Abstract
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.
Highlights
Osteostracans are an extinct group of jawless vertebrates generally considered the sister-group to jawed vertebrates (Sansom, 2009)
The P. oeselensis head shield fragment (GIT 769-17) comes from Silma pank locality, the A. viitaensis shield fragment (GIT 769-29) comes from Elda cliff (Bed 5), the D. gemmifera head shield fragment (GIT 769-25) and scale (GIT 769-24), as well as the T. mammillata (GIT 769-20) head shield fragment come from the Himmiste Quarry, the Tremataspis milleri (GIT 769-1) shield fragment comes from Vesiku creek, and the Tremataspis schmidti (GIT 769-11) shield fragment and the Tremataspis perforata scales (GIT 769-15 and 769-16) come from the Viita trench locality
The tessellate condition of the dermal skeleton is generally considered plesiomorphic for osteostracans (Janvier, 1996)
Summary
Osteostracans are an extinct group of jawless vertebrates generally considered the sister-group to jawed vertebrates (Sansom, 2009). As such, they bridge the phylogenetic gap between living jawless and jawed vertebrates (gnathostomes). Like many stem-gnathostomes, but unlike living jawless vertebrates, osteostracans had a dermal skeleton composed of three layers: a superficial layer formed by an odontogenic component, as well as middle and basal layers formed by an osteogenic component (O'Shea et al, 2019; Smith & Hall, 1993). For a thorough review of previous histological studies of Tremataspis, and osteostracans in general, the reader is referred to O'Shea et al (2019)
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