Specimen description :

• Pygidium, both positive and negative part.
• On matrix
• Total size : 18 mm.
• Upper Devonian - Frasnian.
• Belgium.

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

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.

Genera

Heliopyge

HAAS & MENSINK 1970

• Pygidium : 5 pairs of interpleural "perforations".
• 5 pairs of marginal lateral spines usually well developped, sometimes very long.

Species

helios

RICHTER & RICHTER 1926

• Pygidium : 10 axial rings bearing a strong median tubercle.
• 5 pairs of pleural ribs with broad, complete, pygidial perforations.
• lateral spines very long and thin.
• The 5th pair and the median spines being much smaller than the others.

Synthethogram of the pygidium of Heliopyge n.sp. aff. helios (After P. Lebrun, 2002)