Rhachiosoma bispinosa
Rhachiosoma bispinosa
Common name
Crab
Fossil group
Crustaceans
Order
Decapoda
Genus
Rhachiosoma
Taxonomic status
Accepted
Geological age
Ypresian
Summary
Rhachiosoma bispinosa was a broad portunoid crab with prominent side spines. It was probably an active seabed predator and scavenger, using its claws to seize small animals and process carrion while its spiny carapace provided extra protection.
Spines are costly structures to grow, so their repeated evolution in marine crabs probably reflects the constant pressure of predation. In the London Clay sea they may have helped make this crab harder for fish or larger predators to swallow or handle.
Quick Identification
Look for: a broad portunoid carapace with two especially prominent lateral spines; compare the complete margin with the more extensively spined Rhachiosoma echinata.
Diagnostic Features
- Carapace broad and portunoid in overall outline.
- Two prominent lateral spines dominate the side margins.
- Dorsal surface is regionally organised but less extensively spinose than Rhachiosoma echinata.
- Family is intentionally unresolved within Portunoidea in the current LBPS treatment.
- Species-level identification requires the paired-spine margin together with the full carapace proportions.
Fossil Details
Typical size
Centimetre-scale carapace; a secure species-wide range is not established.
Fossil material
Carapace, Claw / Chela
Preservation
The diagnostic lateral spines are vulnerable to breakage. Specimens missing both side margins may not be safely separable from other portunoid crabs.
Similar Species
Recorded Fossil Sites
No fossil-site occurrences have yet been recorded for this species.
References
- Hiroaki Karasawa; Carrie E. Schweitzer; Rodney M. Feldmann. (2008). Revision of Portunoidea Rafinesque, 1815 (Decapoda: Brachyura) with emphasis on the fossil genera and families. Journal of Crustacean Biology, 28(1), 82–127 View reference
- J. S. H. Collins; C. J. T. Mellish; A. J. Ross; P. R. Crabb; S. K. Donovan. (2020). A guide to the fossil Decapoda (Crustacea: Axiidea, Anomura, Brachyura) of the British Isles. Proceedings of the Geologists' Association, 131(1), 19–50 View reference
Last Reviewed
16 August 2026
Reviewed by
Sam Harris