Fish otoliths from the middle Paleocene (Selandian) of southern Sweden

ABSTRACT

The first fossil otolith association from the middle Paleocene (Selandian) of Scania, southern Sweden is described. Forty-seven otoliths were retrieved from shallow wells representing 14 teleost taxa. Many specimens are small and/or eroded and, therefore, not identifiable to species level. Nevertheless, our findings indicate the potential for further fossil otolith discoveries in the region. The Scanian otolith-based fauna greatly resembles the better-known coeval association from Copenhagen, Denmark, but is relatively rich and diverse in perciform otoliths. The fauna records the first occurrence of Serranus? caribbaeus from the European Paleocene, and of Archaemacruroides ornatus from the Selandian of the North Sea Basin.

KEYWORDS:

• Teleostei
• Gadiformes
• Ophidiiformes
• Perciformes s.l.
• Pterothrissidae
• Scania

Introduction

Late Cretaceous and early Cenozoic (Danian) strata in southern Sweden (Skåne) yield a rich record of fossil vertebrates including chondrichthyans, actinopterygians, turtles, plesiosaurs, mosasaurs, crocodylians, hesperornithid birds, and non-avian dinosaurs (e.g., Lindgren 2004; Rees & Lindgren 2005; Einarsson et al. 2010; Scheyer et al. 2012; Sørensen et al. 2013; Siverson et al. 2015; Bazzi et al. 2016; Kear et al. 2016). In contrast to the wealth of these Late Cretaceous and Danian vertebrate remains, only a single bone fragment, possibly representing a coelacanthid fish, has been described so far from the Scanian Selandian (Ørvig 1986). Middle Paleocene strata of southern Sweden have been studied since the 19th century (Beyrich 1857; Lundgren 1882), and their age was first assessed by Holst & Grönwall (1907). Based on a rich foraminiferal fauna, Brotzen (1948) confirmed the age based on similarities with Selandian faunas of Denmark, Germany, the Netherlands, England, Russia and the Gulf States of the U.S. Middle Paleocene strata of the Malmö and Ystad regions overlie Danian limestone and calcareous sandstone (Holst & Grönwall 1907; Brotzen 1948). The Selandian rocks consist of a basal glauconitic conglomerate incorporating Danian limestone pebbles, phosphatic nodules, bone fragments, and fish teeth, together with re-deposited Danian invertebrate fossils. The conglomerate is overlain by poorly consolidated fossiliferous, dark-green, glauconitic and clay-rich sandstones with many reworked calcareous grains from the underlying Danian deposits. These sandstones represent the Lellinge Greensand in accordance with its occurrence near Copenhagen (Holst & Grönwall 1907; Norling & Skoglund 1977; Wickström et al. 2021). The Selandian deposits are 0.5–2.3 m thick in the Malmö region (Holst & Grönwall 1907; Brotzen 1948) and reach 3 m in thickness in the Ystad region (Brotzen 1948). Magnusson et al. (1962) described a 15-m-thick middle Paleocene fossil-rich succession from a borehole west of Ystad, and Gustavsson & Norling (1973) describe another 15-m-thick Selandian interval rich in benthic foraminifera southwest of Svedala that extends over an area of several square kilometres. There, the Lellinge Greensand fills a depression in the Danian limestone, and was intersected at 58–68.5 m depth in one borehole (No. 2C SO: 390; see Gustavsson & Norling 1973: text-Fig. 1), which is the deepest occurrence of Selandian strata in Scania recorded so far.

