Many believe vertebrates diverged from a common ancestor of chordates and echinoderms. This belief is well supported by the prehistoric marine creature Amphioxus. Amphioxus does not possess bone, making it an invertebrate, but it has common features with vertebrates including a segmented body and a notochord. This could imply that Amphioxus is a transitional form between an early chordate, echinoderm or common ancestor, and vertebrates. Quantitative paleozoology is a process of taking a census of fossil types rather than inventory. They differ in that inventory refers to a detailed log of individual fossils, whereas census attempts to group individual fossils to tally the total number of a species. This information can be used to determine which species were most dominant and which had the largest population at a time period or in a geological region. In the early 1930s, paleontologists Chester Stock and Hildegarde Howard devised special units for quantitative paleozoology and quantitative paleontology. The first unit used, Number of Identified Species (NISP), specified exact quantity of fossils from a specific species recorded. Stock and Howard determined this unit to be problematic for quantitative purposes as an excess of a small fossil—such as teeth—could exaggerate quantity of the species. There was also an amount of confusion as to whether bone fragments should be assembled and counted as one bone or tallied individually. Stock and Howard then devised the Minimum Number of Individuals (MNI), which estimated the minimum number of animals needed to produce the fossils recorded. For example, if five scapulae from a species were found, it might be difficult to determine whether some of them were paired right and left on one individual or whether each came from a different individual, which could alter census, but it could be said that there must be at least three individuals to produce five scapulae. Three would thus be the MNI. In rare cases where enough of a collection of fossils can be assembled into individuals as to provide an accurate number of individuals, the unit used is Actual Number of Individuals, or ANI. Another unit commonly used in quantitative paleozoology is biomass. Biomass is defined as the amount of tissue in an area or from a species. It is calculated by estimating an average weight based on similar modern species and multiplying it by the MNI. This yields an estimate of how much the entire population of a species may have weighed. Problems with this measurement include the difference in weight between youngsters and adults, seasonal weight changes due to diet and hibernation, and the difficulty of accurately estimating the weight of a creature with only a skeletal reference.