Thyestiids are a group of osteostracans (sister-group to jawed vertebrates) ranging in time from the early Silurian to Middle Devonian. Tremataspis is unique among thyestiids in having a continuous mesodentine and enameloid cover on its dermal elements, and an embedded pore-canal system divided into lower and upper parts by a perforated septum. The origin of this upper mesh canal system and its potential homology to similar canal systems of other osteostracans has remained a matter of debate. To investigate this, we use synchrotron radiation microtomography data of four species of Tremataspis and three other thyestiid genera. Procephalaspis oeselensis lacks an upper mesh canal system entirely, but Aestiaspis viitaensis has partially enclosed upper canals formed between slightly modified tubercles that generally only cover separate pore fields. Further modification of tubercles in Dartmuthia gemmifera forms a more extensive, semi-enclosed upper mesh canal system that overlies an extensive perforated septum, similar to that found in Tremataspis. Lower mesh canals in P. oeselensis are radially arranged and buried tubercles indicate a continuous growth and addition of dermal hard tissues. These features are lacking to varying degrees in the other investigated thyestiids, and Tremataspis probably had a determinate growth accompanied by a single mineralization phase of its dermal hard tissues.The previously proposed homology between the semi-enclosed upper canal system in Dartmuthia to the pore-canal system in Tremataspis is supported in this study, but the suggested homologies between these canals and other parts of the thyestiid vasculature to those in non-thyestiid osteostracans remain unclear. This study shows that three-dimensional modeling of high-resolution data can provide histological and structural details that can help clarify homology issues and elucidate the evolution of dermal hard tissues in osteostracans. In extension, this can give insights into how these tissues relate to those found among jawed vertebrates.

A long history of geological research on the island of Gotland, Sweden, has resulted in a detailed biostratigraphy based on conodonts for the Gotland sedimentary succession, but the relation between the Hamra and Sundre formations, the youngest strata on southern Gotland, has remained poorly resolved. These formations have also remained relatively poorly described in terms of vertebrates compared to other parts of the succession. A survey of museum collections and newly sampled material reveal that the taxonomical compositions and richness of vertebrate faunas remain similar compared to the underlying Burgsvik Sandstone and Oolite members. However, the relative abundance of the respective groups changes: Paralogania ludlowiensis and rare osteostracan remains of Tahulaspis sp. only occur in samples from the lower Hamra Formation, while Thelodus sculptilis becomes more common in samples from Sundre Formation. Conodont and isotope data give support to previous suggestions that the Hamra and Sundre formations may be largely isochronous, and it is possible that the observed differences in vertebrate faunas reflect changes in depositional setting. This interval on Gotland has been suggested to represent a hiatus in the East Baltic sections, where younger strata show an increased importance of acanthodians in the vertebrate faunas. Gotland could therefore give insights into the early stages of this diversification of gnathostomes during late Silurian times. However, this has to be done in combination with data from other areas, as well as with a review and revision of the scalebased taxonomy of Silurian acanthodians from the Baltic Basin.

Hypersaline environments are commonly assumed to be barren of metazoans and therefore are avoided by paleontologists, yet a number of early Paleozoic jawless vertebrate groups specialized to live in such settings. Sampling bias against restricted settings resulted in substantial underestimation of their diversity. Rare studies venturing into such environments yielded multiple new species of conodonts, suggesting that the diversity and habitat range of these hyperdiverse predators of the early oceans are equally underestimated. We describe here autochthonous conodont fauna from evaporite-bearing horizons from the middle Silurian of Estonia that provide evidence for efficient osmoregulation in this group. Based on a global compilation of coeval conodont assemblages, we show that marginal-marine, periodically emergent environments were characterized by higher conodont diversity than open-marine shallow settings. This diversity is due to a high number of species occurring in these environments only. The high degree of specialization is also reflected by the highest within-habitat variability (β diversity) in marginal settings. Most conodont species had narrow environmental niches and, unlike in marine invertebrates, extreme environments were inhabited by the most specialized taxa. Such environments represent a large proportion of early Paleozoic tropical epicratonic basins. Our analysis allows quantification of the degree to which mid-Silurian conodont diversity is underestimated as a result of sampling bias against marginal-marine settings.

Vertebrate microremains from the upper Silurian Winnica Formation in the Holy Cross Mountains, Poland are described from the Winnica and Rzepin sections. Both sites record the uppermost part of the Słupianka Member, but represent different depositional environments. The Winnica samples come from a low-energy environment, while the Rzepin sample was taken from a high-energy, oolitic facies. Both sites contain thelodontsThelodus parvidens,Thelodus trilobatus, an anaspid cf.Liivilepisand a number of acanthodian scales of ‘nostolepid’, poracanthodid and ‘gomphonchid’ types. Notable differences between the sites are the addition of the osteostracanTahulaspiscf.ordinata, the thelodontParalogania ludlowiensisand acanthodian scales identified asNostolepis gracilisin the Rzepin section. Placing the vertebrate faunas within the vertebrate biozonation established for the Silurian proved difficult. The suggested late Ludlow age for the Słupianka Member based on sequence stratigraphical and chemostratigraphical correlations cannot be definitely confirmed or refuted, but a late Ludfordian age seems the most plausible based on invertebrate and vertebrate faunas. The much lower abundance of poracanthodid acanthodians in the Rzepin sample supports the notion ofPoracanthodes porosusZone as a deep-water equivalent to a number of vertebrate biozones. The presence ofP. ludlowiensisonly in the oolitic sample confirms a long temporal range, but restricted environmental distribution for this taxon.

Conodonts were a highly diverse and abundant vertebrate group whose fossils are found in marine Paleozoic and Triassic strata around the world. They inhabited environments ranging from lagoons to open oceans and are represented by a wide variety of dental morphologies. Conodonts may have filled many different ecological niches and represent a significant proportion of nekton before the Devonian. Despite this, very little is known about trophic ecology of conodonts. While morphological diversity suggests a complex trophic structure within conodont communities, there is little evidence to support dietary niche partitioning among conodonts. We tested the hypothesis that individual conodont taxa occupied different trophic niches, using Sr/Ca and Ba/Ca ratios preserved in the dental elements of assemblages from Silurian strata of Gotland, Sweden. Sr/Ca and Ba/Ca have been shown to vary in vertebrate skeletal tissues depending on trophic positioning, although biological and environmental conditions can affect these ratios. Environmental influences were minimized by examining entire conodont communities from a tropical epeiric sea and by measuring strontium isotope ratios using thermal ionization mass spectrometry in the most metropolitan taxon (Ozarkodina confluens). Composition of white matter, a tissue unique to conodonts, was also analyzed using microprobe analysis, revealing significantly lower Sr concentrations than in surrounding lamellar tissue, suggesting taxon-specific histology should be considered when analyzing conodonts for geochemical data. Excluding taxa with highly variable quantities of white matter, the results show that each taxon preserves different Sr/Ca and Ba/Ca ratios with limited overlap, indicating variation in trophic positioning.