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A fossil-rich interval in the organic-rich marine black mudstone of the Middle Triassic Botneheia Formation on eastern Svalbard was logged in high-resolution on an extremely well exposed section with emphasis on bivalve beds, taphonomic features, trace fossils and oxygenation proxies. The size distribution, fragmentation, articulation and orientation of three bivalve beds of the epifaunal flat clam Daonella were analysed. The logged section was studied for the recurrent occurrences of trace fossils and bivalve beds, and size distribution of framboidal pyrite. Daonella is most common in the shaly interval, while other taxa seem to outcompete Daonella in the siltier intervals. The comparison of fossil and sedimentological data with geochemical proxies for oxygenation revealed that the bivalve beds formed under dysoxic, conditions and due to low sedimentation rates and winnowing are not mass-mortality assemblages as previously suggested. The deposition on the sea floor was interrupted by anoxic intervals without benthic life. Recurrent beds containing the trace fossil Thalassinoides, however, show that oxygen levels fluctuated. The combination of water currents and oxygen fluctuations is a key to understanding why the black shales of the Botneheia Formation are so rich in benthic fossils.

The marine Middle Triassic sediments of Svalbard are rich in fossiliferous material and are particularly well-known for marine reptile fossils. Here, we present a new specimen of the small-bodied mixosaurid ichthyosaur Phalarodon fraasi from the Botneheia Formation. PMO 235.393 is unusual in being the first three-dimensional mixosaurid skull recovered from this formation, allowing us to use computed tomography to reconstruct the obscured right side of the cranium, resulting in the first 3D model available for a mixosaurid ichthyosaur. Although separated into different slabs, the specimen preserves most of the dermatocranium as well as some partial post-cranial elements. In particular, the rostrum, external naris, dentition, quadrate and sclerotic ring are well-preserved. This methodology gave new insights into the adaptations this taxon has to durophagy, as well as a detailed look at the heterodont dentition present in PMO 235.393. After comparing with other Phalarodon specimens, it was clear that the maxillary heterodonty of this genus is a synapomorphy. As such this was added as a new character in our phylogenetic analysis, supporting the separation of Phalarodon and Mixosaurus.