetitive acts, withdrew to the median tunnel, moved forward, and produced a similar lobe on the other side (Fig. 10). The consistent direction of emanation of the lobes from the median tunnel points to repetitive action initiated from one side of the median tunnel leading to unidirectional propagation of the structure from proximal to distal parts. This suggests a single opening to the seafloor through which the mud was transported into the tunnel and probes below.

Figure 8.

Contours of Polykampton recurvum n. isp., P. eseri, and Dendrotichnium for a comparison of their general shapes.

Gentle curvature of the lobes and lobe–tunnel connections suggests a longer, flexible body of a worm-like organism, probably a polychaete, agreeing with prior interpretation of the tracemakers of the other Polykampton ichnospecies. Some polychaetes are known to drag freshly deposited organic matter into deeper subsurface sediment on the deep-sea floor (Jumars et al., 1990; Levin et al., 1997).

Figure 9.

Geometric differences in schematic contours of Polykampton recurvum n. isp. and P. eseri.

The differences in the width of the lobes within a specimen can be referred to the ontogenetic growth of the tracemaker during formation of the burrow. The differences in width of the median tunnel and the lobes, and values of other morphometric parameters between specimens (Fig. 7.1, Table 1), point to different sizes of the tracemaker body, which can also be related to the ontogenetic development. This is exemplified by: (1) the distinct increase in width of lobes from proximal to distal side of the burrow (Fig. 7.2), (2) the strong (r = 0.62) or very strong (r = 0.71) positive correlation of minimum/maximum values of lobe width (parameters c2, c1) to those of the median tunnel (a2/a1; Fig. 7.3, 7.4), (3) the strong positive correlation (r = 0.68) between the maximal width of the median tunnel (a1) and the widest spacing between the lobes (e1) (Fig. 7.5), (4) the strong positive correlation (n = 0.67) between largest width (c2) and the largest length (d) of the lobes (Fig. 7.6), and (5) the very strong positive correlation (r = 0.78) between the maximal (c1) and minimal (c2) width of the lobes (Fig. 7.7). The moderate positive correlation (r = 0.32) between the smallest and widest spacing between the lobes (e1/ e2; Fig. 7.8) corresponds to the visually poor or moderate regularity of the spacing between the lobes (Figs. 3–6). The width of the narrowest lobe in the proximal part of the burrow may correspond to the width of the body in the initial phase of burrow formation. The maximum width of the tunnel, which usually corresponds to the width of the most distal lobes, reflects the width of the body during the final stage of burrow occupation. This leads to formation of the burrow with a relatively wide median tunnel and lobes whose width increases from proximal to distal parts (Fig. 11). The differences usually do not exceed 50% of the final stage. The ontogenetic development reflected by the differences in size parameters between individual burrows suggests that the tracemaker could form several complete burrows during its life.

Figure 10.

Model of the burrow system and formation of Polykampton recurvum n. isp. The letters (a–e) indicate order of production of the lobes.

According to this interpretation, Polykampton recurvum was formed for purposed nutrition. The organic-rich, dark-gray mud was a source of food, especially during periods when little organic matter arrived on the seafloor (= nonbloom times). Therefore, it can be inferred that the lobes served as pantries for food storage. However, there are no traces of reworking of the lobes. This is a repetitive problem in all ichnospecies of Polykampton, which can be explained by the possibility that the food can be obtained by ingestion without reworking by means of exoenzymes produced by bacteria, as proposed for the other ichnospecies of Polykampton (Uchman and Rattazzi, 2018; Uchman et al., 2019, 2020). Labile organic matter can be released from particulate organic matter by exoenzymes (e.g., Aller and Cochran, 2019) and leached into the main tunnel. As noticed by Boetius (1995) and Chung and King (2001), exoenzymes are concentrated in some polychaete burrows.

Figure 11.

Ontogenetic growth expressed by increasing width of the successive lobes (a, b, c) and width of the median tunnel, which is equal to width of the last lobe.

Because of the storage of organic-rich mud in Polykampton recurvum, it can be treated as another example of the ethological category sequestrichnia, which embraces traces partly or completely filled with sediment utilized as a source of food (Wetzel and Uchman, 2016). Such manner of feeding is typical of the generally oligotrophic deep-sea environments, with a seasonally fluctuating supply of organic matter that is prone to quick oxidization, microbial decomposition, and/or consumption by competitive organisms on the surface (see Druffel et al., 1992). Such predictable supply of organic matter to the deep-sea floor can be seasonal (see Tyler, 1988; Rowe, 2013). It is not excluded that in response to that, the lateral lobes were formed in the same rhythm, as already postulated for P. cabellae (Uchman and Rattazzi, 2018). If so, P. recurvum was formed at least within 8–24 (mean 13) seasons.

The storage of the organic-matter-rich sediment in a burrow prolongs its nutritional utility. Such behavior is in line with the accelerated diversification of deep-sea trace fossils since the Late Cretaceous and continued through the beginning of the Paleogene because of competition for food (Uchman, 2004). It corresponds to the late phase of the Mesozoic Marine Revolution in the deep sea, establishing the benthic–pelagic coupling pattern, which is already typical of the Modern evolutionary fauna (Buatois et al., 2016).

Location of the beds bearing Polykampton recurvum with a depositional system is difficult because of small outcrops and the absence of sedimentary features that would be diagnostic of a common sedimentary model, especially the deep-sea fan with channels, depositional lobes, and associated elements. The abundance of plant detritus in sandy portions of some beds of the sections suggests an extrabasinal origin of the sand, which was supplied from a land by rivers (cf. Zavala et al., 2012). Thick marlstone/mudstone portions in several beds let us suppose that suspension was trapped in some depression. Their material derived probably from a collapsing deeper shelf and slopes. Similar deposition in a deep-sea trench was proposed for the underlying Monte Antola Formation (Fontana et al., 1994). The commonly noncalcareous background mudstones suggest deposition below the calcium compensation depth.

Conclusions

Polykampton recurvum n. isp. is a horizontal, ribbon-like structure composed of a median cylindrical tunnel and sparse, narrow, usually back-curved, lateral lobes, arranged alternately. It occurs 2–3 cm below the top of the Maastrichtian–Paleocene deep-sea turbiditic marlstone beds and is filled with gray mudstone from above. P. recurvum was produced by a “worm,” probably a polychaete. The trace fossil belongs to the category sequestrichnia and reflects a generally oligotrophic environment (deep-sea trench below the calcium compensation depth) with periodic supply of organic matter that is quickly oxidized or consumed. Accumulation of organics in the lateral lobes was evidently a source of food for the tracemaker.