| Necrogammarus Temporal range: Late Silurian,
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| Illustration of In.43786, the holotype (and only known) specimen of N. salweyi. | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Order: | †Eurypterida |
| Superfamily: | †Pterygotioidea |
| Family: | †Pterygotidae |
| Genus: | †Necrogammarus Woodward, 1870 |
| Species: | †N. salweyi
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| Binomial name | |
| †Necrogammarus salweyi Woodward, 1870
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Necrogammarus salweyi is the binomial name applied to an arthropod fossil discovered in Herefordshire, England. The fossil represents a fragmentary section of the underside and an appendage of a pterygotid eurypterid, a group of large and predatory aquatic arthropods that lived from the late Silurian to the late Devonian. The Necrogammarus fossil is Late Silurian in age and its generic name means "dead lobster", deriving from Ancient Greek νεκρός (nekrós, “dead body”) and Latin gammarus ("lobster").
Historically classified as either a millipede or a crustacean (hence the name) and once considered to potentially represent the oldest myriapod in the fossil record, Necrogammarus was first revealed to represent a fragmentary section of a large pterygotid eurypterid in 1986 by the researcher Paul Selden. As two large pterygotids are already known from the same locality and same timespan as Necrogammarus, Erettopterus and Pterygotus, it is possible that the Necrogammarus remains, in reality, belong to one of these genera. As both genera are only diagnosed and differentiated from each other by features in body parts absent in the Necrogammarus fossil, such as the chelicerae (frontal appendages) and coxae (leg segments), it is impossible to assign Necrogammarus to either of them for the time being.
Pterygotids are differentiated from other eurypterids by their flattened telsons (the most posterior segment of the body) and their modified chelicerae (frontal appendages), ending in well-developed chelae (claws). Regardless of its potential identity as either Pterygotus or Erettopterus, these features are likely to have been present in Necrogammarus as well, as all other members of the family possess them.