Cuspidariidae Dall 1886

Cuspidariidae Dall, 1886 Notes With over 260 species (~ 1 / 3 of all Anomalodesmata), typically found in deeper waters (30 to 7,242 m), Cuspidariidae is likely the best-studied family of carnivorous bivalves. The presence of a rostrate shell in most species generally aids in the recognition and identification of its members. Although its taxonomy is relatively well-resolved and it is consistently recovered as monophyletic in most phylogenies (Harper et al. 2006, Combosch et al. 2017, Machado and Passos 2022), the internal relationships between Cuspidariidae and other families of predatory bivalves (with or without a muscular septum) remain a subject of ongoing debate. More recently, however, broader phylogenetic studies (e. g. with a greater representation of taxa) have shown that Cuspidariidae forms a sister group with Spheniopsidae and / or Halonymphidae and Protocuspidaridae (Cuspidarioidea) (Crouch et al. 2021, Machado and Passos 2022) — diverging from previous studies where Cuspidariidae was recovered as the sister group of Poromyidae (Poromyoidea) or Verticordiidae (Verticordioidea) (Dreyer et al. 2003, Harper et al. 2006, Bieler et al. 2014, Combosch et al. 2017). These more recent findings also suggest two possible independent origins of the muscular septum in Anomalodesmata: once in Cuspidarioidea and once in Poromyoidea (Machado and Passos 2022), as well as a deep division into two distinct clades: (i) a non-carnivorous clade and (ii) a generally deep-water marine carnivorous lineage (Machado and Passos 2022, González-Delgado et al. 2024). Although well known for their shell features, more than 90 % of Cuspidariidae still lack detailed anatomical studies. In this regard, M. aleutiana Machado & Sigwart, sp. nov. contributes additional knowledge to this key family, which is crucial for understanding the evolution of carnivorous bivalves.