to Pteropodidae (a name now used for a family of fruit bats) Finally, all these attempts were abandoned and, as more and more species were described as a result of several scientific expeditions, the classification of the Pteropoda into Thecosomata and Gymnosomata was generally adopted. Many of these new species were first described by French zoologists, for example, Jean René Constant Quoy and Joseph Paul Gaimard, Paul Rang, Alcide d'Orbigny and Louis François Auguste Souleyet. The relationship between these two clades is not unequivocally established, but it seems that they are sister taxa. Evolutionary history Pteropods are estimated to have originated during the Early Cretaceous, around 133 million years ago, with the diversification into the major lineages occurring during the mid-late Cretaceous. The oldest known fossil pteropod is a member of Limacinidae from the early-middle Campanian deposits of the San Juan Islands. Phylogeny Threats Vulnerability to ocean acidification A study was conducted on the West Coast of the United States to see ocean acidification's effects on pteropods. Limacina helicina was used to test the sensitivity to decreasing pH. This species of pteropod is potentially vulnerable to the corrosive waters associated with ocean acidification due to their calcium carbonate shell. The shell of a pteropod was immersed in ocean water with the projected pH level that the water will reach by the year 2100. After a month and a half in the water, the shell had almost completely dissolved. Distribution Pteropods are found in all major oceans, usually below the ocean surface and in all levels of latitude. Pteropods can be found lower than 10 meters, but in less amounts in terms of biomass, however, pteropod distribution is more spread out deeper based on findings. This can be explained as Pteropods from tropical areas become more common in deeper areas. They are not found commonly in the deep sea, in fact, few live lower than 500 meters below sea level. Continental shelves, areas containing many opportunities for nutrients, and productivity are locations in which Pteropods are usually populous, according to patterns in data. Springtime is a peak season for pteropoda, as they reach higher populations, though data shows that pteropoda south of the equator are less abundant seasonally. In addition, current data suggests that 93% of the world's pteropods are part of the Thecosomata family, while the 7% are Gymnosomata. References External links Wissenschaftliche Ergebnisse der Deutschen Tiefsee-Expedition auf dem Dampfer