Extinction selectivity and diversity depletion in the Triassic/Jurassic boundary among bivalves

Abstract

Diversity depletion is the result of the combined action of origination and extinction rates. The present study is an attempt to understand this dynamic by the analysis of the end-Triassic extinction event in a very well represented group: the marine bivalves. This is tackled in two steps: a) first we test the selectivity of extinction respect to the main autoecological characteristics of the genera (feeding mechanism, tiering, motility and attachment) among other features (mineralogy, age at the T-J boundary, paleogeographic distribution) by means of generalized linear models and resampling methods; and b) we analyze the evolutionary rates in the orders included. The results of the selectivity analysis show: 1) a positive selection for deep burrowers and a negative selection against reclined habits; 2) a significant association between mineralogy and survival odds, although this could be due to the strong correlation between mineralogy and tiering; 3) a lack of selection for age or geographic distribution (both traits strongly associated), although the last one would require a more detailed examination. From the analysis of the evolutionary rates we can conclude that: 1- all the orders show a negative growth rate during the Rhaetian (in some of them already starting during the Norian), with the main exception of generalist orders (Arcoida, Mytiloida) and deep burrowers (Pholadomyoida); 2- the most affected orders were Hippuritoida (reclined), Trigonioida, Nuculoida (both shallow burrowers) and Modiomorphoida (mostly shallow burrowers and endobyssate). The lack of selectivity for age and distribution points to an indiscriminate extinction. On the other hand, the preferential survival of deep burrowers suggests that some isolated environments were not equally affected by the general disruption; and the differences in growth rates show that the extinction event was strong enough to affect the specialized guilds, but not enough to affect the generalist ones.