Figure 1. Geological map showing the distribution of Paleocene deposits in southern Sweden and the locations of the wells Magnushill (star south of Svedala), Ystad Felix and Ystad 38 (star west of Ystad). Map modified from McLoughlin et al. (2021; this volume)

Here we report a small fauna of Paleocene teleost otoliths from the Lellinge Greensand, obtained from three wells in Scania. They represent the first fossil otolith records from Sweden, except for a single record of a non-teleostean otolith originally described from the Early Jurassic of Bornholm by Malling & Grønwall (1909; reviewed in Schwarzhans 2019) and later recorded from Katslösa in Scania by Troedsson (1951), and a presumed Cretaceous otolith figured by Bazzi (2014), which however likely does not represent an otolith. Paleocene otoliths from the North Sea Basin have been described previously from Copenhagen in Denmark (Koken 1885; Schwarzhans 2003; Schwarzhans & Milàn 2017), Belgium (Nolf 1978), England (Stinton 1965, 1977) and from erratic boulders of northern Germany (Roedel 1930). The early records by Koken (1885) and Roedel (1930) were reviewed by Schwarzhans (2003) and by Nolf (2013).

Materials and methods

The material described here derives from three groundwater drillings performed on behalf of the Geological Survey of Sweden (SGU) in the southwestern part of Skåne (Fig. 1). The borehole Magnushill was drilled in 1973 in the Danian limestone depression south of Svedala described by Gustavsson & Norling (1973) and we investigated an otolith sample from 62–65 m depth. Two drillings west of Ystad also yielded Selandian otoliths: borehole Ystad Felix (773) was drilled in 1973 and yielded otoliths in a sample from 24–25 m depth, and from borehole Ystad 38 (776) we investigated samples from 16.5 m, 20 m and 24 m depth. There are no lithological logs of these boreholes available; the above depth data were obtained from sample labels.

The otolith terminology follows that established by Koken (1884) with amendments by Chaine & Duvergier (1934). Digital photos were taken with a Canon EOS 1000D mounted on a Wild M400 photomacroscope and were taken at regular depth of field increments. Individual photographs were stacked using the HeliconFocus software of Helicon Soft to create a continuously focused image. All otoliths are shown as right otoliths. Left otoliths are mirror imaged and annotated accordingly in the captions.

The materials are housed in the palaeozoological collections of the Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, under the inventory acronym NRM-PZ P.

Results

A total of 47 otoliths were retrieved (Fig. 2, Table 1). Many apparently derive from small specimens or are unique and somewhat eroded so that specific identification is not possible in all instances. Therefore, we have refrained from an extensive descriptive part and instead have summarized the findings in Table 1 and with annotations to specific species in particular when departing from the well-known large association from Copenhagen.

Table 1. Taxonomic assignment, distribution and amount of otoliths in Scanian wells

Taxa	Ystad Felix	Ystad 38	Magnushill
Albuliformes
Pterothrissidae
Pterothrissus sp. juv.		4
Albuliformes family indet.
Protargentinolithus erraticus (Roedel 1930)			1
Anguilliformes
Congridae
Rhynchoconger angulosus (Schwarzhans 2003)		1
Argentiniformes
Argentinidae
Argentina tricrenulata (Stinton 1965)			1
Aulopiformes
Paraulopidae
Paraulopus sp.		1
Gadiformes
Macrouridae
Coelorinchus balticus (Koken 1885)			3
Gadiformes family indet.
Archaemacruroides ornatus Stinton 1965		1
Gadiformes indet. juv.		8
Ophidiiformes
Ophidiidae
Protobythites seelandicus (Koken 1885)	1	20
Beryciformes
Berycidae
Centroberyx sp. juv.		1
Perciformes s.l.
Acropomatidae
?Acropomatidae indet.		2
Serranidae
Serranus? caribbaeus (Nolf & Dockery 1993)			1
?Serranidae indet. frag.			1
?Centrolophidae
?Centrolophidae indet.		1
Total	1	39	7

