Metacanthina issoumourensis

 

Moroccan Devonian Trilobite, here is a specimen which diagnosis was made by P. Morzadec, 2001, in a publication that marked an important step in trilobite knowledge for all moroccan trilobite lovers :) The genus Metacanthina has, in Morocco, 3 species at least : Metacanthina issoumourensis MORZADEC 2001, Metacanthina maderensis MORZADEC 2001 and Metacanthina wallacei (G. & H. TERMIER 1950).

 

2001 Metacanthina issoumourensis nov. sp., MORZADEC 2001, p.66, planche 3, fig. 1 - 3.
 

 
Dorsal view of the cephalon.
Lateral view of the specimen.

Specimen description :

    • Complete trilobite, except for the occipital spine (which is very rarely preserved, or prepared)
    • On matrix, in "living" buried position.
    • Nice dark grey color on a light grey matrix which offers a good contrast.
    • Perfectly symetrical.
    • Very granulous exosqueletton, with eyes very well preserved.
    • Total size : 47 mm.
    • Devonian - Praguian.
    • Ma'Der Region, Morocco.

 

Diagnosis :

 

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.

Super-family

 

 

Acastoiedea

 

 

DELO 1935

 

 
  • Cephalon : Axial furrows slightly to moderately divergent.
  • Anterior glabellar lobes and furrows usually not fused (although furrows may be indistinct).
  • Eyes typically distant from posterior border furrows, but not strongly anterior.
  • Frontal glabellar lobe auxilliary impression system triangular in outline.
  • Maximum width of glabella adjacent to preglabellar furrow.
  • Median region of glabella devoid of muscle scars.
  • Cephalic margin "shouldered".
  • Laterally convex course of genal margin topographically distinct from curvature of axial margin.
  • Thorax : Pleural endings blunt, rounded or angular.
  • Sometimes spinose.
  • Pygidium : Micropygous to subisopygous.
  • Sometimes spinose (expressed as marginal spines).
  • Pygidial axis with deep, apodemal anterior ring furrows, with abrupt transition to shallow posterior ring furrows.
  • Coincident decrease in angle of axial furrow convergence.

Family

 

 

 

 

 

Acastidae

 

 

 

EDGECOMBE 1993

 

 

 

 
  • Cephalon : Semi-circular to rounded subtriangular.
  • Cephalic border narrow in front of the glabella.
  • Dorsal furrows slilghtly to moderately divergent.
  • Anterior glabellar furrows (S3) simple.
  • Medial furrows (S2) moderately deep.
  • Deep posterior furrows (S1).
  • L2 and L3 lateral glabellar lobes generally fused distally.
  • Muscle scars bearing sometimes callosities (Acastinae).
  • Eyes typically distant from the posterior genal furrows.
  • Hypostoma
  • Pygidium : micropygous to subisopygous.
  • Semi-circular to rounded subtriangular.
  • Short posterior spine present.

Sub-family

 Asteropyginae

DELO 1935

Traditionnally included in the Dalmanitidae, Asteropyginae are now included in the Acastids. Mostly benthic trilobites.

  • Cephalon : Closely similar to the Dalamanatid Dalmanitides BARRANDE 1853.
  • Glabella expanding forward.
  • 3 lateral glabellar furrows.
  • Eyes generally large.
  • Genal angles acutely pointed or bearing medium-length to very long genal spines in straight continuation of lateral margins.
  • Pygidium : Rachis never reaching the posterior region of the pygidial border.
  • Generally 5 pairs of lateral spines (sometimes 4 or 6).
  • Unpaired posterior lappet or spine.

Genus

Metacanthina

PILLET 1954
  • Cephalon : Anterior border distinct.
  • Preglabellar area developped
  • Frontal lobe slightly widening.
  • Eyes sometimes fairly developped and distant from the posterior furrow of the Librigenæ.
  • Librigenæ somewhat flat.
  • Long genal spines.
  • Pygidium : Many axial rings.
  • Pleural segmentation type internmediate between boothi and prorotundifrons.
  • Lateral genal spines long, flattened and falciform.
  • Median spine triangular and lanceolated.

Species

issoumourensis

MORZADEC 2001

Metacanthina species characterized by a very granulous exosqueletton.

  • Cephalon : Small flat tongue in front of the cephalon.
  • Occipital spine.
  • Pygidium : 5 pairs of pleural ribs.
  • Median spine broad at its basis, hardly longer than the 5th pair of lateral spines.

 

Discussion :

Complementary description of Metacanthina issoumourensis (After MORZADEC 2001) :

The high number of rachis rings and of pleural ribs, as the strong granulation on the exosqueletton make from Metacanthina issoumourensis a very distant relative of the iberian species Metacanthina aequisulcata GANDL 1972 or the turkish species Metacanthina asiatica (VERNEUIL 1866).

 

Complementary pictures :

Right side view of the cephalon. Study of the librigena and the ocular surface.
Frontal view of the cephalon.

 

Study of the granulations.

 

a) Frontal view of the cephalon. b) Dorsal view of the pygidium.
Dorsal view of the cephalon.

References :

MORZADEC P. 2001. Les Trilobites Asteropyginae du Dévonien de l'Anti-Atlas (Maroc) [Asteropyginae Trilobites from the Devonian of the Anti-Atlas (Morocco)]. Paleontograph. Abt. A 262 (1/3) : pp 53 - 85.

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