Marambionectes

Marambionectes (meaning "Marambio Island swimmer") is an extinct genus of weddellonectian elasmosaurid plesiosaur from the Late Cretaceous López de Bertodano Formation of Antarctica. The genus contains a single species, M. molinai, known from a partial skeleton.

Marambionectes Temporal range: Late Cretaceous (Upper Maastrichtian) PreꞒ Ꞓ O S D C P T J K Pg N ↓
Speculative life restoration
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Superfamily:	†Plesiosauroidea
Family:	†Elasmosauridae
Clade:	†Weddellonectia
Genus:	†Marambionectes O'Gorman et al., 2024
Species:	†M. molinai
Binomial name
†Marambionectes molinai O'Gorman et al., 2024

Discovery and naming

The Marambionectes holotype specimen, IAA-Pv 752, was discovered in 2018 in sediments of the López de Bertodano Formation (unit 9) near on Marambio (Seymour) Island of James Ross Archipelago, Antarctica. The specimen is partially articulated and incomplete, consisting of the basioccipital, mandible, some teeth, cervical, pectoral, dorsal, and caudal vertebrae, ribs, an ilium, the right humerus and ulna, and a femur. The specimen belongs to an adult animal.^[1]

In 2024, O'Gorman and colleagues described Marambionectes molinai as a new genus and species of elasmosaurid plesiosaur based on these fossil remains. The generic name, Marambionectes, combines a reference to Marambio Island, where the fossil material was found, with the Greek word "nectes", meaning "swimmer". The specific name, molinai, honors the Argentinian fossil preparator Omar J. Molina and his Antarctic field work.^[1]

Classification

In their phylogenetic analyses, O'Gorman et al. (2024) recovered Marambionectes as a derived member of the elasmosaurid clade Weddellonectia, as the sister taxon to the Aristonectinae.^[1] Nearly identical results were recovered by O'Keefe et al. (2025) in an updated version of this phylogenetic matrix, with Marambionectes as the aristonectine outgroup. These results are shown in the cladogram below. The ⊞ button can be clicked to expand the node.^[2]

Weddellonectia	Tuarangisaurus Futabasaurus Chubutinectes Aphrosaurus Hydrotherosaurus Morenosaurus Vegasaurus Kawanectes Marambionectes Aristonectinae ⊞ Wunyelfia MLP 99-XII-1-5 (Aristonectinae indet.)[3] Aristonectes parvidens Aristonectes quiriquinensis Kaiwhekea Alexandronectes Morturneria ⊟