Figure 2. Paleocene (Selandian) otoliths from southern Sweden; A, B. Pterothrissus sp. juv., mirror imaged, Ystad-38, 24 m, NRM-PZ P 16439, C, D. Rhynchoconger angulosus (Schwarzhans 2003), mirror imaged, Ystad-38, 20 m, NRM-PZ P 16438, E, F. Protargentinolithus erraticus (Roedel 1930), mirror imaged, Magnushill, NRM-PZ P 16386, G. Argentina tricrenulata (Stinton 1965), mirror imaged, Magnushill, NRM-PZ P 16387, H. Paraulopus sp., Ystad-38, 20 m, NRM-PZ P 16438, I–K. Coelorinchus balticus (Koken 1885), mirror imaged, Magnushill, NRM-PZ P 16386 (I-J) and NRM-PZ P 16387 (K), L, M. Protobythites seelandicus (Koken 1885), mirror imaged, Ystad-38, 20 m, NRM-PZ P 16438.N, O. ?Acropomatidae indet., mirror imaged, Ystad-38, 16.5 m, NRM-PZ P 16441, P, Q. ?Serranidae indet., mirror imaged, Magnushill, NRM-PZ P 16387, R, S. Serranus? caribbaeus (Nolf & Dockery 1993), mirror imaged, Magnushill, NRM-PZ P 16386, T, U. ?Centrolophidae indet., Ystad-38, 17 m, NRM-PZ P 16440. Scale bar equals 1 mm

Pterothrissus sp. juvenile otoliths: Pterothrissid otoliths are common and widespread in Paleocene strata. The Danian and Selandian sediments of Denmark have yielded abundant specimens of Pterothrissus conchaeformis (Koken 1885), which are morphologically mature from about 5 mm length. All specimens studied here are less than 2 mm in length. Even though they are likely from the same species (i.e., P. conchaeformis), their small size does not allow identification to species level. It should be noted that in many fossil communities very small and juvenile Pterothrissus otoliths outnumber large and identifiable ones by a large margin.

Archaemacruroides ornatus Stinton, 1965: This very plesiomorphic gadiform otolith was originally described from the Thanetian of England by Stinton (1965) and was later also identified from the Selandian of West Greenland (Schwarzhans 2004), but has so far not been recorded from the Selandian of the North Sea Basin.

Protobythites seelandicus (Koken 1885): This is the most common species in the samples from Scania, and it is also very common in the Selandian strata of Copenhagen (Schwarzhans 2003). Here, P. seelandicus is represented by consistently diminutive specimens (< 2 mm long) indicating that they originated from juvenile fishes. This species has recently been allocated with the fossil ophidiid genus Protobythites by Schwarzhans and Stringer (2020).

Perciformes s.l. (sensu Nelson 2006): Perciform otoliths are surprisingly diverse in the Selandian samples of Scania and include a putative acropomatid, two putative serranids, and a putative centrolophid. It has to be stressed, as a disclaimer, that Paleocene perciform otoliths are known to show very plesiomorphic morphologies (Schwarzhans & Bratishko 2011) that renders systematic allocation problematic even at family level. Furthermore, most of the specimens described here are eroded to various extents prohibiting specific identification, except for Serranus? caribbaeus (Nolf & Dockery 1993), a species originally described from the Paleocene of the US Gulf Coast as Nemipterus caribbaeus and here recorded for the first time from the European Paleocene. A second, putative serranid otolith probably represents an undescribed species but is incomplete, and hence cannot be identified unambiguously.

Discussion

Here we briefly describe the first Paleocene otoliths from Sweden. Not surprisingly, the association is very similar to the coeval assemblages from Denmark. Minor differences are evident in the relative content of perciform otoliths, which appear to be more abundant and diverse in the Scanian fauna. Another striking aspect is the predominance of small otoliths in the samples from Scania, most likely representing juvenile fishes. It is not known whether this represents a primary signal or whether the drilling techniques applied favored the recovery of small size fossils while larger ones may have been destroyed in the process of drilling. In any case, the few available data described here indicate the potential of finding further otoliths in the Selandian of Scania and opportunities for broadening the recognition of teleost groups in the Paleocene of the boreal European bioprovince. We anticipate that future investigations of Paleocene strata in Scania will retrieve many more otoliths.

Acknowledgments

Jonas Hagström and Steve McLoughlin (both NRM) assisted with literature search and Figure 1. We thank the guest editors of GFF, Christian Skovsted and Vivi Vajda (both NRM), and Gary Stringer (West Monroe, Louisiana) and an anonymous reviewer for their helpful comments.

Disclosure statement

No potential conflict of interest was reported by the author(s).