Specimen description

• Cephalothorax.
• Dorsal view.
• On matrix.
• Important contrast between the matrix and the natural brown color of the trilobite.
• No lateral compression, but cephalon crushed.
• Total size : 25 mm.
• Devonian
• Kawala, Poland.

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

HAWLE & CORDA 1847

Exoskeleton compact.

• Cephalon : Glabella inflated, anterior slope steep or overhanging.
• "intercalating ring" usually separated from anterior part of the glabela at least by shallow depression that commonly is very distinct. Tendency to decay in isolated nodes.
• Subcephalinc (vincular) furrow on doublure continuous along whole cephalon (seometimes reduced mesially : Eophacops, Reedops)
• Hypostoma triangular to trapezoidal.
• Wings inclined vertically.
• Posterior margin denticulated in some genera.
• Pygidium : short.

genus

Trimerocephalus

M'COY 1849

• Cephalon : Eyes absent.
• Marginal ridge wide, convex.
• Vincular furrow wide, deep.
• Doublure short, flat.
• Dorsal furrows rectilinear.
• Pygidium : Very short and broad (tr.).
• Rachis marked, with segmentation not pronounced.

Species

caecus

GÜRICH 1896

Size sometimes important (length : up to 120 mm). Occurence : Lower Devonian, lower Emsian. E and W of the Rhenisches Bergs.

• Cephalon : Semicircular.
• Dorsal furrows not deep.
• "intercalating ring" with 2 very characteristic nodes.
• Occipital furrow distinct.
• Broad occipital ring with median tubercle almost undistinct.
• Vincular furrow very deep, continuous, with small pits and presenting an ornementation made of 2 types of tubercles - large rounded tubercles intercalated with smaller, spiny tubercles.
• Pygidium : 4 marked axial rings.
• broad (tr.) pleural fields.
• 3 pairs of ribs.