Planktonic Foraminifera may be divided into two mayor groups either having spines (=spinose) or lacking spines (=non-spinose). Spines are single crystals planted into the test wall in spine-holes. They do not form part of the wall. In contrast pustules are layered structures of the wall. Spines may be round to triangled in cross-section.
Lack of spines in spinose tests
Commonly spines are only found in living spinose specimens, rarely in tests and almost not in fossil spinose specimens. Still the spine-holes indicate their former existence. Confusion is caused by the fact that the spine-holes may be covered in a later life-stage by calcification. Even experts may not be able to differentiate between such calcifications and pustules. A reason for missing spines is that during the reproductive cycle (=gametogenesis) the spines are dissolved. Furthermore spines in a dead specimen soon tend to get loose or break off.
Bad preservation and how to deal with it
Besides of calcification also bad preservation may lead to wrong conclusions. I recommend to always pick several specimens alike from a sample and base assumptions only on clearly identifiable ones. Samples should be split and treated with different methods in order to find the best method to preserve the subtle surface-features such as broken spines and spine-holes.
First occurence of spines
While pustules already occur in Jurassic species, the first occurence of spines is observed for the earliest Paleocene. This significant change in test morphology leeds to specific lineages of spinose species from the K/T-boundary till today.
Biological function of spines (and pustules)
Spines leed to more stability and greater outreach of the plasma-strangs (=rhizopods). Studies of living spinose species show the significant efficiency of spines in catching prey, especially bigger copepods. Spinose species of today have a more carnivorous diet while non-spinose species tend to have a more herbiferous diet.