Phacopidina micheli micheli TROMELIN 1877

 

This is a trilobite which genera is uncertain, belonging to 2 different families according to the writers. This is why one can read incertæ sedis in the diagnosis. Rábano and Struve class it in the Calmoniidæ but classically, it has been described as belonging to the Phacopidinæ.

 

Phacopidina micheli micheli

Specimen description :

  • Almost complete : small left part of the pygidium and lef eye missing.
  • Dorsal view.
  • On a chip of matrix.
  • Total size : 28 mm.
  • Ordovician - basal Caradoc.
  • Marrolithus bureaui Zone.
  • Guichen, Ille-et-Villaine.
  • France.

 
Complete view of the specimen.

 

Diagnose :

 

Order

 

 

 

 

 

 

 

Phacopida

 

 

 

 

 

 

 

SALTER 1864

 

 

 

 

 

 

 

Occurrence: Lower Ordovician (Tremadoc) to Upper Devonian (Famennian).

  • Cephalon: Proparian (Phacopina and Cheirurina), gonatoparian (Calymenina) or opisthoparian (Calymenina),
  • Preglabellar field often very short or absent.
  • 4 or fewer pairs of glabellar furrows.
  • Eyes : when present, schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina).
  • With rostral plates (Calymenina and Cheirurina) or without (some Phacopina).
  • Hypostome conterminant (all suborders) to impendent (some devonian Phacopina).
  • Exosqueletton generally granulous.
  • Thorax: 8 – 19 segments, sometimes distinctly furrowed.
  • Axis sometimes broad (e.g., Homalonotidae).
  • Pygidium : Typically micropygous (most Calymenina and Phacopina), but variable (e.g., subisopygous in Dalmanitoidea and Acastoidea).
  • May be lobed or spiny (e.g., Cheirurina, some Dalmanitoidea, Acastoidea), or smooth-margined, with round or subtriangular outline (e.g., Calymenina, Phacopoidea).

Sub-order

 

 

 

 

 

Phacopina

 

 

 

 

 

STRUVE 1959

 

 

 

 

 
  • Cephalon : Proparian sutures (sometimes fused).
  • Schizochroal eyes.
  • Glabella expands forwards.
  • Librigena typically yoked as single piece.
  • Hypostome conterminant to impendent, some with no rostral plate.
  • Some with genal spines.
  • Thorax: 10 to (typically) 11 segments.
  • Pleurae furrowed, articulating facets distinct.
  • Rounded, angular, or spinose tips.
  • Pygidium : Typically smaller than cephalon (but subisopygous in Dalmanitoidea and Acastoidea).
  • Smooth or spinose.

Superfamily

 

Phacopoidea

 

HAWLE & CORDA 1847

 
  • Cephalon : Auxiliary impressions short or lacking.
  • Dorsal furrows strongly diverging.
  • "anterior" lobe composite.
  • No genal spines.
  • Vincular furrow generally present.

Family

 

Phacopidæ

 

HAWLE & CORDA 1847

 
  • Cephalon : Glabella broadening markedly forward.
  • S2 and S3 lateral glabellar furrows obsolescent.
  • S1 generally transglabellar, thus forming a more or less distinct "intercalating ring".
  • Auxiliary impressions field ovale à circulaire.
  • Genal angles rounded, lobiform or angular.
  • No genal spines.
  • Hypostoma triangular to trapezoidal.
  • Thorax : Thoracic pleurae with rounded ends.
  • Pygidium : Well-rounded, semicircular or shorter.
  • Margin entire.
  • No spine.

Subfamily

 incertæ sedis (Phacopidinæ ? Calmoniidæ ?)

 

 If one considers this trilobite as a phacopidine, one has to refer to the diagnosis of Chotecops : The Exoskeleton is almost entirely strongly inflated, notably at the cephalon
  • Cephalon : Frontal lobe strongly to very strongly inflated, almost completely hiding the border in dorsal view.
  • Most taxa, particularly from Emsian to Fammenian, present a distinct - sometimes deep - vincular furrow in the anterior part of the cephalon.
  • Pygidium : Smaller to notably smaller than the cephalon.

Genus

Phacopidina

BANCROFT 1949
  • Cephalon : Ogival.
  • Anterior cephalic border distinct, more or less flatenned.
  • Suture slightly isolated of the furrow préglabellaire.
  • Frontal lobe losangic.
  • Glabellar furrows S2 and S3 less marked than the deep and forked furrow S1.
  • Very short genal points.
  • Vincular furrows on each lateral side of the doublure.
  • Pygidium: Small, triangular.
  • Rachis with 5 to 8 rings.
  • Pleural lobes prolonged by a more or less developed terminal spine, incised by 4 to 5 pairs of pleural ribs.

Species

micheli

TROMELIN 1877

 
  • Cephalon: Glabella non claviform, limited by narrow dorsal furrows.
  • Occipital furrow slightly marked in its median part.
  • Tiny genal points.
  • Lateral border of the genae punts with more or less sharp edges.
  • Lateral cephalic furrows marked well at the level of the eyes.
  • Very large kidney-like eyes comprising 28 or 29 dorso-ventral files with nine lenses to the maximum.
  • Narrow cephalic doublure.
  • Very lengthened hypostom, with posterior border strip-like equipped with 2 small round outgrowths.
  • Pygidium: 7 or 8 axial rings.
  • 4 pairs of pleural ribs with obsolete interpleural furrows.
  • Tiny hardly visible terminal spine.

Subspecies

micheli

 

Complementary pictures :

 

Phacopidina micheli micheli Phacopidina micheli micheli
Dorsal view of the cephalon (B&W).
Dorsal view of the pygidium (B&W).

 

Phacopidina micheli micheli
Synthetogram of Ph. micheli micheli (After P. Lebrun, 2002)

 

 

Differential diagnoses :

Phacopidina micheli couyerensis.

The distinction is relatively dfficult to make and one has sometimes to eventually consider the identification via geographic criteria only.

 

Species

micheli

HENRY 1980

 

Occurence : Lower Caradoc (Upper part, "zone à Marrolithus bureaui") of the Andouillé formation - synclinorium of Menez-Belair.

  • Cephalon : Relatively small eyes.
  • 25 ou 26 dorso-ventral rows of 8 lenses max.
  • Pygidium : Medial spine a bit more distinct than Ph. micheli micheli.

Subspecies

couyerensis

 

Phacopidina micheli ssp.

 

Species

micheli

?

 

Occurence : upper part of the Postolonnec formation (Finistère), in the stages just under the "zone à Marrolithus bureaui" (llandeilo ?) and rarely under the "grès de Kerarvail" (Abereidian).

  • Cephalon : Eyes a bit larger than Ph. micheli couyerensis.
  • Probably fewer lenses, with 25 files dorso-ventral rows of 7, maybe 8, lenses max.

Subspecies

ssp.

Back to Home Page