Over magen en staarten van thelodonten

In the collection of the Royal Museum of Scotland, the author came across a thelodont (Loganellia scotica, 1967.-65.18a +b) from the Upper-Llandoverian Jamoytius horizon, which shows in part and counterpart a clear, smooth barrel shaped concave impression. There are remarkable similarities with the barrel shaped stomachs in the recently discovered Canadian fork-tailed thelodonts (Wilson and Caldwell, 1993). If this impression can also be interpreted as a stomach, the appearance has taken place 5-8 Myr earlier compared with the Canadian Silurian species. More inspected specimens of L. scotica and other Scottish thelodonts do not show a trace of a stomach. The complex system of branchial plates, forward pointing denticles in the naso-pharynchial duct form a barrier to the instream of sediment. However, the disarticulated head could have been the reason for sediment infilling in this RMS specimen. The well showing notochord is also an indication that internal structures have been preserved. Is L. scotica the only jawless denticle covered craniate in Scottish Upper-Llandoverian rocks? Intact specimens are rare, but a collection of commonly found tails do show a variation in shapes yet not mentioned before. The denticle covering of the tails are regarding to their orientation, shape and seize clearly divided in zone s. Some tails do show a different orientation of the rays in the epichordal and chordal lobes. This could confirm the theory of Janvier (1981 and pers. comm. 1993) that the epichordal lobe in thelodonts is homologue to the second dorsal fin in gnathostomes and osteostracans. There is evidence that most of the also commonly found nodules containing rather worn out disarticulated thelodont denticles are of coprolithic origin. a) The shape of the denticle concentrations within the nodules. b) More important: nodules have been found containing a mixture of denticles and fragments of the podshrimp Ceratiocaris papilio. The droppings probably came from eurypterids. These coproliths give the impression that eurypterids and probably also the other members of the associated fauna must have swum in life above the pycnocline which separated oxic and anoxic waters.