Abstract
The closure of the late Neogene interoceanic seaways between the Western Atlantic (WA) and Tropical Eastern Pacific (TEP)—commonly referred to as the Central American Seaway—significantly decreased nutrient supply in the WA compared to the TEP. In marine invertebrates, an increase in parental investment is expected to be selectively favored in nutrient‐poor marine environments as prolonged feeding in the plankton becomes less reliable. Here, we examine turritelline gastropods, which were abundant and diverse across this region during the Neogene and serve as important paleoenvironmental proxies, and test whether species exhibit decreased planktotrophy in the WA postclosure as compared to preclosure fossils and extant TEP species. We also test for differences in degree of planktotrophy in extant sister species pairs. Degree of planktotrophy was inferred by measuring the size of protoconchs, the species' larval shell that represents egg size. Protoconch size was compared between extant postclosure WA and TEP species and preclosure fossil species. To compare extant sister species, we reconstructed the phylogeny of available WA and TEP species using one nuclear (H3) and three mitochondrial markers (12S, 16S, and COI). Compared to the preclosure fossils, protoconch size increased in WA species but remained the same in the TEP species. In the two extant sister species pairs recovered in the phylogenetic analysis, the WA species are inferred to be nonplanktotrophic while the TEP species are planktotrophic. This suggests that decreased nutrient availability and primary productivity in the WA may have driven this change in developmental mode, and was the primary selective force resulting in postclosure turritelline extinctions.
Highlights
The closure of the interoceanic seaways between the Tropical Eastern Pacific (TEP) and the Western Atlantic (WA)—commonly referred to as the Central American Seaway (O'Dea et al, 2018,2016)—in the late Neogene resulted in significant changes to the abiotic and biotic oceanographic conditions in the WA
We test these hypotheses by comparing (a) turritelline protoconch size in the postclosure WA with both modern TEP protoconch sizes and preclosure fossil protoconch sizes, and (b) comparing protoconch size changes in extant sister species pairs by generating a robust molecular phylogeny of TEP and WA turritellines based on H3, c oxidase subunit I (COI), 16S, and 12S sequence data
For maximum‐likelihood analysis, sequences were partitioned by codon position in each gene and run under a GTRCAT model with joint branch length optimization using RAxML 8.0.9 on the Cyberinfrastructure for Phylogenetic Research (CIPRES) platform (Miller, Pfeiffer, & Schwartz, 2010) to calculate the ML tree and nonparametric bootstrap node support
Summary
The closure of the interoceanic seaways between the Tropical Eastern Pacific (TEP) and the Western Atlantic (WA)—commonly referred to as the Central American Seaway (O'Dea et al, 2018,2016)—in the late Neogene resulted in significant changes to the abiotic and biotic oceanographic conditions in the WA. It is expected that decreased nutrient availability in the plankton would result in selection for greater nutrient apportionment (larger eggs) which should be reflected in increased sizes for protoconchs in postclosure WA species We test these hypotheses by comparing (a) turritelline protoconch size in the postclosure WA with both modern TEP protoconch sizes and preclosure fossil protoconch sizes, and (b) comparing protoconch size changes in extant sister species pairs by generating a robust molecular phylogeny of TEP and WA turritellines based on H3, COI, 16S, and 12S sequence data
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