Recent basking sharks (family Cetorhinidae) are large plankton-feeding lamniform sharks that filter huge volumes of water through their gills (Compagno et al. 2005). Cetorhinids are represented in the fossil record primarily by isolated teeth, gill rakers, dermal scales, and vertebrae, with partial skeletons being rare (Cappetta et al. 2016; Hovestadt and Hovestadt-Euler 2012; Reinecke et al. 2015; Welton 2013a). The family includes three genera: the extant Cetorhinus Blainville, 1816, the extinct Keasius Welton, 2013, and Caucasochasma Prokofiev & Sychevskaya, 2018. Together, these taxa document the evolutionary history of planktivory among lamniform sharks (Kovalchuk and Barkaszi 2021).

The genus Cetorhinus first appears in the fossil record during the Early to Middle Miocene (Malyshkina et al. 2022; Welton 2013b) and is represented today by a single species, Cetorhinus maximus (Gunnerus, 1765), which inhabits temperate and boreal waters worldwide (Hoelzel et al. 2006; Rigby et al. 2021). Two extinct species of the genus are recognized: Cetorhinus huddlestoni Welton, 2014 from the Middle Miocene of California (USA) and South Korea (Malyshkina et al. 2022), based on isolated teeth, and C. piersoni Welton, 2015 from the Lower Miocene of Oregon (USA), also based on teeth alone.

The extinct genus Keasius ranges from the Middle Eocene to Lower Miocene; it was erected by Welton (2013a) to accommodate a fossil basking shark formerly assigned to Cetorhinus. Four species of Keasius are currently recognized: K. taylori Welton, 2013 from the Upper Eocene of Oregon (USA), K. parvus (Leriche, 1908) from the Lower Oligocene of Belgium, K. septemtrionalis Reinecke, von der Hocht & Dufraing, 2015 from the Upper Oligocene–Lower Miocene of Germany, and K. rhenanus Reinecke, von der Hocht & Dufraing, 2015 from the Lower to Middle Miocene of Germany. Fossil remains of Keasius are widespread, occurring in Europe, North America, Antarctica, and Asia (Bienkowska-Wasiluk 2021; Cappetta et al. 2016; Cione and Reguero 1998; Kovalchuk and Barkaszi 2021; Malyshkina 2006; Nazarkin 2014; Reinecke et al. 2015; Welton 2013a). An articulated partial skeleton from the Lower Oligocene of Germany (Hovestadt and Hovestadt-Euler 2012) provided associated skeletal material that confirmed the anatomical connection between dentition and gill rakers. However, this was already demonstrated by the holotype of K. taylori, which comprises teeth, vertebrae, and gill rakers from a single individual.

Caucasochasma is monospecific, represented by C. zherikhini from the Lower Oligocene of the North Caucasus, Russia (Prokofiev and Sytchevskaya 2018). Prokofiev and Sytchevskaya (2018) interpreted it as a bottom-associated planktivorous shark that fed on plankton near the sea floor, representing a unique ecological adaptation within the family.

Thus, cetorhinids have a nearly continuous fossil record from the Middle Eocene to the present, documenting the gradual re-establishment of pelagic suspension feeding among lamniform sharks. Importantly, cetorhinids were not the first lamniforms to evolve suspension feeding; Cretaceous taxa such as Pseudomegachasma and Aquilolamna also show adaptations indicating planktivory (Shimada et al. 2015; Vullo et al. 2021). The Oligocene diversification of Keasius parallels the radiation of baleen-assisted mysticetes (Fitzgerald 2006; Tsai et al. 2024), reflecting broader restructuring of plankton-based marine ecosystems during a period of increasing oceanic productivity (Bartol et al. 2014).

Here we describe basking shark remains and associated faunal elements retrieved from a bromalitic concentration from the Lower Miocene of northeastern Slovenia and analyze these findings in the context of trophic interactions in a deep-water Paratethyan environment.

The studied specimen represents a bromalitic concentration that includes numerous fragmented gill rakers associated with abundant remains of irregular spatangoid echinoids. The echinoid material consists of disarticulated test fragments and hundreds of complete and partial spines, indicating the breakdown of several individuals. The concentration also includes foraminifera, among which Strictocostella Patterson, 1987 was recognized, as well as small polygonal plates with tessellated structure. Because the echinoid material occurs within the bromalite itself rather than in the surrounding sediment, its inclusion has implications for identifying the producer and supports exclusion of certain predators (e.g., crocodiles), whose digestive processes would have dissolved echinoid tests before regurgitation.

