Salp sp. to be determined

This is one fossil I have only seen once. A salp! What is a salp? It is a pelagic tunicate. (pelagic=free-swimming, tunicate=urochordate) Urochordates are an extrememly interesting group. They are deuterostomes, like vertibrates and echinoderms, they go through a larval stage in which they have a notochord and gills, which puts them in the phylum Chordata, along with vertibrates, hag-fish, and lancelets. They have a heart and a bundle of nerves thought now to be a primitive start to the brain and spinal cord of vertibrates. Urochordates are characterized by having an outer "tunic" of cellulose, secreted by the tunicate for protection. This makes them unique in the Kingdom Animalia as the only group that produce cellulose. Most urochordates, or tunicates, are sessile in their adult forms and are called ascidians (sea-squirts). However, salps are free-swimming and have a straight siphon through which they filter the water. Some fuse their tunics with those of other salps, forming long chains of filter-feeding salps. While the sessile sea-squirts can come in a variety of colors, salps are all clear and colorless. Needless to say, these animals are extremely rare in the fossil record.

About this Specimen

This salp is from the late Mississippian (Namurian), Bear Gulch Limestone, Fergus County, Montana. I believe that the salp extends above a bit from the part that is darker and smooth. I can see a bit of a line of continuation up there, though is not as well preserved. I can faintly see a depression where I believe the siphon tube runs through the salp. I'd love it if I could see a picture of another person's specimen for comparison, so please email me if you have one!

(below, left) A scan of my salp fossil. (below, right) A picture of it with side-lighting, so you can see the outline of the possible, yet less-preserved top part. The siphon, I believe, runs from the center-bottom diagonally up to the right slightly, ending in the upper right corner. I think the bottom might be the excurrent siphon and the upper right may be the incurrent siphon, but I can't be sure.

My salp fossil Salp with side-lighting

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