The oldest frog crabs (Decapoda: Brachyura: Raninoida) from the Aptian of northern South America

Abstract

Raninoida, also known as “frog crabs,” is a clade of extant true crabs (Brachyura) characterized by a fusiform carapace (raninid-type), narrow thoracic sternum, pleon partially exposed dorsally, and paddle-like limbs, all of which are well suited to their cryptic burrowing lifestyle. However, the most basal raninoids from the Cretaceous were morphologically different, with ornamented carapaces that were wider than long (necrocarcinid-type), a broader thoracic sternum, and the pleon fitting between the legs assisted by pleonal locking mechanisms. During Albian times (∼112 to 99.6 Ma.) both body plans flourished worldwide. In contrast, pre-Albian (older than ∼112 Ma.) fusiform families have not yet been reported. The discovery of Notopocorystes kerri n. sp., a fusiform crab from the upper Aptian (∼115 Ma.) of Colombia, South America, and the re-examination of Planocarcinus olssoni (Rathbun, 1937) n. comb., a necrocarcinid-like crab from the same age and locality, extend the record of the two body plans back into the Aptian of the equatorial Neotropics. Notopocorystes kerri is the oldest fusiform raninoid known to date, revealing that the morphological innovation of a fusiform carapace was already evolved in Raninoida before the rapid radiation experienced during Albian times. Our findings are suggestive of a still unresolved Palaeocorystidae, containing the rootstock for the post-Aptian Raninidae/Symethidae clade, with the most basal palaeocorystids lying in proximity to, and possibly derived from, a necrocarcinid-like ancestor.