Macroynis casieri

Macroynis casieri

Beckett, Giles, Johanson & Friedman, 2018

Common name Cutlassfish relative
Fossil group Bony Fish
Order Scombriformes
Family incertae sedis
Genus Macroynis
Taxonomic status Accepted
Geological age Ypresian

Summary

Macroynis casieri was a predatory trichiuroid-grade fish related to the radiation that includes snake mackerels and cutlassfishes. It is known from three-dimensional cranial material that was formerly placed in another genus before modern anatomical study showed that it represented a distinct lineage. Its skull and jaw organization indicate a carnivorous teleost adapted to seizing mobile prey.

The precise family-level position of Macroynis remains unresolved, so its ecology should not be copied directly from any one living cutlassfish or snake mackerel. Nevertheless, its trichiuroid affinities and predatory cranial construction make feeding on fishes and other nekton a reasonable inference. The species illustrates how much hidden diversity is revealed by re-examining old London Clay skulls with modern comparative anatomy.

Quick Identification

Look for: a three-dimensional trichiuroid skull with an elongate low cranium, a long predatory jaw apparatus and a distinctive combination of skull-roof and suspensorial proportions; the head is slender rather than the broad compact form of typical mackerels.

Diagnostic Features

  • Neurocranium is elongate and relatively low in lateral profile.
  • Jaws are long and narrow, forming an extended predatory gape.
  • Frontals are elongate and dominate the dorsal roof in front of the posterior cranium.
  • Posterior cranial region is narrow relative to the length of the snout and jaws.
  • Suspensorial bones are slender and obliquely arranged, producing a long cheek region.

Fossil Details

Preservation
London Clay fish skulls are commonly incomplete, compressed, pyritised or enclosed in concretions. Thin cranial margins and projecting processes are easily lost, and separated bones can be difficult to orient; species-level identification is strongest when several adjoining diagnostic bones are preserved together.

Recorded Fossil Sites

References

  • Beckett, H. T.; Giles, S.; Johanson, Z.; Friedman, M. (2018). Morphology and phylogenetic relationships of fossil snake mackerels and cutlassfishes (Trichiuroidea) from the Eocene (Ypresian) London Clay Formation. Papers in Palaeontology, 4(4), 577–603 View reference
  • Casier, E. (1966). Faune ichthyologique du London Clay. 496 pp.. Trustees of the British Museum (Natural History), London View reference
  • Friedman, M.; Beckett, H. T.; Close, R. A.; Johanson, Z. (2016). The English Chalk and London Clay: two remarkable British bony fish Lagerstätten. Geological Society, London, Special Publications, 430(1), 165–200. Geological Society of London, London View reference
  • Monsch, K. A. (2005). Revision of the scombroid fishes from the Cenozoic of England. Transactions of the Royal Society of Edinburgh: Earth Sciences, 95(3–4), 445–489 View reference
Last Reviewed
20 August 2026 Reviewed by Sam Harris