A unique insight into the growth of necrocarcinoid crabs (Malacostraca: Decapoda: Brachyura): Evidence from the Upper Cretaceous of Siberia (Russia), with a description of a new species

The Upper Cretaceous (Cenomanian to Coniacian, 100.5–86.3 Ma) strata of the northern Krasnoyarsk Krai (Russia) yielded new fossil brachyuran crab material, based on which a new species, Necrocarcinus sibiricus, is described. Although the specimens come from settings of slightly different ages, all of them display dorsal carapace characteristics consistent with variation attributable to ontogenetic changes related to growth, for instance, the narrowing of the orbitofrontal margin, and a continuous smoothing and widening of the carapace. Interestingly, due to these ontogenetic changes, the largest of the studied specimens exhibits a morphology close to that previously described in Camarocarcinus, a related necrocarcinoid crab genus known from Paleocene strata. The newly described species shows that the dorsal carapace morphology typical for Camarocarcinus can be observed in at least some fully grown necrocarcinids, prompting a re-evaluation of the relationship between Necrocarcinidae and Camarocarcinidae. We propose that camarocarcinids may be derived from within Necrocarcinidae, with the particular morphology of Camarocarcinus resulting from heterochronous developmental processes, i.e., peramorphosis.