Fossil History

The fossil record of Herdmania momus, as in many urochordates, is extremely sparse. Although this sparsity is not uncommon for any soft-bodied animal, the characteristic, aragonitic spicules embedded in the test of H. momus indicate that fossilised specimens should occur somewhere. A possible explanation for their absence in the fossil record may be that aragonite is unstable and tends to dissolve to calcite, preventing preservation. Many other aragonite fossils, however, have been found in specific clays or as calcite replacements. An alternative explanation is that ascidian spicules may be mistaken for sponge spicules however given the great difference in spicule morphology, this is also unlikely. Further adding to the mystery are the missing fossilised embryos. Evidence shows that H. momus embryos may be mineralised in similar conditions and on a fitting timescale to what may have historically occurred. This evidence leaves the sparse records of fossilised embryos of these and other dueterostomes a mystery.