Halkieriids and other small shelly fossils are typically, although not always, preserved in phosphate, which may or may not have been their original mineral composition. Preservation by a covering of phosphate only seems to have been common during the early Cambrian, becoming rarer with time as a result of increased disturbance of sea-floors by burrowing animals. Hence it is possible that halkieriids and other small shelly fossils were alive earlier than the earliest known fossils and later than the latest known fossils — paleontologists call this kind of uncertainty the Signor–Lipps effect.