SUSTAINED MORPHOSPACE OCCUPANCY OF NATICID GASTROPODS ACROSS THE K-PG MASS EXTINCTION

The Phanerozoic fossil record is characterized by patterns of sustained increases in biodiversity, interrupted by episodes of sudden drops, known as mass extinctions. Though synonymous with major losses in diversity, mass extinctions also affect the overall distribution of morphological variance (i.e., disparity) in a group, which is commonly reflected in the total morphospace occupied by the group. Typically, during selective extinctions the range of occupied morphospace is reduced in the affected groups; however, the disparity dynamics operating on morphologically conservative taxa which survive extinctions, requires further insight. With this context, here we investigate variation in the disparity dynamics of the morphologically conservative and spatio-temporally diverse gastropod family Naticidae, to study whether this pattern of conservatism was derailed by the K-Pg mass extinction event. Our dataset comprises 111 species, representing 21 genera and five subfamilies, ranging in age from the Santonian to the Thanetian, a time interval spanning 30.3 Myr. A standard landmark-based morphometric approach was used to quantify disparity, incorporating metrics such as Sum of Range, Sum of Variance, Convex Hull Area, Mean Pairwise Distance and Centroid to get a comprehensive understanding of disparity evolution, and for further confirmation of results, PERMANOVA and Kruskal-Wallis tests were used. Our results suggest that naticid disparity remained unchanged across the K-Pg extinction; i.e., neither the total extent of occupied morphospace change, nor did the dynamics within the morphospace change. The reason behind this extinction resistance could be the morphological redundancy of naticids; irrespective of their diversity, the occupied naticid morphospace is inherently small, and the variability within that morphospace is also low. Our results offer a new perspective on the interplay between mass extinctions and morphospace dynamics by suggesting that in low disparity groups with generalized morphology, taxonomic turnover may be operating on different degrees, however, the total morphospace occupancy of the group will not be affected.