The one on the left is a trace fossil - a burrow of some sort, possibly part of a Thalassinoides network (but these things are often difficult to ID with much certainty).
The one on the right shows synsedimentary microfaulting - the multiple sub-parallel steps. That indicates that the siltstone was stiff enough to crack and move (due to disturbance of some sort). The mark across it... I think the two main possibilities for that are:
- a groove (seen in inverse) left by a passing animal (another trace fossil), or
- a small synsedimentary silt dyke.
The little branching feeders tend to support the latter idea, and indirectly that also provides a hint about the reason for the 'disturbance' causing the microfaulting.
It is like this... In some circumstances beneath the sea, where mud/silt/sand-stone are interlaminated, and before they have had a chance to become lithified (turned into rock), the coarser layers (silt/sand) can become liquidified. That could happen due to seismic vibration, and/or a nearby landslide for instance. When that happens, the mudstone layers may be disturbed, producing microfaults, and the liquified silt/sand-stone may break through and move up to the sea bed. That can result in small silt/sand volcanoes appearing on the sea bed, fed from pipes or dykes crossing the interlaminated sediments. I think it is part of one of those dykes that we see in your specimen. I studied such structures in the Carboniferous (Namurian) of the western coast of County Clare, Ireland, in the 1970-80s.
Just to be clear, that microfaulting is not related to the faulting that occurred millions of years later after all the sediments had been turned to rock. But it may be an accompaniment to synsedimentary faulting related to movement of salt deposits in the area (discussed in 'Salt Tectonics, Sediments and Prospectivity', G. Ian Alsop (ed.), Geological Society of London, 1jan2012; http://books.google.co.uk/books?id=dD1eJGuSm_AC).
You might be able to find out more from the Watchet Museum
Thank you so much Mike for your comprehensive and detailed reply. Looking closely at the one on the left, it does appear to be formed from conglomerated material including shells which fits in with the idea of some kind of burrow with infill. I have an icthyosaurus vertebrae which I found at Lyme Regis so I'm not too disappointed that it's not a bone of some kind! I agree the one on the right appears to be what you describe too. Thank you again for your expert opinion which makes me want to read more.