The Echinoid Directory

Acanthechinus Duncan & Sladen, 1882, p. 34

[=Dixieus Cooke, 1948, p. 606, type species Phymosoma dixiei Cooke, 1941]

Diagnostic Features
  • Test wheel-shaped, depressed.
  • Apical disc subcircular, moderately narrow (a little less than one-third test diameter in length). Plates not bound to corona, always missing.
  • Ambulacra straight; plating polygeminate, compounded in phymosomatid style. Pore-pairs undifferentiated; biserially arranged aborally to the ambitus with 8-10 elements to a compound plate, in arcs subambitally and uniserial adorally with five elements to a compound plate - no phyllode expansion.
  • A single primary tubercle to approximately every other compound plate at the ambitus and above; intervening plates with smaller secondaries and granules only. Primary tubercles relatively large, leaving little space for perradial granulation.
  • Interambulacral plates a little wider than tall; with a single large primary tubercle, positioned centrally plus adradial and interrtadial bands of small secondary tubercles and granulation. No well-developed naked interradial zone present aborally.
  • Tubercles imperforate and crenulate (the radial ornament of the boss described in the original is an artefact of weathering). Mamelon relatively fine.
  • Peristome moderately small, circular, a little sunken, with small, but clearly defined buccal notches with thickened lip; no tag.
  • No sphaeridial pits; no basicoronal interambulacral plate.
  • Spines unknown.
  • Lantern unknown.
Latest Palaeocene - Eocene, Pakistan, USA, Caribbean.
Name gender masculine
Acanthechinus nodulosus Duncan & Sladen, 1882, p. 34 [=Cyphosoma abnormale Duncan & Sladen, 1882, p. 32].
Species Included
  • Acanthechinus nodulosus Duncan & Sladen, 1882; latest Palaeocene-Lower Eocene, Pakistan.
  • A. dixiei (Cooke, 1948); Upper Eocene, USA.
  • A. peloria (Clark, 1927); Eocene, Jamaica.
Classification and/or Status
Phymosomatoida, Phymosomatidae.

The type species is a poorly preserved fragment. Its distinguishing feature, namely the radial ornament of the boss, is commonly seen in many echinoids that have been rather heavily weathered from this bed, and is an artefact of preservation. Restudy of this material shows that there are no characters that distinguish it from the better preserved species Cyphosoma abnormale, that Duncan & Sladen described a few pages earlier.

Differs from Phymosoma in having primary ambulacral tubercles on every second plate. In larger specimens there are ca. 10 pore-pairs to a primary tubercle, not the usual five, and two compound plates form a single composite structure.

Duncan, P. M. & Sladen, W. P. 1882. The fossil Echinoidea from the Ranikot Series of Nummulitic starat in western Sind. Memoirs of the Geological Survey of India, Palaeontologia Indica. Tertiary and Upper Cretaceous fossils from Western Sind, Series XIV, Vol. 1.3.