Does mode of early development influence distribution? A study of Eocene gastropods from the Western Indian Province

The relationship between the mode of development and spatial-temporal distribution of gastropods has been under scrutiny, albeit inadequately, for a long time. It is generally believed that the planktotrophic mode of development gives rise to a wider geographical and longer temporal distribution. Hence, gastropods with non-planktotrophic development would have shorter distributions. Here, a case study is presented where spatial-temporal distribution and information about the early developmental modes of all Eocene gastropods from the Western Indian Province are used to understand this purported relation. Information about distribution and developmental mode are gathered from literature as well as direct observation of protoconch, wherever possible. Here, four parameters, including maximum geographical distribution (Rmax), total temporal range (Ttot), time (T1) a genus took to reach their peak geographical distribution Rmax and the time (T2) a genus survived after reaching the Rmax, are investigated. This study failed to find significant differences in these parameters among genera developed by planktotrophy (P), non-planktotrophy (NP) and both (P/NP). Combining it with other known case studies, it is highlighted here that planktotrophy does not ensure wide spatial-temporal ranges, and the absence of planktotrophy does not necessarily restrict these ranges. Sweepstake dispersal and changes in developmental modes during the evolution of a lineage appear to play important roles in distribution. Future studies are needed to fully understand how several non-planktotrophs attain great distributions.