Fossil elements were identified using diagnostic morphological features and comparative analyses of both fossil and extant representatives of the respective taxa. Preparation was carried out under Leica SZ4 and EZ4D stereomicroscopes. Photographs were taken with a Nikon Z7II digital camera fitted with a Nikkor Z MC 50 mm f/2.8 lens. Image stacks were processed using Helicon Focus and subsequently refined in Adobe Photoshop CS6. Micro-computed tomography (μ-CT) imaging was performed with a Phoenix L240 system developed by GE Sensing & Inspection Technologies at the Earth Science Institute of the Slovak Academy of Sciences in Banská Bystrica. We follow the descriptive terminology of gill rakers after Welton (2013a, fig. 11) and comparative morphological characters compiled by Kovalchuk and Barkaszi (2021, Table 1). Additionally, three gill rakers with discernible outlines (SLO, n = 3) were included in a 2D landmark analysis of gill-raker shape variation following Kovalchuk and Barkaszi (2021) and compared against samples of Cetorhinus maximus (n = 14, recent), Keasius taylori (n = 5, Eocene). K. parvus (n = 33, Lower Oligocene–Upper Miocene), K. rhenanus (n = 7, Lower–Upper Miocene), and K. septemtrionalis (n = 10, Upper Oligocene). The Keasius samples contain the respective type specimens (K. taylori – 4 holotypes, K. parvus – 4 syntypes) or those associated with type series (K. rhenanus, K. septemtrionalis). Landmark data were acquired using tpsUtil and tpsDig, and analyzed (Procrustes fit, principal component analysis, and canonical variates analysis) in MorphoJ (Klingenberg 2011). One-way PERMANOVA of PCA scores was carried out in PAST 4.17 (Hammer et al. 2001).

Class Chondrichthyes Huxley, 1880

Subclass Elasmobranchii Bonaparte, 1838

Superorder Galeomorphii Compagno, 1973

Order Lamniformes Berg, 1958

Family Cetorhinidae Gill, 1862

Genus Keasius Welton, 2013

The bromalite described herein contains abundant fragmentary basking-shark gill rakers, disarticulated echinoid test fragments and spines, foraminifera, and polygonal calcified-cartilage tesserae (Fig. 4A, B). The tessellated cartilage plates correspond to the branchial basket of a basking shark (Hovestadt and Hovestadt-Euler 2012; Kemp and Westrin 1979), supporting the interpretation that the material originated from a decaying carcass. Direct predation on basking sharks cannot be entirely excluded (Matthews and Parker 1950), but the fragile preservation of gill rakers and absence of acid etching support ingestion during opportunistic scavenging followed by regurgitation (Walker and Brett 2002).

The co-occurrence of echinoid remains within the bromalite itself is critical, as it demonstrates ingestion near the seafloor. This effectively disfavors crocodiles, which could not have accessed such depths, and pelagic mammals, whose regurgitation at the surface would have dispersed fragile skeletal debris before deposition. Instead, benthopelagic sharks (particularly other Lamniformes or Hexanchidae) represent the most plausible producers. These taxa are known opportunistic feeders (Belben et al. 2017; Reum et al. 2020), potentially capable of ingesting both cetorhinid remains and benthic invertebrates (e.g., echinoids).

Despite the limited number of intact gill-rakers, the preserved material exhibits a combination of morphological and morphometric traits most consistent with Keasius septemtrionalis. Stratigraphically, the late Burdigalian (NN4) age of the Meljski Hrib deposits coincides with the known temporal range of this species (Reinecke et al. 2015). We therefore adopt the cautious designation as Keasius cf. K. septemtrionalis.

In the Early Miocene of the Central Paratethys, a basking shark carcass would have represented as substantial energy-rich resource, attracting scavengers. If the carcass remained buoyant, pelagic scavengers would have dominated (Smith and Baco 2003). Once settled on the seafloor, benthic invertebrates and demersal fishes would have taken over (Higgs et al. 2014), while benthopelagic sharks exploited the remains. Extinct lamniforms such as Carcharodon hastalis and Otodus megalodon are well documented from the Miocene of Slovenia (Mikuž and Šoster 2013; Mikuž et al. 2013). Though lacking serrated teeth, Carcharodon hastalis possessed large, robust, smooth-edged teeth adapted for grasping and slicing soft tissue (Dubikovska et al. 2025), while Otodus megalodon was equipped to dismember large prey or carcasses (Shimada et al. 2025, 2026). Hexanchid sharks, as demersal taxa, also represent a viable candidate, since they could have ingested cetorhinid remains together with echinoids in deep-water settings (Ebert 2002; Reum et al. 2020).

While direct fossil evidence of scavenging on basking sharks is lacking, documented cases of lamniform teeth embedded in marine mammal bones indicate opportunistic feeding or scavenging behavior (Godfrey et al. 2025). The bromalitic nature of the specimen thus records a rare instance of trophic interaction among vertebrates in marine environments. The combination of cetorhinid raker fragments, tessellated cartilage plates, and deep-water echinoids points to deposition within an outer-shelf to upper-slope setting (Cappetta et al. 2016; Gašparič et al. 2025). This assemblage provides rare indirect evidence of the presence of basking sharks (Keasius cf. K. septemtrionalis) in the Central Paratethys and indicates that benthopelagic sharks, including lamniforms and hexanchids, could have exploited their carcasses. The described specimen illustrates the ecological complexity of Miocene marine food webs and highlights the role of scavenging within deep-water ecosystems.